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Old 14th June 2011, 23:51   #46
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Default Re: Does coasting save fuel?

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Originally Posted by dhanushs View Post
The link you gave is of O2 sensor.

Here's how IAC valve works, in laymans terms.
How a Car Engine Idle Air Control Valve Works - 2CarPros
The link was for the wideband sensor. I guess I explained how the IACV works in my post itself.
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Old 15th June 2011, 00:19   #47
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Default Re: Does coasting save fuel?

Obviously one would save fuel. Good in a way, saving the planet etc. More so, at the risk of your life and your co passengers. commendable effort, award could be received by a relative too.

Err, One tiny doubt, are you by any chance related to this guy called Flintstone? :d
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Old 15th June 2011, 01:28   #48
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Default Re: Does coasting save fuel?

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Originally Posted by pranavt View Post
In a wet sump 4 stroke engine, lubrication is taken care of by the oil in the oil sump.
Thanks mate!
The whole thing is kind of complex for me unless I totally focus on it. Which oil are we talking here? Is it the engine oil? Also, is it similar in engines in most cars in India or does it vary? Is this applicable to both Diesel and Petrol engines?

Why I am asking is if it would hurt any car (Diesel, Petrol, Crdi, Mpfi, Carburetor based) if one was coasting, with regards to possible lack of lubrication.

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Originally Posted by dhanushs View Post
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.


The link you gave is of O2 sensor.

Here's how IAC valve works, in laymans terms.
How a Car Engine Idle Air Control Valve Works - 2CarPros
Thanks for the links again. Yes, one has heard that odd burst of sound from (ironically) the silencer of older bikes when going downhill. Thanks mate.
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Old 15th June 2011, 08:59   #49
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Default Re: Does coasting save fuel?

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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.
.......

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.
What does coasting in gear mean exactly? So is ....taking the foot off the pedal when doing XX KMPH in Y gear coasting in Y gear?

If my above statement is correct another question....my car idles at about 800RPM...In the first gear without any accelerator input it still does 800RPM (and goes to about 10miles/hr...taking care not to stall the engine). Doesn't mean that in coasting and in neutral same amount of fuel is consumed? And that no fuel is cut off while coasting?

Some one please clarify.
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Old 15th June 2011, 09:43   #50
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Default Re: Does coasting save fuel?

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Originally Posted by Sanjunair5 View Post
What does coasting in gear mean exactly? So is ....taking the foot off the pedal when doing XX KMPH in Y gear coasting in Y gear?

If my above statement is correct another question....my car idles at about 800RPM...In the first gear without any accelerator input it still does 800RPM (and goes to about 10miles/hr...taking care not to stall the engine). Doesn't mean that in coasting and in neutral same amount of fuel is consumed? And that no fuel is cut off while coasting?

Some one please clarify.
Coasting in gear means using the momentum of the car to move forward instead of gas pedal, while you are in gear. This could happen if you are going downhill, or slowing down from a higher speed. Fuel supply gets cutoff in such scenario. However for idling and in neutral, engine needs to consume some fuel. Idling in gear, 10 mph in your case, also will consume fuel since the car is not moving by momentum but by engine power.
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Old 15th June 2011, 10:09   #51
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Default Re: Does coasting save fuel?

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Originally Posted by vasoo View Post
Coasting in gear means using the momentum of the car to move forward instead of gas pedal, while you are in gear. This could happen if you are going downhill, or slowing down from a higher speed. Fuel supply gets cutoff in such scenario. However for idling and in neutral, engine needs to consume some fuel. Idling in gear, 10 mph in your case, also will consume fuel since the car is not moving by momentum but by engine power.
Thanks Vasoo.....I guess my question wasn't clearly articulated....This is what I wanted to convey through my neutral and first gear example....when I take the foot off the pedal (irrespective of which gear I am in, what speed I am doing) the amount of fuel supplied is same as that for idling. So what does fuel getting cut off mean? Fuel isn't getting cut off, right?
I am pretty certain fuel doesn't get cut off, because engine is still running (if you press the clutch while coasting, tacho will show idling RPM). And unless the car is intelligent enough to know that the car is coasting and has a way to minimize fuel consumption, it will consume the same fuel consumed as in idling.
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Old 15th June 2011, 12:22   #52
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Default Re: Does coasting save fuel?

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Originally Posted by Sanjunair5 View Post
Thanks Vasoo.....I guess my question wasn't clearly articulated....This is what I wanted to convey through my neutral and first gear example....when I take the foot off the pedal (irrespective of which gear I am in, what speed I am doing) the amount of fuel supplied is same as that for idling. So what does fuel getting cut off mean? Fuel isn't getting cut off, right?
I am pretty certain fuel doesn't get cut off, because engine is still running (if you press the clutch while coasting, tacho will show idling RPM). And unless the car is intelligent enough to know that the car is coasting and has a way to minimize fuel consumption, it will consume the same fuel consumed as in idling.
All cars these days are intelligent enough to know that they are coasting (i.e. don't need engine power to do what the driver wants them to do).

If you are going at a good speed in momentum and take your foot off the pedal to slow down OR going downhill and take your foot off the pedal (whether you then slow down or not) - ECU detects it and cuts off the fuel supply.

