does coasting save petrol

Wongsky

tbourner wrote: »

That's what I've been trying to get across.

It's also why I think cruise control isn't good for economy, maintaining a fixed throttle position is better, meaning you slow down up hills and speed up down the other side. There's a whole other argument though!

I think cruise control often suffers from criticisms like this - which in fairness have some degree of truth about it - but ignoring Clarkson's bandwagon, from that big Audi fuel economy thing he did, a few years back, does ignore some rather more soft factors.

Yes, so true enough, cruise control tends not to be able to anticipate like a driver can - in order to, say, gain a bit of momentum going downhill, in order to reduce the need for as much throttle going up a following hill. But, on the other hand, encourages a more constant speed, with more observation and planning, that quite clearly a lot of drivers dispense with whilst on the road / motorway.

And, can encourage you to stay driving at a certain speed, rather than have your cruising speed fluctuate or increase more, as you respond to traffic around.

I did many thousands of miles in a relatively large engined petrol auto, using cruise control a lot on motorways, and for me, at least, it tended to be a more economical method of driving, because I tended to stick to a more modest speed, whilst using it.

Wongsky

mikey72 wrote: »

That's inherently wrong.
The accelerator controls the airflow, either mechanically or electronically on a petrol engine.

If more acceleration is required in gear, that means more airflow is required and hence more petrol will be injected than in neutral at lowest airflow.

Thing is, on EFI petrol cars, you're getting freebie all the time you're off the throttle, and you probably need only light throttle to maintain such speed - maybe sometimes completely off throttle, maybe a very slight amount of throttle, if the engine braking is slowing you down.

Whereas in neutral, you're getting fuelling to maintain idle, plus, as the road speed is above walking space, typically, idle will be ever-so-slightly boosted - so it will likely be a slightly higher idle than you'd get with the car at rest.

mikey72 wrote: »

And if the vehicle would slow down in gear, more energy is required to move the entire car, as opposed to simply keeping the engine idling

You're ignoring the bits where there would be no fuelling, plus probably making more of the amount of throttle required to maintain - or at least reduce the reduction - of road speed.

mkirkby

Tobster86 wrote: »

Yes it does, it's nothing greatly clever just very finely engineered & machined.

Germans eh??

Tobster86

mikey72 wrote: »

The governer simply stops the engine over reving by limiting fuel, it's got nothing at all to do with anything discuissed here.
We're not talking about the governor limiting high revs, we're talking about keeping it in gear with no throttle.

The reason for having a governor is because the engine requires much more fuel under load than it does to rev at maximum speed without load (which actually takes very little fuel); it is not adequate to just have an accelerator maximum position otherwise it would have no power. Hence why it's called a 'governor' and not a 'maximum fuel limiter', and why there is a completely seperate 'full load fuel setting'. It's you that has completely misunderstood what a governor does.

But since you believe you know what you're talking about enough to make arrogant statements like

It's physics, so what you believe won't really affect it. You need to actually understand how they work.

How about sharing some of your knowledge instead of attacking mine by answering the following relevant questions about carburettors:

A) Given that their regulation is air flow, controlled by a throttle; where does this 'air flow' into the engine that you claim occurs come from when the throttle is closed, as we're describing?

If the answer to (A) is 'Because the suction from the engine is able to overcome the closing spring tension of the butterfly valve/similar throttling mechanism'; what actually prevents the engine from overrunning and where does it get it's engine braking properties from?

C) For the laymen, why is a 'throttle' called a 'throttle'?

Tobster86

mkirkby wrote: »

Germans eh??

The tolerances are absolutely crazy; a few microns. Lining the plunger up with the head is difficult; it just doesn't go in at all until it's correct by a fraction of a degree and then pop, it slides in without any resistance.

The lifespan and reliability of these pumps is absolutely incredible considering what they go through!

mkirkby

Tobster86 wrote: »

The lifespan and reliability of these pumps is absolutely incredible considering what they go through!

Including the veg oil abuse I've been giving it for far too long...although to be fair I do try to help it by lowering the viscosity as much as possible.

It's only the seals that seem to give up (the non-engineering bit I guess)

mikey72

Tobster86 wrote: »

A) Given that their regulation is air flow, controlled by a throttle; where does this 'air flow' into the engine that you claim occurs come from when the throttle is closed, as we're describing?

Take something simple like a basic SU carb.
The idle screw keeps the butterfly partially open, if it fully closes as you describe the engine would stall every time you lift off.

