It's a very complex topic, with no 'right' answer I don't believe.

In very basic terms, the lower the throttle the shorter the injection pulsewidth (i.e. the injectors are open for less time, and hence inject less fuel given fixed fuel pressure and flow rates). The lower the RPM the lower the pulsewidth as well.

Conversely, higher throttles and RPMs increase the pulsewidth.

Injector pulsewidth peaks at peak torque (or it should in ideal conditions - obviously with the complex nature of an engine it might be that this isn't exactly the case), but because the engine is spinning faster it's injector more frequently. So peak fuel flow is at peak RPM.

Ultimately, the question is which is better - accelerating from 1500rpm to 3000rpm using around 30% throttle for 50 seconds OR accelerating from 2500 to 4000rpm using 70% throttle for 20 seconds and then cruise at 2000rpm for the remaining 30 seconds. Assume that the road speeds in each test are the same (i.e. different gears).

Personally I believe that the latter is more efficient, and hence in your example the manual mode in a lower gear with more throttle will give better mileage than the auto mode choosing a higher gear.

I could give you the fuelling map of my race car (which I know isn't as complicating or refined as a road car map that will have spent weeks being developed on a dyno and on the road), and you can use that to work out various possibilities...

I dug through a stack of books and found these neat figures in Internal Combustion Engine Fundamentals by Heywood.

It shows that throttle position is the limiting factor for air flow with an intake manifold pressure < 40 cmHg. Limiting the air flow too greatly obviously restricts efficiency.

In the other figure, turbulent flow is shown especially at low throttle openings.

Quote :

Except at or close to wide-open throttle, the throttle provides the minimum flow
area in the entire intake system. Under typical road-load conditions, more than
90 percent of the total pressure loss occurs across the throttle plate.

Obviously it's still not that simple, WOT will most likely not result in the best fuel efficiency. But low throttle positions are bad when accelerating.

Quote from Ball Bearing Turbo :

Will an engine burn more fuel at lower RPM but much wider throttle, for say 6 seconds, or at higher RPM but less throttle for 33% less time.

my take on the question...

if you're below the power band (ie: gear too high, speed too low), and you put your foot on the gas, it's going to try dumping as much fuel into the cylinders as it can. when you take your foot off the gas, it deceases that fuel flow to idle levels. even though the engine isn't rotating as much at lower speeds, i would have to believe the former uses more fuel.

on a side note though, i don't know how EFI would play into this, i'm only going by what i remember from school. i would imagine that EFI throws my theory into chaos because of all the electronic mapping involved.

optimally, you want to keep a road car under 4000rpm. from a dead stop, an 5 speed automatic would probably want to start WOT in 2nd, go to 3rd, then 4th. but by the time you get into 4th, you're probably going ~100-120kmh and most likely, the second you lift your foot off the gas, the overdrive gear will engage. but again, more electronic mapping, it'll vary car-to-car.

Agreed, and I don't think anyone would try to argue that small throttle openings are efficient. But is the decrease in throttle efficiency offset by less time spent at higher speed (i.e. less net drag), and how much less throttle do you need to accelerate 'steadily' in 4th gear as opposed to fairly briskly in 3rd gear? It might be that from 20mph, similar throttle openings in different gears produce noticeably different accelerations.

Quote from bunder9999 :

my take on the question...

if you're below the power band (ie: gear too high, speed too low), and you put your foot on the gas, it's going to try dumping as much fuel into the cylinders as it can. when you take your foot off the gas, it deceases that fuel flow to idle levels. even though the engine isn't rotating as much at lower speeds, i would have to believe the former uses more fuel.

on a side note though, i don't know how EFI would play into this, i'm only going by what i remember from school. i would imagine that EFI throws my theory into chaos because of all the electronic mapping involved.

optimally, you want to keep a road car under 4000rpm. from a dead stop, an 5 speed automatic would probably want to start WOT in 2nd, go to 3rd, then 4th. but by the time you get into 4th, you're probably going ~100-120kmh and most likely, the second you lift your foot off the gas, the overdrive gear will engage. but again, more electronic mapping, it'll vary car-to-car.

Not quite. Fuel is metered. On carbs it is metered by depression (pressure drops across the carburettor relative to ambient (in the case of normally aspirated engines). At low revs, the air velocity isn't that high, as the piston is going down very quickly, so the pressure across the carbs is not as low as full throttle high revs, so hence less fuel is dumped in.

