easier. The compression process is similar to pumping up a tyre with a hand or foot pump; compressing the air generates heat.
In an internal-combustion engine, the fuel mixture contains a high percentage of air (which is a gas) and the compression process therefore causes the temperature of the air to increase. Adding a spark to the already hot mixture causes a small part of the mixture to ignite. The mixture starts to burn and, as with any burning process, the flame will then spread to the remainder of the fuel. During the combustion process of the internal-combustion engine, the flame spreads very rapidly to the remainder of the mixture. Therefore, by compressing the mixture, the temperature increases and it is then only necessary to add a spark at the right time to create full ignition and combustion.
In fact, as detailed later, some engines do not even require the additional spark because the temperatures reached during compression are sufficient to ignite the fuel mixture. These engines are known as ‘compression-ignition engines’ (commonly referred to as diesel engines).
In both types of engine (spark ignition and compression ignition), once the fuel has started to burn (combustion) it generates a substantial increase in temperature. The increase in temperature causes the mixture to expand, and because it is contained within the cylinder, this results in a rapid and substantial increase in pressure. It is this increase in pressure that forces the piston down the cylinder producing the power.
There has always been argument as to whether the combustion process is in fact an explosion or just a very rapid burning process, and no attempt is made to resolve that argument within this book. However, the combustion process is without question very rapid;
when an engine is operating at 6000 revolutions per minute (rpm), the time taken to ignite the fuel mixture and force the piston from the top to the bottom of the cylinder is in the region of four thousandths of a second (4 milliseconds), with the main burning process only taking a small part of that time.
poor Beau de Rochas the fame he deserved, and he was awarded a sum of money by the Academy of Sciences in Paris in recognition of his invention. Even so, the method of operation, which he was the first to describe, and which is the one used in most modern engines, was for many years (and sometimes still is) known as the Otto cycle.
Compression-ignition (diesel) engine history The compression-ignition type of engine is often called a ‘diesel engine’; the name is derived from the German engineer Dr Rudolf Diesel, who in 1892 took out a patent (No. 7241) on an engine, which relied on the heat generated during compression to ignite a fuel of coal dust. This fuel was blasted into the cylinder by air pressure when the piston was at the end or the top of its stroke. The aim of the design, which was successfully applied to an engine five years later, was to achieve a higher thermal efficiency or improved fuel consumption by using a compression ratio higher than that employed on petrol engines. In those early days, pre-ignition (ignition before the spark) occurred in a petrol engine if the compression ratio exceeded a given value. In fact, this problem can still occur on modern engines if the compression is too high.
Many authorities do not recognize Diesel as the inventor of the engine, which was the forerunner of the modern compression-ignition engine. They state that the patent (No. 7146), which was taken out in 1890 by a British engineer (Herbert Ackroyd-Stuart), and put into commercial production two years later, contained all the fundamental features of the modern unit. This patent, which was the result of practical development work on low-compression oil engines, included the induction and compression of air, as well as the timed injection of a liquid fuel by means of a pump.
To avoid taking sides in this controversy, the terms
‘compression-ignition’ (CI) or ‘oil engine’ are often used.
2.1.2 The reciprocating engine
In Chapter 1 we saw that the internal-combustion engine has evolved to become the dominant automobile power source. Although there are variations on the internal-combustion engine such as the Wankel engine, the ‘reciprocating’ engine is by far the most common type used. This chapter therefore deals in great detail with the reciprocating–type engine but it does also briefly examine the Wankel engine.
The reciprocating engine is defined by its use of pistons that reciprocate within a cylinder, i.e. the pistons move up and down to complete the cycle of operation, including the power stroke (see Figure 2.1).
The pistons are connected via connecting rods to a crankshaft, which is rotated as the pistons move up and down. The crankshaft then transmits the power to the gearbox and transmission system.
Although the above paragraph identifies the essential components for creating and delivering power The internal-combustion engine therefore gained
favour with designers because it enabled high power outputs to be achieved in a compact and integrated assembly. The steam engine did, however, remain popular where exceptionally high power was required and size was perhaps less of a problem. For this reason, steam locomotives remained in use pulling trains through to the early 1960s.
As previously mentioned, development of the internal-combustion engine was largely related to use of fossil fuels. Although petrol and diesel fuels are both extracted from crude oil, their characteristics are different when used in an internal-combustion engine.
The differences between petrol and diesel engines are explained later in this chapter but the most obvious difference is that diesel fuel is ignited using only the heat generated by compression. In a petrol engine, although heat is created by the compression process, an appropriately timed electric spark is used to provide the additional heat to start ignition of the petrol/air mixture.
