On the Wings of a Beetle: ~60+ mpg
14 December 2004
Ernie Rogers, an aerospace engineer, set out on a mission “to promote fuel conservation and reduce greenhouse gas emissions by researching, developing and commercializing novel technologies that simply and cost-effectively improve the efficiency of cars and trucks.”
One of those technologies or approaches is the addition of a simple aerodynamic drag reducer. Adding such a device (picture to the right) and slightly oversized, low rolling-resistance Michelin Energy MXV4 S8 tires to his 2003 VW Beetle TDI diesel resulted in an average 58.8 mpg in extended, high-speed highway driving. The EPA rates the diesel Beetle at 49 mpg highway. Here are his latest specific results:
|Fuel Consumption w/ Drag Reducer and Low Resistance Tires|
|Date||Miles Traveled||Fuel (gal)||MPG||l/100 km|
|2 Nov 04||797.6||13.71||58.2||4.04|
|11 Nov 04||793||13.37||59.3||3.96|
|24 Nov 04||786.7||13.45||58.5||4.02|
|8 Dec 04||792.5||13.37||59.3||3.96|
Gasolines have freezing points well below even the most severe winter conditions. Diesel fuels, on the other hand, have both pour points and cloud points (the temperature at which a cloud or haze of wax crystals first appears and separates from the fuel) well within the range of cold temperatures at which they might be used. To guard against this, refiners change blending formulas between summer and winter, working from an ASTM specification for fuel composition based on seasons and latitudes. The cold weather fuels have a lower density and carry slightly less energy—hence winter fuel economy suffers slightly by comparison with a summer blend.
Because the results above are winter results, Rogers estimates he can improve that result by approximately 10% with a summer blend, up to some 64–65 mpg.
Adding the drag reducer lowered the Beetle’s coefficient of drag to about Cd=0.28 from Cd=0.38, and resulted in an increase in fuel economy by 5% to 8% depending on speed. The new tires kicked in another 6% improvement or so. (Rogers says that his baseline highway mileage was approximately 52.5 mpg—higher than EPA figures.)
The story is further evidence that there are numerous different and incremental approaches to reducing fuel consumption that, when aggregated in a vehicle, can provide substantive benefit. I’d say a 12% increase is pretty good. Not everyone can design and install their own drag reducer. But low rolling-resistance tires are definitely an option.
Salon article on the Beetle.
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