Why do Diesels rev so slow?

[benough]

This only came to mind after driving one for a year!

It is because the stroke is long? i.e. Long stroke for torque but too long to spin fast?

[RhysQ]

thats a good question, so i went searching and found this, it should be what you're looking for...look at mainly points 3 and 4.

http://rjmason.com/ramblings/dieselTorque.html

[gregozedobe]

I think the valve timing has something to do with it as well.

The valves have to close much sooner with a diesel to let it get up to proper ignition pressure, whereas a petrol can allow much later closing (exhaust) and earlier opening (intake) and still get correct ignition. Hence seriously modding a naturally aspirated petrol engine almost always ends up with more power at higher revs (think F1 engine at 15,000 to 18,000rpm).

[Greg Roles]

add to that a MUCH higher compression, dual mass flywheels, and a much later "bang" over a sparkplug engine, then add gregoz's correct nil overlap cam timing and your long stroke, and it's a wonder they can rev at all!

[gldgti]

I think the valve timing has something to do with it as well.

The valves have to close much sooner with a diesel to let it get up to proper ignition pressure, whereas a petrol can allow much later closing (exhaust) and earlier opening (intake) and still get correct ignition. Hence seriously modding a naturally aspirated petrol engine almost always ends up with more power at higher revs (think F1 engine at 15,000 to 18,000rpm).

valve timing isnt entirely what is responsible - infact it may be of interest to note that the difference in valve timing and duration is close to nill in the early vw diesels.

from my 1st thesis studies, my understanding became that the main reason diesels rev less is because of the necessities of injection timing, and the diesel combustion process.

contrary to popular belief, the diesel combusion speed is quite fast - however due to the nature of the cycle (excess air, no "mixture") much of the expansion of gas is caused by the initial combusion heating up the rest of the unbrned air charge in the cylinder - this effect is quite slow - and more importantly, powerful - giving diesels the propensity to make large torque at low revs.

as the cycle speed increases, the ability of an expanding excess air charge to make torque is reduced, as the time allowed for combusion to heat cylinder charge is reduced. consequently, more fuel and advance is required to make torque.

there is a limit to the amount of advance that can be used in a diesel engine (as with any internal combustion engine) and because of the speed of diesel combustion, the limit of advance comes up sooner than in an otto cycle engine.

[Preen59]

there is a limit to the amount of advance that can be used in a diesel engine (as with any internal combustion engine) and because of the speed of diesel combustion, the limit of advance comes up sooner than in an otto cycle engine.

.... Due to the ignition of the charge needing to be caused by compression, rather than spark, like in a petrol engine.

ie, you need to compress the charge sufficiently to create enough heat that it will spontaneously combust, rather than giving it an ignition source (such as a spark plug in a petrol engine). This is why Diesels have glow plugs for cold starting. It aids in creating sufficient combustion chamber temperature to ignite the charge.

In a diesel engine, "advance" is in reference to injection advance in crank shaft degrees prior to TDC (Top Dead Centre), rather than ignition advance.

Well put, Aydan, however i thought i'd add that for a bit more clarification and additional information.

[gldgti]

thanks preeny - good clarification there - im distracted by a movie

[gregozedobe]

I must be having a "seniors moment", as these later explanations have undermined my theoretical understanding of the diesel combustion process.

.... Due to the ignition of the charge needing to be caused by compression, rather than spark, like in a petrol engine.

I agree that high compression pressure is needed to create the right conditions for combustion to be possible, but surely actual combustion cannot start until at least when the diesel is actually injected (or sometime after that if the pressure isn't high enough) ? Just plain air can't burn, right ?

The term "spontaneously combust" confuses me (except in the sense that it isn't initiated by a spark plug creating a spark). Combustion can't happen before the fuel is delivered into the combustion chamber, and doesn't it then normally start as soon as the fuel is injected ?

Otherwise it wouldn't matter when the fuel was injected, and so injection "advance" would be meaningless. I understood that the timing of the start of the combustion event is important, and is totally dependent on the timing of the injection of fuel (once the engine is warm).

I'm ignoring (for the purpose of this discussion) the multiple injection events that happen in each combustion cycle in modern diesels.
.

[Transporter]

It would be impossible to ignite compressed air, if it was cold air.
When the piston goes up it compresses the air and as the air is compressing the temperature of the compressed air is increased to some 800 deg C or more, than the diesel fuel is injected and rapidly burns causing the piston to go down.
This is very basically and very shortly explained how the combustion happens in the diesel engine.

[gldgti]

I must be having a "seniors moment", as these later explanations have undermined my theoretical understanding of the diesel combustion process.



I agree that high compression pressure is needed to create the right conditions for combustion to be possible, but surely actual combustion cannot start until at least when the diesel is actually injected (or sometime after that if the pressure isn't high enough) ? Just plain air can't burn, right ?

The term "spontaneously combust" confuses me (except in the sense that it isn't initiated by a spark plug creating a spark). Combustion can't happen before the fuel is delivered into the combustion chamber, and doesn't it then normally start as soon as the fuel is injected ?

Otherwise it wouldn't matter when the fuel was injected, and so injection "advance" would be meaningless. I understood that the timing of the start of the combustion event is important, and is totally dependent on the timing of the injection of fuel (once the engine is warm).

I'm ignoring (for the purpose of this discussion) the multiple injection events that happen in each combustion cycle in modern diesels.
.

indeed you are ofcourse correct..... unless there is a problem, the diesel will generally ignite AS IT IS INJECTED - ie as it comes out of the injector.

what preeny was referring to, was that if you attempt to begin injection BEFORE sufficient cylinder pressure/temperature is reached, then the fuel wont ignite upon injection, and therefore will combust "spontaneously" as the compression builds in the cylinder.

preeny was referring to compression of the AIR charge. once it has reached sufficient compression (for an arbitrary example, 15degBTDC) then you may inject fuel with the knowledge that it will immediately combust, hence giving meaningful practice to the term injection timing.