Pulse and Glide on Interstates? Efficiant highway driving?

While you guys are trying to figure this out I'm going to take my jet ski out and pollute Lake Peris this weekend.

 

OK I understand what P%G is now. Seems to be something that can be applied to all cars.

But is there a term for what I do.

I'll accelerate up to cruising speed, usually around 40 mpg take my foot off the gas until the MPG scale max out then lightly press on the gas to maintain constant speed but under battery power only. It might take 3 or 4 seconds to get it to were the ICE is off and I'm running on batteries but I can usually travel for a couple of minutes under battery power only. Usually change in terrain causes the Prius to run with ICE but once over the hill, i can start over again.

By the time I get to work, (20 miles), my battery is down to 1 or 2 bars and the ICE is practically running all the time but I'm able to keep it around 40- 50 mpg.

My total trip is around 64 mpg.

on cruise on the same trip I'll only get 51 mpg.

But I am able to maintain a constant MPH that doesn't annoy people..

 

I've searched through many website about P&G driving practice but haven't been able to find one from a reputed source. All I can find is the main website "glideandpulse.com" (which is currently for sale) and many forums. Reading through those, I feel like I discovered that there is a new religion out there with so many followers...

I can only offer the theory to dispute the benefit of the P&G driving practice. We need to have a group of college students to do a real experiment. They will need to have a Prius equiped with all required instruments and may need to borrow or rent a race track. We can only believe in real data come from reliable sources. However I doubt that any university or college would fund a project like this. If the result favors P&G, people would change their driving habit and that would affect the traffic flow negatively. (Just imagine there were a guy defending himself before the judge that his erratic driving was only an attempt to save the earth by following the advices from a so-and-so university...)

Even with the proven data which proves that P&G driving does save gas, I will keep driving the way I am driving right now. Prius is amongst the most fuel efficient cars out there. I will try to drive with the flow and not to offend anyone behind me. Instead of watching for cops, I will concentrate on the traffic around me. My goal is to enjoy my car, my trip, my driving companions. My pledge is to drive to work and come home safely with the family. Any pleasure trip on the weekend will have a happy ending. Please don't try to save couple bucks of gas and that would make your driving become a chor. Worse yet, it is also a risk taking business...

 

Last night, I did not do P&G due to traffic condition, my round trip was about 9 miles and the mpg was 60. I normally get 65 mpg on this same route doing P&G. If my car was a EV, doing P&G probably wouldn't have much of a benefit. Since it's a Prius with a gas engine, it does make a difference if I ran the engine at a higher RPM (Pulse) and then shut it off when I get to a target speed (glide).

Kyle - you make it sound like P&G is a difficult thing to do. The fact is it's not hard to do if traffic is not congested. I would never do P&G in the morning nor evening during rush hour. I wouldn't do it on the highway either unless I'm going down hill.

 

Kyle - Where I disagree with you is when you say you will get the best mpg by using cruise. I agree - on a completely flat road (0% grade) - I might buy into it, but how many roads have a 0% grade? When I drive, and I see a hill coming up, I try to gradually increase my speed so that when I start climbing the hill, I can "back-off" the speed as I go up that hill. I guarantee you if I left the cruise on as I climb that hill, I would be decreasing my mpg.

 

Basically you'll want to keep your MPG as low as possible if you want the most efficient driving. If I recall correctly gas consumption increases exponentially after 55 miles per hour. So you'll want to keep your speed as low as reasonable. Contrary to others thoughts you can keep up with traffic without being burdensome to others. My daily route includes a 25 miles stretch of interstate, I was able to achieve 69.5 mpg on this route while maintaining an avg speed of 40mph over 600 miles. Its all about taking advantage of whats available to you. I use every hill on my route to my advantage, see another slow car or semi awesome shack up behind them and cruise along. I was able to get 69 mpg without the aid of cruise control, IMO use of cruise control will hurt mpg. The cruise control primary function is to keep your speed constant not achieve maximum mpgs. Cruise doesn't know when there are no other cars around and when you can slow down a bit and putz along or when a big hill is coming up and you can let of the gas a bit ahead of time.

There is a lot out there regarding theory and ideally speaking, but from an experience stand point just being more aware and taking advantage of hills and slow cars can drastically increase your mpg. Tire pressure and minimizng dead weight is important too, don't carry around those 40lb bags of water softerner salt UNLOAD them .

