Fuel cut off switch
Discussion
The following quote comes from an article I read on eco-driving.
“...When slowing down, it’s important to remain in gear as the fuel cut-off switch in a fuel injection engine is then activated, meaning virtually no fuel is used while braking...”
I worked on engines back in the 70s and I am under the impression that internal combustion engines need fuel to force the pistons down. Right now, I am finding it hard to believe that the engine can run without fuel.
[Move forward 50 years].
I am still finding it hard to believe that an internal combustion engine can run without fuel.
What happens now when the fuel is cut off, does the ECU kick and send a signal to the flywheel to keep the engine ticking over, or is something else taking over?
On another trial, if I leave the engine ticking over when stationary, I can see the range dropping by the minute so it looks obvious to me that the vehicle should be left in gear to activate the injector cut-off.
I am still thinking though that there would be at least, a small thimble full of fuel going through the injectors.
“...When slowing down, it’s important to remain in gear as the fuel cut-off switch in a fuel injection engine is then activated, meaning virtually no fuel is used while braking...”
I worked on engines back in the 70s and I am under the impression that internal combustion engines need fuel to force the pistons down. Right now, I am finding it hard to believe that the engine can run without fuel.
[Move forward 50 years].
I am still finding it hard to believe that an internal combustion engine can run without fuel.
What happens now when the fuel is cut off, does the ECU kick and send a signal to the flywheel to keep the engine ticking over, or is something else taking over?
On another trial, if I leave the engine ticking over when stationary, I can see the range dropping by the minute so it looks obvious to me that the vehicle should be left in gear to activate the injector cut-off.
I am still thinking though that there would be at least, a small thimble full of fuel going through the injectors.
When under power, fuel goes to the engine which, via the transmission, turns the driving wheels.
When slowing, it’s the wheels that turn the engine, via the transmission - so no fuel is needed for combustion. However, if you coast or slow or stop in neutral, or with clutch depressed, fuel will be needed to turn the engine over (assuming no stop/start system is in play).
ETA:
When you are stationary and the engine is running, the instantaneous fuel consumption will be zero mpg. That is you are going nowhere (zero miles) but you are using fuel. Similarly, when you start to slow down in gear, the mpg will rise and tend to infinity. In reality, most instantaneous fuel consumption indicators normally show a blank (usually a series of dashes rather than zero), because the system is set-up so as not to show unrealistic figures.
When slowing, it’s the wheels that turn the engine, via the transmission - so no fuel is needed for combustion. However, if you coast or slow or stop in neutral, or with clutch depressed, fuel will be needed to turn the engine over (assuming no stop/start system is in play).
ETA:
When you are stationary and the engine is running, the instantaneous fuel consumption will be zero mpg. That is you are going nowhere (zero miles) but you are using fuel. Similarly, when you start to slow down in gear, the mpg will rise and tend to infinity. In reality, most instantaneous fuel consumption indicators normally show a blank (usually a series of dashes rather than zero), because the system is set-up so as not to show unrealistic figures.
Edited by Pica-Pica on Friday 5th August 21:17
Pica-Pica said:
When under power, fuel goes to the engine which, via the transmission, turns the driving wheels.
When slowing, it’s the wheels that turn the engine, via the transmission - so no fuel is needed for combustion. However, if you coast or slow or stop in neutral, or with clutch depressed, fuel will be needed to turn the engine over (assuming no stop/start system is in play).
ETA:
When you are stationary and the engine is running, the instantaneous fuel consumption will be zero mpg. That is you are going nowhere (zero miles) but you are using fuel. Similarly, when you start to slow down in gear, the mpg will rise and tend to infinity. In reality, most instantaneous fuel consumption indicators normally show a blank (usually a series of dashes rather than zero), because the system is set-up so as not to show unrealistic figures.
Thanks for that information, it all makes sense now. When slowing, it’s the wheels that turn the engine, via the transmission - so no fuel is needed for combustion. However, if you coast or slow or stop in neutral, or with clutch depressed, fuel will be needed to turn the engine over (assuming no stop/start system is in play).
ETA:
When you are stationary and the engine is running, the instantaneous fuel consumption will be zero mpg. That is you are going nowhere (zero miles) but you are using fuel. Similarly, when you start to slow down in gear, the mpg will rise and tend to infinity. In reality, most instantaneous fuel consumption indicators normally show a blank (usually a series of dashes rather than zero), because the system is set-up so as not to show unrealistic figures.
Edited by Pica-Pica on Friday 5th August 21:17
Now I need to make adjustments to my approach to junctions. As a member of IAM, I have always de-clutched and rolled up to a junction and then taken the appropriate gear. With this new information and thinking about eco driving, I think the way forward is to change gear once on the approach and then select the correct gear when appropriate. This may save quite a considerable amount of fuel over a relatively long rural drive. (Although that does go against the theory of IPSGA).
Sherpa Kev said:
I am still finding it hard to believe that an internal combustion engine can run without fuel.
If your foot is completely off the throttle ( and engine not required to idle ), quite simply it needs no fuel.And for emissions purposes, fuel economy, whatever other BS they say, if it is not running there are no exhaust emissions. Hence it is standard practice on the overrun to use no fuel.
