Maximum torque vs maximum fuel efficiency

[samygero]

Hello,

I would like to ask a question about engines in general.

Manufacturers give the maximum torque point for all engines.

I was wondering first of all if this maximum torque point is the same as maximum fuel efficiency point.

Secondly is it also the point where for one specific gear we have maximum fuel savings? Personnally I think not. I think that if we take the graphic of Torque/rpm the maximum fuel savings point is the intersection between the curve and the tangent passing through the origine. Am I clear enough?

thx

[Jay2TheRescue]

Its walking a fine line, as the maximum efficiency for your engine is at a different RPM than the maximum efficiency for your transmission. Overall vehicle efficiency will be somewhere in the middle.

[theholycow]

Maximum torque RPM is not maximum efficiency RPM, except when it's a coincidence.

In my VW I've found that lower RPM is always equal to better fuel economy. The only exception is if I'm actually losing speed uphill; then I should downshift one gear. Surely other vehicles would vary, but I think that most people would get better fuel economy at lower RPM.

[Jay2TheRescue]

Yes. Transmissions are most efficient at low RPM's in high gears. The transmission usually trumps the engine as far as total vehicle efficiency is concerned.

[bobc455]

Originally Posted by theholycow

Maximum torque RPM is not maximum efficiency RPM, except when it's a coincidence.

There is "conventional wisdom" floating around that the engine's torque-peak RPM is the same as its RPM of maximum cruising efficiency. But like HC said, that's only true by coincidence.

I think there was more validity to that rule-of-thumb back when carburetors were prevalent, there is better technical justification for that IMO.

-Bob C.

[GasSavers_Pete]

For the engine alone on a test dyno YES the torque curve will mirror the BSFC curve so that at maximum torque the fuel consumed will be at the lowest point.
Any engine chart will show you this.

Pu the engine in a car and the whole picture changes as has been pointed out above.
Running an engine at the point of maximum torque will result in most cases with a fall in fuel economy.

Peter.

[theholycow]

Originally Posted by Pete

For the engine alone on a test dyno YES the torque curve will mirror the BSFC curve so that at maximum torque the fuel consumed will be at the lowest point.
Any engine chart will show you this.

Wouldn't even that only be when it's making that maximum torque, rather than merely the same RPM as maximum torque?

Engine efficiency is interesting, transmission efficiency is interesting, but if you're efficiently producing more work than you need then it's all wasted. Regardless of transmission efficiency, making maximum torque likely means you're producing way more power than you need, and then you'll just have to discard it with your brakes.

(Pete, I'm not arguing against your post, just starting there and running with it.)

[pgfpro]

What I have seen is the torque curve and or fuel curve will be the same on an engine dyno and chassis dyno. The leverage from the transmission adds torque multiplication so the torque read out is just higher.

I have, and several others have built fuel maps on chassis dyno's. On a inertia dyno you will have to do some fine tuning when your road testing but the fuel curve will stay the same. So all you do is +/- fuel across the board.

On a load type dyno you can simulate the weight of the car etc. But its a little more complicated then that.

1)I think the question is at lighter load does the torque curve drop to a better part of the island?

2)The other question is if you shift at a lower rpm before MBT will this save fuel based on the fact of only so much torque is needed to accelerate the vehicle to a certain speed?

Combine #1 and #2 together and you will have a much improved driving technique for saving fuel?

[theholycow]

Originally Posted by pgfpro

2)The other question is if you shift at a lower rpm before MBT will this save fuel based on the fact of only so much torque is needed to accelerate the vehicle to a certain speed?

That is one part of the concern. Another is when you're cruising steady, although I suspect that most people don't have tall enough gears to have to worry about it.

[IMG]http://i147.photobucket.com/albums/r312/rrussell_photo/Saturn_99_1_9l_dohc_bsfc.jpg[IMG]

If I'm reading that chart correctly, you have pretty steady maximum torque from 2500 to 5500 RPM and the best BSFC is from 1500 to 3500 RPM with heavy load...which puts best efficiency, on average, below torque peak - though BSFC has a spike at low load 2500 RPM (one of two torque peaks) where it may be most efficient to cruise if you need as much power as it's making.

[samygero]

Hello guys,

I searched in wikipedia and found out that BSFC is fuel consumption divided by Power produced. So is BSFC is low than fuel efficiency is high right?

Now about the graphic. I don not understand it completely. In what conditions can we situate our selfs on one iso-BSFC and not on another?

Also just for my general knowledge. Theoretically speaking BSFC should be the same from one gear to another at same RPM right?

What I was able to conclude (I think...) is that fuel efficiency is increasing while increasing RPM let's say until 3000 RPM.

So the ideal way of driving to achieve better fuel economy(with no traffic) with a car of 3000 RMP maximum torque (my case), would be to change gears at approx 2500 RPM (so I do not fall too low on the RPM of the higher gear), and run at highest gear possible (respecting speed limits) at 2000 RPM approx. If highest gear (fifth in my case) is ON then go untill 2500-3000 to achieve best BSFC (one highway for example). Is my developement correct?