M
Mickymoody
Ask them, I don't understand it!
Is it true
- Thread starter Biguns
- Start date
Ask them, I don't understand it!
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You are driving along in your car, in whatever gear. at aspecific engine revs, lets say 2500rpm. This will (lets imagine this road is level) maintain a certain speed.
If you took your foot of the accelerator The engine is not producing the power to maintain that speed and in effect is creating a drag on the cars momentum thus slowing it down.It is decellerating. It will continue to slow down, helped greatly by the effort of trying to turn the engine, that does not want to turn at that RPM, till it reaches a speed commensurate with the engine power produced by the lesser throttle setting.
If the effort to move the vehicle is greater than the power the engine can produce at this (tick over) throttle setting, the engine will stall.
You are riding your bike along the same road ( you are the engine in this case) You are pedaling at acertain RPM which produces a certain speed. You stop pedaling, the bike will continue to move forward with nothing to impede its progress except gravity and air resistance. Therefore it wuld continue to move forward till its kinetic energy had expired. Therefore unlike the engine in the car. you not producing power has no effect on the speed at which the bike will come to a halt
It is like a ratchet. in one direction there is grip in the other direction there is no grip
Saab produced cars with this feature 40 years ago. See the links on my post of Aug 24
If you took your foot of the accelerator The engine is not producing the power to maintain that speed and in effect is creating a drag on the cars momentum thus slowing it down.It is decellerating. It will continue to slow down, helped greatly by the effort of trying to turn the engine, that does not want to turn at that RPM, till it reaches a speed commensurate with the engine power produced by the lesser throttle setting.
If the effort to move the vehicle is greater than the power the engine can produce at this (tick over) throttle setting, the engine will stall.
You are riding your bike along the same road ( you are the engine in this case) You are pedaling at acertain RPM which produces a certain speed. You stop pedaling, the bike will continue to move forward with nothing to impede its progress except gravity and air resistance. Therefore it wuld continue to move forward till its kinetic energy had expired. Therefore unlike the engine in the car. you not producing power has no effect on the speed at which the bike will come to a halt
It is like a ratchet. in one direction there is grip in the other direction there is no grip
Saab produced cars with this feature 40 years ago. See the links on my post of Aug 24
M
Mickymoody
You are in 1st gear, at idle speed, you let the clutch out, without pressing the accelerator. The car chugs along at idle rpm. It doesn't stall, until coming to an incline.
You are in a multistorey car park, you select 1st gear, at the top of a slope, again with no accelerator, the car doesn't speed up downhill, the engine acts as a brake. Something that 4x4 drivers have been using for years.
So what point are you making?
You are in a multistorey car park, you select 1st gear, at the top of a slope, again with no accelerator, the car doesn't speed up downhill, the engine acts as a brake. Something that 4x4 drivers have been using for years.
So what point are you making?
What on earth are you talking about?
I am trying to explain (to people who don't seem to be able to grasp it) in simplistic terms the workings of the free wheel principal as applied to the motor vehicle
Is there anyone with educational skills can help me out here
I am trying to explain (to people who don't seem to be able to grasp it) in simplistic terms the workings of the free wheel principal as applied to the motor vehicle
Is there anyone with educational skills can help me out here
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I haven't read the whole of this thread, so apologies if I'm just duplicating stuff. However, most modern cars DO have over-run fuel cut-offs. That means that if the vehicle is moving, in gear, and the throttle is released, the injectors stop supplying fuel to the engine. This happens on petrols AND diesels. Over-run fuel cut off started to appear in the 1990s, and the programming was relatively crude, leading to jerkiness as the throtle was re-applied and stalling (sometimes) if the clutch was suddenly depressed. These days, the systems are much more sophisticated and gradually re-introduce the fuel as the road speed drops. It is, therefore, true that you can use more fuel by coasting with the engine idling than on over-run whilst in gear.
There is, of course, a point were one method becomes more fuel-efficient than the other.
As has been said, on over-run, the wheels turning the engine will create a fair bit of drag. Dipping the clutch and coasting, will remove this drag (good for fuel consumption) but then the engine will start idling - and thus use some fuel in the process. Going down a long mountain pass at high speed, it MIGHT be more fuel efficient to coast, but slowing down at 30 for a set of lights in a relatively high gear (4th or 5th), it's likely to be more efficient to leave it in gear and de-clutch at the last minute.