If you want to truly include this into this daft hypermiling kinda thing, then get a basic automotive multimeter and hook it up to an injector to read duty cycle or pulse width ( conventional petrol engine ), then you can actually learn when your specific car does actually cut fuel and when it is restored again approaching idle and under what circumstances.
Or just drive the car and enjoy it, and stop worrying about such things as they are a distraction to safe driving and road use.
And it is not a simple "fuel cut off switch", conditions where fuel is cut will be a variable based on throttle, rpm, vehicle speed, and can be other variables too.
Sherpa Kev said:
Right now, I am finding it hard to believe that the engine can run without fuel.
When you’re coasting the engine isn’t running on fuel it’s effectively an air pump being driven via the drivetrain by the inertia of the car. Old cars with carbs did keep introducing a small amount of fuel on overrun but only as a practically unavoidable by-product of the way they function, it has always been wasteful
Worth noting there is also a point at lower RPM where fuel injection will resume, probably sub 2000 rpm
There is also an argument so say that although you’re not using any fuel in the overrun cutoff you are creating more friction by being in gear turning the engine above idle speed than the car would be idling using fuel.
In some circumstances where you don’t need to reduce speed with enough forward planning (such as rolling down a gentle hill with enough gradient to maintain speed but not require braking) it may be fractionally more fuel efficient to let it idle in neutral. Marginal/questionable maybe but goes to highlight that the overrun cutoff is not simply ‘free’. This is why most modern hybrids and even some automatic ICE cars cut the engine and allow the car to freewheel when off throttle.
The IAM like their dogma but IMHO they're mostly focused on making it easy to train proper awareness and forward planning with a limited understanding of how the car works.
Personally I think it better to take the dogma on board but understand how your own car works and make your own mind up on the best way to operate it.
Sherpa Kev said:
Thanks for that information, it all makes sense now.
Now I need to make adjustments to my approach to junctions. As a member of IAM, I have always de-clutched and rolled up to a junction and then taken the appropriate gear. With this new information and thinking about eco driving, I think the way forward is to change gear once on the approach and then select the correct gear when appropriate. This may save quite a considerable amount of fuel over a relatively long rural drive. (Although that does go against the theory of IPSGA).
This is where ‘block-changing’ comes in. You don’t need to change down through all the gears.Now I need to make adjustments to my approach to junctions. As a member of IAM, I have always de-clutched and rolled up to a junction and then taken the appropriate gear. With this new information and thinking about eco driving, I think the way forward is to change gear once on the approach and then select the correct gear when appropriate. This may save quite a considerable amount of fuel over a relatively long rural drive. (Although that does go against the theory of IPSGA).
When I had a (5-speed) manual, I would approach a fast and empty roundabout in 5th, as I got nearer and slowed, if it was clear, then I would block change down from 5th to 3rd and sail through.
Similarly, if I was approaching a junction, I would be in 4th from way back, then slow to check the junction was clear and block-change down to 2nd to pull out without needing to stop. If I needed to stop, it is then easy to go into 1st to pull out.
Similarly I would accelerate fairly firmly in 3rd to join a motorway, then block-change into 5th.
I never like to coast with the clutch pedal down. Block-changing is definitely within IPSGA philosophy.
https://www.iamroadsmart.com/groups/centralsouther...
https://www.easy-it.co.uk/blog/?p=491
Just be aware that block shifting does put more load on individual synchros
Fine in most cars and if done sympathetically but it can cause issues in certain cars
Also never select a low gear too early, even if you don’t let the clutch out, the input shaft and clutch friction plate are still spinning at the speed the engine would be if you did!
Fine in most cars and if done sympathetically but it can cause issues in certain cars
Also never select a low gear too early, even if you don’t let the clutch out, the input shaft and clutch friction plate are still spinning at the speed the engine would be if you did!
The Wookie said:
Just be aware that block shifting does put more load on individual synchros
Fine in most cars and if done sympathetically but it can cause issues in certain cars
Also never select a low gear too early, even if you don’t let the clutch out, the input shaft and clutch friction plate are still spinning at the speed the engine would be if you did!
Fine in every car I have driven, you just select the gear for the next phase.Fine in most cars and if done sympathetically but it can cause issues in certain cars
Also never select a low gear too early, even if you don’t let the clutch out, the input shaft and clutch friction plate are still spinning at the speed the engine would be if you did!
Now, clutchless gear changes was the thing I liked to perfect!
The Wookie said:
Also never select a low gear too early, even if you don’t let the clutch out, the input shaft and clutch friction plate are still spinning at the speed the engine would be if you did!
Now I'm wondering what the maximum safe RPM for a clutch friction plate and input shaft is. Anybody ever found out?
GreenV8S said:
Now I'm wondering what the maximum safe RPM for a clutch friction plate and input shaft is. Anybody ever found out?
I've selected reverse after 5th before in my work van ( previous van had 6th there LOL )....never got to the stage of letting the clutch out though.But it did select surprisingly easy. That's quite a change for the plate/input.
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