My random thoughts after 1 week of ownership

<div class='quotetop'>QUOTE(SureValla @ Jul 27 2007, 08:22 PM) [snapback]486443[/snapback]</div>

Well, this won't be too technical. I'm not an engineer! But maybe it will help.

Hobbit has researched optimal RPM ranges as described here, among other places. His paper mainly addresses moderate to higher (i.e., highway) vehicle speeds.

For lower speed driving, consider the experience of the Japanese "hypermilers." Though I wouldn't call it technical data along the lines of what Hobbit reports, their results are compelling. Ken1784 (from Japan) has posted multiple reports of him and his colleagues getting 1000+ miles per tank making extensive use of "pulse and glide." He reports they get their best MPG results using an accelerator pedal position, as reported by CAN, of 76-80 (out of a possible 200) during the pulse phase of low speed pulse and glide. I use Can-View, which reports pedal position in percent, so that of course translates to values of 38-40. After seeing his post I've watched the relationship of pedal position to RPM on Can-View during my pulses. At speeds of 30 MPH and less, pedal position of 38-40 generally maintains RPM at <2300. For more technical information, I'm sure Ken can help.

I proposed the rule of keeping instantaneous MPG > MPH x 0.5 in the thread from which I linked Ken's post. Unbeknownst to me at the time, Theorist had previously and independently suggested the same rule. (I acknowledged him in my thread when I discovered his post.) After getting Can-View and observing the relationship between pedal position, RPM and iMPG, I concluded that the Japanese technique validates the (iMPG > MPH x 0.5) rule, with RPM at about 1600-1700 off the line and gradually increasing to 2200-2300 or so at 30 MPH.

A good search here, at GreenHybrid, and at CleanMPG should reveal other reports you'll find helpful. Though most are anecdotal, results are pretty consistent. For more hard data, look at Bob Wilson's work on his page, at GreenHybrid, and here (user name bwilson4web). He too is an engineer and, IIRC, works for NASA. Not sure if that makes him an aerospace engineer, but I bet it's pretty close.

 

Wow, this thread really has taken on a life of it's own since I last checked in this afternoon!
<div class='quotetop'>QUOTE(Wiyosaya @ Jul 27 2007, 02:27 PM) [snapback]486231[/snapback]</div>

Perhaps I was not clear in my choice of words - I was not implying that playing the P&G game is not more efficient than 'normal' driving, but rather driving 'normally' generally got me better results than staring at the MFD and trying to run in EV, without applying proper P&G (i.e. no arrows) technique.
<div class='quotetop'>QUOTE(Wiyosaya @ Jul 27 2007, 02:27 PM) [snapback]486231[/snapback]</div>

Careful... "normal" may not be defined the same for every driver. I don't know how aggressive your wife's driving style may be, but my previous car was a 5-speed Integra LS which I prided myself on driving smoothly and efficiently, regularly scoring >31MPG, which was above EPA for that car. :lol:

By "normal", I was merely suggesting driving in a responsible manner with moderate acceleration and braking, without applying any Prius "tricks".

<div class='quotetop'>QUOTE(Wiyosaya @ Jul 27 2007, 02:27 PM) [snapback]486231[/snapback]</div>

As Danny Hamilton suggested, I believe our experiences are indeed similar, as we were clearly comparing different things...

 

<div class='quotetop'>QUOTE(kidtwist @ Jul 27 2007, 04:58 PM) [snapback]486432[/snapback]</div>

Prius is neither a verb nor an adjective. It is a NOUN, specifically a masucline noun, and therefore,
the plural [in Latin] of a noun is: prii. In Latin, Prius is pronounced Pryus, not Preeus, linguistically speaking to be proper.

 

<div class='quotetop'>QUOTE(GeoDosch @ Jul 27 2007, 09:22 PM) [snapback]486487[/snapback]</div>

Yes, if you have decided to stop the car faster than you can by coasting, then the regen avoids wasting some of the energy that friction brakes would have converted to heat.

