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Old 30-09-2014, 14:26   #1
tat
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Does engine revs alone determine fuel used, regardless of accelerator use?

A couple of years ago there was a bloke in the radio being interviewed. It was something to do with driving a car more fuel efficiently.

This guy was supposed to be an expert, but the interviewer was just the regular radio presenter, asking simply from his own self as an average motorist.

It got quite funny, thought not ment to be funny.....no one was laughing.

The presenter asked this guy a question of his own which he'd wondered about, which was whether it saved fuel to freewheel. This expert said it was, yes, but it was always better and safer to keep the car in gear. So the presenter then asked, 'So is it just the same whether or not you are in gear as long as you don't have your foot on the accelerator?'.

The expert suddenly stumbled slightly, seemed not quite sure, and then says, 'the fuel used depends on how many revolutions the engine is doing.....whether or not you are depressing the accelerator pedal'.

The presenter seemed baffled and confused by that answer as it defied his logic, saying, 'But surely, if you are not pressing the accelerator pedal it can't be using as much fuel as when you aren't, surely?'.

The expert seemed even more unsure now and decides to simply just hold his position that there was no difference and it all depended upon engine revs, and he tried to say it with conviction, but this conviction just got the presenter even more baffled, clearly a clever bloke who was used to understanding things, being able to grasp things, and so he asked again, trying to understand. And so it went on.

As a listener, I was trying to grasp it, too. Surely if you are not spraying petrol into the carb with the accelerator pedal then you would not be using the same as if you was, surely.

But this expert was adamant that it was the same, that usage was determined by engine revs alone.

I keep wondering about this.....my logic tells me that it is not the same at all....going downhill is not the same as going uphill. But how could an expert be soooo wrong? So can someone just check me on this, am I missing a grasp of how engines work?

tat
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Old 30-09-2014, 14:38   #2
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Re: Does engine revs alone determine fuel used, regardless of accelerator use?

In the case of a steady throttle in a low friction environment (like a boat in water) where engine load is mostly the same at given rpm, yes. Fuel usage is proportional to the cube of engine rpm.

Diesel Engine Fuel Consumption - Quick calculation

In the case of a car, where you are varying the throttle opening to maintain a set speed on a much higher friction media, not so much. It's much more a function of engine load. Think 3k rpm throttle down heading up a steep hill vs 3k rpm foot off the throttle coasting down said steep hill.
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Old 30-09-2014, 14:57   #3
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Re: Does engine revs alone determine fuel used, regardless of accelerator use?

Load on the engine determines fuel use not revs alone. Going up a hill at 3000 rpm takes more energy (fuel) then racing down a hill at 3000 rpm.
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Old 30-09-2014, 15:16   #4
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Re: Does engine revs alone determine fuel used, regardless of accelerator use?

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Originally Posted by ontherocks83 View Post
Load on the engine determines fuel use not revs alone. Going up a hill at 3000 rpm takes more energy (fuel) then racing down a hill at 3000 rpm.
Yes, I thought so, too. But would you say that going downhill in gear but not on the accelerator uses the same as freewheeling downhill just on normal standing tickover?

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Old 30-09-2014, 15:26   #5
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Re: Does engine revs alone determine fuel used, regardless of accelerator use?

Take a modern petrol engine in a typical car. The engine is controlled by the ECU.
If you go down a steep enough hill, and take your foot of the accelerator, the ECU will sense low/no load on the engine, and cut off the fuel supply: No fuel used. The engine will not stall as it is in gear and the wheels are turning the engine via the gear box.

Now, if you decide to go down hill in neutral, the ECU will continue to supply fuel to the engine to prevent it from stalling.

This don't work on boats.
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Old 30-09-2014, 15:37   #6
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Re: Does engine revs alone determine fuel used, regardless of accelerator use?

I think your radio expert was mostly right.
The amount of fuel going into the engine depends upon the amount of air going in... and that is determined by rpm.
The fuel/air ratio should remain constant at all times, wether freewheeling or under load. A cylinder will not take in more air just because it's under load. Therefore it shouldn't be taking in more fuel either.
Ignore electronic fuel injected for a second (Simply because you can't see what the computer is thinking) and look at carbs. The fuel is "sucked" in by the passing air. More air means more fuel. Less air, less fuel. The accelerator pedal opens and closes an air valve allowing more or less air to pass through carb into engine. Fuel use is therefore 100% linked to air use. And a cylinder ALWAYS uses the same amount of air.
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Old 30-09-2014, 15:39   #7
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Re: Does engine revs alone determine fuel used, regardless of accelerator use?

In the GOD carbies worked by the air sucking fuel out of a jet as it rushed past. Closing the throttle did slow the flow but not entirely. Very inefficient and wasteful. EFI has allowed much more efficient fuel management. On a boat it doesn't matter; no uphills and downhills just motor load proportional to speed.
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Old 30-09-2014, 15:40   #8
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Re: Does engine revs alone determine fuel used, regardless of accelerator use?

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Originally Posted by nigel1 View Post
Take a modern petrol engine in a typical car. The engine is controlled by the ECU.
If you go down a steep enough hill, and take your foot of the accelerator, the ECU will sense low/no load on the engine, and cut off the fuel supply: No fuel used. The engine will not stall as it is in gear and the wheels are turning the engine via the gear box.

Now, if you decide to go down hill in neutral, the ECU will continue to supply fuel to the engine to prevent it from stalling.

