Petrol and Diesel Cycles

in petrol engines, energy from fuel is released almost instantaneously when piston is at TDC-there is almost no change in volume when this happens. In diesel engines, fuel is introduced at abt TDC and continuosly added while the piston is still moving. In real world what you get is more of combined cycle for both engines., that is an engine encompassing features of both cycles.

Also, since fuel needs finite time to burn, especially at high speeds, petrol engine tends to move closer to constant pressure cycle. In similar condition, for diesel engine, you'll need to inject fuel earlier, so bulk of combustion takes place in near constant volume condition.

please simplify.

1. petrol engines - spark comes on, fuel burns at one go.

2. diesel engines - fuel is injected, fuel burns continously while piston is moving down.

good info here; http://en.wikipedia.org/wiki/Four-stroke_cycle or type in 'air standard cycle' in google.

In theory:

In a petrol engine the combustion is done at a certain time in the cycle(when the spark plug ignites the fuel): the volume does not change while the fuel is burning (this is theory)

A diesel cycle burns it's fuel so that the pressure in the combustion chamber stays constant (while delivering the energy)

In reality, the two cycles come to each other: the burning of the fuel takes some time and the pressure is not constant.

The real difference between the two IC engines is the compression ratio: petrol will auto-ignite when the pressure (and temperature) goes over a certain level this limits the compression ratio. A diesel cycle is compressing pure air (no mixture) so the compression ratio is in theory unlimited. The mechanical solution limits the compression ratio.

Return to your books and dive into the PV diagrams, you will clearly recognize the two differences.

"....The real difference between the two IC engines is the compression ratio: petrol will auto-ignite when the pressure (and temperature) goes over a certain level this limits the compression ratio. A diesel cycle is compressing pure air (no mixture) so the compression ratio is in theory unlimited. The mechanical solution limits the compression ratio."

Actually Gwen, the difference between the 2 cycles is actually the way fuel is inginted ! In petrol engines, spark from a spark plug/glow plug does the job, and in a diesel engine, when the fuel is injected --- it auto-ignites. This is the difference!

The compression ratio merely modifies the final effect, i.e.: too high a compression ratio in petrol engine, and may cause the mixture to auto-ignite(pre-mature combustion - causes knocking). Too low pressure in diesel engines, and temperature at end of compression stroke may not be high enough for fuel to auto-ignite.

That is a technical difference on how the cycle is realised.

You could come up with more differences: Petrol is burning a petrol gas-air mixture where a diesel cycle is burning liquid droplets.

There even exist petrol engines who inject the fuel direct in the combustion chamber, after the compression has taken place. There are so many differences even in the two families.

The compression ratio is set as high as possible, this is the main factor to gain efficacy. And there is the difference between the two cycles that explains why a diesel is consuming less. All the rest are technical jokes to make it happen (and those explain the price and size difference).

It depends on cycle's expansion stroke.