1.2 Engines Classification

Engines for automotive and construction equipment may be classified in a number of ways: type of fuel used, type of cooling used, or valve and cylinder arrangement. They all operate on the internal combustion principle, and the application of basic principles of construction to particular needs or systems of manufacture has caused certain designs to be recognized as conventional.

The most common method of classification is by the type of fuel used, that is, whether the engine burns gasoline or diesel fuel.

Diesel Engine

Diesel engines can be classified by the number of cylinders they contain. Most often, single cylinder engines are used for portable power supplies. For commercial use, four, six and eight cylinder engines are common. For industrial use such as locomotives and marine use, twelve, sixteen, twenty and twenty-four cylinder arrangements are seen.

Engine Cycle Design

The four-stroke cycle diesel engine is similar to the four-stroke gasoline engine. It has the same operating cycle consisting of an intake, compression, power, and exhaust stroke. Its intake and exhaust valves also operate in the same manner. The four-stroke cycle of a diesel engine is as follows:

• Diesel Engine Intake Stroke – The intake stroke begins when the piston is at top dead center. As the piston moves down, the intake valve opens. The downward movement of the piston draws air into the cylinder. As the piston reaches bottom dead center, the intake valve closes.

• Diesel Engine Compression Stroke – The compression stroke begins when the piston is at bottom dead center. As the piston moves upwards, the air is compressed to as much as 500 pounds per square inch (psi) at a temperature approximately 1000°F.

• Diesel Engine Power Stroke – The power stroke begins when the piston is at top dead center. The engine’s fuel injection system delivers fuel into the combustion chamber. The fuel is ignited by the heat of the compression. The expanding force of the burning gases pushes the piston downwards, providing power to the crankshaft. The diesel fuel will continue to burn through the entire power stroke (a more complete burning of fuel). The gasoline engine has a power stroke with rapid combustion in the beginning, but little to no combustion at the end.

• Diesel Engine Exhaust Stroke – The exhaust stroke begins with the piston at bottom dead center. As the piston move upwards, the exhaust valve opens. The burnt gases are pushed out through the exhaust port. As the piston reaches top dead center, the exhaust valve closes and the intake valve opens. The engine is now ready to begin the next cycle.

Cylinder Arrangement

Figure 1-5 shows the most common types of engine designs. The inline cylinder arrangement is the most common design for a diesel engine. They are less expensive to overhaul, and accessory items are easier to reach for maintenance. The cylinders are lined up in a single row.

Typically there are one to six cylinders and they are arranged in a straight line on top of the crankshaft. In addition to conventional vertical mounting, an inline engine can be mounted on its side. This is common in buses when the engine is under the rear seating compartment. When the cylinder banks have an equal number on each side of the crankshaft, at 180 degrees to each other, it is known as a horizontally-opposed engine.


Figure 1-5 — Engine block designs.

V-type engines are another popular engine configuration. Cylinders are set up on two banks at different angles from the crankshaft, as shown in Figure 1-5. A V-type engine looks like the letter V from the front view of the engine. Typical angles are 45, 50, 55, 60 and 90 degrees. The angle is dependent on the number of cylinders and design of the crankshaft. The typical V-type engines are available in six through twenty-four cylinders; however, other configurations are available.

The W-type engine design is like two V-type engines made together and operating a single crankshaft. These engines are used primarily in marine applications, as shown in Figure 1-5.

Combustion Chamber Design

In order to have the best power with low emissions, you need to achieve complete fuel combustion. The shape of the combustion chamber combined with the action of the piston was engineered to meet that standard. Figure 1-6 shows the direct injection, precombustion and swirl chamber designs.


Figure 1-6 — Direct and indirect injection.

Direct injection is the most common and is found in nearly all engines. The fuel is injected directly into an open combustion chamber formed by the piston and cylinder head. The main advantage of this type of injection is that it is simple and has high fuel efficiency.

In the direct combustion chamber, the fuel must atomize, heat, vaporize and mix with the combustion air in a very short period of time. The shape of the piston helps with this during the intake stroke. Direct injection systems operate at very high pressures of up to 30,000 psi.

Indirect injection chambers were used mostly in passenger cars and light truck applications. They were used previously because of lower exhaust emissions and quietness. In today’s technology with electronic timing, direct injection systems are superior. Therefore, you will not see many indirect injections system on new engines. They are, however, still on many older engines.

