Diesel cycle

The Diesel cycle ( constant-pressure process ) is named after the German engineer Rudolf Diesel comparison process for piston engines in which the flow of heat at constant pressure ( isobaric ) takes place in the first phase of expansion. For this is in contrast to the Otto cycle ( constant volume process ) as a comparison process for piston engines with heat at constant volume ( isochoric ).

Process flow

The comparison process consists of four changes of state of an ideal gas. The gas exchange cycle ( isobaric ejection and suction) is not considered. The four phases of the process are:

• Isentropic compression ( 1 → 2)
• Isobaric heat ( 2 → 3 ) (hence constant pressure process! )
• Isentropic expansion ( 3 → 4 )
• Isochoric heat dissipation (4 → 1)

The trace of (1 → 2 → 3 → 4 ) enclosed area corresponds to the specific work.

The four process steps are as follows:

Efficiency

The efficiency of the diesel process is dependent on the geometric compression ratio, the full pressure or constant pressure ratio, which in turn depends on the amount of heat supplied ( the real engine this is the injection ratio ), and the Isentropenkoeffizienten.

Comparison processes for illustrative purposes only fundamental processes and relationships. So is also expected here usually with air as the working medium, simplistic - far removed from reality - temperature-independent material properties (specific heat capacity and isentropic exponent ) are assumed ( ideal gas ). The thermal efficiency of the diesel process can then be determined as follows:

The term including the full pressure ratio is > 1 Therefore, for the same compression ratio, the efficiency with respect to the constant-volume process would be less. However, the pressure and temperature peak is lower by the heat supply in the expansion phase, which is why the same thermal load a larger compression ratio is possible and ultimately a greater efficiency of the constant pressure process.

The equations for the changes of state

The real diesel engine

The deviation of the comparison process of the real process is very large not only because of the simplified assumptions. In a real engine is in part also held constant volume combustion, so that the maximum pressure is at least twice as high as the compaction pressure. Therefore, a better comparison process ( for gasoline engines ) the Seiliger cycle process with a share equal space and another moiety constant-pressure heat. The constant-pressure portion is in the real diesel engine but later and therefore, a higher compression ratio with a higher final pressure is possible with the same thermal loading of the material. This higher efficiency is achieved. The comparison processes also take into consideration not the dissipative processes such as friction and heat by cooling the cylinder, so that the calculation - as in the example - much higher efficiencies than actually achievable results. Accessible today efficiencies of about 42 % for passenger cars, 45% trucks and 50 % for the marine diesel engine. The lower power density ( output per liter ) than the gasoline engine has the cause in the larger combustion air ratio.