4 Physiology of Human Power Generation
The human is the power plant for all human-powered vehicles. Although the human engine is often compared to an internal combustion engine, its operation and performance characteristics are quite different. Both engines convert the chemical energy of fuel into mechanical energy, and both produce about four times as much waste heat as useful power. The similarity ends here, however. The internal combustion engine is a heat engine, operating on a thermodynamic cycle and subject to the second law of thermodynamics. This cycle is limited by the difference in temperatures between the hottest and coolest parts of the cycle. In contrast, the human engine, while producing heat as a by-product, is not a heat engine. The energy conversion is completely chemical, and the human is not subject to the second law limitations. Performance characteristics are quite different as well. The internal combustion engine can produce no zero-speed torque. As the engine speed increases, the torque increases to a maximum, then decreases at high RPM. In contrast, a human achieves maximum torque or force when the speed is zero. Torque decreases with speed to a value of essentially zero at high speed. In this respect, the human is much more like a permanent magnet DC motor than an internal combustion heat engine.