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diesel engines ?
jacko - 26/2/10 at 08:01 PM

Hi All i was just thinking to my self WHY don't diesel engines rev as high as petrol engines
WHY
Jacko

dlatch - 26/2/10 at 08:02 PM

the burn rate of the fuel compared to petrol basicly

[Edited on 26/2/10 by dlatch]

stevec - 26/2/10 at 08:10 PM

And heavy internals.

Steve

daviep - 26/2/10 at 08:11 PM

The components also tend to be much stronger and hence heavier and don't like the sudden changes of direction very much.

DOH too slow

[Edited on 26/2/10 by daviep]

mookaloid - 26/2/10 at 08:26 PM

I believe that it is because you can't advance the ignition timing like you can with a petrol engine. At higher revs the ignition sparks sooner so that combustion is already taking place by the time air/fuel mix reaches it's maximum compression

The diesel/air mix can only ignite when it has been compressed to it's optimum pressure whereupon it ignites spontaneously.

Hope that helps

Cheers

Mark

hillbillyracer - 26/2/10 at 08:32 PM

As said the big thing is the diesel just wont burn fast enough to make use of more revs, besides they make such good power/torque at 2000-3500rpm there's just no need to go further.
I've heard a Volvo tractor puller that went to 5000rpm & it sounded unreal, never heard a diesel scream like it did.

oadamo - 26/2/10 at 08:37 PM

so could you convert it and run some high boost
adam

Miks15 - 26/2/10 at 08:59 PM

i heard that it was because of the higher compression rate needed, therefore a longer a stroke hence wont rev as high

sebastiaan - 26/2/10 at 09:17 PM

burn rate (and only that)

l0rd - 26/2/10 at 09:25 PM

quote:
Originally posted by Miks15
i heard that it was because of the higher compression rate needed, therefore a longer a stroke hence wont rev as high


+1

Completely different philosophy

These engines are made for maximum torque hence the stroke is higher and revs lower.

se7en - 26/2/10 at 09:47 PM

quote:
Originally posted by mookaloid
I believe that it is because you can't advance the ignition timing like you can with a petrol engine. At higher revs the ignition sparks sooner so that combustion is already taking place by the time air/fuel mix reaches it's maximum compression

The diesel/air mix can only ignite when it has been compressed to it's optimum pressure whereupon it ignites spontaneously.

Hope that helps

Cheers

Mark


That's nonsense Mark, with common rail injection the timing is infinitely adjustable. The main reason is the difference in the way that the engines generate their power.

In a petrol engine you will get more torque from a short stroke and thus the engine has to do quite a lot of RPM.
The diesel engine on the other hand uses a long stroke to generate the high heat needed to make the diesel oil explode. Thus the diesel engine runs at a much lower RPM.

Newer diesel engines have drifted away from the original design by Rudolf Diesel and that is one of the reasons for diesels to be very loud. There is talk of designers reverting back to the original designs in order to make the engines quieter.

smart51 - 26/2/10 at 09:54 PM

Petrol is ignited with a spark. You advance the timing of the spark as the revs rise so the fuel is burned at the right time.

Diesel is ignited by the temperature caused by the very high compression. Whilst you can adjust the fuel timing on a direct injection diesel, you're still restricted by the compression.

I've always wondered why you couldn't add a spark plug to a diesel engine and spark the mixture early at high revs. No-one has ever been able to answer that question properly. One of the nice things about a diesel is that the compression is much the same everywhere so the fuel starts to burn everywhere at once. A sparked mixture starts to burn at the spark and a flame front spreads out. Sparking diesel will cause the flame front effect but it doesn't hurt the petrol engine so why not a diesel?

hillbillyracer - 26/2/10 at 10:15 PM

quote:
Originally posted by smart51
Petrol is ignited with a spark. You advance the timing of the spark as the revs rise so the fuel is burned at the right time.

Diesel is ignited by the temperature caused by the very high compression. Whilst you can adjust the fuel timing on a direct injection diesel, you're still restricted by the compression.

