After so much discussion of this subject, CalCars Technology Lead Ron
Gremban examines the issue. This is more technical than most things
we send around. It was first published at Green Car Congress, where
you can see a comparative table that doesn't transmit well by email,
as well as several dozen comments -- most of which, unfortunately,
are off-topic, but a few of which respond to the issues raised by Ron.

CalCars Weighs In on GM Series/Toyota Parallel PHEV Debate
http://www.greencarcongress.com/2008/03/calcars-weighs.html
7 March 2008
Guest piece by Ron Gremban, CalCars

GM and Toyota have been taking public shots at each other, each
claiming that their plug-in hybrid (PHEV) technology--not yet brought
to market--is the best, and implying that the other's plans are
poorly thought out, to say the least.

We at CalCars, if anything, are thrilled to see the two biggest
automakers in the world touting their upcoming PHEV wares and paying
significant attention to each other's. But what is the science behind
the dispute? What follows is a discussion that is aimed at engineers,
but we think will be quite informative also to non-technical
audiences. Thanks to Dr. Andy Frank of UC Davis and Efficient
Drivetrains Inc. for his helpful review and comments.

A preview of my conclusion: It turns out that different battery sizes
have different optimum PHEV architectures, and each company's claims
are basically accurate, but only for its vehicle's battery size.
Since each type of PHEV has its own advantages, disadvantages, costs,
and optimum driving regimes, our expectation is that during the first
few years--maybe a decade--of PHEV production, all types of PHEVs
will compete well in the marketplace.

Then, eventually--as batteries become a cheaper, longer-life,
commodity item, liquid fuels become more dear, renewable electricity
generation proliferates, and CO2 emissions are increasingly
targeted--the PHEVs with the most EV power and range will come to dominate.

First, let's establish what, in our opinion, are the most important
characteristics of a PHEV. Though PHEV technology can improve overall
powertrain efficiency, decrease criteria emissions, provide full
zero-emissions capabilities part of the time, etc.--and other
technologies can and ought to be used to significantly reduce vehicle
mass and drag--the most profound capability of any PHEV is its
ability to displace some of the vehicle's consumption of liquid fuel
(usually gasoline) with stored electricity from the grid, and to do
so without introducing new overall vehicle limitations (e.g. the high
cost, extra weight, and range limitations of pure EVs).

It is this fuel displacement from which all the most important
advantages of PHEVs arise: dramatically reduced oil consumption and
greenhouse gas emissions, low enough liquid fuel consumption that
biofuels may someday fully substitute for fossil fuels, and energy
storage that can eventually enable increased deployment of
intermittent renewable electric generation from sources such as wind.
Therefore, the very most important measure of a PHEV is the extent of
its ability to displace liquid fuels, to do so during normal US
driving cycles, and to do so cost effectively. All else is frosting
on the cake.

When we look at "normal US driving cycles", there are several areas
of general agreement. The average mileage driven per day is around 30
miles. There is a curve available showing percentage of daily driving
vs. distance. Though there is a continuum, driving is broken down
into city and highway driving; standard drive cycles, UDDS (a city
driving cycle) and HWY, have been designed to emulate each. These
standard cycles are obsolete and grossly underestimate required
vehicle energy and capabilities, but are used as the basis of all EPA
and CARB testing anyway. The US06 combined drive cycle is a much more
realistic standard cycle.

Since the first standards for testing and measurement of PHEV
performance are still being written, general references to these
three standard cycles that the upcoming SAE J1711 standards will
reference are our best bet for measuring and comparing PHEV
performance. Dr. Andy Frank suggests that a new "Annual Driving
Cycle" be designed to model annual electricity and gasoline usage,
but for now that doesn't exist.

There are series hybrids, where the internal combustion engine (ICE)
drives only a generator; parallel hybrids, where both the ICE and
electric motor are always connected to the wheels; and power-split or
series/parallel hybrids, where either the motor or the ICE or both
drive the wheels at various times.

Though the Chevy Volt is presented as a series PHEV, and the Toyota
Prius (as well as the 2-mode Saturn Vue, too!) is power-split, the
specific architecture is actually fairly irrelevant to the main issue
that GM and Toyota are addressing. Incidentally, my calculations lead
me to believe that the inherent efficiencies of each of the
architectures are close enough to each other that the quality of
engineering that goes into each vehicle is more likely than the
architecture chosen to determine overall vehicle efficiency.

