At 10:05 AM 3/30/5, Jed Rothwell wrote: >30 hp, by the way, seems a little low even for a lightweight electric car, >based on the performance of my 40 HP Geo Metro. I think you need more like >70 to 100 HP, even with a light, aerodynamic car. The Honda Insight has a >73 hp engine and a 10 kW electric motor. The Toyota Prius has 110 hp, >gasoline-electric combined. [snip]
Gasoline is roughly 20,000 Btu/lb, and 6 lbs/gal, thus has about 120,000 Btu/gal. A car that gets 30 mi/gallon at 60 mi/hr uses 2 gallons per hour, or 240,000 Btu/hr = 94 hp. However, if the motor is only 30 percent efficient, the motor is really only putting out about 21 hp. A car or motorcycle that cruises at 60 mi/gallon at 30 percent efficiency is using only about 10 hp to cruise. For some reason I just find this fascinating. The extra horsepower, which is indeed needed for normal and safe driving, is really needed primarily for accelerating and hill climbing. This is obviously one of the reasons hybrids work so well - they can combine energy sources when a power boost is needed, but cruise on a smaller motor. Another reason is that electric motors are not Carnot limited, so can do a lot better than 30 percent efficiency. The problem is to get the recharge capability running at high efficiency, so that's where fuel cells enter the picture for hybrids. With regard to hill climbing, most journeys are closed circuit, so for each up meter travelled there is a down meter, and regenerative breaking combined with freewheeling coasting should be able to recover hill climbing energy using even moderately fast recharging batteries, unlike the eneryg dissipated in high energy fast breaking. Good driving habits could thus substantially affect the milage that can be obtained from fast recharge battery regenerative breaking. You have pointed out that automated transportation systems (automated cars, etc.) are the way to go. Such a system would eliminate the safety requirements for high vehicle horsepower and result in a much more efficient transportation network. In other words, lead footed idiots really burn the fuel. Regards, Horace Heffner
