Power through revs at partial-throttle
Discussion
Hello,
I was thinking this morning, when gently accelerating I often wind it up to around 4,500 rpm on a narrowly open throttle (~25%), enjoying the feeling of the power building up, then block changing.
Would it be more or less fuel efficient to have wider throttle (~50%) but shifting earlier?
Are there any other considerations I should have over letting the revs rise on partial throttle?
It's not a completely resolved question, I just wondered what other people's thoughts were.
Thanks
I was thinking this morning, when gently accelerating I often wind it up to around 4,500 rpm on a narrowly open throttle (~25%), enjoying the feeling of the power building up, then block changing.
Would it be more or less fuel efficient to have wider throttle (~50%) but shifting earlier?
Are there any other considerations I should have over letting the revs rise on partial throttle?
It's not a completely resolved question, I just wondered what other people's thoughts were.
Thanks
waremark said:
I suspect how you accelerate makes less difference than how fast you go and how much you brake.
I'd go with that.I'd also suggest that keeping the engine near peak-torque is probably more important than the level of throttle-opening...and that varies from engine to engine.
IIRC:
A closed or partially closed throttle causes increased pumping losses, therefore less efficiency. Equally, a fully open throttle will tend to run richer and use more fuel.
To maximise fuel economy you want to be using a maximum of about 90% throttle in the highest possible gear.
(That's my recollection of it anyway! Might be wrong!)
A closed or partially closed throttle causes increased pumping losses, therefore less efficiency. Equally, a fully open throttle will tend to run richer and use more fuel.
To maximise fuel economy you want to be using a maximum of about 90% throttle in the highest possible gear.
(That's my recollection of it anyway! Might be wrong!)
waremark said:
Eco driving experts seem to say - use the highest gear in which the engine is not labouring (however you assess that). They generally seem to like changing up at extremely low revs.
I suspect how you accelerate makes less difference than how fast you go and how much you brake.
I'll go with the braking theoryI suspect how you accelerate makes less difference than how fast you go and how much you brake.
Wasn't there a test done some years ago that proved that in modern cars WOT to cruise speed gave the best economy?
Certainly in a Diesel the most efficient use of fuel to accelerate at a set rate is usually heavy throttle and low revs, as opposed to light throttle and higher revs. Even off boost decent specific fuel consumption is possible, but the highest efficiency usually occurs on boost, but I don't know whether this is affected much by having the boosted air, it might just be due to being in the right rev/load range.
Petrol's are similar in theory, the BSFC map is best at high load, although higher revs may be okay unlike a Diesel - often the best efficiency is close to peak torque (can be high up the rev range on a petrol) and with a heavy throttle, although forced induction could skew this I imagine.
If you can find a BSFC map of your engine, this will clear up the question of what load/revs is using the fuel most efficiently during acceleration, but heavier throttle and lower revs should always be better than using a very light throttle and climbing higher up the rev range. However, the best BSFC during acceleration will only tell part of the story... for overall decent economy, the time spent in acceleration plays only a small part. More important, well during town driving anyway, is anticipation, and driving smoothly. So avoiding storming up to traffic lights and queues of traffic, backing off the throttle early, and aiming to avoid coming to a complete stop all help. On a run, keeping the car in the highest gear that is not bogging down the engine, and keeping cruising speed down, will pay dividends.
You can go a bit OTT, and be sacrificing your enjoyment of your car, or safety even, to eke every last mile out of a gallon of fuel.
ETA - some petrol petrol cars run rich at wide open throttle and high revs I think , i.e. wasting fuel, and if yours has a throttle plate, i.e. is not a direct injection petrol, high throttle will reduce pumping losses and increase efficiency. This is not to say accelerating using heavy throttle uses less fuel all the time, if you have to brake to scrub off the excess speed you have just gained by over-accelerating, it might be the case that accelerating in a less efficient mode and slower speed will have been more efficient, because you used 15hp instead of 30, so the engine used more fuel per HP to produce the 15hp, but overall you used less fuel using less throttle, because you threw away some of the extra HP produced using the more efficieint heavy throttle setting by overdoing it and needing to brake.
