deian wrote:I think the reason why petrol engines use more fuel the faster they turn is down to how they are fuelled. I was think about this the other day...
Dei, absolutely
With an old-school mechanical diesel pump,
you and the governor had complete control of the fuelling. The harder you pressed the throttle, the more diesel was injected. Only an increase in boost would override this to a small extent and if you went too far the governor would rein you in. Result: great economy if you featherd back the throttle and drove carefully.
This held true with the later semi-electronic Bosch VP20. All the electronics did was alter the timing to optimise it under various conditions. You still had full control but with better economy as the timing was more optimised than ever it could be with a mechanical pump.
This all started to change a bit with the 2.1 Lucas EPIC where the electronics controlled fuelling as well as timing All you did was give it a guide as to how much you wanted and it computed what to give the engine to meet your desire. This was taken a stage further in the HDi and it shows. Electronics offer far finer control of injection than a mechanical system and economy goes up. You can see a clear trend here. In economy terms it goes 1.9TD - 2.1TD - HDi and the key is increasingly sophisticated injection systems with ever tighter controls on the injection.
Present day diesel economy has fallen again due to the way manufactures seem to want to make a diesel feel like a petrol engine and do away with the very essence of what makes a diesel a great engine; loads of low-down thumping torque. High torque low in the rev range makes for economy. What is does not make for is a sporty car with a feeling of lots of go. These days it is what people seem to want and the manufacturers give them that, at the expense of fuel economy.
With petrol engines, the electronic injection has increased base economy and power output enormously due to the fine control it exercises over the fuel injected. Quite ordinary modern petrol engines give power outputs nowadays that could only be drempt of years ago and although they are efficient, the power they generate is used badly. Comparatively small engines with high specific outputs haul heavy cars (due to the need of very weighty safety gear) along at high speed. This consumes fuel as a lot of power is needed to give a modern lardy car any performance to speak of. Finally, petrol engines today do not run at their most efficient due to the need to keep the engine fed with a precise 14.7:1 (stochiometric) ratio of air and petrol at all times to keep the catalytic convertor happy. This wastes so much fuel as hardly anywhere is 14.7:1 and ideal ratio. Under light crusie for instance, the ratio could be taken out to beyond 20:1 these days but at the expense of NOx emissions and a cat that no longer works.
Petrol cars could be incredibly frugal and develop high powers with good torque if the full benefits of lean-burn and the Atkinson cycle could be realised but, due to the incredibly foolish and short-sighted requirement to have a cat and the crazy obsession with CO2, development is skewed and hindered by the need to meet low CO2 and other emissions by actually consuming far more petrol than is necessary. It is a scandal.
Who would have thought it 
