DOE-HDBK-1018/1-93Diesel Engine FundamentalsFUNDAMENTALS OF THE DIESEL CYCLEpressure condition within the cylinder and will help pull in the fresh air charge as shownin Figure 16.The piston continues its upward travel through top dead center (TDC) while fresh airenters and exhaust gasses leave. At about 12 after top dead center (ATDC), thecamshaft exhaust lobe rotates so that the exhaust valve will start to close. The valve isfully closed at 23 ATDC. This is accomplished through the valve spring, which wascompressed when the valve was opened, forcing the rocker arm and cam follower backagainst the cam lobe as it rotates. The time frame during which both the intake andexhaust valves are open is called valve overlap (51 of overlap in this example) and isnecessary to allow the fresh air to help scavenge (remove) the spent exhaust gasses andcool the cylinder. In most engines, 30 to 50 times cylinder volume is scavenged throughthe cylinder during overlap. This excess cool air also provides the necessary coolingeffect on the engine parts.As the piston passes TDC and begins to travel down the cylinder bore, the movement ofthe piston creates a suction and continues to draw fresh air into the cylinder.CompressionAt 35 after bottom dead center (ABDC), the intakeFigure 17 Compressionvalve starts to close. At 43 ABDC (or 137 BTDC),the intake valve is on its seat and is fully closed. Atthis point the air charge is at normal pressure (14.7 psia)and ambient air temperature (~80F), as illustrated inFigure 17.At about 70 BTDC, the piston has traveled about 2.125inches, or about half of its stroke, thus reducing thevolume in the cylinder by half. The temperature has nowdoubled to ~160F and pressure is ~34 psia.At about 43 BTDC the piston has traveled upward 3.062inches of its stroke and the volume is once again halved.Consequently, the temperature again doubles to about320F and pressure is ~85 psia. When the piston hastraveled to 3.530 inches of its stroke the volume is againhalved and temperature reaches ~640F and pressure 277 psia. When the piston hastraveled to 3.757 inches of its stroke, or the volume is again halved, the temperatureclimbs to 1280F and pressure reaches 742 psia. With a piston area of 9.616 in2thepressure in the cylinder is exerting a force of approximately 7135 lb. or 3-1/2 tons offorce.Rev. 0ME-01Page 23
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business