Internal Combustion Engines:
Components and Auxiliaries

5

Learning Outcome

When you complete this learning material, you will be able to:

Explain the design, selection, and components of reciprocating internal combustion engine installations including auxiliaries.

Learning Objectives

You will specifically be able to complete the following tasks:

1. 1. Explain design, applications, and selection criteria for the different types of reciprocating internal combustion engines.
2. 2. Explain fuels and combustion processes and fuels used by internal combustion engines.
3. 3. Describe the design of internal combustion engine scavenging and supercharging arrangements.
4. 4. Describe the design and components of internal combustion engine fuel conditioning systems, injection systems, and ignition systems.
5. 5. Describe the design and components of internal combustion engine cooling systems and cooling water conditioning systems.
6. 6. Describe the purpose, design and components of internal combustion engine lubricating oil systems.
7. 7. State the purpose and describe the control of a typical internal combustion engine including the operation of safety devices.

Objective 1

Explain design, application, and selection criteria for the different types of reciprocating internal combustion engines

DESIGN

Reciprocating internal combustion engines are divided into either spark-ignition (SI) or compression-ignition (CI) types. They can operate in either a two-stroke or four-stroke mode. This results in four possible combinations. The two-stroke compression-ignition engine and the four-stroke spark-ignition engine are the most common in industrial applications.

THE FOUR-STROKE CYCLE

The four-stroke cycle occurs over two rotations of the engine, as illustrated in Fig. 1. It consists of the following steps:

• • Induction
• • Compression
• • Power
• • Exhaust

Induction

As the piston moves down, air is drawn into the cylinder through the intake port. The exhaust valve then closes. In spark-ignition engines, a mixture of air and fuel is drawn into the cylinder — unless direct fuel injection is used.

Compression

The intake and exhaust valves are closed and the air (or air-fuel mixture) is compressed. In spark-ignition engines, an electric spark ignites the air-fuel mixture just before top dead centre (TDC) and starts the combustion process. In compression-ignition engines, or fuel injected spark-ignition engines, fuel is injected prior to top dead centre after which combustion occurs.

Expansion

In spark-ignition engines, combustion is largely finished at the beginning of the power stroke. The hot gases expand and force the piston down from top dead centre. The exhaust valve opens just before the end of the stroke. In compression-ignition engines, combustion continues for most of the power stroke.

Exhaust

The exhaust valve remains open and the products of combustion are exhausted to the atmosphere. At the end of this stroke, the exhaust valve closes and the intake valve opens. The process then repeats itself.

Figure 1
Four-Stroke Cycle

THE TWO-STROKE CYCLE

As shown in Fig. 2, the two-stroke cycle takes place over one revolution of the engine with each stroke combining two of the strokes of a four-stroke cycle. To accommodate this, the piston stroke must be longer. One advantage is that no intake or exhaust valves are needed since the piston covers and uncovers the intake (a) and exhaust ports (b).

At the beginning of the first stroke, the intake ports are uncovered. Fresh air then enters the cylinder while the exhaust ports are still open to exhaust the burnt gases from the previous combustion. Once the piston moves up and covers the ports, compression begins. Fuel injection and self-ignition occur before top dead centre. Meanwhile, fresh air is drawn into the crankcase through the non-return inlet valve.

Combustion continues for much of the power stroke at close to constant pressure. The fresh air in the crankcase is partially pressurized during this part of the stroke to assist with induction. Toward the end of the stroke, the exhaust ports, and then the intake ports, are uncovered.

Figure 2
Two-Stroke Cycle

SPARK-IGNITION ENGINES

In spark-ignition engines, a spark ignites the air-fuel mixture. Fuel can be pre-mixed in a carburetor or injected directly into the cylinder. Compression ratios, limited by the need to prevent pre-ignition, or knock , range from 7:1 to 10:1. Supercharging, or pre-compression of intake air, is used to increase power output.

The thermodynamic cycle for spark-ignited engines is also known as the Otto cycle. The ideal description of a thermodynamic cycle is shown in Fig. 3(a) along with the more realistic version in Fig. 3(b). The numbers 1 to 4 correspond to the four strokes of the four-stroke cycle.

Since most combustion takes place while the piston is approaching top dead centre, spark-ignition is said to be a constant volume process. This is not strictly true as can be seen in Fig. 3(b). Once combustion is finished, power is produced by expansion of the hot gases. The combusted mixture is close to atmospheric pressure at the end of the stroke.