If anyone has a VAG car with a trip computer, put it on "instantaneous fuel consumption" and try the experiment. Lifting off the throttle whilst in gear, the display shows a series of dashes - signifying infinite fuel economy, because it can't display more digits than 999 MPG. Push the clutch in and it will read a very high number, but not as high as "zero consumption"!
So you pays your money and takes your choice! You can coast - and travel further at (say) 300MPG, or you can leave the clutch in, not travel as far due to the engine drag, but use zero fuel for a shorter distance!
There is, of course, a point were one method becomes more fuel-efficient than the other.
As has been said, on over-run, the wheels turning the engine will create a fair bit of drag. Dipping the clutch and coasting, will remove this drag (good for fuel consumption) but then the engine will start idling - and thus use some fuel in the process. Going down a long mountain pass at high speed, it MIGHT be more fuel efficient to coast, but slowing down at 30 for a set of lights in a relatively high gear (4th or 5th), it's likely to be more efficient to leave it in gear and de-clutch at the last minute.
If anyone has a VAG car with a trip computer, put it on "instantaneous fuel consumption" and try the experiment. Lifting off the throttle whilst in gear, the display shows a series of dashes - signifying infinite fuel economy, because it can't display more digits than 999 MPG. Push the clutch in and it will read a very high number, but not as high as "zero consumption"!
So you pays your money and takes your choice! You can coast - and travel further at (say) 300MPG, or you can leave the clutch in, not travel as far due to the engine drag, but use zero fuel for a shorter distance!
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Car may use zero fuel when on the over-run, but won't travel very far. Dip the clutch at 50mph on a decline and your sped will hardly be effected, leave it in gear and you will soon have to use the accelerator!
2.2 litre diesel uses 0.75litres per hour idling. there for anything over 8mph is a bonus.
15mph = 90mpg
30mph = 180mpg
45mph = 270mpg
2.2 litre diesel uses 0.75litres per hour idling. there for anything over 8mph is a bonus.
15mph = 90mpg
30mph = 180mpg
45mph = 270mpg
It depends. Lift off in 2nd at 40 going up a hill and you won't get far. Lift off in 6th at 50 going down a hill and you'll get much further! It's almost impossible to provide any kind of "rule of thumb" as to when it's better to coast and when it's better to lift off, because there are so many factors to take into account.
This is fact I Run many a diagnostic live data recordings in previous jobs and when reprogramming my own ecu's
Examples Down hill
THE ECU maintain IDLE
put the car in neutral or dump the clutch. not touching the accelerator
The ECU will have to provide fuel via the injectors,
It input the air via the idle control Valve since the throttle is fully shut.
this means the ECU is imputing fuel just to Idle the engine. Putting the injector on a duty cycle of about 3%
ENGINE maintains IDLE
Leaving the car in gear
The engine does not require fuel as the engine is turned over by the momentum the car. Via the Wheels gearbox.
This is any on hill and Zero throttle position.
The gear you are make no difference to fuel use ( still zero) providing the engine is still being "powered" by the cars momentum.
the gear you are in will just effect you speed you go.
Braking is possible as if you are in 4th
You slow down via the brakes. The rev's will drop till the engine reaches the stall point ( eg about 750rpm depending on engine) when the ECU will chuck loads of fuel in to try and prevent this, However if the revs stay above this point lower gear for example NO FUEL will be used
Examples Down hill
THE ECU maintain IDLE
put the car in neutral or dump the clutch. not touching the accelerator
The ECU will have to provide fuel via the injectors,
It input the air via the idle control Valve since the throttle is fully shut.
this means the ECU is imputing fuel just to Idle the engine. Putting the injector on a duty cycle of about 3%
ENGINE maintains IDLE
Leaving the car in gear
The engine does not require fuel as the engine is turned over by the momentum the car. Via the Wheels gearbox.
This is any on hill and Zero throttle position.
The gear you are make no difference to fuel use ( still zero) providing the engine is still being "powered" by the cars momentum.
the gear you are in will just effect you speed you go.
Braking is possible as if you are in 4th
You slow down via the brakes. The rev's will drop till the engine reaches the stall point ( eg about 750rpm depending on engine) when the ECU will chuck loads of fuel in to try and prevent this, However if the revs stay above this point lower gear for example NO FUEL will be used
M
Mickymoody
And that's when it stalls.
Ohhh dear.
Some people just can't, or don't want to grasp it.
Some people just can't, or don't want to grasp it.
