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Old 14th June 2011, 14:21   #31
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Default Re: Does coasting save fuel?

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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.

I can understand applying the lowest possible gear (i.e. 1st or reverse), but what is the advantage of putting the first on an incline?

I mean is there something (some mechanical arrangement etc.) that prevents the shaft from rotating in both directions (clockwise as well as anticlockwise)?

I always thought that the correct rotation direction is initially determined by starter motor (and ignition timing) and later by flywheel inertia.
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Old 14th June 2011, 14:28   #32
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Default Re: Does coasting save fuel?

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Originally Posted by vina View Post
Sir

I think by coasting here people mean - in gear, clutch engaged (i.e foot off), foot off the pedal (may be on the brake to slow down in emergency)
In my lexicon coasting is driving with the car declutched or in neutral. Sometimes I wonder whether we are living on the same planet!
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Old 14th June 2011, 14:42   #33
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Default Re: Does coasting save fuel?

Coasting in gear will cut off fuel supply in most cars, while in neutral there will be a minimal supply of fuel to keep the engine idling.

However there is another angle to look at this in terms of overall FE. Coasting in gear will have significant engine braking, thereby reducing the speed and overall distance covered in that mode, vs coasting in neutral. So the overall FE savings may not be very different in both cases, for a given length of road including a downhill part and a level part. It may be so possible that to cover the same distance as coasting in neutral, you have to apply gas pedal again, which will consume fuel. However if you have to apply brakes while coasting in neutral, the the above scenario will turn the other side.

Best approach: If you are coasting and see that there is no need to brake in the near future, coast in neutral. If you see a braking situation ahead, coast in gear.

Last edited by vasoo : 14th June 2011 at 14:59.
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Old 14th June 2011, 14:51   #34
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Originally Posted by sgiitk View Post
In my lexicon coasting is driving with the car declutched or in neutral. Sometimes I wonder whether we are living on the same planet!

Well, I may be wrong.

I got the word from internet to mean "riding the already existing KE of the vehicle (not supplying any more power)". this also goes with Merriam Websters definition:

1. (of a person or vehicle) Move easily without using power.
2. Act or make progress without making much effort.

In gear or not has not much to do with the literal definition, though the usual meaning people understand may be different (and as I said, in that case I'm wrong).

Coasting in neutral is definitely dangerous, and coasting with clutch depressed is bad (though sometime unavoidable - e.g. while shfting gear OR when you are about to enter a crowded intersection, you are braking to slow down, want to finish downshifting before you arrive at the crowd so both hands can be on the wheel. In such cases I downshift to 2nd directly from 4th and keep the clutch pressed till the speed drops sufficiently)

Quote:
Originally Posted by vasoo View Post
...

Coasting in gear will have significant engine braking, thereby reducing the speed and overall distance covered in that mode, vs coasting in neutral. So the overall FE savings may not be very different in both cases, for a given length of road including a downhill part and a level part. It may be so possible that to cover the same distance as coasting in neutral, you have to apply gas pedal again, which will consume fuel. However if you have to apply brakes while coasting in neutral, the the above scenario will turn the other side.
From what I have seen on my Figo, in the 4th and 5th gears, even with AC on full blast there is not much engine braking at moderate rpms (below 2300 for Figo). The effect of engine braking becomes noticeable in 3rd and in 1st it almost rivals the brake itself.


Speed doesn't reduce from 70 to even 65 over a 200m distance (have seen it the first few times I was learning to use engine as brake) - and this is on a flat road.

Downhill the speed actually increases even in 2nd gear on sharp declines and 3rd gear on mild declines unless I apply brakes (again I almost panicked the first time I saw this happen - fortunately there was nobody else on the road)



Lately I have been using a "hunch" to reduce fuel consumption - I don't accelerate too fast or decelerate (engine braking part) too fast. Both are relatively easy to guess from the noise coming from the engine - accelerate too fast and it cries out and decelerate too fast (engine braking at 2700+ rpm) and it makes some noise.



Coasting will probably always help but will not be a significant factor in city driving - anticipating traffic is useful more in avoiding braking some biker's skull rather than saving money. Also most o the times it is driving in lower gears where there is not much KE in the car anyway.

Last edited by Technocrat : 15th June 2011 at 01:35. Reason: Please use "Multi Quote" option for quoting Multiple posts, instead of creating another back-to-back post. Thanks
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Old 14th June 2011, 16:17   #35
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Default Re: Does coasting save fuel?

