[euromodder]

Originally Posted by bikenfool

the optimum solution is constant speed.

That's the general thinking.
But it's wrong.
The most efficient is pulse (accelerate @ BSFC for your engine) and glide (engine off). Repeat and repeat ...
Even though your speed varies significantly, it's more efficient..
Constant speed means the engine is running inefficiently - even if its fuel consumption is relatively low (compared to what you're used to).

With Pulse & Glide, you use the engine at its peak efficiency whenever it's on.
That's what makes it so efficient.


Even coasting with the engine on (and driveline disconnected) will improve your efficiency.

I don't see how that comes into play for DWL or CT.

Constant speed is fine - compared to "regular" driving - as long as the road is level. Actually, you'd be DWL at that point.

But you lose efficiency whenever you go uphill or downhill.
Uphill, a CC or the driver simply pours on fuel to keep the speed up.
Downhill, it's actually using engine braking to keep the speed under control - and braking is not good. The car wants to go faster due to the grade, so let it go faster, if it can be done safely.

Slowing down on the hills saves fuel on the hill, but also takes longer. Wouldn't it be better to just slow down the average constant speed?

The slower you go, the better your average - usually.

But there's a point where the engine becomes more inefficient as you slow down, and you may get the same MPG as you would at a higher speed.
The benefit then is in not accelerating to the higher speed - which will be minimal on longer drives.

But even if the slower average is improving your MPG, it'd still be more efficient to slow down uphill and catch up downhill.

Perhaps its just a conservation of energy thing?

That's part of the point.

You trade fuel for potential energy : your car's weight @ height.
The engine load increases, and so does your engine's efficiency.
High load and low rpm is where it gets its BSFC after all ...
Even if it uses more fuel/mile, it's producing the horses required to get up there more efficiently.

Going downhill, you can trade height for speed (and distance) again.
But if you hold it back, you'd be braking and you won't get the return on your invested fuel.

you're gaining economy but losing time.

Despite slowing down considerably, I'm not losing that much time on my 30 mile commute.
On longer trips, the loss can be considerable, so there's usually always a trade-off between time and MPG.

If you accelerate with throttle on the descent I don't think you'll gain much, if any, in fuel economy.

You don't really accelerate with throttle when going downhill while using DWL.
You just get the car back to the speed it wants to make on as much fuel as you're giving it + the downhill grade

Some hills you can even glide down with no throttle at all - and driveline disconnected !

If you brake, you won't be getting as much of the potential energy (i.e. the fuel invested) back though.

Then when the road levels out, you'll be back to your DWL fuel target, the sort of fuel consumption you want to achieve .


Try DWL instead of CC for a few tanks.
It will give you improved MPG