Using similar mechanical com•- ponents to a piston petrol engine, the diesel burns fuel oil. It has no spark plugs— instead the air is compressed in the cylinder until it heats up, and so ignites fuel that is injected into the combustion chamber. This compression-ignition process works as follows:

Suction. Exhaust valve shut, inlet valve open. The descend- ing piston draws in atmos- pheric air.

Compression. Both valves shut. The rising piston highly compresses the air (the com- pression ratio on a diesel engine may be around 22:1), substantially raising its temperature.

Injection/power. As the pis- ton approaches the top of its stroke, fuel is injected into the combustion chamber, meets the hot air and ignites. Com- bustion forces the piston to the bottom of the cylinder, turning the crankshaft.

Exhaust. The exhaust valve opens, and the rising piston pushes out the burned gas. The very high compression ratio of the diesel engine results in greater efficiency, and as a result, diesel engines use less fuel than a comparable petrol engine— particularly in traffic.

The disadvantages of the diesel are higher initial cost, noisier operation, slower acceleration and a lower maximum speed. Despite the drawbacks—some of whi~h are being eliminated by improved design—the diesel engine is being fitted to greater numbers of private cars because of its energy-saving potential.

Exhaust emission requirements have made the two-stroke pis- ton engine obsolete in new cars, although a few older models use it.

Combustion occurs on each downward stroke of the piston. There are no valves; instead the descending piston uncov- ers ports which let out the exhaust gas and then replace it with a fresh charge of fuel/air mixture.

Most two-strokes use a small quantity of lubricating oil in the fuel, which produces a smoky exhaust. They are less efficient than a four-stroke engine.

The Wankel engine

When it was first produced in 1964 the Wankel rotary engine, with its turbine smoothness, light weight and very few mov- ing parts, was expected to challenge seriously the supre- macy of the piston engine. Since then, problems with internal sealing, a heavy thirst for fuel and difficulty in meet- ing exhaust pollution regula- tions have meant that it has become a rarity.

Inside; instead of pistons, it has a three-pointed rotor which makes tip contact continuously with the inside of a chamber which has the shape of a wide- waisted figure-of-eight. As the rotor orbits within the chamber, three working spaces between the rotor sides and the chamber expand and contract.

Two ports in the chamber admit the petrol/air mixture and let out the exhaust. A spark plug (sometimes two) fires the mixture. The engine has no val- ves, and relies on the move- ment of the rotor to provide a four-stroke sequence. On a single rotor engine there are three power strokes for each single rotation of the rotor—it is usual to link two or more rotors together. Anoutputshaft geared to the centre of the rotor turns at three times rotor speed and takes the drive to the gearbox.

Power and torque

The output of an engine is expressed in two ways. One measure is its maximum power output, expressed in bhp (brake horsepower) or kilowatts and is an indication of its rate of doing work. The figure will be quoted alongside the engine speed at which the power is delivered—for inst- ance 60bhp at 5, 500rpm’. The majority of modern engines are capable of 50bhp per 1000cc of engine capacity. Most cars are geared so that the engine develops maximum power at maximum road speed.

The other measure of engine output is torque, loosely defined as ‘pulling power’. A car needs high torque from low engine speeds, helping to eliminate frequent gear chang- ing. Torque is a measure of leverage, and is quoted in lb-ft (pounds-feet) or kg-m (kilo- gram-metres) at the engine speed at which maximum tor- que occurs, usually around half the speed at which maximum power is produced.