TABLE OF CONTENTS
1.0 OBJECTIVES. 2
2.0 INTRODUCTION. 3
3.0 BASIC THEORY OF DIESEL ENGINE. 3
4.0 FOUR CYCLE DIESEL ENGINE. 5
5.0 MAIN COMPONENTS OF INTERNAL COMBUSTION ENGINES. 8
6.0 FUEL SUPPLY SYSTEM 10
7.0 STARTING SYSTEM. 10
8.0 GOVERNING AND SPEED CONTROL. 11
9.0 LUBRICATION SYSTEM. 13
10.0 COOLING SYSTEM. 14
11.0 INJECTION SYSTEM 16
12.0 AIR INTAKE SYSTEM. 16
13.0 EXHAUST SYSTEM. 17
14.0 ALARM AND SHUTDOWN SYSTEM. 17
15.0 INSPECTION AND MAINTENANCE. 18
16.0 OPERATION. 19
17.0 TROUBLE SHOOTING. 20
The trainee will be able to:
- State the definition of an engine, internal combustion engine and diesel engine.
- Demonstrate an understanding of the basic diesel engine theory.
- Demonstrate an understanding of four cycle diesel engine.
- Identify the main components of a diesel engine and state its functions.
- Demonstrate an understanding of Fuel supply system.
- Demonstrate an understanding of starting system.
- Demonstrate an understanding of governing and speed control system.
- Demonstrate an understanding of lubrication system.
- Demonstrate an understanding of engine cooling system.
- Demonstrate an understanding of fuel injection system.
- Demonstrate an understanding of air intake system.
- Demonstrate an understanding of exhaust system.
- Demonstrate an understanding of alarm and shutdown system.
- Demonstrate a good understanding of diesel engine operation.
- Demonstrate a sound understanding of inspection and maintenance of diesel engines.
- Acquire generic knowledge of trouble shooting a diesel engine.
Simply, diesel engine is a machine, which produces power by burning oil in a body of air, which has been squeezed to a high pressure by a moving piston. Since it is a machine that produces power, it is called an engine and since the burning or combustion takes place within the engine itself, it is called an internal combustion engine. Thus heat is converted to power.
Diesel engines are built either two stroke or four stroke. In a two cycle engine it takes two strokes of the piston (one up and one down stroke) to complete a cycle. In a four cycle engine, a complete cycle requires four strokes of the piston; i) the intake stroke, ii) the compression stroke, iii) the power stroke and the iv) exhaust stroke.
In this module, we will learn the basic diesel combustion theory, diesel engine components, auxiliary systems, operation, maintenance and trouble shooting.
3.0 BASIC THEORY OF DIESEL ENGINE.
In a diesel engine, ignition of the fuel is accomplished by the heat of compression alone. To support combustion, air is required. Approximately 14 pounds of air is required for the combustion of 1 pound of fuel oil. However, to insure complete combustion of the fuel, an excess amount of air is always supplied to the cylinders.
The ratio of the amount of air supplied to the quantity of fuel injected during each power stroke is called the air-fuel ratio and is an important factor in the operation of any internal-combustion engine. When the engine is operating at light loads there is a, large excess of air present, and even when the engine is overloaded, there is an excess of air over the minimum required for complete combustion.
The injected fuel must be divided into small particles, usually by mechanical atomization, as it is sprayed or injected into the combustion chamber. It is imperative that each of the small particles be completely surrounded by sufficient air to effect complete combustion of the fuel. To accomplish this, the air in the cylinder must be in motion with good fuel atomization, combined with penetration and distribution.
Figure 1.0 is a reproduction of pressure-time diagram of a diesel engine. The lower curvy part of which is a dotted line, is the curve of compression and expansion when no fuel is injected. At A the injection valve opens, fuel enters the combustion chamber and ignition occurs at B. The pressure from A to B should fall slightly below the compression curve without fuel due to absorption of heat by the fuel from the air. The period from A to B is the ignition delay. From B the pressure rises rapidly until it reaches a maximum at C. This maximum, in some instances, may occur at top dead center. At D the injection valve closes, the fuel is cut off, but burning of the fuel continues to some undetermined point along the expansion stroke.
The height of the diagram from B to C is called the firing pressure rise and the slope of the curve between these two points is the rate at which the fuel is burned.
Theoretically, in a diesel cycle the combustion takes place at constant volume rather than at constant pressure as we can see in Figure 2.0. Most diesels are also four-stroke engines but there are two stroke diesels in operation. The first, or suction stroke (e – a) draws air, but no fuel, into the combustion chamber through an intake valve. On the second, or compression stroke (a – b) the air is compressed to a small fraction of its former volume and is heated to approximately 440° C (approximately 820° F) by this compression. At the end of the compression stroke, vaporized fuel is injected into the combustion chamber (Q1) and burns instantly because of the high temperature of the air in the chamber. Some diesels have auxiliary electrical ignition systems to ignite the fuel when the engine starts and until it warms up. This combustion drives the piston back on the third, or power stroke (c – d – a) of the cycle. The fourth stroke, (a – e) is an exhaust stroke.
The key properties and some useful information about diesel fuel is attached to the end of this module.
Diesel fuel is mainly a mixture of hydrocarbons in liquid form. A hydrocarbon is a chemical compound composed of hydrogen and carbon. When hydrogen combines with oxygen the following reaction takes place:
2H2 + O2 = 2 H2O
Carbon burns with Oxygen in two proportions. First it burns with available oxygen and forms carbon monoxide. If plenty of oxygen is available complete combustion takes according to following equations:
2C + O2 = 2CO
Carbon monoxide, CO, is poisonous gas. It readily combines with oxygen to form carbon dioxide CO2 according to equation
2CO + O2 = 2CO2
If sufficient oxygen is available carbon readily combines with oxygen to form carbon dioxide CO2 according to equation
C +O2 = CO2
Another combustible but undesirable element found in fuels in small amount is sulfur. Sulfur, S, burns with oxygen to form sulfur dioxide SO2 as follows:
S +O2 = SO2
Sulfur dioxide is a corrosive gas in the presence of water. It is responsible for the corrosion found inside the exhaust pipes.
The efficiency of the diesel engine is inherently greater than that of any Otto-cycle engine and in actual engines today is slightly more than 40 percent. Diesel engines are, in general, slow-speed engines with crankshaft speeds of 100 to 750 revolutions per minute (rpm). Some types of diesel, however, have speeds up to 2000 rpm. Because diesels use compression ratios of 14 or more to 1, they are generally more heavily built but this disadvantage is counterbalanced by their greater efficiency and the fact that they can be operated on less expensive fuel oils.
4.0 FOUR CYCLE DIESEL ENGINE
The elementary diagrams in Figure 3 and 4 show the moving parts of a simple four cycle, single cylinder diesel engine. The illustration is divided into several views with moving parts in different position of a single cycle. By studying the different positions in order, we learn how the engine works.
