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
Why Diesel cars have better fuel efficiency than Petrol cars?
Part of the diesels fuel efficiency stems from the fact that most diesels use what’s called direct injection, which is when the fuel is injected directly into the cylinder, instead of into the intake manifold and drawn into the cylinder via the intake valves (hence the name). This allows the fuel to be injected at the right time to automatically combust, and also to inject an exact amount of fuel, giving the ability to inject much less fuel than you would have to otherwise for the same power. This is the reason that gasoline engines are slowly switching over to direct injection. Leaner also means less fuel used. Another reason is that diesels operate at lower engine speeds (RPM) than gas engines, which reduces fuel consumption as well. Yes, the parts that make up a diesel engine are more expensive. For example, if you were to buy a complete setup, fuel pump, fuel injectors, fuel rails, fuel pressure regulator for a gasoline engine, you might spend $500-600. If you buy just a high pressure injection pump for a VW 1.9 TDI, it’s $900.
Lastly, to say that a gasoline is cleaner is also inaccurate. The exhaust gases that come out of a diesel that is running properly is much cleaner than gas.
A engines efficiency is directly related to it’s compression ratio. Since a diesel operates with higher compression than a gas engine, it squeezes more energy from the fuel it burns.
As for the cost, a diesel engine has to be built much sturdier than a gas engine to handle the higher operating pressures and temperatures. I’m guessing here….there’s also the cost of the fuel injection system which contain parts that are machined to within millionths of an inch (cm), and have to handle extreme pressures. I’d have to think that would expensive too