(a) Ideal Cycle

(b) Actual cycle

Figure 3
The Spark Ignited Four-Stroke Cycle

The swept volume is the volume traveled by the piston as it moves from bottom to top dead centre and is equal to the area of the piston times the length of the stroke.

The clearance volume is the volume trapped above the piston at top dead centre. Both of these are illustrated in Fig. 3. The compression ratio can be calculated from the clearance and swept volumes using the following equation:

$$ \text{Compression Ratio} = \frac{\text{Clearance Volume} + \text{Swept Volume}}{\text{Clearance Volume}} $$

Note: Since other factors, such as the timing of the opening and closing of inlet and exhaust valves, are also important, this is only an approximate compression ratio.

COMPRESSION-IGNITION ENGINES

In compression-ignition engines, spontaneous ignition occurs due to the rise in temperature caused by high compression ratios. This results in a more efficient engine. Compression ratios need to be higher than 12:1 to allow spontaneous combustion. Ratios of 15:1 to 20:1 are typical, but can be as high as 25:1.

The ideal description of a compression-ignition cycle is shown in Fig. 4(a) along with a more realistic version in Fig. 4(b). The numbers 1 to 4 correspond to the four strokes of the four-stroke cycle. In a two-stroke engine, induction and compression (steps 1 and 2) are combined into the first stroke, and power and exhaust (steps 3 and 4) are combined into the second stroke.

The rate of combustion in a compression ignition engine is controlled by injection of the fuel in order to limit the peak pressure. Combustion continues as the flame front advances. The process continues at essentially constant pressure although this is an approximation, as can be seen in Fig. 4(b).

(a) Ideal Cycle

(b) Actual cycle

Figure 4
The Compression Ignited Four-Stroke Cycle

Comparison of Different Types of Engines

Although a complete discussion of the merits of two-stroke and four-stroke engines is not required here, it is worthwhile pointing out some key differences.

• • Two-stroke engines do not require intake and exhaust valves and are thus simpler, easier to maintain, and less expensive to build
• • Two-stroke engines produce power every stroke instead of every other stroke, and therefore produce more power for a given engine size and weight. This makes them noisier than four-stroke engines of the same size
• • Two-stroke engines are less efficient than four-stroke engines because the induction and exhaust processes are less complete. However, superchargers can be used to increase efficiency
• • Compression-ignition engines are more efficient than spark-ignition engines because they operate at higher compression ratios
• • Compression-ignition engines that burn diesel fuel produce excessive emissions and exhaust smoke that requires treatment

APPLICATIONS

Reciprocating internal combustion engines provide a cost-effective power source for:

• • Many types of vehicles
• • Standby and base load electrical power generators
• • Compressors
• • Pumps
• • Marine applications
• • Industrial equipment applications

Power output ranges from very small (less than 5 kW) to very large (up to 50 000 kW), but the most important range for oil and gas power generation applications is 500kW-5000 kW. The focus of this module is stationary applications for power generation and mechanical drive equipment using natural gas as a fuel.

Fig. 5 shows a typical 12-cylinder natural gas lean burn engine used to drive a compressor. It has a special fuel system that minimizes exhaust emissions.

Figure 5
Typical 12-Cylinder Natural Gas Lean Burn Engine
(Courtesy of Tom Van Hardeveld)

SELECTION

The successful application of any engine depends upon satisfying requirements related to performance, operating costs, and expected engine life. This requires a thorough understanding of available designs, engine rating systems, and knowledge of tradeoffs that might be necessary.

Selecting an internal combustion engine for a specific application depends on factors such as:

• • Type of fuel available
• • Expected performance ratings and load cycles
• • Configuration options
• • Installation requirements and constraints such as weight and size
• • Maintenance resources available
• • Life-cycle costs - capital, operating, and maintenance
• • Noise and exhaust emission requirements

The type of fuel used is a major consideration. The cleanest and most readily available fuel should be used. Pipeline quality natural gas is desirable because it delivers the most efficient, cost effective and environmentally acceptable solution. Lower quality gaseous fuels, such as landfill or sewage gas, require special considerations and could provide less desirable operation because of poor efficiency and lower power output. Diesel fuels, such as kerosene, provide reliable operation. However, they may be unsuitable where emissions are an issue, or where fuel sources are not easily accessible. Lower grade liquid fuels may be cost-effective for lower speed engines, but require fuel treatment and could result in higher maintenance costs.