In figure A – B, we find the engine with its piston at the top of the cylinder (TDC) and ready to draw in a charge of air. The inlet valve is open and the other valves are closed. The crank is turning right and downward. The crank is pulling down the connecting rod and piston. As the piston descends it draws a charge of fresh air. This is called intake stroke.
In figure C – D, we find the piston moving upwards with both valves closed. Air is being compressed and at the same time the temperature of the compressed air rises. This is called the compression stroke.
In figure E, we see that piston is at the top (TDC), compression stroke is complete and the injector is delivering fuel into the hot and compressed air. Fuel combusts quickly and increases the pressure in the combustion chamber. This pushes the piston down (see figure E). This stroke is known as power stroke or expansion stroke
In figure G the power strokes is complete, exhaust valve opens and spent gases get expelled from the cylinder. When the piston has risen to the top, the spent gases have all been expelled from the cylinder and the exhaust stroke is complete.
This ends one cycle. The inlet valve opens once again and the engine is back to intake stroke as shown in diagram A.
5.0 MAIN COMPONENTS OF INTERNAL COMBUSTION ENGINES.
The essential parts of a diesel engine are similar to a gas engine except for the ignition system. The combustion chamber consists of a cylinder, usually fixed, that is closed at one end in which a close-fitting piston slides. The in-and-out motion of the piston varies the volume of the chamber between the inner face of the piston and the closed end of the cylinder. The outer face of the piston is attached to a crankshaft by a connecting rod.
The crankshaft transforms the reciprocating motion of the piston into rotary motion. In multi cylindered engines the crankshaft has one offset portion, called a crankpin, for each connecting rod, so that the power from each cylinder is applied to the crankshaft at the appropriate point in its rotation. Crankshafts have heavy flywheels and counterweights, which by their inertia minimize irregularity in the motion of the shaft. An engine may have from 1 to as many as 28 cylinders. Entire top engine section is assembled on a frame/bed plate and the frame/bed plate assembly is installed on a foundation and bolted down/grouted. The main parts of a single cylinder diesel engine are shown in Figure 5.0., and their functions are noted below:
- Engine structure holds the cylinders, crankshaft and main bearings in firm relation to each other. This structure usually includes all the fixed parts that hold the engine together i.e. bed plate, base or pan and frame.
- A piston sliding in a cylinder. The piston has two jobs: to compress the air charge and to receive the pressure of the gases while they are burning and expanding. A piston typically carries piston rings (compression rings for sealing the space between the liner and piston and oil control rings for scraping the oil from the liner. Piston is connected to the connecting rod by a wrist pin which moves free inside the piston bushing.
- A cylinder head which closes the top end of the cylinder so as to make a confined space in which to compress the air and to confine the gases while they are burning or expanding. Cylinder head carries inlet and exhaust valves, injection nozzle and sometimes the air starting valve and auxiliary combustion chamber.
- Valves or ports to admit the air and to discharge the gases.
- Connecting rod to transmit forces in either direction between the piston and the crank of the crankshaft. It is connected to the piston through wrist pin bearing and wrist pin and at the crank shaft end by crank pin bearing to the crank shaft.
- Crankshaft and main bearings which support the crankshaft and permit it to rotate.
- Fuel injection pump to force the oil into the cylinder; also a fuel injection nozzle to break up the oil into a fine spray as it enters the cylinder.
- Camshaft, driven by the crankshaft to operate the fuel injection pump and also to open the valves (in engines which use valves). Camshaft is connected to the crankshaft through camshaft timing gears for controlled motion.
- Flywheel, to store up surplus energy on the power stroke and to return that energy when the piston is being pushed upward on the compression stroke.
- Governor or throttle, to regulate the amount of fuel supplied at each stroke, and thus control engine speed and power.
- Auxiliary parts such as piping to supply air and remove exhaust gases, muffler to dampen the exhaust noise, lubricating system to lubricate the moving parts, water jackets to cool the cylinders, starters to start the engine (diesel engines are not self starting), fuel supply system (tank, strainer, transfer pump, fuel piping), engine cooling system and instrumentation for engine control, safeguarding, alarm and shutdown.
- FUEL SUPPLY SYSTEM.
The fuel supply system of a diesel engine consists of a diesel storage tank, a feed pump which feeds the fuel through a filter to a fuel pump which pressurizes diesel fuel to a very high pressure. This high pressure fuel is then sent through high pressure tubing to the fuel injector mounted on the cylinder. Fuel injectors atomize the liquid fuel thereby improving ignitability, resulting in complete combustion. The combustion air is admitted to each cylinder and the waste gases exhausted through mechanically operated poppet valves or sleeve valves. The valves are normally held closed by the pressure of springs and are opened at the proper time during the operating cycle by cams on a rotating camshaft that is geared to the crankshaft.
7.0 STARTING SYSTEM
There are three basic types of starting systems used in diesel engines — electric, hydraulic and compressed air.
Electric starting systems use direct currentbecause electrical energy in this form can be storedin batteries and drawn upon when needed. A typical electrical starting system is shown in figure 7.0. The starting motor for a diesel engine operates on the same principle as a DC electric motor. The motor is designed tocarry extremely heavy loads but, because it drawsa high current (300 to 665 amperes), it tends tooverheat quickly. To avoid overheating, NEVERallow the motor to run more than the specifiedamount of time, usually 30 seconds at a time.Then allow it to cool for 2 or 3 minutes beforeusing it again.
Hydraulic Starting System:Hydraulic starting systemconsists of a hydraulic starting motor, a piston-type accumulator, a manually operated hydraulicpump, or an engine-driven hydraulic pump, and areservoir for the hydraulic fluid.Hydraulic pressure is provided in the accumulator by the manually operated hand pump orfrom the engine-driven pump when the engine isoperating.When the starting lever is operated, thecontrol valve allows hydraulic oil (under pressureof nitrogen gas) from the accumulator to passthrough the hydraulic starting motor, therebycranking the engine. When the starting lever isreleased, spring action disengages the startingpinion and closes the control valve. This stops theflow of hydraulic oil from the accumulator. Thestarter is protected from the high speeds of theengine by the action of an overrunning clutch.
Air Starting System: Most large diesel engines are started when compressed air is admitted directly into the engine cylinders. Compressed air is directed into the cylinders to force the piston down and thereby, turn the crankshaft of the engine. This air admission process continues until the pistons are able to build up sufficient heat from compression to cause combustion to start the engine.
8.0 GOVERNING AND SPEED CONTROL.
Governor is a device that controls the engine speed automatically. They can be mechanical, hydraulic or electronic.
