Shifting and Fuel Economy...

[Christ]

Now that I went back and read them again, I've mis typed that...

One method is high load, high gear w/o lugging, the other is peak bsfc...

[Christ]

Further clarification - the peak bsfc number always occurs at 100%load, and because of the amount of power that most engines produce, they can't actually get to 100% load, which means wasted movement, wasted efficiency.

Either method, the idea is to keep the engine loaded. The bsfc curve for a given engine changes dynamically with varying load... How it changes depends entirely on the engine and the duty.

[PaleMelanesian]

Originally Posted by JMac

Obviously lugging is bad, but even at a speed where the vehicle is moving just fast enough to not lug the engine, say, 35mph in 5th gear on your average car. Let's say you want to accelerate from there to 50mph. In 5th gear, and assuming a flat road, one would have to press pretty far on the gas pedal to get decent acceleration (even for a hypermiler) at those speeds. Where as fourth gear, I believe, would get the car up to 50 much more efficiently.

My thoughts (and actions): I do a 5th gear 35-50 P&G routine every day. I think it works well.

I use ~75-80 LOD, up to 90 at times. Above that and it goes into open loop mode, dumping extra fuel in to keep things cool.

[orange4boy]

CVT's are so much easier to understand. Here's my attempt at manual tranny shifting for best efficiency. Assuming accelerating to top gear and then cruising at X speed. Corrections welcome.

The goal is to create all your energy most efficiently. Never mind the dogma about shift as low as possible. That's the correct method once you are at steady speed. Gears mostly just change the power/time ratio. The limitation of gears is really just a function of the RPM at which engines operate.

Accelerating is the act of creating mechanical energy (HP/time) which is "stored" as kinetic (speed) and potential (top of hills). RR and Drag eat away at this but we will leave that out for now. Accelerating faster in and of itself is not more or less efficient than accelerating slowly.

Power is inversely proportional to acceleration:
100hp 1000lb car:0-60 ~20s
200hp 1000lb car:0-60 ~10s
400hp 1000lb car:0-60 ~5s
Theoretically, anyhow.


In a lower gear the "lever" the engine is acting on is longer so it simply applies it's work faster. Therefore, you should shift only when you will end up in the best bsfc range of the next gear. If you shift too early and end up "off the island" then you will create that energy more slowly at lower BSFC until your RPM gets into the sweet spot.

Is that about it?

I'm just now getting my head around this. I have always thought that accelerating faster to X speed requires more energy but it really only requires more HP which is energy over time. In the end it's still the same amount of energy to get to X speed.

[Christ]

Marcus, my issue with that is: the bsfc map is only totally valid when the engine is fully loaded, under theoretically ideal conditions. That never happens in real life, so the effective peak bsfc changes with throttle and load, making it very difficult to accurately judge one's island status.

[PaleMelanesian]

"bsfc changes with load and throttle"

Isn't that what the vertical axis is? Isn't half throttle the point halfway up the chart? That's how I've always read it. In the following chart, the best bsfc looks like 3,000 rpm (that high?!) and 55 Nm, or about 2/3 of max. Wouldn't that be (about) 2/3 throttle?

[PaleMelanesian]

Originally Posted by orange4boy

In a lower gear the "lever" the engine is acting on is longer so it simply applies it's work faster. Therefore, you should shift only when you will end up in the best bsfc range of the next gear. If you shift too early and end up "off the island" then you will create that energy more slowly at lower BSFC until your RPM gets into the sweet spot.

Is that about it?

I'm just now getting my head around this. I have always thought that accelerating faster to X speed requires more energy but it really only requires more HP which is energy over time. In the end it's still the same amount of energy to get to X speed.

That's the idea. It's a fixed amount of energy to accelerate a car to the same point. The difference is how efficiently the engine converts fuel into that energy. That's where bsfc comes in.

You're right about "falling off the island" of bsfc. It varies by engine, but in most cases you don't want to be below 1400-1500 rpm for accelerating. Steady cruise is different - lower is better.

[bertb]

As so often happens, I get lost in the math and physics. So did we decide that its better for FE but worse on the clutch to start in second?

I found this thread by searching before asking the same question. I have much smoother starts in second and it seems to temper my driving through the rest of the range. I do about 300 miles a week, half hwy half city. I think I'd be interested in saving wear and tear on the clutch.

[PaleMelanesian]

Your EX compared to my DX has a shorter first gear. You'll be just fine starting in 2nd going downhill. Use first for uphill. Flat - your choice. I use 2nd but a (very) quick moment in first isn't terrible. I'm talking about 10 feet or so and then into 2nd - just enough to get rolling.

[Fat Charlie]

Originally Posted by JMac

One might think, "So my clutch might wear out twice as fast, no big deal, I can still make it last 75k miles..."

So, how much is a clutch worth? I will admit one miiiight be able to squeeze a bit more FE, but I'm not convinced of that, especially since all that slippage is wasted energy, and that energy is being converted to heat on the clutch which increases wear.

And that's why I start out in first. Who the hell would think that ever having to replace a clutch would somehow be efficient? How much time does anyone actually spend in first gear anyway? The only possible payoff would be if you're going to ditch the car or it isn't yours in the first place.

Once I'm rolling I'll skip gears as appropriate, but first doesn't get skipped.