Why Diesel Engine Have High Compression Ratio?

Compression ratio of diesel engine refers to the degree of compression of engine mixture, which is expressed by the ratio of total volume of cylinder before compression to volume of cylinder after compression (i.e. combustion chamber volume).

1. Definition of Compression Ratio

Compression ratio is the ratio of total cylinder volume to combustion chamber volume, which is an important structural parameter of internal combustion engine. When the piston is at the bottom dead center, the cylinder has the maximum volume, expressed by Va. When the piston is at the top dead center, the volume in the cylinder is called the combustion chamber volume, which is expressed by Vc.

The compression ratio of internal combustion engine is ε

ε=Va/Vc

Where ε is the geometric compression ratio, which represents the degree of gas compression in the cylinder when the piston moves from BDC to TDC.

When the piston is at BDC, the intake valve or the intake and exhaust ports are not closed, so the concept of effective compression ratio ε0 must be used sometimes. Parameterε0 refers to the ratio of cylinder volume to cylinder compression volume at the moment when inlet and exhaust valves (ports) of internal combustion engine begin to close completely. Any compression ratio not specifically specified refers to the geometric compression ratio.

2. The Influence of Compression Ratio on the Performance of Internal Combustion Engine

Compression ratio has many effects on the performance of internal combustion engine. The higher the compression ratio is, the higher the thermal efficiency will be. However, with the increase of compression ratio, the increase range of thermal efficiency becomes smaller and smaller. The increase of compression ratio makes the compression pressure and the maximum combustion pressure increase, so the mechanical efficiency of internal combustion engine decreases. If the compression ratio of gasoline engine is too high, it is easy to produce knockings. If the compression ratio of KPV420 high speed diesel engine is too low, the end temperature of compression will become lower and the cold start performance will be affected.

3. Why Does Diesel Engine Have High Compression Ratio

Because compression is to ignite the air fuel mixture, and it is described as homogeneous charge compression ignition. High quality 12 cylinder diesel engines do not use spark plugs and compression must be utilized to start the combustion process.

When the air and fuel charge is compressed by the piston on its upper stroke, the heat in the cylinder is compressed to a smaller area and the temperature increases. Once the temperature of compressed air fuel ratio reaches ignition point, homogeneous charge compression ignition will occur. This ignition point occurs when the air fuel is charged and ignited and the energy is released to drive the piston backward, which is a unique property of powerful KPV420 12 cylinder diesel engine.

Diesel and gasoline are two kinds of fuels with different physical and chemical properties. It is easier to light gasoline than diesel. If light a match and throw it into a gasoline puddle, then it will burn quickly at room temperature and pressure. However, diesel does not ignite as easily as gasoline, so a higher autoignition pressure is required (assuming both fuels are at the same temperature).

The typical compression ratio of KPV420 high speed 12 cylinder diesel engine is 18-22. Under the same conditions, gasoline is usually in the range of 9-13 in CR state.