The Mercedes 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, the lower is the cylinder block, which contains the crankshaft assembly. Both the head and the block are usually made out of cast iron, but the aluminum is an alternative material for lightness and good heat dissipation.
In the cylinder head are a combustion chamber, two valve ports and two valves, for each cylinder.
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 valve-operating gear.
The cylinder block is usually in one piece with the crankcase. It contains the cylinders and carries the crankshaft, to which are attached the connecting rods and pistons. It may also contain the camshaft by which the valves are opened and closed. Alternatively, the camshaft may be carried on the cylinder head, in which the engine is known as and overhead-camshaft (OHC) unit.
Both the Mercedes cylinder head on 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 aluminum 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.
How heat becomes driving Power
HEAT-ENERGY produced by the combustion of petrol and air is converted into mechanical power by the pistons, connecting rods and crankshaft of an engine. The engine’s efficiency depends upon on how much of this energy becomes useful power.
The more petrol/air mixture that can be drawn into a cylinder, and the more it can be compressed, the higher will be the specific output of the engine.
The degree of compression is measure by the compression ratio – the ratio of the volumes of gas inside the cylinder before and after compression. The average family car has a compression ratio of about 9:1, which means gas in the cylinder is compressed to one-ninth of its original volume.
The compressed mixture, when ignited by a spark, should burn rapidly but not aggressively and smoothly across the top of the piston. It should not explode.
If the compression ratio is too high for the grade of petrol being used, the burning will not be progressive. Some of the mixture furthest away from the spark plug will explode violently, or detonate. This is known as knocking or pinking.
Apart from loss of power, this detonation can cause over-heating and if allowed to continue will damage the engine.
Loss of efficiency and overheating can also occur in an engine from pre-ignition – ignition before the timed sparking occurs at the spark-plug. This can arise from defective or incorrect spark-plugs, or it can be caused by deposits in the combustion chamber which have grown hot and glow continuously. Pre-ignition, like knocking, can cause extensive engine damage, as well as reduce engine power.
The action cycle in most car engines is the 4-stroke or Otto cycle, with the power stroke of the piston occurring once every four strokes. During one revolution of the crankshaft, the piston descends on its induction stroke and rises on its compression stroke. During the next revolution of the shaft, the piston is forced down on its power stroke, then rises on its exhaust stroke to expel burnt gases.
Since the inlet and exhaust valves must open only once in each cycle, the camshaft operating them is driven at half the speed of the Mercedes crankshaft, which makes two revolutions during the 4 stroke cycle.