Which uses more fuel:

[rowanxx]

Low throttle, high revs (1st gear at 20mph)
Lots of throttle, low revs (4th gear at 20mph)

Ideas or actually evidence?

[kambites]

Neither is likely to be ideal. Which is worse will depend on the car, engine and gearbox in question. I suspect most cars will do best in 4th, though, if they're capable of maintaining 20mph in 4th at all.

Edited by kambites on Sunday 4th May 17:44

[fwaggie]

If friction was non-existant they would both use almost identical amounts.

But as friction does exist and is a horrible sticky bugger, high revs will use more fuel.

[anonymous-user]

rowanxx said:

Low throttle, high revs (1st gear at 20mph)
Lots of throttle, low revs (4th gear at 20mph)

Ideas or actually evidence?

Option 1, by a massive margin, for either a petrol engine or a diesel one!


(reduced engine friction, no overfuelling for component protection, and for a petrol, reduced pumping losses)

[kambites]

fwaggie said:

If friction was non-existant they would both use almost identical amounts.

As I understand it, that's not even close to being true. The huge majority of the losses in an internal combustion engine are not friction related.

[BritishRacinGrin]

Depends on a lot of variables. Being in the correct gear would be best, third probably if just cruising at 20 on a flat road.

[Zad]

Don't forget that most engine management systems go open-loop above 75% throttle and run rich. But still the high revs one.

[kambites]

Zad said:

Don't forget that most engine management systems go open-loop above 75% throttle and run rich. But still the high revs one.

In my experience, most go open circuit above about half of maximum RPM too. From crude experimentation, the Elise seems to go open at anything above about 30% throttle or about 3000rpm.

[BritishRacinGrin]

fwaggie said:

If friction was non-existant they would both use almost identical amounts.

But as friction does exist and is a horrible sticky bugger, high revs will use more fuel.

shirley if engine RPM is higher there'll be more combustion cycles for a given distance ergo more fuel injected?

[kambites]

BritishRacinGrin said:

shirley if engine RPM is higher there'll be more combustion cycles for a given distance ergo more fuel injected?

Fuel injected per combustion cycle is not a constant.

[BritishRacinGrin]

kambites said:

BritishRacinGrin said:

shirley if engine RPM is higher there'll be more combustion cycles for a given distance ergo more fuel injected?

Fuel injected per combustion cycle is not a constant.

Indeed, but if you're talking about 6,500rpm vs 800rpm it has to more or less stack up?

[anonymous-user]

kambites said:

fwaggie said:

If friction was non-existant they would both use almost identical amounts.

As I understand it, that's not even close to being true. The huge majority of the losses in an internal combustion engine are not friction related.

It's not a fixed number, as engine friction is engine speed dependant.

At idle, pumping loses and friction are typically of a similar magnitude. But at 7krpm, friction is much much bigger!

(i'm including support system parasitic losses in both cases (ie oil,water pump etc)

[kambites]

BritishRacinGrin said:

Indeed, but if you're talking about 6,500rpm vs 800rpm it has to more or less stack up?

I think you underestimate how much difference the throttle makes to the amount of fuel injected.

[anonymous-user]

Zad said:

Don't forget that most engine management systems go open-loop above 75% throttle and run rich. But still the high revs one.

Not any longer. Pretty much all EMS systems now only go into component protection when absolutely necessary.

[kambites]

Given that even with zero friction, a primary heat engine can't exceed about 40% efficiency, I don't think you can argue that most of the losses are frictional in any situation.

[anonymous-user]

kambites said:

Given that even with zero friction, a primary heat engine can't exceed about 40% efficiency, I don't think you can argue that most of the losses are frictional in any situation.

Generally, the basic maximum theoretical thermal efficiency of an ICE is used to simplify the loss comparison. This is because you can never avoid the thermal losses in the engine (you can of course harvest and re-invest the wasteheat, but the engine itself cannot avoid producing that waste heat. Frictional and parasitic loses can be reduced to (nearly) zero without breaking any laws of physics!

Of the loses we can attempt to negate, friction, is by far the largest contributor, hence the appearance of multi-ratio transmissions to leverage the principle of downspeeding to the extreme!