I like to think I have a bit of real world experience with getting good results on the interstate

 

The HSI is my guide. On hilly roads, I switch the Prius to "Normal" mode and do my best to keep the bar in the "Eco" zone. When going up a hill, I move to the right lane and find myself a truck that is also climbing slow. By keeping the bar within the "Eco" zone, I will lose speed but as long as I can keep up with the truck in front of me, I'd be fine.

I don't have any data to prove anything but I feel this is the best for the car when comes to mpg and reducing stress on the engine and motor.

 

This is the key, with one caveat; that "energy needed" does NOT factor in the efficiency of the engine. It absolutely takes more energy to the wheels to do P&G; however P&G allows you to use the engine in a manner that produces less waste heat. That waste heat comes from extra friction in the engine. Consider it this way; during a glide, the engine is off, producing zero engine drag. During a pulse, the engine is producing a fairly high amount of power, such that the percent of energy that is dissipated as engine drag is less than if you were producing just a little bit of power.

You seem to think there's some great Pulse & Glide conspiracy, that we should all have our website where we spread our prophecy or something. There really isn't - the closest you might come is to visit cleanmpg.com, which I think tends to have an audience that's more focused on fuel efficiency than on any particular model of car. Some of the best technical explanations of this sort of thing come from hobbit, who lurks on various forums, and has some great writeups.

We don't really care if you do P&G or not. In fact, many of us suggest that most people use the "just drive it" method, which gets excellent mileage. But some of us enjoy the challenge of squeezing the most out of a gallon of gas, so we use the data and experience we have available to greatly exceed the EPA ratings.

All this said, I would mostly agree with you at highway speeds; P&G is not very useful there in the Prius, since the engine can't shut off. There, the preferred method of obtaining efficiency is usually "driving with load", where you hold the load on the engine constant (your speed will vary with the terrain, but by holding the load constant, you get the best possible average efficiency from the engine).

 

Try Wikipedia...

 

I did not want to continue on this subject but after reading your reply, I decide to come back...

Let's look at the video posted on youtube and then we will discuss further about this subject. The reason I want to show this video because according to the conclusion in this video, Willie the tester made a statement saying that the "sweet (-and-sour)" spot for P&G is 37 mph. "Sweet" because that is the speed where you get the most benefit from P&G. For speed above 40 mph the benefit is insignificant. ("Sour" because that is not the speed we prefer.) However the 37 mph is needed for my argument down below

After pulling a few remaining hair off my head trying to understand your statement, I just wonder if you have any scientific or logical explaination why P&G produces less waste heat. I know for sure that the engine friction doesn't give out that much of the heat!

Let's look at the chart here:

(Note that the red curve is for a typical ICE. I do not have the one specifically for Prius engine.)
I chose 2500 rpm as the target engine speed because it may fit the number 37 mph in the video above.

By doing P&G, the engine has to run haft of the pulse cycle at a lower torque than an engine runing at a constant speed. For the full pulse cycle, your car engine should produce at least the same heat as the one runing at a constant speed. In theory, the P&G engine should produce more heat due to the incread load using for accelerating the car.


About the P&G test done and shown in the video above, I suspect that Willie did not take into account of the contribution of the energy come from the traction battery during the acceleration in the pulse cycle. And then during the glide cycle, he deprived the battery from being recharged... If it's true, the test is invalid!

 

OK, that diagram maybe gives me an inkling of what you're thinking.

First, you do know that during the "Glide" part of Pulse and Glide, the engine is OFF, right? As in, zero RPMs, zero fuel, zero friction in the engine. That's where the energy savings comes from. So I might do an 8-second pulse, followed by 16 seconds of gliding, as an example. Doing P&G in a normal ICE vehicle is pointless for exactly the reasons you say; increased aerodynamic drag at higher speeds, as well as generally running the engine at less efficient points on the operating curve.

So I don't know what the lower point on your chart is trying to indicate, but it seems to me like you think the engine is going to be still running, and at that lower point, during the glide, which is not the case. Or maybe you're forgetting that the Prius has a CVT, and can run at the optimal RPMs at any wheel speed - I basically alternate between running the engine at an efficient ~2,000 RPMs and coasting with the engine off. The ideal behavior is to basically sit at the most efficient point, and then coast with the engine off for as long as you can stand it.