I've always wondered why you couldn't add a spark plug to a diesel engine and spark the mixture early at high revs. No-one has ever been able to answer that question properly. One of the nice things about a diesel is that the compression is much the same everywhere so the fuel starts to burn everywhere at once. A sparked mixture starts to burn at the spark and a flame front spreads out. Sparking diesel will cause the flame front effect but it doesn't hurt the petrol engine so why not a diesel?


I'm sure it reacts a bit differently when injected at high pressure & squeezed to buggery like it is inside an engine but it doesnt like to light with a flame or spark, if you drop a lighted match into a dish of petrol you'll stand to loose your eyebrows but diesel will put the match out every time.

Simon - 27/2/10 at 12:36 AM

Modern diesels will have up to 5 pulsed squirts for every power stroke to increase power strike.

As has been said, a diesel prduces so much torque that a lower final drive ratio and lower engine speed make up for higher engine speeds/diff ratios

This might help, dragged off BMW website (and based on BMW 1 series 5 door SE spec):

Petrol Diesel
1995cc 1995cc
4 cyl/16valve 4 cyl/16valve
Stroke/Bore Stroke/Bore
90mm/84mm 90mm/84mm

Identical so far, but
139mph top 142mph
7.8 - 60 7.6 - 60
7.5 - 50 > 75 6.4 - 50 > 75
170hp @ 6700rpm 177 @ 4000
210nm @ 4250 350nm @ 1750

Explains it all really

ATB

Simon

edited torque figs

[Edited on 27/2/10 by Simon]

PaulBuz - 27/2/10 at 09:48 AM

quote:
Originally posted by Simon
Modern diesels will have up to 5 pulsed squirts for every power stroke to increase power strike.

As has been said, a diesel prduces so much torque that a lower final drive ratio and lower engine speed make up for higher engine speeds/diff ratios

This might help, dragged off BMW website (and based on BMW 1 series 5 door SE spec):

Petrol Diesel
1995cc 1995cc
4 cyl/16valve 4 cyl/16valve
Stroke/Bore Stroke/Bore
90mm/84mm 90mm/84mm

Identical so far, but
139mph top 142mph
7.8 - 60 7.6 - 60
7.5 - 50 > 75 6.4 - 50 > 75
170hp @ 6700rpm 177 @ 4000
350nm @ 1750 210nm @ 4250

Explains it all really

ATB

Simon


Simon ,surely those torque figures are the wrong way around?

MikeRJ - 27/2/10 at 11:36 AM

quote:
Originally posted by se7en

In a petrol engine you will get more torque from a short stroke and thus the engine has to do quite a lot of RPM.
The diesel engine on the other hand uses a long stroke to generate the high heat needed to make the diesel oil explode. Thus the diesel engine runs at a much lower RPM.


That's not really true, the stroke of an engine has little or no bearing on torque for a given capacity and state of tune.
It's an oft repeated old wives tale, because it appears to be true, i.e. long stroke engine often develop more torque than the equivalent short stroke, but the reason is simply because they are tuned to do this. Long stroke engines mean low revving engines due to piston acceleration and speed limitations, so the only way to make good power is to tune for high torque, i.e. very mild cams, small ports in head etc.

If you look at modern TDCi engines than you'll find that whilst most are slightly under-square, they are not what would be regarded as long stroke engines, and some are even over-square.

Simon - 27/2/10 at 05:57 PM

quote:
Originally posted by PaulBuz
Simon ,surely those torque figures are the wrong way around?


Er, oops

Corrected!

ATB

Simon

daviep - 1/3/10 at 02:27 PM

quote:
Originally posted by MikeRJ
quote:
Originally posted by se7en


That's not really true, the stroke of an engine has little or no bearing on torque for a given capacity and state of tune.
It's an oft repeated old wives tale, because it appears to be true, i.e. long stroke engine often develop more torque than the equivalent short stroke, but the reason is simply because they are tuned to do this. Long stroke engines mean low revving engines due to piston acceleration and speed limitations, so the only way to make good power is to tune for high torque, i.e. very mild cams, small ports in head etc.



I don't really understand how this statement can be correct. Surely torque is completley dependant on stroke? The crank is like a lever, the longer the stroke the longer the lever.

Davie