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Why is Diesel Engine called as Quality Controlled and Petrol called as Quantity Control.

As far as I understand,

In Diesel Engine the right amount of Diesel for the right amount of Compressed air is more important, Injecting more fuel alone will not increase the torque. Right Combination of Air - Diesel, Fuel Spread, Timing plays important role.

In case of Gasoline/Petrol Engine the air-fuel is mixed external to the cylinder at a more or less a constant ratio. Torque could be increased by sucking more air-fuel mixture into the cylinder during Induction stroke.

Please correct me if I am wrong and add as much more clarity as possible.

  • Well... Your spot on with the concept. – Shobin P Jun 3 '15 at 6:12
  • One more thing , in most modern Direct Injection systems I think fuel is directly inject into the cylinder opposed to the pre-mixture. – Shobin P Jun 3 '15 at 6:18
  • Also, the wrong mixture in a gasoline engine will cause catastrophic damage. Diesel doesn't care. – Captain Kenpachi Jun 3 '15 at 9:39
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You are mostly correct in your assumptions. However, you write: "In Diesel Engine the right amount of Diesel for the right amount of Compressed air is more important, Injecting more fuel alone will not increase the torque." This is not correct, perhaps you meant for a petrol engine?

Anyway, the way a petrol engine works is by having a throttle in the air intake. Its form can vary, but the function is to limit the amount of air that enters the engine. The throttle is controlled with your accelerator.

open throttle

The hard part is then to get the exact amount of fuel mixed into the air. This is the job of the injection system or the carburetor. fuel/air-mixture then enters the cylinder when the inlet valve opens and when compressed by the piston, the spark plug ignites the mixture and a downward force is achieved.

On a Diesel engine, there are no throttle. You don't control the amount of air entering the engine. Instead, you limit the amount of fuel that is injected. There are no spark plugs either, since low octane value of diesel fuel allows it to spontaneously combust in the high pressure of the cylinder. This means the timing of the combustion cannot be controlled by the spark either, and that's why a diesel injects the fuel directly into the cylinder (or pre chamber) exactly when the combustion should start. As you say, the manner of injection play a big role in fuel efficiency and performance. High pressure and several injections in a single combustion is utilized to maximize performance. There are different kinds of diesel engines, but most modern engines are direct injected.

direct injection etc.

Adding more fuel into a diesel gives you better performance up until something breaks or there isn't enough air to combust all fuel, which shows by releasing black smoke in your exhaust.

This is why Diesels are generally better off with turbos than petrol engines. Adding more fuel in a turbo diesel engine gives you more exhausts, which in turn gives a higher turbo pressure, and more air into the engine. More air into the engine gives you the possibility of adding even more fuel, and so on.

I actually haven't heard of diesels being called "Quality controlled", perhaps someone else have an idea? "Quantity controlled" for a petrol engine is easier to guess, the ratio between air and fuel should always be constant. For proper mixture in a petrol engine, you would need about 14-15kg of air for 1kg fuel. In high performance applications you would let it be a bit richer both for more power and better cooling.

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As per my understanding, diesel engines are quantity controlled and gasoline engines are quality controlled. The reason being in gasoline engine for a proper combustion, the mixture formation is very important. There should be homogeneous mixture of air and fuel for a complete combustion to happen. therefore the quality of mixture is very important. That is why in a modern electronically controlled system, both air and fuel are controlled (electronic throttle control and fuel injection) to produce a homogeneous mixture

Whereas in a diesel engine, the air is un-throttled and the mixture is not homogeneous. There is no requirement of metering of fuel based on the air that enters the cylinder in a diesel engine. The power output in a diesel is directly dependent on the quantity of fuel injected into the cylinder.

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