Here's the graph you really want, which IS Prius-specific, unlike yours. (Friendly jab. )



This is the BSFC map, with the 2nd gen on the left and the 3rd gen on the right. The two things this shows that yours does not is the fuel consumption (the shaded areas and solid curves), and the total power output (the dotted lines). The light gray blob on both charts is where it takes 230 grams of fuel to produce one kWh of power from the engine. The dark gray blob is where the Gen 3 takes 220 grams of fuel to produce the same.

You'll notice that you don't want to be at the peak torque; this is actually a very inefficient place to be. What you want to be is right around the middle of that dark gray area; so around 2000 RPMs, producing 20 kW, using less than 220 g/kWh. The "Just Drive It" method is still pretty good; it'll probably put you at the very left edge of the dark gray or into the light gray, so using 220-230 g/kWh.

So yes, it takes more kWh to do P&G; but because the fuel consumed per kWh is lower with P&G, there is an opportunity to save fuel, even if it takes more energy to the wheels.

I've been doing P&G since basically the day I got my Prius, and I can tell you it can get pretty good results. (Not a guarantee - sometimes I do it poorly and I take a hit in the mileage, and other factors like short trips or highway mileage can negate the gains I get from P&G.) When I do P&G for a good distance, I can often get 60-70 mpg without much trouble - and that's in a congested suburban area. My wife does not use P&G, and her mileage on the same car in the same traffic is usually about 50-54 mpg.

And while the subject of the video may not, I do pay very close attention to the power coming from the traction battery. I try to use almost zero power from it, and try to keep the SoC at 58-62%. My wife, on the other hand, often brings the car back with the SoC in the low 50s, so part of the challenge in getting my good MPGs is to replenish what she took out.

 

Do you have any quantitative data to support this? I have read similar posts on other threads, but have not seen any hard data. Toyota engineering papers attribute approximately 30% of the Prius' efficiency increase to the regen/battery/motor system. I aggressively use the battery energy to extend my glides, and rarely park it with more than 2 bars. This time of year, I get 45mpg during the engine warm-up and for segments over 4-5 miles I get 70+mpg on the scanguage, with 80+ not unusual. I get consistent 600 mile tanks, and may get a 700 before the cold weather hits us.

 

There are times that I use only battery power too, like when the car isn't fully warm, but the engine has stopped and I just need a little power to get to the next red light. Getting the engine to turn on again would often mean that it wouldn't stop until I was stopped at the light, so using a little battery power can save 30 seconds or a minute of the engine running.

But in general, it comes down to this; it's more efficient to have the engine drive the wheels, than to have the engine drive the motor to charge the battery, and then discharge the battery to drive the motor to drive the wheels. Both charging and discharging the battery will cause the battery to heat up, as well as other losses in the electrical system. So as much as possible, it's preferable to have zero current in or out of the battery. (Yes, I'm borrowing KyleT's "It's the physics!" argument. )

The car wants to be at 60% SoC. If the engine is running while it's over that number, the engine will run at slightly lower RPMs and some power will be drained from the battery. If the engine is running while it's below 60%, then the engine will run at slightly higher RPMs, and some power will be used to charge the battery. This is unavoidable, and scales with the gap between the current SoC and 60%; if you're at 2 bars, any time the engine is running, the car will desperately try to get some charge into the battery, no matter what you do with the gas pedal.

There are definitely times that it makes sense to use energy from the battery instead of the engine. But in general, you should be able to get better mileage if you keep the SoC around 60%, just from the physics. That's not to say that you can't still get great mileage with other tactics, anywhere from what you're doing to the "just drive it" methodology; Toyota definitely did a good job of optimizing the system's efficiency over a very wide range of operating conditions, and not just a small band.

 

I am familiar with the "physics" discussions. If the engine was running only to charge the battery, they would be correct. That does happen, but only in very specific situations. The battery is primarily charged thru the regeneration systems. The charge rate as part of the engine regime is almost never above 10 amps. Braking can get to 80 amps pretty easily. Once the energy is in the battery, it is no different than the energy in the gasoline. A joule is a joule (it's the physics). The energy stored in the battery is the least costly that you have available, and as such should be the first to be used, if possible. I am also thinking that running a highly charged battery would "turn down" the regen systems to avoid overcharging. Having room in the battery could be better in the long run. I have monitored the charge rate for a lot of years, in many different conditions, and have not found any desperation as you described. I can pulse and glide at 2-3 bars exactly the same as at 6-7 bars. There is some charging done during the pulse, as noted above, but never above 10 amps or so. The regen system is a much larger part of the overall efficiencies of the car than the engine and the aerodynamics combined. The general feeling I get from discussions here is just the opposite.