Configuration options include whether the engine is naturally aspirated (intake air is not compressed) or turbocharged. A naturally aspirated engine is simpler because it has less additional equipment, but performance is affected by altitude and ambient temperatures. A turbocharged engine is more complex because it has a turbocharger and an aftercooler, but it is less affected by external factors and produces more power.

Expected load cycles should be carefully analyzed for feasibility, impact on operation, and maintenance requirements. Many engines can operate for extended periods at peak load, but this increases maintenance effort and costs. Similarly, operation at light loads is possible, but not desirable for longer periods, as operation may become erratic and cylinders might be over lubricated. Estimating load patterns for power generation can be quite complicated because of daily and seasonal load variations (weather and temperature).

Most gas engines are available with low or high compression. High compression is restricted to high quality fuels. Low compression is used for lower quality fuels, and where less stringent emission requirements are in effect.

When selecting an internal combustion engine, it is important to consult with the manufacturer on recommendations for proper application, fuel rating, and approved equipment configuration. Most manufacturers have a method for calculating fuel rating that prevents detonation (engine knocking).

Objective 2

Explain fuels and combustion processes used by internal combustion engines.

FUELS

Most fuels used in internal combustion engines are based on hydrocarbons (hydrogen and carbon). The main source is petroleum, either in the form of natural gas (methane), or a grade of liquid petroleum — ranging from light condensates (e.g. propane) to medium hydrocarbons (e.g. gasoline and kerosene) to heavier oils (e.g. heavy distillates, residuals, and crude oil). Another source is low energy gas fuel obtained from landfills, bio-gas digesters, or coal.

Gaseous fuels are a combination of a hydrocarbon, inert gases such as nitrogen, and possibly contaminants such as sulphur. The overall composition must be carefully considered since it affects combustion processes and emissions.

Liquid fuels may also contain contaminants or other products that can adversely affect engine performance and increase emissions.

Heating value is an important fuel characteristic. This is the amount of energy obtained from a standard amount of fuel when it is fully combusted. It is normally expressed as lower heating value (LHV) . When fuel is burned, water is one of the products of combustion. It is converted to steam during combustion and remains in vapour form in the exhaust. This extra energy cannot be used, and the amount of heat left over for conversion to work is referred to as the lower heating value.

Various approaches are used to determine the suitability of a fuel for combustion and its resistance to detonation, or knock. One common method is the octane rating system developed for liquid fuels. This was adapted for gaseous fuels, but the results have not proven satisfactory for the wide range of fuels in use. Therefore, manufacturers have developed specific methods to evaluate fuels and relate them to engine rating, design configuration, and control limits.

Gaseous Fuels

Many internal combustion engines use natural gas as a fuel source. However, there are many types of natural gas. The specific composition needs to be considered before the gas can be used.

The use of contaminated fuel leads to increased maintenance. The presence of liquids or condensates in natural gas causes pre-ignition, detonation, and other combustion problems. Compounds such as hydrogen sulphide or chlorinated hydrocarbons accelerate corrosion through the formation of acids. Natural gas with high levels of these compounds needs to be treated.

In its original state, natural gas may be referred to as field gas, wet gas (due to the presence of hydrocarbon liquids), or wellhead gas. It is generally unsuitable for use in internal combustion engines. If it contains large amounts of hydrogen sulphide ( \( H_2S \) ), it is referred to as sour gas, and is highly corrosive and damaging to an engine.

Clean natural gas, sometimes called dry pipeline gas or sweet gas, consists of 85%-95% methane. The remainder is usually a mixture of ethane, propane, butane, and other heavier hydrocarbons, mostly in vapour form. It provides the best results for internal combustion engines with respect to efficiency, engine life, performance, and emissions.

Natural gas with a low heating value can be produced from biomass (digester gas), sanitary landfills, or a manufacturing process such as methane recovery from coal. These gases often contain harmful by-products which require special treatment and filtering. In addition, fuel systems may need to be changed to accommodate the higher volume of these fuels before they can be used.

Table 1 shows typical heating values for different gaseous fuels, each of which requires a different carburetion and fuel system configuration.

