01-24-2006, 10:31 PM
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 | Administrator | | Join Date: Oct 2005
Posts: 128
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| The Fuel economy equation
Fuel economy The recent popularity of SUVs is one reason the U.S. population consumes more gasoline than in previous years. SUVs are as a class much less fuel efficient than comparable passenger vehicles. The main reason is that SUVs are classified by the U.S. government as light trucks, and thus are subject to the less strict light truck standard under the Corporate Average Fuel Economy (CAFE) regulations. The CAFE requirement for light trucks is an average of 20.7 mpg (US), versus 27.5 mpg (US) for passenger cars (11.4 and 8.6 L/100 km, respectively). As there is little incentive to change the design, SUVs have numerous fuel-inefficient features. The high profile of SUVs increases wind resistance. The heavy suspension and large engines increases vehicle weight. SUVs also often come with tires designed for off-road traction rather than low rolling resistance. The more car-like SUVs tend to have a somewhat lower profile and better road performance tires, but often still have large, fuel-inefficient engines. The low fuel economy is caused by- high parasitic masses (compared to the average load) causing high energy demand in transitional operation (in the cities)
 where P stands for power, mvehicle for the vehicle mass, a for acceleration and v for the vehicle velocity. - high crossectional area causing very high drag losses especially when driven at high speed
 where F stands for the force, Across for the crossectional area of the vehicle, ρair for the density of the air and vair for the relative velocity of the air (incl. wind) - high rolling resistance due to all terrain tires (even worse if low pressure is needed offroad) and high vehicle mass driving the rolling resistance
 where μroll stands for the rolling resistance factor and mvehicle for the vehicle mass.
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