 

However, a serious hypermiler will avoid getting excess speed in the first place, thereby minimizing the contribution that regeneration makes to charging the battery. (This may or may not apply to either of us - it is usually not something I can avoid, given the density of my area.) Additionally, when traveling at highway speeds, there's basically absolutely nothing you can do to keep the car from charging the battery to 60% SoC. Yes, it'll probably happen at ~5-20 Amps and not 80, but it makes up for it by charging for a much greater period of time. To a lesser extent, this applies to normal driving as well; even though I brake somewhat slowly to maximize the use of regen (and increase the likelihood of timing the lights perfectly), I'm sure I spend far more time accelerating with the engine on than braking; so just because it's charging at a lower rate, I wouldn't assume that it's contributing less total energy to the battery.

 

"It's the physics" is correct, but it is important to understand what the physics is. The energy in the battery has no thermodynamic relationship to the process that put it there. Whether is was the regenerative braking, the engine turning a generator or a charger plugged into the wall, it is all the same when you start taking it out. Consider the energy in the battery- the discharge system of the battery and a motor is about 90% efficient, maybe a bit more. The gasoline engine is around 25% efficient. The energy in the battery is at least three times more efficient to use that the energy in the burning gasoline. There are two ways to charge the battery- by the engine turning a generator and by the various regeneration systems. Contrary to a lot of posts made here, the reclaimed energy is, if not free, at least very, very cheap. When the gasoline burns in the cylinder head, it undergoes an irreversible adiabatic expansion, which means that entropy gets a huge increase. It cannot be causally connected to anything that happens later. All of the energy cost has been sunk into turning the crankshaft in order to get you from point a to point b and it makes no difference if you are in a Prius or a one ton Dodge dually. The fact that the Prius can reclaim some of the braking energy that is normally turned into low grade heat is in no way connected to, or influenced by, the burning of the gasoline. The only real cost incurred in charging the battery is the amount that the engine runs JUST to charge the battery. That is a difficult number to come up with. The engine will, under very specific circumstances, run only to charge the battery....mostly when the car in is P and not moving. The majority of the charging is done under the control of the Synergy firmware, and to some extent it uses engine energy that would be wasted anyway. In urban, suburban and any stop-and-go driving, the aggressive use of the battery energy will result in better mileage in almost all cases, since it is being recharged by the slowing and braking regen systems. Highway driving presents other challenges since there are very few regeneration opportunities. I would maintain that even on the highway, the battery energy is the most efficient because of the Synergy control of the charging system.

 

P&G gets you better mpg because of the inefficiency of the gas engine running at low RPM. I don't have a pure EV car, but I'm willing to bet that P&G yields very little benefit when you're totally running on electricity.

 

First, I'm not a 'hypermiler' by any stretch of the imagination but I do drive efficiently. I found one paper:

Source: Yahoo! Groups

One set of data from the paper looked like an outlier but I agree with their conclusion that Pulse and Glide is not a practical solution in traffic. They did their testing on a track.

My testing is documented here: Wilson PnG Test. Here are my summary metrics:


Compared to the equivalent, cruise control, steady speed, Pulse and Glide is about 11% more fuel efficient. But there is a cost:

In this case, a speed range from 25 to 43 mph. Testing was on an isolated road on the weekend without other traffic.

Using efficient driving, I recently completed a marathon drive using these driving rules:

1. Accelerate manually to cruise control set speed and set it.
2. When car transfers energy from wheel to traction battery, shift into "N".
   • Car will roll faster so monitor speed limit.
3. When car returns to cruise control speed, shift into "D" and "resume".
4. When stopping or slowing down, shift into "D".

Except for initial acceleration or coming to a stop, the feet remain on the floorboard. The rules of efficient driving work at any target speed and do not involve significant speed changes. So they work in traffic.

Good luck!
Bob Wilson