What determines fuel consumption?

It doesn't do anything of the sort. It regulates the amount of fuel delivered which regulates the amount of power produced.

As the revs rise towards this, the governor starts to restrict

Me perhaps?

The amount of fuel injected per cycle is fixed at each "given" throttle setting. It isn't the same at different throttle settings. If that were the case the throttle wouldn't actually do anything. In fact all the throttle on a diesel does is regulate the amount of fuel per cycle as the engine is always operating at WOT with respect to airflow.

It doesn't matter whether an engine is petrol, diesel or any other type of fuel. You get more power by burning more fuel. In a petrol engine the throttle regulates airflow and the carb/ecu matches the fuel delivered to mass of air consumed to keep the A/F ratio fairly constant because petrol engines only operate correctly over a very narrow range of A/F ratios. In a diesel engine the airflow is unthrottled and the throttle just regulates fuel supply. A diesel therefore operates over a very wide range of A/F ratios from very high (very little fuel) at idle/closed throttle to stoichiometric (15:1 A/F by mass) or even a tad richer at full throttle.

The debate about optimum speed for minimum fuel consumption has just been done to death in rec.autos.tech and Deja News/Google will no doubt find it. The usual usenet tale of disinformation, misinformation, misunderstandings and gibberish interspersed with the occasional post from someone who actually knew something worth typing. There are number of facts which are beyond debate.

1) The amount of power required to overcome aerodynamic drag increases in proportion to the cube of velocity. You can't go faster without generating more power. However aerodynamic drag doesn't start to become a major factor until about 50 mph. Below that speed it is rolling resistance tyre losses that determine the major source of drag and fuel used. 2) You need to burn more fuel to generate more power. 3) You will use more fuel as you go faster except for the one specific (and very rare) situation where the engine is operating significantly more efficiently at the higher rpm than the lower one and this is where it gets complex. 4) Small engines operate at higher throttle positions for a given speed than large ones and have lower internal frictional losses. Both of these factors contribute to better economy.

Petrol engines operate more efficiently the more air they process per cycle. i.e. part throttle operation is inefficient operation. Fuel economy is therefore generally better at a given speed in a higher gear than a lower one for two reasons.

A) For a given speed the higher gear means lower rpm and a wider throttle position than a lower gear. The wider throttle position means a higher volumetric efficiency and therefore better BSFC (brake specific fuel consumption)

B) The lower rpm of the higher gear means lower internal engine frictional losses which all have to be overcome by burning fuel before any net tractive power is delivered.

BUT.. camshaft profiles determine where peak torque and peak volumetric efficiency occur (usually at about half maximum rpm) and if rpm drops too low then cylinder filling (volumetric efficiency) suffers and the engine wastes fuel because some of it goes straight down the exhaust pipe unburned during the overlap phase where both inlet and exhaust valves are open close to TDC. For an average std road car this point would be at about 1500rpm or less i.e. about 30 to 40 mph in top gear for a 2 litre car. For sporty cars with long duration (high overlap) cam profiles this minimum efficient rpm would be higher.

At very low rpms, if the benefits of the higher gear (higher VE and lower frictional losses) are outweighed by better BSFC at higher rpm in a lower gear then it will pay to change down a gear. This optimum rpm point is therefore vehicle specific.

In all the cases where I have been able to find actual tested data a vehicle shows better fuel consumption the slower it goes even down to as low as 30 mph. Much of the data available is not actually vehicle specific data though, it is average traffic speed data which is a completely different animal and see my previous post.

To summarise, it is vanishingly unlikely that any normal car will show worse fuel consumption at lower speeds than higher ones down to at least 30 mph. However holding steady speeds that low for long distances is rare and hard to test accurately. Few drivers these days would be concerned about or want to travel at steady speeds lower than 40 to 50 mph. Going any faster than this will result in worse fuel consumption as aerodynamic drag starts to bite. Using an in car fuel computer is not a guaranteed way of knowing your steady speed consumption accurately though as these often don't actually measure fuel flow, they just take data from the ecu, MAP sensor and throttle position and estimate it.

Is this the reason why the ancient BMC A-series engine (especially in its

1275cc form) was able to produce economy figures that far more modern OHC designs couldn't match?

--

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It's being more economical, but efficiency needs to be determined. Efficient in terms of fuel consumption, time, or some factor between the two?

As somebody else has said, it may be wonderfully economical to drive around at 44 mph, but not even I could manage that on the motorway. :)

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If I gave the impression that I thought it did - then it wasn't intentional.

I don't. My old Saab 900 gave figures + 30MPG when driven on motorways arround

70-90mph, but when doing the same kind of drive but at the rush hour 40- 60 I was getting 27

The Celica is an even bigger difference. 140 miles of 70-90 driving + about 40 miles of nose to tail 1st/2nd gear got me 30.5mpg my daily run to work and back equals 180 miles a week normally rolling at an average of 40-60 or stationary (not much in between) last week I got 22 MPG.

My old Skoda Favorit gave exactley 43mpg for 200 miles of driving a week, no matter what speed it was done at, or spread over how many journeys.

Then you weren't driving smoothly enough at 40-60.

See, if you are spending any time stationary, MPG will plummet.

Even in terms of economy, 55mpg is better at 39mph than at 38mph. Since those will probably be measured in good conditions, and adding heating, lights etc will hasve less impact at a higher speed.

Using leaded fuel would help. The best I can find is Maestro 1.3 HLE with 60mpg @ 56mph and 42mpg @ 75mph (4 spd box). Mk1 Mondeo 1.8 matched the 42mpg @ 75mph.

Not according to the Dept of Transport.

states that both fuel economy and CO2 emission data should be made available for all new cars. It doesn't say they are the same thing. There would be no need to publish both if they were the same. Ian

The three or four figures which are published will be measured under different conditions. The amount of CO2 coming out of a car's exhaust at any time is proportional to the amount of Carbon being oxidised in the engine which is proportional to the mass of hydrocarbon fuel being consumed (to suitable accuracy).

Yuk

Asmatic with a long stroke (great with short stroke - I have owned a SS 2V)

In terms of pure mechanical Efficiency saved time is not a factor. It's only when we apply a human-based objective (eg. I *need* to get to that meeting on time) that it becomes so. Just one more of example of there being no truly closed systems.

Seems unlikely, but anything that might get me more than the my current 15mpg or so is worth a try...

;-)

As it will when doing stop start first/second gear crawls on the 30mpg average run, only it didn't.

Still didn't explain the Favorit though, same daily route as now and not much different traffic density. Gave 43MPG.

Did a run to Norwich too, including loads of A road DC and Motorway, at or above the limit. Still gave 43MPG.

It is, of course, also a function of the percentage of carbon in that fuel.

That is why you may see a petrol car and diesel car both rated at around the same CO2/km figure, but with a much better fuel consumption quoted for the diesel (typically about 20%). In this respect at least, HMG are on the right track in taxing cars based on CO2. I'd like to see diesel fuel more highly dutied as a form of carbon tax, but with the rest of the EU taking the opposite view under pressure from certain quarters, it might never happen.

The old " what goes in must come out" principle.

The 'instant' mpg figures on the V40 I had made interesting reading. Driving into the work car park at about 30 in 4th and just slightly accelerating appeared to give 'instants' around 15mpg, but in 3rd it was more like 20-25 mpg. So 'low' revs and high gear may not be the best way to get decent mpg. With the Impreza now nothing makes any difference, its always shit in all gears !