A string of recent vehicle announcements contain miles per gallon fuel efficiency claims that are causing a bit of a stir. The Nissan LEAF, over 300 miles per gallon, the Chevy Volt, 230 miles per gallon (in city driving), and now Raser Technologies is claiming to have built a Hummer that gets 100 miles per gallon. Hummer? Yes, the vehicle emblematic of inefficiency being converted to a hybrid electric drive with 100 miles per gallon fuel efficiency. Makes one think of the LincVolt project where another emblem of inefficiency, the 1959 Lincoln Continental, is being reworked to have a hybrid electric drive train with 100 miles per gallon efficiency.
Understanding a couple factoids helps to understand these claims. Obviously the Nissan LEAF, being pure electric (no gasoline) does not have a fuel efficiency that’s properly represented in miles per gallon. Instead for electric drive trains the better fuel efficiency measure is kilowatt-hours per mile. However most people are accustomed to miles per gallon as the fuel efficiency measure. Portraying electric or plug-in hybrid vehicle fuel efficiency in miles per gallon terminology makes it familiar to people, but it isn’t terribly accurate and has room for fudgery. The more complex scenario is the plug-in hybrid vehicles like the Volt where some of the energy is electricity coming from the grid, and the other energy is gasoline coming from a refinery. The correct conversion of a mix of electricity and gasoline to a single measure of energy efficiency (not fuel efficiency) is a matter of debate.
Raser is very up-front about how they claim 100 miles per gallon. Someone driving 60 miles per day will drive the first 40 on electricity, and the next 25 on gasoline, consume 0.6 gallons of gasoline which equals to 100 miles per gallon. Someone driving less than 40 miles per day has “unlimited” miles per gallon. It’s technically true that one who never uses gasoline has unlimited miles per gallon. But it completely discounts the energy equivalency of the electricity.
A truer way to calculate this fuel economy would be the gallons of gasoline equivalent in the electricity consumed, plus the actual gasoline used.
Their Hummer is a demonstration vehicle for their Series PHEV Drive System. The drive system accommodates several sizes of ICE (internal combustion engine) and battery pack, and includes the controller, traction motor, and electric generator components. In the Hummer the ICE is 2.2 liter in size versus the stock 4.7 liters, the smaller engine is all that’s required to power the on-board 50 kilowatt generator. Further they describe the on-board generator as being sufficient to power a whole construction site. It is clearly meant to be installed in a range of light to medium duty trucks and the target market is hybrid vehicles for fleet use.
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