  There has been many studies on gases which obeys Van der Waal's equation of
state. However there is no specific and direct studies of Van der Waal's gas
which undergoes adiabatic processes are available in the undergraduate text
books and also in literature. In an adiabatic process there is no heat energy
exchange between the system and its surroundings. In this article, we find that
the Van der Waal's equation for the adiabatic process as
$\left(P+\frac{n^2a}{V^2}\right) \left(V-nb\right)^{\Gamma}=\mbox{constant}$,
where $P$ is the pressure, $V$ is the volume, $n$ is the number of moles of the
Van der Waal's gas, $a$ and $b$ are Van der Waal's constant and $\Gamma$ is a
factor which relates the specific heat at constant pressure and at constant
volume. We use this relation explicitly and obtained the efficiency of a Carnot
engine whose working substance obeys Van der Waal's equation of state. Our
simplest approach may provide clear idea to the undergraduate students that
$\Gamma$ is different from $\gamma$ of the ideal gas for an adiabatic process.
We also shown that the efficiency of the Carnot engine is independent of the
working substance.
