Why is 'driving with load' better than constant speed

[twinair]

The benefit of DWL is especially noticeable with my car, since it only has 875 ccm and starts to generate boost very early, which in return and unfortunately increases fuel consumption disproportionally.

On the other hand there are hardly any pumping losses. Even at high rpm engine braking MAP is always at least at 75% atmospheric pressure, but probably more so because the intake valves have substituted the throttle.

[bikenfool]

Originally Posted by 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.

I'm not questioning pulse & glide.
Gravity is not drag, the energy you have to put into the system to climb the hill is regained on the other side. Aero drag is a loss, never regained.
What research did you do for the seminar? Care to share?
It doesn't matter whether I think it works or doesn't, the facts matter.

[bikenfool]

Originally Posted by 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.

What throttle position do you use on the hills now? What is it on the flats? Did you change any other driving behaviors at the same time? WOT is usually enrichened to get maximum power.

[JRMichler]

My engine stays closed loop at WOT up to about 4000 RPM, so no rich mixture penalty at normal speeds.

Throttle position does not matter, what counts is manifold pressure (or manifold vacuum if you look at it the other way). Keep that as steady as possible. With practice, you learn to enter the bottom of the hill 1 or 2 MPH faster and top the hill 5 MPH or so slower. All while minimizing the change in intake manifold pressure. The result, in addition to improved MPG, is that your driving gets smoother and your passengers all fall asleep so you have nobody to talk to.

[ever_green]

Originally Posted by JRMichler

My engine stays closed loop at WOT up to about 4000 RPM, so no rich mixture penalty at normal speeds.

Throttle position does not matter, what counts is manifold pressure (or manifold vacuum if you look at it the other way). Keep that as steady as possible. With practice, you learn to enter the bottom of the hill 1 or 2 MPH faster and top the hill 5 MPH or so slower. All while minimizing the change in intake manifold pressure. The result, in addition to improved MPG, is that your driving gets smoother and your passengers all fall asleep so you have nobody to talk to.

Closed loop does not mean its more efficient. Stoich AFR at higher RPM generates massive amounts of heat which causes knock. Your knock sensors of course will pull timing to avoid damage but this behaviour dramatically reduces your engines volumetric efficiency. Best acceleration method would be open loop with slight enrichment of ~13.5:1 at 3000+ RPM with additional spark advance for gas engines to keep EGT in check. Also there is something called tip in enrichment. Its basically change in throttle position vs time. The faster you hit the throttle the wider injector pulse width becomes to allow more fuel to be mixed. Most manufactorers disable tipin above 3/4 manifold pressure (engine load) though.

Of course the best method would be to just avoid such ungodly engine speeds all together. 2000 rpm is the engine speed that most EFI street engines produce their secondary torque peak at. So for accelerating, keep it at 2k rpm and vacuum as low as possible (load near 90%) and you will be fine.

Most efficient driving is non stop driving. If you are idling you are wasting gas.

[JRMichler]

I'm in good shape then. I rarely exceed 2500 RPM. My truck is geared for 1800 RPM at my normal highway speed of 55 MPH.

[niky]

Originally Posted by bikenfool

I'm not questioning pulse & glide.
Gravity is not drag, the energy you have to put into the system to climb the hill is regained on the other side. Aero drag is a loss, never regained.
What research did you do for the seminar? Care to share?
It doesn't matter whether I think it works or doesn't, the facts matter.

Well, first research was to ask these guys here. And these guys prefer that menthod.

The extra gas spent fighting gravity (whether you call it drag or whatever) cannot be counterbalanced by the extra momentum gained at the top, specifically because of aero drag.

Then I experimented for several days with the car we were to use for the seminar and came to the same conclusion, though my testing was, admittedly, not as thorough as on here. (since it costs $2 in toll to go back and forth along that stretch just once) But over the years, it's held true. About a 1 km/l through that area, depending on the car, and it's become standard practice for me, since the hills are on my daily route.

[bikenfool]

I think there are 2 mechanisms that make what is called DWL work.
1) going slower.
2) The other one is really a side effect of DWL...
its a method to arrive at the top of a hill at a slow speed enabling
you to recover more of the potential energy on the downside. if you're going fast on the
top, then when you coast on the downside you wind up going too fast
and have to waste the PE by braking.

I think it useful to break the hill (or valley) up into the climb and the descent. I think the lowest fuel cost way up the hill is not too much different than the flats, fairly low speed in high gear. I see no advantage to DWL on the climb except to lower your speed. Arriving at the top at a relatively low speed would save gas. DWL is one way to get there.

For the descent it will obviously depend on the length & grade. Coasting being the best unless its too steep. The only place where DWL would make sense is if there is another climb coming up right away. If you need some speed to make it up the next hill without shifting down then DWL would be a technique to accomplish that.

[NachtRitter]

Congrats; sounds like you've got it figured out.

[user removed]

Going slower will increase mileage. That is a separate calculation.

Driving in hills is also a separate scenario.

On a dyno when you increase the load on an engine it will increase the power far in excess of the additional fuel required. 20 HP using 1 unit of fuel, while 50 HP uses 1.5 units of fuel. The additional 30 HP only cost half as much fuel as the first 20, so you are getting much more power for only half as much fuel. This overwhelms the aero drag calculation by a significant amount.This is due to the fact that the lower manifold vacuum under higher loads allows up to double the effective compression in the cylinder and much more power produced for the equivalent amount of fuel.

This is the essence of pulse and glide. My Fiesta will glide down to below 20 MPH and still get over 100 MPG.

Hills offer a chance to minimise the speed variations as long as the hill is not so steep that you can not maintain a safe, or legal speed on the downhill portion. The grade of the hill is crucial to the strategy of climbing and descending grades.

If your downhill coast can be maintained within a legal speed then you have the better choice. Lowest speed at the crest of the hill, highest speed at the bottom of the hill.
Climbing the hill allows you to store energy by increasing altitude. DWL alllows you to pick a load that minimises your mileage hit while climbing with maximum benefit in the downhill coast where you mileage can soar to hundreds of MPG. If you climb at 20 MPG and coast downhill at 200 MPG then it is easy to see that you can average excellent mileage overall.

If your peak speed downhill exceeds safety or legality then just use engine braking instead of brakes, since engine braking consumes no fuel, compared to coasting in neutral, but it is better to coast over engine braking in most cases, again depending on the grade of the hill.

While aero drag does increase as the square of speed, it does not increase to the point where the increased engine efficiency in the pulse overwhelms the gain in economy if you use the pulse and glide technique, as long as you do not exceed safe speeds and or legal speeds. It also does not mean you have to average a lower speed as long as you keep the variations in the range of your target speed.

Using this technique, refined over many miles of experience, I can average over 50 MPG in my Fiesta on the Interstate, while maintaining the average speed of traffic, in a car rated at 38 MPG highway, but the hills here are not steep enough to coast at 65 MPH. To achieve that mileage at that average speed requires focus, work, and concentration, and some drafting also helps.

regards
Mech