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does coasting save petrol
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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.
We've all agreed, some, not all vehicles, eventually cut fuel on overrun. And restart injecting as the engine approached idle again.
But as the the question of whether or not it's more economical to coast further, out of gear with the engine idling continuously in neutral, or to use engine braking later and decelerate faster, then have to apply throttle again, and have the ecu start to use fuel earlier on the ones that do cut fuelling........I've a vauxhall.
On overrun, it does cut fuel eventually, but then engine braking slows the car dramatically on a petrol engine, but after a period of several seconds. It then kicks in again at 1200 rpm or so, and the car will pull on idle, and refuel on load. So cutting fuel isn't for long.
Personally, I find it better to coast, it will run over twice as far, so I can drop into neutral a lot earlier, and offest the cut out.0 -
We've all agreed, some, not all vehicles, eventually cut fuel on overrun. And restart injecting as the engine approached idle again.
But as the the question of whether or not it's more economical to coast further, out of gear with the engine idling continuously in neutral, or to use engine braking later and decelerate faster, then have to apply throttle again, and have the ecu start to use fuel earlier on the ones that do cut fuelling........
Right I get it now.
So you're saying that in a DFCO situation at 3000rpm, with the revs dropping and the car slowing down, if you very slightly touch the throttle to reduce the engine braking force there is no gradient and you're suddenly propelled into full fuelling for 3000rpm? Essentially going from zero fuel to 3 times the idle fuel in an instant?Trev. Having an out-of-money experience!
C'MON! Let's get this debt sorted!!0 -
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 as the the question of whether or not it's more economical to coast further, out of gear with the engine idling continuously in neutral, or to use engine braking later and decelerate faster, then have to apply throttle again, and have the ecu start to use fuel earlier on the ones that do cut fuelling........
Tell us why you don't just switch the engine off, with the clutch in and a high gear selected, ready to bump & go when power is needed (as I used to do before I discovered it's unnecessary)?0 -
Right I get it now.
So you're saying that in a DFCO situation at 3000rpm, with the revs dropping and the car slowing down, if you very slightly touch the throttle to reduce the engine braking force there is no gradient and you're suddenly propelled into full fuelling for 3000rpm? Essentially going from zero fuel to 3 times the idle fuel in an instant?
The fuelling will switch back on.
The ECU controls the amount, and within a set of parameters to smoothly restart the power, but yes.
Then you'll lift off again, and the fuel will eventually shut down again, if you lift off for long enough, and don't drop the revs to low.
Whether the car coasts, or engine brakes, accelerates, brakes again, it'll take the same amount of energy to get to the end of the road.
I figure if I can coast, I'm using the stored momentum of the car to achieve that, plus a little bit of fuel burnt continuously to keep the pistons moving up and down.
Others argue it's better to use the cars momentum to run the engine at a higher speed, then burn more petrol occasionally to replace that momentum lost.
If the fuel isn’t shut off, I believe the second scenario is even worse, as the fuel is also burnt inefficiently to replace the stored energy.0 -
Overrun will be the same as idle on some petrol fuel injection, with air supplied via an idle air control valve in some cases.
I suspect some IACVs have a slightly open position, not really specific to overrun, just when the car is in motion above a certain road speed. I'm a tad unconvinced they're actuated specifically for overrun, though - got any examples?0 -
We've all agreed, some, not all vehicles, eventually cut fuel on overrun. And restart injecting as the engine approached idle again.
But as the the question of whether or not it's more economical to coast further, out of gear with the engine idling continuously in neutral, or to use engine braking later and decelerate faster, then have to apply throttle again, and have the ecu start to use fuel earlier on the ones that do cut fuelling........
And why would you imagine car makers do this? What's their motivation?0 -
The fuelling will switch back on.
The ECU controls the amount, and within a set of parameters to smoothly restart the power, but yes.
Then you'll lift off again, and the fuel will eventually shut down again, if you lift off for long enough, and don't drop the revs to low.
Whether the car coasts, or engine brakes, accelerates, brakes again, it'll take the same amount of energy to get to the end of the road.Others argue it's better to use the cars momentum to run the engine at a higher speed, then burn more petrol occasionally to replace that momentum lost.If the fuel isn’t shut off, I believe the second scenario is even worse, as the fuel is also burnt inefficiently to replace the stored energy.
Because otherwise, this point of detraction, just looks like merely that.0 -
When you say the "same as idle" what do you mean?
I suspect some IACVs have a slightly open position, not really specific to overrun, just when the car is in motion above a certain road speed. I'm a tad unconvinced they're actuated specifically for overrun, though - got any examples?
If you lift off, the throttle will close.
But the ECU will open the idle control valve to keep air supplied to the engine if it hasn't cut the fuelling yet, even if it's above idle.
There must always be an option to keep air supplied, either by a butterfly, by the idle air control valve, or whatever is on the system to do it.
Otherwise everytime you lift off to overtake, change gear, the engine would be starved of air and die.
Even if you lifted off in gear, you've notice a sudden stall if the air was restricted.0 -
Others argue it's better to use the cars momentum to run the engine at a higher speed, then burn more petrol occasionally to replace that momentum lost.
I'm arguing that for any slope where you would increase speed in neutral, if you keep the car in gear and maintain the same speed as you would be in neutral, you'll save fuel.
That's the optimum proof of the concept, rather than saying you'll get more speed out of coasting etc. So how do we prove that without lots of arguing over how cars work in theory?Trev. Having an out-of-money experience!
C'MON! Let's get this debt sorted!!0 -
Depends on how much you've converted to heat by using the brakes.
I don't think that's mere amateur opinion - firstly, why would car manufacturers do this, and why would literature advise it over, over coasting - they really that concerned about people freewheeling and not doing what the IAM advise?
So people keep talking about exceptions from fuel cut off on overrun, on reasonably modern EFI petrol cars. And given some of the very earliest EFI engines did this, let's have some examples of some semi-modern cars / engines that don't cut fuel on overrun (petrol EFI engines / cars) that is.
Because otherwise, this point of detraction, just looks like merely that.
If you're using brakes, don't coast.
Use engine braking as well.
You want to stop.
If you want to travel further, coast.
No one is disputing modern cars do it.
Anything with a cat must, otherwise it'll destroy the cat.
You don't have a choice in it.
It's advertised as economy as people buy that.
And I doubt a car manufacturer would ever advise coasting.0
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