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Old 14th June 2011, 11:33   #16
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Default Re: Does coasting save fuel?

Sometime back, I had gone to Ooty and on my way back I refueled my ANHC. As usual I rest the trip meter after refueling. After driving for some distance (may be 10-15 kms) I noticed the built in FE meter and it was indicating 34 kmpl. Initially I thought the meter must be mal-functioning. later it occurred to me that the stretch I was going was downhill and that is what must have resulted in such unbelievable FE.
But on a level roads, the coasting may not help as the momentum would be lost and the car ends up burning more fuel to gain the momentum. Law of conservation of energy cannot be beaten.
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Old 14th June 2011, 11:36   #17
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Default Re: Does coasting save fuel?

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Originally Posted by alpha1 View Post
... then how do the pistons keep moving?...
Instead of engine driving the wheels this time, the wheels drive the engine.
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Old 14th June 2011, 11:37   #18
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Default Re: Does coasting save fuel?

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Originally Posted by alpha1 View Post
If the fuel is cut-off during deceleration - then how do the pistons keep moving?
Because of vehicle intertia?

Doesn't look possible - because we all know that we can park the vehicle on slope by keeping it in gear.

The engine keeps moving due to inertia (vehicle, transmission, flywheel - but not the reciprocating parts).

You can park the car because of the same reason car will slow down if you try coasting - the engine "applies a break".


By the way, is it a good idea to rely on keeping the car in gear while stopping the vehicle? I thought handbrakes must be engaged.
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Old 14th June 2011, 11:42   #19
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Default Re: Does coasting save fuel?

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Originally Posted by Guna View Post
Sometime back, I had gone to Ooty and on my way back I refueled my ANHC. As usual I rest the trip meter after refueling. After driving for some distance (may be 10-15 kms) I noticed the built in FE meter and it was indicating 34 kmpl. Initially I thought the meter must be mal-functioning. later it occurred to me that the stretch I was going was downhill and that is what must have resulted in such unbelievable FE.
But on a level roads, the coasting may not help as the momentum would be lost and the car ends up burning more fuel to gain the momentum. Law of conservation of energy cannot be beaten.
The fuel would be wasted in idling an engine when you are coasting to stop the car. Instead the engine will go in fuelcut mode. Bringing up the law of conservation of energy is moot because you are using torque from the wheels to spin the engine instead of a chemical reaction. Nobody is implying that energy is being created.


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Originally Posted by vina View Post
The engine keeps moving due to inertia (vehicle, transmission, flywheel - but not the reciprocating parts).

You can park the car because of the same reason car will slow down if you try coasting - the engine "applies a break".


By the way, is it a good idea to rely on keeping the car in gear while stopping the vehicle? I thought handbrakes must be engaged.
To be pedantic, the vehicle keeps moving due to inertia. The engine is being spun by the torque applied by the wheels through the tranny. Inertia cannot keep an engine running, there's way too much friction from the bearings and rings for that to happen.

Stopping the vehicle as in parking it? I engage just the handbrakes unless the car is parked on an incline/decline.

Last edited by pranavt : 14th June 2011 at 11:46.
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Old 14th June 2011, 11:54   #20
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Default Re: Does coasting save fuel?

Just one point to clear out a popular misconception that the ECU cuts off fuel completely while coasting in gear. It does not. The ECU cuts of the fuel supply from the injectors. Yes this is correct. But, when it does this, it also activates the idle air control valve through which some amount of fuel is still going into the engine. Just enough to keep it running and to prevent it from stalling. Also depending on the vehicle's speed the ECU will also bring in the injectors into play if the IAC valve cannot supply enough fuel and the ECU detects that the engine is running very lean.

As for the discussion on this thread, yes coasting will save you fuel, but only when you do it with the vehicle in gear.
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Old 14th June 2011, 12:04   #21
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Default Re: Does coasting save fuel?

A few years back, I used to leave the car in neutral and coast to a stop when I notice, from a distance, that the traffic lights had turned red. It's only after reading up about such practices that I changed my habit.

Coasting in neutral is a dangerous thing to do. Not only does the engine consume more fuel than it would while in gear, it does not allow you to react quickly enough when you need to. Leaving the car in the highest gear, at the lowest possible RPM (but not so low that the engine would start knocking) is the perfect way to coast to a stop.

You will not only save fuel, but also have control over the car at all times.

