= Solution
Label the initial state 1, the end of compression 2, the end of constant-pressure heating 3, and the end of adiabatic expansion 4. The <volume (thermodynamics)> sequence is $V_1\to V_2\to V_3\to V_1\to V_1$, with $V_2<V_3<V_1$. The <Diesel cycle> has a compression <adiabat> $1\to2$, a horizontal constant-pressure heat-input segment $2\to3$, an expansion <adiabat> $3\to4$, and a vertical constant-volume cooling segment $4\to1$.
\Image[/past-exam-of-the-physics-b-course-of-the-university-of-cambridge/2017/paper-2-2017-diesel-cycle.png]
{title=Clockwise ideal Diesel cycle with its four thermodynamic states}
{description=Original dimensionless example with V2/V1=0.2, V3/V1=0.4 and gamma=1.4. The curved segments are reversible adiabats, the upper horizontal segment adds heat at fixed pressure and the right vertical segment rejects heat at fixed volume. The enclosed area is the positive net work output.}
For an <ideal gas>, a reversible <adiabat> obeys $pV^\gamma=\mathrm{constant}$, so it falls more steeply than an <isotherm>. The expansion <adiabat> lies above the compression <adiabat> at equal <volume (thermodynamics)> between $V_3$ and $V_1$, giving a clockwise cycle with positive <work> output.
Back to article page