Petrol engine history
A Frenchman, Etienne Lenoir, made the first commercially successful internal-combustion engine in 1860. It ran on coal gas, but worked on a cycle of operations, which did not include compression of the gas before ignition: as a result it was not very efficient.
In spite of this it was in some respects superior to small steam engines of the time, and a great many were sold and did useful work driving machinery in factories. In 1862 Lenoir made a ‘horseless carriage’ powered by his engine and possibly drove it on the roads, but he lost interest in this venture and nothing came of it. In fact, a method of carrying out the cycle of operations using compression of the gas, was described in a patent dated 16 January 1862 taken out by a French civil servant, M.
Beau de Rochas. Since he did not have the means to develop the patent himself, the patentee offered it to Lenoir who, failing to realize its importance, turned it down.
However in Germany, Dr N. A. Otto started the manufacture of gas engines around 1866. Although the first Otto engines were extremely noisy, they were quite effective. Around 1875 Otto took out a patent describing a method of carrying out the cycle of operations, which was in fact identical with that of Beau de Rochas’ thirteen years earlier (it is however, most unlikely that Otto had heard of the Frenchman or his patent). Otto’s new engine was an immediate success. It was much more efficient than Lenoir’s and was very quiet, a characteristic which led to its being named ‘Otto’s silent gas engine’.
Lenoir, realizing his mistake, began to manufacture engines working on the same principle. Otto, of course, sued him for infringing his patent rights, but Lenoir had no difficulty in proving that his engines were made under the earlier patent of Beau de Rochas, which had by now lapsed. The court proceedings at last brought
from the engine, there are numerous other components and systems that are required to make the whole process work. These components and systems are dealt with in great detail in the following sections.
Petrol and diesel reciprocating engines: the main differences
The reciprocating engine can be divided into two main types, depending on the fuel used: petrol and diesel.
Although there are fundamental differences between the two types, they share many common principles of operation and many common components. The main differences relate to fuel supply and ignition.
Note: The petrol and diesel types can also be divided into two sub-types, depending on which operating cycle they use, i.e. two-stroke or four-stroke.
Because these two sub-types are both commonly used, they are covered separately within this chapter, but note that the four-stroke operating cycle is the most popular and is therefore generally used as the starting point in the explanation process.
Diesel engines require fuel to be delivered under very high pressures with very accurate timing of the fuel delivery, and have therefore traditionally made use of high-pressure pumps, which meter and time the fuel delivery. A petrol engine, however, is not as critical with regard to the timing of fuel delivery and delivery pressure, so simple devices known as carburettors were (until relatively recently) the main means of metering and delivering fuel.
A diesel engine makes use of heat created by the compression of a fuel and air mixture to ignite the fuel.
The compression ratio and pressures within the cylinder are therefore high. A petrol engine however, does not generally have so high a compression ratio and so requires the addition of a spark or arc to ignite the fuel.
In recent years the petrol engine has been the subject of much change because of exhaust and other emissions legislation. This has resulted in the simple carburettor being replaced by fuel injection systems, which are now electronically controlled. Additionally, the simple-type ignition systems have been replaced by more advanced, electronically controlled systems. In fact, as well as fuel and ignition control, many other aspects of petrol engine operation are precisely managed by a computer (electronic control). Such control is referred to as ‘engine management’.
Since the end of the 1990s, diesel engine design has also been influenced by emissions legislation, and the latest diesel fuel systems are much more closely related to the petrol injection systems. The diesel engine does still require higher fuel injection pressures and still relies on heat generated by compression to ignite the fuel. However, today’s diesel engine has engine management-type controls, very similar to those fitted to petrol engines. However, the basic differences between petrol and diesel engines remain.
Because the petrol and diesel engines share many common design features and components, the initial
sections of this chapter cover both types of engine. Any differences between diesel and petrol engines are identified where appropriate and later sections deal with issues specific to petrol and diesel systems.
2.1.3 The main components of a reciprocating engine
Figure 2.1 shows the main parts of a reciprocating engine.
Figure 2.1 The main parts of an engine
1 The cylinder, in its simplest form is a circular tube, which is closed at one end.
2 The piston fits closely inside the cylinder. Ideally it would be perfectly gas-tight yet perfectly free to move up and down inside the cylinder.
3 The connecting rod connects the piston to the crankshaft. At the piston end of the connecting rod is a swivel pin called the ‘gudgeon pin’. The gudgeon pin is fitted into holes in the piston and the connecting rod, thus coupling them together.
