Recharge Plug-In Prius With Gasoline? Why?

By Bill Moore

Posted: 16 May 2010

Recently, a blogger on another web site wondered why he couldn't recharge the battery pack in Toyota's PHV Prius from the car's gasoline engine. Certainly, it's feasible. The standard, off-the-showroom-floor model already does that in order to maintain the battery pack's state-of-charge in what is called "charge-sustaining" mode. Ideally, of course, you want to use the car's kinetic energy (momentum) and regenerative braking to do the task. Using energy from the car's engine only reduces its overall fuel efficiency.

That's certainly the case with LIVN GRN, our 2009 converted plug-in Prius. It is possible, in fact, to recharge our 6.1kWh battery pack on the fly, so-to-speak, while driving. I have not done so for a couple of reasons. First, I have to connect my laptop computer to the car and run a special program that talks to the Prius' engine control system, which is, as the Brits would say, "a bit of a bother."

More importantly, even if it weren't a hassle to do so, it doesn't make economic sense. Here in Omaha, we pay just 6.5¢ per kilowatt hour of electricity. That means it takes, roughly, 4 kW to recharge the pack from its depleted PHV/EV mode. At this point, there is still nearly 25% of the Goldpeak NiMH pack's energy left in the battery. This enables the car to function like a normal, charge-sustaining hybrid once we pass its theoretical 20 mile PHEV range. That means it costs us 25¢ worth of electricity to recharge the car when fully depleted from PHEV mode.

Now let's assume that we wanted to charge it while we're driving. Nationally, the average price of a gallon of gasoline this week here in America is about $3, and heading north of that as we enter the peak summer driving season. There is approximately 32kWh of equivalent energy in a gallon of gasoline (petrol). That works out to be approximately 9.3¢/kWh; not a huge difference between what I pay locally for electricity, but enough to sway me to use grid power instead of petrol power.

Psychologically, too, once you begin driving a plug-in hybrid, your perspective begins to change, not necessarily for economic reasons, but for more intangible ones: you really start to hate seeing the car's fuel economy sink into the 50-60 mpg range. Even 70 mpg is a bit of a disappointment.

Last month, I had to use LIVN GRN for a couple trips out of the city that meant the car operated more in conventional hybrid mode, so when we refueled the car a couple days ago, after driving 688 miles for nearly a month on just over 9 gallons of fuel, I calculated we'd gotten just over 73 mpg. But that was down from the previous month when we averaged just over 75 mpg. I was disappointed. Seriously, I was. Now most people would be drooling with envy to get that kind of fuel economy. Since we don't have any long trips planned for the coming month, I am hoping we can improve on that 75 mpg average.

Even my wife, who drives the car daily, is disappointed when she doesn't come home with the display maxed out at 99.9 mpg; and she's looking forward to the next upgrade of Plug In Conversion Corporation's system what will move their system display from the cumbersome laptop to the Prius central display screen. Then she can see what her actual MPG was each day, which should be well into three-digit numbers.

So, while it's entirely possible for us to recharge our PHEV Prius battery pack while we're driving, we know it will dramatically impact our average fuel economy and we just don't want to see 35-40 mpg on the display as we do so. There might be circumstances where we'd want to charge up the pack while driving, say while visiting relatives in Kansas City where we might not be able to plug-in the car overnight, and are planning to do some local driving the next day, such as driving over to the Plaza or Electric Light District for lunch. But as a general rule, on a day-by-day basis, we just don't see the need to use gasoline when grid power is so much more convenient and economic in our situation.

Finally, as to the issue of CO2 emissions and air pollution, Toyota's Atkinson-cycle engine is an amazingly clean power plant. I don't know how it compares to average grid power, but they are likely pretty comparable by the time you take grid distribution system losses into consideration, as well as the emissions generated on the oil refinery side of the supply chain. It would be interesting to compare the emissions per kilowatt hour of their engine versus the power grid in my part of the country where coal accounts for 65% of power generation, but that'll be an exercise for another time.

For now, I'll happily plug into American-made power, thank you.

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