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Old 9th November 2011, 14:26   #16
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Default Re: Is Fuel efficiency strictly a function of RPM or is it speed+gear+RPM?

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Originally Posted by SDP View Post
As per my understanding, 4th gear delivers 1:1 rpm. i.e. for the engine's one revolution, you will get one revolution of the wheel in the 4th gear. In 5th gear, you are in over-drive and for each revolution of the engine, you get more than 1 revolution of the wheel.
In some cars even the 4th gear is 'overdrive' apart from the 5th.
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Old 9th November 2011, 14:34   #17
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Default Re: Is Fuel efficiency strictly a function of RPM or is it speed+gear+RPM?

When I am on economical drive, I used to shift to neutral when there is a good slope. The RPM will drop to about 800 but car will keep the same momentum for a good distance. Again, switch back to 5th itself when the speed is down at about 70-80.

Similarly, shift to neutral before stopping especially at toll booths/bumps. I tried it several times when the road is clear without affecting average speed. I am not sure if there any risks involved in this other than braking.
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Old 9th November 2011, 14:51   #18
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Default Re: Is Fuel efficiency strictly a function of RPM or is it speed+gear+RPM?

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Originally Posted by coolclouds View Post
I have a small question here.

Case 1: Car in 5th gear, RPM 2000, speed - 80 km/hr
Case 2: Car in 4th gear, RPM 1800, speed - 60/km/hr

Which one will return better mileage? (pardon me if its a nonsense question)

Edit: Thanks, above post gets me the answer.
This is not reasonable

Let me assume it a bit different. (50 instead of 60)
Assuming that both cars are the same.
Car in 5th gear at 2000 RPM would go at 80 Kmph
and in 5th gear at 1800 RPM would go at 70 Kmph

In 4th at 1800 RPM, it would probably go at 50 Kmph
So in 4th gear at 2000 RPM it would have done 65 Kmph

Assuming 4th gear at 2000 RPM and 5th gear at 2000 RPM take in the same amount of fuel, you will have travelled 80 Kms in 5th and only 65 Kms in 4th

And assuming 4th gear at 1800 RPM and 5th gear at 1800 RPM take in the same fuel, you would have travelled only 50 Kms in 1 hour.

With this, 5th gear at 2000 will take you 80 Kms in an hour
and 4th gear at 1800 will take you 50 Kms. To travel the extra 30 Kms, you will have to spend 40 more minutes in 4th gear which means 40 minutes worth of extra fuel. So in all, 5th gear at 2000 is more economical

Actually, any taller gear is always economical.

So, when are we going to get the 6th gear into our car?
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Old 9th November 2011, 14:59   #19
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Default Re: Is Fuel efficiency strictly a function of RPM or is it speed+gear+RPM?

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Originally Posted by coolclouds View Post
When I am on economical drive, I used to shift to neutral when there is a good slope. The RPM will drop to about 800 but car will keep the same momentum for a good distance. Again, switch back to 5th itself when the speed is down at about 70-80.

Similarly, shift to neutral before stopping especially at toll booths/bumps. I tried it several times when the road is clear without affecting average speed. I am not sure if there any risks involved in this other than braking.
I believe that it's better to keep your gear engaged even while coasting/ driving down a slope, not just because it's safer (more traction and control) but also because it's better for FE. I read somewhere that in modern cars, when we shift to neutral and take our foot off the accelerator, the fuel pump keeps injecting fuel to keep the engine spinning at idle rpm, however if we keep the car in 5th gear and take out foot off the accelerator, the fuel pump doesn't inject fuel, instead the rotation of the tyres keeps the engine spinning. I may be wrong here, but that is what I seem to remember, maybe others can chip in with more info.
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Old 9th November 2011, 15:04   #20
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Default Fuel efficiency strictly a function of RPM

Attached is the Specific Fuel Consumption Graph for an engine (Source: internet).

It shows that the Fuel Consumption is least at the curve 250 that lies on either side of 3000 rpm.

Now the Gear Ratios for this engine will be decided at this particular engine speed range. Max torque and speed is tuned at this engine rpm rate only.

Thus engine speed (RPM) is a direct function related to the Fuel economy.
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Old 9th November 2011, 15:09   #21
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Default Re: Is Fuel efficiency strictly a function of RPM or is it speed+gear+RPM?