EDIT: The clutch must be engaged only when one decides to change gear. Engaging the clutch any other time (often) would inflict a certain degree of wear and tear on its running parts and one will have to release the clutch a great degree in order to achieve its bite.

Last edited by suhaas307 : 14th June 2011 at 12:09.
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Old 14th June 2011, 12:05   #22
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Default Re: Does coasting save fuel?

I have a doubt whether it is ok if i travel in neutral particularly when there is bumper to bumper traffic in Blore road traffic condition, mostly when it is on the slope downhill. I am of the impression if we keep pressing the clutch, the wear and tear on the clutch plate will be more expensive proposition, please clarify if there are any experts on the issue. I never coast in ghat section, that is dangerous proposition.
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Old 14th June 2011, 12:35   #23
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Default Re: Does coasting save fuel?

Quote:
Originally Posted by vina View Post
By the way, is it a good idea to rely on keeping the car in gear while stopping the vehicle? I thought handbrakes must be engaged.
Yes Vina, parking brakes should be engaged EVERY TIME the vehicle is parked, irrespective of slope.

In most cars rear drum brakes are connected to the hand brakes, and are not so effective in slopes. So, yes, in slopes you need to engage the gear as an aid to the parking brakes. If on a downward slope engage the lowest reverse gear, and if on backward slope, engage the lowest forward gear.
Quote:
Originally Posted by vina View Post
The engine keeps moving due to inertia (vehicle, transmission, flywheel - but not the reciprocating parts).
Again, is it inertia that keeps the pistons alive?. IMO, its the PE, created by the mass of the car on a height.
Quote:
Originally Posted by vikram_d View Post
.... idle air control valve through which some amount of fuel is still going into the engine.
======
IAC valve cannot supply enough fuel
AFAIK, IAC valve CANNOT supply any amount of fuel. When the TB is closed, it provides a bypass with just the right amount of AIR for idling.

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Originally Posted by ukderebail View Post
... whether it is ok if i travel in neutral particularly when there is bumper to bumper traffic in Blore road traffic condition...
As a matter of fact, it is NOT OK to travel in neutral on any kind of surfaces. For you to be in optimum control of your car, the vehicle's wheel's should be driven, and not coast freely. Just ease off the throttle and you will even save your brake pads. Coasting is utter waste in any circumstances.

Last edited by dhanushs : 14th June 2011 at 12:40.
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Old 14th June 2011, 12:38   #24
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Default Re: Does coasting save fuel?

Quote:
Originally Posted by vikram_d View Post
Just one point to clear out a popular misconception that the ECU cuts off fuel completely while coasting in gear. It does not. The ECU cuts of the fuel supply from the injectors. Yes this is correct. But, when it does this, it also activates the idle air control valve through which some amount of fuel is still going into the engine. Just enough to keep it running and to prevent it from stalling. Also depending on the vehicle's speed the ECU will also bring in the injectors into play if the IAC valve cannot supply enough fuel and the ECU detects that the engine is running very lean.

As for the discussion on this thread, yes coasting will save you fuel, but only when you do it with the vehicle in gear.

I didn't know about IAC, thanks.

How does this work for a turbo-diesel? You can not just inject fuel into the inlet air for diesel.
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Old 14th June 2011, 12:46   #25
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Default Re: Does coasting save fuel?

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Originally Posted by dhanushs View Post

In most cars rear drum brakes are connected to the hand brakes, and are not so effective in slopes. So, yes, in slopes you need to engage the gear as an aid to the parking brakes. If on a downward slope engage the lowest reverse gear, and if on backward slope, engage the lowest forward gear.
I do not understand this.

Most cars sold in India are front-wheel driven. So engaging any gear while parked on a slope is do the job, since the power goes to the front wheels. The gear will not allow the car's front wheels to move, however, the car might slide off, and for that you need to engage the e-brake as well.
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Old 14th June 2011, 13:14   #26
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Default Re: Does coasting save fuel?

Quote:
Originally Posted by dhanushs View Post
Yes Vina, parking brakes should be engaged EVERY TIME the vehicle is parked, irrespective of slope.

In most cars rear drum brakes are connected to the hand brakes, and are not so effective in slopes. So, yes, in slopes you need to engage the gear as an aid to the parking brakes. If on a downward slope engage the lowest reverse gear, and if on backward slope, engage the lowest forward gear.
thanks, I'll keep this in mind from now on


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Originally Posted by dhanushs View Post

Again, is it inertia that keeps the pistons alive?. IMO, its the PE, created by the mass of the car on a height.

inertia means (for an already moving vehicle) KE - that is what keeps the vehicle moving, and via the transmission provides the energy to compensate for the engine's (and other) losses as well as for the alternator, AC etc.

