Difference Between Petrol and Diesel Engine

Petrol Vs Diesel Engine

Petrol and Diesel Engines are known to be internal combustion engines. The crucial difference between petrol and diesel engine is based on the way the combustion is performed by the two types of engines.

People generally consider that petrol and diesel engines are somewhat the same as both of them are internal combustion engines exhibiting similar anatomy but in reality, various factors differentiate the two. Internal combustion engines mean that both perform combustion internally. Thus, we can say mainly, petrol and diesel engines are differentiated on the basis of the way they ignite the mixture of fuel and air.

What is an Engine?

An engine is a device that transforms heat energy into mechanical motion. Basically, it changes force into motion as here, by applying force the fuel is burnt and that energy is utilized for moving vehicles. Petrol and diesel engines are known to be internal combustion engines as in both of them the combustion occurs with the cylindrical chamber.

Petrol Engine

A type of internal combustion engine which is based on spark ignition and that operate with petrol or similar volatile fuels is known as a petrol engine. In these engines, with the use of a spark plug, the pre-mixed mixture of fuel and gas is compressed so that ignition can occur.

It is also known as a gasoline engine. It operates in a way that initially; the combination of petrol and air is inserted within the cylinder. Further, the gaseous mixture of the two compacted up to a certain level within the cylinder during the compression stroke. Once this is done, then with the help of a spark plug the mixture is ignited during the ignition stroke. Lastly, removal of exhaust takes place during the exhaust stroke.

Diesel Engine

A type of internal combustion engine that is based on compression of air within the fuel cylinder to produce heat is known as a diesel engine. This engine rather than using a spark plug utilizes high-temperature air, obtained from compression in order to ignite the diesel fuel.

It operates in a way the initially, only air is inserted within the chamber during the intake stroke. Further, the air within the chamber is highly compressed leading to give rise high temperature. It is to be noted here that this temperature must be greater than the one which can ignite the injected fuel. Once a high temperature is obtained, diesel is sprayed into high-temperature air. This ignites the fuel leading to cause combustion and lastly, the exhaust is removed from the valve.

Difference Between Petrol and Diesel Engine


The way ignition takes place inside the two engines is different. A major component of a petrol engine is the spark plug that ignites the mixture of fuel and air leading to cause combustion. On the other side, in the case of a diesel engine, spark plugs are not present, and so, here higher level of compression causes the fuel and air mixture to get ignited and thereby causing combustion.

Introduction of Fuel

In a petrol engine also known as an SI engine, the fuel-air mixture in gaseous form is introduced within the cylinder during suction stroke. On the contrary, only air is introduced during the suction stroke however, fuel is injected at high pressure within the chamber during the compression stroke.

Compression Ratio

The compression offered by diesel engines is nearly 14 to 23 times the actual volume whereas the same is simply 7 to 10 times in the case of petrol engines. Due to the lower compression ratio of petrol engines spark plug becomes necessary to ignite the fuel. Also, the higher compression ratio of diesel engines produces more torque.

Thermal Efficiency

The thermal efficiency possessed by diesel engines is comparatively more than petrol engines. The high thermal efficiency corresponds to a high level of extraction of energy from the fuel-air mixture. However, the reason is as in diesel engines the heat of compressed air ignites the fuel-air mixture, thus are considered to be more thermally efficient. Hence, are known to be more powerful than petrol engines.


Diesel engines are more durable than petrol engines. The reason is diesel is considered light oil that helps in lubricating the components of the engine when burnt. As against, petrol acts as a detergent, the presence of which within the engine removes oil from various parts leading to faster wear and tear. Hence, the life span of the diesel engine is more than that of a petrol engine.


The weight of petrol engines is lesser than that of diesel engines.


Along with the high efficiency offered by diesel engines, diesel fuel is comparatively cheaper than petrol and thus, it is said that diesel engines are more cost-effective than petrol engines.


As we have mentioned that the operation of the diesel engines is based on the level of compression of air and fuel mixture, thus, these are needed to possess high physical strength so that they can handle high compression of gases.

Energy offered

Diesel engines are considered to be more efficient than petrol engines. One of the reasons for high efficiency is that diesel is composed of a comparatively long chain of hydrocarbons than petrol that implies more energy.

Fuel Consumption

In diesel engines, less amount of fuel is consumed in comparison to petrol engines. The reason for the same is that the diesel engine exhibit a higher compression ratio with a longer stroke leads giving rise to more torque at lower rpm than the petrol engine. This helps in less fuel utilization in diesel engines.

Quick Comparison: Petrol Vs Diesel Engine

BasisPetrol EngineDiesel Engine
Operating PrincipleBased on Otto CycleBased on Diesel Cycle
Another nameSI Engine (Spark-Ignition Engine)CI Engine (Compression-Ignition Engine)
Spark PlugExistNot Exist
Fuel introductionAir-fuel as gaseous mixture together in suction stroke.Injection of fuel within the chamber at high pressure during compression stroke.
Power producedLessComparatively more
IgnitionThrough spark plug.Self-ignition by compression.
Compression RatioLowComparatively high
Fuel UsedPetrol or gasoline or high-octane fuel.Diesel or high cetane fuel.
Maintenance CostHighLow
Fuel ConsumedMoreComparatively Less
Thermal efficiencyNearly 26%Around 40%
Engine speedHighLow

Thus, from this discussion, one can conclude that in the long runs diesel engines are superior. But if you are concerned regarding environmental imbalance caused due to burning of the fuel then petrol engines are to be chosen as they are more eco friendly.

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