As the petrol flow is controlled by the amount of air passing over the jet, an engine idling at low rpm will have less airflow, and use less petrol, than the same engine with the same partially open butterfly pulling more air through by being turned faster, and trying to fill the cylinders more times per minute.

mikey72

Tobster86 wrote: »

The reason for having a governor is because the engine requires much more fuel under load than it does to rev at maximum speed without load (which actually takes very little fuel); it is not adequate to just have an accelerator maximum position otherwise it would have no power. Hence why it's called a 'governor' and not a 'maximum fuel limiter', and why there is a completely seperate 'full load fuel setting'. It's you that has completely misunderstood what a governor does.

Load makes no difference to the governor.

Regardless of what load is on the engine, the governor will simply cut fuel when maximum rpm is reached.

If it's on no load, the givernor will eventually open, fuel will cut off, the engine will slow, and fuel will resume.
If it's on full load, the governor will eventually open, fuel will cut off, the engine will slow, and fuel will resume.

The only difference will be more fuel is required to be pumped to keep the engine running at maximum rpm, at full load, than at no load. So it'll probably cut off faster with no load, as it'll get to maximum rpm faster, and will indeed then let less fuel through to sustain the rpm than it would under full load, but that's purely based on rpm, and where the plunger ends up.

Tobster86

mikey72 wrote: »

Load makes no difference to the governor.

Regardless of what load is on the engine, the governor will simply cut fuel when maximum rpm is reached.

If it's on no load, the givernor will eventually open, fuel will cut off, the engine will slow, and fuel will resume.
If it's on full load, the governor will eventually open, fuel will cut off, the engine will slow, and fuel will resume.

The only difference will be more fuel is required to be pumped to keep the engine running at maximum rpm, at full load, than at no load. So it'll probably cut off faster with no load, as it'll get to maximum rpm faster, and will indeed then let less fuel through to sustain the rpm than it would under full load, but that's purely based on rpm, and where the plunger ends up.

I didn't say load made a difference to the governor, I said the governor allows for greater fuel quantities than those required to sustain max RPM. Rotation speed controls the governor through the force exerted by the centrifugal governor weights. The governor isn't an on/off switch; it applies a force to the fuel quanitity lever connected to the plunger control sleeve which is proportional to engine speed. As engine speeds increase above idle, for the same accelerator position, the governor will compress the idle spring further; hence resulting is less fuel quantity than at idle.

Your description of a carburettor is adequate; however I remember my old CFI Ford Escort (what an awful car that was) having a vacuum pipe which resulted in rough idling/stalling when disconnected. I could speculate that this mechanism allowed the throttle to fully close at above-idle revs, but in the absense of the equipment or literature will give you the benefit of the doubt.

An engine will not just stall when the fuel is cut unless there is nothing turning it. With the engine switched off completely, at driving speeds you will have smooth engine braking until the last few lurches of cylinder compression as it comes to a halt.


Incidentally, I've devised a test that will end this argument once and for all, and answer on a per-car basis the question "Does my car use more fuel when coasting?"

Before doing this, ensure that you know how the steering wheel lock works and that it won't engage during this test.

In a safe and convenient downhill location, or flat location with sufficient momentum, engage a gear that gives adequate feel of engine braking. Take your foot off the accelerator and perceive the effect of engine braking.

Leaving the car in gear, switch the engine off. If there is an increase in engine braking, your car does not 'overrun' as described, and continues to use fuel when in gear with no throttle. If this is the case, compare the engine braking between coasting and being in gear with no throttle. If there is more relative engine braking from switching the engine off, compared to just lifting the clutch and allowing in-gear engine braking, your vehicle will save fuel by coasting out of gear (I should add, at the expense of brake wear and possibly safety).

If no difference is observed, your car was injecting NO FUEL before you switched the engine off. Coasting out of gear would use more fuel.

Returning the ignition position to 'run' should end the test and return the car to a normal driving state.


It would be nice to see some results against car types if anyone feels like trying this test. Please observe good safety considerations.

mkirkby

Tobster86 wrote: »

In a safe and convenient downhill location, or flat location with sufficient momentum, engage a gear that gives adequate feel of engine braking. Take your foot off the accelerator and perceive the effect of engine braking.

Cool. I'm gonna try it.

Tobster86 wrote: »

Please observe good safety considerations.

Down Sutton bank.....do you think "0800 no win no fee ltd" would take my case :mad: