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Old 11-18-2013, 02:26 PM   #1 (permalink)
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Why is 'driving with load' better than constant speed

Why is 'Driving With Load' (DWL) or 'constant throttle' (CT) better than constant speed? Given that we want to minimize fuel consumption but still arrive at our destination in a reasonable time, if we only consider the losses (aero and friction), the optimum solution is constant speed. There is a trade off between speed and economy. This is because the aero loss increases as the square of speed.

Is there some engine efficiency factor I'm missing? I see how pulse & glide works by putting the engine into an efficient regime on the pulse, and drastically reducing engine friction on the glide, but I don't see how that comes into play for DWL or CT.

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?

Perhaps its just a conservation of energy thing? If you go up a hill and arrive at the top at 0 speed, then coast down the other side you've minimized the energy consumption for that event (up and down the hill). In practice you'd want to keep it in high gear and roll over the top at your minimum high gear speed. If you can't coast down the hill faster than your normal cruise speed and make up the time you've lost on the climb you're gaining economy but losing time. If you accelerate with throttle on the descent I don't think you'll gain much, if any, in fuel economy.

The articles I've found didn't (including the wiki here) did not explain the mechanics of DWL at all. If someone can explaing it or point me to something I'd appreciate it.

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Old 11-18-2013, 02:48 PM   #2 (permalink)
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Quote:
Originally Posted by bikenfool View Post
Why is 'Driving With Load' (DWL) or 'constant throttle' (CT) better than constant speed? .......
Driving with load allows the gas engine to run in a more efficient regime.

If you can find it, look for a fuel consumption map for your particular engine.

I drive a Honda Insight, and the BMSC map for my engine shows that I should run between 1500 and 2500 rpm, and about 80% throttle for the greatest engine efficiency.

I use this approach when going up hills and getting up to speed.

Using DWL is the most effective way to work with EOC, Engine Off Coasting. When I do that effectively, I get over 150 mpg in the summer months.

HTH, Jim.
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Old 11-18-2013, 03:18 PM   #3 (permalink)
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Quote:
Originally Posted by bikenfool View Post
Why is 'Driving With Load' (DWL) or 'constant throttle' (CT) better than constant speed? Given that we want to minimize fuel consumption but still arrive at our destination in a reasonable time, if we only consider the losses (aero and friction), the optimum solution is constant speed.
That might be true for a Diesel engine. That is decidedly not true for a gasoline engine.

A significant amount of the work done by a gasoline engine is to create and maintain a vacuum in the intake manifold. A higher intake manifold vacuum requires more work from the gasoline engine. Also, varying the intake manifold vacuum is much the same as speeding up and slowing down - gasoline is wasted in shooting for a target speed when going up and down hills.

DWL is an attempt to provide a constant intake manifold vacuum, regardless of speed. Of course, not a whole lot can be done with going down a hill (other than to go into neutral), but for level roads or going up hills, it's better than maintaining a constant speed.
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Old 11-18-2013, 03:41 PM   #4 (permalink)
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Quote:
Originally Posted by 3-Wheeler View Post
Driving with load allows the gas engine to run in a more efficient regime.

If you can find it, look for a fuel consumption map for your particular engine.
I wish I could find one. Yes I've seen the BSFC map page here, and searched elsewhere.
Quote:
Originally Posted by 3-Wheeler View Post
I drive a Honda Insight, and the BMSC map for my engine shows that I should run between 1500 and 2500 rpm, and about 80% throttle for the greatest engine efficiency.

I use this approach when going up hills and getting up to speed.
I'm familiar with the concept and my toy v6 is probably most efficient in that area also. (What is BMSC?)

I agree with that, but that isn't DWL as defined in the wiki here (sorry I'm a noob & it won't let me post a link):
"In other words, you will back off the accelerator and lose speed (possibly also downshifting) as you climb, and gain that speed back on the descent."
For most cars, when cruising we're at low throttle opening, either backing off, or constant throttle doesn't get the engine close to peak efficiency.
Quote:
Originally Posted by 3-Wheeler View Post
Using DWL is the most effective way to work with EOC, Engine Off Coasting. When I do that effectively, I get over 150 mpg in the summer months.

