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Old 10th November 2011, 14:49   #31
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Default Re: Is Fuel efficiency strictly a function of RPM or is it speed+gear+RPM?

I am more confused now

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Originally Posted by nipunkul View Post
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!)
You are giving example of car with ACC i guess. Most cars dont have that. So in simple words, if you increase speed of the AC, FE will go down, right?

Quote:
Originally Posted by vasoo View Post
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.
What you are saying is right but i guess by electrical usage like ICE or head lights etc, FE will have very less impact, may be less than 1-2 %. But AC will make big impact, specially in the city running as car is idle more time like on signals and gear shifting is too much.
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Old 10th November 2011, 15:05   #32
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Default Re: Is Fuel efficiency strictly a function of RPM or is it speed+gear+RPM?

Even for manual AC's when you set it on recirculate, the AC needs less power once the car cabin has already cooled down, which might be achieved faster by having a higher fan speed at the start. So maybe more fuel is consumed when you start the AC, over the period of the drive (say 1 hr), the consumption may be no higher than keeping the fan speed medium for the whole time. Of course if the temperature outside is only say 30 deg C, it doesnt matter, and higher fan speed consumption will not average out lower than low speed consumption.

In the core of it, the AC to consumption ratio is not directly related to fan speed alone, but depends on cabin temp. before you start, outside temp., AC compressor efficiency and vent placement plus other controllable factors like opening windows or boot at mall/office entrance etc.
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Old 10th November 2011, 15:11   #33
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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
I am more confused now



You are giving example of car with ACC i guess. Most cars dont have that. So in simple words, if you increase speed of the AC, FE will go down, right?



What you are saying is right but i guess by electrical usage like ICE or head lights etc, FE will have very less impact, may be less than 1-2 %. But AC will make big impact, specially in the city running as car is idle more time like on signals and gear shifting is too much.
Whether the impact is big or small is subjective. As mentioned earlier by somebody here, for a more powerful engine the impact will be lower. For example a BMW which gives 8 kmpl normally may give 7.8 kmpl with a/c while an Alto which gives 17 kmpl will give 13 kmpl with a/c. (just an example).

Impact of ICE and electricals is also subjective. If you have a real high end ICE with two subs, three amps etc. then the impact on FE will surely be higher and comparable to switching on the a/c.
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Old 10th November 2011, 15:14   #34
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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 Astleviz View Post
Would I get similar FE figures if I drove the car at 80 kmph in 5th gear or 60 kmph in 4th gear or 42 kmph in 3rd gear or would it vary at different gear/speed combos assuming RPM is maintained at 2000 RPM?
Hmm..look at it this way. It will take longer to cover the same distance in lower gears at those speeds, eg: other things remaining constant, you will cover 60 KM in 45 minutes, 60 minutes and 86 minutes respectively! Which means the engine operates for a longer duration for each lower gear and on that count alone, your FE will decrease.
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Old 10th November 2011, 18:18   #35
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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
I am more confused now
What you are saying is right but i guess by electrical usage like ICE or head lights etc, FE will have very less impact, may be less than 1-2 %. But AC will make big impact, specially in the city running as car is idle more time like on signals and gear shifting is too much.
I notice that the same questions are asked in different guises. So I will try to explain it with numbers

. Assume your AC consumes 2KW, a perfectly normal number
. You have an engine producing 50KW peak (MUL K10 engine), normal for a hatch
. At each speed your engine produces (numbers are approximate)
- 20 km/h ~ 10KW
- 40 km/h ~ 20KW
- 60 km/h ~ 30KW
- 120 km/h ~ 50KW

so the proportion of power consumed by AC is
20 ~ 20%
40 ~ 10%
60 ~ 6.5%
120 ~ 4%

The same holds for Electrical accessories
100+100W head lamps = 0.2KW. Add 4-6 high power external lights to your SUV, and you are now approaching 1KW in lights only.
1000W ICE = 1KW, put in a really high power system and you will consume as much power as an AC, if you let it BLAST!.

