Spark Ignition - Petrol/Gasoline Engines - EngineKnowHow

In a petrol / gasoline engine the fuel / air mixture is ignited via a spark towards the end of the compression stroke by a spark plug. In a spark ignition engine the air and fuel mixture a premixed prior to ignition, with the fuel either injected into the inlet ports, known as Port Fuel Injection (PFI) or directly into the cylinder, known as Direct Injection (DI) in fuel injection engines. In older and cheaper engines, the air / fuel mixture is supplied to the engine via a carburettor. In a spark ignition engine the torque, or load is controlled by throttling the air supply to the cylinders, typically by a throttle.


Spark Ignition


In a spark ignition engine:

  • Maximum engine performance and efficiency is limited by knock, which is the ignition of the fuel / air mixture in front of the flame front.
  • Increasingly in turbocharged engines due to higher cylinder pressures, maximum performance and efficiency is limited by Pre-Ignition, which is ignition of the fuel / air mixture prior to the spark.
  • Typically there is just enough air to completely consume all of the fuel, (Lambda = 1).
  • Compression ratios are typically between 8 – 14, dependent upon fuel and boosting.
  • Relative to compression ignition engines, higher fuel consumption due to lower compression ratios and throttling of the air supply under part load conditions.
  • Low emissions with exhaust after treatment.


Since the first Otto engine, improvements to the spark ignition engine have largely come from:

Fuels – Increased octane numbers have allowed the increase of compression ratios. Additionally, increased vaporisation characteristics of fuels result in a more homogeneous fuel / air mixture.

Materials – Increased strength of materials and reduced masses allow higher compression ratios and higher piston speeds.

Computer Modelling – Increased understanding of fluid dynamics and computing capabilities allowed optimisation of the combustion chambers, fuel / air mixture delivery and engine breathing capabilities.

Variable Valve Timing and Lift – Have allowed increased control of the air to, and the exhausting of the cylinder by tailoring the valve timing and valve lift to specific engine speeds and load.

Forced Induction – Turbo and supercharger technology has increased power levels due to the higher mass of air they can deliver but equally importantly reduce the pumping work of the engine.

Electronics – Electronics and sensor development has allowed for increased accuracy in fuel delivery via fuel injection systems, combustion timing via spark control and the intake / exhaust process via variable valve timing and lift systems.

Emission Treatment Systems – 3-way catalysts have resulted in a large reduction in engine emissions.