3 Petrol and diesel engines

Petrol engines

The petrol-fuelled spark-ignition engine is characterised by the use of a spark plug to initiate the combustion process. The engine uses a piston which is driven up and down inside a cylinder and connected to the drive section by a rotating crankshaft. At the top of the engine there is a cylinder head containing a number of valves controlling the flow of gas in and out. The four 'strokes' are: induction, compression, power and exhaust. An animation of this process can be found on the Animated Engines website [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] . As is explained in the animation, on the induction stroke a small amount of fuel and air is drawn into a cylinder through the open inlet valve, which then closes. On the next stroke this mixture is then compressed into a smaller volume. This reduction in volume is a rather critical factor called the compression ratio. In a modern car it is about 9:1, i.e. the fuel/air mixture is squeezed into one-ninth of its original volume, creating a highly inflammable mixture. This is then ignited using an electric spark on a sparking plug. The gases then burn very rapidly reaching a high temperature (750 °C or more) and expand, pushing down the piston on the power stroke. Finally, on the exhaust stroke, the burnt gases are pushed out into the exhaust system through the open exhaust valve. The whole cycle then repeats.Higher compression ratios of 13:1 or more are possible using petrol, with careful engine design or by the use of fuels with a high octane rating, such as ethanol, methanol, natural gas or hydrogen. These can allow a higher combustion temperature and increased engine efficiency.

Diesel engines

The diesel engine works using the same four-stroke cycle as the petrol engine, but with two major differences involving the air–fuel mixture and injection systems (again, see the Animated Engines website). In the diesel engine, only the air is compressed in the cylinder instead of an air–fuel mixture, and at the end of the compression stroke, the fuel is directly injected into the combustion chamber by a fuel injection pump. A typical compression ratio of 20:1 is used, which is sufficient to raise the air temperature to over 400 °C. Once the diesel fuel is injected into the cylinder, it immediately vaporizes and spontaneously ignites.

Modern diesel engines tend to use direct-injection fuel-delivery systems, as they can be closely controlled by the use of computerized engine management systems.

In general, the fuel efficiency of a diesel engine is higher than that of a petrol engine. This is primarily due to the fact that the combustion temperature (and pressure) within a diesel engine is higher than in a petrol power unit. This increases the engine's efficiency according to Carnot's equation for a perfect heat engine. The higher combustion temperature also leads to different exhaust emission profiles between vehicles with a petrol engine and those with a diesel engine, which will be considered when I cover emissions in Section 4. In addition, although diesel fuel has almost the same energy content per kilogram as petrol, it is denser and so contains more energy per litre.For further details on petrol and diesel engines see Chapter 8 of Everett et al (2012).

Petrol and diesel vehicle energy efficiency

In a diesel engine about 32% of the heat energy is delivered to the crankshaft, whereas in a petrol engine only about 24% becomes delivered work (Everett et al, 2012). As this kinetic energy is delivered to the wheels via the mechanical drive-train, energy is 'lost' owing to friction between the transmission components and to aerodynamic drag of the vehicle. As a result, in theory only about 24% of diesel fuel's energy ends up being used for moving the car; in the case of petrol this figure is only 18%. In practice, the actual values found vary enormously with the vehicle type and with the driving conditions. ICEs are particularly inefficient in slow stop/start urban motoring and in situations of high acceleration; they work most efficiently running at a constant speed (e.g. on motorways).

If we consider how much of the fuel's energy is actually used to move the payload (as opposed to the whole vehicle), the situation is even worse. Only around 1–2% of the fuel's energy is used to move the vehicle's occupants.