Mpg dropped considerably in the past week...

[hogweed]

Originally Posted by T-Cut

In simplistic terms, heat energy generated by combustion needs to be used for driving the pistons. All heat absorbed by the surroundings in warming up the engine is wasted. Diesels need heavy construction and therefore have high mass. This makes the warm up period protracted compared with spark ignition engine, but once hot they're more efficient.Clearly, the colder the engine initially, the longer it will take to get to optimal efficiency. Some diesels never become warm enough no matter how long they're running. Ideally, no heat should be transferred to the surrounding mass. That's why heat engines run most efficiently at high temperatures. For piston engines, the upper limit is relatively low because of the characteristics of petrol and diesel fuels, lube oils and low cost metallurgy. They have 20-40% efficiency (petrol-diesel resp.). Other engine types are more efficient because they run hotter. Gas turbines for example run extremely hot, even above the melting point of the turbine blades. These can achieve 98+% efficiency.

TC

Thanks for that, TC - I like to know these things

[bl52krz]

Originally Posted by T-Cut

Surely the expansion coefficient of fuel (which is tiny) will have a miniscule effect on fuel consumption compared with the large variation in running temperature seen in diesels? If it had, the fuel producers would take whatever steps necessary to avoid you getting more than your money's worth in cold weather. I know fuel pumps are calibrated/certified accurate at 20C or whatever, but the economic effect of delivery temperature must be negligible. The fuel actually entering the engine doesn't vary much, it's always pretty hot and the backflow from the injectors warms the tank contents over a long drive. That would alter the fuel consumption in the other direction. It's an interesting thought anyway.

TC

The coefficient of Diesel thermal efficiency expansion is 0.00083 per degree c. Therefore 1000 gallons of diesel at-7deg c, would expand to 1037 gallons @ 38deg c . With the return to the tank from the injectors this would, I imagine, only make a small difference to the expansion of the diesel. But it remains that under the compression in the injector pump, temperature would rise quite high hence the need for a cooler radiator to be fitted. On the above info:- 69 fill ups would give you approx .5 gallon per fill extra.

[T-Cut]

Originally Posted by bl52krz

The coefficient of Diesel thermal efficiency expansion is 0.00083 per degree c.

Indeed, but as noted this can't explain the rise in fuel consumption from 47 to 35mpg over one very cold week.

69 fill ups would give you approx .5 gallon per fill extra.

Wow! But I can understand why wholesalers dealing with millions of gallons will be mindful.

TC

[bl52krz]

I would think that one very good reason would be lower gears that have probably been in use over the last two weeks or so during the cold weather. Fbh running nearly all the time( half a kg per hour on full burn, quarter kg on half power) during a similar period. Colder running engine does not help either as you know. Also if people use all their accessories at the same time,electric rear window, heater and headlights during the cold weather, the alternator takes some power to generate electric. I am aware that you yourself are well aware of these effects.

[klarzy]

Originally Posted by bl52krz

I would think that one very good reason would be lower gears that have probably been in use over the last two weeks or so during the cold weather. Fbh running nearly all the time( half a kg per hour on full burn, quarter kg on half power) during a similar period. Colder running engine does not help either as you know. Also if people use all their accessories at the same time,electric rear window, heater and headlights during the cold weather, the alternator takes some power to generate electric. I am aware that you yourself are well aware of these effects.

Would any of that effect the calculated mpg through the injectors...?

[T-Cut]

Originally Posted by klarzy

Would any of that effect the calculated mpg through the injectors...?

I've always been intrigued by that. Is there any MGR info to refer to? Is it a generally employed method to calculate range? Wouldn't the fuel used according to tank volume be a simpler and accurate enough method?

TC

[HarryM1BYT]

Originally Posted by T-Cut

I've always been intrigued by that. Is there any MGR info to refer to? Is it a generally employed method to calculate range? Wouldn't the fuel used according to tank volume be a simpler and accurate enough method?

TC

My last injection Ford, which had a range/consumption gauge, said it was calculated from injector opening period, plus the measured tank level. I suspect it maybe also included fuel pressure in the equation too.

Present consumption obviously has to be involved in the formulae and the easy way is via injector opening periods.

[klarzy]

Originally Posted by T-Cut

I've always been intrigued by that. Is there any MGR info to refer to? Is it a generally employed method to calculate range? Wouldn't the fuel used according to tank volume be a simpler and accurate enough method?

TC

not if you fill up..... it would reset the calculation.... and skew the average figure...

[klarzy]

Originally Posted by HarryM1BYT

My last injection Ford, which had a range/consumption gauge, said it was calculated from injector opening period, plus the measured tank level. I suspect it maybe also included fuel pressure in the equation too.

Present consumption obviously has to be involved in the formulae and the easy way is via injector opening periods.

Also if you reset the consumption meter, drive or a few minutes then put your foot down the figures drop quite quickly... although the tank measurement would not have moved hardly at all...
I also think its pressure x opening time = flow rate / miles travelled = average consumption and pressure x opening time for instantaneous calculation...

[T-Cut]

The Handbook says the Range function is (my quotes)
- - based on a 'combination' of current driving style and the fuel consumption recorded by the computer during the previous few minutes.

Not sure what the 'current driving style' actually means. Presumably whether it's a constant or frequently varying in terms of fuel being transferred through the injectors.

The consumption over the last few minutes seems to be what people refer to as the 'current consumption' - as distinct from the 'momentary consumption' displayed via the Diagnostic Mode (Test 4).

Diagnostic Test 5 gives the 'momentary range', which doesn't seem that useful. For example, if you were accelerating hard at the time or maybe parked at tick-over, the range would be way out. I guess this display varies rapidly as you vary the driving conditions.

Interestingly, there's no 'current range' available via Diagnostic Mode, but could this is related to the 'average consumption' also displayed under Test 5? This 'average consumption' is the figure you can adjust for accuracy using a manually calculated consumption over several hundred mile of 'default' style driving. So, if this data is involved in the range calculation, it'll be inaccurate without the manual calibration described in the Diagnostic Mode notes.

Elsewhere, the same questions been asked and a few people have looked at it a bit deeper. These aren't MG Rover cars.
One poster said:
I have a Scanguage, a wonderful piece of technology that tells me all kinds of stuff. Among others, it shows instantaneous MPG, current trip MPG, and current tank MPG, as well as distance-to-empty. The DTE is calculated based on the current tank MPG, which may or may not be the best method, but at least it's obvious what it's based on. Fill the tank, pull away, and as you accelerate away from the gas station, with correspondingly dismal tank MPG, the DTE is maybe 100 miles. Get up to speed, and it increases. Sit at a long red light and you can see it gradually go down.

And another post from a software developer:
The computer's fuel consumption statistics don't actually come from the tank float. The consumption estimate comes from the injector duty cycle and fuel pressure.
The computer probably has a map or graph between duty cycle and fuel pressure which tells how much fuel the injector will spray at any given cycle and pressure. Using this it continually estimates how much fuel it has sprayed since the mpg calculator was last reset. It then takes this amount and divides it into the miles traveled since reset to give you the miles per gallon.
There are definitely going to be inaccuracies in the graph between the expected fuel spray and the actual spray that occurred so the MPG estimate will never be 100% accurate but I wouldn't expect it to be anymore than 5-8% off.

My brain hurts.

TC