Table 1
Typical Heating Values for Gas Fuels
(Courtesy of Finning-Caterpillar)

Liquid Fuels

Diesel fuel is the most common fuel used in compression-ignition engines. The most important characteristics of good diesel fuel are cleanliness, self-ignition capability, viscosity, volatility, and temperature.

Diesel fuels are generally sufficiently clean when produced at the refinery, but there is substantial opportunity for contamination by dirt, water, or other substances during transportation and storage.

The self-ignition capability of a diesel fuel is indicated by its cetane number. This is the measure of the ignition quality of the fuel, and is more important for higher speed engines than for lower speed ones. If a low cetane number fuel is used in a high speed engine, a considerable quantity of liquid will accumulate before ignition takes place, resulting in engine knock.

Viscosity is important for heavier fuels or at lower temperatures. Heating may be required in these cases.

At low temperatures, fuels can form waxy deposits which plug filters and cause gummy deposits to build up in the cylinder. Fuels with a higher volatility prevent the build up of deposits on cylinder walls.

Higher speed diesel engines are only suited to run on No. 1 and No. 2 distillate (kerosene) but lower speed engines can use heavier fuels such as low grade residual oils and Bunker C.

Additives may be used to:

• • Improve cetane number
• • Inhibit the formation of combustion deposits
• • Absorb water
• • Reduce foaming

COMBUSTION

Every fuel requires a precise amount of air to produce combustion. The stoichiometric ratio is the chemically perfect air/fuel ratio (AFR) that results in complete combustion. The equivalence ratio is the ratio of the stoichiometric ratio to the actual air/fuel ratio. If the equivalence ratio is less than 1, the mixture is lean. If it is greater than 1, it is rich . Maximum power is generated by a mixture that is about 10% rich (equivalence ratio of 1.1). The best fuel consumption is produced by a lean mixture (equivalence ratio of 0.9, or 10% lean).

The inverse of the equivalence ratio ( \( 1/R \) ), called the Excess Air Ratio or Lambda ( \( \lambda \) ), is also used. In this case, values greater than 1 are lean and values less than 1 are rich.

Spark Ignition

One of the main differences between spark-ignition and compression-ignition engines is the type of combustion. In spark-ignition engines, the fuel is pre-mixed with air in a carburetor using an air/fuel ratio that is close to stoichiometric. If the mixture is too lean or too rich, ignition and combustion may not occur, might be delayed, or could be erratic. Compression-ignition engines use fuel injection instead.

When the spark occurs, the initial onset of combustion is quite slow, and there is a short delay before rapid combustion spreads through the cylinder. Thus, the point of ignition is always in advance of top dead centre as shown in Fig. 6.

Figure 6
Cylinder Pressure for a Spark-ignition Engine

As combustion occurs, the expanding burned gas compresses and heats the remaining unburned gas. This can cause detonation, or knock, if the unburned gas spontaneously self-ignites ahead of the flame front. The severe pressure wave caused by detonation can be very destructive to mechanical components.

Knock should not be confused with pre-ignition, which happens when a hot surface, such as the tip of a spark plug, ignites the unburned gas prior to spark-ignition. Increasing inlet air temperature decreases the knock margin; therefore, detonation may occur more frequently in the summer.

Compression Ignition

In compression-ignition engines fuel is not pre-mixed with air; it is injected into the cylinder. Combustion occurs spontaneously along a flame front where stoichiometric conditions exist. Since combustion is caused by compression, not by a spark, pre-ignition cannot take place.

Objective 3

Describe the design of internal combustion engine scavenging and supercharging arrangements.

SCAVENGING

Scavenging is the removal of combusted gases and the replacement with intake of fresh air (or air-fuel mixture).

The intake and exhaust processes and the geometry of the cylinder, create turbulence. Turbulence is important in speeding up combustion.

In four-stroke engines, because complete strokes are dedicated to intake and to exhaust, scavenging can take place almost completely. Fig. 7(a) shows loop scavenging. Fig. 7(b) shows cross scavenging while Fig. 7(c) is the uniform method of scavenging with exhaust valve.

Figure 7
Two-Stroke Mixing Approaches

SUPERCHARGING

Several methods, including supercharging, are used to increase mixing and swirling, and thus improve the intake, exhaust, and combustion processes. Supercharging pre-compresses the intake air to increase mass flow through the engine. Increasing mass flow directly increases power output. Supercharging can increase the power output for a given engine size by 20%-40%. However, it has almost no effect on efficiency. In two-stroke engines, supercharging also improves scavenging.

