Fuel economy optimum cruise

[mejermy]

Following on from the 220cdi fuel con thread, *** suggests keeping a diesel under 2200rpm for best economy (thx ***). Seems about right, but I had always understood that for pure economy the best speed was the lowest you could run at in top gear. If you actually want to get anywhere - i.e. time is a factor - then the best cruise speed was at the peek of the torque curve (although this may attract speed related profile and induced drag penalties). I have come across a graph on this site - Powertrain Performance Graphs - Rototest Research Institute - (my car is a 2004 S211 220cdi auto). I expect then that 2500 rpm is about the pace. Does this make sense? I will be doing a long run tomorrow in conditions which should allow extensive constant speed running and hope to try this out.

 

[MB-BTurbo]

No idea on the diesel front but my petrol turbo gives 45mpg when driving at 40mph in 6th. Any lower and I have to change gear. So as you say the lowest speed without labouring the engine for me.

 

[Dieselman]

With a direct injection diesel, the lowest constant speed over about 1,500 rpm should give the best mpg figure.

 

[st4]

With a direct injection diesel, the lowest constant speed over about 1,500 rpm should give the best mpg figure.

So in a 7g merc in 7th 60mph or 55 in 6th. Sounds good to me

 

[Dieselman]

You need to be in top gear for the best economy unless driving uphill.

 

[Bellow]

Page #6 here >> http://www.motor-talk.de/forum/aktion/Attachment.html?attachmentId=675478

Contours map of SFC for 1.5 litre 3cyl OM639 as fitted to smart forfours.
Scale as 3/4 of a 2.0 litre four, 1/2 a 3.0 litre V6 etc.

What surprised me was how much grunt they are delivering at BSFC. It's at low loads that it falls away.

 

[DSM10000]

With a direct injection diesel, the lowest constant speed over about 1,500 rpm should give the best mpg figure.

I have read that for peak efficiency you should be driving at an engine speed that matches the torque peak is this correct? Of course most diesel engines have a fairly flat curve that peaks relatively early.

 

[Bellow]

I have read that for peak efficiency you should be driving at an engine speed that matches the torque peak is this correct? Of course most diesel engines have a fairly flat curve that peaks relatively early.

For best specific fuel consumption (BSFC) which is the least fuel for the most hp, the engine has to be run at peak torque - not merely the same rpm that peak torque occurs. Foot to the floor then. But that's more hp than required (unless you're in some kind of hurry that is).

For a Diesel though, your statement will be more truthful than for a petrol engine.

 

[st13phil]

For best specific fuel consumption (BSFC) which is the least fuel for the most hp, the engine has to be run at peak torque - not merely the same rpm that peak torque occurs.

And that's why the question "will my car consume less fuel if driven at the quoted peak torque rpm" is never possible to answer. Also, unless someone is just going to drone along a flat section of road at a constant speed, the driver's use of acceleration and braking will have a more significant effect on fuel consumption than their choice of engine RPM.

My experience of the 220CDI engine coupled to an autobox is that best economy is achieved not by using very gentle acceleration, but by accelerating to the speed desired fairly briskly (though not using full throttle) and anticipating the need to slow down and coming off the throttle early rather than using the brakes excessively. As far as motorway cruising speed is concerned, around 60-65mph tended to give the best compromise of journey time and fuel economy - probably partly because it was easier to maintain a constant speed.

 

[mejermy]

Thanks for your views everyone. Some new info for me to digest there. The ability to cruise at a steady speed in much of Europe is why I felt this useful. Latest data shows 978km run up France at a steady 135kph indicated (130true) - 2600rpm on the flat (auto) - and 38.30 mpg computer, 36.70 brim to brim. OAT 20c-30c. The car is decatted and I'm adding 300ml of 2 stroke to each tank, which appears to have had a beneficial effect. Unimpressed with my results, although there was a strong headwind for the first 150ish km, and some have suggested this has an effect even on cars. Only other excuse is that we are heavily laden and she's done 148000 miles. The quest continues.

 

[mejermy]

Forgot to add it's also EGR shunted as per A Crow's CDI3 whizzardry.