Engines have become a vital part of all vehicles. Nowadays, no vehicle can move without an engine. There are different types of engines in the market, and a petrol engine is one of them. A petrol engine is also known as a SI engine (spark ignition engine). This article explains the petrol engine working, types, parts, and applications.
What is a Petrol Engine?
A petrol engine, also known as a gasoline engine, is an internal combustion (IC) engine in which the air-fuel mixture is ignited due to the spark provided by a spark plug. Therefore, it is also known as a SI engine. It uses petrol as its fuel source.
In 1876, Nikolaus August Otto designed the first petrol engine. A petrol engine works on the basic principle of the Otto cycle.
The spark plug of the engine takes electric current from the vehicle battery to generate a spark. This spark plug is usually located on the combustion chamber’s top for a quick ignition of the air-fuel mixture.
The ignition process of the fuel produces heat, which is transformed into mechanical work in the power stroke of the piston.
In the old version of the SI engines, air and fuel (petrol or gasoline) are mixed before sending them into the combustion chamber for compression and ignition. While the latest SI engines use a fuel injector that injects fuel directly into the combustion chamber, the mixing process occurs there. This mixing process is regulated electronically by a fuel injector.
The petrol engines have high auto-ignition temperatures. Therefore, a petrol engine has a low compression ratio than a diesel engine.
The compression ratio of a petrol engine is usually 6:10. The gasoline engines can also run on fuels other than petrol like natural gas (CNG), methanol, Autogas (LPG), compressed hydrogen, ethanol, nitromethane (in drag racing), and bioethanol.
Petrol engines are most commonly used in different vehicles, including cars, motorcycles, boats, and airplanes. However, they have poor fuel efficiency and higher emissions than diesel engines and electric motors.
Petrol Engine Working Principle
The spark ignition (SI) engine working principle is similar to the compression ignition (CI) engine, but there is little difference. In a diesel or CI engine, the ignition process happens due to high compression of the air-fuel mixture, while ignition occurs due to a spark in a petrol engine.
A petrol engine works according to the Otto cycle. A petrol engine works in the following way:
- Suction stage
- Compression stage
- Power stage
- Exhaust stage
1) Intake or Suction Stroke
- For the intake stroke, the piston moves downward. As it moves down, a vacuum creates inside the combustion chamber; due to that, the air-fuel mixture starts coming from outside into the combustion chamber.
- In this stroke, the suction valve opens, and the exhaust valve remains closed.
2) Compression Stroke
- When the suction process of the air-fuel mixture completes according to the requirements, the piston travels upwards for the air-fuel mixture compression.
- As the piston moves up, it pressurizes the mixture into the combustion chamber. During this stroke, the intake valve and exhaust valve are closed.
- Due to the compression process, the temperature and pressure of the air-fuel mixture become very high.
- At the end of the compression process, a spark plug fires a spark and ignites the air-fuel mixture.
- Due to the provided spark, the combustion process of the air-fuel mixture occurs inside the combustion chamber. Due to this combustion, the piston further moves up, which further increases the temperature and pressure of the mixture. During this process, heat is produced.
3) Expansion Stroke
- The expansion stroke is also known as a power stroke.
- In this stage, the generated heat in the previous stroke (compression process) forces the piston to move downward (TDC to BCD) and turns the crankshaft.
- Due to the piston’s downward motion, the air-fuel mixture expands inside the chamber, and the pressure of the mixture decreases.
4) Exhaust Stroke
- In this stroke, the piston moves upward, opens the exhaust valve, and releases useless gases from the combustion chamber.
- After completing the exhaust stroke, the piston again moves down, and all four strokes repeat.
Read More: Working of Diesel Engine
Parts of the Petrol Engine
The main parts of the petrol engine are given below:
- Spark plug
- Cylinder or Combustion chamber
- Carburetor
- Piston
- Inlet or Suction valve
- Connecting rod
- Exhaust valve
1) Intake Valve
The intake valve is one of the most important parts of the spark ignition engine. The air-fuel mixture enters the engine cylinder through an intake valve.
2) Exhaust Valve
This valve is used to discharge the exhaust gases. It is a one-way valve. It also stops the backflow of exhaust gases.
3) Spark Plug
The spark plug is used to start the combustion of the compressed air-fuel mixture. It is located at the top of the engine cylinder.
The spark plug is one of the most important parts because, in a petrol engine, the ignition process can’t start without a spark.
Read More: Bad Spark Plug Symptoms and Causes
4) Combustion Chamber
This is an empty cylinder with a rotating piston. The piston reciprocates inside the combustion chamber. The combustion of the air-fuel mixture takes place inside the combustion chamber.
Read Also: Combustion Chamber Working and Construction
5) Piston
A piston is a moving part of the petrol engine that reciprocates to suck air-fuel mixture and generates power during the power stroke. After power generation, it transmits this power to the crankshaft.
Read More: Piston Construction and Parts
6) Connecting Rod
It is one of the most important components of the petrol engine. The connecting rod connects the piston and crankshaft of the engine. It delivers the movement of the piston to the crankshaft.
