Difference Between Petrol Engine and Diesel Engine

There are quite a few similarities between diesel and petrol engines, regardless of an ensemble of differences from their ideal power cycles to their real-time operation. These similarities include:

  • Three out of four processes in the ideal Otto and diesel cycles are the same: Isentropic compression, Isentropic expansion, and isochoric exhaust of burned gaseous byproducts.
  • Both engines are commercially available in two-stroke and four-stroke designs.
  • Both engines are available with natural and charged aspiration mechanisms.

Difference Between Petrol Engine and Diesel Engine:

Petrol EngineDiesel Engine
The combustion process is spark-ignited.The combustion process is self-ignited.
As can be observed from the TS plot of the ideal diesel cycle, the net enclosed area or the net work output is lower than that of the petrol engine.During the compression stroke, only the air is compressed.
As discernible from the PV diagram of the ideal Otto cycle, the isentropic compression and expansion ratio is equal.As discernible from the PV diagram of the ideal Otto cycle, the isentropic compression and expansion ratio are equal.
As can be observed from the TS plot of the ideal Otto cycle, the net enclosed area or the net work output is higher than the Diesel engine.Because of the higher compression ratio, there is a strong likelihood of increased stress and engine damage, thereby necessitating the need for a robust, heavier, and sturdier engine design.
The amount of heat rejected during the exhaust process is lower with the same heat input and compression ratio.The amount of heat rejected during the exhaust process is higher with the same heat input and compression ratio.
The detonation considerations such as surface ignition and combustion knock, do not exist with diesel engines. Therefore, higher mean effective pressure and compression ratio do not affect the thermal output of diesel engines.The detonation considerations, such as surface ignition and combustion knock, do not exist with diesel engines. Therefore, higher mean effective pressure and compression ratio do not affect the thermal output of diesel engines.
If the amount of heat rejected is the same (in both the engines), then the petrol engine does have higher thermal efficiency.The fuel used in the petrol engine should have a higher propensity to mix with the air fully. This quality of the fuel is its volatility. The higher the volatility of the fuel, the better the homogeneity of the air-fuel charge, and the higher the thermal efficiency of the engine.
Modern, lower speed, large capacity petrol engines have thermal efficiency of almost 41% on LHV basis and 37% on HHV basis.
LHV: The lower heating value of the fuel
HHV: The higher heating value of the fuel
The compression ratio and mean effective pressure in the petrol engine are limited by two detonation aspects: surface ignition and combustion knock.
The fuel used in the petrol engine should have a higher propensity to mix with the air fully.  This quality of the fuel is its volatility. The higher the volatility of the fuel, the better the homogeneity of the air-fuel charge, and the higher the thermal efficiency of the engine.The compression ratio and mean effective pressure in the petrol are limited by two detonation aspects: surface ignition and combustion knock.
The compression ratio and mean effective pressure in the petrol are limited by two detonation aspects: surface ignition and combustion knock.Higher throttling at part-loads means more pumping work and,, ultimately, lower efficiency.
The detonation considerations, such as surface ignition and combustion knock do not exist with diesel engines. Therefore, higher mean effective pressure and compression ratio do not affect the thermal output of diesel engines.Higher throttling at part-loads means more pumping work and, ultimately, lower efficiency.
It is governed by throttling the air-fuel mixture into the engine cylinder.The abuse of engine combustion chamber caused by relatively less refined fuel causes soot deposits and, thereupon, higher maintenance costs.
Because of the higher compression ratio, there is a strong likelihood of increased stress and engine damage, thereby necessitating the need for a robust, heavier, and sturdier engine designBecause of the higher compression ratio, there is a strong likelihood of increased stress and engine damage, thereby necessitating the need for a robust, heavier, and sturdier engine design
Because of the higher compression ratio, there is a strong likelihood of increased stress and engine damage, thereby necessitating the need for a robust, heavier, and sturdier engine designAs discernible from the PV diagram of the ideal Otto cycle, the isentropic compression and expansion ratio are equal.
The abuse of engine combustion chamber caused by relatively less refined fuel causes soot deposits and, thereupon higher maintenance costs.The fuels used in the petrol engine, such as natural gas or gasoline are environment friendly and cause less emissions.
Because of the higher compression ratio, there is a strong likelihood of increased stress and engine damage, thereby necessitating the need for a robust, heavier, and sturdier engine designThe maximum compression ratio of the diesel engine is 20:1. It may exceed it.
With cogeneration cycles, petrol engines, due to rejecting more heat at the sink are favorable for recovering lost heat.Diesel combustion engines produce higher emissions, CO, soot, etc. Due to the less refined character of its fuels, they are less environmentally conducive.