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Power torque and short runs - ernest


Hi all

If a 100hp petrol engine gives 92 Lb.ft torque and a 1.9 TDI
gives 100hp but 176 Lb.ft of torque. They both produce the same power but one gives more torque why?
I thought power was i.e the ability to do work. confused over this.

What is the definition of a short run one that does not get the engine up to normal operating temp?

Bye for now
Diesels - David Lacey
TORQUE & SPECIFIC FUEL CONSUMPTION - That is precisely why trucks/boats/construction machinery/generators/static pumps use the Diesel engine. It's inherent lugging ability (low down torque capability) is suited to many applications - how many trucks/earthmovers do you see with petrol engines?
Re: Diesels - steve paterson
Don't think you really answered the question David. Say a petrol engine is doing 6000rpm and producing 100bhp. Why does it's 'twisting' power increase at lower revs when it's producing less bhp ?
Re: Diesels - here goes! (VW) - footsfitter
different makes of engines tend to produce different torque outputs for similar hp outputs, depends on what the designers like to do, but if you slap a turbo on either petrol or diesel engines then you can be sure that the "power band" will be much shorter & when it peaks! it peaks!
for reference, things like combines tend to run at fixed speeds because they need to be at a constant rpm for driving what they are meant to be doing, i/e threshing grain, therefore they tend to use hydrostatic (variable-hydraulic) transmissions or if its an old-un then they might use variable speed pulleys & belts similar to the old orginal 300 series daf variomatic drive belts!
Diesels - here goes! (VW) - Guy Lacey
Dave Lacey never answers the question.

I'm no expert but I'll give it a go and wait for the experts from XYZ Polytechnic University to tell me I'm wrong! :-)

Diesels produce more torque as a result of the slower burn of the fuel and the much (4 x) higher compression ratio needed. Petrol 5:1 and diesel near 20:1.

Diesels have a peak power band that is very narrow when compared to petrol and this is, usuall, at much lower revs, say, 3,500rpm. This is why many new diesels have 6-speed boxes. Also, if you look at large plant diesels, such as a Combine Harvester, they operate at one set rev for all manouvres - there is no rev range to run through.

Diesels cannot rev as high as a petrol due to the slow burn and in the case of early engines, i.e. *not* direct injection, the rev range is even lower.

Only the *real* innovators in engine design can give petrol power and diesel torque - VW for example. They are the only manufacturer I am aware of that rallies a diesel (Golf TDI) and circuit races a diesel (Bora TDI).
Re: Diesels - here goes! (VW) - Dave
Guy Lacey wrote:

> VW for example. They are the only
> manufacturer I am aware of that rallies a diesel (Golf TDI)
> and circuit races a diesel (Bora TDI).

I was a Marshall in the last network Q. (A forest bit in wales 'Marshams' or summit like that)

I thought the VW's were crap.

If they're so bloody good when are we going to see a Diesel entry in formula 1, F3, Formula 3000, WSB, BSB????

Just ensuring this thread runs...
Re: Diesels - here goes! (VW) - David Butler
Power is defined as the rate of work and is measured by the amount of work expended in a given period of time. Torque isa measure of the moment an engine can continuously exert. It,s not unusual in agricultural diesels to have 2 engines with the same horse power rating but totally different torque ratings.I will try to get back with an example later. Dave .
Re: Diesels - here goes! (VW) - Alex. L. Dick
Or, twice the torque, half the revs, same power. You'll need to gear up double!
Guy got it wrong - David Lacey
Guy

Now sit down and listen

A petrol engine with a compression ratio of 5:1 as you state probably would not start.
Petrol engines typically run a compression ratio of about 11:1, some more, others less

No rgds

David
Re: Power torque and short runs - Cliff Pope
Someone said a petrol engine with a CR of 5:1 probably would not start. Tractors from the early 50's had very low CRs, 5:1 or lower, to run on TVO or even Lamp oil.
My 1949 Ferguson starts perfectly.
Cliff Pope
Re: Power torque and short runs - John Slaughter
Saturday's motoring Telegraph had an article about a replica Ford car with a 3:1 compression ratio. With the right mixture, they will start, but don't expect power or economy!

Cheers

John
Re: Guy got it wrong - Chris
He was wrong about VW as well. BMW raced a diesel in 24 hour race in Germany a couple of years back - and won.