On EFI systems you can control how much fuel at any given moment is fed into the engine. You could have 8ms worth of fuel injected at 60% throttle and 4000rpm. And you could set it so that at 60%/4250rpm it inject no fuel at all (which would be silly, but what I'm trying to say is that the fuel amount can be controlled independently of throttle and speed).

Carbs on closed throttle always feed a bit of fuel in (unless they have some form of solenoid that halts fuel delivery in certain conditions like overrun... but they were measures taken by manufacturers before the widespread adoption of injection in the 70s), whereas on injection you can choose what to more easily.

My race car always injects fuel when the engine is running, even on the overrun. It helps pickup, it helps keep things cooler. Thus I go faster and my engine lasts longer.
Most road cars cut the fuel completely on the overrun until about 500rpm above idle. But modern engine managements are so complex that under various conditions - a hot catalyst for example, although I'm sure there are countless others - it might choose to inject some or lots of fuel on the overrun.

A final (ish) word - when my Dad and I drive somewhere together, he often drives there and I often drive back. In his car, we do an economy run, to see who can get the biggest number on MPGs. Not because I worry about climate change, but because we're competitive, and it's a fun game. He drives gently, changes gear as little as possible, but makes good time. I tend to do the same for the first 5 minutes, then get bored and drive home normally. Our journeys take about the same time. My MPG is usually 2 higher than his. It could be I get better traffic, or the conditions of the day help, and 2mph is too small to be statistically useable really... but I've won every time bar one, and we've played that game lots.

Quote from tristancliffe :

A final (ish) word - when my Dad and I drive somewhere together, he often drives there and I often drive back. In his car, we do an economy run, to see who can get the biggest number on MPGs. Not because I worry about climate change, but because we're competitive, and it's a fun game. He drives gently, changes gear as little as possible, but makes good time. I tend to do the same for the first 5 minutes, then get bored and drive home normally. Our journeys take about the same time. My MPG is usually 2 higher than his. It could be I get better traffic, or the conditions of the day help, and 2mph is too small to be statistically useable really... but I've won every time bar one, and we've played that game lots.

that's not exactly a fair test, because it's not the same trip both ways, between weather and the road's elevation changes... even if you did drive the same path twice it still wouldn't be a fair test because of the weather... if you did the same trip at the same time using two cars, it would still be unfair because each car isn't built (and operates) equally.

I know it isn't entirely fair - I think I mentioned that. But having done it many many times now, it seems that whatever unfairness there is always (except once) favours me. So either I drive slightly more economically, or the weather becomes more favourable for me every time.

I played the same kind of game with my granddad a couple of times and although he's pretty anal to get the lowest consumption as possible, the difference was never quite as big as one would expect. I tend to disregard speed limits on open roads and motorways a lot and I'm certainly not paying any excessive attention to how much fuel the engine needs. The only things I do is trying not to brake when I see an upcoming obstacle (a city, major speed limit, heavy traffic..) but rather gradually slow down. I also try to stay at high gear when I know I won't need to accelerate anytime soon (like when I'm trapped in heavy traffic with no chance to overtake). Even though the car has a 1.2 litre 3-banger engine that just won't get the car moving unless you really put your foot down, I've never had more than 1 l/100 km higher consumption than my granddad, which isn't bad at all IMO.
I wonder how would the difference change if we tried it in a car with a less pathetic engine though...

Quote from MadCatX :

but rather gradually slow down

wouldn't it be more economical to just put your foot on the clutch the instant you know you need to slow down, and brake it out? i know "they" tell you not to do that because you might need engine power to avoid an accident, but i don't see the difference between taking your foot off the brake and taking your foot off the clutch...

I've always wondered this as well, and I think it's something that can't easily be answered with just "this is how it is".

I had always held the belief that WOT acceleration, then dropping to the lowest possible RPM for cruising. However, this theory was ruined when I discovered that my MR2 got considerably better mpg when at 85 compared to 70. 85mph had the engine speed very close to that of peak torque, which is what I attributed the improvement to.

I haven't added anything useful to the discussion.

Quote from bunder9999 :

wouldn't it be more economical to just put your foot on the clutch the instant you know you need to slow down, and brake it out? i know "they" tell you not to do that because you might need engine power to avoid an accident, but i don't see the difference between taking your foot off the brake and taking your foot off the clutch...

