An adiabatic process is generally obtained by surrounding the entire system with a strongly insulating material or by carrying out the process so quickly that there is no time for a significant heat transfer to take place.
Applying the first law of thermodynamics to an adiabatic process, we obtain:
delta-U = -WSince delta-U is the change in internal energy and W is the work done by the system, what we see the following possible outcomes:
- A system that expands under adiabatic conditions does positive work, so the internal energy decreases.
- A system that contracts under adiabatic conditions does negative work, so the internal energy increases.
The compression and expansion strokes in an internal-combustion engine are both approximately adiabatic processes. What little heat transfers outside of the system is negligible and virtually all of the energy change goes into moving the piston.