Though the details can vary and/or the mode distinctions blur, all
plug-in hybrids basically have a charge-depletion mode and a
charge-sustaining mode. After a grid charge, the charge-depletion
mode is activated first, during which time as much of the vehicle's
propulsion energy as possible is pulled from the battery, while as
little liquid fuel as possible is used. If this charge-depletion mode
is 100% electric, the vehicle is considered a "pure-EV PHEV",
otherwise, it is a "blended-mode PHEV". Once the battery is
discharged to its target depth-of-discharge (DOD), the battery's
state-of-charge (SOC) is maintained at this level and the vehicle
functions in charge-sustaining mode, just as an ordinary hybrid.

A PHEV can either have some pure EV range, be "blended mode", or, of
course, employ some combination of the two. For example, a PHEV may
start out with some pure EV range. Near the end of that range, the
ICE may be started more and more often, providing some blended-mode
driving before full DOD, at which time the vehicle shifts to
charge-sustaining mode. Or charge-sustaining mode may consist of
alternating periods of pure EV driving and significant ICE power,
causing the SOC to vary rather than stay steady at maximum DOD.

Also, there are various kinds and degrees of power blending. A PHEV
may be able to drive purely electrically only up to a specific speed,
such as the 34 mph/55 kph limit imposed by the hybrid system on
converted Prii. Also, only limited electric propulsion power may be
available, like the 21 kW limit also imposed on converted Prii by the
hybrid system.

The extent of a blended-mode PHEV's blending is expressed as a
"Utility Factor" that is a percentage of the wheel energy that is not
supplied by the ICE. A vehicle's Utility Factor can be quantified
over each of the standard drive cycles talked about above. Its
"Effective EV Range" is its depletion-mode range multiplied by its
Utility Factor, which is conceptually the EV range it would have if
its depletion mode were pure EV.

A PHEV with pure EV range has a Utility Factor of 100% and an
Effective EV Range equal to its real EV range. Of course, this is
also complicated by the fact that Utility Factor and Effective EV
Range can each be very different when measured using each of the
three standard driving cycles. In general, both parameters will be
highest on the UDDS cycle and lowest on US06.

Another measure of a PHEV's capability&madsh;in some ways even more
useful than Effective EV Range--is the usable capacity of its battery
pack in kilowatt-hours or kWh, as this indicates how much energy is
available after each charge to displace liquid fuel. A 12.5 kWh
battery pack, allowed to charge fully but discharge only to 80% DOD,
will have 10 kWh usable capacity.

Since a gallon of gasoline holds about 33 kWh of heat energy and the
most efficient hybrid drivetrains approach 30% efficiency, 10 kWh of
usable battery capacity can potentially displace a gallon of gasoline
after each (often <$1.00) grid charge, or up to 365 gallons/year when
the vehicle is charged every night and driven to the end of depletion
mode every day. However, a PHEV whose battery is regularly not fully
depleted between charges is leaving money on the table (the battery
could have been smaller and less expensive), and a PHEV that is
regularly driven significantly beyond charge depletion mode into
charge sustaining mode could potentially gain from having a larger battery.

What we want, of course, is, on the average, the most displacement of
liquid fuels for the least incremental cost over that of a standard
ICE propulsion system. Motor, power electronics, and ICE costs are
all fairly proportional to maximum power output. Battery cost, which
for now dominates PHEV costs, is set by energy storage capacity,
maximum input/output power, and cycle life, which is itself dependent
on maximum DOD and other factors.

As everyone else does (but without acknowledging it), we will ignore
the fact that until PHEVs become ubiquitous, people who buy and drive
PHEVs will in general be those whose driving regimes are most suited
to them, meaning that generalizations based on average US driving
patterns will, possibly greatly, underestimate the amounts of liquid
fuels likely to actually be displaced by a particular model of PHEV.

Now we can finally get to the meat of the matter. GM's Volt is
reportedly capable of driving all three standard cycles, including
the US06, purely electrically. GM states, accurately no doubt, that a
PHEV that cannot do that is really a blended-mode PHEV, with one or
more engine starts during most people's normal driving. The company
goes on to say that only a PHEV with 40 miles of pure EV range (which
it calls an Extended Range EV or ER-EV) can obtain maximum PHEV
benefits. Toyota, who admits that its prototype Prius PHEVs are
blended-mode, does not disagree but says that pure EV PHEVs are too
expensive and not cost-effective.

