Question 3 AEL01 - Assistant Engineer - Limited

The Correct Answer is C ### Explanation for Option C (Correct) **C) Exhaust ports will uncover before the air intake ports** The **crank lead** refers to the angular separation (measured in degrees of crank rotation) between the two crankshafts in an opposed-piston engine. Typically, the exhaust-side crankshaft (connected to the piston controlling the exhaust ports, often the lower piston) leads the intake-side crankshaft (connected to the piston controlling the intake/scavenge ports, often the upper piston). The purpose of this lead is to ensure **blowdown**—the process of releasing residual high-pressure exhaust gases—occurs *before* the intake ports open. If the **lower crank lead** (which typically controls the exhaust piston) is *increased*, it means the lower piston reaches its port-opening position (and its BDC) **earlier** relative to the upper (intake) piston. * **Result:** The lower piston retracts sooner, causing the **exhaust ports to uncover earlier** in the expansion stroke. Since the intake piston timing has not been similarly advanced, the exhaust ports will open earlier, increasing the *angular separation* between exhaust opening and intake opening. Therefore, the exhaust ports will uncover **before** the air intake ports (and even earlier than they did before the timing change). ### Explanation for Other Options (Incorrect) **A) Exhaust ports will uncover after the air intake ports** This is the opposite of what is required for proper scavenging and is the opposite of the effect of increasing the exhaust crank lead. Increasing the lead ensures the exhaust opens *earlier* (before the intake), not later. If the exhaust opened after the intake, hot high-pressure exhaust gas would rush into the intake manifold, causing damage and poor combustion. **B) Duration of air intake will increase** The duration of the port opening is determined by the design of the port height and the geometry of the connecting rods/crank throw. Increasing the crank lead affects the *timing* (when the port opens/closes relative to the other port), but generally does not affect the *duration* (the number of crank degrees the port remains open) for that specific set of ports, assuming no other physical changes are made. **D) Duration of air intake will decrease** Similar to B, increasing the angular lead primarily affects the *timing* (phase relationship) between the two pistons, not the angular duration of the individual port events. The air intake duration is determined by the physical design of the intake ports and piston travel.