4 The crankshaft is the main shaft of the engine and is carried in bearings in the crankcase. Offset from the main part of the shaft is the crankpin to which the connecting rod is fitted and is free to turn.
The arrangement is such that rotation of the crankshaft causes the piston to move up and down inside the cylinder. Lines A and B in Figure 2.1 indicate the limits of travel of the top of the piston. As the piston moves upwards the space between its top surface and the closed end of the cylinder is reduced, i.e. the gas trapped in this space is compressed. As the piston moves downwards the space above it is increased, i.e.
the gas in this space expands.
The crankshaft can be rotated by pushing the piston up and down in the cylinder. Starting with the position shown in Figure 2.1, the crankshaft rotates clockwise as the piston is pushed downwards until the piston reaches
the lowest point of its travel. At this point the crankpin will be directly under the centre of the crankshaft, and the centres of the gudgeon pin, crankpin and crankshaft will all lie in a straight line. In this position pressure on the piston will have no turning effect on the crankshaft, and this position is therefore called a ‘dead centre’.
Another dead centre occurs when the piston is at the extreme top of its travel.
These two dead centres, which are known as bottom dead centre (BDC) and top dead centre (TDC) respectively, mark the extreme limits of the piston’s travel. They are illustrated in Figure 2.2a and 2.2b respectively. Movement of the piston from one dead centre to another is called a stroke, and there are two strokes of the piston to every revolution of the crankshaft.
Since this is the volume displaced or swept by the piston, it is called the displacement volume or swept volume of the cylinder. If the engine has several cylinders, as most have, the total swept volume of the engine equals the swept volume of each cylinder multiplied by the number of cylinders.
Example 1: using formula (a) πr2h
Cylinder bore d = 68.2 mm (6.82 cm) (Cylinder radius) r = 34.1 mm (3.41 cm)
Stroke h = 68.2 mm (6.82 cm)
π (3.142) × r2(3.41 × 3.41) = 36.535
36.535 × (h 6.82) = 249.17 (cubic centimetres) cc If the example is a four-cylinder engine, then the total capacity of the four cylinders = 4 × 249.17 = 996.68 cc.
This engine would be referred to as a 1 litre (1000 cc) engine.
Example 2: using formula (b) πd2h/4
Cylinder bore d = 68.2 mm (6.82 cm) (Cylinder radius) r = 34.1 mm (3.41 cm)
Stroke h = 68.2 mm (6.82 cm)
π (3.142) × d2(6.82 × 6.82) = 146.14
146.14 × (h 6.82)/4 = 249.17 (cubic centimetres) cc for each cylinder.
Note that when the bore is equal to the stroke (as in the above example), the engine is called a ‘square engine’.
In a similar way, when the bore is larger than the stroke the engine is called ‘over square’, or if the bore is smaller than the stroke it is called ‘under square’.
Compression ratio
An important feature of the dimensions of an engine is a comparison between the swept volume (as described above) and the space into which gas is compressed.
When the piston is at TDC, there is a small space above the piston, which is called the clearance volume. When the piston rises, it compresses the gas into this space.
The clearance volume will be much smaller than the swept volume of the cylinder and therefore the gas will be compressed into a much smaller space.
If for example the swept volume (volume swept by the piston from BDC to TDC) is 100 cubic centimetres (100 cc – for practical purposes identical to 100 cm3) and the clearance volume is 10 cc, the total volume above the piston when the piston is at BDC is 110 cc. In effect, the cylinder could be filled with 110 cc of gas or air.
If when the piston rises to TDC, the 110 cc of gas is now compressed into only 10 cc of clearance volume, the gas has been compressed to one eleventh of its original volume. Because 10 cc is one eleventh of 110 cc the relationship or ratio of total volume to clearance volume is 11:1.
Figure 2.2 Top and bottom dead centres
Engine size
The usual method of indicating the size of an engine is to state the volume of air and fuel taken into the engine during each complete cycle of operations. In effect, this is the usable volume within the cylinder between the TDC and BDC positions of the piston.
The volume of a cylinder can be calculated by using the following formulae:
(a) V = πr2h
where V is the volume of the cylinder, r the cylinder bore radius and h the stroke (between TDC and BDC). Note that π (pi) = 3.142
(b) V = πd2h/4
where V is the volume of the cylinder, d the cylinder bore diameter and h is the stroke (between TDC and BDC). Note that π (pi) = 3.142
The internal diameter of the engine cylinder is called the bore, while the distance the piston moves between TDC and BDC is called the stroke.
With reference to Figure 2.3:
Compression ratio =
=
= 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