The simple way is we all had a fixed wheel bike as kids,
Peddle your legs ( engine) you go forward
stop peddling and you slow down but
your legs still turn with no effort from you ( you use no energy)
Big hill means legs spin very fast with no effort from you ( till your feet fall off the peddles and you destroy you legs
)
Peddle your legs ( engine) you go forward
stop peddling and you slow down but
your legs still turn with no effort from you ( you use no energy)
Big hill means legs spin very fast with no effort from you ( till your feet fall off the peddles and you destroy you legs
)You are very welcome. May we ask what your interest in it is.
M
Mickymoody
So one of the links stems from a link to Jeremy Clarkson.....great! And the other... the opening statement says, this isn't true......
That's the evidence?
WOW, I really am not concerned with anything you say in the world, as you got it so badly wrong?
You post evidence, and the evidence is so badly wrong? and you believe it?
That's the evidence?
WOW, I really am not concerned with anything you say in the world, as you got it so badly wrong?
You post evidence, and the evidence is so badly wrong? and you believe it?
Micky, what WOULD it take to convince you? Plenty of people have tried to explain why a car won't stall if you cut the fuel off on over-run. Many, like me, have some connection with the automotive industry. Modern injection systems DO cut the fuel off on over-run. As the engine approaches idle speed, they gradually restore the supply of fuel - precisely to prevent stalling. Lift off the throttle at (say) 3000 RPM and no fuel will enter the engine on most modern cars. As you slow down (still in gear), the revs drop. As they approach idle speed, the fuel supply is gradually reinstated.
Anyway, I completely agree that Mr. Clarkson should not be taken as a font of knowledge on matters technical, but a quick "Google" suggests that he is not alone in thinking this!
Here's what the AA have to say about it:
"...Old car with carburettor - take your foot off the accelerator pedal with the car in gear and fuel is still drawn through into the engine. Fuel savings could be made by coasting out of gear.
Modern car with electronic engine management - fuel and ignition systems are effectively combined and controlled by one Electronic Control Unit (ECU). Take your foot off the accelerator and the ECU cuts the fuel supply to the injectors anyway so there's nothing to be gained by coasting.
Modern diesel engines - these also have the ability to shut off the fuel when you take your foot off the accelerator."
http://www.theaa.com/motoring_advice/fuels-and-environment/drive-smart.html
Also worth a look at this book, which explains a fair bit about engine management systems:
http://books.google.co.uk/books?id=...=onepage&q=overrun fuel cut off Bosch&f=false
Lastly, I can recommend the "Bosch Automotive Handbook" (I have an ancient 4th edition from the mid 1990s!) also mentions how it works - and that's going back a while!
Anyway, I completely agree that Mr. Clarkson should not be taken as a font of knowledge on matters technical, but a quick "Google" suggests that he is not alone in thinking this!
Here's what the AA have to say about it:
"...Old car with carburettor - take your foot off the accelerator pedal with the car in gear and fuel is still drawn through into the engine. Fuel savings could be made by coasting out of gear.
Modern car with electronic engine management - fuel and ignition systems are effectively combined and controlled by one Electronic Control Unit (ECU). Take your foot off the accelerator and the ECU cuts the fuel supply to the injectors anyway so there's nothing to be gained by coasting.
Modern diesel engines - these also have the ability to shut off the fuel when you take your foot off the accelerator."
http://www.theaa.com/motoring_advice/fuels-and-environment/drive-smart.html
Also worth a look at this book, which explains a fair bit about engine management systems:
http://books.google.co.uk/books?id=...=onepage&q=overrun fuel cut off Bosch&f=false
Lastly, I can recommend the "Bosch Automotive Handbook" (I have an ancient 4th edition from the mid 1990s!) also mentions how it works - and that's going back a while!
Avocet
Do not get drawn in. I am embarrassed to admit that it has taken me a while to realise that this individual has one of two problems. (See some of his other posts)
1/He has not got the capacity to understand how this simple electro/mechanical feature operates, and is therefore in denial. Which is sad.
2/He does understand, and it is his pathetic attempt to have sport with genuine posters. Which is very sad.
Do as I do now, and just ignore.
Do not get drawn in. I am embarrassed to admit that it has taken me a while to realise that this individual has one of two problems. (See some of his other posts)
1/He has not got the capacity to understand how this simple electro/mechanical feature operates, and is therefore in denial. Which is sad.
2/He does understand, and it is his pathetic attempt to have sport with genuine posters. Which is very sad.
Do as I do now, and just ignore.
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