1. Dec 2, 2008

### dlw902

1. The problem statement, all variables and given/known data

There has been some interest in automobiles that run on compressed air. Let's use thermodyanmics to investigate the claims of air-car entrepreneurs. The air c ar is suppsoed to have a tank with a capacity of 300 liters, at a pressure of 300 bar. While details of teh engine are hard to come by, (likely some type of high efficiency turbine, or perhaps based on hydraulics), lets obtain an upper estimate of the energy output by assumign taht the engine operates by reversible expansion of a monotomic ideal gas. Let's also assume that the initial temperature of the gas is 300K.

(a) How many moles of monatomic, ideal gas are present in the tank?

(b) Assume the gas expands by reversible, adiabatic expansion to a final pressure of 1 bar. What are the fianl volumes and temperature? How much work is perforemd? Assuming an engine power of 1 kilowatt (1000J/s) and a speed of 30 miles per hour, what is the range (how far will the car go on one tank of air)?

(c) A somewhat more optimistic estimate is obtained if we assume that the expanding air can absorb heat from teh surroundings. If we assume this as a reversible, isothermal expansion to a final pressure of 1 bar ( at 300 K) what is the range, also assuming the engine power is 1 kilowatt and a speed of 30 mph.

(d) Of course, we expect the efficiency of an actual air car would be somewaht less than this, and there appears to be a little independent confirmation that such cars would work and be reliable, but based on these numbesr, does this car seem practical for some use such as short communities less than 45 miles?

2. Relevant equations

3. The attempt at a solution

(a) PV=nRT... n=296(300)/(.08206*300)=3,602 moles

not sure how to do the other three :(