The fuel efficiency of any vehicle is a direct function of the amount of throttle given. You can consider the Accelerator pedal as a Faucet. The more you press it, the more fuel you release. (Thats the reason it is also called the 'Gas Pedal' ).

Fuel usage at any speed when going along at a 'constant' speed is actually minimal. Fuel is used up most whenever you increase your speed.

The RPM is only an Indicator and can be easily done without if you know what is happening under the hood.

Now, if you watch closely at your throttling habits, you will realize that it takes less throttle AND less time to take the car from 60-80 in 4th as compared to in 5th. Even though the RPM this happens at might be higher for 4th than in 5th, you will actually use up less fuel in 4th since you were throttling less and for less time (i.e. you opened the tap less, for less time).

But when I state the previous para, I am not stating that you will get more FE in 4th when going at a constant speed of 60 OR 80. In both cases, you will in fact better FE in 5th than in 4th as long as 60 in 5th comes at over idling RPM.. But only if you are going along at a CONSTANT speed with minimal throttle input just to overcome the wind / road drag.

An even better way to get better Fuel Efficiency is to use the technique of Pulse and Glide (P&G)(which most hyper-milers use). This involves feathering the throttle ever so slightly once in a while just to take it to a speed 5% higher than the speed you want to cruise at.

Eg. You want to cruise at 80kmph. You feather the throttle slightly and gently take your speed upto 83-84 and leave the throttle. Since you are in top gear, there is very little resistance due to the internals and the car takes some time to slow down back to 80. Even more time to slow down to 77-76. When you eventually get down to this speed, just feather the throttle verry gently (and take almost the same amount of time it took you to slow down) and get back up to 83-84.

What the above exercise will do is, whenever you leave the throttle altogether, as long as the engine RPM stays above idling, the ECU cuts fuel supply to all cylinders and the engine keeps on coasting OR 'Glide'ing using just the inertia of the flywheel and cylinders and the car(since you are in gear) to keep running. It is like running without using any fuel (infinite kmpl). Also, when you feather or 'Pulse' the throttle to get back upto speed, since you are using so little throttle, the fuel used is always less.

In effect, even though you still maintain an average cruise speed of 80, you consume lesser fuel than what you would have had you been continuously cruising at 80.

P.S.: The P&G technique works better with cars with bigger engines since they have better low-end-torque required for a successful 'P' Stage.
P.P.S.: P&G will not work with older carb cars.

I use P&G on our Civic AT extensively on empty+straight 4-lane roads and return ~14-15 kmpl on the straights (only to switch to sport mode as soon as I hit the curves and ruin all my figures ).
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Old 9th November 2011, 15:15   #22
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Default Re: Is Fuel efficiency strictly a function of RPM or is it speed+gear+RPM?

Quote:
Originally Posted by coolclouds View Post
When I am on economical drive, I used to shift to neutral when there is a good slope. The RPM will drop to about 800 but car will keep the same momentum for a good distance. Again, switch back to 5th itself when the speed is down at about 70-80.

Similarly, shift to neutral before stopping especially at toll booths/bumps. I tried it several times when the road is clear without affecting average speed. I am not sure if there any risks involved in this other than braking.
Actually, you lose more fuel by Coasting in neutral.

Check this: http://www.team-bhp.com/forum/techni...save-fuel.html (Does coasting save fuel?)
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Old 9th November 2011, 15:23   #23
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Default Re: Is Fuel efficiency strictly a function of RPM or is it speed+gear+RPM?

Lowest RPM in the highest (or is it the lowest) gear gives the best mileage. Remember the gear sizes. 1st gear is the biggest in size (therefore it has maximum pulling power) hence lowest FE while you progressively shift up the size of the gear decreases with consequent loss in power (that's why the car stalls) and this aids FE.

Coming to the RPM the lowest speed a car can travel in 5th gear is around 45 (your RPM metre is the best guide), therefore you will get max FE in this speed in 5th gear.

Remember a news in which a Ford Fiesta had given a mileage of 40+ (I don't remember the figure correctly but it was phenomenal. There is also a thread on TBHP on this). The car as I had suspected had clocked an average speed of ~45 km/hr, all the way from Mumbai to Goa.

So for max FE, at least in theory, drive at the highest gear and lowest RPM (which will also translate into min possible speed in that gear which also means only a slight press of foot on the accelerator).

Last edited by anandjha : 9th November 2011 at 15:24.
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Old 9th November 2011, 16:26   #24
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Default Re: Is Fuel efficiency strictly a function of RPM or is it speed+gear+RPM?