Chris
Re: Guy got it wrong - Ben Lacey
David Lacey said:
"Guy
Now sit down and listen
A petrol engine with a compression ratio of 5:1 as you state probably would not start.
Petrol engines typically run a compression ratio of about 11:1, some more, others less"

And Chris said:
"He was wrong about VW as well. BMW raced a diesel in 24 hour race in Germany a couple of years back - and won."

So, in summation, Guy 'I went to Kings' Lacey made no correct and/or valid points in his post. Hahahahahahhahaha.

Ben 'Bristol Polytechnic' Lacey

PS. David, why are you online posting here at 10 to 7 in the morning? Harriet keeping you up lately?
I got some of it right! - Guy Lacey
I got the jist of it!

Never was meant to be a grease monkey anyway..........
Power/torque - Engineer please! - David Woollard
It seems we all know what effect power and torque have on the performance of a car, and some make attempts to explain.

But actually the original question still stands and I can't put the answer into words either.

If torque is a twisting force at 3000rpm on a particular engine what is maximum power that may be at 6000rpm? If power isn't a twisting force (which is after all what propels the car) what is it?

David
Re: Power/torque - Engineer please! - John Slaughter
David

Engineer at your service!

I'm going to use imperial units here. For any engine the HP at any point can be calculated from:

HP = 2πNT/33000

Where N = rev/min

T = torque in lb-ft

Torque is a function of the pressure in the combustion chamber and the physical dimensions of the engine components.

As you can see power is a direct function of both torque and rev/min. Engines usually have their torque peak below max power, usually due to the fact that the cylinders don't fill so efficiently at higher speeds. However, this doesn't mean that max. torque = maximum power. Above the maximum torque point if, say, torque drops by 20% whilst the engine speed increases by 30%, then at that raised speed, the HP will be greater.

So depending upon the particular engine characteristics, it is perfectly feasible for two engines to have identical maximum power outputs, but very different maximum torque figures.

Diesels tend to run at greater combustion pressures, due to their not being throttle governed, and so tend to have higher torque outputs. However, their torque curve tends to be very 'peaky', so the power drops off at higher revs.

Hope this all makes sense (and I didn't get the text book out once!).

Regards

John
Re: Power/torque - Engineer please! - John Slaughter
In case it's unclear from the font the symbol after '2' in the equation is pi

Regards

john
Re: Power/torque - Engineer please! - Ian Cook
John

You beat me to this one - our server went down so I couldn't get to it. You are spot on with the formula, of course.

Torque is related to BMEP (brake mean effective pressure), this being an average value of the pressure in the cylinder. At any one instant the pressure in a cylinder will vary, depending on the combustion cycle. Torque is related to BMEP by the bore and stroke of the engine. For an engine to produce more torque (at the same RPM, same bore, and same stroke) it has a higher BMEP, and this is the case with a diesel at lower RPM.

Diesel fuel burns more slowly than petrol, and this is what limits the maximum useful RPM of a diesel engine - high pressure common rail direct injection allows a little higher RPM than older indirect injection. Petrol engines have much less restriction on RPM, as witness F1 grand pix engines that typically rev to 18,000RPM, and can produce higher horsepower but not necessarily higher torque.

I hope my small addition helps (or confuses!)
Re: Power torque and short runs - Dave
As a rule of thumb horsepower is Torque multiplied by revs.

This is why diesels are so gutless - they just won't rev.

The sole determinant of torque at the rear wheels is the gear you're in.

HTH.
Re: Power torque and short runs - Chris
Isn't this to do with the different kinds of combustion?

Petrol engine combustion occurs at constant volume (i.e. as near as possible to the the split second when the piston is at the top of the stroke), and after that the piston is pushed down *only* by the expanding hot gases. Diesel combustion occurs at constant pressure (i.e. throughout the stroke), so the piston is pushed down by the constantly burning fuel. Hence the diesel stroke produces equal pushing force halfway down the stroke as it does at the start. The petrol stroke gets weaker as the piston gets nearer the bottom. And that is why the diesel produces greater twisting force (torque); there is always pressure on the piston.

