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The cost of


The world is driven by energy. It is what makes the world go 'round. The energy needed to move a vehicle from location to another depends on the weight, size and shape of the vehicle, the terrain over which it is driven, and the vehicle's speed. The energy to do this is available from many sources. Some sources of energy cost more than others. The cost varies with supply and demand. The object of this web page is to compare the cost of energy from various sources. The energy content of most fuels is measured in BTUs [British Thermal Units]. One kilowatt-hour is the same energy as 3413 BTUs. One Horse Power-Hour is the same energy as 2545 BTU.

The problem is, the conversion from one form into another is a lossy operation. To convert hydrocarbon fuels into mechanical energy requires the use of an internal combustion engine [ICE]. The form most of us are familiar with is the "gasoline engine" that uses the otto cycle. It is not very efficient in normal use when only a small fraction of the maximum available power is needed. During this time, only about 20% of the energy in the fuel is converted into useful work. [At 90% power, efficiency is closer to 25%.]

A plain diesel is more efficient at about 30%,

A turbo-diesel is much more efficient at part throttle, somewhere around 35%. It is because of this high part throttle efficiency that makes the turbo-diesel a favorite engine of large trucks. [At 90% power, efficiency is closer to 40%.]

Conversion of electric power into mechanical power is the most efficient. DC electric motors often have efficiencies greater than 85%. AC induction motors often have efficiencies greater than 90%. Other losses in electric vehicles include the controller and the batteries. A 120 volt lead-acid battery pack would be charged at 135 volts. During operation, under heavy load, that battery pack migh only deliver 100 volts. Obviously that would mean getting back less than 74% of the energy put into the battery. Under a more moderate load, that battery pack would probably deliver about 120 volts for an efficiency of about 88%.

In the table below, June 2005 prices are shown for electric power from SDG&E, the posted prices from the Regional Transportation Center, and a typical gasoline price in Arizona. Note that Arizona gas has more BTU per gallon because it is all hydrocarbon. [No oxygenates] The large amount of oxygen in the fuel is the reason for the much lower BTU content of ethanol.

Fuel when $/U1 U1 BTU/U2 U2 U2/U1 $/MBTU Efficency $/hp-hr comments
Electric Power - SDG&E Jun-05 $0.153 kw-hr 3413 kw-hr 1.00 $44.83 75% $0.152 battery & motor loss
Natural Gas - SDG&E Jun-05 $1.232 therm 100000 therm 1.00 $12.32 20% $0.157 internal combustion
CNG 90+% methane Jun-05 $2.049 5.66 lb 22813 lb 5.66 $15.87 20% $0.202 internal combustion
LPG 90+% propane Jun-05 $1.689 Gallon 20772 lb 5.00 $16.26 20% $0.207 internal combustion
Diesel Jun-05 $2.499 Gallon 18800 lb 7.94 $16.74 30% $0.142 turbo-diesel
Bio Diesel Jun-05 $2.679 Gallon 18800 lb 7.94 $17.95 30% $0.152 turbo-diesel
E85 85% Ethanol Jun-05 $2.509 Gallon 11620 lb 6.84 $31.57 20% $0.402 internal combustion
Premium MTBE gasoline 91 Jun-05 $2.749 Gallon 17000 lb 6.15 $26.29 20% $0.335 internal combustion
Super MTBE gasoline 89 Jun-05 $2.629 Gallon 17000 lb 6.15 $25.15 20% $0.320 internal combustion
Regular MTBE gasoline 87 Jun-05 $2.509 Gallon 17000 lb 6.15 $24.00 20% $0.305 internal combustion
Reg. non-oxygenated gasoline Jun-05 $2.229 Gallon 20750 lb 6.15 $17.47 20% $0.222 Arizona gas

This chart is an attempt to calculate the most cost effective source of energy for vehicle propultion. Hopefully the calculations are correct.

Having determined the cost of energy from different sources, the next step is to determine the energy needed by a vehicle. Instantaneous energy use is power. In other words, power is the rate that energy is used. Power is the product of the force needed to push a vehicle and the speed of the vehicle. The speed is given by the speedometer. The force can be determine by how fast the vehicle slows down with the engine disconnected by selecting 'neutral' as the gear. This can only be done safely on a road without other vehicles. It should never be done in traffic. It should be done on a level road if an average power for a given speed is needed. If 'DS' is the desired speed, accelerate to "DS+11' mph, put the car in neutral and see how long it takes to coast to 'DS-11' mph. Divide the weight of the vehicle by this time in seconds to get the force in pounds needed to maintain a steady speed of 'DS'. The product of this force [lbs] and the speed [mph] divided by 375 is the horsepower needed to maintain a constant speed of 'DS'. The product of horsepower, time [hours] and '$/hp-hr' [see table] gives the cost of running your vehicle at speed 'DS', on the road testted, for the amount of time given. Note that in real life, the power changes with speed and road conditions.

In looking at the cost of moving vehicles, heavier vehicles cost more to run, it costs more to drive fast, and gasoline has become one of the more expensive sources of energy.