Floored throttle vs. High Rev fuel efficiency

[Freeskier]

So I had kind of a random thought today. Which scenario burns more fuel.

a) Cruising at high revs but steady speed and throttle, say 50 in 2nd gear or whatever.

b) Bogging down in a gear and flooring the throttle to prevent stalling. Say 30 in 4th.

In comparison, which burns more fuel. Does the system just dump more fuel in regardless of revs when you push on the gas? Dunno if it's really a clear question, just was wondering.

[93VR6]

Hight revs low throttle will burn more fuel.

At high revs the engine is dumping in less fuel but much more frequently

At low revs the car will be dumping in more fuel per cyl fire but not more as a ratio of revolutions. What I mean is at say 5000rpm in second you are pumping in 1 cc per rpm but at 1500 rpm in fourth you are putting in 2cc per rev, do the math driving in 4th is more efficient.

Also if it was more efficient to cruise at high revs then why would cars even bother with anything past third?

[Mibz]

Originally posted by 93VR6
Hight revs low throttle will burn more fuel.

At high revs the engine is dumping in less fuel but much more frequently

At low revs the car will be dumping in more fuel per cyl fire but not more as a ratio of revolutions. What I mean is at say 5000rpm in second you are pumping in 1 cc per rpm but at 1500 rpm in fourth you are putting in 2cc per rev, do the math driving in 4th is more efficient.

Also if it was more efficient to cruise at high revs then why would cars even bother with anything past third?

You're assuming based on arbitrary numbers. If you were dumping 8cc @ 1500 in 4th then high revs would be better. How do we know it's not 8cc? We don't. We don't know because nobody here has any idea how much fuel is injected per cycle on any of our vehicles. Except maybe Toma.

[Freeskier]

I don't know how realistic the question was. I ended up pulling away from a red light in third cause i missed first and was too lazy to change it once I realized. Just floored the gas and chugged up to speed

Just got me wondering about the efficiency issue.

[89s1]

What you did was called "lugging" the motor and it is very hard on it.

which one uses more fuel?

The angle of the throttle has a lot to do with how much fuel is delivered to the cylinders, so it would be pouring quite a bit in there while floored in fourth gear at idle I would think.

That being said, it is impossible to cruise at high rpm without some degree of throttle input (the higher the revs, the more throttle.) So really, it's arguable in both ways.

[J-hop]

Originally posted by 93VR6
Hight revs low throttle will burn more fuel.

At high revs the engine is dumping in less fuel but much more frequently

At low revs the car will be dumping in more fuel per cyl fire but not more as a ratio of revolutions. What I mean is at say 5000rpm in second you are pumping in 1 cc per rpm but at 1500 rpm in fourth you are putting in 2cc per rev, do the math driving in 4th is more efficient.

Also if it was more efficient to cruise at high revs then why would cars even bother with anything past third?

I'd have to pull the fuel map off my megasquirt to be sure but I'm 99% sure you are wrong.

As a comparison, If I cruise on the highway at 120 (I think at about 3k in 6th on my car) I am more fuel efficient then cruising at 90 in the same gear, because at 120 I am right in the power band and barely have to touch the gas for hills or passing, where as at 90 you'd have to mash the gas and lug it to get up hills and to pass

Also,I am not exactly sure how stock ECUs deal with it but my megasquirt system has what is called accel enrichment so if you are to floor it from say 1k (or any rpm for that matter) it will dump a fair amount of extra fuel, where as this will not occur under gradual accel. it basically reads the TPS and determines a threshold accel of the pedal at which to dump extra fuel.

I can't give much more of an explanation as I can't remember what my fuel maps looked like but also consider the fact that an engine out of its power band will be much less efficient per volume of gas so you will have to factor that in. Also contrary to what someone has stated, the amount of gas input into a cylinder is not primarily a function of throttle position, it has a lot to do with load etc (I sure as hell wish it was just based on throttle position because that would make for easy creation of fuel maps!!!!!) I have only tuned a MAP based system, so I can't speak for MAF, but in MAP based systems it is a function of rpm and MAP reading (manifold air pressure) which does have a relationship to throttle position but it is more highly dependent on load- ie: the fuel curve for bringing a car to 5000rpm in neutral with no load is completely different than flooring a car to 5000rpm in 3rd gear while cruising. I don't think it is as simple as the straight linear equation that you are proposing.

