Question about MPG

Steve Pardoe wrote:


Point of clarification:
The Prius does not wait for the application of the brake to begin the regeneration cycle. I've seen the display indicate regeneration at 50 miles per hour when the conditions are adequate. This is why steady driving is a good thing. If the electronics detects a request for 50 steady mph and there is a down slope of as little 2%, the controller may turn off the engine, turn on the motor, and may even begin regeneration. If the down slope is even more severe, but does not require braking to control the speed of the Prius, the electronics will still take advantage of the situation.

Your second point is offset somewhat by the continual running of a non-hybrid to support the ancillary requirements. The Prius air-conditioning is run off the traction battery, thus allowing complete shutoff of the engine at any complete stop. Given that some of our intersections require minutes of stopped cross traffic, that can be a major savings.
Probably the reason we don't have hybrid diesels for the U.S. is the high sulfur content of the petroleum we receive from our sources as opposed to the lower concentration in the supplies sent to Europe. There is a new requirement to lower that content at the refinery that should lead us to diesel hybrids as the best of both technologies.
In fact, I hope someone takes a close look at a serial hybrid (the Prius is a parallel hybrid in that both sources directly drive the wheels). A constant rotation diesel can be more efficient than a variable speed diesel. It would seem a diesel that charges a battery or capacitor that runs the motor would be the ultimate combination. And then, if we plug it in for the first twenty or forty miles of power requirements, we may have the best energy balance compromise.
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What? If the Prius is charging its battery while the engine is running, but you're not applying the brakes, that's got to be generation, not regeneration.

Again, if the (electric) motor is turned on, it's hard to see where the regenerative energy is coming from! Either the car is slowing and converting some of the energy (which in a conventional car would be lost as heat from the brakes) into battery charge, or it's drawing power from the battery to run its electric motor. It can't be doing both at the same time.

I presume this refers to cruise control 'braking' the car?

But surely no _net_ saving at all, since all the charge taken from the battery during rest has to be replaced by energy from the gasoline engine at some point (unless you believe in perpetual motion)?

Plus of course the fact that until recently gasoline in the US has been so cheap and plentiful that there's been no incentive to look for alternatives.

Yes, though it will always be more efficient to use the internal combustion engine (gasoline or diesel) directly, rather than using it to drive a generator to charge a battery to drive an electric motor to drive the transmission, since you have significant conversion losses at every stage. As I understand, it the main advantage of the battery in the Prius is that it boosts short-term acceleration (especially in the urban cycle) without a corresponding increase in instantaneous power demand (and hence emissions) from the IC engine. Energy recovery from regenerative braking is a bonus, but only of secondary importance.
Good debate, thanks for your input. SP
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wrote in message

The motor/generator used to propel the Prius is a motor when accelerating, a generator when decelerating. Applying the brakes with light-to-moderate pressure increases the charging rate which in turn increases the generator load. Sort of like this:
Take foot off accelerator pedal = 1/3 charge rate. Apply light pressure to brake = 2/3 charge rate. Apply moderate pressure to brake = full charge rate. Apply heavy pressure to brake = mechanical brakes engaged.

Yes it can. The "(electric) motor" is an (electric) motor/generator. Take your foot off the accelerator pedal and it reverts from motor to generator. The generator is driven by the front wheels, not by the gas motor.

With or without the cruise control engaged, the motor/generator reverts to generator whenever kinetic energy can be recovered.

Actually the energy taken from the batter during rest was kinetic energy recovered as the car coasted and/or braked to a stop. With a good tail wind, some of it is actually wind energy.

Steve, your last paragraph doesn't describe the Prius drive system. I believe Toyota calls it "hybrid synergy drive" because the gas engine, the electric motor/generator or both simultaneously propel the car. Remember, energy is recovered from both braking and coasting.
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a
Yes, I understand that.
<snip more details about regeneration which I also understand>

engine
...but surely, surely, you see that all that energy came from the gasoline engine in the first place, which was my point? Sure, in a conventional car slowing / braking energy is wasted, but even with a Prius there is no free lunch.