While we are at the intelligence of the ECU - if you are sedately cruising at a non-earth-shattering speed, car doesn't need much power from the engine. ECU detects this also and regulates the fuel supply to the engine pretty well.
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Old 15th June 2011, 12:26   #53
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Default Re: Does coasting save fuel?

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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.
True, the IAC cannot inject fuel. But when you are coasting in gear the engine is still kept running by using air from the IAC and very tiny amounts of fuel is still being sent in to the engine through the injectors. I had a wideband on my turbo swift and I know what happens. Have done umpteen number of tests on how the stock ECU behaves while coasting in gear.

On my swift while coasting in gear the AFR would go down to 17:1 and as soon as the car went below 1500 rpm, the ECU would resume its normal duty cycle and bring the AFR to about 14.5 - 15:1. Note these readings are without the throttle being applied. As soon as you step on the throttle the ECU would maintain a duty cycle of about 13 - 14:1. And I am talking about the stock ECU here and not the aftermarket one that I had on my car also. Oh and on idle with the stock ECU the AFR would 15.5 - 16:1.

Based on these readings I am very sure that at least in the Swift petrol coasting in gear is more economical than coasting in neutral and I am also pretty positive that this is true for most modern cars.

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Originally Posted by Poitive View Post
Could some of the experts on the forum clarify if the fuel is indeed totally cut off?
No when you coasting in gear, fuel is not completely cut off. There is still very minute amounts of fuel going in to the engine. Just enough to keep the engine running on this zero load condition and also just enough to keep the AFR from getting too lean. The only load on the engine while coasting in gear is that of engine braking.

I'm sure the gurus will agree with the disastrous effects of what will happen to the engine if the fuel supply is completely cut off. The AFR would go super lean and the temperatures inside the cylinders would shoot up like crazy.
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Old 15th June 2011, 12:34   #54
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Default Re: Does coasting save fuel?

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Originally Posted by vikram_d View Post

I'm sure the gurus will agree with the disastrous effects of what will happen to the engine if the fuel supply is completely cut off. The AFR would go super lean and the temperatures inside the cylinders would shoot up like crazy.

How would it effect diesel engines? they do not have any fuel injected with the air supply anyway, and temperatures do go crazy.

Also if the throttle can be controlled to regulate the input air pressure below atmospheric (i.e. deliberately poor VE) then also I'm not sure the compression will cause the temperatures to rise much.

and third - temperatures probably rise much more during ignition stroke anyway - without fuel that is not going to happen.
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Old 15th June 2011, 12:42   #55
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Default Re: Does coasting save fuel?

Yesterday I was running at 50 km/h on my Vento. While coasting in gear it showed 80kmpl. It remained the same when I depressed the clutch also. So, didn't really see any difference. Did this a few times with the same result.

Also, I have tried going up the parking ramp in 1st gear without foot on accelerator, after a brief confusion, the ECU adjusts the fuel supply to take it forward. Is this coasting in gear? I don't think so.

The ECUs being very intelligent behave differently based on conditions

Last edited by srishiva : 15th June 2011 at 12:46.
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Old 15th June 2011, 12:44   #56
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Default Re: Does coasting save fuel?

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Originally Posted by vina View Post
How would it effect diesel engines? they do not have any fuel injected with the air supply anyway, and temperatures do go crazy.

Also if the throttle can be controlled to regulate the input air pressure below atmospheric (i.e. deliberately poor VE) then also I'm not sure the compression will cause the temperatures to rise much.

and third - temperatures probably rise much more during ignition stroke anyway - without fuel that is not going to happen.
I'm not sure about how diesels function in such a scenario. As far as your third point is concerned I have to disagree. A richer mixture normally tends to bring down cylinder and exhaust gas temperatures in petrol vehicles.

Quote:
Leaner AFR results in higher temperatures as the mixture is combusted. Generally, normally-aspirated spark-ignition (SI) gasoline engines produce maximum power just slightly rich of stoichiometric. However, in practice it is kept between 12:1 and 13:1 in order to keep exhaust gas temperatures in check and to account for variances in fuel quality. This is a realistic full-load AFR on a normally-aspirated engine but can be dangerously lean with a highly-boosted engine.
Source: TurboByGarrett.com - Turbo Tech102

They are talking about a turbo engine here, but the same holds good for NA engines also.

Edit: Isn't the input air pressure already below atmospheric in a petrol car? Isn't this what generates the vacuum? Not sure about this and hoping you can enlighten me.

Last edited by vikram_d : 15th June 2011 at 12:47.
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Old 15th June 2011, 12:47   #57
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Default Re: Does coasting save fuel?

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Originally Posted by Sanjunair5 View Post
.... when I take the foot off the pedal (irrespective of which gear I am in, what speed I am doing) the amount of fuel supplied is same as that for idling.
=========
And unless the car is intelligent enough to know that the car is coasting and has a way to minimize fuel consumption, it will consume the same fuel consumed as in idling.
Hey Sanjunair, NO fuel is required to run the engines when going downhill in a gear. The energy of the car itself will propel the vehicle, tyres and even keep the engine running.

