Spark Ignition Engines: Importance of Proper Fuel Quality

Petrol or gasoline is usually used as fuel for spark ignition (SI) engines. Gasoline is a mixture of a number of low boiling point paraffins, naphthenes, and aromatics of varying proportions. There are certain properties that a particular gasoline should have to qualify as SI engine fuel. These properties have been discussed below:

1) Volatility: The gasoline should be volatile; a certain part of it should vaporize at room temperature to allow easy starting of the engine. Better vaporization of the fuel facilitates its even distribution inside the cylinders, which in turn leads to better acceleration of the vehicle.

2) Dilution of the lubricating oil in crankcase: As the fuel is splashed in the cylinder, some lubricating oil from the crankcase is also washed away with it. This leads to overall decrease in the quantity of the lubricating oil and poor lubrication of the engine’s moving parts. To prevent such possibilities, it is important that the type of gasoline used for the engine should vaporize before it gets combusted.

3) Antiknock qualities of the fuel: Abnormal burning or detonation of the fuel inside the engine leads to the effect known as engine knock. During detonation large amounts of heat is released inside the engine which excessively increases the temperature and pressure inside the engine, drastically reducing its thermal efficiency. The fuel should have the tendency to avoid creating the situation of detonation; this quality of the fuel is the antiknock property of the fuel.

The antiknock property of the fuel depends greatly on the self-ignition properties of the fuel, the fuel’s chemical composition, and its chemical structure. The fuel most suitable for the SI engines is the one that has highest antiknock property, enabling the engine to work with high compression ratios of fuel, which in turn leads to higher fuel efficiency and higher power production.

4) Gum deposits formed from the fuel: When gasoline is stored for longer periods of time, it has the tendency to oxidize and form gummy, solid substances. When used with an engine, such gasoline will cause sticky valves and piston rings, carbon deposits in the engine, gum deposits in the manifold, clogging of carburetor jets, and enlarging of cylinders and pistons. The gasoline used in the engine should have a tendency to form lower gum content and have a lower tendency to form gum during storage.

5) Low sulfur content: Hydrocarbon fuels may contain sulfur in various forms like hydrogen sulfide and other compounds. Sulfur is corrosive in nature and it can cause fuel line corrosion, carburetor parts, injection pumps, etc. Sulfur also promotes knocking of engine; hence its content in the gasoline fuel should be kept to a minimum.