Precombustion chamber design involves a separate combustion chamber located in either the cylinder head or wall. As Figure 1-6 shows, this chamber takes up from 20% - 40% of the combustion chambers TDC volume and is connected to the chamber by one or more passages. As the compression stroke occurs, the air is forced up into the precombustion chamber. When fuel is injected into the precombustion chamber, it partially burns, building up pressure. This pressure forces the mixture back into the combustion chamber, and complete combustion occurs.

Swirl chamber systems use the auxiliary combustion chamber that is ball-shaped and opens at an angle to the main combustion chamber. The swirl chamber contains 50% - 70% of the TDC cylinder volume and is connected at a right angle to the main combustion chamber. A strong vortex (mass of swirling air) is created during the compression stroke. The injector nozzle is positioned so the injected fuel penetrates the vortex, strikes the hot wall, and combustion begins. As combustion begins, the flow travels into the main combustion chamber for complete combustion.

Energy cells are used with pintle type injectors. As shown in Figure 1-7, the system consists of two separate chambers connected with a passageway. As injection occurs, a portion of the fuel passes through the combustion chamber to the energy cell. The atomized portion of the fuel starts to burn. Due to the size and shape of the cell, the flame is forced back into the main combustion chamber, forcing the complete ignition. Because of the smooth flow and steady combustion rate, the engine runs smooth and the fuel efficiency is excellent.


Figure 1-7 – Energy cells.

Fuel Injection System Design

The heart of the diesel engine is the injection system. It needs to be designed to provide the exact same amount to each

cylinder so the engine runs smooth, and it needs to be timed correctly so peak power can be achieved. If it is delivered too early, the temperature will be down, resulting in incomplete combustion. If it is too late, there will be too much room in the combustion chamber and there will be a loss of power. The system also needs to be able to provide a sufficient pressure to the injector; in some cases as much as 5,000 psi is needed to force the fuel into the combustion chamber. A governor is needed to regulate the amount of fuel fed to the cylinders. It provides enough pressure to keep the engine idling without stalling, and cuts off when the maximum rated speed is achieved. The governor is in place to help from destroying the engine because of the fuel pressure available.

There are six different types of fuel injection systems: individual pump systems; multiple-plunger, inline pump systems; unit injector systems; pressure-time injection systems; distributor pump systems, and common rail injection systems.

• The individual pump system is a small pump contained in its own housing, and supplies fuel to one cylinder. The individual plunger and pump barrel, shown in Figure 1-8, are driven off of the engine’s cam shaft. This system is found on large-bore, slow speed industrial or marine diesel engines and on small air- cooled diesels; they are not used on high speed diesels.


Figure 1-8 – Individual pump system.

• Multiple-plunger, inline pump systems, shown in Figure 1-9, use individual pumps that are contained in a single injection pump housing. The number of plungers is equal the number of cylinders on the engine and they are operated on a pump camshaft. This system is used on many mobile applications and is very popular with several engine manufacturers.


Figure 1-9 – Multiple-plunger, inline pump system.

The fuel is drawn in from the fuel tank by a pump, sent through filters, and then delivered to the injection pump at a pressure of 10 to 35 psi. All pumps in the housing are subject to this fuel. The fuel at each pump is timed, metered, pressurized, and delivered through a high-pressure fuel line to each injector nozzle in firing order sequence.

• Unit injector systems utilize a system that allows timing, atomization, metering, and fuel pressure generation that takes place inside the injector body and services a particular cylinder. This system is compact and delivers a fuel pressure that is higher than any other system today.

Fuel is drawn from the tank by a transfer pump, is filtered and then delivered. The pressure is 50 – 70 psi before it enters the fuel inlet manifold located within the engine’s cylinder head. All of the injectors are fed through a fuel inlet or jumper line. The fuel is pressurized, metered, and timed for proper injection to the combustion chamber by the injector. This system uses a camshaft-operated rocker arm assembly or a pushrod-actuated assembly to operate the injector plunger.

• Pressure-time injection system (PT system) got its name from two of the primary factors that affect the amount of fuel injected per combustion cycle. Pressure or “P” refers to the pressure of the fuel at the inlet of the injector. Time or “T” is the time available for the fuel to flow into the injector cup. The time is controlled by how fast the engine is rotating.

The PT system uses a camshaft-actuated plunger, which changes the rotary motion of the camshaft to a reciprocating motion of the injector. The movement opens and closes the injector metering orifice in the injector barrel. Fuel will only flow when the orifice is open; the metering time is inversely proportional to engine speed. The faster the engine is operating, the less time there is for fuel to enter. The orifice opening size is set according to careful calibration of the entire set of injection nozzles.

