does coasting save petrol

Tobster86

mikey72 wrote: »

So either coasting in neutral, or on overrun in gear, the butterfly is closed, if your foot is off the accelerator.

The only thing that pulls in more air per minute, is more rpm.

You still haven't answered where this supposed air comes from when the butterfly is closed.

mikey72

Tobster86 wrote: »

You still haven't answered where this supposed air comes from when the butterfly is closed.

Several times.
You just won't accept it.

Google is your friend now.

Tobster86

mikey72 wrote: »

Several times.
You just won't accept it.

Google is your friend now.

I've read through all your posts and you haven't answered that question once.

Air enters cylinders via valves in cylinder head, connected to inlet manifold, connected to carburettor, connected to butterfly valve. If the butterfly valve is shut, by what other path does air reach the cylinders? If there is another path, you have logically nothing to lose by telling us and no need to try to hide behind Google. If there isn't another path, then the only logical way to extend your rapidly crumbling argument is to, as you've done, hide behind Google.

Allow me to point out a few more gaping holes in your story.

but then engine braking slows the car dramatically on a petrol engine

Anyone that's ever driven a diesel knows that there is much less engine braking on a petrol. Even my 1.9 litre diesel 306, for the same speed and revs, has massively more engine braking than my 2.5 litre BMW; the engine braking of which is far from dramatic.

so we're back to the type of governor. Either max, min, or constant speed

You mean mechanical centrifugal, servo-mechanical, hydraulic, pneumatic, electromagnetic, etc?
There is no such thing as 'max, min and constant speed' governors where diesel injection is concerned. They all do the same thing; they allow for very high fuel quanitites throughout the rev range, but do not allow the engine to over-rev.
Those little screws that set the minimum & maximum accelerator lever positions on your injector pump are NOT governors.

The ecu won't allow a mixture out of spec.
So it must use more fuel.

No it must not. Any air quantity can enter the engine, controlled by some kind of valve depending on injection system (for petrols). All the ECU has to do is match this with the correct ratio of petrol. Ratio and quantity are different things.

But the lambda sensor can never let it underfuel.

The lambda sensor measures the ratio of oxygen in the exhaust gases. See my previous point.

So, with the butterfly closed, it has to depend on rpm.
Faster rpm, more fuel.

But with the butterfly closed, more fuel would cause it to run rich.

Petrol burns at a ratio of about 14:1
It doesn't change if the pistons push the engine, or if the engine pushes the piston.
It just burns 14:1

The stoichiometric ratio (i.e. for the most complete combustion) of average composition atmospheric air to petrol is about 14:1. It will burn either side of this, provided there is oxygen, it just isn't desireable.
And what the hell is 'pistons pushing engine or engines pushing piston'? There's either combustion happening in the cylinder, exerting effort on the pistons, or there isn't.

EazyDuz

I think a good old test is the only way to prove, its going to take more than one volunteer though.

I dont actually coast normally, more because im lazy. To a brimmed tank in my car, i get around 400 miles before the fuel light flashes.

My fuel is very low now, so im going to brim it again today or tomorrow, and always ALWAYS coast at any given oppertunity.
My fuel lasts about a month so you wont get the results for a while, but i will report back. You guys can sit and play keyboard warrior in the meantime:D

mikey72

Tobster86 wrote: »

I've read through all your posts and you haven't answered that question once.

Air enters cylinders via valves in cylinder head, connected to inlet manifold, connected to carburettor, connected to butterfly valve. If the butterfly valve is shut, by what other path does air reach the cylinders? If there is another path, you have logically nothing to lose by telling us and no need to try to hide behind Google. If there isn't another path, then the only logical way to extend your rapidly crumbling argument is to, as you've done, hide behind Google.

Allow me to point out a few more gaping holes in your story.


Anyone that's ever driven a diesel knows that there is much less engine braking on a petrol. Even my 1.9 litre diesel 306, for the same speed and revs, has massively more engine braking than my 2.5 litre BMW; the engine braking of which is far from dramatic.


You mean mechanical centrifugal, servo-mechanical, hydraulic, pneumatic, electromagnetic, etc?
There is no such thing as 'max, min and constant speed' governors where diesel injection is concerned. They all do the same thing; they allow for very high fuel quanitites throughout the rev range, but do not allow the engine to over-rev.
Those little screws that set the minimum & maximum accelerator lever positions on your injector pump are NOT governors.