HTH, Jim.
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Old 11-18-2013, 03:49 PM   #5 (permalink)
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Quote:
Originally Posted by t vago View Post
That might be true for a Diesel engine. That is decidedly not true for a gasoline engine.

A significant amount of the work done by a gasoline engine is to create and maintain a vacuum in the intake manifold. A higher intake manifold vacuum requires more work from the gasoline engine. Also, varying the intake manifold vacuum is much the same as speeding up and slowing down - gasoline is wasted in shooting for a target speed when going up and down hills.

DWL is an attempt to provide a constant intake manifold vacuum, regardless of speed. Of course, not a whole lot can be done with going down a hill (other than to go into neutral), but for level roads or going up hills, it's better than maintaining a constant speed.
Gasoline is wasted driving anywhere whether up or down or flat.
Can you explain why constant intake manifold vacuum is the most efficient? Show me on a BSFC map, or equations, or a simulation.
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Old 11-18-2013, 05:15 PM   #6 (permalink)
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Quote:
Originally Posted by 3-Wheeler View Post
Using DWL is the most effective way to work with EOC, Engine Off Coasting.
This is a description of "pulse & glide", but not "driving with load" as it's generally understood.

Quote:
Originally Posted by bikenfool View Post
Why is 'Driving With Load' (DWL) or 'constant throttle' (CT) better than constant speed?
DWL isn't the same as constant throttle (not sure if you're equating them there).

A better way of thinking of DWL is "target driving", where your target is an instant fuel economy number (ie. on your MPG display) that you can hold at constant speed on the flat. You work to maintain that particular number through elevation changes by playing the throttle.
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Old 11-18-2013, 05:21 PM   #7 (permalink)
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Quote:
Originally Posted by bikenfool View Post
For most cars, when cruising we're at low throttle opening, either backing off, or constant throttle doesn't get the engine close to peak efficiency.
Peak engine efficiency isn't the same as best cruising fuel economy.

Peak efficiency on a BSFC map shows at what load & RPM your engine makes the most power/torque per unit of fuel burned. It's usually when making a lot of power, which is why it's applicable to accelerating and using the pulse & glide technique.

But it's not applicable when cruising in top gear because your power requirement then is a small fraction of power needed for accelerating (or climbing).

So you have to operate the engine outside of the "ideal" BSFC zone.
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Old 11-18-2013, 05:22 PM   #8 (permalink)
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Quote:
Originally Posted by MetroMPG View Post
DWL isn't the same as constant throttle (not sure if you're equating them there).

A better way of thinking of DWL is "target driving", where your target is an instant fuel economy number (ie. on your MPG display) that you can hold at constant speed on the flat. You work to maintain that particular number through elevation changes by playing the throttle.
I wasn't equating them, just comparing them to constant speed. My question remains, why is that more efficient than constant speed?
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Old 11-18-2013, 05:32 PM   #9 (permalink)
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Quote:
Originally Posted by MetroMPG View Post
Peak engine efficiency isn't the same as best cruising fuel economy.

Peak efficiency on a BSFC map shows at what load & RPM your engine makes the most power/torque per unit of fuel burned. It's usually when making a lot of power, which is why it's applicable to accelerating and using the pulse & glide technique.

But it's not applicable when cruising in top gear because your power requirement then is a small fraction of power needed for accelerating (or climbing).

So you have to operate the engine outside of the "ideal" BSFC zone.
I get all that. It still doesn't explain DWL. I'm not trying to be annoying, just trying to understand the fundamentals.
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Old 11-18-2013, 05:45 PM   #10 (permalink)
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Constant speed is less fuel-efficient than DWL because at a constant speed:

- More fuel is burned ascending.
- While descending, the energy invested in the climb is then "wasted" through increased engine braking.

One thing to keep in mind is that while target driving, you are typically exceeding the average (constant) speed by the bottom of the descent while still maintaining the target consumption.

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