CalCars Weighs In on GM Series/Toyota Parallel PHEV Debate

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:

<>

<>

<>

<>

<>

<>

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.

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!

—Dr. Andy Frank