Quote:
Originally Posted by HighlandPete
Besides the drivers and user conditions, we are talking of two different cars, what is the mileage difference? One could be performing better than the other, simply on mileage and is now loosened up.
Also how do you both record/report the mpg? Long term brim to brim figures, or OBC readings? If it is the OBC, there could be quite a variation in accuracy over two cars.
I go along with Pottsy in running a test at constant speed, when you are out on the motorway, reset the OBC, see what you get.
On driving styles, because someone looks as if they drive harder, still many factors which skew the consumption. They may drive quite hard, but often their conditions can give better mpg simply that they may be cruising more miles at steady speeds, between their harder accelerations.
Also remember quick efficient accelerations can be the best way to gain speed. Load the engine harder in the most efficient range, for less seconds than lower loads at less efficiency. If not slightly better mpg, it is often no worse than under accelerating "inefficiently". Plus it is more fun. 
HighlandPete |
Pete,
I agree with your point about comparing OBC readings, they are not that accurate. I've been checking brim to brim vs OBC for the past two years and my conclusion is that at least in my car, the OBC is always optimistic and the optimism varies between 2% and 7%.
Regarding the driving technique, I have to disagree with you.
A hard acceleration to reach a given speed is one of the worst forms of fuel wasting with a turbo diesel.
Any cyclist will tell you that it takes a lot of energy to accelerate hard. A gentle acceleration to reach the same speed saves a lot of energy.
Your definition of "efficient" and "inefficient" may have to be revised.
That used to be the case with normally aspirated petrol (low compression) engines. Thermal efficiency is maximum for normally aspirated engines, or any engine for that matter, when cylinders are fully filled with air fuel mixture.
Why is that? Is because the thermal efficiency of internal combustion engines increases with the true/actual compression. If the cylinder is only partially filled then the actual compression is lower and efficiency is lower.
This argument does not apply for turbo diesel engines, they always work close to the maximum compression.