Spark-ignition engine

For details of the spark-ignition system, see Ignition system.

The term spark-ignition engine refers to internal combustion engines, usually petrol engines, 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 is enough to initiate the combustion process, without needing any external spark.

Fuels

Spark-ignition engines are commonly referred to as "gasoline engines" in America, and "petrol engines" in Britain and the rest of the world. However, these terms are not preferred, since 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.

Air/fuel mixture

The air/fuel mixture has to be close to stoichiometric(chemically correct)for satisfactory spark ignition and flame propogation.The eqivalence ratio or mixture strength of air/fuel mixture also affects pollutent emission and influences the susceptibility to spontaneous self-ignition.A lean air/fuel mixture will burn more slowly and will have a lower maximum temperature than a less lean mixture.Slower combution will lead to lower peak pressure ,and both this and the lower peak temperature will reduce the tendency for knock to occure .The air/fuel mixture also affects the engine efficiency and power output .

Carburetor

The air/fuel mixture can be prepared either by carburattor or by fuel injection.In a carburettor air flows through a venturi,and the pressure drop created causes fuel to flow through an orifice,the jet.There are two main types of carburator -fixed jet and variable jet.Fixed jet carburettors have a fixed venturi ,but a series of jets to allow for a diffrerent engine-operating conditions from idle to full throttle.Variable jet carburettors have an accurately profiled needle in in the jet.The needle position is controlled by a piston which also varies the venturi size.The pressure drop is approximately constant in a variable jet carburettor,while pressure drop varies in a fixed jet carburettor.

The carburettor and(or fuel injection system )and manifold have to perform satisfactorily in both in steady state and transient conditions.When an engine is started ,extra fuel floods into the inlet manifold .Under these conditions the engine starts on the very rich mixture and the inlet manifold acts as a surface carburettor;often there will be small ribs to control the flow of liquid fuel.

Fuel injection

The alternative to carburettors is fuel injection.Early fuel injection system were controlled mechanically ,but the usual form of control is now electonic .Fuel is not normally injected directly into the cylinder during the compression stroke.This would require high-pressure ingection equipment,and it would reduce the time for preparation of homogeneous mixture.Also,the ingector would have to withstand the high temperature and pressure during combution and be resistant to the build up of combustion deposite.With low-pressure fuel injection system,the fuel is usually injected close to the inlet valve of each cylinder.Alternatively a single injector can be used to inject at the entrance to the inlet manifold.As there are two types of fuel injection system used on spark ignition engines,multi-point injection and single point injection.The multi-point injection system employs injectors usually mounted close to the inlet ports of each cylinder.The single point injection system can look very much like a carburettor , and as with a carburettor ,the throttle plate and the inlet manifold play an important part in a mixture preparation.Even with multipoint fuel injection system ,a liquid fuel film will develop on the walls of the inlet manifold.

Working cycle

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 following four strokes: suction or intake stroke, compression stroke, expansion or power stroke, exhaust stroke. Each stroke consists of 180 degree rotation of crankshaft rotation and hence a four-stroke cycle is completed through 720 degree of crank rotation. Thus for one complete cycle there is only one power stroke while the crankshaft turns by two revolutions.

See also

• Compression-ignition engine
• Multifuel engine