Supercharging can be accomplished in two ways:

• • Turbochargers
• • Superchargers

Turbochargers

Turbochargers use a compressor which is attached to a turbine driven by exhaust gases. Turbochargers are common on many engines even though they increase the mechanical complexity of the engine and its control. Fig. 8 shows a turbocharger layout with a centrifugal compressor and an axial turbine.

Figure 8
Turbocharger Layout
(Courtesy of Waukesha Engine)

On V-type engines, twin turbochargers are often used as shown in Fig. 9.

Figure 9
Typical Turbocharger for a Natural Gas Engine
(Courtesy of Tom Van Hardevelde)

An intercooler, shown in Fig. 9, is often inserted before the intake manifold because compression increases air temperature and reduces the effect of increased density.

The amount of boost provided by the compressor is controlled by a wastegate in the exhaust which dumps or bypasses unneeded exhaust air before it reaches the turbine. The wastegate can be installed before the exhaust turbine.

Superchargers

Superchargers make use of a blower or compressor that is directly coupled to the engine. Superchargers are not common in industrial applications because they are less efficient than turbochargers. However, they respond faster to load changes (this is more important in auto racing than in power generation). Superchargers usually consist of a positive displacement compressor, such as the ROOTS™ blower shown in Fig. 10.

Figure 10
ROOTS™ Blower

Objective 4

Describe the design and components of internal combustion engine fuel conditioning systems, injection systems, and ignition systems.

FUEL CONDITIONING SYSTEMS

Spark-ignition engines pre-mix air and fuel using carburetors. Compression-ignition engines use fuel injection systems. Mechanical engine control has largely been replaced by flexible and adaptable electronic control systems that optimize engine operation and efficiency. These systems adjust ignition timing to minimize fuel consumption without causing knock, and may use an oxygen sensor in the exhaust to optimize efficiency (see Fig. 14). They provide protection against abnormal conditions, such as overspeed, and ensure that engine operation does not exceed various limits.

FUEL INJECTION SYSTEMS

Fuel injection systems are used on the following types of internal combustion engines:

• • Spark-ignition
• • Compression ignition

Spark-Ignition Engines

Many fuel systems burn a lean mixture to reduce emissions such as nitrogen oxides (NO _x ). Since lean fuels can cause combustion problems, a prechamber, which burns a rich mixture, is added to provide a torch that ignites and combusts the lean mixture. An example of a prechamber design, also called stratified combustion, is shown in Fig. 11.

Figure 11
Pre-Chamber Design with Stratified Combustion
(Courtesy of Waukesha Engine)

The design of a lean-burn fuel system is shown in Fig. 12. The main air/gas mixer (carburetor), which has a governor controlled throttle, mixes the fuel and air. A pressure balance line between the carburetor and main gas pressure regulator maintains a constant gas-over-air pressure differential. The main gas pressure regulator ensures that natural gas is provided to the main air/gas mixer, and to the prechamber air/gas mixer, at the correct pressure. The prechamber air-fuel mixture is admitted into the cylinder through a separate manifold and special admission valves.

Figure 12
Lean Burn Fuel System
(Courtesy of Waukesha Engine)

Fig. 13 shows a close-up of the carburetor on the engine shown in Fig. 5. An air filter ensures a clean supply of air. The fuel supply has a fuel filter. It may also have a heater to keep the temperature of the natural gas above the dew point, and thus ensure that liquids are not introduced into the engine.

Figure 13
Example of a Carburetor for a Natural Gas Engine
(Courtesy of Tom Van Hardeveld)

Some engines have a separate air-fuel ratio control. This control, shown in Fig. 14, measures the amount of free oxygen in the exhaust and adjusts the air and fuel flows accordingly.

Figure 14
Example of an Air-Fuel Ratio Control
(Courtesy of Finning-Caterpillar)

Air-fuel controlled engines have several advantages:

• • They control emissions at constant levels despite variations in ambient temperature, fuel quality, speed, and load
• • They operate efficiently using a lean mixture that is close to misfire (failure to ignite properly)
• • High compression lean burn engines operate with a narrow margin between misfire and detonation
• • Air-fuel control ensures that engine operation stays within this band

Compressions-Ignition

Toward the end of the compression stroke, most compression-ignition engines inject fuel directly, or indirectly, into the cylinder using a solid (airless) injection system.

