Diesel economy - peak torque or lowest RPM?

[BigToe]

This one has confused me for a while and I haven't managed to find any decent info on it.

I was under the impression that an engine is most efficient at peak torque - creating the most force (not work) for a given amount of fuel. Peak torque on my dCi runs at around 2,000rpm. In 3rd gear, this is 30mph. Am I better sitting in 3rd at this speed, or 4th (or 5th?) where the rpm is below turbo boost? Is it possible to actually use more fuel in these higher gears as the engine struggles to produce the force needed? At first I was inclined to think the taller gear would always make for best economy when comparing exact speeds, but I'm not so sure now.

Lastly, while I'm here I may as well ask this: why is it that lots of people insist that to get the best acceleration/performance out of a diesel you have to "keep it on the boil" ie. around maximum torque? Is it not true that peak acceleration for any given road speed occurs towards peak power (where the engine is doing the most work), whilst peak torque just provides the greatest acceleration for a particular gear? Which would mean you basically redline a diesel like you would a petrol when pushing to absolute limits. I haven't done any timing, but I'm sure my car accelerates faster when pushed to the limit rather than using the torque. I had a friend who visited one of these advanced driver courses with his HDi, and the instructor told him he could drive faster if he changed gear at 3k

 

[Jonny69]

So what you ideally want to do is put a flow rate meter on your fuel line and do some simple calculations in various gears at a fixed speed on the same bit of road. Does your car not have a gauge that reads out instantaneous fuel economy? I thought all cars had that these days?

 

[Katoom]

Engines are more efficeint at peak torque, i think that's why top gear at 70mph is usually at that figure, 1900rpm in 6th in my golf TDI, about 3500rpm in the 306 1.4 petrol.
The engine is under quite some load at this speed, pushing against the wind, but at 30mph, there is hardly any load on the engine, so you should be better of using a higher gear to keep the revs lower and using less fuel.
But if your going up hill at 30mph, the load would increase and by being under the peak torque, you would then use more fuel.

 

[BigToe]

So what you ideally want to do is put a flow rate meter on your fuel line and do some simple calculations in various gears at a fixed speed on the same bit of road. Does your car not have a gauge that reads out instantaneous fuel economy? I thought all cars had that these days?

Mine doesn't unfortunately. It's a MK2 facelift Clio Van and doesn't have many of the extras. It does have alloys and a sport pack body kit though

Engines are more efficeint at peak torque, i think that's why top gear at 70mph is usually at that figure, 1900rpm in 6th in my golf TDI, about 3500rpm in the 306 1.4 petrol.
The engine is under quite some load at this speed, pushing against the wind, but at 30mph, there is hardly any load on the engine, so you should be better of using a higher gear to keep the revs lower and using less fuel.
But if your going up hill at 30mph, the load would increase and by being under the peak torque, you would then use more fuel.

So when loaded and outside of boost, the ECU determines that more fuel needs to be injected to create the force? Or is it much simpler than that?

 

[Katoom]

Yes basically, put your foot down, more fuel is injected (increasing the torque output of the engine) and it will build up speed. Unless the load is to great, then you have to change down.

 

[Dogbreath]

Lastly, while I'm here I may as well ask this: why is it that lots of people insist that to get the best acceleration/performance out of a diesel you have to "keep it on the boil" ie. around maximum torque? Is it not true that peak acceleration for any given road speed occurs towards peak power (where the engine is doing the most work), whilst peak torque just provides the greatest acceleration for a particular gear?

Peak torque at the flywheel will also give peak torque at the wheels in any gear. Torque is what provides the froce to accelerate the car, so maximum acceleration will be at peak torque. However, the gearbox is a torque converter, it can trade RPM for more torque so in any given gear you will usualy get more torque at the wheels by dropping down a gear and moving the engine towards it's peak power RPM.

 

[triggerthat]

and the instructor told him he could drive faster if he changed gear at 3k

Generally the power goes after 3K so there's no point in reving the nuts of it. Some diesels have power all the way upto the red line (335D).

 

[BigToe]

Generally the power goes after 3K so there's no point in reving the nuts of it. Some diesels have power all the way upto the red line (335D).

I understand what you are getting at there, but the way I saw it was that it's the torque that dies out after around 3K, whilst the power continues to climb (albeit slower) until around 4K. Unless you have one of those beautiful twin-turbo engines such as the BMW you mentioned where the torque is plentiful throughout the range. Which I will own one day, oh yes - it will be mine etc...

Therefore, surely you must get the most absolute performance by taking the engine to peak power (or past it so the next gear change brings you close again), even in a diesel? If I'm correct, why would people prefer to stay in boost when driving spirited?