This will act as "engine brake" - AC, etc. will also contribute to the braking

This is true irrespective of whether the vehicle is going uphill (imagine you go into a flyover at very high speed and then see slow traffic ahead so take your foot off the pedal), downhill or flat road.

While going downhill, PE will keep getting converted into KE, and this means rather than slowing down you may still speed up (engine brake is not enough) if the KE gained this way is more than KE lost in the engine etc.

Quote:
Originally Posted by dhanushs View Post
AFAIK, IAC valve CANNOT supply any amount of fuel. When the TB is closed, it provides a bypass with just the right amount of AIR for idling.


As a matter of fact, it is NOT OK to travel in neutral on any kind of surfaces. For you to be in optimum control of your car, the vehicle's wheel's should be driven, and not coast freely. Just ease off the throttle and you will even save your brake pads. Coasting is utter waste in any circumstances.
Well, coasting in neautral should be distinguished from coasting in gear with clutch engaged. The former is a waste, the latter is highly recommended (it is called engine braking, going downhill with foot off pedal and what not)
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Old 14th June 2011, 13:25   #27
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Default Re: Does coasting save fuel?

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Originally Posted by suhaas307 View Post
I do not understand this.

Most cars sold in India are front-wheel driven. So engaging any gear while parked on a slope is do the job, since the power goes to the front wheels. The gear will not allow the car's front wheels to move, however, the car might slide off, and for that you need to engage the e-brake as
well.
Hi Suhaas. Any gear will not do the job. 1st gear and reverse gear provide maximum engine braking while parking. While parking the engine braking is due to the compression inside the cylinder, and lowest gear does the best to extract max out of it. Try engaging 5th gear in a steep slope and you'll know.

Quote:
Originally Posted by vina View Post
inertia means (for an already moving vehicle) KE
What I meant is PE is the source of energy, that keeps the vehicle moving.

Quote:
Originally Posted by vina View Post
... The former is a waste...
Yep!. you got it.
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Old 14th June 2011, 13:48   #28
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Default Re: Does coasting save fuel?

Quote:
Originally Posted by suhaas307 View Post
I do not understand this.

Most cars sold in India are front-wheel driven. So engaging any gear while parked on a slope is do the job, since the power goes to the front wheels. The gear will not allow the car's front wheels to move, however, the car might slide off, and for that you need to engage the e-brake as well.
It does not matter if the car is front-wheel drive or rear-wheel drive. The direction of parking on the slope matters.

If your car is parked on an upward slope with the front end faced towards the incline, then engaging first gear while parking is advised. This is because all rolling forces due to gravity act in the opposite direction, i.e. pull the car in the reverse direction.

If the situation is reversed, then it means the rear end of the car is facing the incline. In this case, the car could roll forwards. So apply the reverse gear to provide an anti-force.
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Old 14th June 2011, 14:12   #29
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Default Re: Does coasting save fuel?

Quote:
Originally Posted by dhanushs View Post
Hi Suhaas. Any gear will not do the job. 1st gear and reverse gear provide maximum engine braking while parking. While parking the engine braking is due to the compression inside the cylinder, and lowest gear does the best to extract max out of it. Try engaging 5th gear in a steep slope and you'll know.
Quote:
Originally Posted by lucifer1881 View Post
It does not matter if the car is front-wheel drive or rear-wheel drive. The direction of parking on the slope matters.

If your car is parked on an upward slope with the front end faced towards the incline, then engaging first gear while parking is advised. This is because all rolling forces due to gravity act in the opposite direction, i.e. pull the car in the reverse direction.

If the situation is reversed, then it means the rear end of the car is facing the incline. In this case, the car could roll forwards. So apply the reverse gear to provide an anti-force.
Thanks for clearing that up fellas.

So on a downward slope, one must engage reverse gear while parked along with the handbrake and on an upward slope one must engage 1st gear while parked.
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Old 14th June 2011, 14:18   #30
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Default Re: Does coasting save fuel?

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Originally Posted by vina View Post
Well, coasting in neautral should be distinguished from coasting in gear with clutch engaged. The former is a waste, the latter is highly recommended (it is called engine braking, going downhill with foot off pedal and what not)

By clutch engaged, do you mean pressing the clutch pedal while car is in gear and rolling down.
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