• Distributor pump systems are used on small to medium-size diesel engines. These systems lack the capability to deliver high volume fuel flow to heavy-duty, large displacement, high speed diesel engines like those used in trucks. These systems are sometimes called rotary pump systems. Their operating systems are similar to how an ignition distributor operates on a gasoline engine. The rotor is located inside the pump and distributes fuel at a high pressure to individual injectors at the proper firing order.

• Common rail injection systems are the newest high-pressure direct injection system available for passenger car and light truck applications. This system uses an advanced design fuel pump that supplies fuel to a common rail and then delivers it to the injectors by a short high-pressure fuel line. This system utilizes an electronic control unit that precisely controls the rail pressure, timing, and duration of the fuel. The injector nozzles are operated by rapid-fire solenoid valves or piezoelectric triggered actuators. This is the only system designed to be operated by an electronically-controlled fuel injection system. This is necessary to meet modern performance, fuel efficiency, and emission standards. Of all of the systems available today, the common rail injection system has emerged as the predominant choice for diesel engines today.

Gasoline Engine

Operational Cycles

In the four-stroke cycle gasoline engine, there are four strokes of the piston in each cycle: two up and two down. The four strokes of a cycle are intake, compression, power, and exhaust. A cycle occurs during two revolutions of the crankshaft.

• Intake Stroke – The intake stroke begins when the piston is at top dead center. As the piston moves downwards, the intake valve opens. The downward movement of the piston creates a vacuum in the cylinder, causing the fuel and air mixture to be drawn through the intake port and into the combustion chamber. As the piston reaches bottom dead center, the intake valve closes.

• Compression Stroke – The compression stroke begins when the piston is at bottom dead center. As the piston moves up upwards, it compresses the fuel and air mixture. Since both the intake and exhaust valves are closed, the fuel and air mixture cannot escape. It is compressed to a fraction of its original volume.

• Power Stroke – The power stroke begins when the piston is at top dead center. The engine ignition system consists of spark plugs that emit an electrical arc at the tip to ignite the fuel and air mixture. When ignited, the burning gases expand, forcing the piston down. The valves remain closed so that all the force is exerted on the piston.

• Exhaust Stroke – The exhaust stroke begins when the piston nears the end of the power stroke and the exhaust valve opens. As the piston moves upwards, it pushes the burnt gases out of the combustion chamber through the exhaust port. After the piston reaches top dead center, the exhaust valve closes. The next cycle begins when the intake valve opens. Figure 1-10 shows the operations of a four-stroke cycle gasoline engine.


Figure 1-10 – Four-stroke cycle gasoline engine in operation.

Number of Cylinders

Engines come with a variety of cylinder configurations. Typically in automotive settings, engines have either four, six or eight cylinders. A few may have three, five, ten, twelve or sixteen. Usually the greater the number of cylinders an engine has, the greater the horsepower is generated with an increase of smoothness of engine. Generally a four or five cylinder engine is an inline design while a six cylinder can have an inline or V –type. Eight, ten or twelve are usually a V-type design.

Cylinder Arrangement

 


Figure 1-11 – Cylinder arrangements.

The position of the cylinders in relation to the crankshaft determines the cylinder arrangement. Figure 1-11 depicts the five basic arrangements:

In an inline engine the cylinders are lined up in a single row. Typically there are one to six cylinders arranged in a straight line on top of the crankshaft.

A V-type engine looks like the letter V from the front view of the engine. There are two banks of cylinders at an angle to each other on top of the crankshaft. The benefit of this design is a shorter and lighter engine block.

A slant engine is similar to an inline except the bank of cylinders is off to an angle over the crankshaft. This is done to save space in the engine compartment.

The W-shaped engine looks like the letter W from the front view of the engine. Two banks of cylinders form the V shape, except the cylinders are slightly offset, forming a very narrow V. This allows the manufacturer to make an engine with a bigger displacement without making a bigger engine block.

The opposed cylinder engine lies flat on its side with the crankshaft between the cylinder banks; because of the way the engine looks, it is sometimes referred to as a pancake engine.

Valve Train Type

The valve train consists of the valves, camshaft, lifters, push rods, rocker arms and valve spring assemblies as shown in Figure 1-12.


Figure 1-12 – Valve train parts.