The main components in a governor are;
1) Speed sensing mechanism, usually a fly-ball assembly for mechanical hydraulic governor and a frequency transducer for electro-hydraulic unit,
2) Control mechanism of either mechanical linkages connecting to the fuel control unit in a mechanical unit or a hydraulic unit with linkages in a hydraulic system.
Flywheels are used to regulate speed. We know that kinetic energy of a rotating part is proportional to its inertia. So, heavier the flywheel and larger its diameter, the greater its inertia. Flywheels thus absorb a given amount of surplus power depending on its weight and diameter and make the engine run steadier and release the stored power during the suction, compression and exhaust cycle. Flywheels thus assist in the speed control of the engine.
In centrifugal flyweight governors (fig. 8.0), two forces oppose each other. One of these forces is tension spring (or springs) which may be varied either by an adjusting device or by movement of the manual throttle. The engine produces the other force. Weights, attached to the governor drive shaft, are rotated, and a centrifugal force is created when the engine drives the shaft. The centrifugal force varies with the speed of the engine. Transmitted to the fuel system through a connecting linkage, the tension of the spring (or springs) tends to increase the amount of fuel delivered to the cylinders. On the other hand, the centrifugal force of the rotating weights, through connecting linkage, tends to reduce the quantity of fuel injected. When the two opposing forces are equal, or balanced, the speed of the engine remains constant
In hydraulic governors (fig. 9.0), the power, which moves the engine throttle, does NOT come from thespeed-measuring device, but instead comes from ahydraulic power piston, or servomotor. This is a pistonthat is acted upon by fluid pressure, generally oil underthe pressure of a pump. By using appropriate piston sizeand oil pressure, the power of the governor at its outputshaft (work capacity) can be made sufficient to operatethe fuel-changing mechanism of the largest engines.
9.0 LUBRICATION SYSTEM.
Figure 9.0. Shows a typical lubrication system of a medium size compact diesel engine. The lubrication system is fully enclosed and the oil pan acts as the oil reservoir.
The lubricating system delivers oil to the moving parts of the engine to reduce friction and to assist in keeping the parts cool. Most diesel engines are equipped with a pressure lubricating system that delivers the oil under pressure to the bearings and bushings and also lubricate the gears, camshaft and cylinder walls. The oil usually reaches the bearings through passages drilled in the framework of the engine.
All of the engine parts are lubricated with oil delivered by a gear-type oil pump. This pump takes suction through a filter from an oil pan or sump. From the pump, the oil is forced through the oil filter and the oil cooler into the main oil gallery. The oil is fed from the main gallery, through individual passages, to the main crank- shaft bearings and one end of the hollow camshaft. All the other moving parts and bearings are lubricated by oil drawn from these two sources. The cylinder walls and the teeth of many of the gears are lubricated by oil spray thrown off by the rotating crankshaft. After the oil has served its purpose, it drains back to the sump to be used again. The oil pressure in the line leading from the pump to the engine is indicated on a pressure gauge. A temperature gauge in the return line provides an indirect method for indicating variations in the temperature of the engine parts.
10.0 COOLING SYSTEM.
When fuel burns in the cylinders, only a third of the fuel’s heat enrgy is converted to mechanical energy. Rest of the heat shows up in exhaust gases and heat the metal walls which include combustion chamber, the cylinder head, cylinder and piston. The cooling systems job is to remove this unwanted heat from these parts. In removing the heat, the cooling system achieves the following:
- Prevents overheating and resulting breakdown of lubricating oil,
- Overheating and resulting loss of metal and
- Excessive stresses in or between the engine parts
Different type of cooling systems are used depending upon the size and service of the engine such as air cooled or water cooled. Water cooling systems are further subdivided into open type and enclosed type. A typical enclosed type of water cooling system is shown in Figure 10.0 above.
Figure 11.0 shows two typical open cooling systems. In sketch (A), cooling water is received from an external source and return water is dumped. Sketch (B) is a typical closed cooling water system in which water is recycled through a cooling tower and re-circulated by a cooling water pump. The major problem with this kind of system is the scale and sediment build up in the engine jackets.
In Figure 12.0 we see the closed cooling systems with engine jacket cooling water recirculated through a closed system and an external source employed to remove the heat. These closed systems are more efficient than the open systems, more cost effective and reduce the deposit build up in the cooling jackets.
11.0 FUEL INJECTION SYSTEM.
Injection system is the most important part of a diesel engine. The main functions of the system are:
- Meters or measures the correct quantity of fuel to be injected,
- Time the fuel injection,
- Controls the rate of fuel injection,
- Atomizes, or breaks up the fuel into fine particles,
- Properly distributes the fuel in the combustion chamber.
A schematic showing a typical single cylinder injection system is shown in Figure 13.0:
12.0 AIR INTAKE SYSTEM.
Air intake system provides the air required for the combustion of the fuel. Intake piping should be as short as possible and the bends should have a long sweep to reduce resistance to air flow. An air filter is installed in the intake system to prevent ingestion of solid particles such as sand and dust. The common type of filters are i) dry type filters, ii) viscous impingement filters, iii) oil bath filters and iv) water sprays.
Dry type filter is made from cloth, felt, fiberglass or synthetic such as polyester. The surface dirt is blown off with air when clogged. The cloth or felt filter element should be replaced when the pores become permanently clogged. Regular maintenance of the filter and cleaning/replacement of the filter element is critical as negligence may cause loss of power due to reduced air flow and damage to engine from solid particles entering the engine.
In the normally aspirated diesel engine, the atmospheric air passes through an air shut off valve and in a turbocharged engine a fan, blower or compressor provides compressed air through an air shutoff valve and aftercooler to the engine combustion air distribution manifold. The turbocharger could be standalone or driven by engine either by gear, belt drive or by engine exhaust turbine. (see figure 14.0)
Turbochargers are a type of forced induction system. They compress the air flowing into the engine. The advantage of compressing the air is that it lets the engine squeeze more air into a cylinder and proportionately more fuel. This produces more power from each explosion in each cylinder. A turbocharged engine produces more power overall than the same engine without the charging. This can significantly improve the power-to-weight ratio and efficiency of the engine.
Intake silencers are installed at the entrance to the intake system when the noise produced by the engine is objectionable.
13.0 EXHAUST SYSTEM.
Exhaust systems function to i) carry the products of combustion to a safe point for discharge, ii) Reduce noise produced by discharge pulsation and iii) impose minimum backpressure on the engine. Back pressure reduces engine power.
Steel or cast piping is used for exhaust system and the piping is minimized to reduce back pressure. An exhaust muffler is provided to absorb the noise. Insulation for personal protection is installed in the operator access area to prevent personal injury.