Thus at 20 km/h the AC is draining a lot of power compared to the engine output, hence you feel it. In my Alto K10 you can do 20 in 2nd gear without Ac, but not with AC.

At 100+ the proportion of power used by AC is small, but still significant, thus you do not feel that drastic drop in speed, but it reflects in FE.

Now take a larger engine, and assume the AC is same.

. The AC in 100KW (Toyota Altis) engine will have half the effect compared to 50KW engine
. Increase the power to 150 KW (E 250 CDI), and now the AC is taking much less power, at 200KW (E 350 petrol) even at 20km/h the AC does not matter much.

As city running is in the band of 20-50 km/h, you can see that the AC will make a big impact on both performance as well as on FE, in a hatch, but minor impact of a big 150KW vehicle.
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Old 10th November 2011, 19:04   #36
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Default Re: Is Fuel efficiency strictly a function of RPM or is it speed+gear+RPM?

Very very interesting thread.
Doesn't the weight of the car (there must be other factors too) come into picture at all?
If everything as mentioned in the thread subject line is considered to be same, would an Indica & Dicor give the same mileage?
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Old 10th November 2011, 19:12   #37
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Default Re: Is Fuel efficiency strictly a function of RPM or is it speed+gear+RPM?

Please read up post number 45. This is as regards engine efficiency and not taking aerodynamic criteria into the picture. In the city aerodynamics play very little role especially if you are following a bus or truck which is making a hole in the air for you.

http://www.team-bhp.com/forum/techni...ml#post2298035 (All you enthusiastic drivers : Do NOT OVER-REVV)
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Old 10th November 2011, 22:45   #38
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Default Re: Is Fuel efficiency strictly a function of RPM or is it speed+gear+RPM?

Quote:
Originally Posted by Astleviz View Post
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.

Yes it is true, the ECM will be confused as the Vehicle speed sensor gives a different reading and the clutch and gear sensor gives another. This would make the car run rich as it will default to idle maps/mode / being in gear will lean out the fuel supply if the foot is off the pedal. and coasting in neutral wears the clutch and has poor braking response.
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Old 11th November 2011, 11:39   #39
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Default Re: Is Fuel efficiency strictly a function of RPM or is it speed+gear+RPM?

What about the effect of turbo? I mean which consumes more fuel-driving at 60kmh in 5th gear with rpm of 1500 or 80 kmh in 5ht gear with rpm of 2000?
Also all the TV ads and environmentalists advice to switch off the car when stopping for traffic signals for more than 60 sec. Now I've heard that a lot of fuel is consumed during starting of a car. Also the A/c which would have been switched off would have to start at a higher speed to cool the car(our hot summers can warm a car in 30 sec). So is what they say really true?
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Old 12th November 2011, 17:50   #40
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Default Re: Is Fuel efficiency strictly a function of RPM or is it speed+gear+RPM?

Very informative thread,thanks to all for their info,in these times of high petrol prices,driving with a light right foot is the order of the day!

I have question,lets say i am in a situation where i can drive at 45-50 kmph(this is not too fast not too slow,like in crowded four lane traffic),for an extended period of time.So if i drive in 4th gear i am doing at about lets say 1,600 rpm,car feel relaxed and the just in the start of the powerband.
But if i shift to 5th the rpm falls to 1,200 rpm,but still above my idling speed of 950ish rpm.Its ok to drive,not a problem.

But in the second case,am i lugging the engine?Eitherways if i have to accelerate to 60 it will take time because its out of the powerband,so i think in this case keeping it in 4th should be more efficient?
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Old 12th November 2011, 18:23   #41
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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 avishar View Post
I have question,lets say i am in a situation where i can drive at 45-50 kmph(this is not too fast not too slow,like in crowded four lane traffic),for an extended period of time.So if i drive in 4th gear i am doing at about lets say 1,600 rpm,car feel relaxed and the just in the start of the powerband.
But if i shift to 5th the rpm falls to 1,200 rpm,but still above my idling speed of 950ish rpm.Its ok to drive,not a problem.

