Power Vs. Torque Question

[astonmartinandy]

I appreciate that torque is effectively the turning force of the engine which is generated just after the fuel/air mix is detonated, but what I don't understand is what is going on with all the current diesels you can buy, E.g. the VW V10 diesel.

Why is it that diesel cars are able to produce so much more torque than their petrol counterparts and still have lower emissions and less bhp? Could somebody also please explain how it is possible to adjust the torque curves by an engine so that you have bucket-fulls of torque lower down the rev-range to make a car more usable, but then still have a bhp peak much higher in the rev range?

Your help with this is much appreciated..

[maalox]

A diesel engine's torque advantage results from two factors:
1) the burn duration during the power stroke is longer in a diesel than in an otto (i.e. gasoline) engine
2) diesels are more often long stroke designs

The lower bhp is partly a result of the lower redlines on diesel engines (linked with the longer stroke). As for emissions, altough diesels have better mpg figures, in terms of particulate emissions they are still worse than petrol counterparts.

[veilsidebr]

Diesel engines have more compression ratios than ottos engine. This is one of the factors. What increases torque, in any engine, is the bore. How more bore you have on an engine, how torque you will have, because the area of explosion would be higher. Less bore and much stroke increase the revs of a car.

:D

[astonmartinandy]

So why do they not just make the petrol equivalents with a longer stroke to get the power of the petrol, but torque of the diesel?

[coombsie66]

Diesel engines have more compression ratios than ottos engine. This is one of the factors. What increases torque, in any engine, is the bore. How more bore you have on an engine, how torque you will have, because the area of explosion would be higher. Less bore and much stroke increase the revs of a car.

:D

Wrong way round i think mate, larger stroke more torque and revs less freely. As the con rod has a larger moment about which to turn the crank with a larger stroke. But there is a higher rotating inertia in the bottom end then , so it doesnt rev as freely.
Power = torque x angular velocity, so due to diesels not reving high, due to the high compression ratio and no real need to, the bhp figures are low.
As was stated before, the combustion in a diesel cycle is assumed to take place under constant pressure, and is ignited by detonation. So the force downwards takes place over a longer period of time. Where as the otto (petrol) cycle is not considered to be constant pressure combustion, but constant volume, so the combustion is more or less immediate.
If u ask again in a years time i mite b able to b of more help, after ive started the specialist course on combustion!!
cheers

[coloradosilver]

Veilsidbr is right on. It's more about the compression ratios. A diesel engine has no spark plugs, it relys on sheer compression to generate the heat needed to combust the diesel fuel. This gives a much more powerful stroke. Diesel fuel also will take much more pressure and heat to combust than normal unleaded. However, once it combusts, it goes with like 10x the BTU of normal gasoline. This is what give it all that power.

[zevolv]

Diesel engines create more torque because they are a long blocks not short blocks since Torque=force times distance and power is force times distance times time if you have a longer distance but you use the same amount of force you will create more torque. the explosion in the combustion chamber is what creates the power not the torque and the more power pushing on the crank which is longer equals more torque but takes up power to create. That is the reason that powerful diesels use turbochargers to push more HOT air into the engine since there is no spark plug creating the explosion but it's the pressure creating heat that ignites it. And to create so much pressure the area inside the diesel has compression ratios of 16:1 to 21:1 instead of 8:1 to 11:1. And Diesel engines are Otto cycle engines all four strokes are.

[coombsie66]

They dont force HOT air in mate, cus thats detrimental to power. Hence why they have intecoolers as well as turbos!
Yes the combustion creates power downwards on the piston, but this exerts a torque on the crank, which is what is harnessed from the engine. And when you dyno an engine it is the torque that is measured, then the power is calculated from that.
In our thermodynamics course, otto cycle is the name given to the petrol engine cycle, and diesel cycle the name to the diesel engine. Not all 4 strokes, as thermodynamically they are treated differently.

[zevolv]

they don't intercool Turbo Diesel's since Diesels are ignited using heat and not a spark that is why they use hotter air than normal cars use. You will never see an Intercooler on a Turbo Diesel since it would defeat the purpose of creating heat in the combustion chamber. you will learn this once you take your combustion class I've already taken engine specialist classes. Diesels are the easiest engine to ever work on since there isn't any complex ignition system and the engines are built so much stronger than most.

[coombsie66]

U r wrong, get ur facts straight. Have u owned a diesel?? Cus i have and we put a larger intecooler on it, so that you can therefore burn more fuel, as there is more air in the inlet charge.

[deth]

coombsies got it down pat...studying this exact stuff in my thermo II class now. the large intercooler allows the air to become more dense...more dense=more air/m^3=bigger boom!

[coombsie66]

Im sorry, i just read the whole of ur post again. R u taking the piss!!??? Which 'engine specialist' classes did you take exactly?? Early learning center big book of engines?

[deth]

talking to me?

[coombsie66]

nah mate, the bloke who got it wrong!!!!! U r exactly right!! Cheers 4 backing me up!

[deth]

alright here's deth 'the peace keeper' with the technical answer

http://www.freepixhosting.com/uploadedfiles/page1.jpg
http://www.freepixhosting.com/uploadedfiles/page2.jpg

[coombsie66]

lol, nice, pretty much what we have been learning in thermo classes last year. U study mechanical engineering 2??

[deth]

yup 3rd year, u? 4th i'm guessing?

[coombsie66]

nah, im 2nd year MEng degree at imperial college london. They work us hard!!!

[deth]

hahaha it seems so..but then again thermo isnt my concentration...i'm doing mechatronics, u?

[coombsie66]

We aint specialised yet, start that next year, ive got a nice mechatronics 2-3 hour lab 2morrow evening!!!! Oh yippee!!! But i only find i can really get into the applicable parts of the subject.

[antonioledesma]

OOouch! you didn't have to put the Cengel thermodynamics book to explain the diesel cycle

maybe a little more... summarized, but and excellent book to explain it!

i'll go and check it a while

and for the record, a mechanical and electrical engineering, 8th semester of nine. in december i get out!

thanks deth

[astonmartinandy]

Well thanks alot guys - I had no idea we had so many experts on combustion here! Now all you have to do is learn all the formulae and recite them infront of your friends at the pub!! haha

[coombsie66]

Yeah, although i recon the exams r gunna b a bit more important than earning brownie points down the local boozer!!! :wink:

[zevolv]

U r wrong, get ur facts straight. Have u owned a diesel?? Cus i have and we put a larger intecooler on it, so that you can therefore burn more fuel, as there is more air in the inlet charge.

Yes we own a Turbo Diesel F-350 (it's a big ass truck for all you european people) and Heat is what ignites the Diesel fuel. Cold is the enemy of diesel diesel fuel gets thick and sloshy when it gets really cold outside.

[coombsie66]

I sugest u go and get 'Cengal & Boyles' book on thermodynamics, and get reading!!
U will probably find ur 'big ass truck' has a 'big ass intecooler' as well, as the air from the turbo is a few hundred degrees centegrade after compression. U cant burn shit in that! So cool the air, it becomes denser, therefore u can burn a larger amount of fuel in it, and you get more power.
Intecoolers cool the charge up to 40% HUGELY increasing both the efficiency and power of the engine.
I cant b arsed to explain ny more, just go and read up on it urself, and stop patronising me!
Oh yeah and im pretty sure that diesel doesnt become 'sludgy' untill about 50 degrees below zero!!!!!!!!