[euromodder]

Originally Posted by bikenfool

Hills seem like P&G enhancement. Pulse on the climb, glide on the descent.

The climb can be seen as a pulse (but in potential energy, not kinetic / speed), though you'd still be wanting to slow down uphill.

You usually need to carry some speed though
There are situations where you can crest a hill or bridge with very little speed, and catch back up downhill. Traffic allowing.


Sticking to the same speed while going uphill is already a fuel drain.
You really don't want to have to accelerate uphill, it's a double lose-lose situation.

DWL implies constant load on the descent, in other words using the engine to increase speed.

Regain the speed you had, and let gravity add a bit on top of that, recovering some of the energy wasted going uphill.
The extra speed will then slowly wear off once you're on level ground.

I've found this to be a condition that engines respond quite well to: slowly letting speed decay a wee bit, and fuel consumption drops, then stays low at the slightly slower constant speed.


As you DWL uphill, engine load increases while rpm are reduced, this both boosts your engine's efficiency
(as long as it can make enough power at those rpm - if it can't, you have to shift down)

I know you'd be using more fuel, but going uphill & keeping the speed up would usually be more efficient.

It's not, and it's definitely not the case when it means the higher cresting speed means overspeeding (going beyond the posted or your own limits) on the downhill stretch.

While you're making the HP efficiently at the high load required to go up, the HP required means you also burn more fuel - even if it's done efficiently.

Requiring less HP to start with, but still in an ara of high engine-efficiency, translates in requiring less fuel.

When you're loafing along on flat ground your below the optimum region on a BSFC map. Higher load would bring you up and be more efficient.

When loafing along on flat ground, you're certainly well below the BSFC ...
Higher load would make the engine more efficient, but it'd also use more gas.

You got to disconnect the engine's best fuel-to-HP efficiency from the overal fuel efficiency.

At best BSFC, you're typically using a lot of gas - yet doing it so efficiently to make a lot of HP, that you can use it to save gas overall (i.e. using Pulse & Glide)

At your best fuel consumption speed, you'd be well below the best BSFC of your engine, so your engine isn't using the fuel efficiently, but it needs so little HP to sustain level constant speed that you're still using only a little bit of fuel.


A vehicle's engine is sized for peak performance, but using DWL or CC, it typically runs quite inefficiently (@ converting fuel to HP) at low average power output to sustain a constant, level ground, speed.
The low HP requirement is really the only saving grace in this situation.


That's what makes P&G so efficient :
When it needs to make power, it's done at the best fuel-to-HP conversion rate.
When it doesn't need power, the engine is off , not using any fuel.
The average power requirement is the same for the same average speed.
But with P&G, this power is made far more efficiently when needed.


It also means DWL will be less fuel efficient overall than P&G.
With DWL you mostly run the engine at low power requirement, low load, but that's where it's not running efficiently @ making HP.
It's only needing so little HP it uses little gas.


The really astonishingly high MPG results, are most often achieved using P&G.
It's a shame really that P&G is a high workload driving technique, and not always compatible with other traffic.
It'd drive other ppl mad around here