XM versus Xantia fuel economy

[HST]

One reason for diesel efficiency is that they don't have a "throttle" at all. The have an accelerator! A throttle being a valve in the inlet restricting air input as fitted to a petrol engine. Diesel engines are always at full throttle i.e. wide open, but have a control to vary the amount of fuel injected: the accelerator pedal. Calling it a throttle is a misnomer.

Because the inlet is unrestricted at low accelerator positions the engine doesn't have to work so hard sucking air in. Result - higher efficiency at low "throttle" positions than a petrol engine which is restricted by its throttle.

With foot to the floor, diesel and petrol engines are much more similar in efficiency because both are wide open. Owners of diesel BMWs have found this to there cost when they thoght they could have it both ways. Drive a powerful diesel foot to the floor all the time and your mpg plummets to near petrol levels!

In terms of energy in vs energy out, running at high power outputs may be more efficient, but this doesn't translate into being more economic in the real world because for any decent sized engines this would equate to strong acceleration or very high speeds. The reason very small cars are more economical is because their little engines are working harder, in the more efficient range. For a given car, working the engine harder will result in lower economy because you are doing more work, beyond any efficiency gains & are working against aerodynamics.

[Peter.N.]

Kowalski

Thanks for the graph but I dont really understand some of the abbrevations. This is an engine in a more or less constant speed application, so we are not really looking at the same parameters. If you look at the torque curve of the XUD engine, maximum efficiency is achieved at around 2000 rpm, past this figure the torque drops off as the fuel consumption rises and the faster it goes the less efficient it gets. At full throttle diesel engines, especially older ones, start to smoke, indicating reduced efficiency, because the ammount of fuel being injected is becoming more than the available air can burn, they could be set up not to, but that would seriously reduce the maximum power available.

Hst. No, I know they dont have a 'throttle' in the true sense of the word, but you have to call it something! Gas pedal would be slightly more appropriate but still not accurate. How about 'foot operated fuel oil control valve'?

[Kowalski]

Kowalski

Thanks for the graph but I dont really understand some of the abbrevations. This is an engine in a more or less constant speed application, so we are not really looking at the same parameters. If you look at the torque curve of the XUD engine, maximum efficiency is achieved at around 2000 rpm, past this figure the torque drops off as the fuel consumption rises and the faster it goes the less efficient it gets. At full throttle diesel engines, especially older ones, start to smoke, indicating reduced efficiency, because the ammount of fuel being injected is becoming more than the available air can burn, they could be set up not to, but that would seriously reduce the maximum power available.

I do know its not the best of graphs, I did have some better ones bookmarked but the links are no longer working. It took a lot of googling for find them too. The majority of the graphs I found were for lower reving engines, i.e. industrial, agricultural, truck, but there was the odd car one.

I have been one to experiment with the old Xantia and its fuel economy, and I've done some fairly long term experiments (changing my driving style and maintaining it as consistently as possble for 6 weeks...). My latest experiment consited of being gentle with the accelarator and avoiding using the turbo, avoiding the turbo without actually driving any more slowly was worth about 1.5 mpg, thats not really enough to be anything more than a statistical blip.

Hst. No, I know they dont have a 'throttle' in the true sense of the word, but you have to call it something! Gas pedal would be slightly more appropriate but still not accurate. How about 'foot operated fuel oil control valve'?

To be strictly correct, a diesel with mechanical injection pump has a "accelarator" pedal.

I think gone a little bit off topic again...

[Peter.N.]

Yes, I agree it is going a bit of the topic, so these are my last words on the subject.

Accelerator pedal...thats what it always used to be called.

Mpg... I can only say that with my style of driving, gentle, rural and rarely foot to the floor, my XM averages 40 mpg + with a best of around 55, you dont get that with your foot down hard!

[BonceChops]

How about 'foot operated fuel oil control valve'?

I think mine operates a potentiometer so I think I will call it the 'Potty Pedal'

[Peter.N.]

Oh! I had forgotten about those...Well, in your case 'remote electromechanically operated fuel control valve'