Compression pressure in diesel engines.
Comparing the data in the table of required compression pressure values, we can easily notice, that the compression pressure in diesel engines is in some cases up to three times higher than in gasoline engines. These differences result from differences in the principles of engine operation, or more precisely from different methods of ignition of the charge. In a diesel engine, ignition occurs automatically (hence the other name – compression ignition engine and abbreviation – ZS) due to temperature, which is produced when the air in the cylinder is heavily compressed. In the TDC position of the piston, the temperature in the cylinder of the diesel engine reaches 900 ° C, which is sufficient to ignite the fuel injected into the combustion chamber just before the piston reaches TDC.
This mixture formation method is also different than in a traditional gasoline engine (carburetor, or about indirect injection), in which air and fuel mix outside the cylinder, in the intake manifold. In a diesel engine (but also in gasoline direct injection engines) fuel and air only mix in the combustion chamber. Unlike the previous ZS drive units with indirect injection into the swirl chamber (making their work calmer) modern Volkswagen diesel engines mostly have direct injection, conducive to more efficient operation of drive units and better dynamics of vehicles equipped with them. Further information on direct injection can be found in the chapter on engine controls.
Excessive speed and engine durability.
Operating the engine at excessive speed will shorten its life. Over-rotating engine becomes noisy due to vibration of the crankshaft and moving parts of the valve drive. These vibrations can lead to rupture and fracture of the valve lifters. A faulty mechanism causes the valve and cylinder to be off-cycle and a loss of power. In extreme cases, the valve spring may break and the valve will hit the moving piston, which means very serious engine damage. As a rule, this qualifies the engine for scrap.
Finding faults.
■ When the compression pressure is too low, then pour some engine oil into the spark plug socket hole with an oil can. The oil will seal the space between the walls of the piston and cylinder. Then measure the compression pressure again.
■ If the pressure value is still too low, faulty valves may be the cause, valve guides, head, or a head gasket.
■ If the treatment with oil has brought improvement, this indicates wear of the piston rings, or smoothes the walls of the cylinder.