The thermal efficiency of petrol engine as compared to diesel engine is: A lower B higher C same for same power output D same for same speed

Thanks En.Alex...

It is simply to consider basic thermodynamics - Ideal Otto cycle (for SI engines) and Diesel cycle (for CI engines). At the same compression ratio and under the cold-air standard assumption, thermal efficiency of Otto cycle is always greater than that of Diesel cycle. When the cutoff ratio (the ratio of the cylinder volumes after and before the combustion process) equals to1, both Otto and Diesel cycles have identical efficiencies. In gasoline engines, a mixture of fuel and air is compressed during the compression stroke and the compression ratios are limited by the auto-ignition (fuel property) or engine knock. For diesel engines, only air is compressed during the compression stroke eliminating the fuel's auto-ignition or pre-ignition. Then fuel is sprayed into the combustion chamber where it vaporized, atomized and mixed with high pressure and high temperature air, the combustion process takes place. Diesel engines can therefore operate at higher compression ratio and using heavier fuel.

Option A

The correct answer is (A) : The thermal efficiency of petrol engine is lower than that of diesel engine

As every one know that it is theoretically & both practically proven that Otto cycle have more thermal efficiency than diesel cycle, and petrol engines are based on otto cycle ,so petrol engine have More Thermal Efficiency than diesel engine.It is different thing that Diesel Engine is used often in the Heavy as well as Light Vehicle, for this other reasons are responsible .   

Agree with Mr. Omair.

A lower  

Diesel engines can therefore operate at higher compression ratio and using heavier fuel.

That's way Diesel Engines perform so well in the heavy equipments, trains, trucks, ships,..........

I am going with answer A as well

Agree with an eng. Omair

why efficiency of diesel cycle can't equal zero