14.0 ALARM AND SHUTDOWN SYSTEM.
Alarm and shutdown systems are safeguarding systems and fall into two classes: Alarms to warn the operator and shutdown devices to stop the engine before damage results. The following are some of the common safety devices installed on the engine:
- Cooling water flow (high water outlet temperature and low water pressure)
- Lubricating oil flow (low oil pressure and high oil temperature to indicate oil pump failure or to indicate any oil leaks in the system.)
- Engine speed (Regular governor to control the engine speed and an over speed trip to shutdown the engine)
- Bearing temperature (Either as alarm or trip when the bearings run hot)
- Exhaust temperature (exhaust thermo couples are installed either to give visual indications or as alarms or to shutdown the engine if the cylinder temperature runs abnormally high)
- Day tank level ( high and low level alarms to indicate problems with fuel storage)
- Suction filter blockage (To indicate the suction filter element blockage)
Diesel engines may be shutdown by
- Stopping fuel supply to injection pumps,
- Stopping the action of injection pumps,
- Holding the exhaust valve open and
- Shutting off the air supply.
Engine safety devices and trip systems should receive regular inspection, testing and maintenance to ensure reliable engine operation.
- INSPECTION AND MAINTENANCE.
Inspection and maintenance are vital to ensure smooth operation and to minimize engine failures. Through continuous and detailed inspection procedures, damaged parts or components can be discovered thus preventing early failure of the engine. Underlying conditions will generally include maladjustment, improper lubrication, corrosion, erosion, and other causes of machinery damage.
Particular attention to be paid to the following abnormal condition in an engine:
- i) Unusual noises,
- ii) Vibrations,
iii) Abnormal temperatures,
- iv) Abnormal pressures,
- v) Abnormal operating speeds,
- vi) Any leaks in the engine,
Familiarization with the specific temperatures, pressures, and operating speeds of the engine required for normal operation will help detect any departure from normal operation.
If any gauge or other instrument for recording operating conditions of the engine gives an abnormal reading you must fully investigate the cause immediately and rectify the cause.
House keeping is important for a safe and healthy operation. Ensure the engine and the surrounding area is cleaned at regular intervals.
Promptly attend to any leaks observed. This can prevent major mechanical problems later.
Any changes in the operating speeds (those normal for the existing load) of pressure-governor-controlled equipment, variations from normal pressures, lubricating oil pressure and temperatures, system pressures, quality of exhaust gas and abnormal cooling system temperatures often indicate either improper operation or poor condition of the engine.
Always remember to promptly inspect all similar units to determine whether there is any danger that a similar failure might occur. Prompt inspection and remedial action may eliminate a wave of repeated failures in other equipment.
Pay strict attention to the proper lubrication of all equipment, including frequent inspection and sampling to determine that the correct quantity of the proper lubricant is in the unit. It is good practice to make a daily check of samples of lubricating oil in all auxiliaries. Allow samples to stand long enough for any water to settle. When auxiliaries have been idle for several hours (particularly overnight), you should drain a sufficient sample from the lowest part of the oil sump to remove all settled water. Replenish with fresh oil to the normal level.
The following pointers on starting, operation and stopping of an engine are elementary and are useful reminder and guide. The manufacturers operating manual should be fully read and understood; personnel properly trained in operations and understand the safety requirements associated with the particular engine prior to starting, operating and shutting down of an engine.
- Ensure all safety requirements associated with the engine and driven equipment are complied with and Occidental/ Ipedex HES safety guidelines and the Safah field work permit system are met.
- Carry out a general check of the engine and driven equipment. Complete the manufacturers and OXY pre-start check list.
- Positioning (Check mechanical interference by turning full cycle and put the cylinders in crank position if required)
- Cooling system (make sure water pump is healthy and cooling media tank is full, rid the system of air)
- Lubricating system (Check the oil level, operate the hand pump or turn engine over several times to ensure oil film at the bearings. Where the independent pumps are installed on the engine, run the pumps before starting the engine. Lubricate hand lubricated parts and also grease/oil from cups; operate mechanical lubricators a few times)
- Fuel system (Prime the fuel oil lines and make sure the fuel reaches the injection nozzles)
- Starting system (ensure that the starting battery or the air starting system is fully charged)
- Follow the manufacturers start up procedure and start the engine. (Start up requires i) air and fuel supply to the combustion chamber, ii) compression of the air during the cranking operation)
- If engine does not start promptly, stop cranking to avoid unnecessary loss of starting air or battery charge. Find and eliminate the cause of failure before cranking again.
- Immediately after starting check the lubricating oil pressure, cooling water flow and fuel supply. Watch the entire engine to see if all components are functioning properly. If possible run the engine at light load till it reaches running temperature. Recharge the starting system.
It is important to inspect the engine regularly. Look for leaks and loose fastenings, listen for mechanical noises and check temperatures by instruments or feel. Test emergency devices frequently. Prevent engine overloading as over loading causes combustion troubles and overheating. Ensure uniform temperatures at each cylinder for uniform cylinder loading.
Regulate cooling water temperature and flow within the range recommended by the manufacturer. Low cooling temperature causes condensation on lower cylinder walls and causes liner wear. It also causes incomplete combustion. High outlet temperature promotes scale formation and may breakdown the oil film on the cylinder walls. Scale in jackets causes growth of cylinders resulting in warping, cracking and burning of liners.
Keep the lubricating system clean. Renew filter and purifier elements regularly. Check the condition of the oil by having a sample analyzed and renew on a regular schedule. Keep the lube oil at proper temperature. High temperature promotes oxidation and sludging; it also tends to increase leakage from crankcase. Investigate any increase in crankcase temperature; it may indicate a hot bearing. Investigate any oil pressure change. It signifies clogging of the system. Slow pressure fall indicates bearing or pump wear; a sudden pressure drop means a burnt out bearing.
Excessive lube oil reaching the combustion chamber will cause blue smoke in the exhaust. Keep a log of lube oil consumption and if the consumption is excessive check the piston rings or mechanical defects.
Check combustion conditions. Abnormally high exhaust temperature or rate of fuel consumption means that combustion is poor and is causing waste of fuel.
17.0 TROUBLE SHOOTING.
This chapter is concerned with problems that occurboth when an engine is starting and running. Theproblems are chiefly the kind that can be identified byerratic engine operation, warnings by instruments, orinspection of the engine parts and systems, which canbe corrected without major repair or overhaul. Keep in mind that the troubles listed here are generaland may or may not apply to a particular diesel engine.When working with a specific engine, check themanufacturer’s technical manual and any instructionsissued within OXY.An engine may continue to operate even when aserious casualty is imminent. However, symptoms areusually present. Your success as a trouble-shooterdepends partially upon your ability to recognize thesesymptoms when they occur. You will use most of yoursenses to detect trouble symptoms. You may see, hear,smell, or feel the warning of trouble to come. Of course,common sense is also a requisite. Another factor in yoursuccess as a trouble-shooter is your ability to locate thetrouble once you decide something is wrong with theequipment. Then, you must be able to determine asrapidly as possible what corrective action to take. In learning to recognize and locate engine troubles,experience is the best teacher.
