= Swept volume + 1 Clearance volume
Swept volume + Clearance volume Clearance volume
Total volume Clearance volume
2.2.1 Basic operating process
As mentioned previously, the piston is pushed down the cylinder by the pressure on its upper surface. The pressure is produced by the principle that if a gas is heated it will expand. However, if the gas is held in a confined space then when the gas is heated, there is no room for expansion. The result is that the heated gas suffers an increase in pressure and forces the piston down.
The greater the amount of heat passed to the gas, the greater its expansion. If, however, there is no room for expansion, then the greater the amount of heat passed to the gas, the greater its pressure will be. In an engine, the air is heated to a very high temperature and therefore a correspondingly high pressure is created inside the cylinder. The high pressure is used to exert a considerable force on the piston.
If the piston is at the top of its stroke, pressure above the piston can only push it downwards. There may still be some pressure left when the piston reaches the bottom of its stroke and this must be released before the piston is moved back to the top of the cylinder again.
The pressure is released by opening a hole in the cylinder called the exhaust port. Because of the rotation of the crankshaft, the piston is then able to pass back to the top of the stroke without much opposing pressure.
It would be possible to heat the air inside the cylinder by applying a flame to the outside of the cylinder. To reach the air however, the heat would have to pass through the cylinder wall. Much of the heat would be lost by heating the cylinder and the air outside it, so this would be an inefficient process.
A more efficient method is to directly heat the air inside the cylinder, and this can be achieved by a burning process inside the cylinder. To achieve this, a suitable combustible fuel is mixed with the air then ignited and burnt within the cylinder. The cylinder will of course still absorb a good deal of heat, and
Figure 2.3 Cylinder volume and compression ratio
2.2 WORKING PRINCIPLES OF THE FOUR-STROKE AND TWO-STROKE ENGINE
arrangements must be made to prevent it getting too hot, but the contents can be raised to a much higher temperature and so develop a correspondingly higher pressure.
Engines that use heat produced by burning a fuel to develop mechanical power are called heat engines. An engine where fuel is burnt internally is referred to as an internal-combustion engine. Internal-combustion engines can use any one of a variety of fuels, but in most cases for automotive applications, petrol or diesel fuels are used. Note that some engines are now fitted with conversion systems to enable them to operate on Liquid Petroleum Gas (LPG).
Petrol is a liquid refined from crude petroleum, and is particularly suitable as a fuel for motor vehicles. It is clean to handle and relatively cheap. At normal temperatures, petrol is a liquid, which can be carried in a vehicle in quite a small tank. However, petrol gives off a flammable vapour even at quite low temperatures thus enabling ignition and burning of the vapour to take place even within a cold engine with little difficulty.
Diesel fuel is also refined from crude oil and has similar properties to that of petrol (relatively cheap, easy to handle and is a liquid at normal temperatures).
Diesel differs from petrol in that it gives off a flammable vapour less readily and therefore requires a different ignition process to enable the fuel to be burnt.
2.2.2 Cycle of operations
Before petrol or diesel can be burnt they must be mixed with a suitable quantity of air to enable them to burn.
Many fuels require a specific amount of oxygen before they will ignite and burn efficiently. In a petrol engine, the fuel has traditionally been mixed with the air outside of the cylinder and then drawn into the cylinder as a mixture. In the diesel engine, the fuel is introduced
(injected) directly into the cylinder, and therefore requires air to be drawn into the cylinder.
In many petrol engines now, fuel is injected into the cylinder after the air has been drawn in; this is similar to the diesel engine process.
As illustrated in Figure 2.4, the air/fuel mixture (petrol engine) or air (diesel engine) commonly referred to as the ‘charge’ is introduced into the cylinder through a hole in the cylinder called the ‘inlet port’
which can be opened and closed as required. Once inside the cylinder the charge is then compressed by the upward movement of the piston before burning. If the charge is already compressed before it is ignited and burnt, this greatly increases the pressure after burning.
In the case of the petrol engine, ignition is achieved by an electric arc or spark, which is made to jump across the small gap of a sparking plug. The sparking plug is located so that the spark is exposed to the mixture thus enabling ignition. Sparking plugs are generally screwed into the top of the cylinder (cylinder head). Note that the spark must occur at precisely the correct time to ensure good combustion of the fuel/air mixture and thus achieve good power.
In a diesel engine, the piston compresses the air to a higher pressure than in a petrol engine. This compression process creates a considerable amount of heat, resulting in a very high temperature within the cylinder. When diesel fuel is injected into this high temperature air, the temperature is high enough to ignite the fuel. As with the petrol engine, the start of the combustion process must occur at the correct time and this is achieved by precisely timing the injection of the fuel.
Burning of the fuel creates heat, thus causing a substantial pressure rise, and so pushing the piston down the cylinder (the power stroke). The burned gases and any remaining pressure are then released through the exhaust port.
During the compression stroke and the power stroke, both the inlet and exhaust ports are closed off by
valves. The valves are opened (usually by a mechanical system) when the fresh charge is drawn into the cylinder and when the burned gasses are released from the cylinder. The timing of opening and closing the valves and ports is critical and the valve operating mechanism is designed so that the valves open and close at precisely the correct time. Valve mechanisms are covered in greater detail later in this chapter.