This don't work on boats.
When you let off the gas going down a hill in gear the engine is still getting fuel. It is very minimal and its just enough to keep it running but it is still running. If you dont believe me try going down a hill and turn off the key. You're going to notice it get quieter very quickly.
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Old 30-09-2014, 15:44   #9
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Re: Does engine revs alone determine fuel used, regardless of accelerator use?

RPM's by itself has very little to do with consumption. 2500 rpm with a clean hull will go faster and use less fuel use than the same hull with heavy growth using the same rpm's. A mechanical governor will sense the added load through the (centrifugal weights & speeder spring) requiring the operator of a dirty hull to bump the throttle to a slightly higher rpm position to maintain 2500 rpms which in turn, pumps more fuel into the cylinders.
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Old 30-09-2014, 15:47   #10
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Re: Does engine revs alone determine fuel used, regardless of accelerator use?

First off you can't compare Diesels to Spark ignition engine and you can't compare carbureted to fuel injected.
Diesel have no throttle valve, a full cylinder of air is drawn in regardless of throttle position, Diesels are throttled by fuel alone, Diesels have no vacuum in the intake.
Modern spark ignition will not completely turn the fuel off on coast, they could but don't in order to protect the catalytic convertor.
Brake specific fuel consumption is the number used to determine fuel consumption per HP made, Generally the lower the RPM an engine can turn and still make the required HP, due to less friction, the more efficient the engine, A Prius for example will often operate at very low RPM but at very high throttle settings, this decreases pumping loses as there is no vacuum in the intake and lower RPM reduces friction.
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Old 30-09-2014, 15:50   #11
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Re: Does engine revs alone determine fuel used, regardless of accelerator use?

Fuel mixture will vary somewhat in a spark ignition engine, it is not always stochimetric (sp?) At high power settings a much richer mixture is desired both for power and to protect the engine from detonation, low power and you can lean the snot out of it for better fuel consumption, but due to emissions lean burn engines aren't very common anymore.
Fuel air ratio in a Diesel varies tremendously
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Old 30-09-2014, 15:53   #12
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Re: Does engine revs alone determine fuel used, regardless of accelerator use?

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... And a cylinder ALWAYS uses the same amount of air.
A cylinder always uses the same VOLUME of air, but the pressure at which that air is inserted determines the total MASS of the air. In gasoline engines the air/fuel ratio is generally somewhat fixed, so the amount of air (and fuel) pulled into the cylinder is controlled by the throttle plate, which changes the manifold pressure and thus the mass of air (and therefore fuel) pulled into the cylinder.

In a (normally aspirated) diesel the air gets pulled into the cylinder directly, and so the mass and volume is fairly constant. The amount of fuel per revolution is controlled by the sleeve in the injection pump. The governor on the pump tries to maintain the same RPM for a given throttle setting, and thus advances or retards the sleeve depending on load. So, the air/fuel ratio varies. Diesels in general will run anywhere from about 14:1 to 60:1 air/fuel ratio, but most engines have a much narrower range (usually around 15:1 to 20:1 or 24:1).

Either way, there is some variability in fuel usage at a given RPM, the amount of variability depends on the programming and/or mechanical design of the engine.
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Old 30-09-2014, 15:56   #13
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Re: Does engine revs alone determine fuel used, regardless of accelerator use?

In short Old Snipe pretty much has it right, in short if I down shift my car and turn twice the RPM, but keep speed the same I will not double my fuel consumption, there will be a little increase in fuel consumption, just enough to offset the increased friction of the engine at higher RPM, but since speed and therefore power made is the same fuel consumption doesn't go up as much as you might think.
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Old 30-09-2014, 15:58   #14
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Re: Does engine revs alone determine fuel used, regardless of accelerator use?

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Fuel use is therefore 100% linked to air use. And a cylinder ALWAYS uses the same amount of air.
Ummm no. With gasoline engines you have a useable fuel air mixture ratio ranging roughly from 10:1 all the way to 17:1 so the amount of fuel does vary with load. if you need power fuel consumption increases closer to 10:1 if you are coasting it goes closer to 17:1. hence fuel consumption varies in relation to load
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Old 30-09-2014, 15:58   #15
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Re: Does engine revs alone determine fuel used, regardless of accelerator use?

Thanks for settling that for me. So there you go, learn something everyday.

Whenever there has been a fuel supply crisis, not so much now, certainly in decades gone by, they all used to freewheel, in fact the government recommended it, if I remember rightly.

I'm going to start doing it, knocking it into neutral whenever I can. I actually like driving very steady and gently, which is also fuel efficient, of course, but it is difficult to do that because everyone is push, push, push, hurry, hurry, hurry.

My car is an old citroen xsara, 1600. I've had it from new but always changed the oil twice a year. It is a good car. I've been going up to Scotland quite a bit on the Motorways, crusing at 60mph, at the same speed as the big waggons do, and I topped the tank right up to the brim so that I could see the petrol, and then zeroed the counters and topped up again straight away when I got their. I was getting over 60 mpg! This car is fourteen years old, not one of the newer more fuel efficient ones. But I was driving very carefully with a light and steady touch on the gas on purpose to see just how good i could do. I think it makes a big difference how you drive. Anyway, thanks for settling that in my mind, that it does use more than freewheeling with the engine idling. I'm quite surprised.
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