Fuel Supply, Carburetion and Combustion
This domain covers how petrol reaches the engine, how a carburettor meters fuel into the air stream, what air-fuel ratios mean, and how combustion proceeds - including abnormal combustion such as detonation.
Fuel properties
The two key petrol properties are volatility (how easily it vaporises for cold starting and even mixing) and anti-knock quality, expressed as the octane number. Iso-octane is 100 and normal heptane is 0; a fuel rated 91 has the same knock resistance as a 91:9 blend of those two. Octane number is not a measure of energy content. Diesel quality is rated by cetane number.
Simple carburettor
Its jobs are to atomise fuel, mix it with air in the right proportion and supply a variable quantity for all speeds and loads. Air passes the venturi, the narrow throat where velocity is highest and pressure lowest; this pressure drop draws fuel through the main jet. The float chamber and needle valve keep the fuel level constant. The throttle valve controls the amount of mixture entering the engine, and the choke richens the mixture for cold starting by restricting air.
Air-fuel ratios
Chemically correct (stoichiometric) for petrol is about 14.7:1 by mass. Roughly 12:1 gives maximum power (rich), 15 to 16:1 gives best economy (lean), and idling needs about 10 to 12:1. The carburettor uses a float circuit, idling circuit, main metering circuit, acceleration pump, power or economiser valve and choke. Modern engines use fuel injection, including multi-point injection, controlled electronically.
Combustion in the petrol engine
After the spark there is an ignition lag, then flame propagation across the chamber, then after-burning. Detonation (knocking) is the spontaneous explosion of the end gas ahead of the flame front; it is worsened by low octane fuel, high compression ratio, advanced spark and overheating. Pre-ignition is ignition before the spark, usually from a hot spot such as a glowing carbon deposit.
Worked example
A vehicle pings under load on a hot day. Reasoning: the end gas is exploding ahead of the flame front. Remedies include using the recommended higher octane fuel, retarding the spark slightly, and correcting overheating or heavy carbon deposits - not simply opening the choke.
Traps that cost marks
Thinking a higher octane number gives more power by itself; confusing the venturi with the throttle; saying the choke is used when the engine is hot.