The point is that using the brakes and then reusing the electrical energy reduces the MPG as compared to getting your foot off the accelerator sooner and coasting to a stop.

As I have pointed out, the battery and MGs are there to improve acceleration and deceleration in a car that is far more efficient if you don't use the battery or MGs at all. Just run the ICE as efficiently as possible any time you accelerate and then turn off the ICE and coast the rest of the time.

If you accellerate at a rate that doesn't put any charge into the battery, and then glide with no power sent to or drawn from the battery for propulsion or regen, the battery will still drain very slowly just to keep the various electrical components of the car working (lights, computer, MFD, radio, climate control, various pumps, spark plugs, etc.) The very minimal amount of regen that you get for the few times that you have to stop faster than can be accomplished by coasting will help offset these requirements. So "the most efficient" way to drive the Prius is without using the battery and MGs for propultion, and with the bare minimum amount of regen possible.

Now, that said, the most efficient way to drive the Prius isn't typically the most reasonable way to drive it. Most of us choose to sacrifce MPG for a more reasonable driving experience. We accelerate at a rate that uses some battery or regen, but keeps up with surrounding traffic better. We stop at a rate that uses some regen, but doesn't annoy the heck out of the cars behind us and gets us to our destination in a reasonable amount of time. We maintain a particular chosen speed both drawing from the battery and pushing excess energy back in as the Prius determines is best at the moment rather than oscillating between acceleration and gliding.

These things reduce our MPG from what is possible, and yet still we beat nearly every other car on the road.

The point behind all of this discussion though has been that focussing on trying to keep the Prius using electric propulsion as much as possible will reduce your MPG overall as compared to trying to use EV propulsion as little as possible. Yes when you do use regen and electric propulsion you are reducing the waste as compared to a non-hybrid, but you are increasing the waste as compared to driving the Prius without electric regen and propulsion.

Sure your instantaneous MPG, and perhaps even your MPG for the particular trip might look GREAT, but later on some other trip the MPG will suffer as the Prius trys to get the battery charge back up to where it needs to be.

 

HTTP 404 Not Found

 

<div class='quotetop'>QUOTE(kimgh @ Jul 27 2007, 09:07 AM) [snapback]486072[/snapback]</div>

If you buy a prius in Denver, drive it gently to Los Angeles and sell it... It's possible you could get some of that descent energy for free.

What do you think I should do with my 50cents when I get done with proving you wrong?

Now I suppose you're going to tell me that the semi that hauled my prius to denver spent the fuel to get it up the hill and.... you're still right... Doh!!

What if I steal gas out of my neighbors hummer in the night... is that free?

:lol: Darn PHD's...

 

Catgic,

Here is an analogy that may help you "digest it to your satisfaction", or it may just add to the confusion.

Other than a tailwind, all energy used by the Prius originates with gasoline. I hope that we are at least in agreement on this. Furthermore, I hope that we can agree that (unless you plan on dropping a detonator into your gas tank) the ICE provides the only way to get motion from the gasoline. The ICE is the originator of the resource you will use to travel a distance.

Ok so for the analogy,
Think of the ICE as your sole source of cash (a job that pays cash at the end of each day?)
Think of the distance you wish to travel as a commodity you wish to acquire for use.(coffee?)
Think of the battery as a line of credit with a fair interest rate and no grace period.

The interest rate represents the thermodynamic losses that occur for every energy conversion.
The lack of grace period represents the fact that every time you draw from the card, there is no way to avoid paying the interest.

So in general in the long run you can get the maximum commodity by paying cash as you earn it.

If you spend some cash (use some fuel) and purchase none of the commodity (travel 0 miles) you have a pretty good representation of 0 MPG

If someone gives you some of the commodity as a gift and you don't have to spend any money you have a pretty good representation of a very strong tailwind pushing the car down the road with the engine off and no battery use.