Instruments play an important part in detectingengine problems. You should read the instruments andrecord their indications regularly. If the recordedindications vary radically from those specified byengine operating instructions, the engine is notoperating properly and some type of corrective actionmust be taken. You must be familiar with thespecifications in the engine operating manuals,especially those pertaining to temperatures, pressures,and speeds. You should know the probable effect on theengine when instrument indications vary considerablyfrom the specified values. When variations occur ininstrument indications, before taking any correctiveaction be sure the instruments are not at fault before youtry corrective actions on the engine. Check theinstruments immediately if you suspect them of beinginaccurate.Periodic inspections are also important in detectingengine troubles. Such inspections reveal the failureof visible parts, presence of smoke, or leakage of oil,fuel, or water. Cleanliness is probably one of the greatestaids in detecting leakage.When you secure an engine because of trouble, yourprocedure for repairing the engine should follow anestablished pattern, if you have diagnosed the trouble.If you do not know the location of the trouble, find it by followinga systematic and logical method of inspection.
Once you have associated the problem with a particular system, the next step is to trace out the trouble until you find the root-cause of the problem. Problems generally originate in one system, but may cause damage to another system or to other basic engine parts. When a problem involves more than one system of the engine, trace problems in associated systems separately and make repairs as necessary.
To satisfactorily locate and remedy problems,you must know the construction,function, and operation of the various systems as wellas the parts of each system for a specific engine.
Even though there are many problems that may affect the operation of a diesel engine, satisfactory performance depends primarily on the correct compression of air and injection of the right amount of fuel at the proper time. Proper compression depends on the pistons, piston rings, and valve gear, while the right amount of fuel obviously depends on the fuel injectors and their actuating mechanism. Such troubles as lack of engine power, unusual or erratic operation, and excessive vibration may be caused by either insufficient compression or faulty injector action. You can avoid many problems by following the prescribed instructions for starting and operating the engine.
Troubleshooting in a diesel engine can be difficult and requires detailed analysis of the problem and experience to resolve the problem. On the following pages there is a list of generic problems and corrections. These problems do not comprise a complete list, nor do they all necessarily apply to all diesel engines because of differences in design and construction. Manufacturer’s operation, technical and maintenance manuals should be referred to, in order to resolve the problem and if there is a doubt, manufacturer’s service department should be contacted for assistance.
IMPORTANT NOTE: Always work within the guidelines of the Occidental/ Ipedex HES safety guidelines and the Safah field work permit system. If in doubt stop work and contact the HES department or the departmental supervisor for guidance.
IF IN DOUBT STOP WORK AND SEEK YOUR SUPERVISORS ADVISE
- Engine fails to start
|1||Not enough fuel, air in fuel line||Ensure tank is full and the valves are open; Make sure the pumps and piping is primed and vented. Check the transfer pumps are working and filters are clean.|
|2||Water or dirt in fuel||Drain fuel system and tanks; clean tanks; prime and vent pumps and lines properly. Replace filter.|
|3||Starting valve out of order||Make sure valve admits air just as piston passes top dead centre. Does valve open fully? Check for starting valve leakage and if the valve is stuck open.|
|4||Low compression||Inlet and exhaust valves may not seat properly. Look for stuck or damaged piston rings. Check for cylinder head or valve cage gasket leak and rectify as required.|
|5||Cranking speed too low||Check starting air pressure or battery charge. Bearings may be too tight or cylinder walls and need lubrication. Check for low compression as above. Check the instrumentation faults, rectify faults and reset.|
|6||Fuel injection improperly timed||Make sure fuel injection occurs at or just before top centre, reset timing. Check the manual for correct setting.|
|7||Starter battery flat||Check charging system, recharge or replace battery|
|8||Faulty electrical starter circuit, solenoid or motor.||Check and clear faults in electrical circuit(s), replace faulty motor or solenoid.|
|9||Engine crank does not turn due to internal problem||When the engine crankshaft does not turn after disconnecting the engine from driven equipment, remove the fuel nozzles and check for fluid inside cylinders. If inside fluid is not a problem, engine must be disassembled to check for other inside problems such as bearing seizure, piston seizure etc,.|
|10||Dirty fuel filter||Replace fuel filter|
|11||Dirty or broken fuel lines||Clean or replace fuel lines|
|12||Faulty fuel transfer pump||Check the pump is pumping at the minimum fuel pressure. If not replace fuel filter element. Prime and vent pump and piping. If problem persists replace the pump.|
|13||Bad quality fuel||Drain out the fuel, replace the fuel filter and fill the tank with clean, good and recommended quality fuel.|
- Misfiring or running rough
|1||Fuel pump suction pressure is low||Ensure tank is full and the valves are open; Make sure the pumps and piping is primed and vented. Check the transfer pumps are working and filters are clean. Check for clogged piping and clean. If fuel pressure is still lower than the specified, replace the fuel filter. If the pressure is still low replace the transfer pump.|
|2||Air in fuel system||Check and rectify leak in fuel line, prime pump and bleed air from fuel system. Generally air will enter the system from the pump suction side.|
|3||Incorrect timing||Re-set timing|
|4||Fuel delivery pressure low||– Check fuel in tank, fill
– Check for leak(s) or bad bends in fuel lines, rectify as necessary
– Check for air in fuel system and bleed off.
– Ensure delivery pressure of feed pump is as specified in manufacturers operations manual and replace fuel filter.
– If problem persists replace pump
|5||Leak in high pressure line||Rectify leak|
|6||Incorrect timing||Re-set timing|
|7||Incorrect valve clearance||Re-adjust valve clearance to correct specification|
|8||Faulty injection pump or constant bleed valves||Identify the cylinder that has the maximum misfiring and rectify defects on that cylinders injection element and the constant bleed valve.|
- Engine stalls or stops at low speed or RPM
|1||Fuel pressure low||– Check fuel level in tank, fill
– Check for leak(s) or bad bends in fuel lines, rectify as necessary
– Check for air in fuel system and bleed off.
– Ensure delivery pressure of feed pump is as specified in manufacturers operations manual and replace fuel filter.
