Why is 'driving with load' better than constant speed

[bikenfool]

Originally Posted by MetroMPG

Constant speed is less fuel-efficient than DWL because at a constant speed:

- More fuel is burned ascending.

Sure, but let say you slow down your constant speed target just a little bit, so the time to destination is the same with either method. Then you will use more on the climb, but use less the rest of the time.

Originally Posted by MetroMPG

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

If the hill is steep enuf you can coast. If it is not steep enuf to coast you will waste more energy by increasing your speed due to due to aero drag, velocity squared.

Originally Posted by MetroMPG

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.

[sheepdog 44]

Think of it this way. If you bleed speed going up your trading kinetic energy (speed) for potential energy (Height with gravity). Going down down hill, you want to trade your potential energy for kinetic energy and regain your original speed. That is the most efficient way.

Say your going 55mph and you want to get to the top of a hill. Driving with load allows you to use the least amount of energy to climb that hill. You crest the top at 40mph, and use your throttle to accelerate downhill. It's more efficient to accelerate WITH gravity.

Now if you keep a steady 55mph up that entire hill you use more fuel because energy consumption is a function of speed (and something else can't remember). Ignoring engine load, aerodynamics and rolling resistance, going 40 mph up a hill requires less energy than, going 55mph, than going 70mph. The faster you go up hill the more energy it requires.

Now your just fighting gravity which is pointless. Think back on how much more power you have to use to gain 5mph uphill. That same power (generalizing without physics equations here) will get you 10mph acceleration on the flat, and 15mph going downhill. The more you fight it the worse it gets.

If you use you power with gravity, you can convert that 15mph going downhill to potential energy going back up the next hill. This is why pumping on a swing swings you faster and higher. I like to imagine as rolling marbles.

[bikenfool]

Originally Posted by sheepdog 44

Think of it this way. If you bleed speed going up your trading kinetic energy (speed) for potential energy (Height with gravity). Going down down hill, you want to trade your potential energy for kinetic energy and regain your original speed. That is the most efficient way.

I think that is the answer, and DWL is just a way to get there. Minimize the kinetic energy at the top of the hill so you can convert the potential energy to KE without going too fast.

Originally Posted by sheepdog 44

Say your going 55mph and you want to get to the top of a hill. Driving with load allows you to use the least amount of energy to climb that hill. You crest the top at 40mph, and use your throttle to accelerate downhill. It's more efficient to accelerate WITH gravity.

Now if you keep a steady 55mph up that entire hill you use more fuel because energy consumption is a function of speed (and something else can't remember). Ignoring engine load, aerodynamics and rolling resistance, going 40 mph up a hill requires less energy than, going 55mph, than going 70mph. The faster you go up hill the more energy it requires.

Not true. If you ignore the engine, aero, and rolling resistance it takes the same amount of energy to get up the hill not matter what the speed. The change in potential energy is mass*gravity*height. It takes more power to get up the hill faster, but not more energy.

Originally Posted by sheepdog 44

Now your just fighting gravity which is pointless. Think back on how much more power you have to use to gain 5mph uphill. That same power (generalizing without physics equations here) will get you 10mph acceleration on the flat, and 15mph going downhill. The more you fight it the worse it gets.

If you use you power with gravity, you can convert that 15mph going downhill to potential energy going back up the next hill. This is why pumping on a swing swings you faster and higher. I like to imagine as rolling marbles.

[sheepdog 44]

I would suggest you get a scangauge to make it clearer in your daily driving.

One of your misconceptions is that driving slower (and therefore longer) going uphill wastes more fuel than keeping a constant and faster speed. Such is not the case. You are in fact easing the throttle to gain more mpg as you use momentum to crest the hill.

For extended hill climbs you do want to maintain a minimum mph speed so you keep your engine and transmission in an efficient zone. For example, 40mph would be good. A constant 5mph going up a long hill is terrible! For smaller hills you can bleed more speed.