Now the tricky part comes in when you use the line of credit to purchase some of the commodity. You haven't laid out any cash for it so for the moment it looks like you just got the commodity for free. You've just drawn some energy out of the battery to gain some miles and not used any fuel. But that credit has to be paid back eventually, with interest. For the same amount of cash earned eventually, you are going to get less commodity overall because of the loss to the interest.

Now there may be situations where you need more commodity than you can afford with your source of income, and other situations where you will generate enough income to purchase more commodity than you need. The line of credit allows you to get that extra commodity when you need it, and you can pay it back later with the extra income generated when you don't need as much commodity.

However, everytime you use that line of credit, you are giving up some future commodity that could have been bought because of the loss to the interest.

Need a representation of regen using the same analogy?
Ok, whenever you carry a balance on the line of credit the issuer of the line of credit is willing to buy some commodity from you at a price slightly less than what you paid for it. So if you have accidentally wasted too much cash buying more commodity than you need, you can sell it at a slight loss and use the cash generated to pay back the line of credit and interest.

Need a representation of going down hill?
You had to get to the top of the hill in the first place. In order to do this you either used a lot of cash or your line of credit or both to buy up extra commodity that you needed to have possesion of for a while but knew you wouldn't use up (verticle distance traveled up), and then sold it at a slight loss later (verticle distance traveled back down).

Need a representation of gliding? (horizontal distance traveled without ICE or battery)
You use cash or line of credit or both to purchase today all the commodity you'll need for the next week. Then you slowly use up the commodity over the next week while taking a vacation from your job. If you predict it exactly right you use up all the commodity just as your vacation ends (you glide right up to the point where you start using the ICE again). More likely you purchase extra commidity up front to make sure you don't run out, and then sell the excess at a slight loss when you start working again. (glide for a while and then regen to a stop at a light)

Need a representation of a full battery?
You line of credit is paid in full. Any cash that is generated and not used gets flushed down the toilet each day. You are better off just working less for a while and drawing off the line of credit a bit so you have a place to use future excess cash. Better to spend a bit on interest in the future but get the use of most of the cash than to flush it all. The problem with working less is that the job doesn't pay as well for part time hours as it does for full time hours. (the ICE isn't as efficient outside it's optimal RPM range).

Need a representation of a nearly empty battery?
You are approaching your credit limit. If you don't get some of it paid back soon, you may have a situation where you need more commodity than you can afford. (Your acceleration will suffer, because there is no more to draw from the battery)

Hopefully this analogy will help you see that when you use the battery, you are not getting miles for free, you are just borrowing against future ICE use at a slight loss as compared to using the ICE only.

Hopefully it will also help you see that regen isn't free energy, it's excess energy originally generated by the ICE and paid back to the battery at a bigger loss than if the energy had gone straight to the battery in the first place.

 

<div class='quotetop'>QUOTE(Danny Hamilton @ Jul 28 2007, 06:03 AM) [snapback]486619[/snapback]</div>

All us common plebes is just waitin' to hit the lottery (oil well in back yard)

 

HTTP 404 Not Found

 

<div class='quotetop'>QUOTE(Danny Hamilton @ Jul 28 2007, 01:49 AM) [snapback]486587[/snapback]</div>

I guess my problem is that I don't do all of my driving on a test track. When I have 'decided' to stop the car, it isn't a whimsical though; it's more likely because a traffic light has turned red, or the car just ahead of me has suddenly slowed, and I've decided not to slam into the back of him. I may also need to slow when descending a hill. Don't forget that you don't need to touch the brake pedal to get regen: just taking your foot off the go-pedal does that. When you say 'coasting', that means no arrows on the energy meter, and in that mode it will take the Prius a very long time indeed to stop. It may not be the most efficient way of driving, but it is driving within the constraints of the public roads.

<div class='quotetop'>QUOTE(Danny Hamilton @ Jul 28 2007, 01:49 AM) [snapback]486587[/snapback]</div>

Yes, that's my point. The original poster was asking how he could best drive the car, and I make the assumption that he meant in real-world conditions. If we were not constrained by the realities of driving with other cars, traffic controls, etc., then the Prius is far from the ideal set-up. But if you've ever seen the competitions where they go all-out to see who can make the highest MPG car, they are run on closed roads without any constraints of traffic.