Possibly offtopic, but here is a discussion on whether mileage (odo reading) is a good indicator of wear and tear or not: http://www.team-bhp.com/forum/techni...wear-tear.html (Does the odometer really reflect engine wear and tear?)

Where they are talking about specifically the engine, there's a discussion that it's the RPM and not the mileage which determines wear and tear.
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Old 9th November 2011, 17:42   #25
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Default Re: Is Fuel efficiency strictly a function of RPM or is it speed+gear+RPM?

This is what I think

Highest FE = maximum speed at minimum fuel consumption

1. In an engine there is a peak torque generated within a range of RPM. The specific fuel consumption is constant in this range. So the maximum distance traveled within this RPM will require minimum fuel/RPM, hence distance.

1a. With every gearbox there is a minor loss of power due to the gear action. This is minimum at 1:1 (normally 4th), that is the input and output shaft are coupled directly with no intermediate shafts. Thus the overdrive will loose some power, and higher the overdrive ratio the more power it looses, but you go further for the same engine RPM.

2. As speed increases the air and rolling resistance increase, and you require more fuel to maintaining the speed. This is similar to the case of moving up a slight incline.

Normally if point 2 was not considered then the maximum FE would be in an overdrive ratio just below below the point where the power required to maintain the speed (as though the gear box had 1:1 ratio) is matched by the power loss in the over drive mechanism. But as the speed increases point 2 manifests it self. Once you are beyond the peak torque curve the fuel consumption also increases.

The sweet point between point 1 and 2 is where you get maximum FE. That is the speed (within the peak torque curve) beyond which wind and tyre resistance dominate. For modern cars with mildly streamlined bodies, the sweet spot is around 80km/h. For SUV it may go down as low as 60 and for highly streamlined performance cars it may be 120+, for speed record attempts it crosses 300!

Coming to the different versions of questions asked.

. Driving at the same RPM in different gears within the peak torque band. Below the sweet spot the lower the gear the lesser the distance traveled, hence less FE.

. Driving at same speed in different gears. As long as the speed is within the peak torque band (min consumption), the 1:1 gear will give the best FE. If you are below or above the peak torque band in one gear and in band in the other, the gear in which you are within band gives best FE.

My attempt at quantifying what I said (don't kill me if it makes no sense)
FE(S,R,G) = (f(rpm)*f(GB))/S - f(air,S) - f(tyre,S), where
S = unit speed
R = RPM
G = Gear used
FE = fuel efficiency for a given speed, engine RPM and Gear.
f(rpm) = fuel consumed at that RPM.
f(GB) = loss ratio of the Gear Box in that particular gear at the stated RPM. taken as 1 for 1:1 ratio at cruising speed.
f(air) = loss due to air resistance at speed S
f(tyre) = loss due to tyre rolling resistance at speed S
To make the units same,
- f(air) & f(tyre) km/L.
- S in km/h
and a few constants to make the formula consistent.
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Old 9th November 2011, 23:57   #26
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Default Re: Is Fuel efficiency strictly a function of RPM or is it speed+gear+RPM?

All,

I am no expert. But drive a car with a Instantaneous Fuel Consumption Indicator(IFCI).
Have been observing this over the last two years now.
So this is empirical. I have come to trust the instantaneous readout which leads to the averaged readout. Which bears out the distance covered for a given amount of fuel in the tank. I alwasys go tankful to almost empty to tankful. (I Know i have been told not to drive to empty).

1. When in the city, (I live in Bangalore), since i cannot speed, i thought that i will at least derive the max mileage i can without acting silly.

2. So the quest began of driving in the highest possible gear, lowest rpm and every other combination that has been listed above. (The IFCI actually shows that the mileage drops like a rock, if you try high gear low rpm).

3. So my finding is that the above does not work at least in my car.

4. What works? Funnily keeping the car from coming anywhere close to lugging, which means that you are actually shifting in the powerband and driving very rationally, read "Not Slowly".

5. I get in Bangalore traffic, with at least 95% AC on, at least 16.5 KM/L going up to 18.5 KM/L at the highest.
This is for every tank of diesel that i have filled, with the exception of the first few, where in i was still experimenting.

6. So feel your car, let it ride comfortably. Fuel consumption is a factor of load.
So if the car is feeling loaded, shift to a lower gear immediately.