Read Also: Working of Connecting rod
7) Crankshaft
It is connected to the piston via a connecting rod. It transforms the reciprocating movement of the piston into the rotary/circularity motion.
Read Also: Working of Crnakshaft
Speed and Efficiency of Petrol Engine
A petrol or gasoline engine runs faster than a diesel engine. This is because these engines have a light crankshaft, connecting rod, and piston (because lower compression ratios increase design efficiency) and gasoline burns faster than diesel.
In general, the thermal efficiency of most petrol engines is (on average) around 20%, which is about half that of the diesel engine. However, some latest petrol engines are more efficient (approximately 38% thermal efficiency) than an old spark-ignition engine.
Types of Petrol Engines
The petrol engine has two basic types:
- Two-stroke Petrol Engine
- Four-stroke Petrol engine
1) 2-Stroke Petrol Engine
This engine uses only 2 piston strokes for the completion of one power cycle. It is faster than the 4-stroke engine.
Read More: 2-Stroke Engine Working and Types
2) 4-Stroke Petrol Engine
A 4-stroke petrol engine is a type of internal combustion engine that uses petrol as fuel and operates on a 4-stroke cycle, which includes four phases: intake, compression, combustion, and exhaust.
They are usually used in motorcycles, automobiles, and many other types of vehicles because of their relatively high efficiency and power output.
Read More: 4-Stroke Engine Working and Applications
Advantages of Petrol Engines
- It has a high auto-ignition temperature as compared to the CI or diesel engine.
- Petrol engine is a light engine than a diesel engine.
- SI engine produces low noise as compared to the CI engine.
- It requires less maintenance.
- It is less expensive than the diesel engine.
- It is easy to start than a CI engine.
- Petrol fuel is less expensive than diesel fuel.
Disadvantages of Petrol Engines
- These engines require a spark for a start; otherwise, they cannot start.
- The petrol engine has a high consumption of fuel than the diesel engine.
- It has more moving parts than the diesel engine.
- It has low efficiency as compared to the CI or diesel engine.
- The SI engines have knocking problems.
- It has a low speed.
Applications of Petrol Engines
- Automobiles: Petrol engines are commonly used in trucks, cars, tractors and other types of vehicles. These engines are available in different sizes and configurations, ranging from small 3-cylinder engines to large V8 engines.
- Small machinery: Petrol engines power small machinery like generators, chainsaws, and lawnmowers.
- Motorcycles: They are the main power source for different motorcycles, ranging from small scooters to powerful sports bikes.
- Racing: Petrol engines are commonly used in racing, especially in high-performance cars and motorcycles. They can be tuned to produce more power and run at higher RPMs than regular road-going engines.
- Boats: They are usually employed in boats, especially smaller ones that don’t require the power of a diesel engine.
- Aircraft: Some smaller aircraft use petrol engines, which are lighter and less expensive than diesel engines or turbine engines.
Difference between Petrol Engine and Diesel Engine
The major differences between petrol and diesel engines are given below:
| Petrol Engine | Diesel Engine |
|---|---|
| It uses petrol as fuel. | It uses diesel as a fuel. |
| In a petrol engine, ignition occurs due to a spark delivered by a spark plug. | In a diesel engine, ignition occurs due to the high compression of the fuel and air mixture. |
| It operates on the Otto cycle. | It operates on the diesel cycle. |
| It needs a spark plug for ignition. | It doesn’t need a spark plug. |
| It is less efficient. | It is more efficient. |
| They are most commonly depleted in small vehicles like vans and bikes etc. | They are depleted in heavy-duty automobiles like buses, tractors and cars etc. |
| They have a low cost. | These are very expensive. |
| In this engine, the fuel-air mixture is compressed in the combustion chamber. | In this, only air is compressed while fuel is injected at the end of the compression stroke. |
| A petrol engine uses petrol which is less expensive. | A diesel engine uses diesel which is very expensive. |
| It has a relatively low compression ratio. | It has a high compression ratio. |
| These engines have low maintenance and initial costs. | These have high maintenance and initial costs. |
| The petrol fuel is easy to ignite. | Diesel fuel is more difficult to ignite. |
| It uses a piston for the compression process. | A diesel engine also uses a piston for compression, which reciprocates inside the combustion chamber. |
| It produces less noise. | It produces high noise while working. |
| A petrol engine has higher fuel consumption. | A diesel engine has low fuel consumption. |
FAQ Section
Who invented Petrol Engine?
In 1876, Nikolaus August Otto designed the first petrol engine.
Petrol Engine works on which cycle?
A petrol engine works on the Otto cycle.
What happens if you put petrol in a diesel engine
If you put petrol into a diesel engine, it will decrease the lubrication functionality of the engine. Due to this reason, high knocking will occur, and it can damage your fuel pump.
What is SI engine?
An engine in which combustion occurs due to the spark is known as a SI engine.
What happens when petrol is used in diesel engine
When petrol is used in a diesel engine, knocking problems start producing, damaging the fuel pump.