I've read about people's getting very high mileage numbers on secondary highways while driving in the 50 mph speed range using the glide technique. In this condition, the car is powered by the battery and electric motor only, and the ICE (Internal Combustion Engine) is mostly off.



I have no doubts that those cars did reach high mileage numbers during that period, but something bothers me.



The only energy source for the Prius (at this writing) is gasoline. On the average over time, the battery will be charged to 80% full. If the car is running on stored electricity for some period, then the battery's energy debt must eventually be repaid. That repayment will come as a result of burning gasoline.



Okay, but can't we drive more efficiently? I'm not sure. I think Toyota has set things up to maximize efficiency and the driver doesn't have a lot of choice.



One thing is clear--over 20 mph or so, using the electrical-drive system to turn the wheels wastes energy. Here's why:

Every step involved in generating energy wastes energy.



How much? Here's an estimate:

Generator: 60%? I couldn't find Prius numbers and took a guess based on other generators.

Battery: 95%, maybe a little higher.

Electric motor: 95%. Another guess, but probably not far off.

Total electrical-system efficiency: about 54%.

This means approximately 46% is wasted.



Conclusion: For best mileage, use the ICE to drive the wheels directly and avoid using the electrical system to drive the car. Again, not that we have that much choice in the matter.



The ICE's efficiency is about 35% efficient over the upper 70% of its power output range. That's pretty good, partly as a result of the unique Atkins-stroke engine. In the lower 30% of its output range, efficiency is less, down to about 25%. (See http://techno-fandom.org/~hobbit/cars/prius-curves.gif.)



This is a simplistic theoretical analysis. How realistic is it?

--

Greg Gunther

 

I believe the idea of pulse and glide is to accelerate with the engine alone, and minimize use of the elecrtic system for precisely the reasons you gave -- after a few pulses, the battery output tends to reach an equilibrium from what I understand where there is no net charging or assisting. The electric system is basically just used to instantly re-start the engine on demand from rest (impossible on a standard vehicle without shifting into neutral and using the key).

The idea is to run with the engine at its optium powerband, and use the excess momentum of the car as a "battery", then coast with the engine completely off for as far as possible. This is done to minimize drivetrain resistance, rather than operating at partial load and reduced efficiency continually.

From what I understand these are reversed -- round trip charging/discharging the battery is probably in the 60s, but I believe both the motor and the generator are in the 90% range. Most of the energy that goes through the two motor/generatiors is generated directly off the engine, and shunted to the other motor simultaneously without ever passing through the battery. The CVT works by positioning these motors at different spots on a planetary gearset, and varying the amount of power sent either through the electrical or mechanical pathways -- sending power from one MGset to the other has the effect of "upshifting" or "downshifting" depending on direction. If the generator was only 60%, that trasnmission would waste a huge amount of energy, which doesn't appear to be the case.

 

Don't forget, alot of the battery regeneration comes from braking, where on most cars, this is 0 percent efficient.

When I drive my FEH in EV mode, it is ususally after a long down hill run of charging the batteries.

So, it is best to USE the battery.

Also, while the ICE is on, it is also turning the generator. To not use this energy to recharge the battery would also be a waste.

I think the battery and EV mode is a win-win situation, not a lose-lose situation.

 

I agree with the second post, the motor and generator are more than 90% effient. The battery is less. I get my best P&G milage at between 42 and 30 MPH. At 48 to 52 MPH, if the battery has a good SOC, and if you are on a fairly flat road, you can get up to 52,take your foot of the accelerator, gently put it back on and go for miles at 60-80MPG. Hills and stop lights are the only limiting factor. The computers use the best combination of ICE and Motor in this mode.

https://www.greenhybrid.com/compare/mileage...7/signature.png

 

The technique used by the Prius hypermilers in Japan is to avoid using the electric motor (charge either in or out) at all. They aim to use gasoline power alone and coast at just the right rate when they have to so that the engine is off, but no regen occurs either (to coast for the maximum distance).

 

The reason for making 42 a max speed for pulse and glide is for the reasons you state. Any energy transfer has inefficiencies. If you pulse (accelerate using the ICE) to 42 mph, you're acceleratin using the ICE and reaching a speed at which the ICE will turn off. Gliding is not the same as coasting. When you coast you simply take your foot off the gas. That will cause some regen braking, slowing you down quicker. Yes you'll get some energy back from regen, but again the inefficiencies. So instead you give it a little gas, enough that you won't cause regen, but not enough to power the car using the motors and battery. Your charge should stay almost constant.

