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Old 12-11-2009, 07:43 AM   #8 (permalink)
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Originally Posted by Arragonis View Post
Interesting. My car is manual and diesel. I'm experimenting between pulsing and grinding (nise phrase ) and also which gear to use. Higher gear, lower RPM means I sometimes have to prod harder to maintain speed and as the Diesel is fuel controlled that means more fuel going in. But a lower gear means higher engine speed, less fuel but more cycles.

Pulsing is harder as the roads here are very heavy on traffic, and there is usually some muppet in a van about an inch or so off my rear end.

I shall have to practice technique more to maintain momentum

Thanks guys.
In heavy traffic where close proximity is unavoidable, use drafting to improve your mileage.

"Lugging" an engine refers to the old carbureted gasoline engines where if you gave it more gas pedal the accelerator pump would squirt a given amount of fuel to the engine. This would be too much fuel for low RPM and high load, which is not good for power or emissions.

A modern Diesel or Gas engine can not really suffer from "Lugging" in the classical sense, because the fuel delivery is precise, regardless of the load.
In order to lug a modern engine you would have to go very low in the RPM range, down below 1100, give or take, depending on the engine being a performance type or an economy type.

I agree with downshifting if you have to apply full throttle in the higher gear to maintain your speed. Actually it depends on your RPM and your BSFC map for your specific engine. A good general speed would be 2000 RPM for almost any engine that was designed for economy. If you drop below that point in your climbing phase, then downshifting would probably give you better mileage.

A Diesel is different in this aspect from a gasoline engine since it has no manifold vacuum, but even with no manifold vacuum, lower RPM will be more efficient as long as you do not go below about 1500 when climbing hills. Even with that said, it still depends on the size of you engine in relation to your vehicles gross weight. Higher power to weight lower RPM and vice versa.

It's not a "written in stone" tactic and can change somewhat even based on the number of passengers in your car, but with a Diesel as long as you stay in the general area of best BSFC you will get best mileage as long as you use the least average RPM it takes to do the job.

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