<div class='quotetop'>QUOTE(Danny Hamilton @ Jul 28 2007, 01:49 AM) [snapback]486587[/snapback]</div>

As mentioned before, my stopping rate is more often than not dictated by factors other than annoying people behind me, such as coming to a stop before I get to the stop line, or before I hit the car in front of me.

<div class='quotetop'>QUOTE(Danny Hamilton @ Jul 28 2007, 01:49 AM) [snapback]486587[/snapback]</div>

Actually, I check my MPG by how far I drive and how much I need to refuel, which takes everything into account.

<div class='quotetop'>QUOTE(Danny Hamilton @ Jul 28 2007, 06:03 AM) [snapback]486619[/snapback]</div>

It's an interesting analogy, but it misses the point that we need to compare how a Prius operates in relationship to other cars on the road.

To borrow from your cash analogy, there's a sidewalk, and everyone walking on it has money on them, which represent their energy. At the corner is a goon who hits everyone up for money when they pass by. The corner represents the need to stop, and the money the goon takes represents the energy wasted by braking.

Now, I've got a bodyguard walking with me (my battery). When I see I'm coming up to the goon, I take the money he would have gotten and hand it to my bodyguard. Then when I get to the goon I just need to toss him a few pennies (when my mechanical brakes finally do engage). Later, when I need some of that money, the bodyguard hands it back to me, after he takes a stiff percentage for himself. So I'm still losing some of my money by putting it into my 'battery', but I've lost far less than the folks who gave it all to the goon. And when I get to where I'm going, it has cost me less then everyone else.

 

<div class='quotetop'>QUOTE(JimboK @ Jul 27 2007, 10:44 PM) [snapback]486494[/snapback]</div>

excellent, thanks for all the links.

wow didnt know about bwilson4web thats awesome!

 

Good thread. I can only add:

<div class='quotetop'>QUOTE(AXavier @ Jul 26 2007, 09:44 PM) [snapback]485872[/snapback]</div>

You have a drafting table? That's awesome.

 

<div class='quotetop'>QUOTE(Sarge @ Jul 28 2007, 12:16 AM) [snapback]486533[/snapback]</div>

Yes, seems to take on a life of its own. However, from my experience, you can do so safely and gain valuable experience in the process.

My wife's driving is definitely more aggressive than mine. :lol:

Interesting on the Integra. Before I got my 06 Prius, I drove a 94' Integra LS 5-speed for 12 years. I, too, got better than EPA mileage, however, most of my driving was highway. As well, I often skipped 4th and went to 5th at about 35 MPH or slightly above - i.e., city driving. That worked well for gas mileage.

Well, perhaps they are similar. Thinking about it, my current driving style may be a "toned-down" hypermiling style. However, I use the techniques in a manner that is not obstructive to traffic, although, when I'm doing the speed limit, there may be drivers behind me that want me to go faster.

 

<div class='quotetop'>QUOTE(Wiyosaya @ Jul 30 2007, 02:33 PM) [snapback]487531[/snapback]</div>

Nice! Mine was a '96, also in LS trim with 5-speed. Was yours red too? :lol:
<div class='quotetop'>QUOTE(Wiyosaya @ Jul 30 2007, 02:33 PM) [snapback]487531[/snapback]</div>

Sounds pretty similar to my style... I drive in a conservative manner, but do not obstruct traffic and generally drive the speed of surrounding traffic. There are a few places in my regular commute where conditions are ripe for the no-arrow-glide for a reasonable distance (residential area, very slight downward grade), but I would not attempt it in any situation I feel I would be an impedence to traffic...

 

LOL Danny Hamilton! You forget that such a large percentage of people manage their finances very poorly, so your analogy may be lost on them

Actually I was going to suggest the same analogy earlier, since someone mentioned that using battery power was like borrowing against future gas use... but they forgot to mention the part about paying interest. That's really the key in understanding why battery power is best thought of as an assist in the Prius.

