A thermodynamic system is a mass of fluid separated from its surroundings by a boundary, such as the gas contained in a car engine cylinder during compression. A thermodynamic system can be closed, with only heat and work crossing the boundary during non-flow processes, or open, allowing the fluid itself to cross the boundary along with heat and work during flow processes like the gas in a combustion chamber during induction.

1. Thermodynamic Systems
The thermodynamic system is a mass of fluid which may be liquid, gas
or steam, which separated from its surroundings by a boundary.
For example the gas contained between the piston crown and the
cylinder walls in a car engine during the compression stroke can be
regarded as a thermodynamic system.
A thermodynamic system may be closed as in the diagram adjacent, in which case only heat
and work may cross the system boundary and the processes are term non-flow.
A thermodynamic systems may also be open, in which case the fluid itself is allowed to
cross the system boundary in addition to heat and work, and the processes are called
flow processes.
An example of an open system is the gas contained in the combustion chamber of a car
engine during the induction stroke. Fuel is let in and exhaust gases are let out, both heat
and work cross the boundary.