I was wondering if smaller (below 1.9 Litres) turbo diesel engines need to be driven harder to achieve better fuel economy.
I was chatting to a bloke down the pub the other night and he thinks that changing up each gear at 2.500 rpm is no good as he thinks at least 3.000 rpm to get the right revs for the next gear.
I was always led to believe that driving a diesel on torque and not revs was the best.
I.E change up at 2.500 rpm so that the next gear is at 2k rpm.
so what's best ?
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Depends on the engine's characteristics, the specific gear ratios etc.
For example, my Mondeo is geared so that changing up from 2nd / 3rd / 4th at 2,600 drops the revs back to 2,000, which is around peak torque.
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I agree, it depends on the engine.
I've had two near identical capacity (1.9 litre) diesels in succession and both were completely different.
Renault 1.9 dCi (120) - linear delivery which built progressively with revs. Happy to pull right to 4500 RPM, didn't like lugging below 1500 RPM at all, and would outright "strop" if asked to do so in anything other than first. Beautifully smooth engine.
VAG 1.9 TDI PD (130) - Massive bottom end and midrange stomp, tailing off sharply past 3500 RPM (fixed by remapping apparently). Can be driven quite briskly without ever exceeding 2500 RPM, and very briskly without going much past 3k. Not an engine that wants (or needs) to rev, even by diesel standards. Rough and ready feel compared to the super smooth Renault unit.
Outright performance pretty similar - Golf quicker due to a tad more power and a lighter shell, but really not much in it. VW engine about 20% more economical in equivalent driving conditions than the Renault engine. The Renault engine didn't have the sheer clout of the VW unit at the bottom, but pulled far harder in the upper reaches. Swings and roundabouts.
Both fantastic engines, just very different. Smaller capacity engines will vary in a similar way.
Edited by DP on 19/05/2009 at 10:24
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There was a gap between second and third on my brother's diesel Cee'd, so it was best to hang on in second an extra 500-750rpm to make up for it.
I've driven a diesel Panda a few times and tend to rev that a bit - characteristics of the engine as posted above.
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Small diesel = big diesel
The bigger 1.9 will have more (pre turbo boost) torque available at very low (sub 1,500) revs due to its cylinder capacity.
The smaller 1.3 diesel will have less as its cylinder capacity is smaller. Once turbo boost comes in though its a different story.
Change gear so you don`t fall off the peak torque - there`s nothing to be gained by revving a turbo diesel further - other than now and then to keep the engine free of soot.
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