[Shan2nu]

Wait a second. Where did you get the idea that peak torque = peak efficiency?

Actually its true. The reason why an engine produces max amount of turning force (torque) is bcoz the combustion inside the cylinder is most efficient at that rpm.

This is why they say that an engine provides the greatest amount of acceleration at max torque rpm.

What you are talking about is peak fuel economy which is different from peak fuel efficiency.

Shan2nu

[revtech]

Originally Posted by Shan2nu

What you are talking about is peak fuel economy which is different from peak fuel efficiency.

Shan2nu

This doesnt make sence.im guessing its a typo.

[Shan2nu]

This doesnt make sence.im guessing its a typo

What doesn't make sense?

Shan2nu

[revtech]

sorry my bad.for some reason i read both as fuel economy..

[watashi75]

Originally Posted by Shan2nu

What you are talking about is peak fuel economy which is different from peak fuel efficiency.

Originally Posted by revtech

This doesnt make sence.im guessing its a typo.

Suppose your car gives peak fuel efficiency (all fuel burns cleanly) at 4500 RPM (max torque). But if you are in 1st gear your kmpl will be less than, say, if you were in 4th gear. So fuel economy (fuel needed to cover a particular distance) will be less.

So for peak fuel economy, the highest gear should be engaged that can support the load at that speed. I say speed because at higher speeds the aerodynamic load becomes more significant.

[Shan2nu]

Suppose your car gives peak fuel efficiency (all fuel burns cleanly) at 4500 RPM (max torque). But if you are in 1st gear your kmpl will be less than, say, if you were in 4th gear. So fuel economy (fuel needed to cover a particular distance) will be less.

So for peak fuel economy, the highest gear should be engaged that can support the load at that speed. I say speed because at higher speeds the aerodynamic load becomes more significant.

Exactly. There is no fixed rpm or speed for fuel economy.

The best we can do is try out diff driving styles wrt rpm, speed and throttle pressure, to see which combination works for us.

Shan2nu

[sukrit7]

There's nothing negative apart from a bad fuel economy if you changes gears at 5000 pm is it?
I don't do it regularly, sometimes just to feel the baleno's power.

[Eddy]

Engine wear and tear is higher at higher RPMs. They are all moving parts after all.

My last few tankfuls have given me 16.5 to 16.7 in the city with the AC on.
This while "lugging"(idle in 3rd gear)
Midway through my current tankful, this thread came up, and I decided to change my city driving style.I try now not to let the rpm drop to idle,Downshift at 1500rpm, upshift at 2200, It makes my drive jerky.

I never use the clutch except to change gears. That stays the same.
Filled my tank yesterday and got 17.5kmpl. Just a normal variation? or better fuel efficiency at work? Anyways, this is supposed to be better for my EGTs, so I think I'll stick to it,even if I lose a kilometer or 2 per liter. After all, it is diesel.

I'm still not sure if I have been lugging.
can someone answer that please?
The car does never did shudder/lag or vibrate when it was driven in 3rd gear at idle
I have been going by feel rather than using the tacho, till I saw this thread.

[anupmathur]

Originally Posted by Eddy

Engine wear and tear is higher at higher RPMs. They are all moving parts after all.

Eddy, it is not that intuitive!
They are moving parts, but of a prime mover. (It is different from a driven machine).
You'd be surprised how rapidly they can self destruct at unsuitably low rpms!

[Shan2nu]

My cousin's Indigo Diesel which has never been revved hard, failed at very low rpm.

I asked him how it happened and he told me that he was in 5th gear and the moment he used full throttle to overtake, the engine made some sound and died out.

The bill has come to around 25K and they have filed a case against TATA.

So engines dont need to be revved for them to fail. Driving you car at very low rpms and flooring the throttle puts lot more stress on the internal parts than being in the right gear and at high rpms.

A higher gear provides less leverage to the engine, which is why when you floor the throttle at 1500rpm in 1st, the rate of acceleration is far greater than when you floor the throttle at 1500rpm in 5th.

If an engine produces 50nm@1000rpm, the amount of turning force at the wheels will be "crank torque x gear ratio x final drive ratio". So if your car has a 3.2 1st gear ratio, 0.7 5th gear ratio and 3.8 final drive ratio.

Wheel torque in 1st@1000rpm will be 50 x 3.2 x 3.8 = 608nm, where as wheel torque at the same rpm in 5th will be 50 x 0.7 x 3.8 = 133nm (only 21% leverage compared to 1st gear)

Shan2nu

[Eddy]

Originally Posted by anupmathur

You'd be surprised how rapidly they can self destruct at unsuitably low rpms!

I totally agree sir. Unsuitably low or high, both should be avoided.

[sukrit7]

everyone talks about high revving & redlining here yet it's not good. wow!

[KSM-Vtec]

never do that. kills the engine life i am sure.

[aaggoswami]

Originally Posted by watashi75

So for peak fuel economy, the highest gear should be engaged that can support the load at that speed. I say speed because at higher speeds the aerodynamic load becomes more significant.

From what I have read, in any object moving above 60 kmph, maximum energy is spent on cutting through air. So at any speeds above 60, the aerodynamic resistance increases at a very high rate.

Lugging also depends upon engine to engine. If you are accelerating slowly and steadily in 5th gear on plain surface, then the engine components wont get stress out much. Sudden demand from low rpm in high gear = max lugging. This actually is a time where the engine will be having maximum stress.