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Old 28th June 2013, 20:15   #226
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Default Re: Does coasting save fuel?

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Originally Posted by Jeroen View Post
Actually, not true for carburator engines. When using the engine to brake you aredrawing a higher vacuum which results in more fuel to the engine than just idling.
Jeroen
Actually only very slightly more than when idling. I know it sounds counterintutive.

Proper manipulation of the antidiesel valve (if fitted) can stop even that little flow.

Regards
Sutripta
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Old 28th June 2013, 20:40   #227
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Default Re: Does coasting save fuel?

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Originally Posted by Sutripta View Post
Actually only very slightly more than when idling. I know it sounds counterintutive.

Proper manipulation of the antidiesel valve (if fitted) can stop even that little flow.

Regards
Sutripta
Yes, I agree, counterintuitive, and only slightly more.
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Old 29th June 2013, 00:35   #228
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Default Re: Does coasting save fuel?

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Originally Posted by Sutripta View Post
Actually only very slightly more than when idling. I know it sounds counterintutive.

Proper manipulation of the antidiesel valve (if fitted) can stop even that little flow.

Regards
Sutripta
Unless I am mistaken, the anti-dieseling solenoid kicks in only when the power is cut, via key. Therefore, the small flow will remain.
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Old 29th June 2013, 20:02   #229
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Default Re: Does coasting save fuel?

^^^
That's why I said 'proper manipulation of'. The anti diesel valve is a means of stopping fuel flow in the pilot circuit. Use it as you will.
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Old 19th September 2014, 06:50   #230
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Default Re: Does coasting save fuel?

The new Mercedes S500 Plug-in Hybrid has a neat feature called Haptic Accelerator. It operates in combination with the stereo camera mounted within the windscreen, providing an impulse within the throttle to signal you are too close to the car ahead and can recuperate energy by lifting off at which the S-class automatically switches into a freewheeling state with the petrol engine and electric motor disengaged from the gearbox.

Read it in this autocar review.
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Old 21st June 2015, 17:20   #231
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Default Re: Does coasting save fuel?

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Originally Posted by Jeroen View Post
It's been mentioned before, and the relevant links are mentioned to threads on this forum, that keeping the gear in idle and coasting shuts down the fuel to the engine, safe the occasional burst. And if you coast in neutral the engine will still use fuel to keep it idling.

So here's an interesting youtube video to prove that exact point. The engines uses less fuel in gear than in idle going down the hill. Anybody with an on board computer that give you continuous actual fuel use age can do this experiment for themselves.

Jeroen
This is my understanding... please correct if I'm wrong on something:

Just because fuel supply is cut off when coasting in gear, it does not mean that fuel is saved. It is only saved at the sacrifice of momentum, so actually you will end up spending more fuel to regain momentum later
https://en.wikipedia.org/?title=Momentum

Please check this out:


To put it in simple terms, while cycling if you stop pedaling (similar to coasting in neutral), you are efficiently using the momentum generated by pedalling... but if you stop pedaling and slightly keep applying the brakes(similar to foot-off-accelerator in gear), later you will need to put in more energy pedaling to regain the same speed

So when driving on a slight slope / level road, coasting in neutral is better that using engine braking to maintaining the same speed... it keeps engine RPMs lower (reducing heat due to piston friction on cylinders, etc) and also you save more fuel... But when going down a STEEPER slope, it is better to use engine braking, otherwise controlling momentum is tricky !

Last edited by Gannu_1 : 22nd June 2015 at 09:06. Reason: Removing Youtube URL from the quoted post.
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Old 22nd June 2015, 08:37   #232
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Default Re: Does coasting save fuel?

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Originally Posted by AjayJoshuaN View Post
This is my understanding... please correct if I'm wrong on something:

Just because fuel supply is cut off when coasting in gear, it does not mean that fuel is saved. It is only saved at the sacrifice of momentum, so actually you will end up spending more fuel to regain momentum later
https://en.wikipedia.org/?title=Momentum

To put it in simple terms, while cycling if you stop pedaling (similar to coasting in neutral), you are efficiently using the momentum generated by pedalling... but if you stop pedaling and slightly keep applying the brakes(similar to foot-off-accelerator in gear), later you will need to put in more energy pedaling to regain the same speed

So when driving on a slight slope / level road, coasting in neutral is better that using engine braking to maintaining the same speed... it keeps engine RPMs lower (reducing heat due to piston friction on cylinders, etc) and also you save more fuel... But when going down a STEEPER slope, it is better to use engine braking, otherwise controlling momentum is tricky !
I thnk you might be mixing a few things, that are somewhat related, but different never the less:

1) When you leave the engine idling whilst coasting, it still uses fuel
2) When you coast say down a hill, with the engine in gear, it still uses a tiny bit of fuel, but less than just idling.
3) Whether and when to use engine braking going down hill is more a driving technique issue. Mostly used for avoiding prolonged braking and thus overheating brakes.
4) Momentum is a combination of speed and weight nothing else.

Jeroen

Last edited by Gannu_1 : 22nd June 2015 at 09:06. Reason: Removing Youtube URL from the quoted post.
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Old 22nd June 2015, 09:26   #233
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Default Re: Does coasting save fuel?

Coasting is a very dangerous practice, so fuel savings, if any are secondary. The car should be in the appropriate gear at all times for safety and control.
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Old 22nd June 2015, 09:49   #234
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Default Re: Does coasting save fuel?

