One very basic question, why petrol engine needs a spark plug, and disel engine doesnt need?

Question

One very basic question, why petrol engine needs a spark plug, and disel engine doesnt need? Sounds contradictory, because the petrol is more inflamable than Disel, when Disel can be fired without spark, why the petrol cant be?

Answer 1

I am an automotive engineer so hopefully I don't get too technical here....

The difference between the two is the combustion process. When the engine compresses the fuel/air mixture in the cylinder the temperature increases (ideal gas law pv=nrt). The diesel compresses the mixture at a much higher ratio than gasoline and the heat of the compression process causes the mixture to ignite (think of simultaneous combustion). When you hear a gasoline engine "pinging" it is actually the fuel/air mixture igniting before the spark plug lights it off! - this is really bad in a gasoline engine as the piston has to work against the force of the burning mixture and it is not built like a diesel to sustain this type of operation)

In contrast, the gasline engine requires a spark to ignite the fuel/air mixture.



For this reason the diesel engine makes a lot more low end torque than the gasoline engine, b/c much more of the force of the combustion process occurs prior to the piston fully compressing the fuel/air mixture.

The gasoline combustion process (burning and expansion) occurs later in the combustion process so you therefore lose some of the torque you get from the combustion process.

Hope this helps!

auto1





So when you see in car magazines the compressor ratio like 10.0:1 this means that the cylinder takes an amount of air and compresses it to 10% of its uncompressed volume. Diesel engines have a much higher compression ration ie. 24:1 etc.



The gasoline engine

Answer 2

well you pretty much answered your own question, petrol needs a spark plug to ignite (since in your terms its more inflamable than disel), so they put a spark plug in there, to ignite. where as disel (being more flamable than petrol) can ignite itself under high compression at the temprature of the engine. its just the way things work because of the atomic composition itself. you cant argue with it. so that didnt sound contradictory at all, you just probaly understook it in the wrong way.

Answer 3

Gasoline has a lower BTU rating then Diesel, in othe words less energy. The lower energy per liter the more energy it takes to ignite. Gasoline needs the high heat source of the spark plug to get going. Diesel on the other hand only needs the mid temp of high compression.

Answer 4

Diesel engines work by compressing the fuel/air mix. As the compression increases, the mix heats-up until it's hot enough to ignite the mix, giving you the power. But, this method is a bit unpredictable. The exact timing of when the mix will finaly go BANG varies a tiny bit, as the air & fuel components vary. Because of this variance, a diesel engine is a bit slower to make rpms, & it doesn't go as 'fast' as a gas engine. Plus, because the compression ratio of a diesel is much higher then a gas engine, the diesel is heavier. To compare to a gas engine, just basically the opposite.

Answer 5

I frequently spend my half an hour to read this blog's posts daily along with a mug of coffee.

Answer 6

diesel has high compression...so high it heats fuel to combustion...gas motor has low compression so needs spark

Answer 7

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