Hope I'm making some sense...
Petrol's are similar in theory, the BSFC map is best at high load, although higher revs may be okay unlike a Diesel - often the best efficiency is close to peak torque (can be high up the rev range on a petrol) and with a heavy throttle, although forced induction could skew this I imagine.
If you can find a BSFC map of your engine, this will clear up the question of what load/revs is using the fuel most efficiently during acceleration, but heavier throttle and lower revs should always be better than using a very light throttle and climbing higher up the rev range. However, the best BSFC during acceleration will only tell part of the story... for overall decent economy, the time spent in acceleration plays only a small part. More important, well during town driving anyway, is anticipation, and driving smoothly. So avoiding storming up to traffic lights and queues of traffic, backing off the throttle early, and aiming to avoid coming to a complete stop all help. On a run, keeping the car in the highest gear that is not bogging down the engine, and keeping cruising speed down, will pay dividends.
You can go a bit OTT, and be sacrificing your enjoyment of your car, or safety even, to eke every last mile out of a gallon of fuel.
ETA - some petrol petrol cars run rich at wide open throttle and high revs I think , i.e. wasting fuel, and if yours has a throttle plate, i.e. is not a direct injection petrol, high throttle will reduce pumping losses and increase efficiency. This is not to say accelerating using heavy throttle uses less fuel all the time, if you have to brake to scrub off the excess speed you have just gained by over-accelerating, it might be the case that accelerating in a less efficient mode and slower speed will have been more efficient, because you used 15hp instead of 30, so the engine used more fuel per HP to produce the 15hp, but overall you used less fuel using less throttle, because you threw away some of the extra HP produced using the more efficieint heavy throttle setting by overdoing it and needing to brake.
Hope I'm making some sense...
Edited by Turkey on Friday 30th April 21:53
Lower revs makes better use of the exploding fuel too, since the flame front has time to ignite the full fuel charge before the piston moves away too much. It's for this reason in the main that you should boot the throttle at the minimum revs possible, and then change up ASAP into the highest gear possible for cruise.
davepoth said:
Lower revs makes better use of the exploding fuel too, since the flame front has time to ignite the full fuel charge before the piston moves away too much. It's for this reason in the main that you should boot the throttle at the minimum revs possible, and then change up ASAP into the highest gear possible for cruise.
Or you could just drive to the conditions.If you're really worried about fuel consumption the things you should be looking at is what car you drive and what type of journeys you make.
In my experience, how hard I accelerate and in what gear doesn't make a great deal of difference unless I'm really going for it - and even then it's more to do with higher top speeds and harder braking.
davepoth said:
Lower revs makes better use of the exploding fuel too, since the flame front has time to ignite the full fuel charge before the piston moves away too much. It's for this reason in the main that you should boot the throttle at the minimum revs possible, and then change up ASAP into the highest gear possible for cruise.
Hmm, isn't it a bit more complicated than that? At low revs/high load/high gear you'll likely be on the throttle longer to get to a given speed than if you're high revs/less load/low gear...so is 10 seconds of the former more efficient than 5 seconds of the latter (say)?If there's a need to get up to speed quickly (motorway sliproads etc) then obviously you need to use full power, in which case running all the way up the rev counter is sensible. But in a partial throttle situation (i.e. where you are not constrained by traffic, gradient or other road conditions and are just driving for economy) the most economical way of burning fuel is at low revs and full throttle.
Z.B said:
davepoth said:
Lower revs makes better use of the exploding fuel too, since the flame front has time to ignite the full fuel charge before the piston moves away too much. It's for this reason in the main that you should boot the throttle at the minimum revs possible, and then change up ASAP into the highest gear possible for cruise.
Or you could just drive to the conditions.If you're really worried about fuel consumption the things you should be looking at is what car you drive and what type of journeys you make.
In my experience, how hard I accelerate and in what gear doesn't make a great deal of difference unless I'm really going for it - and even then it's more to do with higher top speeds and harder braking.
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