pulse-n-glide: what speeds can you maintain?

I can assure you no one has so much as looked sideways at me yet. This is Kansas, after all. I get high mileage when conditions allow. So, how do you go about performing on your guarantee?

 

It really depends on the traffic. In my regular commute route, I can easily get 70+ MPG in a stretch where the overall speed is 30-40 MPH.

I-495 LIE Traffic Congestion Utopia (70 MPG in Prius)

 

Just send in your Prius sales receipt along with a full recording of anyone who was driving within 500 feet of you at any time you were driving your car.

We appreciate your business.

This posting may be monitored for quality control purposes.

 

You can P&G at any speed, it just takes getting a little handier
with warp-stealth. Above 50 - 55 MPH it's not really worth
trying as the engine is already under high [thus efficient] load
steady-state to pull the car along, at least on the flat.
.
P&G will *not* be the desired strategy for electric drivetrains.
I expect that low and steady pack current will be the order of
the day. The whole point of P&G is to cater to the efficiency
curves of *heat engines*, where leaving it on but at low output
power is far less efficient than pulsing intermittently at
heavier loads.
.
_H*

 

If you look at the engine efficiency graph, P&G is less effective/necessary in the Gen III because the sweet spot is bigger and starts almost as soon as the engine turns over. I think any improvement in MPG due to P&G in the Gen III is marginal, at best.

In the Gen II, the sweet spot is significantly higher in the rpm range, so you had to pulse to get the best efficiency from the engine, and then had to glide (no ICE) to get back down to average.

In the GenIII, you just need to get up to speed (without gunning it) and then back off the throttle until you're just maintaining speed - the hybrid system will do the rest. Optimum fuel economy on the GenIII is when the RPM is fairly low (i.e.: overdrive - low RPM, low torque load)

 

I find that the ICE kicks in at exactly 51% on the HSI. If the battery has 3 bars or more showing, and if I can maintain the speed I want with the HSI anywhere in the lower half of the ECO band, the ICE is off. At even one pixel above half, I feel that little hesitation when the engine kicks in and the instantaneous mpg indicator drops down from off-scale high to about 50. I've experienced this up as high as 55mph (the max speed limit anywhere here is 50).

There are roads that have ever so slight a grade. So subtle you can't really tell it isn't level until you drive both directions trying to maintain speed without the ICE. And these kinds of conditions are when you become more aware of the prevailing wind as well.

I use this while pulsing and gliding for as long as the battery holds up. It delays the time before you need the next pulse. Sometimes it just seems to go and go at 50 mph and the HSI at 45-50%. If it continues long enough, the battery goes down to 2 bars, and then it's time for pulsing and true gliding till the battery charges up. For a normal round-trip to town of 30 miles each way (which is about 2/3 highway, 1/3 town driving), I normally get between 60 and 65 indicated mpg. My wife pays no attention to the HSI and she normally gets between 55 and 58 mpg. (Reality checks say that my indicated mpg is about 5% higher than actual.)

I find that one of the biggest differences between what I do and what my wife does is that after speeding up (like leaving a lower speed zone or the slow poke in front of you finally looks at his speedo) she will just hold the new speed often with the HSI hovering at 50-55% while getting 45-50 mpg. If I'm driving, I speed up to a couple mph above the new speed then take my foot off the gas for a couple of seconds to give it time to decide to shut off the ICE and then come back down on the gas to just below 50% on the HSI and hold that till the speed is too low. Then I start P&G.

 

Do you have such a graph at low throttle? I thought all the common graphs reflected full throttle, rendering them somewhat irrelevant to normal driving. I find P&G superior to steady state mpg, but certainly don't have sufficient data to back that up.

 

First, I didn't mean to imply that P&G was inferior, or incapable of delivering a mileage difference. I said I think the difference in the Gen III is more marginal and less necessary - although I will retract and say that in a certain range of speeds it may in fact be more than a marginal difference.

http://priuschat.com/forums/gen-iii...2010-prius-2zr-fxe-engine-efficiency-map.html
The 2nd image is a bit clearer, and shows the difference in where the sweet spot is located (the darker blob on the Gen III chart is supposed to represent the 225g/kW range, I think - the extra-sweet spot)

On Page 5 is another graph (which doesn't exactly match the first ones...) but which may imply better results from P&G than sustained speed. However, the curve for the Gen III is still not the same as the Gen II, so I think the benefit of P&G is less now than it may have been in the past - although this chart may hold some clues for maximizing P&G on the Gen III which could completely destroy my original hypothesis.

