Difference between Petrol and Diesel Engine

Car, truck, bus and other automobiles run on engines based on fossil fuels. Many types of engines have been developed by scientists and available in the market. Two most common types of engine are petrol engine and diesel engine. Most of the vehicles run either on petrol engine or diesel engine. The main difference between petrol engine and diesel engine is that in petrol engine fuel and air both are compressed with a spark while in diesel engine only air is compressed, and fuel is injected in compressed air. Petrol engine and diesel engine work on otto cycle and diesel cycle respectively. 

Petrol Engine -

Petrol engines are mostly used in cars, scooters, machines etc. It is an internal combustion engine with spark ignition. It is also known as gasoline engine. It was 1st invented in Europe, 1876. It was a very basic model of petrol engine. After this many improvisations have been done and the present model which people are using is the latest one. In petrol engines, air and fuel are mixed before compression and then an electric spark is used for ignition. Ignition temperature is the lowest temperature at which a fuel or combustible substance when heated catches fire. It works on Otto cycle. Otto cycle explains how in gasoline engines chemical energy converted into thermal energy and then into motion. 

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Petrol Engine 

How Petrol Engine works?

Working of petrol engine can be expressed by the following four steps shown in the image below – 

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Step 1. Fuel (petrol) and air are added in the cylinder and mixed. 

Step 2. The mixture of petrol and air is compressed by the use of crankshaft. 

Step 3. Now spark is used in the mixture and mixture forces the piston down. 

Step 4. Gases go out from the exhaust valve. 

Diesel Engine-

Generally, trucks and other heavy vehicles run on diesel engines. It is also an internal combustion engine that works on diesel. In these engines air is compressed, so it becomes very hot and then fuel (diesel) is injected in it. Thus, diesel reaches to its ignition temperature by the heat of compressed air. It is also known as compression ignition engine. It was 1st developed by Rudolf Diesel in 1893 in Germany. Diesel engines were mainly used in trains at that time. 

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How diesel engine works – Diesel engine works on diesel cycle. It can be explained by the following steps shown in the figures below – 

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Step 1. Induction Stroke – Piston of the engine goes down and air comes into the cylinder.

Step 2. Compression Stroke – Piston goes up and compress the air. Now air is getting hot. 

Step 3. Power Stroke – Now fuel or diesel is added in the cylinder.

Step 4. Exhaust Stroke – Fuel or diesel starts burning due to ignition temperature provided by hot air. Smoke goes out from the valve.

Petrol Engine vs Diesel Engine 

Diesel and petrol both are fossil fuels but working of petrol engine and diesel engine differ from each other. Difference between petrol engine and diesel engine can be explained by the following points – 

Petrol vs Diesel Engine in Tabular Form

Petrol Engine 

Diesel Engine 

Petrol engine works on otto cycle. 

Diesel engine works on diesel engine cycle. 

In this type of engine fuel and air mixture is compressed together. 

In this type of engine fuel and air are not compressed together. 

These engines work with highly volatile fuels. 

These engines work with low volatile fuels. 

In this type of engine ignition is induced by spark. 

In this type of engine ignition is induced by heat generated by compression of air. 

This engine has low compression ratios (7-10). 

This engine has high compression ratios (11-20). 

This engine has lower efficiency than diesel engine.

This engine has higher efficiency than petrol engine. 

In this type of engine fuel consumption is higher. 

In this type of engine fuel consumption is lower. 

While working this engine makes less noise. 

While working this engine makes more noise. 

This engine has high running cost. 

This engine has low running cost. 

It is mostly applicable for light duty applications. 

It is mostly applicable for heavy duty applications. 

In this type of engine fuel and air are injected together. 

In this type of engine first air is injected and compressed then fuel is added.