Difference Between Petrol and Diesel Engine

Difference Between Petrol and Diesel Engine

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The primary difference between Petrol and Diesel engines is that the Petrol engine works on the Otto cycle whereas the Diesel engine works on the Diesel cycle. Other differences will be attributed to the structure, types, and uses of those engines. The important parameter they are classified on is that the sort of fuel they use. Generally, Engines run on the principle of heat transfer.

The diesel and petrol engines that are used in most cars are highly similar. In essence, they are internal combustion engines that work using a two or four-stroke cycle. In an internal combustion engine, the power cycle is made up of four phases: intake, compression, power, and exhaust.

Difference Between Petrol and Diesel Engine

In the intake phase, the air is drawn into the cylinder through the opening intake valve. In the compression phase, the intake valve closes and the air is compressed with fuel. At this point, the mixture of fuel and air is ignited to cause an explosion. It is this explosion that causes the piston to downwards and drive the crankshaft to produce motion. This is the power phase. The final phase is exhaust where the spent air-fuel mixture is expelled out of the cylinder through the opening exhaust valve so that a new cycle may begin.

Petrol Engine

Petrol engines are internal combustion engines that have spark-ignition. They run on relatively volatile fuels like petrol. In these engines, air and fuel are generally mixed post-compression. Petrol engines work on the Otto cycle, which consists of two isochoric processes and two isentropic processes. In petrol engines, air and petrol are usually mixed in a very carburetor supported on play fuel types also play carburetor before being introduced to the cylinder. Once the air and petrol are compressed, the fuel is ignited via an electrical spark.

Diesel Engine

The Diesel engine is additionally an indoor combustion engine which is additionally called the compression-ignition engine. It’s named after Rudolf Diesel. In these engines, the fuel is injected into a combustion chamber and is then ignited by the hot temperature of the air within the chamber. The high temperature of the air within the cylinder is due to the adiabatic compression. These engines only compress the air and not the fuel. When injected into the combustion chamber, the Diesel oil undergoes spontaneous ignition. These engines work on the Diesel cycle, which consists of a relentless pressure process, a constant volume process, and two isentropic processes. 

Difference between Petrol and Diesel Engine Table

Apart from the sort of fuel used, the engines also are divided on the premise of plenty of things, like the presence of plug-in Petrol engines and a fuel injector in Diesel engines. We also know that lighter vehicles like motorcycles, scooters, and cars typically use petrol in their engines whereas Diesel is employed in much heavier machinery like tractors, trucks, and buses. Thus, the kinds of fuel used also play a significant role in defining the most important difference between Petrol and Diesel engines. More differences between these types of engines are listed within the tabular column below.

Diesel Engine Petrol Engine
These engines work on the Diesel cycle Works on the Otto cycle
Fuel and air
The fuel is mixed with air inside the cylinder Air and fuel are mixed in a carburetor
Ignition is achieved with the help of hot, compressed air. Fuel is ignited with an electric spark
compression ratio
High compression ratio Relatively low compression ratio
High power production Relatively low amounts of power are produced in a Petrol engine
These engines work with fuels that have low volatilities Highly volatile fuels are used in these internal combustion engines
fuel consumption
Relatively low fuel consumption High fuel consumption.
Maintenance cost
High initial and maintenance costs Comparatively low initial cost and maintenance cost
Generally used in heavy vehicles such as trucks and buses Used in light vehicles such as motorcycles and cars.


The main difference between diesel and petrol engines is that petrol engines use spark plugs to ignite the air-fuel mixture, while diesel engines rely solely on heavily compressed air. As mentioned earlier, Rudolf Diesel found that the temperature of air could be made to increase high enough if it was heavily compressed. The temperature would rise so high to the point where it could cause the ignition of diesel fuel. 

Therefore in diesel engines, the air in the cylinder would be very heavily compressed, typically to around 14 to 23 times its original volume. In petrol engines, the compression ratio is generally much lower, because they rely more on the spark plug to begin the power phase. The compression ratio in petrol engines is typically only around 7 to 10, with high-performance vehicles having higher compression ratios of up to 13.

do check about the Difference Between AC and DC

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