Let's look at two PHEVs, as much like a Volt and a possible Prius
PHEV as I can estimate based on public data (but both, for ease of
calculation, with a 250 Wh/mile US06 power requirement at the wheels)
and estimate US06 performance. Note, as we explain below, that this
is not an apples-to-apples comparison, since the battery capacity is different:

[TABLE -- see http://www.greencarcongress.com/2008/03/calcars-weighs.html ]

Note that, just as GM claims, the Volt-like PHEV's ICE remains unused
for average daily driving, making the PHEV's benefits very often
perfect: no cold ICE starts, no liquid fuel use, and no ICE emissions
when daily use does not exceed 32 mi. On the other hand, though it
never displaces liquid fuel 100%, the Prius-like PHEV provides
approximately as much fuel displacement per usable battery capacity
(88-117%) as the Volt-like PHEV.

A Volt-like PHEV with a Prius-sized battery could do a better on
daily driving distances up to 16 miles, but at a high cost of double
the relative battery power requirements: 12.5C vs. 6.25C. And
Prius-like PHEV with a Volt-sized battery would make poor use of the
battery capacity below a daily driving range of 48 miles, 160% of the
30 mile US average. This means that different battery sizes have
different optimum PHEV architectures, and each company's claims are
basically accurate, but only for its vehicle's battery size.

Toyota claims that blended PHEVs like its 2.5 kWh-capacity prototype
Prius PHEVs provide more liquid fuel displacement per battery
capacity and power than those like the Volt that have pure EV range,
that a blended-mode PHEV's motor and electronics can cost less, and
that the battery pack may see an easier and therefore a longer life.
What the chart above shows is that Toyota's claim of more
displacement per battery capacity is true only for PHEVs with EV
range less than the US daily average driving distance of 30 miles.
What a blended-mode system can do, with only proportional
disadvantage, is allow the proportional scaling down of battery and
electronics power requirements for vehicles, like Toyota's Prius PHEV
prototypes, with Effective EV Range of less than 30 miles.

Dr. Andy Frank states that the GM and Toyota cost arguments are not
very meaningful at this stage because of unsteady costs due to low
volume production of all parts, especially the batteries.

In conclusion, it is clear that PHEVs with pure EV range of at least
the average US daily driving range of 30 miles can displace the most
liquid fuel, as well as have other advantages like zero tailpipe
emissions in normal daily driving. However, these examples do bear
out Toyota's claims that the relative power requirements of
blended-mode PHEV batteries can be much less than for pure EV
PHEVs--but only for PHEVs with very short Effective EV Range. On the
other hand, Toyota's claim of better utilization of expensive battery
resources can be true, too.

What neither company has stated is that it is following its quickest
and least expensive way to build its first PHEVs by taking advantage
of its own existing hybrid and/or EV technologies and tooling. For
each to do this is highly desirable for all of us. Since each type of
PHEV has its own advantages, disadvantages, costs, and optimum
driving regimes, our expectation is that during the first few
years--maybe a decade--of PHEV production, all types of PHEVs will
compete well in the marketplace. Then, eventually--as batteries
become a cheaper, longer-life, commodity item, liquid fuels become
more dear, renewable electricity generation proliferates, and CO2
emissions are increasingly targeted--the PHEVs with the most EV power
and range will come to dominate.

Dr. Andy Frank:

There is no doubt that it will be completely dominated by the
cost of oil. Remember that the cost of oil doubled in the last five
years and it will double again in less than five years and double
again in even less time! So we can reach $20/gallon in the time frame
that these guys are arguing over. At that time (6 to 8 years from
now) it means an SUV 30 gallon tank will cost $600! This costs will
make all this nit-picking costs argument seem insignificant! I agree
that at this time, let the big guys argue about who is better or more
cost effective, we need to focus on what is good for the people on
earth as the cost of fossil fuel rises.

That is the main reason for the PHEV! To displace fossil fuel
with electricity that can be generated from a plethora of sources
including renewables at a very high efficiency with low to zero emissions!

The Oil companies will eventually throw their wishes into the
pot as well soon. And I think they will be much more vocal because
they have the money! This may be where we should be bracing
ourselves! The [recent] USA today article [inaccurately claiming
PHEVs cause higher emissions] is an example!

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Felix Kramer fkramer@...
Founder California Cars Initiative
http://www.calcars.org
http://www.calcars.org/news-archive.html
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