I had the bitter experience in Mumbai within one year i had to change the entire clutch plates due to bumper to bumper traffic in my maruti 800, i used to drive with depressing the clutch and it costed me 10 k to change over, now with A Star i am afraid the cost of changing the clutch assembly would be much more. Once again this doubt of mine has to be clarified for bumper to bumper traffic say on bellary road what i practise is to engage the gear and put the vehicle on momentum for say 5 meters. I don t mind burning 1 litre fuel more but end up saving or prolonging the life of clutch assembly which can put me down by 30 k, what do you all feel guys.

Last edited by ukderebail : 14th June 2011 at 16:18. Reason: cost
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Old 14th June 2011, 16:34   #36
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Default Re: Does coasting save fuel?

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Originally Posted by ukderebail View Post
I had the bitter experience in Mumbai within one year i had to change the entire clutch plates due to bumper to bumper traffic in my maruti 800, i used to drive with depressing the clutch and it costed me 10 k to change over, now with A Star i am afraid the cost of changing the clutch assembly would be much more. Once again this doubt of mine has to be clarified for bumper to bumper traffic say on bellary road what i practise is to engage the gear and put the vehicle on momentum for say 5 meters. I don t mind burning 1 litre fuel more but end up saving or prolonging the life of clutch assembly which can put me down by 30 k, what do you all feel guys.

I don't get it - do you mean you drove in the first gear with clutch plate pressed, right foot on the brake (but not pressing the brake) and clutch released from time to time to move the car a little?

If so then what alternatives do you have in bumper to bumper traffic anyway? the traffic moves in jerks and the speed is usually less than idling speed in the first gear.
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Old 14th June 2011, 16:41   #37
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Quote:
Originally Posted by vina View Post
I don't get it - do you mean you drove in the first gear with clutch plate pressed, right foot on the brake (but not pressing the brake) and clutch released from time to time to move the car a little?

If so then what alternatives do you have in bumper to bumper traffic anyway? the traffic moves in jerks and the speed is usually less than idling speed in the first gear.
Yes i changed between first and second gear and buggered up the clutch assembly i feel on bumper to bumper traffic, that is when i discovered coasting engaged the gear and allow the front vehicle to move ahead and coast. Hurray this time around the clutch assembly lasted for 8 years.
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Old 14th June 2011, 16:51   #38
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Default Re: Does coasting save fuel?

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Originally Posted by ukderebail View Post
Yes i changed between first and second gear and buggered up the clutch assembly i feel on bumper to bumper traffic, that is when i discovered coasting engaged the gear and allow the front vehicle to move ahead and coast. Hurray this time around the clutch assembly lasted for 8 years.

Well what I do is - release the clutch slightly to get the car moving, press it again fully and let it coast without clutch engaged - it either stops by itself or I have to break.

I do not engage the second gear unless the space from the vehicle in front has grown just too large AND that vehicle and all vehicle in front are going fast enough so that just to maintain that gap I have to speed up - this allows me to avoid sudden braking, usually the car coasts to a stop on its own and the right point.


That said sometimes I apply brakes very lightly even though I don't need to. This is when some moron is tailgating me in heave traffic at close distance - to tell him (via the brake lights) that I do have brakes and the traffic may make me use them anytime. In general if such morons persist I slow down (slowing down even slightly helps - these guys are really impatient), forcing them to pass me and become somebody else's problem.



The traffic moves a few feet each time (sometimes less) and nobody on the road can maneuver, so it doesn't matter from the point of view of safety anyway.
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Old 14th June 2011, 19:10   #39
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Default Re: Does coasting save fuel?

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Originally Posted by vina View Post
... but what is the advantage of putting the first on an incline...
I'm a bit confused about the mechanisms. Will get back to it later on.

For now, assume a downward slope and car is parked front downwards. Something makes the car move forward/downward (Might be a rear ending), so... Will 1st gear or reverse gear be better in stopping the forward moving car?

As for reverse gear assume a vice-versa scenario.
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Old 14th June 2011, 19:15   #40
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Default Re: Does coasting save fuel?

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Originally Posted by dhanushs View Post
I'm a bit confused about the mechanisms. Will get back to it later on.

For now, assume downward slope and car is parked front downwards. Something makes the car move forward/downward (Might be a rear ending), so... Will 1st gear or reverse gear be better in stopping the forward moving car?

As for reverse gear assume a vice-versa scenario.