Two main differences that I see (comparing Gen II to Gen III, using the graph from p5):
1) The first part of the curve (between 10k-20k) is less steep on the Gen III, so you may do nearly as well by holding speed at moderately low RPM rather than P&G (so long as the holding RPM isn't too low). The cost per kW may be a bit higher, but there are startup costs for the ICE and penalties for using battery power to extend a glide, and FE is worst at the lowest rpm's (800-1000W) so passing through that range repeatedly will offset some of the gains.
If nothing else, this implies the need for fewer, longer glides with the Gen III, although in colder temps, I would think it better to keep the ICE running to avoid the heat loss associated with gliding.
2) The later part of the curve (25k-35k) is more steep on the Gen III, so the risk of overshoot on the pulse is slightly higher than on the Gen II. This would suggest the need for more modest/gradual (relatively speaking) pulses.

Much food for thought, but I think those charts are the key - the mystery is how to spend the most time possible inside the sweet spot, while wasting the least amount of energy getting there.

Unrelated, but another item of interest is that (according to the p5 chart) the curve for the Gen III is flat at 230g/kW from 40k-50k! In other words, the 1.8L engine won't exceed 230g/kW - even when you floor it...

 

Not exactly. 1.8L ICE peaks at 60kW. We don't know BSFC between 50kW to 60 kW but the graph looks like it is dropping (more efficient).

 

Tom183, see post #31 of the linked thread in your post above. The chart is at full throttle. It does not speak to the use of lower throttle. I have not had any car full throttle since the 1970's.

After reading several threads on BSFC charts over the years, it seems they fall short of the meaning we all WANT to ascribe to them. Before you get to full throttle, the PSD draws assist from the pack which is above optimal pulse rate according to Wayne Gerdes and others. What we need are charts at the loafing steady state throttle we need at x speed (which is NOT steady since the car decides how the engine is used), along with another chart at the pulse throttle we use in P&G around that speed.

Disclaimer: I know nothing but what I have read; no training in this...

 

We know the best fuel economy is achieved at approx 30 mph average speed by Prius.
It is known we need less than 5 kW average power to maintain Prius speed at 30 mph.
The 5kW zone by Prius engine is very inefficiency on both Gen2 and Gen3.

I did 30 mph constant speed driving by cruise control, and the engine did not run continuously. It did engine-on and engine-off on both Gen2 and Gen3.

I don't know how marginal is marginal in your mind, but I saw at least 10% gain by P&G on Gen3. For me, it's huge.

Ken@Japan

 

Interesting:

• 30 mph - seems a bit high for maximum range. I'll go back and do some regression testing to see if I can replicate this result. At these low speeds, I may have made a procedure error.
• 5kW @ 30 mph - seems a little high, I would expect it to be closer to 3.7 kW
• 10% P&G gain - close enough to the 11% I've measured

Thanks,
Bob Wilson

 

I'm not one to accept established doctrine simply because the church elders say it is so - I need to see more data (which I can reproduce) before I make up my mind either way. (in other words, I don't drink the Kool-Aid until/unless I know what's in it - and why I should drink it) The assumption I find most objectionable is that the Gen III is the same as the Gen II - it isn't.

So I did a short test yesterday in freezing temps with a cold engine on a short (8-mile) trip in speeds between 35-45mph - and saw an improvement using P&G. Not that I consider that conclusive, but it does suggest a time and place where P&G is more than a marginal improvement. (It also suggests that P&G is more important in that range than having a warm ICE, at least for short trips - definitely food for thought.)

What I want to know is exactly where and when P&G is more than a marginal improvement, and how to get the greatest benefit from it. Then I will consider how much that impacts the flow of traffic and how to make use of it without driving anyone else crazy (or off the road).

