1 The reciprocating piston petrol engine
1.1 MODERN REQUIREMENTS
General background
The motor vehicle engine is basically a device for converting the internal energy stored in its fuel into mechanical energy. It is classified as an internal combustion engine by virtue of this energy conversion taking place within the engine cylinders.
Since the term âenergyâ implies the capacity to perform work, the engine is thus able to propel the vehicle along the road and, within limits, overcome unwanted opposition to its motion arising from rolling friction, gradient resistance and air drag. To facilitate this process the engine is combined with a transmission system, the functioning of which is discussed later.
The vast majority of car engines are of the reciprocating piston type and utilize spark ignition to initiate the combustion process in the cylinders. However, the compression ignition or diesel principle to initiate combustion is increasingly challenging the petrol engine for car applications, especially in Europe. Both petrol and diesel engines operate on the four-stroke principle in which the piston travels one complete stroke for each of the successive events of induction, compression, combustion and exhaust.
The late Laurence Pomeroy, a distinguished motoring historian, once summarized the early history of the motor car as follows: From 1885 to 1895 men struggled to make the car go. From 1896 to 1905 they contrived to make it go properly. Between 1907 and 1915 they succeeded in making it go beautifully! What then are the requirements for the engine of the modern passenger car as reflected in many decades of further development and, not least, in the light of present-day energy conservation and environmental pollution considerations? These requirements can now add up to quite a formidable list. As we pursue our studies into the whys and wherefores of engine construction and operation, it will become evident that although some of the requirements are complementary, others are not, and therefore (as in most engineering) some compromise has generally to be accepted in the final product.
Modern requirements
Optimum performance
With modern advances in engine design it is not particularly difficult to obtain sufficient power to give the car a high top speed, especially since the recent trend towards car bodies of lighter construction and more efficient aerodynamic shape. Today, however, a more important engine requirement than a further increase in top speed is an improved accelerating capability together with better flexibility in the low to middle speed range, or what is sometimes termed âdriveabilityâ. A further performance requirement of a new engine design is that it must usually allow for possible future increases in cylinder size.
Good fuel economy
The overall aim of improving the fuel economy of cars is to minimize the amounts of crude oil used to provide petrol for their engines, because of constraints imposed by limited petroleum resources and rising costs. Fuel economy may also be made the subject of legislation, as it already is in America, where each manufacturer has to comply with corporate average fuel economy standards (or CAFE standards, as they are generally termed). For these reasons, further engine requirements are those of minimum weight so as to reduce total car weight; improved combustion efficiency, better to utilize the fuel; and reduced friction losses between the working parts.
Low pollution
Since the late 1960s increasingly stringent legislation has been applied to limit the levels of atmospheric pollutants emitted from car engines, especially the American FTP (Federal Test Procedure), the Japanese and later the European Community ECE/EEC test cycles, all of which differ in their requirements and are therefore not directly comparable. In Britain The Road Vehicles (Construction and Use) Regulations are also now such that there is a requirement for every motor vehicle to be so constructed that no avoidable smoke or visible vapour is emitted therefrom, and another that makes it an offence to use a vehicle which emits substances likely to cause damage to property or injury to persons. In general, legislation is concerned with carbon monoxide, which has toxic effects; unburned hydrocarbons, which contribute to atmospheric smog; and nitrogen oxides, which cause irritation to the eyes and lungs, and also combine with water to produce acid rain that destroys vegetation. To reduce these harmful emissions, not only is very careful control of the combustion process required in modern engine design, but also various sophisticated devices may have to be added for after-treatment of the exhaust gases. Of further concern to the environmentalist is the emission of carbon dioxide which, although non-toxic, is nevertheless an unwanted contributor to global warming. This has to the development of systems for deactivating half the number of cylinders on some large capacity V8 and V12 engines, to reduce fuel consumption and therefore the emission of carbon dioxide when full power is not required.
Minimum noise level
Noise is generally defined as unwanted sound. Reducing interior noise makes a car more attractive to the buyer. Reducing exterior noise to socially acceptable limits has been the subject of increasingly stringent legislation in the European Community and other countries since the early 1980s, and in Britain is included in the Provision of the Motor Vehicles (Construction and Use) Regulations relating to noise. A similar function is performed in America by the EPA (Environmental Protection Agency) noise regulations. Since the engine is an obvious source of noise an important requirement is that its design and installation should minimize noise emission, not only that directly radiated from the engine itself to the exterior, but also that arising from vibrations transmitted through its mounting system to the car body interior.