The purpose is to open and close the valves at the correct time to allow gases into or out of the combustion chamber, as shown in Figure 1-12. As the camshaft rotates, the lobes push the push rods that open and close the valves.

The camshaft is connected to the crankshaft by belt, chain or gears. As the crankshaft rotates, it also rotates the camshaft. There are three common locations of the camshaft that determine the type of valve train the engine has. These are shown in Figure 1-13: the valve in block or L head, the cam in block (also called the I head or overhead valve), and the overhead cam.


Figure 1-13 – Valve train type.

Cooling System

The cooling system has many functions. It must remove heat from the engine, maintain a constant operating temperature, increase the temperature of a cold engine and provide a source of heat for the passengers inside the automobile. Without a cooling system, the engine could face catastrophic failure in only a matter of minutes.

There are two types of cooling systems: liquid, the most common, and air. Although both systems have the same goal, to prevent engine damage and wear caused by heat from moving engine parts (friction) the liquid system is the most common.

The air cooling system uses large cooling fins located around the cylinder on the outside. These fins are engineered to use the outside air to draw the heat away from the cylinder. The system typically uses a shroud (enclosure) to route the air over the cylinder fins. Thermostatically-controlled flaps open and close the shroud to regulate air flow and therefore control engine temperature.

There are two types of liquid cooling systems; open and closed.

The closed cooling system has an expansion tank or reservoir, and a radiator cap with pressure and vacuum valves. There is an overflow tube that connects the radiator and the reservoir tank. The pressure and vacuum valve in the radiator cap pushes or pulls coolant into the reservoir tank instead of leaking out onto the ground. As the temperature rises, the fluid is pressurized causing the fluid to transfer to the reservoir tank. When the engine is shut off, the temperature decreases, causing a vacuum and moving the coolant to the radiator.

The open system does not use a coolant reservoir. There is simply an overflow hose attached to the radiator; when the coolant heats up and expands, the coolant overflows the radiator and out onto the ground. This system is no longer used; it has been replaced with the closed system because it is safer for the environment and easier to maintain.

The liquid cooling system, as shown in Figure 1-14, is comprised of several components which make it a system. The most common are the water pump, radiator, radiator hoses, fan, and thermostat.


Figure 1-14– Closed cooling system.

• The water pump does just what the name says-it moves water/coolant through the engine to the radiator. It is often driven by a belt, but in some cases it can be gear-driven.

• The radiator transfers the heat from the coolant inside it to the outside air, and is normally mounted in front of the engine. The radiator core is made up of tubes and cooling fins. As the air moves over these fins, the heat is transferred to the outside air, thereby lowering the temperature of the coolant.

• Radiator hoses are a means to transfer the coolant from the engine to and from radiator. The upper hose usually connects the radiator to the engine via the thermostat housing. The lower hose usually connects the radiator to the water pump inlet housing.

• The cooling system fan pulls air across the fins in the radiator to transfer the heat from the coolant. Its main function is to prevent overheating when the vehicle is not moving or not moving very fast and the air transfer across the radiator is decreased. There are two basic types of fans, engine-powered and electric- powered. The engine-powered fan is run off a drive belt from the crankshaft pulley. There are also three types of engine-powered fans. A flex fan has thin flexible blades. As the engine is at idle, requiring more air, the blades are curved and draw a lot of air; however, as the engine speeds up, the blades flex until they are almost straight, drawing little air but at the same time reducing used engine power.

The fluid coupling fan is designed to slip at higher engine speed. As the engine is at idle, the fluid engages the blade to turn it; when the engine speeds up, the fluid is not able to keep up and allows the blade to slip. This allows for a reduction of engine power consumed.

The thermostatic fan clutch has a temperature sensitive metal spring that controls the fan speed. The spring controls oil flow in the fan clutch. When the spring is cold, it allows the clutch to slip. As the spring heats up, the clutch locks and forces air circulation.

The thermostat senses the temperature of the engine and opens or closes to control water flow as required. The thermostat has a wax-filled pellet contained in a cylinder. A spring holds the piston and valve in a normally closed position. As the temperature increases, the wax heats up and expands, allowing the valve to open. As the temperature decreases, the wax cools, retracts, and closes the valve.

Fuel Type

An engine burns fuel as a source of energy. Various types of fuel will burn in an engine: gasoline, diesel fuel, gasohol, alcohol, liquefied petroleum gas, and other alternative fuels.

