What determines fuel consumption?

The message from "Clive George" contains these words:

No, you're pumping in the /same/ amount of fuel per rev - there's just fewer of them at low revs.

Actually, that's not strictly true as injector pumps have a governor system which limits the fuel as the engine approaches the chosen revs.

The message from "neutron" contains these words:

He seems to get a kick out of being rude - so in the KF he goes.

Every time you brake you are effectively using fuel, i.e. the fuel you used to get to the speed from which you need to brake from. I'm not saying you should never brake, but nevertheless it costs you to do it.

Rob Graham

Nah Newcastle, but he half a tonne of rolling backy would be for my personal use. I spoke about 3-3.5 ounces a week.

Could buy a years worth at once and offset my petrol costs.

I always liked those russians. women are a bit hairy though ;)

That simply isn't true. Remember we're talking about doing a speed at which the engine is happy in two gears - I chose 30mph and 3rd and 4th gear as a typical example, your car may have the same example at a lower gear. The fuel consumption in the lower gear (3rd) isn't higher by the gearing ratio, so it must be putting in a different amount of fuel per rev in the different gears. Next question is how does it know to do that - simplest is that you just press the pedal a different amount, but it could also adjust the fuel volume based on engine speed automatically, but I don't know about this.

Chosen revs? What chooses those revs? Certainly not pedal position, otherwise it would not require me to press the pedal a bit more when going up hills on motorways.

cheers, clive

However you increase the load. I disproved this in my artic which shows engine load and fuel consumption. I got better fuel consumption doing

50MPH in 15th @ 1600RPM than 50MPH in 16th at 1300RPM.

Engine load.

Yeah...for a bloody lorry. I get better economy on motorways at 70-

75MPH in my car than 56MPH.

Yes, you might well squirt less fuel per rev. But there would be more revs per mile, so it might work out more expensive.

- is this done by the driver not

With these pumps the pedal actuates a rack which turns each piston in the pump one way or the other as it goes up and down. All the pistons turn the same amount. There's a scroll cut into the circumference of each piston (right up to the head of the piston but not right down to the bottom of it) which meets up with a hole in each barrel. When the scroll meets the hole the fuel (which is under pressure, and is being injected) can escape back into the low pressure pipe or tank and not be injected. Depending on the rotational position of the piston/s the scroll meets the hole sooner or later and so less or more fuel is injected before the pressure is lost.

Rob Graham

The message from "Clive George" contains these words:

Thanks. Nice to know that since you know enough about how diesels work to be able to tell me I'm wrong.

Here...

for example is the pump used in my car. As you can see the amount of fuel injected per stroke is the fixed for any throttle setting until the governor cuts in. There's one injector pump stroke per cylinder for every two revolutions. Thus at lower revs there are fews strokes and thus less fuel.

The throttle does indeed set an engine speed that the engine strives to attain. As the revs rise towards this, the governor starts to restrict the amount of fuel available so that it doesn't overspeed. The reason you have to push the pedal harder to get up a hill is rather more complex - I'm not entirely sure I fully understand it properly myself, so I'll not lecture you about it - I'm sure someone else will be along soon who will.

The message from Conor contains these words:

Talking of lorries, Conor. I was on the M54 yesterday plodding along behind a pack of lorries all doing 56mph because there wasn't really a safe gap to pull out into to overtake for a while.

When I did get the chance I was a tad surprised by the '03 registered artic which pulled out in front of me and overtook the lot at a little over 70. Being curious, I followed him for a while. On the flat he was doing nearly 70, up hills he seemed to hit the 56 limit quite accurately, but downhill he was stonking along at nearly 80.

Hey whats wrong with a hairy woman. Not enough of them now! :(

Kev

You're welcome. Anytime you want one of your myths exploded please feel free to drop by.

Haha! Agreed - not easy but it helps if you at least *try* ;-)

Indeedy. Add to that the cost of brake pads and discs, and in the very long term, caliper overhaul (or at least sliders regreasing), and other associated braking costs, like rubber flexi-hoses needing occasional replacement, and brake pipes rusting over time.

Having said that, front bumpers are expensive as well. As are other people's back bumpers/panels, and insurance claims.

Peter

-- "Diamonds are what I really need - think I'll rob a store, escape the law, and live in Italy. Lately, my luck has been so bad, you know the roulette wheel, it's a crooked deal, I'm losing all I had."

Look, I'm only stating the obvious. Constant road speed, different gears - the amount of fuel per rev just isn't the same. Neither is the pedal position. I think you may be missing the constant road speed condition here. Doesn't help that you keep snipping it from your reply...

If it was the same, the fuel use in 4th would be directly proportional to the fuel use in 3rd multiplied by the difference in gear ratios. Also the engine would have to become miraculously more efficient in the higher gear - the power produced by the engine is the same in both gears (since the load, primarily aerodynamic, is the same), so the extra must be dissipated somewhere, probably as heat. Surely you're not claiming at the efficiency of an engine is related to the output gear?

Doesn't disagree with what I wrote at all.

In the lower gear, higher engine speed case, there is less fuel delivered per rev. The question is, how is that controlled : manually (driver varies pedal position - there is certainly some of this going on, hence varying pedal position as you climb/descend) or automatically via a governor. I suspect it's actually a combination of the two, ie the governor can do a bit of it, but it needs manual intervention beyond a certain point.

Unfortunately the website you mention only says the governor is in charge of maximum revs, thus we're both still in the dark on how the governor works. As I said above, I suspect it's merely an imperfect system (deliberately), but I'm happy to be shown wrong. BTW feel free to say what you know - I think you're merely being confused by misunderstanding what I'm saying, rather than us disagreeing.

cheers, clive

Ah ok. Probably was in 5th then.

IME, it's not as simple as that.

Using my MPGOmeter in my car, I can get about 58mph doing 70 in 5th. If I do the same speed in 6th, on the same bit of road, I get about 53.

Obviously the car prefers revving at about 2.5k as opposed to being loaded at 2k.

Pete.

That's not quite what was said. I'm sure a car could be engineered to be more efficient at 60 relative to 50, depending on gearing.