Energy in Cars

Cars are a common feature of our life here in the US (even if not everyone drives) and the same is true of Europe. Because Europe operates on the metric system it is sometimes hard to make sense of the numbers that David MacKay uses in his book. Part of what I want you to do on this homework is consider how those numbers look here in the US as well as some details of how energy is consumed by cars.

Conversion Constants: It may be helpful to know/remember that there are 36.6 kWh of energy in an standard gallon of gasoline. It may also be helpful to know that there are 1.6 km in a mile and 3.8 liters in a gallon.

1) In this country we tend to think of the efficiency of a car in terms of mpg (which means miles moved for each gallon of fuel). David bases his calculations on 12 km/liter. How many miles/gallon is this? Another chance to practice unit conversion from previous weeks.

2) David also assumes that each person who has a car drives it, on average, 50 km/day. How many miles is this and how many miles is this during the whole year?

3) Here is a link to the data for a VW Golf TDI. This is a small car with a diesel engine. Find the rating for the highway miles/gallon and calculate the kWh/d used by this car if you drive it 50 km a day? You will have to dig through the information on this site to find what you need.

4) Here is a link to a database that shows a whole raft of information about vehicles and drivers in this country. The second table down has a column at the right hand edge which tabulates the miles driven by each licensed driver. What is the annual number of miles driven by licensed Oregon drivers in this database? What is the average number of miles/day driven in Oregon by such a driver? How does this compare to the 50 km/day that David MacKay uses in his book?

5) In the same database from the previous problem it gives (in the very first table) the number of drivers in Oregon. How many miles a day do Oregon drivers drive on average? If the average car gets 25 miles/gallon how many gallons of gasoline does Oregon use each day?

6) If your car only uses 25% of it's energy to move the car forward how much of the energy in a gallon of gasoline is actually used to move the car? Assuming electric motors are 90% efficient how much electric energy (kWh) do you need to provide to move the car the same distance as a gallon of gasoline? Remember that the energy used to move the car is the same in each case so start by figuring out how much of the energy in a gallon of gasoline actually is used to move the car down the road. Then ask how much electric energy (kWh) do I need to put in so that 90% of my input is the same.

7) Why would a 'jet fighter' style car get better gas mileage on the highway? Asked another way, what makes air resistance get smaller? I'm asking you to be clear about cars like the VW 1 liter concept car are seeking to reduce the effect of air drag. Remember our discussion of the various terms in the air drag equation. The link is in the breadcrumbs.

8) Consider the plots that we looked at for cars and bikes from the tech chapter on cars. An important difference between cars and bikes is that a car is mostly vehicle and a little bit passenger while a bike is mostly passenger and a little bit vehicle. Of course the car takes a lot more energy to go 100 km which makes an honest comparison difficult. A car has a mass ('weight') of 1200 kg which is the same as about 12 bikes with rider. Divide the numbers on the car plot by 12 and compare them to the bike. Is the bike really that much more efficient than a car when you look at it this way? What do you think this means?

9) This is just a very complicated unit conversion problem. See if you can set it up. Riding a bicycle is another way of getting from place to place. It takes about 50 Wh/10 min (calculator) to ride a bike at 10 miles/hour (convert to km/hr?). How many kWh does it take a cyclist to ride the same 50 km that a typical driver goes each day? How does this compare with driving a car and the previous question?