[anupmathur]

Originally Posted by watashi75

I think you are talking about "Power".

Quite right, but in this context, when you want to cover a distance in a much shorter time, you need much higher power/energy.
It would not be possible to do at same power at 40 kmph or at 120 kmph.
The fuel consumed will be much more!
Another example perhaps?

[clevermax]

Aw.... the 'too much sceintific' debate here hurts my brain.

Forget about work done being equal no matter what speed the vehicle was driven from A to B. The IC engine efficientcy is not the same at all times. It is least efficient (in terms of fuel consumption) while accelerating under load. It is most efficient while running at a constant speed producing enough torque to keep the car running at that speed.

And I don't buy the argument that efficiency at 80 Kmph in 5th gear being equal to the effciency at 60 Kmph in 3rd gear and so on. Can we say that efficiency at 20 Kmph in second gear is also the same? No way.

Constant acceleration or breaking is gonna reduce fuel efficiency. Using the wrong gear as well. - downshift instead of flooring the gas pedal in a higher gear for example. IMO, a car is most fuel efficient when driven by keeping the rpm in the sweet spot ~2000 rpm, as far as possible in the higher gear, downshifting whenever necessary. And, predictive driving goes a long way in increasing the fuel efficiency.

[watashi75]

Originally Posted by anupmathur

It would not be possible to do at same power at 40 kmph or at 120 kmph.
The fuel consumed will be much more!
Another example perhaps?

I am not sure how to give a better example...this is all from high school physics. But let me try to explain.

The difference between a car moving at 40 and a car moving at 120 is that the latter has more kinetic energy. By the law of conservation of energy, this energy has to come from somewhere, and in this case it comes from fuel burnt. But once the car reaches its cruising speed, the only forces that act on it are the frictional forces and air drag. Now, frictional forces do not depend on speed. Only air drag depends on speed. At low speeds where air drag is negligible, the amount of energy required to transport car from point A to point B will be same.

At high speeds the force required to overcome air resistance might be double of that required at low speeds, in which case the work that needs to be done is also double, which means twice the amount of fuel burnt.

So, yes, the amount of fuel required will be significantly more but only if you are going at high speeds. There won't be much difference in the fuel consumed if you are driving at 60 instead of 40.

A note for guys who are only considering speed for fuel economy and ignoring engine RPM - if you are operating the engine at less that peak torque RPM, the fuel efficiency is less and some fuel is left unburnt. It is common sense to see that if fuel is not burnt completely, more fuel will be required to travel the same distance.

[sinhsha]

Hmm Very interesting.
Let me revive my Hybrid Automobile Technologu Thesis from my MS.

Want to add few points
1. IC engines best efficiency is at constant RPM but not on any constant RPM only at a particular RPM and the value of the same changes from engine to engine (depends on various things like number of cylinders, long stroke or short stroke, materials used, engineering used, fuel used, ignition timing, cam control etc)
2. Apart from engine while calculating efficiency (suppose engine as a unit is same efficient all the time) the vehicle in real condition the effects of friction both external like between tire and road, between car body and air as well as internal like bearing friction, axle friction etc has to be taken into account. They all have different coefficient of drag and more speed and more acceleration mean more drag means more load on the engine.
3. Gears play a role to make the IC engine drivable and to maintain and extract it's optimum efficiency. My two small visio diagrams might clarify the same.
Sorry if I confused more. My intentions were noble.

[watashi75]

Originally Posted by sinhsha

3. Gears play a role to make the IC engine drivable and to maintain and extract it's optimum efficiency. My two small visio diagrams might clarify the same.

So, does this mean an engine running at near peak torque RPM is more fuel efficient and give better fuel economy than an engine running at low RPMs?

[sinhsha]

theoretically yes in a no friction/drag perspective.
But practically you mean the efficiency of the car and not the engine only. And a yes/no answer to the questions you asked is not possible. There is a optimal speed after calculating all the parameters to be arrived.
In my thesis I calculated and tested for different vehicles in american driving condition and found 55MPH to be optimal in most of the cases.

[anupmathur]

Originally Posted by sinhsha

In my thesis I calculated and tested for different vehicles in american driving condition and found 55MPH to be optimal in most of the cases.

So, obviously all the manufacturers work backwards, considering that's the most widely posted speed limit, LOL!

[watashi75]

Originally Posted by sinhsha

In my thesis I calculated and tested for different vehicles in american driving condition and found 55MPH to be optimal in most of the cases.

Now this is interesting. But can this be generalized without taking into consideration the vehicle load factor, gear and engine RPM?

FYI, we have another thread here with some more debates on fuel economy with respect to load -

http://www.team-bhp.com/forum/techni...ur-engine.html

[abhibh]

Just back from 1400 KM trip.

Total Petrol filled Rs. 2600.
Avg Speed with AC - 70 KMPH Touched 140 for a few minutes just to check out the performance.

Still 1/4 tank left LOL. Amazing FE

[akshay4587]

Originally Posted by abhibh

Just back from 1400 KM trip.

Total Petrol filled Rs. 2600.
Avg Speed with AC - 70 KMPH Touched 140 for a few minutes just to check out the performance.

Still 1/4 tank left LOL. Amazing FE

kaha gaya tha(where did u go)
pics; log????????

[abhibh]

Originally Posted by akshay4587

kaha gaya tha(where did u go)
pics; log????????

arey no report or pics as such but I WENT TO ICE HEAVEN a.k.a Delhi 6

Chandigarh > Jalandhar > Delhi > Gurgaon > Delhi > Gurgaon > Delhi > Noida > Gurgaon > Delhi > Gurgaon > Delhi > Jalandhar > Chandigarh

[Zahir]

I believe its more about the right rpm range. Don't over rev dont under rev. For example, with my VDi, I drive the cold engine upto 2000 rpm and a warm engine between 2000-3000 rpm. This falls within the Torque range of the car too so its fun to drive while still maintaining good FE

[amateurpro]

Originally Posted by vasudeva

Short trips means high consumption. Short trips at cold ambient temperature result in the greatest fuel economy penalty. During the first kilometre of travel from a “cold” start even at 20°C, the fuel consumption is up to three times higher than with a fully warmed-up engine.

You had given some excellent dope on how short trips affect fuel economy in petrol cars. Can you throw some light on how short trips affect FE in the following scenarios.

Let us say I have to drive 10 km in city (roughly 35-40 minutes of drive time) with a 5 minute stopover at an ATM in between. I have to choose between two ATM's, the first one is 2 km from the starting point (or 8 km before the destination) and the second one is 8 km from the starting point (or 2 km before the destination). Which ATM is better for me to stop purely from an FE angle? I guess the second one is better but I am not sure if it makes any difference.

[vasudeva]

You may save on FE but with regards to ATMs, I will go to the one which I think works or which I know from experience has a shorter queue. Fuel is money and so is time and so is ease of use. That is why humans rule because we can make judgments.

Hope I have not evaded your query.

[pganapathy]

From an FE point of view, the second ATM is better. Your car engine warms up fully by the time you do 8 km. Switching it off for a few minutes will not cool it down too much and will be at high temperature for the remaining 2 kilometres.