Car Parts and Accessories

Seven steps to understanding how it works

The mass of tubes, pipes, wires and components scrambled together under the bonnet of a car can present a bewildering picture.

The average family sedan is assembled from over 13 000 different parts—some 1 500 of them synchronised to move together—many of them working to within tolerances of 0,01 mm. Nearly 60 different materials go into a car’s construction—varying from steel to straw-board, from nickel to nylon.

The understanding of a car’s mechanics can be simplified by splitting it up into seven groups; for despite the enormous range of differences in styling, performance and cost, modern cars follow the same principles.

It has been claimed that the economical life of the average car is 10 years or

160 000 km. Obviously, however, this depends on many factors such as the condition and nature of use, how well the vehicle has been cared for, whether spare parts are readily available at reasonable cost etc. The AA operates a car examination and advisory service in most of the larger centres in South Africa. For a modest fee members can have the condition of any vehicle accurately assessed and obtain advice on purchasing, selling, maintenance and repairing.

How the engine produces power

The engine is the powerhouse of the car. It converts the heat produced by burning fuel into mechanical energy to turn the wheels. The fuel, normally a mixture of petrol and air, is burnt in closed cylinders inside the engine.

Petrol and air are mixed in the carburettor and drawn into a combustion chamber at the top of each cylinder. Pistons, inside the cylinders, compress the mixture, which is then ignited by a spark plug. As the mixture burns it expands, forcing the piston down on its power stroke.

The up-and-down movement of the pistons is transformed into rotary movement to drive the crankshaft, which in turn transmits power to the wheels through the clutch, gearbox and final drive. Connecting rods link the pistons to the crankshaft.

A camshaft, driven by the crankshaft, controls inlet and exhaust valves at the top of each cylinder.

Initial impetus to set the engine in motion comes from the starter motor. This is connected to a starter ring which is fitted around the edge of the flywheel — a heavy disc bolted to the end of the crankshaft. The starter motor, which is geared to the flywheel, is operated electrically and turns the flywheel and crankshaft, which starts the pistons and connecting rods moving up and down.

In addition to providing a smooth and robust surface onto which the clutch assembly is mounted, the flywheel smoothes out the power impulses of the pistons and gives a relatively smooth rotation of the crankshaft.

Because of the heat produced by an internal combustion engine, the metal parts would seize without a cooling system.

In most cars, water is circulated through channels in the engine called a water- jacket. The hot water then passes through a radiator where the heat is dispersed to the atmosphere. The dispersal of heat is speeded by a fan, which draws cooling air through the radiator.

Other cars, mostly those with engines at the rear, are cooled by air which circulates over fins cast in the cylinder head.

To prevent wear and overheating, the engine has a lubrication system. Oil, kept in a sump underneath the cylinder block, is pumped to the moving parts.

Separating the main parts

The engine must be a rigid structure in order to withstand the heavy loads applied to the crankshaft bearings and other internal parts.

It is made of two basic parts bolted together: the upper is the cylinder head which houses a combustion chamber, two valve ports and two valves for each cylinder. The lower is the cylinder block, which contains the crankshaft assembly, pistons, connecting rods and the oil pump.

Both the head and the block are usually made of cast iron, but aluminium is an alternative material for lightness and good heat dissipation.

Practically all modern engines have their camshafts carried in the cylinder head and are known as an overhead camshaft (OHC) unit.

The engine draws in the petrol / air mixture through one set of valves (the inlets) and expels burnt gases through the other (the exhausts). On top of the cylinder head is the gear that operates the valves.

In older engines, the camshaft is sometimes contained in the cylinder block and operates a set of valves located in the cylinder head by the use of push- rods and rockets.

When this system is used, the engine is known as a side cam overhead valve (OHV) unit.

Both the cylinder head and the block contain passages, known as the water jackets, through which water circulates to cool the engine.

The sump, which is the reservoir for the engine lubricating oil, is made of sheet steel, cast aluminium or magnesium, and is bolted to the bottom of the crankcase.

A cover, usually of similar material to the sump, is fitted over the valve gear to exclude dust and retain oil.

Cylinder head

This carries the valves and the springs which close them. It usually carries the camshaft and rocker gear to open the valves. On a push-rod engine, the head carries the rocker shaft and has holes for the push-rods. The head also contains the inlet and exhaust ports and at least part of the combustion chamber.

Cylinder block

The largest part of the engine it has cylinders (bores) for the pistons, passages for water to cool- the cylinders, oilways for the lubrication system, tunnels for the crankshaft bearings and tunnels for push-rods if these are used to operate the rocker gear.

Crankshaft assembly

Pistons moving up and down in the cylinder bores are linked to the rotating crankshaft by connecting rods. The crankshaft is carried in main bearings at the base of the cylinder block. At one end of the crankshaft is the flywheel which smooths out the power impulses of the individual pistons

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