The point is that, with newer cars, it's more economical to engine brake as no fuel is injected. If you depress the clutch, fuel will have to be injected to keep the engine idling.

Quote from RasmusL :

The point is that, with newer cars, it's more economical to engine brake as no fuel is injected. If you depress the clutch, fuel will have to be injected to keep the engine idling.

That's exactly it. If you see there is a village coming from the distance, the best way to save fuel is to leave the car in gear in and take the foot off the gas. If you know your car well enough, it's quite easy to reach the speed limit just as you enter the village, shift to a proper gear and carry on without using a bit of fuel while slowing down.

Exactly. The wheels drive the engine, and with no fuel it's just an air pump (with the throttle restricting the intake) so no need to worry about being lean. That's been covered to death on this and every other 'car forum' though.

How come there's no appreciable change in noise when you're engine braking (and so no fuel is being burnt)?

Is it just that most of the noise of a 'normal' engine is mechanical, rather than due to combustion?

Noise from the engine would be similar but the noise from the exhaust should be very different.

Quote from MadCatX :

It's not, it can't be, the fundamental physical principles behind CVT make it better. CVT has a bit more complex construction than an ordinary gearbox and it would probably take some getting used to and tweaking if you wanted to use it in a race car, but as far as fuel economy and acceleration goes, CVT is the way to go...

still wouldnt want it in my car.
my scooter was bad enough (acclerating at constant same rpm is just...annoying).
i rather "waste" more fuel but have fun driving

In full auto the car probably goes into too high a gear for the situation then rides the torque converter/slips the clutch wasting fuel.

Ive had 2 autos and drive my parents now and again and its damn obvious. Even driving thru town it will go into 5th and then slip and unless u step on the gas hard to get into a reasonable gear, it will go around the place sounding like a scooter or go-kart gearbox...

I just keep the car in 3rd or 4th to keep it with locked torque converter rather than scooter mode...

Quote from [RCG]Boosted :

still wouldnt want it in my car.
my scooter was bad enough (acclerating at constant same rpm is just...annoying)

Nor would I, shifting properly to keep the engine at optimal RPM is part of the fun You're also pretty much at the mercy of the gearbox's electronic which decides what RPM will your engine run at.
I mentioned it 'cause I'm getting a tiny bit annoyed with S14 stating obvious BS (again).

kay my opinion is BS cause I've not driven autos or had family and friends with autos....ok no-one I know or a car I have driven has been after 2007 but yeah.

Quote from tristancliffe :

Exactly. The wheels drive the engine, and with no fuel it's just an air pump (with the throttle restricting the intake) so no need to worry about being lean. That's been covered to death on this and every other 'car forum' though.

yeah, i think we had that conversation here before, talking about driving downhill... but since we're talking about engine braking, doesn't that wear your clutch out more?

why should it? The clutch is there to transfer forces from the engine to the wheels/gearbox, why not the other why around? You keep it fully engaged, not letting it slip.

The problem with longtime engine breaking (like on a mountain pass) is iirc that diesel cars tend to overheat. Their was an explanation somewhere but I can't find it now.

If you're engine braking downhill, the clutch should only wear if you downshift for nicreased braking, and you can reduce that massively by blipping the throttle appropriately for rev matching.

Quote from ACCAkut :

The problem with longtime engine breaking (like on a mountain pass) is iirc that diesel cars tend to overheat. Their was an explanation somewhere but I can't find it now.

You mean overheating by running too lean? The fuel vapor actually does cool the engine to a degree before igniting, but I doubt any modern engine with an intact cooling system would have a problem with that...

Wondering how the off-throttle fuel usage works with an auto? When I let off the throttle at speed in my wife's car, the engine comes down close to idle speed. It's not at idle, but maybe about 1200 RPM (idles at 900 I think) or so no matter what speed I'm running at. Thus, is the fuel flowing to keep it running or is a little bit of wheel speed still being transfered back to the engine keeping it running?

If you have a classic A/T with a torque converter, the engine speed falls to something close to idle, because the engine is not locked to the gearbox, but rather spins somewhat independently to it's speed. Therefore it needs to burn some fuel to stay running. My guess is that a car with A/T would travel a bit further with a foot off the throttle, because some torque is transferred to the wheels even when the engine is idling, but at the expense of burning some fuel. I don't dare saying which saves you more fuel, my bet is on M/T, but I guess the difference is quite negligible...