Compression:

At 35° ABDC (after bottom dead center), the intake valve begins to close. At 43° ABDC (or 137° BTDC), the intake valve is on its seat and is fully closed. At this point the air charge is at general pressure (14.7 psia) and ambient air temperature (~80°F), as described within Figure. At about 70° BTDC the piston has traveled about 2.125 inches and on half of its stroke therefore decreasing the volume in the cylinder through half. The temperature has now doubled to ~160°F and pressure is ~34 psia.

At about 43° BTDC the piston has traveled upward 3.062 inches of their stroke and the volume is once again halved. As a result, the temperature again doubles to about 320°F and pressure is ~85 psia. While the piston has traveled to 3.530 inches of their stroke the volume is again halved and temperature reaches ~640°F and pressure 277 psia. While the piston has traveled to 3.757 inches of its stroke and the volume is again halved then the temperature climbs to 1280°F and pressure reaches 742 psia. Along with a piston area of 9.616 in2 the pressure in the cylinder is exerting a force of around 7135 lb. or 3-1/2 tons of force.

Figure: Compression

The above numbers are ideal and give a good instance of what is occurring in an engine during compression. In an original engine, pressures reach only about 690 psia. This is because of primarily to the heat loss to the surrounding engine elements.