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Wednesday, May 13, 2009

The 100+ miles-per-gallon, plug-in hybrid Hummer

Raser Technologies have debuted a Hummer H3 converted into a plug-in series hybrid to co inside with the listing of Raser company stock on the New York Stock Exchange. With the aerodynamics of a house (Cd .43) and a curb weight of 4700 lb (2132 kg) in standard trim, the H3 hardly makes for the ideal vehicle to base a hybrid conversion on. Raser chose the Hummer on the logic that trucks and SUVs are/were the best-selling vehicles in America and can benefit the most from increased fuel economy and reduced emissions. The recent economic slump and fuel price volatility have led to the Hummer brand being put up for sale (with no takers) while SUV sales have stalled with many plants former SUV plants now closed.

The Plug in technology demonstrator has the original 5.3 Liter V8 14 mpg (17.l/100 km) removed and replaced with a much smaller 2.2-liter turbocharged four-cylinder EcoTech engine. Because this is a series hybrid there is no mechanical connection between the engine and the wheels, the EcoTech drives a 100-kilowatt generator only which charges three lithium ion battery packs of 30 kWh battery capacity which Raser say will give an electric mode of 40 miles. Although the engine capacity more than halved the addition of the battery packs, electric motor and generator raises the curb weight by over 20% to 5720 lb (2594 kg).

The standard Hummer four wheel drive automatic transmission and transfer case is retained but moved back in the chassis to accommodate a 200kw AC Induction motor bolted on where the petrol engine would normally connect. Mechanical losses through a 4WD transmission system are in the region of 40% and the use of an automatic transmission pretty much eliminates one of the main benefits of a hybrid, brake regeneration.

To explain briefly, because electric motors have 100% torque from zero rpm with a flat torque curve throughout their rev range they do not need a multi speed gearbox like a combustion engine. As a comparison, the Chevy Volt is a series hybrid where the combustion engine charges the battery and an electric motor provides the drive to the wheels but it has only a single speed reduction gear between the motor and the drive shafts. The reason this is done is the achieve maximum drivetrain efficiency. Each time power is transmitted through a pair of gears 10-15% of the energy is lost to heat. To achieve maximum range in an electric vehicle the fewer number of gears between the motor output shaft and the wheels the more energy efficient the vehicle is. In a 4WD system there are more gear that 2WD cars because of the transfer case gearing and you most certainly don't want to put an automatic gearbox in an EV as they are much less energy efficient than even a manual gearbox. On top of the huge power loss in the transmission the lack of regeneration just makes this hybrid conversion half baked.

While the Raser claims of 100MPG sounds impressive it is only achieved in electric only mode where energy consumption is usually measured in watt-hours / mile. As a comparison an electric vehicle like the Wrightspeed X1 consumes 200 wh/mi which they calculate is equivalent to 170 MPG. Although the X1 is a much lighter vehicle the 5 door Chevy Volt also gets approx 200 wh/mi.

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