Most of my local roads are 35mph (and not very busy), so I should be able to play around with this quite a bit.

 

I'll use the same disclaimer, but you're talking about the first chart rather than the one I attached, correct? The article you refer to implies that we should floor the pedal for best FE, which seems beyond counter-intuitive - but may be true for traditional engines. It certainly seems to assume a manual transmission, where the engine can be forced to spin at non-optimum RPM by under-/over-revving. And it makes sense on a certain level - if you downshift at high speed and don't apply adequate throttle, power is very low but fuel consumption is still high.

The article also shows the Prius graph and states that the CVT keeps the ICE in/near the optimum fuel input range (varying the RPM to match the fuel input, rather than glutting the engine at low RPM or leaning it out too much at high RPM) - which increases the validity of the charts we're looking at.

I guess what that article really shows is that most traditional engines are vastly oversized for the demands normally placed on them. (which lines up with the Top Gear comparison of a BMW supercar with the Prius - the BMW was near its optimum consumption because they were stomping it the whole time. City MPG for the BMW is much worse). Of course, if those traditional engines had some way to glide without turning the engine and flowing fuel, then they could see a significant improvement at non-highway speeds via P&G

I wouldn't take any of the charts too literally, but as a rule of thumb I think they have some usefulness - and using those charts to compare Gen II to Gen III is perfectly valid, since they're using the same methodology for both cars.

 

I like that kind of thinking. Not to slide this to off-topic. You must be a Buddhist. We'll at least that's what Buddha was teaching over 2,600 years ago.

Buddha: Believe nothing, no matter where you read it, or who said it, no matter if I have said it, unless it agrees with your own reason and your own common sense.

ICE won't shut down if the temp is not high enough, therefore you can't do P&G. The traffic speed, coolant temp and driver's input are needed to utilize P&G.

 

I much prefer knowing over "believing" - "belief" is something you fabricate when you don't have enough information to know, like "what happens when we die" or "was OJ guilty".

In other words, belief is connecting the dots when you don't have enough dots - some people draw a car, some a horse, others a face. Until you have more dots, nobody knows who is right, and the intelligent, rational thing to do (in the absence of more dots) is to recognize all of the possibilities, rather than exclude all but one.

Because when dots appear which don't fit on the lines you've drawn, you'd better be prepared to change your answer... Christians once believed that the sun orbited the earth, until astronomers uncovered some dots which changed the picture.

Unfortunately, too many people today choose instead to find ways to exclude/ignore the dots which don't fit their drawings (as did the church leaders in Galileo's day), instead of acknowledging new information and other possibilities. When you've already chosen to exclude all other possibilities, it's much more difficult to admit when you chose incorrectly - and much easier to insist that the dots are wrong...

Unfortunately, some people think think "belief" is something they're supposed to cling to despite the facts, rather than because of them.

Me, I'd rather get more dots, acknowledge all the possibilities, and wait until the picture is clearer. In the meantime, I may speculate and/or form opinions, but will revise them instantaneously when new data paints a better picture - and not rule anything out until the dots show that can be done with some certainty. The horse I draw may transform into a camel or a unicorn, but I won't insist that it's still a horse when the dots show a hump on its back and a horn coming out of its forehead - unless the hump turns out to be a knight and the "horn" his lance. (at which point, my description would be slightly more informed than simply "a horse")

But I will admit that some people can't handle that kind of ambiguity and desperately seek a black&white, everything-revolves-around-the-earth kind of simplicity and certainty (God DOES exist), rather than seeing a technicolor universe where the earth doesn't simply revolve around the sun, the sun revolves around a galaxy, which itself is travelling through a universe of incomprehensible dimensions (wherein one or more gods might exist or have once existed).

Give me technicolor reality over monochrome dogma any day - the possibilities are so much greater (and far more interesting to contemplate).:rockon:


But I digress...

I think the temp threshold must be pretty low, because the engine was entering shutdown mode after only a couple of miles. But you make a good point - there's no use attempting P&G until the engine is warm enough to shut off. But I think the ICE reaches that temp (70F?) a lot sooner than the temp needed for max ICE efficiency, so P&G is still better than fully warming the engine.