– If problem persists replace pump
|2||Engine idle speed low||Adjust governor to increase idle speed and check if the governor linkage is sticking and rectify as appropriate.|
|3||Faulty injection pump||Service injection pump.|
- Exhaust temperature too high
|1||Air inlet leak||Check manifold depression|
|2||Exhaust leak||Rectify exhaust leak|
|3||Incorrect timing||Reset timing|
- Sudden changes in engine speed
|1||Air in fuel system||Check and rectify leak(s) in fuel lines, prime pump and bleed air from fuel system|
|2||Faulty injector pump||Service injection pump as necessary|
|3||Control links sticking, not moving freely||Check governor for broken springs and control links for free movement, repair or replace as necessary. Check governor oil pump and by pass valve.|
- Not enough power
|1||Air in fuel system||Air generally enters the system from the suction side, check and rectify leaks
– Check air filter, clean or renew
– Check for correct manifold installation or gasket leak and rectify.
|2||Fuel pressure low at pump suction||– Check fuel level in tank, fill
– Check and rectify fuel leaks or bad bends in fuel lines
– Check for air in fuel system
Check delivery pressure of the fuel feed pump is in accordance with vendor specification. As a first step replace fuel filter and if fuel pressure is still low, replace feed pump
|3||Faulty injection pump, governor and mechanism||Check governor, control links and injection pump, repair or replace as necessary|
|4||Injector faulty||Replace injector.|
|5||Bad quality fuel||Drain fuel tank, change fuel filter, refill good fuel, prime pump and bleed air from fuel system|
|6||Incorrect valve clearance||Adjust valve clearance to vendor specification|
|7||Incorrect timing||Re-set timing|
|8||Fuel adjustment or fuel to air ratio incorrect||Make adjustments as necessary|
|9||Excessive carbon deposits on turbocharger and other components||Check and clean the deposits, identify the cause of carbon deposition and rectify.|
- Engine stalls frequently or stops suddenly
|1||Air in fuel system, clogged fuel filter, unsatisfactory functioning of injection equipment||– Check the fuel system for air ingestion and repair.
– Replace clogged fuel filter.
– Check the injection system and rectify as appropriate
|2||Improper cooling water temperature||Low cooling temp. increases the ignition lag, causing detonation and higher temperature causes heat damage. Check the thermostat or automatic temperature regulator and replace if faulty|
|3||Loss of compression||Refer to engine technical manual for the procedure and check the compression pressure of each cylinder. Rectify any of the following problems:
– leaking cylinder head gasket,
– leaking or sticking cylinder valves,
– worn pistons,
– liners or rings, or
– a cracked cylinder head or block
|4||Insufficient intake air due to blower failure or to a clogged air silencer or air filter.||Clean or replace the air filter, clean the silencer or repair/replace blower. Note: Do not use solvents to clean filters as they may cause fire or explosion if remain in filter|
|5||Obstruction in combustion space||Due to broken valve heads, valve stem locks or broken springs. Open the head and repair the unit as appropriate.|
|6||Piston seizure||The principal causes of piston seizure are insufficient clearance, excessive temperatures, or inadequate lubrication. Repair or replace piston and rings as required. Rectify lubrication problems|
- Excessive engine vibration
|1||Loose bolts and nuts||Identify the loose bolts and nuts in the engine and tighten. If the bolts or nuts are damaged, replace them. Identify the underlying cause for the bolts getting loosened and rectify.|
|2||Loose, out of alignment components in the engine||Identify the loose/out of alignment components, identify the base cause for looseness or out of alignment and rectify problem|
|3||Unbalanced rotating elements||Check fan blades and other rotating components for unbalance and re balance where necessary|
|4||Misfiring or running rough||Refer to item 2 above|
|5||Broken or loose foundation bolts||Major maintenance required to replace the broken foundation bolts|
- Engine knock (loud noise)
|1||Bad quality fuel||Drain fuel tank, change fuel filter and refill with correct grade of quality fuel in the tank|
|2||Defect in injection pump||Test and repair, re-adjust or service as necessary|
|3||Incorrect timing||Re-set timing|
|4||Worn engine bearings||Inspect the con. rod and crankshaft bearings for wear and damage and repair as necessary. (major overhaul)|
|5||Worn pistons||Replace pistons and piston rings (major overhaul)|
|6||Worn valve mechanism, gears, incorrect valve tappet clearance||Replace worn components as necessary (semi major overhaul)|
|7||Loose or worn engine mount, broken or loose foundation bolts||Replace worn engine mount|
|8||Damage to crankshaft||Check crankshaft for damage and deflection and repair or replace.|
- Clicking (loud noise) from valve mechanism
|1||Lack of lubricant||Check lubricant pressure, lubricant flow and passages. Correct as necessary|
|2||Damaged valve springs or locks||Replace the damaged parts. Defective locks can cause the valves to drop inside cylinders, which can damage the engine extensively.|
|3||Excessive valve clearance||Readjust valve clearance to vendor specification|
|4||Damaged valves rocker arms and push rods||Remove cylinder head if required, inspect valve mechanism, re-lap valves seats, replace damaged parts and reassemble with correct setting.|
- Oil in cooling system
|1||Defective oil cooler||Replace oil cooler or identify and plug the leaks in the exchanger|
|2||Head gasket blown||Replace the head gaskets|
|3.||Piston liner “O” rings damaged, water leaks into crankcase||Pull the liner out and replace “O” rings. Replace lubricating oil and coolant after thoroughly cleaning the lube oil and coolant system if contaminated.|
- Engine has early wear
|1||Lube oil contamination||Remove dirty oil, flush the entire system and fill correct grade of fresh oil. Replace the oil filter|
|2||Air inlet leak||Check the inlet system. Clean or replace filter. Repair any leaks in the inlet system.|
|3.||Fuel or water leak into the lubricating oil||Identify the source of leak and carryout necessary repairs.|
- Fuel consumption too high
|1||Fuel system leaks||Large changes in fuel consumption may be the result. Inside leaks probably will cause low engine oil pressure and an increase in oil level. Identify the leaks and rectify|
|2||Incorrect injection timing||Make an adjustment to timing|
|3||Damaged combustion system internals||Identify the damaged internals (valves, injector, piston rings etc) and repair or replace as required.|
- Knock (mechanical noise in engine)
|1||Defect in connecting rod bearings||Identify the damaged bearing and replace (engine overhaul)|
|2||Damaged timing gears||Replace damaged gears and reset timing|
|3||Damaged crankshaft||Disassemble and repair engine|
- Low oil pressure
|1||Defective oil pressure gauge||Recalibrate or replace oil gauge.|
|2||Dirty oil filter or oil cooler||Replace oil filter element. Check the filter bypass valve and repair if required. Clean the cooler tube bundle. If the cooling oil is dirty, replace the oil.|
|3||Diesel fuel in lubrication||Find the source of leak and repair. Replace the contaminated oil and oil filter element.|
|4||Excessive bearing clearance at the crank-shaft, camshaft and connecting rod||Inspect bearings and replace if necessary.|
