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.

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.

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.

I wish I could find one. Yes I've seen the BSFC map page here, and searched elsewhere.
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.

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.

This is a description of "pulse & glide", but not "driving with load" as it's generally understood.

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.

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 wasn't equating them, just comparing them to constant speed. My question remains, why is that more efficient than constant speed?

I get all that. It still doesn't explain DWL. I'm not trying to be annoying, just trying to understand the fundamentals.

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.

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.
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.

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.

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.
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.

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.

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

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.)

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

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.

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

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

I understand what you're saying about increasing the load on the engine makes the engine more efficient. Perhaps you can clarify this: "DWL alllows you to pick a load that minimizes your mileage hit while climbing with maximum benefit in the downhill coast where you mileage can soar to hundreds of MPG." What is that load that you use? Efficient load is 1/2-full throttle. But most descriptions of DWL say to back off on the throttle. For most of the hills around here, to get up them in high gear I'm at a high load on the engine.
To be clear, I'm not questioning P&G. I still think DWL is a bit of a special case, or perhaps just misnamed.

Throttle position and load are two different things. Manifold vacuum determines load, higher vacuum=lower load. When you transition from downhill to uphill the load increases beyond the percentage of throttle position. Efficiency is measured by the actual compression in the cylinder when combustion begins is directly related to manifold vacuum and can vary significantly with very small changes in throttle position.

My Insight had an instant economy gauge which I used to refine my acceleration technique by accelerating with the lowest loss in instant MPG while providing an increase in speed which could then be used for a period of coasting. The Insight would coast in neutral at 150 (max measurement) in instant MPG down to 16 MPH which indicated and idle consumption of .11 GPH. When coasting at multiples of 16 MPH the instant MPG would be 150Xspeed divided by 16, so at 64 MPH the instant MPG would be 600 MPG. Using this on slight grades I could average over 90 MPG in the Insight and it was with a CVT transmission.

If you go by throttle position then at a point in load the AT will downshift and you will suffer a dramatid loss in instant MPG. Without precision instrumentation the best technique is to avoid any downshifting in a auto transmission to the point where you are loosing to much speed (when climbing uphill). This is where you would wantt to consider allowing your speed to slowly drop in order to maintain top gear. The same also works with manuals. Avoid downshifting to minimize the loss in MPG climbing the hill, but not to the point where your speed drops below about 75-80% of your desired average or you take so much longer to get to the top of the hill at lower instant MPG.

I will even accelerate before reaching the bottom of the hill, if I risk to long on the climb.

As I said before it really depends on the hill and you strategy changes as the grades increase, but if you do it correctly you should do fine until the downhill portion does not allow you to coast without engine braking or any use of friction brakes. Only when the grades get beyond that percentage will you suffer loss of average MPG.

I call it the roller coaster scenario. The coaster climbs the grade initially and from that point onward it is all inertia. The distance the coaster travels is many times greater coasting than the initial climb. I remember one member here telling about coasting 35 miles coming out of the Rocky mountains. My longest coast has been about 3.5 miles.
That was also enhanced by drafting on I85 coming east from Blacksburg Va. I had to used engine braking, in my Insight and the battery charge dropped dramatically on the lcimb but recovered completely on the downhills. I average 70 MPG with an average speed of 55 MPH in the Insight on that trip. I think it was 635 miles on 9.65 gallons of fuel.

regards
Mech

Best load is 75-80% with out full throttle enrichment. I will drive many tanks of fuel in my Fiesta and never see over 3500 RPM, becasue that is where it shifts out of first gear even when accelerating gradually. I find that when I am in heavy traffic it is actually best to accelerate at the average speed of the traffic and it also creates less issues with other drivers wanting to tailgate you if you accelerate at a slower pace.

regards
Mech

If your RPM's and load are in an efficient zone, why slow down at all on a hill with a manual? If you slow down you risk having to shift down.

If you can not maintain speed in top gear then you are better off to downshift than go into full load enrichment with throttle at 100%, but it is close either way.So close that it would depend on your priority, higher average speed or higher mileage. A good rule is the opportunity for vastly better mileage is what you do on the downhill portion, not the uphill climb.

If you can coast downhill and your terminal speed will not exceed your safety or legality threshold, then you objective should be principally based on your speed at the BOTTOM of the hill. It's like thinking ahead in a Chess game. You want you speed at the top to be at the exact point where your speed at the bottom is the highest practical speed (legal and safe) and you have coasted dowhilll to reach that speed, while your mileage jumps to way over 100 MPG, in some cases over 300 MPG, or infinite MPG if you use engine off coasting, which I do not do in my driving.

Then you have more speed to help with climbing the next hill (assuming there is one). Maintaining the best top gear load will cause your speed to gradually drop as you climb hill #2 in order to again reach your terminal velocity at the bottom of hill 2. Rinse and repeat for additional hills.

regards
Mech

That's the general thinking.
But it's wrong.
The most efficient is pulse (accelerate @ BSFC for your engine) and glide (engine off). Repeat and repeat ...
Even though your speed varies significantly, it's more efficient..
Constant speed means the engine is running inefficiently - even if its fuel consumption is relatively low (compared to what you're used to).

