Revs vs MPG

[McSam]

motco said:

You might well think so, but I regularly manoeuvre mine on idle relying solely on the anti-stall. Mine is not a common rail TDCi but a humble TDDi though.

Yeah, if you're gentle you can do that with the TDCis as well. What I meant was - I've just noticed it's not very well explained! - if you mess up a hill start or do something else that abruptly causes the revs to dip below the idle speed, by say 100-200rpm, you'll get a momentary massive rattle and then it'll stall very quickly. My petrol Audi, on the other hand, is quite smooth at very low revs and has recovered from 200rpm or even lower before . This makes me guess the Ford diesel engine is designed to stop itself running at anything much below idle to avoid vibration damage.

Wouldn't be surprised if an RX8 did 60mpg at a constant level 30mph in fifth, to be honest.

[otolith]

Meoricin said:

An RX8 that gets 60mpg?

That graph originally came from the owners club magazine, and was based on data collected using a CANScan device. They are constant speed readings on level ground, not achievable numbers in reality - you don't get many opportunities to complete an entire journey at a constant 30mph, fifth gear, warm engine, no junctions, speed bumps, etc. As soon as you throw any significant changes of speed or a cold engine in, you can forget it.

[Rollcage]

I'd say it's more to do with throttle opening than actual RPM........

On my normal commute to work I get 28mpg from my Mondeo, on a mixed route of 32 miles of NSL and D/C. Last couple of days I have tried a different route, avoiding the D/C and economy has gone up to 35mpg, and journey time has stayed the same. Thats pretty good for an ST24, and it wasn't achieved by driving like a granny.

[motco]

McSam said:

motco said:

You might well think so, but I regularly manoeuvre mine on idle relying solely on the anti-stall. Mine is not a common rail TDCi but a humble TDDi though.

Yeah, if you're gentle you can do that with the TDCis as well. What I meant was - I've just noticed it's not very well explained! - if you mess up a hill start or do something else that abruptly causes the revs to dip below the idle speed, by say 100-200rpm, you'll get a momentary massive rattle and then it'll stall very quickly. My petrol Audi, on the other hand, is quite smooth at very low revs and has recovered from 200rpm or even lower before . This makes me guess the Ford diesel engine is designed to stop itself running at anything much below idle to avoid vibration damage.

Wouldn't be surprised if an RX8 did 60mpg at a constant level 30mph in fifth, to be honest.

Ah yes, I see. I once had a 2.5 litre turbo-diesel Ford Granada (Scorpio type) with a Peugeot 505TD (I think) engine in it. That was almost impossible to stall even if you tried. I agree that the modern diesels will collapse if you abuse the torque on idle.