What is a Spark-ignition engine?

A spark-ignition engine (SI engine) is an internal combustion engine, generally a petrol engine, where the combustion process of the air-fuel mixture is ignited by a spark from a spark plug.

This is in contrast to compression-ignition engines, typically diesel engines, where the heat generated from compression together with the injection of fuel is enough to initiate the combustion process, without needing any external spark.

Spark-ignition engines are commonly referred to as “gasoline engines” in North America, and “petrol engines” in Britain and the rest of the world.

Spark-ignition engines can (and increasingly are) run on fuels other than petrol/gasoline, such as Autogas (LPG), methanol, ethanol, bioethanol, compressed natural gas (CNG), hydrogen, and (in drag racing) nitromethane.

Parts Of Spark Ignition Engine

The main components of a spark-ignition engine are as follows:

Inlet Valve: Air-fuel mixture enters the cylinder through the inlet valve.
Exhaust Valve: The burnt or exhaust gases produced in the power stroke escape out through the exhaust valve.
Spark Plug: It produces a spark at the end of the compression stroke, which ignites the compressed air-fuel mixture.
Cylinder: It is a hollow cylinder in which the piston reciprocates.
Piston: It is a moving part of the engine that performs reciprocating motion and transmits the power generated during power stroke to the crankshaft through the connecting rod.
Connecting Rod: It is that part of the engine which connects the piston to the crankshaft.
Crankshaft: It is used to convert the reciprocating motion of the engine into rotary motion.

How Does a Spark Ignition Engine Work?

In a spark-ignition engine, the fuel is mixed with air and then inducted into the cylinder during the intake process. After the piston compresses the fuel-air mixture, the spark ignites it, causing combustion. The expansion of the combustion gases pushes the piston during the power stroke.

The working cycle of both spark-ignition and compression-ignition engines may be either two-stroke or four-stroke.

A four-stroke spark-ignition engine is an Otto cycle engine. It consists of the following four strokes: suction or intake stroke, compression stroke, expansion or power stroke, and exhaust stroke.

In suction stroke, the air-fuel mixture enters the cylinder, and in compression stroke compression of the air-fuel mixture takes place. In power stroke combustion of fuel and power generation, and in exhaust stroke escaping of burnt gases out of the engine.

Each stroke consists of a 180-degree rotation of crankshaft rotation and hence a four-stroke cycle is completed through 720 degrees of crank rotation. Thus, for one complete cycle, there is only one power stroke while the crankshaft turns by two or more revolutions.

Application of Spark Ignition Engine

Two-stroke SI Engines are used in small vehicles like mopeds and scooters.
These engines are used for lawnmowers.
These are used in small electricity-generating sets and pumping sets.
Four-stroke SI engines are largely used in automobile industries.
CNG engines are used in buses.
Radial SI engines are used in small aircraft.

Advantages Of Spark Ignition Engine

SI Engines are low in cost
SI Engine produces less pollution as compared to CI Engine
These engines are smaller as compared to the CI engines and hence require less space.
These engines are lightweight.
The cost of an SI engine is lower compared to CI Engine.
These engines require less maintenance cost.

Disadvantages Of Spark Ignition Engine

At high loads, these engines are less economical.
These engines have high fuel consumption.
It has less efficiency as compared to the CI engine.
This engine has a knocking problem.