Burning Petrol is very inefficient

Petrol engines are able to use only 20% of the energy to produce useful work-turn the crank shaft. About 50% of it goes into atmosphere mainly through exhaust gases and engine cooling system. The remaining energy is lost by friction of moving parts, turning generators, pumping air into & out of cylinder etc.

Efficiency of internal combustion engines is a function of expansion ratio, which is generally the same as compression ratio. The compression ratio of piston engines generally varies between 1:8 to 1:10. Higher compression ratio engines use high octane fuel to prevent detonation—self ignition at high temperature before spark. This is a limiting factor on petrol engines, hence their low efficiency.

Diesel engines, on the other hand, do not suffer much from limitation on compression ratios since they do not need spark plugs to ignite fuel. The slow burning diesel fuel self ignites at high compression ratio of 1:14 to 1:25. The resulting expansion ratio increases its efficiency upto 45%.

Another factor is dearth of oxygen in air. It forms only 21% of the atmosphere by mass. Hence air runs out of oxygen without completely burning the fuel inside cylinder. Unburnt fuel, therefore, is pumped out of engine into atmosphere through exhaust. Additionally fuel gets less time to burn at high RPMs, leaving more fuel unburnt. VVTI (Variable Valve Timing Intelligence) relatively recent technology has solved this particular problem. This allows fuel more time to burn at higher RPMs, by changing valves opening and closing timings at high RPMs.

Credit: Muhammad Afsar Malik