Long post, my experience.
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Old 10th November 2011, 11:08   #27
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Default Re: Is Fuel efficiency strictly a function of RPM or is it speed+gear+RPM?

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Originally Posted by vinya_jag View Post

Actually, any taller gear is always economical.

So, when are we going to get the 6th gear into our car?
6th gear is not additional gear above the 5th gear (taller gear). Actually the gear ratio of the 6th gear in a 6 ratio gear will be the same as the 5th gear of in 5 ratio system (for the same vehicle). They only add a gear in between 4th and 5th to make it 6 gears, so that the 4th to 5th is less tall
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Old 10th November 2011, 12:50   #28
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Default Re: Is Fuel efficiency strictly a function of RPM or is it speed+gear+RPM?

One question here related to FE and RPM. Is there any effect of AC speed on the FE? Means if i put AC on 1,2 or 3 will it affect my mileage? Also in this case does RPM matter? Some people say that it doesnt matter on FE whatever speed you run AC on i.e. FE will be same even if speed of AC is 1,2 or more, which i dont agree. More the AC more power it will consume from the engine. So FE will go down for sure. people say once AC is on, it doesnt make any difference what speed it is. I always experience the lack of power when AC is on full speed.

Will running car on constant rpm of 2000 will make more FE even if AC speed is high? Please confirm.
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Old 10th November 2011, 13:26   #29
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Default Re: Is Fuel efficiency strictly a function of RPM or is it speed+gear+RPM?

The speed is related to the fan, this may or may not have any bearing on AC power consumption. To elaborate:

- In a car with low power (say <100 hp, near 5-10% of the power at any time is taken by the AC), since you drive at around half or third of max revs, meaning half or so of the power output. So if on a really hot day, you select the temperature at 16 deg C, the AC will consume maximum power, and with the fan speed the interior temperature rate of change will vary (faster fan can help cool faster), so maybe the AC can operate at lower power after 20-25mins, when the car is reasonably cool, even if not at 16 deg C. Alternatively, on low fan settings, the AC will keep trying to make air cooler and resending it to the fan, which is not doing much to spread it around, so the air the AC gets remains warmer. So actually low fan speed may hurt your efficiency when the temperatures outside are on either extreme.

- In a high powered car (say >200 hp), driving it at sane rpms would still produce 20-30% more power than a 70bhp car, especially in the city drives. This means the AC is using only 4-6% of the power, and the effect of extreme temperatures is proportionally lower. However as the cabin becomes larger, vent placement becomes highly important in effectively cooling it, and thus allowing AC to operate at lower power levels.

If fuel economy is the sole goal, shun the AC, but if you are a convenience seeking human, adjust the fan speed based on number of occupants, so that the cooling time of the cabin can be reduced, and adjust the temp. setting as per outside temperatures (dont run it on lowest from Feb-Nov!)
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Old 10th November 2011, 14:31   #30
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Default Re: Is Fuel efficiency strictly a function of RPM or is it speed+gear+RPM?

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Originally Posted by aniketi View Post
One question here related to FE and RPM. Is there any effect of AC speed on the FE? Means if i put AC on 1,2 or 3 will it affect my mileage? Also in this case does RPM matter? Some people say that it doesnt matter on FE whatever speed you run AC on i.e. FE will be same even if speed of AC is 1,2 or more, which i dont agree. More the AC more power it will consume from the engine. So FE will go down for sure. people say once AC is on, it doesnt make any difference what speed it is. I always experience the lack of power when AC is on full speed.

Will running car on constant rpm of 2000 will make more FE even if AC speed is high? Please confirm.
Usage of A/c will have an impact on FE, in any car. Here is what I think happens when you switch on A/c at a particular RPM or when idle.

1. Engine idles at 1100 RPM (example idle scenario)
2. A/c powered on
3. More load added to engine since it has to now run the compressor
4. Engine RPM drops since there is more load
5. ECU senses this and opens the fuel system more
6. Engine uses extra fuel and tries to maintain the RPM

So my assumption is that the fuel consumption will increase with higher load to maintain the RPM. I feel the same will happen if you have high powered electricals as well. For example an ICE, or extra/high power head lamps etc. When you switch them on, the battery will drain more, ECU senses this and increases the engine RPM by supplying more fuel to maintain the charging rate of the battery.

Last edited by vasoo : 10th November 2011 at 14:37.
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