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Old 31st July 2012, 01:51   #16
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Originally Posted by ramzsys

John sir, don't they need to go to lower gears at higher rpms to climb an incline?
You are right, that is true for ghat sections, not our city roads, right ?
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Old 2nd August 2012, 02:21   #17
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Default Re: Fuel Efficiency determined by rpm, gear or both?

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Originally Posted by esteem_lover View Post
RPM is what defines, but it also matters at what gears, isn't this a basic fact of science ? I do not understand fitting speed governors on city buses, governors should be based on rev limits and not on speed limits to avoid rash driving.
I'm not sure but won't the buses in our city revv almost to its peak when loaded and in peak city traffics for the start-stop traffic (correct me if I'm wrong).

Atleast here in Hyderabad most of the RTC buses are now the TATA Marcopolo's which are auto trannies so revv limiting for these would be an issue. Revv a lot and make a hell of a racket when started from standstill.
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Old 2nd August 2012, 10:28   #18
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Default Re: Fuel Efficiency determined by rpm, gear or both?

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Originally Posted by pganapathy View Post
Therefore the answer to my question is a combination of engine revs and gear.
Here's another related thread that might have some info :
http://www.team-bhp.com/forum/techni...-gear-rpm.html (Is Fuel efficiency strictly a function of RPM or is it speed+gear+RPM?)

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Old 2nd August 2012, 10:46   #19
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Default Re: Questions about fuel consumption at engine level

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Originally Posted by esteem_lover View Post
You are right, that is true for ghat sections, not our city roads, right ?
Always remember what a gearing system does. It manipulates the load as seen by the power source. In this case the power source is the engine and the load is the car's weight.

Also remember what newton said, energy is only required to accelerate/decelerate and not to maintain constant speed.

So combining these two when we are in lower gears (1st/2nd) the engine is seeing a very low load (actually just divide the weight of the car by the gear ratio and this shows the actual weight as seen by the engine) and hence is able to rev easily to 2K/3K where it generate enough torque.

As you gain speed, you need lesser and lesser energy to accelerate and hence you can present a larger load to the engine and reduce the rpm while increasing the speed.

(Think about someone at home asking you to move a big sofa/bed. The initial energy required to get it to move is high. Once it moving to keep it moving its always less. Well in cars we we have gears but at home we just need to push push push )

The above is simplistic and a basic understanding. There are other complexities to worry about such as wind resistance at higher speeds, optimum rpm ranger where the engine is most efficient etc)

Last edited by ash_blore : 2nd August 2012 at 10:47.
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Old 5th August 2012, 23:09   #20
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Default Re: Questions about fuel consumption at engine level

Maybe Off topic, but relevant to the conversations had before.

Why I said that there has to be a rev limiter more than a speed limiter on our buses is because driving rash (especially wrt buses) is that even 40 kmph is too fast on a busy market place and without a rev limiter it is so easy to do 30-40 kmph in 2nd or 3rd gear. If we were to control rash driving (over speeding is often confused with rash driving), rev limiter is the way to go.
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