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Old 11th October 2013, 00:31   #61
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Originally Posted by a4anurag View Post

Just tried checking for fuel supply when in gear and when in neutral add discussed in our earlier parts. Used my ELM 327 OBD II reader.

Kept the car constant at 80 kmph:

Kept the car constant at 100 kmph

At 80 kmph with my leg OFF the accelerator pedal

At 100 kmph with my leg OFF the accelerator pedal

In neutral and idling. Check the fuel flow per minute:

When in gear at off the accelerator pedal the dials read '0 cc/min' be on idle the dials give a reading of '10 cc/min'

Anurag.
Thanks for doing this little experiment and sharing. Confirms what was said before. The zero fuel flow is prbably in reality a little higher, but probably not high enough to show up here. still idle fuel flow is considerable higher then coasting in gear, as stated.
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Old 11th October 2013, 10:32   #62
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Default Re: Do High RPMs result in unburnt fuel?

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Originally Posted by auto.enthusiast View Post
Actually, I think that should be re-phrased. At peak torque (high load), you will have minimum fuel consumption i.e. Brake Specific Fuel Consumption (BSFC). But that does not translate to maximum FE for the car. The BSFC/fuel consumption is minimum since it is relative to the power produced at peak torque, which means that fuel consumed is less 'per kW' power, since the engine is efficiently producing high power (i.e. the engine efficiency is maximum in this region).

Now, this high power makes the vehicle go faster,and gives it good acceleration to surge ahead, but if you see the overall fuel economy of the vehicle (i.e. the vehicle efficiency), that will not be the best FE you can get from the vehicle (remember:aerodynamic losses also increase at higher speeds,gear ratios, etc also decides where/at what speed you can get the best FE in your car).

In short, it would be fair to assume that max FE will normally be at some point lower than at the peak torque engine speed, though this point will be different for every car.
You are correct when the torque is peaked. Where the torque curve is relatively flat; as in detuned engines; you have a wide RPM range where the torque is maximum, and that is when FE is maximum (barring air and rolling resistance at high speeds). For city driving, say at < 70km/h, a flatter torque will give better FE and more driving pleasure, while a peaked torque will suit spirited driving.
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Old 11th October 2013, 11:09   #63
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Default Re: Do High RPMs result in unburnt fuel?

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Originally Posted by ahmad.007 View Post
No, I am not that deep into technical stuff.
What I did was I took my Beat D to 50 kmph in 2nd gear and let go off the accelerator, the car started deaccelerating at a constant rate, then I turned the ignition to 'off' and there was absolutely no percievable difference in deacceleration or any sound coming from the engine. I turned it back on and both the speedo and tacho climbed to their respective positions without any jerk or anything, and the process was seamless.
I tried this multiple times in different speeds and rpms, and I only found jerks when the the ignition was turned on near idling rpms (>1000).
This I feel is sufficient for the conclusion I made earlier.
I had tried the same experiment in my bike which has a carburretor & not a fuel injections system and my observation was same. Though, I believe you had carried this experiment in a controlled environment, I sincerely advice that you do not carry out this experiment in a car since you can loose your brakes & power steering in case if you shift to neutral accidentally. That's the reason I tried this on bike as there are no hydraulics involved.

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Old 11th October 2013, 11:42   #64
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Originally Posted by saket77 View Post

I had tried the same experiment in my bike which has a carburretor & not a fuel injections system and my observation was same. Though, I believe you had carried this experiment in a controlled environment, I sincerely advice that you do not carry out this experiment in a car since you can loose your brakes & power steering in case if you shift to neutral accidentally. That's the reason I tried this on bike as there are no hydraulics involved.

Regards,
Saket
There is a dead end road near my place where there is virtually no traffic and no people. The road is quite wide. I did this over there so as to ensure safety of others.

Its interesting that the carb bike also displayed the same behavior. Maybe the carburretors have become smart these days. But I am sure this experiment would not succeed on a 2 stroke RX 100 or a bajaj scooter.

After reading so much into fuel efficiency factors and efficient driving techniques, I think the best and the simplest would still be-"lowest possible rpm, highest possible gear, light foot on accelerator and anticipation of events".
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Old 11th October 2013, 11:59   #65
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Default Re: Do High RPMs result in unburnt fuel?