No it must not. Any air quantity can enter the engine, controlled by some kind of valve depending on injection system (for petrols). All the ECU has to do is match this with the correct ratio of petrol. Ratio and quantity are different things.


The lambda sensor measures the ratio of oxygen in the exhaust gases. See my previous point.


But with the butterfly closed, more fuel would cause it to run rich.


The stoichiometric ratio (i.e. for the most complete combustion) of average composition atmospheric air to petrol is about 14:1. It will burn either side of this, provided there is oxygen, it just isn't desireable.
And what the hell is 'pistons pushing engine or engines pushing piston'? There's either combustion happening in the cylinder, exerting effort on the pistons, or there isn't.

Long post.

I've given up trying to explain about the butterfly never fully closing now.
And governors seem to still confuse you.

You can carry on if you want to.

Tobster86

mikey72 wrote: »

I've given up trying to explain about the butterfly never fully closing now.
And governors seem to still confuse you.

You can carry on if you want to.

Since I have meaningful points to contribute to the debate, I will do.

So you're (unclearly) suggesting that the butterfly returns to a partially closed, 'idle' position of which, you suggest, it cannot close any further than, and that the engine forcefully sucks a greater-than-idle air mass through this gap?

And regarding your opinion of my knowledge relating to governors; frankly, sticks & stones. I must say I didn't feel confused in the slightest last time I stripped a governor down to it's flyweights.

Instead of attacking other peoples knowledge, would you care to share some of your past experiences with us that qualify yours?

mikey72

Tobster86 wrote: »

So you're (unclearly) suggesting that the butterfly returns to a partially closed, 'idle' position of which, you suggest, it cannot close any further than, and that the engine forcefully sucks a greater-than-idle air mass through this gap?

I'll repeat it for you

mikey72 wrote: »

..........The idle screw keeps the butterfly partially open.........

Maybe you need to see one before you can understand what I've explained to you.

Tobster86 wrote: »

........Instead of attacking other peoples knowledge, would you care to share some of your past experiences with us that qualify yours?

12 years in automotive engine management.
All the way throught the decade from introducing the first single point injection systems, through lean burn, then into catalytic conversion technology used now.
Mechanical diesel injection, onto indirect, then direct injection as it progressed.

But, no matter, you've got a fuel pump in pieces.
To be fair though, I have a garage full of spare carbs for the four cars I still have them on, and a spare 80's supercharger to rebuild to match you with.

tbourner

Interesting:

Since unburned fuel into the converter can cause catastrophic failure, misfire detection is a must for safe converter operation. Misfire detection is described previously. Converter also must have an over temperature detection algorithm to detect excessive temperature in the converter. This is done by decreasing the A/F ratio’s lambda value to less than 1. This algorithm cannot work for coastdown conditions or overrun conditions. Therefore Deceleration fuel cutoff (DFCO) is used to control catalyst temperature during vehicle coastdown, when the engine intake manifold pressure is too low to allow complete combustion. To prevent unburned fuel from entering the converter, the fuel injectors are shut off by the engine controller. Spark advance is filtered and thresholds are set to control torque reversal ”bump” while still protecting the converter.

From http://groups.engin.umd.umich.edu/vi/w2_workshops/OBD_ganesan_w2.pdf

mikey72

tbourner wrote: »

Interesting:



From http://groups.engin.umd.umich.edu/vi/w2_workshops/OBD_ganesan_w2.pdf

Overrun will be the same as idle on some petrol fuel injection, with air supplied via an idle air control valve in some cases.
So the normal map can't cope, as the rpm is too high, but the throttle is closed, and shuts off the fuel.
It switches back on when you approach idle rpm.

Others can achieve the correct mix if enough airflow is present.

Some non cat ones don't need to worry about it, and reduce the injector duty cycle, or switch to an idle injector.

It depends on the age of the car, and the type of injection system.

tbourner

mikey72 wrote: »

Overrun will be the same as idle on a petrol fuel injection, with air supplied via an idle air control valve in some cases.
So the normal map can't cope, as the rpm is too high, but the throttle is closed, and shuts off the fuel.
It switches back on when you approach idle rpm.

Isn't that what we've been saying? It shuts off the fuel on overrun, and keeps it flowing at idle - therefore leaving in gear uses no fuel and coasting in neutral uses the same amount as idle.