TM-9-2815-220-24 Maintenance Manual for AVDS-1790-2CA AVDS-1790-DA and AVDS-1790-2DR Engines Download
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TM 9-2815-220-24 THEORY OF OPERATION 0004 00 GENERAL THEORY (Continued) Basic Four-Cycle Engine Operation: Four-cycle (four-stroke) engines fire every other time the piston reaches Top Dead Center (TDC). This means that the crankshaft makes two revolutions and the piston makes four strokes, one stroke up, and one stroke down per revolution. The first cycle is the intake stroke, or suction stroke. The stroke begins with the exhaust valve closed and the intake valve open. As the crankshaft rotates, it pulls the connecting rod down and piston down. A vacuum is created as the piston moves down the cylinder, and this vacuum draws a charge of fresh air into the cylinder. When the piston reaches Bottom Dead Center (BDC), the intake valve closes. The second cycle is the compression stroke. The stroke begins as the crankshaft rotates enough to force the connecting rod and piston upward. Since the charge of air cannot escape, it is compressed until the piston reaches TDC. The temperature of the air charge is raised to 1,000 °F or more as a result of the high compression. The third cycle is the power stroke. With the charge of air compressed and hot, fuel is sprayed into the cylinder through the fuel injection nozzle. The temperature causes the fuel and air to ignite beginning the power or working stroke. The resulting heat greatly increases the temperature, causing the gases to expand. As the gases expand, the piston is forced down. As the piston approaches BDC the fuel has completed burning and the exhaust valve opens. The gases are cooled considerably because of their expansion and release from pressure. The fourth cycle is the exhaust stroke. When the piston starts upward, the exhaust stroke begins. With the exhaust valve open, all the burned gasses are expelled completing the four cycles and preparing the cylinder for the next cycle. Timing: For efficient operation, the timing of the valve action is engineered so that opening and closing do not exactly coincide with the piston reaching Top Dead Center (TDC) or Bottom Dead Center (BDC). The intake valves open 25 degrees before the piston reaches TDC and remains open for 260 degrees, closing at 55 degrees after the piston has passed BDC. Fuel is injected into the cylinder from 11 to 37 degrees before TDC depending on the injection advance unit, which in turn is controlled by engine speed. The exhaust valves open 45 degrees before BDC and remain open for 245 degrees, closing at 20 degrees after TDC. There are times during which both intake and exhaust valves are open (overlap). ENGINE SYSTEMS The AVDS-1790-2 series engine has been divided into six interrelated major systems. 1. Fuel System The overall function of the fuel system is to deliver the proper amount of clean fuel to the cylinders at the correct time. The system must also provide a safe place to store the fuel and a safe method of transferring it from storage to the cylinder. The major components of this system include: WP 0004 00-3