热力工程中常遇到理想气体定容变化、定压变化、变化、绝热变化（定熵）四个变化过程。 In thermal engineering, there are four processes of constant volume change, constant pressure change, constant temperature change and adiabatic change (constant entropy) of ideal gas.

在上述四种典型热力过程的特征是，某一个状态参数保持不变（例如v=定值、p=定值等），或在过程中热力系与外界没有热量交换（绝热过程）。The characteristic of the above four typical thermal processes is that a certain state parameter remains unchanged (e.g., v= constant, p= constant, etc.) or there is no heat exchange between the thermal system and the outside world during the process (adiabatic process).

实际的过程往往是所有的参数都在变化，并且也不完全绝热。The actual process tends to be all parameters changing and not completely adiabatic.

在一般的实际过程中，气体的状态变化往往遵循一定的规律。In the general practical process, the change of gas state often follows a certain rule.

此时，过程方程式可通过实验，测定过程中一些状态点的p、v值而近似整理成常量的形式。At this point, the process equation can be measured through experiments, p, v values of some state points in the process and approximately sorted into the form of = constant.

这样的过程称为多变过程，式中指数n叫作多变指数，可以为任意常数。Such a process is called a polytropic process, where the exponential n is called a polytropic exponential and can be an arbitrary constant.