The running of the engine involves the continuous repetition of four operations that make up what is called the cycle of operations. These operations are continuously repeated (as long as the engine is running) in the following order:
1 The space in the cylinder above the piston is filled with the charge (either a mixture of petrol vapour and air or just air for the diesel engine and some petrol engines). The mixture or air is able to enter the cylinder when the piston moves down (this is generally referred to as the induction stroke).
2 The charge is compressed into the top end of the cylinder (called the combustion chamber) thus raising its temperature. This is achieved by the upward movement of the piston (this is generally referred to as the compression stroke).
3 For petrol engines, the petrol vapour is ignited by an electric spark and burned. For diesel engines, the injection of diesel fuel into the heated charge causes ignition and burning of the fuel. The resulting increase in pressure drives the piston down the cylinder (this is referred to as the power stroke).
4 The burned gases are expelled from the cylinder.
This is achieved by the upward movement of the piston (exhaust stroke).
Note: The four cycles coincide with four strokes of the piston (top to bottom or bottom to top of piston travel), therefore the cycle of operations is referred to as the four-stroke cycle. However, a two-stroke cycle engine is also common in motor vehicles (mainly motor cycles) and although there are major differences in the details
Figure 2.4 The operation of an engine on the four-stroke cycle
of the way the four-stroke and two-stroke types work, the principle of induction, compression, ignition and exhaust remain applicable to both types. Four-stroke and two-stroke cycles are dealt with in later sections in this chapter.
Notice that the gas helps the piston to move only during one of these cycles of operations (the power stroke). The remaining operations provide no power or help to the movement of the piston. The induction and exhaust strokes, and more especially the compression stroke (where energy is required to compress the air or mixture), require energy to move the piston. The energy needed to complete the induction, compression and exhaust strokes is actually opposing the rotation of the crankshaft.
Flywheel effect
To assist in keeping the crankshaft rotating between power strokes and thus enable the induction, compression and exhaust strokes to be performed, a flywheel (a large weight or mass) is attached to the end of the crankshaft.
Note: The flywheel is effectively a weight that when turning helps to keep the crankshaft rotating. This is no different to spinning a pushbike wheel or a car wheel when it is free of the ground; the more energy used to initially spin the wheel, the longer the wheel will rotate.
Also, the heavier the wheel the greater the amount of energy it can store and therefore it will rotate for longer.
When a single cylinder engine operates at a very slow speed, there is a relatively long time between power strokes of the piston; the flywheel action helps to maintain the crankshaft rotation until the next power stroke.
The energy provided by the flywheel to keep the crankshaft turning is therefore not simply used to push the piston back up the cylinder. It is also necessary for the flywheel energy to keep the crankshaft rotating against the opposing forces created by the compression, induction and exhaust strokes.
Note that for multi-cylinder engines there are a greater number of power strokes per turn of the crankshaft, one provided by each of the cylinders. The amount of flywheel action needed is therefore not so great. A twelve-cylinder engine is much less dependent on a flywheel than a four-cylinder engine for example.
2.2.3 The four-stroke cycle
The four-stroke cycle for the petrol engine (spark ignition) is very similar in principle to that of the diesel engine (compression ignition or CI). The following explanation for the working operation of an internal-combustion engine highlights the four-stroke cycle; for further details on the operation of the spark ignition and compression-ignition engines and their fuel systems, refer to the relevant petrol or diesel engine section within this book.
During the four-stroke cycle (Figure 2.4), one complete stroke of the piston (from top to bottom or bottom to top of the stroke) is used to carry out each of the four operations which comprise the complete cycle of operations. To complete the cycle, four strokes of the piston are needed, hence the term ‘four-stroke cycle’.
To allow the fresh charge (fuel mixture or air) into the cylinder and to allow the burnt gases to exit the cylinder, inlet and exhaust ports are provided. Valves are located at the end of the ports where they meet the cylinder. The valves are closed for most of the cycle but open at the appropriate time to let in the fuel mixture or let out the burnt gases. Valves are generally opened by a mechanical system and closed by a spring, although some variations do exist.
Starting with the piston at TDC, as the crankshaft rotates, the next four strokes of the piston will result in a number of different processes and actions taking place:
First stroke (induction) When the piston moves down the cylinder, the inlet port is open and the exhaust port is closed, a fresh charge is drawn into the cylinder. Most petrol engines draw in a mixture of air and fuel, however a diesel engine draws in a charge of air only. At the end of the stroke, the inlet port closes.
Second stroke (compression) The piston moves up the cylinder with the inlet and exhaust ports both closed. The charge is therefore compressed and forced into the combustion chamber which is located at the top end of the cylinder.
Third stroke (power) Towards the end of the compression stroke, the highly compressed charge is hot due to being compressed.
For a petrol engine, an electric spark or arc is provided by a sparking plug and ignition system. The spark ignites the air/fuel mixture, which then burns.
For a diesel engine, the diesel fuel is injected at high pressure into the hot compressed air and mixes to form a combustible air/fuel mixture. The heat of the air ignites the fuel mixture, which then burns.