|5||Oil pump relief valve does not operate properly||Clean and recalibrate the valve to correct pressure|
|6||Lube oil piping leak||Identify the leak and repair.|
- Oil at the exhaust
|1||Too much oil in the exhaust compartment||Oil seeping into the exhaust from valve compartment. Identify the source of leak and rectify.|
|2||Worn valve guides||Replace valve guides, recondition the cylinder head.|
|3||Worn piston rings||Inspect the ‘O’ rings and replace with new rings|
- Engine coolant too hot
|1||Not enough coolant in the system||Add coolant to the system.|
|2||Clogged radiator or heat exchanger||Clean the radiator or the heat exchanger. Check the cooling fan and fan belt. Check if the piping and hoses are clogged and clean or replace them.|
|3||Defective temperature gauge, thermostat or temperature controller||Identify the defective instrument and repair or replace as appropriate.|
|4||Scale build-up in the cooling system||Descale the cooling system. Source and replace the existing cooling water with demineralised water.|
|5||Defective cooling water pump||Repair the water circulation pump|
|6||Too much load on the system||Identify the cause of the load and amend the system as appropriate.|
|7||Wrong fuel injection timing||Make adjustment to the timing.|
- Incorrect exhaust
|1||Blue smoke||Blue smoke occurs when oil is entering the combustion chamber and is burning along with the fuel. Blue smoke also smells like oil burning. Possible causes include valve seals or cracked piston rings. Blue smoke usual indicates a condition which should be corrected a.s.a.p.|
|2||Black smoke||Black smoke is caused by an over rich mixture (more fuel and less air) and normally occurs whenever the engine is working hard. A dirty air filter is also another cause of excessive black smoke. If black smoke is noticed while the engine is under normal operating conditions this condition should be rectified immediately to prevent engine damage.|
|3||White smoke||White smoke is fuel not being burned completely. Extreme white smoke can be caused by the combustion chambers cooling down. Incorrect injection pump timing and coolant getting into the combustion chamber can cause white smoke. Causes are blown head gaskets, cracked heads, cavitation, etc.|
|1||Damaged valve spring(s) or locks||Replace spring(s). Check the clearances and set correct clearance. Check the locks in their slots and if the groves are enlarged, may need replacement of valves. Check other springs in the engine and replace if there are nicks, cracks or surface corrosion.|
|2||Damaged or worn camshaft||Replace camshaft (or cams if the cam alone is damaged), check the rocker clearance and reset with correct clearance. Check the lubrication to camshaft. Check the drive gear train|
|3||Little movement due to damaged rocker arms and push rods||– Make adjustments according to specifications
– Check lube oil passages and flow in vavle lifter assembly, rectify as appropriate
– Check for wear on valve stem end, rocker arm face and push rods and replace worn item(s)
– Check valve clearances and reset with correct clearance.
– Check for wear and tightness in other associated parts.
|4||Damaged valve(s)||Replace damaged valve(s). Check the cylinder combustion temperature, Check the valve setting and entire valve lifting assembly. Check the cooling around valves. Check inside the cylinder, piston head and inside of cylinder head for damage if the valve is fallen inside the cylinder. Check other valves if the damage is repetitive.|
|5||Damaged piston and piston rings||Replace worn and broken rings. Free the jammed rings in piston, check and reinstall if found good. Check grove clearance in piston. Ensure ring clearances (both gap and clearances are correct)
Check the clearance between piston and liner for wear. If wear is excessive check piston lubrication, (both pressure and passages). Check coolant temperature
|6||Damaged piston pins and sleeve bearing||Pull out the pin. Check the bushing and pin contact area for wear with a micrometer. Replace if the wear is excessive. Check lubrication pressure and passages for clogging and clean as necessary.|
|7||Connecting rod damage||If the rod is cracked, replace the rod. Check the connecting rod bore for out of roundness. Replace if found excessive out of roundness. Clear the oil passages|
|8||Crankshaft damage||If broken or bent, replace crankshaft. Check web deflection using a deflection gauge. Check for scoring and wear. If mild scoring, dress the scoring using a oil stone. Protect the oil passages while dressing to prevent abrasives entering the oil passage. If heating or burning observed, replace crankshaft. If deflection is outside the acceptable range, recheck alignment if still outside the range, replace crankshaft.|
|9||Faulty air starting system||– Check the starting air charging system and service
– Check the starting air tank RV’s and service them.
– Check for clogged air passages and clean.
– Check if the packing nut is over tightened on starting valve and ease off.
– Check for stuck open valve and release.
– Check for damaged or broken valve and springs, replace.
– Completely disassemble the valve and service if problem persist.
|10||Fuel system problems||– Check if the tank is empty and replenish fuel.
– Check if there is fuel contamination and rectify.
– Replace clogged fuel filter.
– Check the transfer pump and service
– Check high pressure pump and service or replace
– Check the injection timing and rectify faulty timing
– Check the injector functioning, service or replace.
|12||Governor problems||– Check the control linkages and free them.
– Check oil in the hydraulic reservoir and replenish.
– Check oil leakage and repair leaks, replace leaky oil seals
– Always refer to governor manufacturer’s service manual for servicing the governor.
|13||Over-speed trip problem||– Check over speed governor if accidentally tripped and reset.
– Check for improper adjustment, faulty linkages or a broken spring and service as appropriate.
|14||Irregular engine operation problem||– Check for abnormal changes in the supply, temperature, or pressure of the lubricating oil or cooling water.
– Check for colour and temperature of the exhaust as they may also indicate abnormal conditions.
– Check for abnormal sound and vibrations from an engine
Check engine instruments if they vary from the specified operating condition. Before taking corrective action, check if the instruments are faulty. Diagnose the problem correctly before taking remedial action.
USEFUL INFORMATION ON DIESEL FUEL
Most modern diesel engines operate on any good commercial diesel fuel oil of 40 cetane or above. A typical diesel fuel oil specification is shown in below.
|Fuel oil physical properties||Limits|
|API gravity||30 min.|
|Cetane number||40 min. (Note 1)|
|Sulphur %||0.7 max.|
|SU viscosity Sec.@ 100°F||30-50|
|Water and sediment %||0.1|
|Pour point °F. Min.||10°F below amb. air|
|Ash % Max.||0.02|
|Alkali or mineral acid||Neutral|
|50%||475 to 550|
|End point max.||725|
|Cloud point||not more than 10°F above Pour Point|
Fuel Oil Specifications
The cetane number is the ‘ignition quality’ of the fuel and is a major factor of ‘ignition delay period’. High ‘ignition quality’ fuels have a short ‘ignition delay period’.
Excessive ‘ignition delay period’ cause large quantities of fuel to collect in the combustion space, and when ignition commences it burns very rapidly, giving an instantaneous temperature and pressure rise with resultant shock waves. This condition causes ‘diesel knock’.