Easy cold starting
An essential driver requirement of any engine, whether it be of past or present design, is that it should possess good cold starting behaviour and then continue to run without hesitation during the warming-up period. A present-day additional requirement is that the cold starting process should be accomplished with the least emission of polluting exhaust gases and detriment to fuel economy. To monitor the required enrichment of the air and fuel mixture for cold starting, increasingly sophisticated controls were applied first to carburettor automatic choke systems and then later to fuel injection cold start systems. These controls form part of what are now termed âengine management systemsâ.
Economic servicing
An important owner requirement of a car is that its engine design should acknowledge the need to reduce servicing costs. This aim may be approached by minimizing the number of items that need periodic attention by a service engineer. For example, the use of hydraulic tappets eliminates altogether the need for adjustment of the valve clearances. It is also promoted by allowing ready access to those items of the engine involved in routine preventive maintenance, such as the drive-belt tensioner, spark plugs, and petrol and oil filters.
Acceptable durability
In order to reduce fuel consumption while still maintaining good car performance, it is now the trend to develop engines of smaller size with relatively higher power output. Furthermore, the installation of a turbo-charger permits an increase in power without imposing a corresponding increase in the size or weight of the engine itself. However, the greater heating effect on certain engine components may require changes to their material specifications and also the addition of an oil cooler. The components of modern engines have therefore tended to become more highly stressed, so that engine testing of ever-increasing severity by the manufacturers is now required to maintain durability in extremes of customer service.
Least weight
Another important design requirement of the modern petrol engine is that it should be made as light as possible. This is because a corresponding reduction in car weight can make significant improvements not only in fuel economy and acceleration capability, but also in general handling and ease of manoeuvring the car. Since reducing engine weight is not always consistent with maintaining durability, the need for adequate testing of the engine components is confirmed. Also special manufacturing techniques may have to be adopted to avoid damage to such items as castings with very thin walls.
Compact size
For the modern car, the manufacturer strives to provide the maximum interior space for the minimum possible exterior dimensions. Thus the trend is inevitably towards having the front wheels driven, with the power unit (engine and transmission) installed transversely between them; the conventional arrangement was to have a longitudinally mounted power unit from which the drive was taken to the rear wheels. It follows that the requirement now is for a more compact engine. This is because the engine length is controlled by the distance available between the steerable front wheels, less that required by the transaxle (combined gearbox and final drive); its width by the distance available between the radiator and the dash structure, less that required by the engine auxiliaries; and its height by the need for a low and sloping bonnet line, which contributes to an efficient aerodynamic body shape.
Economic manufacture
This is clearly a most important requirement for any new design of engine, since putting it into production demands a massive initial investment on the part of the car manufacturer. It is, of course, for this reason that the smaller specialist car manufacturer generally uses an existing engine from a volume producer. For economic manufacture a new design of engine should lend itself as far as possible to existing automatic production processes and require the minimum of special tooling. The cost of materials will be reduced in building a smaller engine, and the construction should be as simple as possible to minimize the number of parts to be assembled and thereby further reduce manufacturing costs. Similarly, to produce a range of large capacity V6 and V8 engines a modular design approach may be adopted, so that their major components can be produced on the same machinery.
Aesthetic appearance
In early years the under-bonnet appearance of high-grade cars of the 1920s and 1930s, such as Bugatti, Hispano-Suiza and Rolls-Royce, was much admired for the elegant proportions and beautiful finish of their engines. More recently manufacturers have recognized the customer appeal of a pleasing under-bonnet appearance. Not so much of the engine itself, which is usually buried deeper within the engine compartment, but of the neat arrangement and smooth contours of the modern comprehensive air intake system and its manifold runners that now lie above the engine.
1.2 ENGINE NOMENCLATURE
To understand the information given in an engine specification table, such as those included in a manufacturerâs service manual or published in the motoring press, it is necessary to become familiar with some commonly used terms (Figure 1.1). The âlanguageâ of the reciprocating piston engine is summarized in the following sections.
Figure 1.1 Engine nomenclature (Yamaha)
Top dead centre
The top dead centre (TDC) is of general application in engineering; it is any position of a hinged linkage in which three successive joints lie in a straight line. In the case of a motor vehicle engine, top dead centre refers to the position of the crankshaft when the piston has reached its closest point to the cylinder head. This results in the main, big-end and sma...