In both petrol and diesel engines, the air/fuel mixture burns very rapidly within the confined cylinder, which heats the gas to an even higher temperature and considerably increases its pressure. Both the inlet and exhaust ports are closed off by the valves, so the ignited mixture cannot escape and its pressure therefore forces the piston down the cylinder.
Fourth stroke (exhaust) The rotation of the crankshaft causes the piston to return up the cylinder.
The inlet port remains closed but the exhaust port is now opened. The movement of the piston towards the top of the cylinder therefore forces the burnt gases out of the cylinder via the exhaust port. At the end of the exhaust stroke, the exhaust port closes and the inlet port re-opens for the next induction stroke, which follows immediately.
By referring to the above explanation and to Figure 2.4 it is possible to note the relationship between the position of the pistons on the different strokes and the opening/closing of the inlet and exhaust valves.
Petrol four-stroke
For petrol engines (spark ignition), the air and fuel mixture enters the cylinder via the inlet port. In most petrol engines the fuel is mixed with the air before entering the cylinder, although there is an increasing trend to inject fuel into the cylinder after the air has been drawn in.
The amount of air or air/fuel mixture allowed into the cylinder is usually regulated by some form of valve located in the intake port system. The valve operation is controlled by the driver and is usually referred to as a throttle valve or throttle butterfly.
In most modern petrol engines, the petrol is delivered via a fuel injection system, by which fuel is delivered under relatively low pressure into the inlet ports via injectors. However, there is an increasing use of direct injection, where fuel injectors deliver the fuel direct into the cylinder (at a relatively high pressure).
The throttle butterfly regulates the amount of air passing through to the cylinders and the fuel injection system regulates the amount of fuel injected, thus ensuring that the mixture of petrol and air is correct.
Note: Until the early 1990s most engines made use of a carburettor to mix the air and fuel into the correct proportions. The carburettor was located at the outer end of the inlet port, and the mixture would therefore pass from the carburettor via the inlet ports to each of the cylinders. The inlet ports for each of the cylinders generally joined together and this assembly was referred to as an inlet manifold.
Diesel four-stroke
If the engine runs on diesel fuel (compression ignition), air is drawn into the cylinder through the intake ports and manifold, but fuel is injected into the cylinder at very high pressure via a fuel injector.
2.2.4 The two-stroke cycle
It has always been considered a disadvantage of the four-stroke cycle that there is one working stroke but three ‘idle’ (non-working) strokes.
Between 1878 and 1881 a Scotsman, Dugald Clerk, developed an engine in which the cycle of operations was completed in only two strokes of the piston, thus providing a power stroke for every revolution of the crankshaft. However, this engine used a second cylinder and piston to force fresh mixture into the working cylinder. The second piston therefore acted as a pump to force the charge into the working cylinder.
In 1891 Joseph Day invented a modified form of Clerk’s engine, in which he dispensed with the second cylinder but made use of the space in the crankcase underneath the piston as a pumping chamber.
Additionally, Days’ engine also avoided the use of valves to close the inlet and exhaust ports by making use of the piston to cover or uncover the ports.
Although petrol and diesel two-stroke engines are similar in operation, it is necessary to describe them separately.
The two-stroke petrol engine
Instead of using valves to open and close the inlet and exhaust ports, two-stroke engines generally make use of ports located at strategic positions along the length of the cylinder. As the piston rises and falls within the cylinder the ports are covered and uncovered. There are generally three ports: an inlet port, an exhaust port and a transfer port (which connects the lower chamber beneath the piston to the upper chamber above the piston).
The operation of the two-stroke engine, which is illustrated in Figure 2.5, is as follows.
Figure 2.5 The operation of an engine on the two-stroke cycle
1 Beginning with the piston about halfway on its upward stroke, all three ports are covered. The upward movement of the piston compresses a fresh charge in the top of the cylinder, but at the same time the upward movement of the piston causes the pressure underneath the piston to fall below atmospheric pressure (note that this lower chamber underneath the piston is sealed). Near the top of the stroke the lower edge of the piston has risen sufficiently to uncover the inlet port, and because the lower chamber pressure is lower than atmospheric pressure, a fresh charge is forced or drawn into the lower chamber.
2 Just before the piston reaches the top of the stroke, the charge above the piston is ignited in the same manner as in the four-stroke engine, and with the same result: the high pressure of the burned gases drives the piston down the cylinder. When the piston is then on its way down the cylinder, at a point which is a little below TDC the piston re-covers the inlet port thus blocking of the lower chamber or crankcase. Further downward movement of the piston then compresses the charge in the crankcase. Near to the bottom of the stroke,
2 Just before the piston reaches the top of the stroke, the charge above the piston is ignited in the same manner as in the four-stroke engine, and with the same result: the high pressure of the burned gases drives the piston down the cylinder. When the piston is then on its way down the cylinder, at a point which is a little below TDC the piston re-covers the inlet port thus blocking of the lower chamber or crankcase. Further downward movement of the piston then compresses the charge in the crankcase. Near to the bottom of the stroke,