SI

A spark ignition engine is one in which a timed electrical spark ignites the fuel-air mixture inside the cylinder. The spark is generated by a spark plug, which fires at a precise moment during the compression stroke, typically a few degrees before top dead centre. This distinguishes SI engines from compression ignition (diesel) engines, where fuel ignites spontaneously from heat alone.

The spark plug voltage in a typical SI engine ranges from 5,000 to 20,000 volts depending on the ignition system and engine load. Modern SI engines use either inductive ignition coils or capacitive discharge systems to generate this high voltage. The timing of the spark must be carefully controlled: fire too early and peak combustion pressure occurs before the piston reaches top dead centre, wasting energy; too late and combustion continues past the power stroke, reducing efficiency and producing excess heat.

SI engines have lower compression ratios than diesel engines, typically between 8:1 and 12:1, because the fuel-air mixture would self-ignite if compressed too far. This allows the use of lighter fuels and simpler fuel systems, but at the cost of lower thermal efficiency. Petrol, liquefied petroleum gas, and hydrogen can all be burned in SI engines, making them flexible for different fuel sources.

The main failure modes in SI ignition are spark plug fouling, which can occur from oil accumulation or carbon deposits; plug gap erosion from repeated spark discharge; and coil or module failures. Cold starting in SI engines presents a particular challenge because the mixture must be enriched to ignite reliably at low temperatures, requiring a choke or electronic fuel enrichment system.

SI engines dominate light-duty vehicles and small equipment because of their simplicity, cost, and responsiveness to rapid throttle changes. In industrial and marine applications they are common in smaller generators and auxiliary engines. The term SI itself is primarily used in engineering and regulatory contexts; it appears in emissions standards, engine classification schemes, and technical specifications rather than in everyday speech.