Comparison of Petrol and Diesel Engines

Petrol vs Diesel engines: A comparison - Basic difference between a petrol engine and a diesel engine based on working, pressures, combustion, compression ratios, speed, efficiency, maintenance, and running costs. The following points are important for the comparison of petrol and diesel engines:

SN	Petrol Engines	Diesel Engines
1.	A petrol engine draws a mixture of petrol and air during suction stroke.	A diesel engine draws only air during suction stroke.
2.	The carburetor is employed to mix air and petrol in the required proportion and to supply it to the engine during suction stroke.	The injector or atomiser is employed to inject the fuel at the end of compression stroke.
3.	The pressure at the end of compression is about 10 bar.	The pressure at the end of compression is about 35 bar.
4.	The charge (i.e. petrol and air mixture) is ignited with the help of a spark plug.	The fuel is injected in the form of fine spray. The temperature of the compressed air (about 600° C at a pressure of about 35 bar) is sufficiently high to ignite the fuel.
5.	The combustion of fuel takes place approximately at constant volume. In other words, it works on Otto cycle.	The combustion of fuel takes place approximately at constant pressure. In other words, it works on Diesel cycle.
6.	A petrol engine has compression ratio approximately from 6 to 10.	A diesel engine has compression ratio approximately from 15 to 25.
7.	The starting is easy due to low compression ration.	The starting is little difficult due to high compression ratio.
8.	As the compression ratio is low, the petrol engines are lighter and cheaper.	As the compression ratio is high, the diesel engines are heavier and costlier.
9.	The running cost of petrol engines is high because of higher cost of petrol.	The running cost of diesel engines is low because of the lower cost of diesel.
10.	The maintenance cost is less.	The maintenance cost is more.
11.	The thermal efficiency is upto about 26%.	The thermal efficiency is upto about 40%.
12.	Overheating trouble is more due to low thermal efficiency.	Overheating trouble is less due to high thermal efficiency.
13.	These are high speed engines.	These are relatively low speed engines.
14.	The petrol engines are generally employed in light duty vehicles such as scooters, motorcycles, cars. These are also used in aeroplanes.	The diesel engines are generally employed in heavy duty vehicles such as buses, trucks and earth moving machines etc.