How to Calculate Energy Lost in a Moving Car

Every drop of gasoline pumped in here ultimately ends up as heat in the environment.

The easiest way to characterize the energy lost in a moving car is that ultimately, all of its energy is lost as heat, according to the Second Law of Thermodynamics. This is further evidenced by the fact that if any car's energy source is removed, it will come to a dead stop in a short time. Published statistics show that a scant 5.8 percent of all energy generated by a car's engine actually ends up propelling the vehicle, and even that is dissipated as heat as the brakes bring the vehicle to a stop.

Things You'll Need

• Calculator

Instructions

Calculating Total Energy Applied

1. 

   Vehicles leave all the energy behind as heat.

   Define the automotive energy applied-to-lost scenario. In this example, a 3600-pound car moving at 60 miles per hour burns a gallon of regular fuel in 30 miles. From this you can calculate the total amount of energy applied to moving the car.

2. Look up the amount of energy in one gallon of regular gasoline. Published values average 115,000 British Thermal Units per gallon (Btu/gal) which can be converted to horsepower (hp) by dividing 115,000 Btu/gal by 2,545 Btu/hr/hp = 45.18 hp per hour. Dividing 45.18 hp by 0.746 kW-hr/hp = 60.57 kW-hr (kilowatt-hours), which expresses the power in electrical unit equivalency.

3. Calculate the total energy applied to the car's engine by the fuel in one hour by doubling the power expressions from Step 2 to account for the 2 gallons of gasoline burned in the 1-hour period traveling at 60 miles per hour. So, 115,000 Btu/gal X 2 gal = 230,000 Btu. The engine uses the 2 gallons of gasoline to produce 45.18 hp/gal X 2 = 90.36 horsepower. Converted to electrical terms, 90.36 hp/0.746 kW-hr/hp = 121.14 kW-hr.

Calculating the Energy Lost

1. Determine each of the areas representing the greatest losses in driving the car from published research data. According to the U.S. Department of Energy, most of the power is lost in the engine itself at 62.4 percent, idling and standing at 17.2 percent, driveline losses at 5.6 percent, accessory loss at 2.2 percent, with only 12.8 percent making its way to the drive wheels. Of this, rolling resistance of the flexing tires eats up 4.2 percent, and aerodynamic drag 2.6 percent. This leaves only 5.8 percent as inertia (moving the car), which is entirely absorbed by the brakes later and dissipated as heat.

2. Calculate the amount of energy lost in the moving car by multiplying (100 percent -- 5.8 percent inertia) = 94.2 percent lost. On the basis of 2 gallons of gasoline, (128 ounces x 2) X 5.8 percent, only 14.8 ounces of the 256 original ounces supplied ever go into providing motion, which means that 256 minus 14.8 = 241.2 ounces is lost as heat.

3. Convert to other energy units by multiplying the total energy applied terms from Section 1 of 230,000 Btu, 90.36 horsepower, or 121.14 kW-hr by 0.942 (94.2 percent expressed as loss fraction) this is 85.12 hp lost, 216,660 Btu lost, and 114.36 kW-hr lost.

Tips & Warnings

• You can use the published figures contained herein to calculate the individual contributions of each of the car's loss components.

• Soft tires, clogged filters and poorly tuned engines can make these numbers even worse.

References

• Oak Ridge National Laboratories: Bioenergy Conversion Factors/Fossil Fuels
• California Energy Commission/Consumer Energy Center: Energy Losses in a Vehicle
• U.S Department of Energy/fueleconomy.gov: Advanced Technologies and Energy Efficiency
• Argonne National Laboratory: Parasitic Energy Loss Mechanisms Impact on Vehicle System Efficiency

Resources

• McMaster University: Analysis of Gas-Electric Hybrid Fuel Efficiency