With Pulse & Glide, you use the engine at its peak efficiency whenever it's on.
That's what makes it so efficient.


Even coasting with the engine on (and driveline disconnected) will improve your efficiency.


Constant speed is fine - compared to "regular" driving - as long as the road is level. Actually, you'd be DWL at that point.

But you lose efficiency whenever you go uphill or downhill.
Uphill, a CC or the driver simply pours on fuel to keep the speed up.
Downhill, it's actually using engine braking to keep the speed under control - and braking is not good. The car wants to go faster due to the grade, so let it go faster, if it can be done safely.

The slower you go, the better your average - usually.

But there's a point where the engine becomes more inefficient as you slow down, and you may get the same MPG as you would at a higher speed.
The benefit then is in not accelerating to the higher speed - which will be minimal on longer drives.

But even if the slower average is improving your MPG, it'd still be more efficient to slow down uphill and catch up downhill. :)

That's part of the point.

You trade fuel for potential energy : your car's weight @ height.
The engine load increases, and so does your engine's efficiency.
High load and low rpm is where it gets its BSFC after all ...
Even if it uses more fuel/mile, it's producing the horses required to get up there more efficiently.

Going downhill, you can trade height for speed (and distance) again.
But if you hold it back, you'd be braking and you won't get the return on your invested fuel.

Despite slowing down considerably, I'm not losing that much time on my 30 mile commute.
On longer trips, the loss can be considerable, so there's usually always a trade-off between time and MPG.

You don't really accelerate with throttle when going downhill while using DWL.
You just get the car back to the speed it wants to make on as much fuel as you're giving it + the downhill grade :)

Some hills you can even glide down with no throttle at all - and driveline disconnected !

If you brake, you won't be getting as much of the potential energy (i.e. the fuel invested) back though.

Then when the road levels out, you'll be back to your DWL fuel target, the sort of fuel consumption you want to achieve .


Try DWL instead of CC for a few tanks.
It will give you improved MPG :)

Euromodder, I agree with most of what you're saying, but most of what you're saying is about P&G, not DWL which is what I question. Hills seem like P&G enhancement. Pulse on the climb, glide on the descent.
DWL implies constant load on the descent, in other words using the engine to increase speed. It's obvious gliding is best.

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


That sounds like a pulse in the climb, not DWL, they are opposing techniques.

I've been playing with it, but I don't typically use CC locally anyway.

When I read what you wrote, it seems to me that you are under the impression that a hypermiler sticks to exactly one technique no matter what the condition. What you seem to be trying to say is that when the hypermiler crests the hill, he (or she) will press on the accelerator to maintain a specific load ... when legal speed is reached, the hypermiler will continue to press on the accelerator to maintain that specific load ... when a sharp curve is approaching, the hypermiler will continue to press on the accelerator to maintain that specific load ... no, that's just silly. The hypermiler will do what he or she knows how to do or is able to do which fits the current conditions. That means DWL up the hill if that's appropriate for the conditions and then switching to a different (appropriate) technique (e.g. coasting, engine on or off, in gear or out of gear) when going down the hill. This "switching of tactics" does not violate the description of DWL given on this site. You can read whatever you want into it, but apply some logic to your assumptions.

Please explain your reasoning here. How will using more fuel per mile on the uphill get you better fuel economy, keeping in mind that you know you have a downhill once you've climbed the uphill? In your scenario, how would you maintain your speed within legal limits on the downhill? Engine braking? Brakes? Somehow, the net amount of fuel consumed on the uphill, downhill combination needs to be lower than technique of using DWL on the uphill (possibly losing speed) and then coasting on the downhill (gaining the speed back "for free"). Consuming more fuel on the uphill than one would consume using DWL won't net you better FE overall.

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.

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)


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 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 :(

Enough talk, lets see some analysis. :eek:
Lets look at 2 cases. Perhaps someone can compute the fuel consumption for each case with a typical car. Manual tranny, non hybrid.

The road is a long flat section, followed by a climb. The time to cover the course has to be the same for each method. You can pick the parameters for the length and grade. Try a couple of different courses. The intial and final conditions are the same for each method.
Method 1. DWL. It would be way simpler to do this in top gear, no down shift.
Method 2. Constant speed, begin a coast before the top of the hill so the speed at the top is the same as for the DWL case. This takes the descent out of the equation since you could use the same method (glide) for either case on the down side. The speed on the flat section will be slower for this case than for the DWL case since you don't lose time on the climb.

I know that you can save fuel by arriving at the top of the hill with a slower speed, but there are other ways to accomplish that such as the glide I just proposed in 2)
I know that you can save fuel on the climb by going slower, but you can also save fuel by going slower on the flats, so there is a trade off.

Maybe I'm overthinking this, but I don't think the solution is as obvous as you guys think it is.

My brief search didn't show up any open source simulations, but it seems like there's probably something out there somewhere. Perhaps some government project. It wouldn't be trivial, but it wouldn't be a huge task to code something up in matlab or octave. If someone knows of a sim out there point me to it. It doesn't have to be very complicated. This simple case could probably be done with some fairly simple script, but I won't be able to try it for a while. I'm sure one of you guys already have the tools ready to go ;)