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Difference Between Petrol and Diesel Engine

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Unveiling the Distinctions: Petrol Engine vs. Diesel Engine

To explain petrol and diesel engine: People frequently argue about the differences between petrol and diesel engines. When we talk about it in technical terms, the main difference is that the petrol engine operates on the Otto cycle. Similarly, diesel engines use the diesel cycle. Furthermore, there are numerous differences in the types, uses, and structure of both of these engines. The type of fuel we use in them is the most important factor. Other differences include spark plugs, fuel injectors, and other components. Furthermore, the vehicles that use them. To better understand these differences, we must first learn about them.

What is Petrol and Diesel Engine?

Petrol Engine:

The auto-ignition temperatures of petrol engines are high. As a result, a petrol engine has a lower compression ratio than a diesel engine.

A petrol engine's compression ratio is typically 6:10. Other than gasoline, gasoline engines can run on natural gas (CNG), methanol, Autogas (LPG), compressed hydrogen, ethanol, nitromethane (in drag racing), and bioethanol. 

Petrol engines are most commonly found in automobiles, motorcycles, boats, and airplanes. However, they consume more fuel and emit more pollution than diesel engines and electric motors.

Working Principle of Petrol Engine:

With a few minor exceptions, the operation of a compression ignition (CI) engine and a spark ignition (SI) engine is essentially the same. In a diesel or CI engine, high compression of the air-fuel mixture triggers ignition, whereas a spark triggers ignition in a petrol engine. 

The Otto cycle governs the operation of a petrol engine. A petrol engine works as follows:

1. Stage of suction

2. Stage of compression

3. Powerful stage

4. Stage of exhaustion

Diesel Engine:

The piston-cylinder diesel engine uses intermittent combustion. It can run on a two-stroke or four-stroke cycle (see figure), but unlike gasoline engines with spark ignition, diesel engines only introduce air into the combustion chamber during the intake stroke. Compression ratios for diesel engines typically fall between 14:1 and 22:1. Engines with bores (cylinder diameters) less than 600 mm (24 inches) can have two-stroke or four-stroke engine designs. Nearly all engines with bores bigger than 600 mm use two-stroke cycle systems.

Fuel that has been injected or sprayed into the compressed, hot air charge inside the cylinder is burned to produce energy for the diesel engine. The temperature of the air must be raised above the point at which the fuel injection can ignite. However, since the temperature of the air within the cylinders is influenced by both the engine's compression ratio and its current operating temperature, supplemental heating of the cylinders is occasionally used at engine start-up. Because air heated by compression rather than an electric spark initiates combustion, diesel engines are sometimes referred to as compression-ignition engines.

Working Principle of Diesel Engine:

When the piston is moving upward, the fuel injection valve is opened, which causes fuel to be pumped into the cylinder (the amount of fuel depends on the load). The piston used to move in a downward motion at first by the air entering the cylinder, at the time of the return stroke all the valves are closed by this and the compressed air inside the cylinder is heated to a high temperature.

The combination and ignition of this fuel and the hot air aids in the burning of the fuel that is introduced into the cylinder.

Due to the expanding gases in this fuel, the valve closes after the fuel is supplied to the cylinder.The piston then moves on.

Following that, the piston began to move downward as a means of putting the fuel's energy to use. When the piston moves all the way to the bottom of the cylinder, the exhaust valves open, allowing the exhaust gases to escape.

Characteristics of Petrol and Diesel Engine

Diesel Engine:

  • An internal combustion engine, or compression-ignition engine, is what the diesel engine is. It bears Rudolf Diesel's name.

  • In these engines, fuel is injected into a combustion chamber, where it is later ignited by the air's high temperature.

  • The adiabatic compression is what causes the air in the cylinder to be so hot. These engines do not compress fuel; they only compress air.

  • Diesel fuel spontaneously ignites when injected into the combustion chamber.

  • These engines operate using the Diesel cycle, which entails two isentropic processes, a constant volume process, and a constant pressure process.

Petrol Engine:

  • Internal combustion engines with spark ignition are petrol engines. 

  • They are powered by relatively flammable fuels like petrol.

  • In these engines, the fuel and air are typically combined after compression.

  • The Otto cycle, which includes two isochoric and two isentropic processes, is how petrol engines operate.

  • Prior to entering the cylinder in petrol engines, air and fuel are typically blended in a carburettor.

  • After the fuel and air have been compressed, an electric spark is used to ignite the fuel.

Petrol and Diesel Engine Difference

S No.

Comparison Basis

Petrol Engine 

Diesel Engine



These engines use the Otto cycle to operate.

The Diesel cycle generates diesel engines.


Compression ratio

Compression ratio of petrol engine is moderately low.

Compression ratio of diesel engine is high.



In a carburetor, fuel and air are combined.

Inside the cylinder, the fuel and air combine.


Fuel Consumption

It consume a lot of fuel.

The consumption of fuel is less.


Cost of Maintenance

It has a low maintenance cost.

It has a high maintenance cost.


Power Generation

These generate comparatively small amounts of power.

Compared to others, they produce more power.


Vehicles that uses

Car , Motorbikes

Trucks , Buses


Petrol and diesel engines differ in several key aspects. Petrol engines rely on a spark for ignition, while diesel engines use adiabatic compression. Petrol engines operate on Otto cycles, while diesel engines utilize diesel cycles. Diesel engines excel in providing torque for uphill climbs and heavy loads, making them suitable for SUVs and larger vehicles. On the other hand, petrol engines are favored in sports cars due to their superior acceleration despite lower torque. Petrol engines are more cost-effective and require less maintenance, making them popular in lighter vehicles. However, diesel engine production and maintenance tend to be more expensive.

Last updated date: 26th Sep 2023
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FAQs on Difference Between Petrol and Diesel Engine

1. What are the fuel consumption differences between a petrol engine and a diesel engine?

Diesel engines use less fuel than petrol engines.

2. Name the measurement unit used to express a fuel's calorific value.

Kilojoules per kilogramme (kJ/kg), the calorific value of a fuel.

3. What are the four power cycle steps that both kinds of engines use? 

The power cycle's four steps are as follows:

  • Intake- Air and fuel are injected into the cylinder during the intake phase.

  • Compression- Air and fuel are compressed during compression.

  • Power- Fuel is burned to produce energy, or power.

  • Exhaust- Removal of the burned fuel from the exhaust.

4. On which principle does engines run?

Engines run on the principle of heat transfer.

5. List the circumstances in which combustion is possible.

The following circumstances must exist for combustion to occur:

  • 1. The availability of fuel.

  • 2. For combustion to occur, oxygen is required.

  • 3. Minimum temperature at which a substance ignites.