Oh, come on... ;-)

...and it all comes from the gasoline engine to start with! Look, all I'm trying to find out (as per my first post) is what mileage drivers such as you actually get per (US) gallon you put in the tank, as opposed to what Toyota's dash gauge (whether or not we agree it's a trip computer) tells you. I don't think the Prius "knows" how much gasoline you put in at any particular top-up, so the only reliable way (as the OP asked) is to keep a log and do the math yourself. Looking at the table at GreenHybrid.com it seems most drivers are relying on the dash indicator, "by display" in the jargon. You may say that "Frankly, both methods are so close as to render further discussion pointless" but I'd like to see some evidence to back that up. That's all. I've told you what I get in my European diesel car, but (unless I missed it) you haven't told me what you _really_ get in your Prius. All I get in this forum are explanations of how the Toyota system works - which I accept is very clever - but that's not my point. I'd be interested in a car that was truly as economical as claimed but not if (in fact) it's easy to get the same economy with a much cheaper diesel engine. I'm just trying to make a fair comparison, is all.
Steve
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Do you have any idea what a 30 mph wind does to mileage? Where I live, wind is the rule, not the exception.

I don't think the Prius "knows" how much gasoline you put in at any

I calculated my own mileage for the first few tanks. My calculated mileage was essentially the same at my displayed mileage plus or minus a couple of percentage points. My worst tank was 48 mpg. My best tank was 55 mpg. There. You have it.
To see what our EPA is contemplating, go here:
http://www.epa.gov/fueleconomy/420d06002.pdf
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That is true, but gasoline isn't used at constant efficiency. Idling is 0% efficient and moving slowly is scarcely any better... worse as the engine displacement increases. Hybridization allows the very low efficiency modes to be replaced by electricity generated when the engine is developing power and getting much better efficiency. In combination with allowing downsizing of the engine (the electric side determines the acceleration) that accounts for the tremendous increase in in-town efficiency and modest increase in open road efficiency of hybrids.
Mike
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In the old days the slightest downward movement of the gas peddle caused the fuel injector to squirt extra raw fuel. I remember experimenting with disconnecting it. There would be a momentary bog down as I tried to rev the engine. With the hybrid the fuel accelerator is replaced by a momentary boost from the electric motor. Another fuel savings is from the lack of a mechanical linkage between accelerator peddle and engine.
The electric motor provides for an extended coasting (to a red light for example) that if it weren't for the motor the car would come to a halt very short of reaching the light. With a traditional power train anticipating a red light up ahead by letting up on the gas peddle saves almost nothing. Actually, it rather comical to watch people zoom right up to a light when they knew it was red maybe 500 feet back or more.
Hybrids aren't for everyone. I dare say they are more suited for those of us who need not show our emotions by the way we drive. You can't rev the engine. You can't pass another car and make a lot of noise. These things don't cut it in the Animal Kingdom where growling and snarling cars helps people let off steam. So I guess if I have something to say, I step out of the car. Amazing how the other driver's safe heaven now becomes their prison while I lean on their hood and ask, " Do you have a problem?" Last time I did that (guy was flashing his high beams on a 35 mph road) he nearly crapped his pants. He denied flashing his lights.
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Couldn't this be fixed with gas pedal controlled sound effects switch-selectable between, say, Hog, Mac, & Titanic? :-)
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I apologize if my message is a little curt... IE just went and ate my longer and much thought out response. (GRR!) Hopefully I get it all right this second time around.
Steve Pardoe wrote:

http://autos.groups.yahoo.com/group/toyota-prius/surveys?idD4783 Question On average, what is the difference between your mpg calculated by the car's computer and by the gas station fill-up method? 499 responses: Choices Votes % computer 5mpg or more higher 17 3 computer 4mpg higher 38 7 computer 3mpg higher 70 14 computer 2mpg higher 107 21 computer 1mpg higher 12 2 both methods about the same 128 25 computer 1mpg lower 5 1 computer 2mpg lower 9 1 computer 3mpg lower 15 3 computer 4mpg lower 4 0 computer 5mpg or more lower 6 1 varies too much to give meaningful average 88 17

