Question 47 QMED01 - Junior Engineer

The Correct Answer is D. ### Explanation for Option D (Correct Answer) The primary reason a two-stroke cycle diesel engine of equal displacement requires less starting air than a four-stroke cycle diesel engine is that the two-stroke engine utilizes ports (or valves combined with ports) for scavenging and exhaust, and completes a full power cycle in two piston strokes (one revolution). The four-stroke engine must overcome the resistive forces (pumping work) during two full strokes—the intake stroke (drawing air in) and the exhaust stroke (pushing combustion products out). These two strokes represent "energy absorbing intake and exhaust strokes" where the piston is performing work against fluid pressure differentials and flow resistance. In the two-stroke engine, the work required to introduce fresh air (scavenging) and expel exhaust gases is primarily performed by an external blower or turbocharger, meaning the engine itself does not have to dedicate two full piston strokes (the third and fourth strokes in the four-stroke cycle) to this pumping work during starting. By eliminating these resistive pumping strokes, the two-stroke engine requires less torque (and therefore less high-pressure starting air) to achieve starting speed. ### Explanation for Incorrect Options **A) has little or no internal friction:** This is incorrect. Both two-stroke and four-stroke engines have significant internal friction (piston rings, bearings, etc.). While the friction characteristics might differ slightly (e.g., in valve train complexity), a two-stroke engine does not inherently have so little friction that it would be the primary reason for requiring significantly less starting air. **B) has a lower effective compression ratio:** This is incorrect. The effective compression ratio in a two-stroke engine is generally defined from the point the exhaust port closes, which is lower than the geometric compression ratio. However, overall diesel engines require high compression ratios (typically 14:1 to 20:1) to ignite the fuel. While the effective ratio might be slightly lower, two-stroke engines are designed to operate efficiently at high power densities, and reducing the compression ratio would make starting harder, not easier, as it would be more difficult to achieve ignition temperature. **C) operates with scavenge air under a positive pressure:** This is incorrect as the primary explanation. While true (scavenging requires positive pressure), this positive pressure is supplied by an external device (blower/turbocharger). This arrangement *contributes* to the engine requiring less starting air by offloading the pumping work to the external device, but the fundamental mechanical difference is the absence of the pumping strokes, which is explained by option D. Furthermore, many modern four-stroke engines also utilize positive pressure charging (turbochargers/superchargers).