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Originally Posted by ahmad.007 View Post
Its interesting that the carb bike also displayed the same behavior. Maybe the carburretors have become smart these days. But I am sure this experiment would not succeed on a 2 stroke RX 100 or a bajaj scooter.
I think CV carburetors are much smarter than old carbs. Looks like my dad's old Bajaj Scooter needs a sprint now to repeat this experiment

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Originally Posted by ahmad.007 View Post
After reading so much into fuel efficiency factors and efficient driving techniques, I think the best and the simplest would still be-"lowest possible rpm, highest possible gear, light foot on accelerator and anticipation of events".
That of course, without the engine struggling. To cut short, must be in the right RPM band. My petrol car, a 1000CC Zen manages well over 20-21KMPL when I drive with a light foot on highways.

Last edited by saket77 : 11th October 2013 at 12:00.
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Old 11th October 2013, 12:21   #66
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Default Re: Do High RPMs result in unburnt fuel?

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Originally Posted by ahmad.007 View Post
...

After reading so much into fuel efficiency factors and efficient driving techniques, I think the best and the simplest would still be-"lowest possible rpm, highest possible gear, light foot on accelerator and anticipation of events".
I would modify it to the lowest possible usable RPM. Too low and you are in low torque area and the FE would just dip.

At one time every car had a "cruising speed" advertised. This was the speed in the top gear which would return the best possible FE. I guess that is around 50 in 4th and 70 in fifth for most of the Maruti petrols.
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Old 11th October 2013, 12:34   #67
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Originally Posted by saket77 View Post

I think CV carburetors are much smarter than old carbs. Looks like my dad's old Bajaj Scooter needs a sprint now to repeat this experiment
Do share the results

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Originally Posted by saket77 View Post
That of course, without the engine struggling. To cut short, must be in the right RPM band. My petrol car, a 1000CC Zen manages well over 20-21KMPL when I drive with a light foot on highways.
Yup, I forgot to mention that, without lugging the engine.
My 15 year old Matiz gave me 18-20kmpl in city with a light foot until recently the signs of aging showed up. Now it gives 16-18 kmpl, maybe because my foot has swollen a little.

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Originally Posted by Aroy View Post
I would modify it to the lowest possible usable RPM. Too low and you are in low torque area and the FE would just dip.

At one time every car had a "cruising speed" advertised. This was the speed in the top gear which would return the best possible FE. I guess that is around 50 in 4th and 70 in fifth for most of the Maruti petrols.
Yes, you are right, there is a small low torque range just above the idling rpm where a car's engine usually struggles. One should not stay in that range.
That low torque area is usually below 1400-1500 rpm mostly.
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Old 11th October 2013, 13:15   #68
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Default Re: Do High RPMs result in unburnt fuel?

Firstly, high rpms should theoretically produce slightly high efficiency than the lower rpms (~1000-1400). But in turbocharged diesel engines, i had a crude test that i tried long ago and noticed mileage was very good when driven in the turbo band with consistent upshift and downshift. the max i extracted was close to 26-27 kmpl and a top speed of a 100. When you over rev, there is only so much air that can be sent into the engine and only so much fuel that can be injected and hence, the higher rpms will definitely have better mileages than the lower 'lugging-around' rpms but not comparable with the turbo zone, as in the sweet spot between 1800-2500. Also, if you know how to rev match and shift into the correct gears, you'll place lesser stress and therefore slightly increase longevity and mileage too (not very significant, but yes, noticeable nevertheless).

These are my observations and looking forward to other's observations.
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Old 11th October 2013, 19:40   #69
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Default Re: Do High RPMs result in unburnt fuel?

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Originally Posted by Arch-Angel View Post
Firstly, high rpms should theoretically produce slightly high efficiency than the lower rpms (~1000-1400). But in turbocharged diesel engines, i had a crude test that i tried long ago and noticed mileage was very good when driven in the turbo band with consistent upshift and downshift. the max i extracted was close to 26-27 kmpl and a top speed of a 100. When you over rev, there is only so much air that can be sent into the engine and only so much fuel that can be injected and hence, the higher rpms will definitely have better mileages than the lower 'lugging-around' rpms but not comparable with the turbo zone, as in the sweet spot between 1800-2500. Also, if you know how to rev match and shift into the correct gears, you'll place lesser stress and therefore slightly increase longevity and mileage too (not very significant, but yes, noticeable nevertheless).

These are my observations and looking forward to other's observations.
The reason is that with turbo, the torque is more peaked compared to NA diesels, hence you have a sweep spot which is quite narrow, and towards the upper RPM region. This is one of the reasons that modern Trucks with turbo diesel have so many gears - 10 to 18, to ensure that the vehicle is always in the peak torque region, thus having optimum FE.
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