The only fair comparison is between similar-sized vehicles, using a standardized test, such as offered by various governments.
And it's not what is "claimed" - the manufacturers are required by law to post the fuel economy results of the governmental tests and no other figures.
post from Toyota GB press relations: http://autos.groups.yahoo.com/group/Prius-UK/message/7619
Prius Fuel Economy: Explaining the EPA Ratings Toyota explains what the EPA ratings actually mean, and lists ways to improve your MPG http://autos.groups.yahoo.com/group/toyota-prius/message/71431 http://autos.groups.yahoo.com/group/Prius-2G/message/2742 http://www.priusonline.com/viewtopic.php?t 79
Prius Fuel Economy Factsheet: http://www.toyota.com/images/vehicles/prius/Understanding_Fuel_Economy.pdf
For comparisons: US: http://www.fueleconomy.gov / Canada: http://oee.nrcan.gc.ca/transportation/personal / UK: http://www.vcacarfueldata.org.uk /
US 2006 best mid-sized cars: http://www.fueleconomy.gov/feg/best/bestworstNF.shtml 1: Toyota Prius (Hybrid), 4 cyl, 1.5 L, Automatic(Variable), Regular gasoline, 60 US MPG city, 51 US MPG highway 2: Hyundai Elantra, 4 cyl, 2 L, Manual(5), Regular gasoline, 27 US MPG city, 34 US MPG highway
Canada 2006 best mid-sized cars: http://oee.nrcan.gc.ca/transportation/tools/compare/compare-search-one.cfm?attr=8 1: TOYOTA PRIUS (HYBRID), regular gasoline, 4.0 l/100km (71 imp MPG) city, 4.2 l/100km (67 imp MPG) highway 2: MERCEDES-BENZ E320 CDI TURBO, diesel, 8.9 l/100km (32 imp MPG) city, 5.9 l/100km (48 imp MPG) highway
Unfortunately, the UK site doesn't break out the size classes, but they will sort by fuel economy. The Prius is in the highest band of cars, in the 60-70 imp MPG range or 1-5 l/100km range, along with a bunch of tiny diesels. http://www.vcacarfueldata.org.uk/search/fuelConSearch.asp
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Sorry to top-post but I just wanted to compliment you on your excellent, comprehensive reply to Steve's question. Scroll no further.

http://oee.nrcan.gc.ca/transportation/tools/compare/compare-search-one.cfm?attr=8
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<snip>
Agreed, very comprehensive and thanks for taking the trouble to post again.
It's contrary to anecdotal evidence from respected consumer organisations and road test magazines in the UK, which have consistently stated (but without numerical evidence) that the mileage achieved by typical drivers is significantly worse than the 66 mpg (Imp) headline figure I've seen claimed for the Prius. Perhaps it's to do with the kinds of journeys we typically make over here.
Steve (signing off)
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See also : http://test.nepsecure.co.uk/ADMIN/PropMotorsPlatform/newportal/motoringnews/fullstory.asp?siteid=&storyid 0
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Steve Pardoe wrote:

Based on what I've been reading on http://autos.groups.yahoo.com/group/Prius-UK/ , I didn't think that there were any "respected" organizations/magazines, only entertainment avenues.
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I went from Pittsfield MA and then onto MA Pike, drove 62 mph, and got 63.9 mpg.
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Steve Pardoe wrote:

Let me try this again. Obviously I haven't communicated the complexity of the control model implemented in the Prius' computers.
When you get into the Prius and turn on the system, all ancillary devices are initially run off the traction battery. If this were a gasoline only vehicle, we would immediately begin consuming gasoline and get no miles credited for it.
The Prius' gasoline engine is turned on briefly to warm the catalytic converter. This does not get us any miles, either, but if we haven't wasted any time getting into motion, the converter will be warmed up by the same engine heat used to move the car.
As we move down the road, the engine is only used when we exceed about 15 mph or the traction battery needs a charge. If there is a down slope and the engine is not yet running, the potential energy will be used either to increase speed or recharge the traction battery or both, recovering a portion of the energy used to climb the hill. It is not necessary to apply the brakes for this to occur. It is merely necessary to request less speed increase than the hill can provide. A gasoline only vehicle would still be consuming gasoline on this same route.
When we reach a controlled intersection and come to a complete stop, the gasoline engine is turned off. All ancillary devices are run off the traction battery. Again, we aren't consuming gasoline to go nowhere.
It is my feeling that using an idling gasoline engine to run ancillary devices and never recovering any energy used to climb hills is what costs the gasoline engine only vehicle the mileage that the Prius delivers. The Prius is a better overall compromise of a design. It is the total of small changes that gains the Prius significant mileage improvement. What the Toyota engineers have done is make it far more probable that a turning gasoline engine is causing the wheels to turn, thus improving the miles for each gallon it consumes.
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You and Bill are wasting your time. He isn't paying attention.

