Higher or lower revs?

[ste_bla]

I noticed on the flat in me escort in 3rd gears doing 35 it does 22oo rmp if i change into 4th using the same amount of throttle it does less revs but surely it uses the same amount of petrol. So do I use 3rd or 4th?

 

[Morba]

4th

if you can, do it in 5th

 

[GravyMonster]

lower revs = fewer piston cycles = less petrol used.

 

[S@njay]

lower revs = fewer piston cycles = less petrol used.

Ok, but what if, you h ave it in 5th, your going 30mph, the revs are low, but you have to press the accelerator down more? Would that use more petrol? (hyperthetical question)

 

[penski]

Ok, but what if, you h ave it in 5th, your going 30mph, the revs are low, but you have to press the accelerator down more? Would that use more petrol? (hyperthetical question)

*Hypothetical.

If you press the accelerator down more, you will up the revs, go faster and use more fuel.

*n

 

[benneh]

Probably use less fuel, but you'll be labouring the engine (causing it undue stress). Best to stick to the correct gear, 3rd/4th.

 

[penski]

The only issue you may find is that at lower revs you lack the power to overcome both mechanical and aerodynamic resistance for the gear/speed you wish to have.

*n

 

[GlasgowTitan]

If you press the accelerator down more, you will up the revs, go faster and use more fuel.

*n

Not on my car, if I'm in 5th doing 35mph, I press the accelerator to the floor, bugger all haoppens, maybe a little more noise, but to get to 40mph would take about 10 seconds.

 

[penski]

Not on my car, if I'm in 5th doing 35mph, I press the accelerator to the floor, bugger all haoppens, maybe a little more noise, but to get to 40mph would take about 10 seconds.

See my post above about overcoming mechanical resistance

*n

 

[Morba]

thats called learning to change down a gear to be in a better gear to perform the next move....

 

[volospian]

What happens when you press the accelerator is that the air inlet opens. If the engine is labouring it will not draw in any more air "per second" (for want of a better method of explaination) as it is not physically capable of running faster.

Fuel is added to the amount of air taken it to give the correct mix. If you are on a carb and the air being pulled in remains fairly constant, then the fuel being pulled in with that air will remain fairly constant.

If you have a electronic fuel injection system, it may detect a Wide Open Throttle situation and start adding more fuel to feed the expected air flow, however, that should then back off as it receives a "rich mixture" signal from the lambda.

While labouring in too high a gear will not cause much extra fuel consumption, there will be some increase when you try to rev as the air flow will be capable of being increased, even if the engine can't increase it's revs quickly, but it won't be as if you are pouring fuel through the engine by having the throttle open.

Bah, this is difficult to explain what I mean. Basically fuel is used up in accordance with the amount of air used over a given period. That is, if two cars have the same engine efficiency, and one is driving at 30mph in 5th and the other at 30mph in 3rd, you will use more fuel in the higher revving one as it is "eating" more air (and therefoer air/fuel mix) per second than the lower revving engine. If a car is not capable of using a lot of air in one period, i.e. the engine is labouring against the friction and mavity and so on of the car, gearbox and so on, it won't use much fuel either.

I think!

 

[Jonny69]

Get an auto, it will decide for you My girlfriend's auto uses rugger ball fuel around town compared to any manual I've used, you're just not on and off the accelerator and up and down the revs all the time. It sits at its optimum for the speed you're doing and that's that.

 

[Jonny69]

Volospian, only if both are at the same throttle position. If the labouring one is on wider throttle then it will be taking bigger gulps.

But yeah, I now what you mean, you'd need to look at the flow rate through a synchrometer in each gear to really know.

 

[Nozzer]

Get an auto, it will decide for you My girlfriend's auto uses rugger ball fuel around town compared to any manual I've used, you're just not on and off the accelerator and up and down the revs all the time. It sits at its optimum for the speed you're doing and that's that.

The last auto I drove got 9MPG in town

 

[Morba]

auto and town driving?
not in my bm auto thanks, drinks like a fish

 

[volospian]

Volospian, only if both are at the same throttle position. If the labouring one is on wider throttle then it will be taking bigger gulps.

But yeah, I now what you mean, you'd need to look at the flow rate through a synchrometer in each gear to really know.

I'm not sure, the higher revving engine will be taking smaller gulps as it's throttle may be closed more than the labouring engine, but it will be taking them more often resulting in a similar level of fuel/air mix being consumed to the larger gulping, slower breathing labouring engine (depending, as you said, on flow rate).

Also, when the throttle is opened wide in both cars the "unlaboured" engine will rise in revs faster and therefore use more fuel in the same time period as the laboured engine which cannot rise in revs as fast.

 

[Jonny69]

auto and town driving?
not in my bm auto thanks, drinks like a fish

That's because it's a BMW and you're supposed to be able to afford it

 

[CGrieves]

Incidentally the most fuel-efficient driving method may not be as you expect.... almost all vehicles at "green" races, i.e. when the most fuel-efficient win, or the rallies where the vehicles are given a set amount of fuel, all generally use the same method:

1. Select highest gear (most only have one fixed ratio), at minimum revs without stalling.
2. Pedal to the floor, keep it there until peak revs are reached.
3. Turn off engine, disconnect gearbox and coast down to minimum speed.
4. Repeat until won/out of fuel.

Just goes to show that driving around just feathering the throttle all the time will actually waste fuel. Most engines are at their most efficient when they are at peak power (and therefore usually maxiumum throttle). Under most other conditions they are generating excess heat without maximum motive force. And heat is the biggest enemy in terms of fuel waste.

Volospian hit the nail on the head with his excellent description- putting your foot to the floor does not necessarily increase the rate of fuel consumption. It merely opens the throttles. The carb/FI decides how much fuel to provide. Many modern engines, particularly motorcycle engines, use secondary, computer controlled throttle vanes that override what your foot may be doing in order to provide the most efficient flow (i.e faster mixture flow, not more mixture) so the driver is essentially taken out of the equation anyway!

Another interesting fact- in most road engines, at the stoichiometrically ideal mixture, the air provided to the cylinder in one induction cycle weighs more than the fuel.

 

[Morba]

That's because it's a BMW and you're supposed to be able to afford it

that will be why it was on ebay and will be on autotrader from tomorrow!! haha

 

[Smiley Man]

i use 4th at 30 and 5th at 40, can do 30 in 5th but at about 1300rpm and when you boot it the whole car shakes