Indirect injection systems use a prechamber to speed up combustion and allow engines to run faster. Many different prechamber designs are used. Initial combustion occurs in the prechamber, and then the burning fuel-air mixture is injected into the main cylinder. This action produces swirling and turbulence that speeds up the rate of combustion. Indirect injection is not as effective in two-stroke engines since the increased turbulence interferes with the exhaust portion of the stroke and starting the engine is more difficult.

For slower engines, direct injection is more effective because rapid combustion is less important at lower speeds.

A typical direct fuel injection system for a small engine is shown in Fig. 15.

Figure 15
Fuel Injection System

Fuel pumps control the load and speed of diesel engines by metering the amount of fuel supplied to the fuel injectors. An example of a mechanically controlled fuel pump is shown in Fig. 16. Fuel injectors are an important component of the ignition system since they provide an accurate high-pressure spray that is easily combustible.

Figure 16
Fuel Pump

IGNITION SYSTEMS

Battery operated ignition systems are adequate for low compression engines. In Fig. 17, a primary coil, or transformer, boosts the voltage, and a distributor provides high voltage pulses to the spark plugs.

Figure 17
High Tension Battery Distribution Circuit

In Fig. 18, individual coils are used for each cylinder to minimize the length of the high voltage lines.

Figure 18
Low Tension Battery Distribution Circuit

High compression (1700 – 4300 kPa) spark-ignition engines require high voltage (25 000 to 30 000 volts) to produce an adequate spark. Special spark plugs are used that operate at low enough temperatures to prevent pre-ignition, yet high enough to promote rapid combustion and prevent carbon build up on the electrode.

High compression engines commonly use a magneto. This is a rotating magnet, driven from the engine that does not require a battery because it uses a changing magnetic field to produce its own current. The alternating current generated by the magneto is rectified to direct current and stored in a capacitor. Silicon controlled rectifiers release this electrical energy to high voltage coils located close to each cylinder. A pickup sensor, which reads magnetic reference marks on a timing disc, records the exact position of the crankshaft and tells each coil when to fire. The coils can be seen on top of each cylinder in Fig.5.

Fig. 19 shows a typical spark plug with a coil (high energy ignition transformer) mounted on each cylinder.

1. 1 Coil
2. 2 Tube
3. 3 Extender with Spring Loaded Aluminum Rod
4. 4 Spark Plug

Figure 19
Spark Plug and Ignition Coil

A magneto is shown in Fig. 20.

Figure 20
Solid State Magneto
(Courtesy of Finning-Caterpillar)

Modern ignition timing systems are electronically controlled by variable ignition to enhance engine performance and prevent detonation. An example of an ignition timing system is shown in Fig. 21.

Figure 21
Ignition Timing System
(Courtesy of Finning-Caterpillar)

Objective 5

Describe the design and components of internal combustion engine cooling systems and cooling water conditioning systems.

PURPOSE OF COOLING

The purpose of engine cooling is three-fold:

• • To promote efficiency
• • To enhance combustion
• • To ensure mechanical reliability

Engine efficiency is improved when more air is inducted into the cylinder. When the cylinder walls are cooled, more air can be drawn into the cylinder. In spark-ignition engines, combustion is enhanced by having cooler cylinder walls which will also inhibit knock and detonation.

Mechanical reliability is adversely affected by high metal temperatures and thermal strain. In addition, if the temperature of the top rings on the cylinder exceeds 200°C, lubricants will degrade and fail to provide adequate protection. Thus, it is very important that the cooling system function properly since it has to remove about 20%-40% of the energy input into the engine.

COOLING WATER SYSTEMS

Most internal combustion engines use a mixture of water and industrial grade antifreeze (such as ethylene glycol) which contains various inhibitors and corrosion protectors. A 50/50 water to antifreeze mixture provides the best overall protection against freezing and boiling, but this can reduce the cooling efficiency by as much as 15%. A minimum of 30% antifreeze is usually recommended, but local conditions and manufacturer recommendations should be carefully checked. Cooling water samples should be taken periodically and checked for contaminants and antifreeze strength.