Gasoline is the most common type of automotive fuel. It is abundant and highly flammable. Extra chemicals like detergents and antioxidants are mixed into it to improve its operating characteristics. Antiknock additives are introduced to slow down the burning of gasoline. This helps prevent engine ping, or the knocking sound produced by abnormal, rapid combustion.

Gasoline has different octane ratings. This is a measurement of the fuel’s ability to resist knock or ping. A high octane rating indicates that fuel will not knock or ping easily. High- octane gasoline should be used in high-compression engines. Low-octane gasoline is more suitable for low-compression engines.

Diesel fuel is the second most popular type of automotive fuel. A single gallon of diesel fuel contains more heat energy than a gallon of gasoline. It is a thicker fraction or part of crude oil. Diesel fuel can produce more cylinder pressure and vehicle movement than an equal part of gasoline.

Since diesel fuel is thicker and has different burning characteristics than gasoline, a high-pressure injection system must be utilized. Diesel fuel will not vaporize as easily as gasoline. Diesel engines require the fuel to be delivered directly into the combustion chamber.

Diesel fuel has different grades as well: No. 1, No. 2, and No. 4 diesel. No. 2 is normally recommended for use in automotive engines. It has a medium viscosity (thickness or weight) grade that provides proper operating traits for the widest range of conditions. It is also the only grade of diesel fuel at many service stations.

No. 1 diesel is a thinner fuel. It is sometimes recommended as a winter fuel for the engines that normally use No. 2. No. 1 diesel will not provide the adequate lubrication for engine consumption.

One of the substances found in diesel fuel is paraffin or wax. At very cold temperatures, this wax can separate from the other parts of diesel fuel. When this happens the fuel will appear cloudy or milky. When it reaches this point it can clog fuel filters and prevent diesel engine operation.

Water contamination is a common problem with diesel fuel. Besides clogging filters, it also can cause corrosion within the system, and just the water alone can cause damage to the fuel pumps and nozzles.

Diesel fuel has a cetane rating instead of an octane rating like gasoline. A cetane rating indicates the cold starting ability of diesel fuel. The higher the rating, the easier the engine will start and run in cold weather. Most automakers recommend a rating of 45, which is the average value for No. 2 diesel fuel.

Alternative fuels include any fuel other than gasoline and diesel fuel. Liquefied petroleum gas, alcohol, and hydrogen are examples of alternative fuels.

Liquefied petroleum gas (LPG) is sometimes used as a fuel for automobiles and trucks. It is one of the lightest fractions of crude oil. The chemical makeup of LPG is similar to that of gasoline. At room temperature, LPG is a vapor, not a liquid. A special fuel system is needed to meter the gaseous LPG into the engine. LPG is commonly used in industrial equipment like forklifts; it is also used in some vehicles like automobiles and light trucks. LPG burns cleaner and produces fewer exhaust emissions than gasoline.

Alcohol has the potential to be an excellent alternative fuel for automobile engines. The two types of alcohol used are ethyl alcohol and methyl alcohol.

Ethyl alcohol, also called grain alcohol or ethanol, is made from farm crops. Grain, wheat, sugarcane, potatoes, fruits, oats, soy beans, and other crops rich in carbohydrates can be made into ethyl alcohol.

Methyl alcohol, also called wood alcohol or methanol, can be made out of wood chips, petroleum, garbage, and animal manure.

Alcohol is a clean-burning fuel for automobile engines. It is not common because it is expensive to produce and a vehicle’s fuel system requires modification to burn it. An alcohol fuel system requires twice the amount burned as gasoline, therefore cutting the economy in half.

Gasohol is a mixture of gasoline and alcohol. It generally is 87 octane gasoline and grain alcohol; the mixture can be from 2-20% alcohol. It is commonly used as an alternative fuel in automobiles because there is no need for engine modifications. The alcohol tends to reduce the knocking tendencies of gasoline; it acts like an anti-knock additive. A 10% alcohol volume can increase 87 octane gasoline to 91 octane. Gasohol can be burned in high-compression engines without detonating and knocking.

Synthetic fuels are fuels made from coal, shale oil rock, and tar sand. These fuels are synthesized or changed from solid hydrocarbons to a liquid or gaseous state. Synthetic fuels are being experimented with as a means of supplementing crude oil because of the price and availability of these fuels.

Hydrogen is a highly flammable gas that is a promising alternative fuel for the future, and it is one of the most abundant elements on the planet. It can be produced through the electrolysis of water. It burns almost perfectly, leaving only water and harmless carbon dioxide as a by-product.