The 100+ miles-per-gallon, plug-in hybrid Hummer
May 3, 2009 Raser Technologies has debuted a Hummer H3, converted into a plug-in series hybrid, to coincide with the company's listing on the New York Stock Exchange. With the aerodynamics of a house – it has a drag coefficient of .43 (Cd .43) – and a curb weight of 4700 lb (2132 kg) in standard trim, the H3 hardly makes an ideal vehicle to base a hybrid conversion on. Raser chose the Hummer because trucks and SUVs have been among the best-selling vehicles in the US and stood to benefit 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 SUV plants now closed.
The plug-in technology demonstrator has the original 5.3-liter, 14-miles-per-gallon (17.l/100 km), V8 engine removed and replaced with a much smaller 2.2-liter turbocharged, four-cylinder EcoTech engine. As 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. This, says Raser, will give an electric mode of 40 miles. Although the engine capacity more than halved the addition of the battery packs, the electric motor and generator raises the curb weight by more than 20 percent 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 normally connects. Mechanical losses through the 4WD transmission system are about 40 percent and the use of an automatic transmission mostly eliminates one of the main benefits of a hybrid, brake regeneration.
To explain briefly: because electric motors have 100 percent 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. However, it has only a single-speed reduction gear between the motor and the drive shafts, the same transmission layout as used in the Tesla model S and Roadster.
This is done to achieve maximum drive-train efficiency. Each time power is transmitted through a pair of gears 10-15 percent of the energy is lost to heat. To achieve maximum range in an electric vehicle (EV), the fewer number of gears there are between the motor output shaft and the wheels the more energy-efficient the vehicle. In a 4WD system, there are more gears than 2WD cars because of the transfer case gearing; and you certainly don't want to put an automatic gearbox in an EV, as it is much less energy-efficient than 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 Raser claims of 100 mpg sounds impressive, it is only achieved in electric-only mode, where energy consumption is usually measured in watt-hours per mile. As a comparison an EV such as the Wrightspeed X1 consumes 200 wh/mi, which is equivalent to 170 mpg. Although the X1 is a much lighter vehicle, the five-door Chevy Volt also achieves about 200 wh/mi.