There are still times to use battery power, since the game is not "finding free energy" but rather, making the best use of otherwise wasted energy.

I would add one scenario to your analogy:

Your job is seasonal, so you make a lot sometimes, and very little at other times. Rather than live well during busy times and starve during slow times, you're better off living moderately when your income is good, and maybe even borrowing against the known and expected influx of income when income is low.

In Prius terms, that means things like, if you know you have a long downhill ahead, but you have to go uphill a little before you crest the hill, you can use stealth to get there and recapture much of what was wasted. It's not an overall strategy, but there are times when borrowing a little pays off. It also means that if you know you'll be finishing your drive in under a mile with relatively good terrain, you may want to stealth as much as possible since you know the engine will have to run for a few minutes the next time you start up. The startup cycle is all about generating heat (waste) so you might as well be recapturing as much as possible during that time.

 

<div class='quotetop'>QUOTE(SureValla @ Jul 28 2007, 11:12 PM) [snapback]486914[/snapback]</div>

I'll give you my two cents as well.

The Prius is a complex mechanical system, so the technique that maximizes MPG, for a given set of driving conditions, is going to have to be determined empirically.

As far as I know, there is no good systematic analysis of this issue for regular traffic, though I read that Toyota is installing an "eco-light" on 2008 (?) Japanese hybrids to show when the car is being driven for optimum efficiency. So, I think Toyota's going to program something into the car, but its not here in America yet.

You can find Argonne National Labs and others' tests and simulations of the Prius for various standard driving cycles (e.g., what the EPA uses here), but I think that for almost anything else, you are left with whatever casual empirical data you can get.

So, I settled for second best, and rely on the rules as stated for two reasons. First, several independent groups -- Japanese hypermilers and some senior posters here (at different times) all developed more-or-less the same rule for the optimal rate of acceleration. Second, the rules are in accord with what I understand about how the car works. (In other words, I can rule out some other alternatives just because the basic science suggest they are unlikely to be correct).

Here are some other points of reference.

First, the ICE in isolation is roughly equally efficient over a broad range of outputs -- graphs courtesy of Hobbit, who posted 1500 RPM as the point at which the curve flattens.

http://www.techno-fandom.org/~hobbit/cars/prius-curves.gif

In fact, the whole point of the hybrid system is to allow the ICE to run a larger fraction of the time at efficient loads. (Tried to find a quote from Toyota on that but failed, but that's my understanding. All the energy comes from the gasoline. Ultimately, for a given amount of output from the car, the efficiency is a function of how much of that output was obtained at the peak of the energy efficiency curve for the ICE. The engine was specifically designed to have that flat efficiency curve, then the car was built to keep the engine running on that flat part of the curve. Until you get to the point where you can plug it into the wall, the electrical components are just a way of avoiding running the engine below or above the peak efficiency RPMs.

There's a lot more by Hobbit that I don't even pretend to understand, accessible here for example:

http://www.techno-fandom.org/~hobbit/cars/...s-linkfarm.html

Second, if you had to trust any one group of people to know how to get the absolute max MPG out of a Prius, it would be the folks who held the Prius marathon and set the US (and at that time world?) record for Prius mileage. Reading this interview, they specifically tried not to move current into or out of the battery, other than the input occasionally required to recharge the battery due to drawdown while running, and the energy required for ICE restarts. In other words, the people who set the US record deliberately avoided moving current into or out of the battery.

Key quote:

"The pulse and glide technique seeks to minimize the amount of time that the engine runs, and minimizes power flowing to and from the battery."

http://www.toyota.com/html/hybridsynergyvi...l/marathon.html

But you can't run the car like that in regular traffic. Hence the narrower question: given that you're going to "glide" once up to speed, what's the optimal rate of acceleration (or, really, engine load, I guess, as you might be accelerating uphill) for best fuel economy.

