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Old 03-30-2012, 09:08 PM   #11 (permalink)
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Quote:
Originally Posted by serialk11r View Post
Ah I see you're talking about a turbo diesel. I'm not too clear on how different a diesel's powerband looks, but if you shift down, it doesn't hurt economy as long as you're operating the engine near peak efficiency right? Shifting down reduces the load requirement anyways.

I guess the turbocharger + diesel combination changes the rules a little since diesels pass a larger volume of air through at low load than gas engines, and would build up boost fairly quickly. I suppose I didn't think of the fact that higher rpm on a diesel means a lot more gas going through the turbine.
It can be sometimes very confusing with our large variety of vehicles

I think that lower gear means less speed, which in turn gives less distance for similar rpm and load, thus it could be same consumption if larger wheels would give same rpm and load with 4th gear than smaller wheels with 5th gear, but it would mean that speed should be relatively similar too, I guess.

With my case there is only difference of bit over 100rpm with tires, but it is enough to cause too much load on steep hills.

Going bit faster and pushing it more would give more power, but with price of boost and very much higher consumption.

I think that petrol cars get away bit better with this, also NA diesels to some extent as higher rpm is not causing boost which is reason for high increase of consumption. Old tech turbo diesel is bit problematic in there, all mechanical so boost enrichment is made to be safe for all situations, so it is relatively lot of enrichment.

I have also observed bit of same with modern diesel with fuel computer onboard, even engine has low end torque quite a bit, using that torque puts really huge numbers to screen, better use lower loads and more rpm.

So I try to drive so that I don't need to shift down, which seem to be working well, only few hills where I need to shift down as I also try to avoid using high loads, makes driving rather interesting with only 400rpm to play with

Example of boost up would be like this; I drive at 0 boost on 5th gear, I shift 4th, continue at same speed and I have 0.2bar boost, if I accelerate even little it climbs easily up from there and this is not even VNT which would boost much earlier. 2500rpm is boosting 0.1bar when engine braking. That is indeed because diesel has no throttle butterfly, it is wide open constantly.

One thing this topic made me think is that it is really different techniques that different vehicles require and it is same as with aero, there is no single way or single method to cover all cars or even all diesels, there are too many variables to make generalizations I believe. Only thing is certain that all cars can be made to use less fuel, methods might vary.

My experience is mostly from fully mechanical turbo diesel with small turbo, all applies of course to that, maybe some other kind of vehicle that has similar characteristics, variables, but certainly does not apply to all cars.

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Old 03-31-2012, 12:22 PM   #12 (permalink)
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my tires are a bit larger than OEM size. my speedo shows 100kmh when I am actually travelling 103 by GPS. I hadn't thought of this before. if my speedo is off, my odometer is off too! does it mean I am actually getting less or more economy than I am calculating!
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Old 03-31-2012, 02:09 PM   #13 (permalink)
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a wheel correction tool also . calculated my mileage to be off by 3percent .

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