Comparison of SI and CI Engine

 

Comparison of S.I. and C.I. engines is made from various aspects is made below:

Fuel economy 

The chief comparison to be made between the two types of engine is how effectively each engine can convert the liquid fuel into work energy. Different engines are compared by their thermal efficiencies. Thermal efficiency is the ratio of the useful work produced to the total energy supplied. Petrol engines can have thermal efficiencies ranging between 20% and 30%. The corresponding diesel engines generally have improved efficiencies, between 30% and 40%. Both sets of efficiency values are considerably influenced by the chosen compression-ratio and design.

Power and torque 

The petrol engine is usually designed with a shorter stroke and operates over a much larger crankshaft-speed range than the diesel engine. This enables more power to be developed towards the upper speed range in the petrol engine, which is necessary for high road speeds; however, a long-stroke diesel engine has improved pulling torque over a relatively narrow speed range, this being essential for the haulage of heavy commercial vehicles.

At the time of writing, there was a trend to incorporate diesel engines into cars. This new generation of engines has different design parameters and therefore does not conform to the above observations.

Reliability

  Due to their particular process of combustion, diesel engines are built sturdier, tend to run cooler, and have only half the speed range of most petrol engines. These factors make the diesel engine more reliable and considerably extend engine life relative to the petrol engine.

Pollution

  Diesel engines tend to become noisy and to vibrate on their mountings as the operating load is reduced. The combustion process is quieter in the petrol engine and it runs smoother than the diesel engine. There is no noisy injection equipment used on the petrol engine, unlike that necessary on the diesel engine. The products of combustion coming out of the exhaust system are more noticeable with diesel engines, particularly if any of the injection equipment components are out of tune. It is questionable which are the more harmful: the relatively invisible exhaust gases from the petrol engine, which include nitrogen dioxide, or the visible smoky diesel exhaust gases.

Safety 

Unlike petrol, diesel fuels are not flammable at normal operating temperature, so they are not a handling hazard and fire risks due to accidents are minimized.

Cost 

Due to their heavy construction and injection equipment, diesel engines are more expensive than petrol engines.