The higher the engine speed, the higher the ‘ignition quality’ of the fuel required. (Note 1)
Cetane No for low speed engines = 30
Cetane No for high speed engines = 47
This index may now be used instead of Cetane No and is quoted as the Cetane No + 3 , that is:
Diesel index for low speed engines = 33
Diesel index for high speed engines = 50
Viscosity of diesel fuels affects:
The ease of atomization
The pumpability of the fuel
High viscosity leads to an increase in droplet size when the fuel is atomized consequently the rate of combustion is reduced.
This is the amount of non-combustible material in a fuel oil. Gas oils and other distillates are almost free from ash. In residual fuel oils the percentage could be up to 0.2 per cent, depending upon the source of the crude oil supply, the refining process and market requirements. Efficient centrifugal purifying or clarifying usually removes this ash content.
Water is negligible in gas oils and distillates, but could be up to 1.0 per cent in residual fuel oils. Water should separate out when the fuel oil is heated to 100 deg. F., but can also be removed by centrifugal purifying.
Carbon Residue (Conradson Value)
This is the weight of the residue expressed as a percentage of the weight of the original sample of fuel oil after it has been vaporized and vapors burned. For gas oils the residue is under 0.1 per cent, whereas for residual fuel oils the percentage may be as high as 15. It will be appreciated that the higher the Conradson value the higher the amount of carbon which fuel oil will deposit. However, other factors in a diesel engine and a fuel oil affect the amount of carbon deposited. To mention a few, there is the mechanical condition of the engine, cleanliness of the intake air, and other properties of the fuel oil.
Sulfur percentage in fuel oils varies from negligible quantities in gas oils to upwards of 8 per cent in residual fuel oils. The calorific value of sulfur is low, and generally it is undesirable. However, it is usually present in all crude oils and cannot economically be removed completely. Combined with water sulfur is corrosive. Boiler oils can be expected to have up to 3 per cent sulfur content. With perfect combustion in diesel engine cylinders sulfur appears to have no detrimental effects. Leaking piston rings and water leakages must be avoided, especially if the products of combustion can enter the engine crankcase.
There are three ways in which detrimental matter can be removed from fuel oils:
Pass the fuel through a fine mesh,
In a fuel installation supplying the engine, all or a combination of the above systems may be used. However, the most commonly found is the fine mesh filter which is fitted on the fuel inlet upstream of the fuel pumps.
Fuel Specifications – Gas
Most gas engines operate on any of the following range of gas fuels; the type of fuel however affect performance and efficiency of the engine:
- Natural gas is the most common type of natural gas engine fuel, it is a gas that has had impurities removed.
- Sewage gas is formed from sewage stored in special tanks from which the gas is drawn off to run engines that power the treatment plant.
- Propane is a fuel that is normally stored in a liquefied state. It has more heat value than natural gas, is more expensive and has to be vaporized before entering the engine. Propane is heavier than air.
- Butane is also a fuel that is stored as a liquid and must be vaporized before entering the engine. It contains even more heat value than propane. It is little used because of it’s low critical compression value and it is also heavier than air.
- Field gas is the name for gas as it comes from the gas well. It is not called natural gas because it still contains impurities such as water, propane and/or butane.
- Sour gas is field gas which is contaminated with hydrogen sulfide. It is very corrosive and its use as a fuel would shorten engine life and pollute the environment.
- Wet gas is field gas with a large amount of water.
Most of these gases above are odorless and colorless (except sour gas which smells like rotten eggs) so usually a ‘tracer’ is added which gives the gas a smell and allows leaks to be detected. The ‘tracer’ is not burned in the engine, but goes out through the exhaust stack. The heat values of the above gases are stated in Btu’s, (British Thermal Units). One Btu will heat one pound of water through one degree Fahrenheit. The higher the Btu content of the fuel gas the more horsepower the engine will produce.
Detonation is identified by a sharp “pinging” or knocking noise coming from the engine cylinders. In normal combustion, the fuel burns with a flame front that progress outward from the spark plug. Under certain conditions, the fuel can be made to ignite instantly throughout the cylinder volume. Detonation occurs due to the temperature of the air fuel mixture exceeding the self-ignition temperature (spontaneous combustion). Detonation is primarily controlled by compression ratio, ignition timing, and intake air temperature. Detonation causes excessively high stresses to be developed in the components of the I.C. engine and as such must be avoided at all times.
Critical compression is the amount of compression required to cause detonation of a fuel/air mixture. Table 2 shows the critical compression ratios for various fuel gases. Natural gas has a critical compression of 15:1 which is above the 12:1 compression ratio of the high compression gas engine. However, if the cooling system or fuel supply system of the engine is faulty, it is possible the fuel/air mixture will detonate. Butane, with a critical compression of 6.4:1, is usually only used in low compression naturally aspirated engines, but because of it’s low critical compression and high cost it is rarely used.
Per cu. ft. (H.H.V)
|Sewage gas||500 – 650||Above 15:1|
|Wet gas||Above 1000||Below 15:1|
|Field gas||Above 1000||Below 15:1|
|Sour gas||1,000||Below 15:1|
Propane has a higher critical compression than butane i.e. 12:1, which equals the compression ratio of a high compression gas engine. However, providing the engine is not overloaded and a sufficient supply of cooling water to the after-cooler is available no problems are experienced.
Wet gas and field gas have a lower critical compression than natural gas because of the propane and butane mixed with it. These amounts can vary from well to well so a critical compression of below 15:1 is only given as an approximate figure.
The above gases are given another rating which is called the ‘equivalent octane rating’, and this is a measure of how fast the fuel burns. The higher the octane rating the slower the fuel burns. Each type of fuel reacts differently during combustion so it is very important that the timing of ignition is correct. The ignition timing will also need to be adjusted if the engine is to be run using a different fuel gas from that originally specified. Ignition timing that is too early will cause loss of power. Retarded timing will also cause loss of power but also a very hot exhaust system.
A few important definitions
Flash point. The temperature, to which oil must be heated before the oil vapour over the oil ignites when a small flame is passed across the surface of the oil.
Pour point. Pour point is the lowest temperature at which petroleum fraction flows or can be poured. It is an indication as to how suitable a fuel is for cold-weather operation.
Viscosity. Internal resistance to flow in a liquid or gas. In practice, for oils it is measured by the number of seconds required for a definite quantity to flow through a standard orifice under stated test conditions.
Combustion. The rapid oxidation, or combination, of a combustible such as carbon, hydrogen, or sulphur, with oxygen.
Compression. The act or result of pressing a substance into a smaller space. One of the events of a combustion-engine cycle.
Compression ignition. Ignition of a fuel charge by the heat of the air in a cylinder, generated by compression of the air, as in the diesel engine.
Compression pressure. The pressure of the air charge at the end of the compression stroke.
Compression ratio. The ratio of the volume of the charge in the engine cylinder at the beginning of the compression stroke to that at the end of the stroke.
Volatility. Ability of a liquid to turn into vapour. Influence of heat increases the volatility of a fuel.