工程热力学--aps审核

This course introduced some basic definitions and laws, for example the temperature(P9), the specific heat capacity(), the famous first（the heat is a kind of energy, the heat and mechanical energy can transform between each other）and second(the power is produced by the difference of temperatures, it’s impossible to make a machine which can absorb heat from one only heat source and transform the heat to power completely) law of thermodynamics. It also tells us how to analyze or describe the every process of the energy conversion (transformation).
For example we use the Carnot cycle to comprehend the working processes of an ideal engine (P123). The Carnot cycle is composed by two reversible constant temperature processes(可逆定温过程) and two reversible thermo insulation processes（可逆绝热过程）. The working medium(工质) absorbs heat from a constant temperature source T1 and releases heat to another constant temperature source T2. the process from d to a is thermo insulation compressing process(绝热压缩), form a to b is constant temperature heat absorbing process（定温吸热）, from b to c is thermo insulation expanding process（绝热膨胀）, from c to d is constant temperature heat releasing process(定温放热). And its efficiency is 1-T1/T2, we can reach a conclusion that the efficiency of Carnot cycle is only decided by the two temperatures T1 and T2. this is a very important conclusion in the improvement of internal combustion engine. It indicates that if we want to increase the efficiency of the engine, first of all we need to increase the heat absorbing temperature T1. The way is to increase the compression ratio of the engine and that’s why that the efficiency of a diesel engine is much higher than a gasoline engine.
Ignition point(燃点) transpire(蒸发)
In practice, the processes are not as same as the Carnot cycle. The practical working processes can be generalized in such three traditional cycles: the constant volume heating cycle, the constant pressure heating cycle and the synthesis cycle. We assume the compression and work stroke as a thermal insulation process. We assume the combustion process as a constant volume absorbing heat(endothermic) process or a constant pressure absorbing heat(endothermic) process. We assume the exhaust stroke as a constant volume heat release process. And the real cycle is like this: this part is the pumping loss, because the exhaust pressure is larger than the atmospheric pressure and the intake pressure is smaller than the atmospheric pressure. And we can use some technology to decrease this part of loss such as the VVA system.。