72 Diesel Technology Copyright Goodheart-Willcox Co., Inc. Intake Stroke During the intake stroke, the intake valves are open and the exhaust valves are closed. The piston begins the intake stroke near its top dead center (TDC) position in the cylinder. As the piston travels down the length of the cyl- inder, air is drawn into the combustion chamber. Compression Stroke During the compression stroke, both the intake and exhaust valves are closed. The piston moves upward in the cylinder, compressing the trapped air, causing it to heat. Common compression levels inside the cylinder, 425–550 psi (2930.4–3792.3 kPa), will elevate air tem- peratures to 1000–1200°F (537.7–648.8°C). The actual pressure and temperature levels inside the cylinder are affected by many factors, including compression ratio, tur- bocharger boost pressure, ambient air temperature, valve timing, engine speed, and load. At the end of the compression stroke, the piston is once again at its top dead center position. The crankshaft has completed a full revolution and is now 360° through its 720° cycle. Just before the piston reaches the top of the cylinder, the fuel injector is triggered, fuel is sprayed into the cylinder, and combustion begins. This creates a major increase in the pressure and temperature inside the combustion chamber. Fuel is sprayed into the combustion chamber for a specific length of time to maintain this high pressure. The length of fuel injection is based on engine design and the engine’s load and speed at the time of injection. Power Stroke The combustion chamber is located between the top or crown of the piston and the cylinder head. As combustion gases expand in this confined space, they push on the cyl- inder head and piston. Since the cylinder head is stationary and airtight, the force directed on the piston pushes it down- ward in the cylinder during the power stroke. Peak cylinder firing pressures in modern, high-speed, heavy-duty truck engines can reach 2300 psi (15,858 kPa), with temperatures ranging between 3000–4000°F (1648–2204°C). The motion of the piston is transferred through the connecting rod to the crankshaft. The length of the power stroke is controlled by the amount of time the exhaust valves remain closed. As soon as the exhaust valves open, pressure inside the cylinder is lost, and the piston stops generating pushing power. During the power stroke, the piston moves from the top of the cylinder to the bottom. The crankshaft has completed 540° of its 720° cycle. Exhaust Stroke As the exhaust stroke begins, the exhaust valves are open and the high pressure exhaust gases rush out of the combustion chamber, drawn to the lower atmospheric pressures outside of the cylinder. The upward motion of the piston helps push these gases out of the cylinder. At the end of the exhaust stroke, the piston is in its top dead center position, the crankshaft has completed another half turn for a total of two full revolutions (720° of rotation), the intake valves open and the exhaust valves close. All components have returned to their original positions, and the sequence (cycle) is ready to be repeated. Four-Cycle Engine Valve Timing In a four-cycle engine, the camshaft turns at one-half crankshaft speed. Therefore, each valve is opened and closed once during two revolutions (720° rotation) of the crankshaft. The exact time in the cycle the intake and exhaust valves open and close is extremely important to proper engine per- formance. Engine manufacturers take great care in calculating these times and in manufacturing the camshaft and the valve train parts to exacting specifications. It may seem logical for the valves to open and close at exactly the top dead center piston positions, but this is not the case. To make certain all exhaust gases are removed or scavenged from a cylinder before the beginning of an intake stroke, the engine is timed to open the intake valves slightly before the upward-moving piston has finished its exhaust stroke. This creates a ram air effect that helps to remove exhaust gases from the cylinder and draw fresh air in through the intake valves. Similarly, the exhaust valves are timed to remain open until the piston passes top dead center (TDC) on its exhaust stroke and travels a very short distance downward on its intake stroke. The intake air helps push the remaining exhaust gases out of the cylinder through the exhaust valve. The condition when the intake and exhaust valves are open at the same time during the combustion cycle is called valve overlap. Valve overlap is measured in degrees of crankshaft rotation. For example, if the intake valves open 16° before top dead center (BTDC) and the exhaust valves do not close until 16° after top dead center (ATDC), the valve overlap is 32°. Similar timing is designed into the intake valves at the bottom dead center position. Bottom dead center (BDC) is the position when the piston is at the very bottom of the cylinder. The intake valves do not close until the piston passes BDC and begins its upward stroke on the compres- sion cycle. Closing the intake valves slightly after bottom dead center (ABDC) ensures that the largest possible air charge is drawn into the cylinder. The true compression stroke can only begin when the intake valves are closed. Fuel is injected earlier in the com- pression stroke (piston farther from TDC) when the engine is operating under increased speed or light load. Fuel injection is timed to begin later in the compression stroke (piston closer to TDC) when the engine is operating at low speeds or under heavy loads to compensate for the lag time between the start of injection and the resulting increase in expansion pressure. The power stroke continues until the exhaust valves open. Once again, the exhaust valves are timed to open shortly before bottom dead center (BBDC). This gives the exhaust gases extra time to begin moving out through the exhaust ports and into the exhaust manifold. The piston