The Automobile


U.S. Energy consumption by sector

U.S. Energy consumption, by economic sector

U.S. Transportation energy

U.S. direct transportation energy consumed by each transportation mode.

Internal combustion engines

Mustang150 kW internal combustion engine.

Best theoretical engine efficiency ~ 30%.

But typically, 15-20%.


Energy flow for gas-fueled car

What is the efficiency of this car?

eff=10/70 = 1/7 ~14%

Electric cars

Comuta-Car... in Goshen

Is an electric car a zero-emission vehicle? If not, where are the emissions happening?


Comuta-Car'electric' mileage: $(5 mi) / (kwh)$ works out to 103 miles for the 2009 cost of 1 gallon of gas.

$CO_2$ emissions: equivalent to a 48 mpg gasoline car.

Top-speed: ~35 mph (30 mph in the winter).

Range: 40 miles fully charged. (Average 2nd car in the U.S. driven 27 miles per week).

Conversion of ElectricE -> MechanicalE in a DC motor: >90% efficient.

Cost in 1993: used 1980s model ~$2500.

Cost of replacing batteries ~ $800.

2010: Chevy Volt estimated to cost $33,000 after rebates has a small gas engine (to charge batteries) as well as batteries.

Cost of replacement batteries... unknown. For Honda Civic hybrids, costs range from $2,000-4,300.


Efficiency - revisited

Prius vs Hummer

Is there a better way than "miles per gallon" to measure transportation efficiency?

Camello vs Prius

Particularly if you want to take into account how many people are being transported?

Choose between mpg / # of people or # of people $\cdot$ mpg ...?



people $\cdot$ miles per gallon is the measure of efficiency we want! It's a number that gets larger the more people we put in the vehicle.

Comparing transportation


* The energy content of gasoline is converted to food Calories to figure the energy efficiency of humans.

Sample test question

"Which is more efficient: 3 people driving in a van that gets 21 mpg, or 1 person driving a Prius that gets 48 mpg?"

The van is getting 3 passengers $\cdot 21 mpg$
$ = 63 $(passenger miles) $ / (gal)$.

The Prius is getting 1 passenger $\cdot 48 mpg$
$ = 48 $(passenger miles) $ / (gal)$.


Sample life question

"Which investment is more likely to save more energy: buying a 48 mpg Prius or buying a house within less than a mile from where you work?"

Moving freight

The equivalent efficiency criteria for moving freight is to multiply the "gas mileage" times the weight (or, ahem, mass) moved.

Why might the barge come out the best?

* TDM encyclopedia from

Animal kingdom

Comparing many different animals by the number of kg $\cdot$ km they can move on 1 MJ of energy

Maybe that's why he's smiling?
Einstein bicycling

Small group discussion:
  * Other than technical improvements to vehicles, how could you reduce the amount of energy you personally expend on transportation?   * Which of these suggestions involve *no* negative "quality of life" impact?

Steam-electric plant

80% of electricity is produced in steam-electric power plants. This includes both coal- and nuclear- plants.

Steam turbine

The steam expands in several stages as it moves through the turbine blades.

CO2 emissions from coal

Howstuffworks addresses the cost of powering one lightbulb. They find that when burning 714 lbs of coal, a total of 1852 lbs of $CO_2$ is produced. How is it possible to have a greater weight of $CO_2$ emitted tham the weight of what we started burning??

$CH_n + $ several$ O_2 \rightarrow CO_2 + $ several$ H_2O$

Carbon comes from coal, Oxygen comes from atmosphere.

Energy flow: electric generator

Efficiency : 900 MW / 2500 MW = 0.36 = 36%


Efficiency: (900 + 1200)/2500 = 84%

Also called "Combined Heat and Power" (CHP).

Mirant Kendall Cogeneration Station (1949), in Cambridge, Massachusetts.

Suggested Exercises

Conceptual exercises, Chapter 7: 25, 27, 28, 30, 33, 35

Image Credits

Chad Conway, MissMass, Fletcher