The engine - how it drives its ancillary parts

The engine is, in many respects, self-sustaining: it supplies the power that drives a number of ancillary - subordinate - components without which it could not work.

It requires a controlled fuel supply, a timed electric spark to ignite the air-and-fuel mixture, a means of dispersing heat, and lubrication to reduce friction . These ancillary functions are carried out mainly by mechanically driven components. The crankshaft is their main power source, through gears and sprockets or pulley-driven chains or belts.

A powerful electric motor connected directly to the battery is used to rotate the crankshaft at a speed high enough to initiate the four-stroke cycle and start the engine.

Starting the car, particularly from cold, therefore makes the heaviest demand on the battery , since the starter motor has first to overcome the inertia of the engine. There is also a high demand from electrical equipment such as flashers and lights, so the battery needs constant replenishing in order to maintain its charge of about 12 volts .

Charging is accomplished by a generator - a dynamo in earlier cars, or an alternator in later ones - driven by the crankshaft. The generator output is controlled by a charging circuit which ensures that the battery receives the correct amount of current necessary to keep it fully charged.

To produce the timed electric spark needed to ignite the fuel mixture, the low voltage from the battery is boosted to a very high voltage, about 30,000, by the ignition coil - a form of transformer.

The low-tension (LT) voltage passes through a primary winding in the coil and then to the contact-breaker points in the distributor .

Each time the low-tension-current circuit is interrupted by the opening of the contact-breaker points in the distributor, the electrical surge as the current suddenly collapses induces high-tension (HT) voltage in a secondary winding in the coil.

The distributor then feeds the high-tension voltage to each of the spark-plugs in turn at the correct time. Each sparkplug has two electrodes at its tip, with a gap between them. The high-tension voltage jumps this gap and produces the spark that ignites the air and fuel mixture.

The petrol pump that feeds the carburettor is either a mechanical one operated by an off-centre disc - a sort of circular cam - on the engine camshaft , or an electric pump remotely mounted, sometimes close to the petrol tank.

In water-cooled cars, the water pump that circulates water through the cooling passages of the engine is belt-driven from a pulley on the crankshaft.

The oil pump that pressurises oil for engine lubrication works directly from the crankshaft or camshaft.

Friction between moving metal parts in the engine is minimised by a thin film of oil.

Stored in a reservoir , called the sump , at the bottom of the engine, the oil is sucked into the pump, which sends it under pressure through various feed pipes and channels to the moving parts of the engine and then back to the sump.

The oil is circulated under pressure at a rate of several gallons per minute. The pressure is controlled in the pump by a relief valve ; when the pressure is excessive, it leaks some of the oil back to the sump.

Oil forced out of the crankshaft journals is thrown against the cylinder walls. This is known as 'splash lubrication'.

Before reaching the engine, the oil passes through a filter attached to the pump, which removes sludge and potentially abrasive particles such as debris resulting from engine wear.

If the pressure drops - usually through mechanical failure - or if there is an oil deficiency, the moving parts of the engine wear rapidly and eventually seize .

There are two basic types of oil pump - a rotor type and a gear type.

The rotor pump has two multi-lobed rotors - an inner one and an outer one, revolving on different axes. The gear type has two adjacent meshing gears. The pump may be mounted internally or externally.

Oil sucked from the sump is pressurised as it passes through the decreasing gap between the rotor lobes or meshing gears.

The commonest filter element used to trap the sludge and debris that collects in the sump is made from resin-impregnated, pleated paper.