Nerdy question for the academics..

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tomsheppard
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Joined: 19 Dec 2002, 15:46

Nerdy question for the academics..

Post by tomsheppard » 29 Oct 2003, 03:21

Off load, an engine burns least at idle. On load, am I right in thinking that, given more torque, fewer horsepower are used to move the load? If that is true, the body of the car imposes the least loading on the engine at peak torque (ignoring aerodynamics just for the moment!) Is this also the point of lowest fuel consumption? I can't quite see why not, but I am not entirely sure! Can anybody enlighten me, please?

Jon

Post by Jon » 29 Oct 2003, 03:52

Um, er thanks Tom. I will of course attempt to answer your post.
On Friday night when I come in from a night at the Pub. [:D]
But for now I would think that optimum fuel economy would of course be obtained once the car is rolling, and held at a certain speed on a light throttle. That is, with barely any load on the engine, and using low gearing and of course higher torque. In which case, the lower the throttle opening can move the load (the car) in the most economical manner.
This is why some amazing "constant 56mph" figures used to be branded about in the old Govt MPG figures.In reality, and in "real world" conditions, its simply not possible to sit at the sort of speed which require minimum and steady throttle. Either you would be going too slow, or else you have to use the throttle to accelerate past other slower moving traffic. Unless of course you fancy the M25 at a constant 56mph with hordes of maniacs bearing down on you.
I tried this once as the "official" figures state that my ZX TD could obtain 62MPG at a constant 56MPH. I was on my way to the Lakes to meet some friends and was in absolutely no rush. After one junction on the M5 I was so frightened by lorries trying to push me from the inside lane that I fired the turbo up and drove at around 75MPH and still did Swindon to Junc 40 of the M6 and back on the on tank.
Best economy I have ever achieved was in a BX 19D on a light throttle coming back from Gibralter. [;)]

RichardW
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Post by RichardW » 29 Oct 2003, 18:54

Tom,
Horsepower and torque are inextricably linked: HP = torque x rpm / 5250. At a given speed, on a level road, the amount of power required to maintain speed is constant, and does not therefore vary with engine speed. However, the specific fuel consumption of the engine does vary (eg kg of fuel per horsepower) with varying revs. I know that (in a diesel engine anyway!) min specific fuel consumption equates with maximum torque (but I have not managed to find an explanation for this I understand!), so for maxiumum economy at a given speed you should be aiming at the torque peak. For XUD units this is around 2000 rpm, and you therefore use LESS fuel if you drive at 40mph in 4th (2000 rpm) than you do if drive in 5th (1600 rpm) - this effect is marked enough to show up on the fuel computer on my mate's A4 TDi 130.
Richard

tomsheppard
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Post by tomsheppard » 30 Oct 2003, 03:08

Thanks, What units are the torque in?

RichardW
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Post by RichardW » 30 Oct 2003, 13:52

Torque units are force length eg lbf ft in imperial and N m in metric

mbunting
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Post by mbunting » 06 Nov 2003, 15:53

So, to revive this thread:
1) Why is maximum torque of an engine at x revs, and not y revs ?
2) Presumably the bore/stroke has an effect on this, so increasing the bore would increase torque ?

andycarter
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Post by andycarter » 07 Nov 2003, 00:38

Just going back to the question about units, if you're using the factor 5250 then torque must be in lbf.ft