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<snip>

Sorry...
Sure.
With you so far.

The last two cars I've owned (one petrol / gasoline, one diesel) consume no fuel at all on the over-run (but I agree they are not conserving any energy while slowing down).

Yes, although a diesel engine consumes a remarkably small amount when idling compared with a gasoline engine, since it can run very much leaner.

I think it's fair to say that whether or not a gasoline / electric hybrid such as the Prius is "a better overall compromise of a design" depends on what kind of driving you mainly do. If you mainly drive in the city and spend a lot of time stopped at intersections, it makes good sense. Several European manufacturers make city cars with a stop-start system with a conventional engine. On a long steady journey with little opportunity to regenerate, the savings must be very slight, if any, compared with the same car without the 'hybrid' bit (but for the sake of argument still using the very efficient Atkinson-cycle engine and planetary transmission), and so under such conditions there's no reason why the car should be any better than a diesel, which was my original question.

Well made point, and all credit to them, and indeed to the far-sighted customers who care enough about all this to buy and run a Prius. Iadmire them, especially since I now understand that diesel isn't a viable alternative in much of the USA. However, as in my first post, I was really more interested in a straightforward user-based mpg comparison.
Thanks anyway,
Steve
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Again, which vehicle are you comparing the Prius to? Until you compare EPA mileage figures, this is speculation.

Which user? I presume that would be you. We are back to my earlier suggestion that you rent a Prius to compare with the mystery vehicle to which you constantly allude.
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There is one other Prius operational mode not yet discussed called Electric Vehicle (EV) or "stealth mode." At speeds below 42 mph, the vehicle can operate on just the electric motor with the engine off. The distance increases significantly at speeds below 30 mph. For example, the last 1-2 miles into work or returning home, I take streets with 25-30 mph limits. This last mile is run with almost no gas consumption unless I have to react to other traffic. This includes 2-4 m. rises, which can be taken at 10 mph without the engine.
This EV mode is a unique to the stock Toyota and Ford hybrid systems. Some models of Prius in Europe and Japan come with the EV button and in the USA, there is an after market kit to fit an EV button. So, are any other vehicles able to climb 2-4 m. without turning on the engine?
My 03 Prius has one well documented EV mode, reverse. In fact, even cold, the engine can not run when I back out of my driveway. The reason has to do with the planetary gears and engine. I can also climb steep hills in reverse.
It is possible to use the EV capability to considerably extend the "glide" part of "pulse and glide" driving. What this means is the speed decay is countered by the EV so the hybrid vehicle 'glides' further than an engine-off non-hybrid. For example, two vehicles starting at 40 mph start a glide down to 30 mph. The non-hybrid will go about 1/2 mile before reaching 30 mph and having to start the engine to speed back to 40 mph. The hybrid vehicle can stretch that energy-off glide to nearly a mile using a small amount of electrical power. The pulse back to 40 mph quickly add that energy back and you're back in glide again. But in the meanwhile, the hybrid has gone a considerably further distance.
Hybrids are also designed for frequent engine off and on operation. This means the start-engine fuel burn is extremely low and efficient. There is no Bendix gear that has to reach out and engaged the flywheel but effectively a directly connected motor that engages electronicly. This minimizes starting friction losses and engine wear. Unlike traditional 12 VDC starter motors drawing a hundred or more amps, the hybrid motors are 200-270 VAC motors drawing a few tens of amps. This lower current minimizes the resistance losses and heat suffered by ordinary starter motors.
Bob Wilson
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He meant that, when you're coasting, the motors/generators are taking energy from the wheels and putting it back into the battery. One does not have to be using the brakes to be regenerating.

It's a net savings because when the engine runs to recharge the battery it does so in its most efficient operating range, which produces much more power than is needed to run the A/C and other accessories. When the engine is off is obviously uses no fuel at all. Overall, even with the conversion losses, it's more fuel efficient to run the engine intermittently.

The beauty of Toyota's HSD system is that it can use a variable fraction of the engine's output with a direct mechanical connection, and allows sizing the engine to something much less than the maximum power requirement.
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