In this way you use the ICE for a short period of time and then glide for a while giving you high mileage.

 

The all ICE is probably the way to go if you are driving in a no braking environment.
You are right that whenever you change energy from chem/ mech/ electrical/mech you lose a bit.
The whole idea behind the Pulse and Glide is to use as few engine revolutions as possible to go a certain distance in a given amount of time.
Run the ICE for a very few Revs at a high torque/hp rpm and get up to the speed you will need to be at to cover the distance in a given amount of time.
Somewhere Near the torque peak you probably get max mechanical efficiency.Maybe you will have to use 500 revolutions at that power(them the ICE goes off).This is probably more efficient that poking along using 2000 revolutions with the throttle plate nearly closed to go the same distance/same time.500 high power cylinders full waste less energy than 2000 barely filled cylinders.
Now before we had the ability to completely shut the engine off and take it out of gear Pulse and Glide wouldn't work so well -especially with the automatics we all have. Being able to automatically shut the engine off while still moving is what makes Pulse and Glide work. Many very adept folks can P&G normal cars-but it takes a lot of practice-and Cojones-to shut off your engine at speed and feel your brakes/power steering "tighten up".
Oh well.Charlie

 

Hi Greg,

First, welcome to GreenHybrid.com. You might wander over to the Prius forum where we have a FAQ with some helpful pointers. But you have asked a very important question about modeling Prius performance:
Perhaps this chart might give you some ideas:


These data were assembled using the earlier generation Prius. The most recent one, like yours, has better performance in some areas, especially above 70 mph. BTW, I just finished a trip from Washington DC and ran two full tanks with 566 and 562 miles.

The second tank had a block speed of 59.4 mph at 53.1 MPG. I used cruise control set to 62-63 mph but traffic, road construction and the on-off ramps and two breaks brought the average speed to just under 60 mph.

I've also been looking at low-speed performance with:


All of these data were collect using ordinary cruise control speed management. But some of the recent data suggests there may be techniques that could improve performance:

http://hiwaay.net/~bzwilson/prius/pri_power.html

I have been monitoring the Pulse-and-Glide 'buzz' but have other tasks to finish before investigating this approach. I noticed that the block speeds of some publicly reported, Pulse-and-Glide centered tests are not far off from the ranges I've been able to plot in steady state with my NHW11 Prius. The effectiveness needs to be measured but I'm not ready to run that test, yet.

My most recent effort has been looking at the effect of 'N' on Prius MPG performance. It looks like there are cases where 'N' is an effective glide IF the limitations are understood. For example, if you are going faster than 42 mph and put the car in 'N' for a glide, the ICE will continue to run at idle speed. Once you get below 42 mph, in fact all the way to stop, the ICE continues to run. It is important to at least briefly put the car in 'D' long enough for auto-stop to turn off the ICE so you won't have the ICE idle fuel burn:

http://hiwaay.net/~bzwilson/prius/pri_neutral.html

BTW, there is a Prius simulator that runs on a Palm Pilot. This might help in your efforts to understand what is going on.

Bob Wilson

 

As I didn't realy know exactly what "pulse and glide" was, I Googled it. This site explains it very well: http://hybridcars.about.com/od/owner...seandglide.htm.

I agree completely that P&G is the way to get the best mileage, as it uses the ICE under heavy load, and then runs with ICE off (or idling over 40 mph) and no indicated electric motor usage. It makes sense.

The biggest problem with it has to do with "social conscience." According to an article in the L.A. Times, freeway drivers don't like Priuses because they drive too slowly. Probably so.
--
Greg (Whose love of fast driving doesn't allow great mileage numbers.)

 

There is also an issue of steady-state driving vs. real life. Its rare that you drive under the same conditions for long periods of time, except for long highway trips. Thus, it is relevant to consider what your driving conditions will be like 15 minutes from now when considering how much to favor the ICE vs. electric motor. For example, on my 70 min commute, I'm often stuck in sluggish (20-30 mph) traffic on the DC beltway, but I know that in 5 or 10 minutes I'll be cruising at 70+ mph and the ICE will be able to efficiently recharge the battery. Under that scenario, I think its favorable to go full-electric in the slow traffic. Not to mention safer, too. Longer traffic jams ultimately take their toll because you have to recharge the battery under low-load conditions.

Maybe pulse & glide at all times would be more efficient, but its not always practical. I do know that I get better mileage on the days I'm stuck in traffic and favoring stealth mode.

-Terence