COOLING SYSTEM DESIGN AND COMPONENTS

Cooling systems normally use forced circulation. The coolant pump is powered by the engine, either by a gear or by belts. As shown in Fig. 22, the coolant circulates through the cylinder walls, the cylinder head, and the exhaust manifold. A thermostat (or multiple thermostats) divides the coolant between a direct return line and a cooling circuit that passes through the heat exchanger. The heat exchanger may use air or oil. A top-up reserve tank is often included. Fig. 22 also shows an auxiliary water pump that is used to feed the oil cooler.

Figure 22
Cooling System
(Courtesy of Waukesha Engine)

There are many cooling system configurations. Fig. 23 shows an example of an air to air aftercooler engine. The type used depends on the application, and on whether or not there are other cooling requirements, such as gas compression. For cogeneration applications, the cooling system may be used to heat water sources for domestic or hot water heating.

Figure 23
Cooling System for Compressor Application
(Courtesy of Finning-Caterpillar)

Objective 6

Describe the purpose, design and components of internal combustion engine lubricating oil systems.

PURPOSE OF LUBRICATION SYSTEMS

Lubrication is critical to engine operation for the following reasons:

• • Minimizes friction losses of sliding and rotating surfaces
• • Reduces friction wear on moving components
• • Cools engine parts such as pistons that cannot be cooled directly by cooling water
• • Cleans the engine by flushing away wear particles
• • Helps seal piston rings in the cylinders

OIL PROPERTIES

Oil has several properties that are important for successful engine operation including:

• • Viscosity
• • Additives
• • Acidity
• • Contaminants

Viscosity

Viscosity measures the resistance of a fluid to deformation under pressure. Oil, with a higher viscosity, is better able to withstand the friction forces from two adjacent components. However, friction losses are higher with a higher viscosity, so the proper level of viscosity has to be determined for each application. Since viscosity decreases with temperature, operating temperatures have to be taken into consideration.

Additives

Additives are present in lube oils to improve performance, to prevent deterioration, and to combat contaminants. Common additives are:

• • Detergents to clean engine surfaces by reacting with oxidation products
• • Oxidation inhibitors to prevent increases in viscosity, organic acids or other compounds
• • Dispersants to prevent the formation of sludge by keeping contaminants in suspension
• • Alkalinity agents to neutralize acids
• • Anti-wear agents to reduce friction
• • Pour-point dispersants to counteract the formation of waxes at low temperatures
• • Viscosity improvers to increase viscosity at higher temperatures

Acidity

Acidity must be closely controlled because acids can corrode wetted oil system surfaces.

Contaminants

Oil quality can deteriorate over time due to heat and use. It can become contaminated by particles caused by the internal wear of engine components, or by external contaminants such as dirt or glycol.

Oil can also be affected by fuel contaminants such as hydrogen sulphide ( \( H_2S \) ). If sulphur compounds cannot be totally removed from the fuel, additional precautions, such as enhanced oil sampling and reduced oil replacement intervals, need to be taken. The engine manufacturer should be consulted on recommended lube oil type.

OIL SYSTEM DESIGN AND COMPONENTS

The internal oil flow system is quite extensive, as shown in Fig. 24. A header distributes pressurized oil to the main bearings. The oil then flows through drilled passages in the connecting rods to the connecting rod bearing and the piston rod. From the piston rod, it is sprayed onto the underside of the piston crown for cooling, and then drains into the sump. The cylinder head has a separate oil supply that lubricates the camshaft assembly and rocker arms. Oil is supplied to the turbocharger and the gear train.

Figure 24
Internal Oil Flow System
(Courtesy of Waukesha Engine)

A typical external oil system schematic is shown in Fig. 25. The engine crankcase, or sump, serves as the oil reservoir. Oil is drawn from the lowest part of the sump through a screen that prevents foreign material from entering the lube oil circuit. A positive displacement pump, gear-driven from the engine, is usually used as a main oil pump. Excess oil is dumped back into the sump by the oil pump relief valve. Then, the oil flows to the cooler where a temperature control valve allows the correct amount of oil to be cooled. The final oil pressure is adjusted to compensate for installation differences.

Figure 25
External Oil System Schematic
(Courtesy of Waukesha Engine)

The main oil filter is usually a full-flow type, and typically filters up to 10-20 microns. With a clean filter, the differential pressure is about 15-20 kPa. Maximum allowable differential pressure is normally about 100 kPa. An example of a cooler and filter assembly is shown in Fig. 26.

