@Alfredo: some comments on your previous post-
I once tried calculating the caloric efficiency of a human on a bicycle, and I recall that the number came out somewhere in the low 20% range,
I've heard that the best one can expect to get from an internal combustion engine is 25%. I had also heard that a bicycle is the best widely available machine for converting human power to kinetic energy. (I don't want to imagine the calculation you tackled to get your above number)
Besides the efficiency of the gasoline engine, there are other factors that conspire to kill gas mileage:
* People tend to like to accelerate quickly from stops, and getting the car up to speed takes much more energy than maintaining a constant speed.
As you indirectly point out, if people didn't have the "need for speed", or at least the need for quick pick-up in their vehicles, then smaller and more economical engines could easily replace what we now use. Most of the power (and inefficiency) designed into an engine is for snappy acceleration.
* Many people would rather use air conditioning in the summer than open the windows or just ignore the heat, and the A/C compressor uses a lot of energy.
This concept may have become outdated. When cars weren't aerodynamic and engines were inefficient pigs, an open window made little difference in mileage. Today, with cars being wind-tunneled for near-zero resistance, and engines and compressors being ultra efficient, it's actually becoming more economical to keep the windows up a all times and run the AC if it gets too hot.
* In some cars, engaging the windshield defroster automatically engages the A/C compressor (to dehumidify the air).
Despite a degree of fuel use, this is a safety feature that I wouldn't be willing to see discontinued.
* Americans love their automatic transmissions. Automatic transmissions lose power in the torque converter, and they limit how the driver can maximize use of the car's momentum.
Again, what was previously common knowledge is being upended by computers and engine/transmission design. Fuel efficiency with automatic transmissions today matches and often exceeds that of stick shift cars. Computers can identify the torque curve of the engine much more accurately than the driver, and can execute shifts at exactly the right points during acceleration for optimal power and fuel efficiency. There is no longer any practical reason to build cars with manual gearing. Only enthusiasts who love to shift (ME!) have a reason to buy a stick shift car.
Posted on March 2, 2011 - 03:36 PM