Well my point is - if the engine shaft has more resistance towards free rotation in one direction versus the other (i.e. some special mechanism that either prevents the rotation in one direction or offers more resistance) then I should follow your advice.

If the engine on the other hand will offer same resistance in both directions when ignition is off - then I should use the gear with a smaller gear ratio no matter the direction of the incline - reverse should usually be the choice as it has a smaller ratio than even the first gear in most cars.



Of course your advice on reverse on a decline and first on an incline will be very helpful (safety wise) should someone get in the car and try to start it without disengaging the engine (via changing to netural or pressing the clutch) first - in each case the car will go uphill when the starter jerks it.
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Old 14th June 2011, 20:13   #41
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Default Re: Does coasting save fuel?

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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.
The IACV cannot inject fuel. It is a simple PWM valve that allows a vacuum leak big enough to let the engine idle when no throttle is applied. When using engine braking, the IACV will be open to let in some air (or the intake stroke would start pulling in oil through the rings and head because of massive negative pressure), but no fuel is injected. Use a wideband, you will see what I'm saying.
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Old 14th June 2011, 20:57   #42
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Default Re: Does coasting save fuel?

Anyone tried doing push start for an MPFI car?

So does it matter if its the reverse or the first gear, which is engaged.
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Old 14th June 2011, 21:29   #43
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Default Re: Does coasting save fuel?

Quote:
Originally Posted by pranavt View Post
When using engine braking while doing downhill, the ECU goes into fuelcut mode and no fuel is injected.
Quote:
Originally Posted by vasoo View Post
Coasting in gear will cut off fuel supply in most cars, while in neutral there will be a minimal supply of fuel to keep the engine idling.
It has been said a few times on this thread that the fuel supply is totally cut off when one is coasting downhill.

I read about engines a long time back and with respect to 2 stroke mototbikes. They used to need a 2T oil to keep the the cylinder/piston rings lubricated. Am not sure of this but, in 4stroke petrol engines, isn't the lubrication from the fuel itself? (In Diesel engines, I presume some of the engine oil is used for it).

If there is no fuel supply to the chamber (cylinder/piston rings), how does it get lubricated?

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.
Vikram's explanation above takes care of the lubrication aspect as well.

Could some of the experts on the forum clarify if the fuel is indeed totally cut off?

Quote:
Originally Posted by pranavt View Post
The IACV cannot inject fuel. It is a simple PWM valve that allows a vacuum leak big enough to let the engine idle when no throttle is applied. When using engine braking, the IACV will be open to let in some air (or the intake stroke would start pulling in oil through the rings and head because of massive negative pressure), but no fuel is injected. Use a wideband, you will see what I'm saying.
@Pranavt, Could you please put it in simpler words for non-techies like me? (IACV? PWM? Wideband?)
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Old 14th June 2011, 21:42   #44
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Default Re: Does coasting save fuel?

In a wet sump 4 stroke engine, lubrication is taken care of by the oil in the oil sump. Fuel is not used for lubrication. A 2 stroke engine is different because air and fuel travels through the crankcase into the combustion chamber due to lack of valves. Fuel is indeed totally cut off in a 4 stroke engine during engine braking.

PWM = pulsewidth modulation. Basically an IACV is rapidly moving valve which controls the amount of air being bled into the cylinders when the throttle plate is completely closed. In the absence of an IACV, or one which is blocked, the engine will not be able to hold a stable idle or shut off completely due to the lack of air/fuel which is required to keep the engine running.

Oxygen sensor - Wikipedia, the free encyclopedia
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Old 14th June 2011, 22:05   #45
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Default Re: Does coasting save fuel?

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Originally Posted by Poitive View Post
If there is no fuel supply to the chamber (cylinder/piston rings), how does it get lubricated?
AFAIK, unlike four-stroke ECU controlled engines, fuel is NOT CUT OFF in two-stroke engines. In carburetor engines, as long as vacuum is created in the piston, even on engine braking, 'idle amount' of fuel is supplied.

This is one of the main reasons, for attaining max fuel efficiency, the idle speed is set very very low in carburetor bikes. Also, if one observes a two stroke bike going downhill in gear, one can see the occasional burst of excess fuel.

Quote:
Originally Posted by pranavt View Post
The link you gave is of O2 sensor.

Here's how IAC valve works, in laymans terms.
http://www.2carpros.com/articles/how...ol-valve-works

Last edited by dhanushs : 14th June 2011 at 22:10.
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