Posted by NeoPhyte_Rep on August 15, 2006, 12:23 am
Steve Pardoe wrote:
>>> The Prius is a reasonably economical car, especially in the urban cycle,
>>> but
>>> it still ultimately gets _all_ its energy from a gasoline engine, rather
>>> than a diesel, which is what we Europeans have come to know and love.
>>>
>>> Steve P
>>>
>> I didn't know there was a hybrid diesel on the market.
>
> Presuming that enough people continue to think hybrids are the way forward
> (I'm not yet convinced), Europe will go for diesel every time. In many
> countries, diesel cars far outnumber gasoline already. Several
> manufacturers who area talking about doing hybrids in Europe are talking
> only diesel, rather than gasoline. However, there are also makers such as
> VW who are releasing super-economical (non-hybrid) diesels with very low
> emissions and exceptional fuel mileage (aiming for 3 litres per 100km).
> It's interesting that in several cases recently the most powerful model in a
> maker's range is a diesel - e.g. the VW Touareg, Skoda Fabia, etc.
>
>> If you are comparing the Prius to the conventional diesel then your
>> statement regarding energy deserves some clarification. In both cases the
>> energy comes from the engine HOWEVER the Prius is capable of recovering
>> some of it's own energy whereas the conventional diesel is not.
>
> Of course, that's what the batteries are for (though I have met people who
> say that the reason for the Prius's economy is that some of the power is
> ultimately "electric", without having thought much about where the
> electricity comes from).
>
>> Climb a hill with both and an efficient diesel will use less fuel than an
>> efficient gas engine.
>
> Correct.
>
>> On the other side of the hill, on the way down, the Prius will recapture
>> some of that energy and store it in it's battery bank to be used when
>> needed. When a Prius is slowing to a stop, it is recovering some of the
>> energy required to get it up to speed in the first place. The
>> conventional diesel, unable to recapture energy, is less efficient
>> overall.
>
> There are two caveats. The first is that in normal driving (not in
> stop-start traffic) how much time do you actually spend pressing the brake
> pedal?
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.
> Not much, as a proportion of a journey, unless you are a driver with
> exceptionally poor anticipation. This is evidenced by the very low capacity
> of the Prius battery pack, only able to power the car for a few seconds at
> full output, which in turn explains why Toyota provided no facility to
> charge it from the mains. There's no point.
>
> Second, lugging the additional mass of the battery pack, electric motor /
> generator set, power and temperature control units, etc around all the time
> has an impact on economy.
>
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.
Posted by Steve Pardoe on August 15, 2006, 1:35 pm
> Steve Pardoe wrote:
>> There are two caveats. The first is that in normal driving (not in
>> stop-start traffic) how much time do you actually spend pressing the
>> brake pedal?
> 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.
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.
> 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.
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.
> 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.
I presume this refers to cruise control 'braking' the car?
>> Not much, as a proportion of a journey, unless you are a driver with
>> exceptionally poor anticipation. This is evidenced by the very low
>> capacity of the Prius battery pack, only able to power the car for a few
>> seconds at full output, which in turn explains why Toyota provided no
>> facility to charge it from the mains. There's no point.
>>
>> Second, lugging the additional mass of the battery pack, electric motor /
>> generator set, power and temperature control units, etc around all the
>> time has an impact on economy.
>>
> 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.
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)?
> 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.
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.
> 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.
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
Posted by Bill on August 15, 2006, 4:37 pm
>> 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.
> 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.
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.
>> 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.
> 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.
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.
>> 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.
> I presume this refers to cruise control 'braking' the car?
With or without the cruise control engaged, the motor/generator reverts to
generator whenever kinetic energy can be recovered.
>> 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.
> 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)?
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.
>> 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.
> 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
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.
Posted by Steve Pardoe on August 16, 2006, 2:08 pm
> 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, I understand that.
<snip more details about regeneration which I also understand>
> > 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)?
> >
> Actually the energy taken from the batter during rest was kinetic energy
> recovered as the car coasted and/or braked to a stop.
...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.
> With a good tail
> wind, some of it is actually wind energy.
Oh, come on... ;-)
> 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.
...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
Posted by Bill on August 16, 2006, 4:34 pm
> ...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.
>> With a good tail
>> wind, some of it is actually wind energy.
> Oh, come on... ;-)
Do you have any idea what a 30 mph wind does to mileage? Where I live, wind
is the rule, not the exception.
> 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.
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
XML site map
>>> but
>>> it still ultimately gets _all_ its energy from a gasoline engine, rather
>>> than a diesel, which is what we Europeans have come to know and love.
>>>
>>> Steve P
>>>
>> I didn't know there was a hybrid diesel on the market.
>
> Presuming that enough people continue to think hybrids are the way forward
> (I'm not yet convinced), Europe will go for diesel every time. In many
> countries, diesel cars far outnumber gasoline already. Several
> manufacturers who area talking about doing hybrids in Europe are talking
> only diesel, rather than gasoline. However, there are also makers such as
> VW who are releasing super-economical (non-hybrid) diesels with very low
> emissions and exceptional fuel mileage (aiming for 3 litres per 100km).
> It's interesting that in several cases recently the most powerful model in a
> maker's range is a diesel - e.g. the VW Touareg, Skoda Fabia, etc.
>
>> If you are comparing the Prius to the conventional diesel then your
>> statement regarding energy deserves some clarification. In both cases the
>> energy comes from the engine HOWEVER the Prius is capable of recovering
>> some of it's own energy whereas the conventional diesel is not.
>
> Of course, that's what the batteries are for (though I have met people who
> say that the reason for the Prius's economy is that some of the power is
> ultimately "electric", without having thought much about where the
> electricity comes from).
>
>> Climb a hill with both and an efficient diesel will use less fuel than an
>> efficient gas engine.
>
> Correct.
>
>> On the other side of the hill, on the way down, the Prius will recapture
>> some of that energy and store it in it's battery bank to be used when
>> needed. When a Prius is slowing to a stop, it is recovering some of the
>> energy required to get it up to speed in the first place. The
>> conventional diesel, unable to recapture energy, is less efficient
>> overall.
>
> There are two caveats. The first is that in normal driving (not in
> stop-start traffic) how much time do you actually spend pressing the brake
> pedal?