Diesel Engine Fundamentals
FUNDAMENTALS OF THE DIESEL CYCLE
pressure condition within the cylinder and will help pull in the fresh air charge as shown
in Figure 16.
The piston continues its upward travel through top dead center (TDC) while fresh air
enters and exhaust gasses leave. At about 12 after top dead center (ATDC), the
camshaft exhaust lobe rotates so that the exhaust valve will start to close. The valve is
fully closed at 23 ATDC. This is accomplished through the valve spring, which was
compressed when the valve was opened, forcing the rocker arm and cam follower back
against the cam lobe as it rotates. The time frame during which both the intake and
exhaust valves are open is called valve overlap (51 of overlap in this example) and is
necessary to allow the fresh air to help scavenge (remove) the spent exhaust gasses and
cool the cylinder. In most engines, 30 to 50 times cylinder volume is scavenged through
the cylinder during overlap. This excess cool air also provides the necessary cooling
effect on the engine parts.
As the piston passes TDC and begins to travel down the cylinder bore, the movement of
the piston creates a suction and continues to draw fresh air into the cylinder.
At 35 after bottom dead center (ABDC), the intake
Figure 17 Compression
valve starts 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 normal pressure (14.7 psia)
and ambient air temperature (~80F), as illustrated in
At about 70 BTDC, the piston has traveled about 2.125
inches, or about half of its stroke, thus reducing the
volume in the cylinder by half. The temperature has now
doubled to ~160F and pressure is ~34 psia.
At about 43 BTDC the piston has traveled upward 3.062
inches of its stroke and the volume is once again halved.
Consequently, the temperature again doubles to about
320F and pressure is ~85 psia. When the piston has
traveled to 3.530 inches of its stroke the volume is again
halved and temperature reaches ~640F and pressure 277 psia. When the piston has
traveled to 3.757 inches of its stroke, or the volume is again halved, the temperature
climbs to 1280F and pressure reaches 742 psia. With a piston area of 9.616 in2 the
pressure in the cylinder is exerting a force of approximately 7135 lb. or 3-1/2 tons of