Chris
Re: Power torque and short runs - Alex. L. Dick
Chris wrote:
>
> Isn't this to do with the different kinds of combustion?
>
> Petrol engine combustion occurs at constant volume (i.e. as
> near as possible to the the split second when the piston is
> at the top of the stroke), and after that the piston is
> pushed down *only* by the expanding hot gases. Diesel
> combustion occurs at constant pressure (i.e. throughout the
> stroke), so the piston is pushed down by the constantly
> burning fuel. Hence the diesel stroke produces equal pushing
> force halfway down the stroke as it does at the start. The
> petrol stroke gets weaker as the piston gets nearer the
> bottom. And that is why the diesel produces greater twisting
> force (torque); there is always pressure on the piston.
>
> Chris

True for low speed diesels. High speed, like automotive, diesels give an indicator diagram much nearer to that of a petrol engine. I think a discourse on thermodyamics is inappropriate here and besides you forget an awful lot in half a century!

Cheers, Alex
Re: Power torque and short runs - John Slaughter
Alex

Yes, you're absolutely right here!

Regards

john
Re: Power torque and short runs - Chris
Alex. L. Dick wrote:

> True for low speed diesels. High speed, like automotive,
> diesels give an indicator diagram much nearer to that of a
> petrol engine.

Presumably this is why diesels as a rule don't perform well at high revs - the combustion process just isn't quick enough (I realise that's simplistic, but as you say, this is not the place for thermodynamics). Incidentally, is this also the reason why petrol cylinders become progressively less efficient beyond about 500cc, whereas diesel cylinders can be as big as you can build them (but don't work well below about 300cc)?

Chris (not an engineer but an interested amateur)
Re: Power torque and short runs - John Slaughter
Chris

Yes, I think the issue is one of gas flow rates, and the time needed to scavenge and fill the cylinders. So, the larger the cylinders, the lower peak revs will tend to be. Classic example is marine engines - pistons the size of dutbins, but probably <200rpm

cheers

john
Re: Diesels - torque and power curves - Mike Humpherson
I have seen torque and power data for a couple of car diesel engines (older VW and Peugeot 1.9 turbos), and the thing that struck me was that the POWER remains remarkably constant between max torque RPM and max power RPM.

This gives the slightly unexpected result that if you stay within that range of RPM, it doesn't matter what gear you are in, the torque AT THE WHEELS will be the same!! However, they are presumably burning more fuel at higher RPM, so the efficiency must be dropping steadily with RPM.

Does anyone have access to curves for any other engines to see if this is generally true?

What is the thermodynamic explanation for this phenomenon.

Mike Humpherson.
Re: Diesels - torque and power curves - ernest
Well you where all good if some of you got
off topic. However I will have to read them all
again so it all makes sense.
But what about sort runs guys?
Ernest a new Fabia TDI estate owner
the first diesel i have owned. Great fuel
economy get 475 to the tank and 275 on
my previous Fiesta, same size tank.
Re: Diesels - torque and power curves - Chris
Your average engine takes between ten and twenty minutes to really warm up (not just the coolant) depending on the kind of driving it's doing and the ambient air temperature. Quite a lot of cars never make it in their whole lives, I suspect. Diesels are good for short runs - no catalyst to knacker (on most of them), no plugs to collect filth, and the economy isn't really affected much by running cold. BUT it might get smoky, and will definitely need frequent oil and filter changes (3-3.5k miles) if all it does is go down the shops. To cure the smoke situation give it thirty miles on the motorway every now and again, and especially before an emissions test once it needs MOTs.

Best thing for really short runs (a couple of miles or so) is the bicycle. For 100 quid you can get a trailer that will carry all your shopping - probably not far off the size of a Fabia's boot, in fact.

Chris
Re: Diesels - torque and power curves - John Kenyon
ernest wrote:
>
> Well you where all good if some of you got
> off topic. However I will have to read them all
> again so it all makes sense.
> But what about sort runs guys?
> Ernest a new Fabia TDI estate owner
> the first diesel i have owned. Great fuel
> economy get 475 to the tank and 275 on
> my previous Fiesta, same size tank.

You don't quote any engine sizes, but from my experience
moving from a 1.3l petrol to to a 1.9l Tdiesel initially resulted in
vastly improved fuel consumption down to the fact that I wasn't
caning the a**e out of my car to get it up to speed.

I would reconsider your position once the novelty factor has worn
off.

To give you an idea:

1.3 Escort - 30-35mpg
1.9TD 306 - 45-55mpg (first few months)
1.9TD 306 - 35-40mpg (Now)

(Figures are for "open road" driving, with next to no urban use)

/John
 

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