07 Mar 2009
Many people could halve their transport fuel bills tomorrow if they "took-onboard" one or two facts.
Allthough diesel engines CAN be more fuel-efficient than their petrol counterpart, how efficient they actually are depends greatly upon how they are used/misused. That is to say, that the diesel is relatively efficient only when supplying "light" torque. A fraction of what it is capable of, that is.
If we "overdo" this, of course, efficiency drops again because friction starts to become a significant fraction of the "light" torque produced by the engine. Using more than 1/4 "welly"/"throttle"/"accelerator" only in occasional circumstances, will double MPG over "putting the foot in the tank" on every occasion - particularly in a high gear.
Petrol engines are exactly opposite. Their thermo-dynamic cyle is different to diesel, since the heat is added to a constant a Volume (at the top of the piston's stroke) whereas diesel is injected as the piston moves down, which makes the the Pressure more like "constant". When the fuel-injection stops, heat is converted to Work for the remainder of the stroke. And so the sooner the injection is cut-off, the more of the heat from the fuel will be made use of.
To return to the Petrol engine, here there is a waste of energy at LOW throttle setting, because the pressure in the cylinder/manifold goes way below atmosheric, which of course means the piston must be pulled-down against this pressure differential. The situation has been described, many years ago, as the "Volumetric efficiency", I believe. The Petrol-engine, then, is most efficient with large throttle settings, high % of maximum torque.
Apart from this, it is as well to realise that fuel is used to push the car along against both gradient
and friction from air and road, and putting-on the brakes adds to this. More fuel-efficient to be going at a sane speed for the prevailing road !