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Difference between Petrol Engine and Diesel Engine
A machine which consists of moving parts and converts one or more form of energy into mechanical energy (usually in the form of motion) is known as an engine.
Depending on the fuel used as the source of energy, the engine may be classified in following the types −
- Steam engine
- Diesel engine
- Petrol engine, etc.
In this article, we will discuss all the major differences between petrol engine and diesel engine by considering different parameters such as working principle, fuel-air mixing, ignition, power, applications, etc. But, before discussing the differences, we will learn the basics of diesel and petrol engines so that it becomes easy to understand the differences between them.
What is a Petrol Engine?
A petrol engine is a type of internal combustion (IC) engine which is usually used to powering the light vehicles. The petrol engine uses a spark-ignition technology, i.e. it uses an electric spark arc to ignite the fuel. The first practical petrol engine was invented by a German engineer Nicolaus August Otto in 1876.
The petrol engine works on the Otto cycle (named in the honor of its inventor). The Otto cycle comprises two isochoric processes and two isentropic processes. As we known that presence of air is required for burning of a fuel. In case of a petrol engine, the fuel (petrol) and air are mixed before compression.
The part of petrol engine where the air and fuel mixed is called carburetor. Once the fuel and air mixed thoroughly, this mixture is launched into the engine cylinder where it is compressed by a piston and ignited by an electric spark. In this way, a petrol engine converts the chemical energy of petrol fuel into mechanical energy.
The petrol engines are usually used to drive the light weight and high-speed vehicles such as cars, bikes, etc.
What is a Diesel Engine?
A diesel engine again a type of internal combustion (IC) engine which is used for powering the relatively heavy machines and vehicles. The diesel engine is also known as compressionignition engine. The diesel engine was invented by a German thermal engineer Rudolf Christian Karl Diesel in 1890s.
The diesel engine works on the diesel cycle. The diesel cycle comprises a continuous pressure process, a constant volume process and two isentropic processes. In case of diesel engine, the fuel (diesel oil) is injected in a combustion chamber and then it is ignited by the temperature of hot and compressed air supplied to the combustion chamber. The high temperature of air is due to the adiabatic compression. Therefore, in case of a diesel engine, only air is compressed and not the fuel.
The diesel engine involves the spontaneous ignition of diesel fuel when it is injected into the combustion chamber. The diesel engine is a low-speed engine and is usually used to drive the heavy machineries and vehicles such as buses, trucks, generators, etc.
Difference between Petrol and Diesel Engines
Diesel engine and Petrol engine are both internal combustion engines. However, there are several differences between them that are given in the following table:
| Basis of Difference | Petrol Engine | Diesel Engine |
|---|---|---|
| Definition | A petrol engine is an internal combustion engine which uses spark-ignition technology to ignite the fuel. | A diesel engine is an internal combustion engine which uses compression-ignition technology to ignite the fuel. |
| Working cycle | The petrol engine works on the Otto cycle. | The diesel engine works on the diesel cycle. |
| Major parts | The main parts of a petrol engine are: engine cylinder, piston, spark plug, carburetor, and intake and exhaust valve. | The main parts of a diesel engine are: combustion chamber and cylinder, fuel injector, piston, intake and exhaust valve. |
| Air-fuel mixing | In a petrol engine, the air and fuel are mixed in a carburetor before entering into the engine cylinder. | In a diesel engine, the air and fuel are mixed inside the engine cylinder. The carburetor is not used in the diesel engines. |
| Mean of ignition | The petrol engine consists of an electric spark plug to produce an electric arc for igniting the fuel. | In a diesel engine, the fuel is ignited by the high temperature of hot and compressed air. |
| Volatility of fuel | Petrol engine uses highly volatile fuels like petrol to work. | Diesel engine uses comparatively fuels of low volatility to work. |
| Fuel consumption | Petrol engine consumes relatively more fuel to produce the same amount of power. | The fuel consumption is less in case of diesel engine. |
| Engine weight | Petrol engine is light in weight. | Diesel engine is heavy. |
| Initial cost | The initial cost of a petrol engine is less. | The initial cost of a diesel engine is high. |
| Maintenance | Petrol engine requires frequent maintenance. | Diesel engine requires relatively less frequent maintenance. |
| Maintenance cost | The maintenance cost of petrol engines is low. | The maintenance cost of diesel engines is high. |
| Power produced | A petrol engine produces relatively less amount of power. | A diesel engine produces high power. |
| Compression ratio | The compression ratio of a petrol engine is comparatively low. | A diesel engine has high compression ratio. |
| Starting | It is easy to start a petrol engine due to its low compression ratio. | It is slightly difficult to start a diesel engine because it has high compression ratio. |
| Thermal efficiency | The thermal efficiency of a typical petrol engine is around 26 %. | The thermal efficiency of a diesel engine is around 40 %. |
| Speed | A petrol engine is a high-speed engine | A diesel engine is a low-speed engine. |
| Noise & vibrations | Petrol engine produces less noise and vibrations. | Diesel engine produces high noise and vibrations. |
| Impact on environment | Petrol engines emit relatively less pollutants in environment. | Diesel engines cause relatively high pollution in environment. |
| Torque availability | There is low torque availability in the petrol engines. | Diesel engines produce relatively high torque. |
| Applications | Petrol engines are generally used in light vehicles like bikes, cars, etc. | Diesel engines are used in heavy vehicles like buses, trucks, trains, etc. |
Conclusion
The most significant difference that you should note here is that a petrol engine requires a spark plug to start the ignition, whereas a diesel engine ignites the fuel by using the temperature of hot and compressed air.
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