Your second misconception is that the increased aerodynamic resistance going downhill is ENOUGH to NEGATE the benefits of the increased speed and kinetic energy. That is not the case until you get past 75-80mph depending on how aerodynamic your car is. Cresting a hill at 40mph, then reasonably accelerating downhill to 65mph doesn't increase aerodynamic resistance enough to be unbeneficial.

[bikenfool]

Originally Posted by sheepdog 44

I would suggest you get a scangauge to make it clearer in your daily driving.

One of your misconceptions is that driving slower (and therefore longer) going uphill wastes more fuel than keeping a constant and faster speed. Such is not the case. You are in fact easing the throttle to gain more mpg as you use momentum to crest the hill.

I don't have that misconception. I agree that easing the throttle will gain more MPG. It will under constant speed method too.

Originally Posted by sheepdog 44

For extended hill climbs you do want to maintain a minimum mph speed so you keep your engine and transmission in an efficient zone. For example, 40mph would be good. A constant 5mph going up a long hill is terrible! For smaller hills you can bleed more speed.

On any highway with traffic 40 MPH would be obnoxious. In my Sube that would be lugging. In my rav4 w AT it would shift down to 3rd

Originally Posted by sheepdog 44

Your second misconception is that the increased aerodynamic resistance going downhill is ENOUGH to NEGATE the benefits of the increased speed and kinetic energy. That is not the case until you get past 75-80mph depending on how aerodynamic your car is. Cresting a hill at 40mph, then reasonably accelerating downhill to 65mph doesn't increase aerodynamic resistance enough to be unbeneficial.

I fail to see your point. Energy is lost due to aero drag proportional to velocity squared. It makes more sense to coast and let gravity do it's thing.

[MetroMPG]

Originally Posted by bikenfool

Sure, but let say you slow down your constant speed target just a little bit, so the time to destination is the same with either method. Then you will use more on the climb, but use less the rest of the time.

I think you can still beat constant speed (cruise control style) driving with DWL, and still have the same time to destination. The lower average ascending speed of DWL is offset by the higher than average speed by the bottom of the descent. (I'm assuming that in the constant speed approach, there is higher manifold vacuum to hold speed on the downhill part.)

If the hill is steep enuf you can coast.

True. And that's more efficient because now we're effectively talking about pulse & glide.

[NachtRitter]

I'm not sure I understand your definition of "constant speed". When I read "constant speed", I think of (let's say) 65mph on the flat, 65mph up the hill, and 65mph down the hill. If you are talking about a "target average" speed of 65mph for the trip, where you are under the average on the uphill and over the average on the downhill, then that sounds like it is compatible with DWL.

In my case, I also do Pulse & Glide... despite going faster than my average speed on the pulse, and gliding down below my average speed afterwards, I still have a target average speed.

[deejaaa]

good explaining guys.
bikenfool, get a UG or SG and try it yourself.

[niky]

In practice, reading off of instant meters on various cars, it's more efficient to pursue load or to use a pulse-and-glide with hills than to maintain a constant speed over them... even a slower speed.

Aero drag going downhill hurts. Gravity drag going uphill hurts more.

Tried it several ways when researching for a Shell eco-seminar. It's more effective to allow speeds to vary on the hills on my regular highway route than to go over them at a constant slow or fast speed.

It works. Never mind whether you think it does or doesn't, it works. Gravity > Aero.

[JRMichler]

Back when I drove "normal", I would maintain 55 MPH uphill and down. That resulted in WOT throttle uphill and throttle closed downhill. When I learned DWL, and after practice, I still average 55 MPH. My speed now drops to 50 MPH uphill and reaches 60 MPH downhill. My manifold pressure (MAP on the Scangauge) mostly stays between 7 and 10 PSI. The secret to DWL is keeping MAP within the smallest practical range.

Result is that I gained a full 10% MPG while driving at exactly the same average speed.