That's where it gets to be an empirical question based on how the entire drive train functions, not just the engine. For example, the efficiency curve for the engine is pretty flat, but if you floor it, you'll draw a lot of current from the battery. That's bad, for reasons discussed earlier in this thread. And rapid draw is very bad, as heat losses are proportional to the square of the current. So, people who look at the flat engine efficiency curve and, from that, decide that acceleration at full throttle is as efficient as acceleration at lower throttle setting are probably not correct. (Not to mention that there most of the internal losses in the engine and HSD seem scaled to rotation speed, so you losses in the drivetrain are higher at higher RPM).

Ideally, you'd like to see fuel consumption, for a given acceleration and coast cycle, with total cycle time held constant. In other words, if I'm going to get from this stop sign to the next one in 45 seconds, what's my optimal rate of acceleration? (So that if you loaf up to speed you must accel longer, and so on.) This is what I've never seen the empirical data for. All you get is the rough empiricism from people who've tried various things, and the consensus of the people whom I think have it right is as I stated earlier.

 

<div class='quotetop'>QUOTE(catgic @ Jul 28 2007, 02:44 AM) [snapback]486600[/snapback]</div>

Sorry, JP, I missed this in my first reading of your post. The trip was so-so, FE-wise, hurt by the hills and mountains of VA, WV, and KY; and less than stellar weather -- stiff headwinds on the way up in OH, IN, and IL, and showers for a good part of one day of the return trip. I'm estimating overall FE was in the mid-50s, but I didn't figure it precisely. Instead I tracked individual logical (to me) segments based on days, terrain changes, and/or fillups. I chronicled the trip here.

 

<div class='quotetop'>QUOTE(Sarge @ Jul 30 2007, 04:52 PM) [snapback]487593[/snapback]</div>

Mine was the lighter green (as opposed to the darker green). I think '94s were, perhaps, one of the few years that got that color.

After test-driving the Prius, I was convinced that it handled virtually as well as the Integra, however, I think the Prius has a tendency to over steer, and I think that is responsible for the drift that some people report. If I use two hands on the steering wheel, I find that the Prius is very stable. Also in comparison to the Integra, the Prius seems a bit top-heavy. Acceleration seems very similar; I have never been impressed by "hot rods" and turned down one person's "desire" to race my Integra. I bought it mainly because of its repair reputation and resale value, though I never did resell it.

Me neither; however, I do tend in many circumstances to stick to the speed limit, though, I have been known to exceed it at times.

 

<div class='quotetop'>QUOTE(Wiyosaya @ Jul 31 2007, 10:21 AM) [snapback]487976[/snapback]</div>

You must be referring to the 'teal' color that I know was available in '94. I had test-driven a used GS-R in that color, but it was high mileage and expensive so I passed.
<div class='quotetop'>QUOTE(Wiyosaya @ Jul 31 2007, 10:21 AM) [snapback]487976[/snapback]</div>

Well... I realized immediately that the Prius generally doesn't 'handle' as well as the nimble Integra did (stock suspension, but lower-profile/higher performance tires), which I think is largely because of the stiff suspension on the Integra. But I wasn't expecting it to, either. I know I can add the stiffening plate and upgrade the tires, which I plan to do in due time. However, the bottom line is that I see the Prius as more of the family car for me that it will be, whereas my Integra coupe was my bachelor car. B)

On a completely different note (but still qualifying as a "random thought" as the thread title implies), I used to consider the rear drums on the Prius as a "drawback" for a car of this price range, but once I gained a good understanding how HSD & regen braking works, I realize this was a really intelligent move on Toyota's part. Rear discs are generally considered a higher performance upgrade over drum brakes, but since they are exposed they also carry the drawback that they are more prone to corrosion from splashing and as a result require more servicing than drums (especially in northern, snowy climates). Of course, corrosion will only be magnified by the reduced duty due to the regen braking. Since they aren't used that much in an HSD vehicle, I am glad Toyota saved the cost of rear discs and opted for lower-maintenance drums instead. B)