It is often mentioned in this thread that if a car is in motion and in gear with foot off the accelerator, the ECU will cut off the fuel supply to the engine and the engine will then be 'run by the wheels' in turn.

My experience is, I start the car in first gear, no accelerator input, slowly release the clutch, car starts rolling. As it gains a little speed maybe 10kmph, I press the clutch, shift to second gear and slowly release the clutch. It continues to roll on at maybe 20kmph. All this while I have never touched the accelerator and all this in level road. I can move like this the whole day at max of 20kmph.

Now the question if the fuel is cutoff and no fuel is burnt where did the car get its power from, whole of day?

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Old 22nd June 2015, 10:05   #235
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All this while I have never touched the accelerator and all this in level road. I can move like this the whole day at max of 20kmph.

Now the question if the fuel is cutoff and no fuel is burnt where did the car get its power from, whole of day?
AFAIK, this process as described will use up fuel.

And the reason is "anti - stall" feature that the ECU will be fueling it and not allowing the car to switch off until it is moving on a flat ground without much restrictions. Hope I am correct.
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Old 22nd June 2015, 10:13   #236
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Default Re: Does coasting save fuel?

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Originally Posted by fordday View Post
It is often mentioned in this thread that if a car is in motion and in gear with foot off the accelerator, the ECU will cut off the fuel supply to the engine and the engine will then be 'run by the wheels' in turn.

My experience is, I start the car in first gear, no accelerator input, slowly release the clutch, car starts rolling. As it gains a little speed maybe 10kmph, I press the clutch, shift to second gear and slowly release the clutch. It continues to roll on at maybe 20kmph. All this while I have never touched the accelerator and all this in level road. I can move like this the whole day at max of 20kmph.

Now the question if the fuel is cutoff and no fuel is burnt where did the car get its power from, whole of day?
.
In the scenario you describe the engine is just idling, so it just uses a bit of fuel. As you put it in first and let out the clutch, it is essentially still idling, although the engine now needs to produce a bit of torgue the ECU will command a little extra fuel to prevent the engine from stalling.

So this is different from a scenario where as you say, the engine will be run by the wheels. In theory the fuel into the engine can be completely turn off, however, as explained before in practice most ECUs still inject a tiny bit of fuel every now and then.

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Old 22nd June 2015, 11:11   #237
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Default Re: Does coasting save fuel?

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Originally Posted by Jeroen View Post
1) When you leave the engine idling whilst coasting, it still uses fuel
2) When you coast say down a hill, with the engine in gear, it still uses a tiny bit of fuel, but less than just idling.
3) Whether and when to use engine braking going down hill is more a driving technique issue. Mostly used for avoiding prolonged braking and thus overheating brakes.
4) Momentum is a combination of speed and weight nothing else.
Hi Jeroen, I have a technical question

While coasting in neutral, the engine would be idling at idle speed (say 1000 rpm).

While engine braking (or slowing down at a lower gear) the engine would always be in 1400-2200 rpm.

Will not the higher rev account for a (relatively) greater fuel consumption?
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Old 22nd June 2015, 11:22   #238
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Quote:
Originally Posted by Kumar R View Post
While coasting in neutral, the engine would be idling at idle speed (say 1000 rpm).

While engine braking (or slowing down at a lower gear) the engine would always be in 1400-2200 rpm.

Will not the higher rev account for a (relatively) greater fuel consumption?
Neutral will use fuel as it has to keep the engine running whereas in gear deceleration will not use fuel as the ECU will cut off supply irrespective of the RPM it is.
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Old 22nd June 2015, 11:37   #239
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Default Re: Does coasting save fuel?

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Originally Posted by Kumar R View Post
Hi Jeroen, I have a technical question

While coasting in neutral, the engine would be idling at idle speed (say 1000 rpm).

While engine braking (or slowing down at a lower gear) the engine would always be in 1400-2200 rpm.

Will not the higher rev account for a (relatively) greater fuel consumption?
In modern engines the ECU controls the fuel flow based on a few parameters.
. While idling (includes coasting in neutral), ECU will supply sufficient fuel to maintain the idling RPM
. While coasting in gear, the ECU will supply fuel just enough to stop stalling. As the wheels are running the engine, the fuel supplied will be extremely low, as no fuel is required to maintain the car's momentum. Once the speed is low enough to stall the car, the ECU will start increase the fuel to prevent stalling.

So in conclusion, idling generally consumes more fuel than coasting in gear. I checked this concept with our Honda City, and the fuel average shown by MID, goes beyond the scale while coasting in gear
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Old 22nd June 2015, 12:42   #240
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Default Re: Does coasting save fuel?

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Originally Posted by Aroy View Post
idling generally consumes more fuel than coasting in gear.
+1.

Coasting in neutral, esp in modern engines, wont save anything than a repair of early worn out clutch. Moreover the engine load on a proper gear coasting is far less than idling. This saves fuel and can maintain the proper speed, its possible even at high speeds too.

Quote:
Originally Posted by Kumar R View Post
Will not the higher rev account for a (relatively) greater fuel consumption?
The momentum from the wheels keep the high rpm which gradually decrease with the physical params of a vehicle.

Last edited by ::CMS:: : 22nd June 2015 at 12:49.
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