When the piston nears TDC in view D, there has been a marked decrease in volume. Also, both pressure and temperature are much greater than at the beginning of compression. Notice that the pressure has gone from 0 psi to 470 psi and Figure 2-4. Volume, temperature, and pressure relationships in a cylinder. The temperature of the air has been raised sufficiently to cause ignition of the fuel that is injected into the cylinder.
Further changes take place after ignition.
However, there is a sharp increase in both pressure and temperature shortly after ignition takes place. The increased pressure forces the piston downward. These changes in volume, pressure, and temperature are representative of the changing conditions in the cylinder of a diesel engine. The reason for this is that the heat of compression of a diesel engine would ignite a gasoline and air mixture before the piston could approach the top of its stroke. This pre-ignition would tend to drive the piston back down the cylinder and place an excessive, damaging strain on the engine.