But in the second case,am i lugging the engine?Eitherways if i have to accelerate to 60 it will take time because its out of the powerband,so i think in this case keeping it in 4th should be more efficient?
You have to be in the peak torque band, which in most cases is beyond 1500 RPM in a Diesel and 2000 RPM in a Petrol. So going down to 1200 is "Lugging", you will be spending more fuel.

The idling speed is the minimum speed an engine can do in a "no load condition" and not an indication of the optimum speed for the vehicle.
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Old 13th November 2011, 10:32   #42
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Default Re: Is Fuel efficiency strictly a function of RPM or is it speed+gear+RPM?

@Aroy; What you're saying is that 45-50kmph at 4th gear @1600rpm is more efficient than 5th gear @1200rpm.
Another question- in turbo diesels where the turbo kicks in at say 1700rpm, in a similar scenerio, would 3rd gear @1800 rpm be better?
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Old 13th November 2011, 10:55   #43
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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 wildsdi5530 View Post
@Aroy; What you're saying is that 45-50kmph at 4th gear @1600rpm is more efficient than 5th gear @1200rpm.
Another question- in turbo diesels where the turbo kicks in at say 1700rpm, in a similar scenerio, would 3rd gear @1800 rpm be better?
Maximum efficiency is achieved when you are in the RPM band where the torque is flat (and maximum). Out side this range and the efficiency drops fast.

. In my experience, at least with petrols, 45-50 km/h is too low a gear for 5th gear. Yes 4th will be more efficient in this band.

. As turbo boosts the power, the efficiency with a turbo on, will definitely be more than without a turbo.

The 40-50 speed band is one tricky situation. Most vehicles can run in 3rd or 4th gear. Depending on where the peak torque band starts it will be 4th or 3rd that is more efficient. If it starts around 40 in fourth then definitely fourth is better, else third will be more efficient. In my case if it is short distance then I use the third for speeds below 45. If I have to sustain it for long then I shift to fourth but try to be above 50. This is a rare case as in the city it is either < 40 or > 50. On highways I like to maintain 80-90, hence I downshift to second for overtaking and then shift directly to fifth, as both my cars - Esteem and Alto K10 can easily do 90+ in second (done 100+ in both for testing the limits). For normal diesels you can downshift to third instead, for overtaking.
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Old 13th November 2011, 11:07   #44
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Default Re: Is Fuel efficiency strictly a function of RPM or is it speed+gear+RPM?

FE is certaily a combination of factors.
Speed, Right Gear changes, RPM, correct Tyre Pressure, Laden Weight in the car, usage of AC and other electrical equipment.

Excessive acceleration and hard braking etc to be avoided. One should not ride the clutch. One should ideally switch off the engine when parked at a traffic light and particularly when it is clear that the wait is going to be in excess of one minute.

Also better not to take the vehicle for very short trips to the shops etc - better to bicycle or walk in case the shopping load is not going to be too heavy. (bread and other basic stuff that is.)

There are many ways one can save Fuel and improve the vehicle's FE.

If all of us would simply follow some of these, we would see a distinct drop in the monthly Fuel expense statement.
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Old 13th November 2011, 13:21   #45
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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 shankar.balan View Post
Also better not to take the vehicle for very short trips to the shops etc - better to bicycle or walk in case the shopping load is not going to be too heavy. (bread and other basic stuff that is.)

The additional benefit is that in this day of high pressure jobs and lack of time, this is the only time you get to relax and build up your health. In fact at times I have noticed that walking down to the neighbourhood shop (less than 15 minutes round trip walking) for small items is faster than taking a vehicle and finding a parking slot for it.
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