Figure 26
Oil Cooler and Filter Assembly
(Courtesy of Tom Van Hardevel)

Since the main oil pump cannot supply sufficient pressure until the engine is rotating, a separate electric prelube pump activates on startup to provide initial lubrication prior to and during startup.

Some engines (those used for backup power generation) have quick-start capability aided by a low pressure pump that operates when the engine is not running to minimize startup time.

Objective 7

State the purpose and describe the control of a typical internal combustion engine including the operation of safety devices.

ENGINE CONTROL

The control of a reciprocating internal combustion engine consists of a number of different systems including:

• • Speed governing
• • Ignition control
• • Fuel system control
• • Cooling system control
• • Lubrication system control
• • Safety and engine protection
• • Startup and shutdown sequencing

Most systems incorporate extensive electronic and computerized control and monitoring devices. Various aspects of control, such as oil cooling, are controlled by independent devices such as thermostats. If the equipment is unattended, additional supervisory control and monitoring systems may need to be installed at a remote location.

SAFETY AND PROTECTION SYSTEMS

Protection can be provided by:

• • A local alarm that activates an indicator or audible horn but causes no additional actions
• • A remote alarm that is transmitted to a control centre, pager, or other remote device
• • A normal shutdown with local or remote indication
• • An emergency shutdown with local or remote indication
• • A manual emergency shutdown of the engine via a local panel

Most protection systems use both an alarm and a shutdown, but every situation has to be considered separately. Factors such as the criticality of the equipment, local regulations and conditions, and company practices need to be considered when planning a protection system.

A typical electronic engine protection system is shown in Fig. 27.

Figure 27
Engine Protection System
(Courtesy of Finning-Caterpillar)

General Engine Operation Protection

Protection for general engine operation may include:

• • Intake air restriction caused by plugging of the intake air filter or blockage of the intake
• • High intake air temperature caused by high ambient temperature or inadequate cooling by the intercooler (for turbocharged engines)
• • Engine overspeed caused by loss of load
• • High vibration caused by a number of different factors such as mechanical failure, unbalance, or misalignment
• • High crankcase pressure due to wear or failure of the piston ring or cylinder
• • High main bearing temperature caused by long term wear or high oil temperature

Fuel System Protection

Fuel system protection may include:

• • Low fuel temperature resulting from failure of the fuel heater
• • High fuel pressure due to failure of the fuel regulator
• • High fuel filter differential pressure due to clogging of the filter

Cooling System Protection

Cooling system protection may include:

• • Low coolant level in the coolant reservoir
• • High jacket water temperature due to failure of cooling or inadequate coolant flow
• • Cooler vibration due to unbalance, or misalignment of the cooler fan

Oil System Protection

Oil system protection may include:

• • Low oil pressure due to failure of the oil pump or a restriction
• • Low oil temperature resulting from failure of the oil heater
• • Low oil level in the sump
• • High oil pressure caused by failure of the oil pump relief valve
• • High oil temperature caused by failure of the oil cooling system
• • High oil filter differential pressure due to clogging of the filter

Combustion System Protection

Protection for combustion systems may include:

• • High exhaust gas temperature (measured by a pyrometer -usually one per cylinder)
• • High exhaust gas temperature spread (difference between the highest and lowest temperature)
• • Activation of a detonation sensor (usually one per cylinder)

Safety Parameters

During startup, relevant safety parameters include:

• • Low starting gas pressure (for an air or gas starter)
• • Excessive cranking time due to a bad starter or insufficient start pressure
• • Low oil pressure caused by cold oil or failure of the prelube pump

Additional protective shutdowns may be added for fire and gas leak detection if the engine is located in a hazardous location.

Chapter Questions

B1.5

1. 1. Describe the steps of a four-stroke cycle for a spark-ignition engine.
2. 2. What are the differences between a spark-ignition and a compression-ignition engine?
3. 3. List the two types of supercharging and describe how they function.
4. 4. With the aid of a simple sketch, describe the design of a lean burn fuel system used in a spark ignition engine system.
5. 5. Describe three major purposes for engine cooling.
6. 6. What are three aspects of oil quality that need to be monitored?
7. 7. Discuss four operating conditions for which engine protection is required?