AC Propulsion Battery Management System moduel
AC Propulsion VP of Engineering Paul Carosa holds BMS module that monitors the condition of individual cells in an electric car's battery pack. This is the module that E-Drive is testing for implementation in future plug-in Prius conversions.

3-Speed Prius

E-Drive's Greg Hanssen explains the why and how of a plug-in Prius' all-electric operation

By Bill Moore

I have long been under the impression that a Prius will only operate in all-electric mode up to around 35 mph and that beyond that point, the Atkinson-cycle gasoline engine turns on and you're quickly back to burning petroleum, albeit leanly and cleanly, but there's still emissions coming out of that tailpipe.

It turns out that I have been wrong: imagine that.

I recently bumped into Greg Hanssen while visiting AC Propulsion's (ACP) shops in San Dimas, California. They're located at the end of a curving culdesac, the entrance to which is guarded by a high performance car conversion shop -- talk about being poles apart on the same street. Greg was there testing ACP's other product, its microprocessor-based battery management system or BMS. While the company is best known for its alternating current electric drive and control system -- and more recently, its amazing eBox (which I'll report on later) -- they also sell clients like E-Drive the BMS, which monitors individual cell voltage and temperature of battery packs used in electric vehicles like their eBox and E-Drive's plug-in Prius conversions.

Greg took me for short spin around the culdesac in his own modified Prius, this one utilizing NiMH batteries instead of lithium. He explained to me that a Prius can actually drive faster than 35 mph (technically its 34 mph) without its engine burning fuel, though as he explained, the engine will be turning over and for good reason.

First, a little background on Greg. He's been hacking into automotive software programs since the days of the EV1, when he successful figured out how to read and interpret GM's computer controls. He was instrumental in helping develop the first practical plug-in Prius by cracking Toyota's control codes. His efforts -- without Toyota's blessings -- and those of his associates, have played a critical role in demonstrating the feasibility of plug-in electric hybrids, which companies like GM, Ford and others are now starting to embrace.

Bottom line: he probably knows as much about the inner workings of the Prius than anyone at Toyota.

As we finished the loop around the culdesac, he explained the Prius has, in effect, three speed regimes: 0-34 when the car is being powered only by its electric motor and battery, 35-to-mid-40ish, when it can switch back to zero emission mode once the catalytic converter has warmed up, and mid-40ish to around 54 mph when the engine is actually turning over, but there is no combustion taking place.

How can the engine be turning and not burning gasoline, you ask? Apparently the valves are left open to reduce mechanical resistance as much as possible, but no fuel is being pumped into the injectors. According to Hanssen, spinning the engine with the electric motor is necessary to keep the motor, the internal generator and the IC engine rotating in sync. That makes sense since you want to have it ready to respond in a fraction of a second and not from a dead stop, which could be catastrophic.

Hanssen told me that he's seen his own Prius running along level ground at close to 55mph with the engine spinning, but burning no fuel, though climbing small rises in the road will initiate engine ignition. In effect, the car is "pulsing" down the road running on electric power, then briefly on gasoline and then back to electric power. With the plug-in conversion that upgrades the car's smaller 1.4kW battery closer to 9kW, this translates into fuel economies of 150 mpg and better since very little fuel is actually being burned. The car's extremely low coefficient of drag helps too, requiring less energy to overcome wind resistance.

Achieving this level of performance is harder in the unmodified Prius because its smaller battery doesn't offer the same reserve of electric energy with which to propel the car. Once the battery starts to deplete towards the lower reserve limit of 40 % state of charge, the gasoline engine will ignite to propel the car and recharge the battery.

So, there you have it -- as best I can remember it. It is possible to drive a Prius in zero emission mode faster than 34 mph. In fact, assuming you buy an E-Drive conversion someday -- and Hanssen is working on it despite some recent complications -- you'll be able to drive at freeway speeds in EV-only mode a fair share of the time; saving fuel, saving the country, saving the planet.

Times Article Viewed: 15312
Published: 03-Mar-2007


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