As far as how the car can know, with these many sensors going into a car these days, its pretty easy to differentiate when the engine drives the wheels and the wheels drive the engine. For example, with TB closed, if the rpm is higher than idle rpm, its pretty obvious that the wheels are driving the engine.

What you say, might be true for carburetor engines.
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Old 15th June 2011, 12:52   #58
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Default Re: Does coasting save fuel?

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Originally Posted by vikram_d View Post
I'm not sure about how diesels function in such a scenario. As far as your third point is concerned I have to disagree. A richer mixture normally tends to bring down cylinder and exhaust gas temperatures in petrol vehicles.

Source: TurboByGarrett.com - Turbo Tech102

They are talking about a turbo engine here, but the same holds good for NA engines also.

What you have written and quoted holds good if there is sufficient amount of petrol.

Very lean mixtures (that is what you are referring to) will not even combust - petrol's specific heat of vapourisation is out of picture.


Several BHPians have seen huge kmpl numbers on their cars (there is a video posted on this thread actually showing 0 supply)- petrol supply is cut off heavily.

Now you may be right for some vehicles, but the reason you give (cooling of the engine) doesn't make sense. Air-only will almost certainly run cooler than petrol-air mixture ignited (no matte how much petrol there was - and also very lean mixtures can not be ignited anyway) - the only heat you are adding is via compression, none added via combustion. Since the engine easily works off petrol-air ignition it should have no problem with air-only kind of temperatures.

Injecting some petrol would be mandatory in 2-strokes but that is for the reasons of lubrication. For cars I don't know why you would have to inject fuel.
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Old 15th June 2011, 12:54   #59
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Default Re: Does coasting save fuel?

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Originally Posted by vikram_d View Post
I'm sure the gurus will agree with the disastrous effects of what will happen to the engine if the fuel supply is completely cut off. The AFR would go super lean and the temperatures inside the cylinders would shoot up like crazy.
There is no load on the engine when coasting. The temperatures inside the cylinders are not going to shoot up when there is no combustion happenning in the engine because fuel IS completely cut off. On hitting fuelcut, the wideband shows completely lean (22.4 in my wideband), the duty cycle of the injectors shows 0% and I have seperately turned off the fuel pump when coasting (have a seperate toggle switch for it) and the engine keeps running as long as the pump is turned on when coming out of fuelcut mode.

Quote:
Originally Posted by vikram_d
True, the IAC cannot inject fuel. But when you are coasting in gear the engine is still kept running by using air from the IAC and very tiny amounts of fuel is still being sent in to the engine through the injectors. I had a wideband on my turbo swift and I know what happens. Have done umpteen number of tests on how the stock ECU behaves while coasting in gear.

On my swift while coasting in gear the AFR would go down to 17:1
That 17:1 is probably the leanest your wideband can show. The injectors are shut off during fuelcut mode.
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Old 15th June 2011, 13:08   #60
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Default Re: Does coasting save fuel?

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Originally Posted by vina View Post
What you have written and quoted holds good if there is sufficient amount of petrol.

Very lean mixtures (that is what you are referring to) will not even combust - petrol's specific heat of vapourisation is out of picture.


Several BHPians have seen huge kmpl numbers on their cars (there is a video posted on this thread actually showing 0 supply)- petrol supply is cut off heavily.

Now you may be right for some vehicles, but the reason you give (cooling of the engine) doesn't make sense. Air-only will almost certainly run cooler than petrol-air mixture ignited (no matte how much petrol there was - and also very lean mixtures can not be ignited anyway) - the only heat you are adding is via compression, none added via combustion. Since the engine easily works off petrol-air ignition it should have no problem with air-only kind of temperatures.

Injecting some petrol would be mandatory in 2-strokes but that is for the reasons of lubrication. For cars I don't know why you would have to inject fuel.
So what you are saying is that fuel is completely cut off? If this is the case then what is happening with the spark plugs? Are they firing?

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Originally Posted by pranavt View Post
and the engine keeps running as long as the pump is turned on when coming out of fuelcut mode.
Didn't understand this part.

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Originally Posted by pranavt View Post
That 17:1 is probably the leanest your wideband can show. The injectors are shut off during fuelcut mode.
I had an Innovate LM2 which also used to read till 22.4. But on my turbo petrol swift I never saw that number with the stock ECU. I did see it only once on the after-market ECU when we were tuning the car.

Did some research and found this. i believe this is true.

Quote:
depends if it's petrol of diesel
petrol cars use air when you prod the throttle so the fuel used matches the air going in
diesels use fuel, so the more throttle you use the more fuel goes in and the air is open in any case so it will match it
so, for petrol cars, when you shut off the throttle, you virtually eliminate the need for the engine to need to provide power to keep itself turning over as the momentum keeps it going so it basicly provide minisclue amounts of fuel to keep things going
diesels do the same thing
so when you are coasting and in gear, you are using less fuel than you would be if you were keeping your speed at a steady rate using the accelerator pedal
it's also why you can se better fuel economy when using your feet rather than the cruise control

Last edited by vikram_d : 15th June 2011 at 13:13.
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