Fuel and Power

Fuel efficiency isn't just about miles per gallon; it's about using power wisely.


Wow, $3.50 a gallon diesel. That was a shocker. Fuel economy has taken front burner in the public eye. Of course if you run a fleet it has never left the front burner. Fuel has always been the number one operating cost. At these recent prices the fuel cost approaches the labor cost for the driver!

To manage fuel there are some basic questions that you need to know the answer to:

  1. What are my total fuel purchases?
  2. Where do I purchase my fuel?
  3. What are the different prices by source?
  4. How much do I use?
  5. How much is missing?
  6. Why don't I fuel In-house?
  7. What is my fuel specification?

Simple arithmetic can show the savings: 100,000 miles per year at 6.0 mpg cost about $46,000 (not including fuel taxes). Last year it cost about $32,000.

Every 2 percent reduction in consumption is worth almost $1,000 per year per vehicle.

Thank Jimmy Carter for the basic research. Much of the data for this section is derived from materials generated from the US government's Voluntary Truck and Bus Fuel Economy Improvement Program (VTBFEIP). This program was set-up under President Carter during the last oil crisis.

This program conducted research and disseminated information from manufacturers, users and associated parties. All parties involved published their own studies on the influences of fuel consumption and the components of a fuel savings program. (Unfortunately this valuable source was cut out of the Federal budget and has been discontinued. Too bad, because with 30 additional years under their belt who knows what our efficiency would be?)

The VTBFEIP and its members commissioned excellent research into the factors of fuel consumption and thoroughly investigated fuel savings techniques and devices. We will first review the factors of fuel consumption.

Power: Fuel is consumed to generate power to turn the engine. Power is needed for four purposes:

1. It takes power to overcome inertial resistance to start the unit moving, or to accelerate the vehicle to needed speed. Power to overcome inertial resistance is determined by acceleration and gross mass. In addition to standard inertial resistance there is grade resistance. The amount of power required for long constant grades is:

HP Grade = 0.03645 * GM * G * V Where GM is gross mass, G is grade in %, and V is velocity

Note: This is a metric formula, expressed in Metric Tons, KM/HR.

2. Friction causes rolling resistance. Rolling resistance is the tendency for the unit to slow down unless engine power is applied. It is the internal resistance of the bearings, drive-train friction and the visco-elastic resistance of the tires against the pavement. The vehicle mass, road surface, and design all impact rolling resistance.

3. As the unit moves faster aerodynamic drag resistance (wind resistance) becomes the big factor. At 63 MPH wind resistance consumes 50 percent of the available power. The standard square shape (contrast truck shapes to airplanes or speed boats) requires high amounts of fuel to merely part the air. The height, width, shape and speed determine this resistance. Doubling the height or width (for the same shape) will double the wind resistance. Speed of the vehicle (or a head wind) has a major effect. Doubling the speed will increase the drag by a factor of eight.

4. Accessories include air conditioning, thermostatic fans, alternator, air compressor, power steering pump, etc. These items' use of power varies with engine speed (rather than Road speed). The total of these loads varies from 3-4HP under light conditions to a maximum of about 35HP.

The total of the four factors is the required power to do the job. The margin between the power needed and the power available is apparent for hills and acceleration. Note that the margin decreases as speed increases. Items that will impact fuel consumption will impact power. The sum of 1, 2, 3, and 4 is the minimum total power requirement.

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