[Freeskier]

Originally posted by 89s1
What you did was called "lugging" the motor and it is very hard on it.

Yeah, i know. Don't ever do it normally.

[Mibz]

J-Hop's example also brings up aerodynamics. Just because your flat-front Jeep is more efficient in a vacuum at a certain gear/speed doesn't mean that air resistance is going to keep it that way.

I really don't think there's an objective way to answer this question without people who deserve to wear lab coats.

[Darell_n]

Lots of throttle at lower RPM will almost always have a lower fuel consumption than the same power output at a higher RPM (BSFC). This is one of the factors contributing to the fuel efficiency of a diesel engine, always having a wide open air supply.

[Darell_n]

Here is an excellent article for everyone to nerd out on:

http://autospeed.com/cms/A_110216/article.html

[msommers]

A distinction from efficiency to net consumption really needs to be made here.

[georgemagana]

This is what another guy said in another forum....

How much gas you use is dependent on a lot more than just what gear you are in.

I don't want to get into a lengthy discussion, but it is all based on load...

too much load and you are killing the engine and using much MORE gas because of that

too little load and then a big portion of your load it to overcome the internal friction of your transmission and motor. At idle, your load is like 20% ... that's ALL friction!

For NA (*This is an oversimplification*) You can think of load in terms of how much air is entering into your engine .... 100% possible air entering at the given rpm = 100% load... therefore, load is directly related to how much you are pressing the gas pedal

Ideally you want to have your load at around 60% -75% for best MPG at all times

High revs, low load = bad mpg due to friction losses

low revs, high load (85%+) = bad mpg due to MORE fuel being used to offset detonation etc...

For best mpg on a flat surface at cruise, you want to keep ur rpms in the 2250 - 3250 range ... with my experience, but DONT stomp on it ... if you are steadily accelerating, higher rpms are preferred due to increased load

.... others, feel free to expand

edit: Again not my opinion just what other guy said in a different forum.
I agree since almost any other if not every single IC engine is more efficient at higher load ranges like 60% load and higher

[Cos]

I wondered this a long time ago. I always assumed that because a vehicle with an auto transmission downshifts to rev higher, the ECU would be engineered to be the most efficient as possible.

Therefore I would assume that higher revs and lower throttle angle would yield a better result in most situations.

[crapstixs]

im going to say high rpm light load will get you better mpg.
your engine is the most efficient at around 3000 to 4000rpm.
throttle wide open= no vacuum= fpr commanding full fuel pressure.
pulling some numbers out of my ass, at full throttle low speed the maf will be reading lets say 40g/sec. at high rpm low throttle the maf reading will be like 25g/sec. it has all to do with the load on the engine.

[revelations]

I have a ScanGauge 2 in my car and I tried this once.

My readout shows in litres per hour.

Driving at a steady state speed, (eg in 4th gear):
If I were to shift down to 3rd and maintain same speed, I use more gas than if I went to 5th and stomped on the throttle harder - in fact I use LESS gas that 4th gear if I upshift to 5th.

[VaN_HaMMeRSTeiN]

so how does this all work for turbod engines, seems like for best mileage I should just make sure that I'm not driving around in boost, AKA less than 3000 RPM. If I just keep around 2500 rpm regardless of gear I seem to get the best mileage, if I need to go faster I just up shift and get down to 2500, but if I go under 2500 I have no acceleration (lugging) so I stay out of that area.

[rage2]

Here's a stock fuel map for a b18b Honda motor.



Red is full throttle lugging at 1200rpm.

Blue is low throttle at 4600rpm.

The number is % duty cycle. At high revs, the injectors are open longer, AND for more cycles, so it's a no brainer that high revs low throttle uses much more fuel than lugging at low rpms.