Question 51 DDE01 - Designated Duty Engineer - Unlimited HP

The Correct Answer is D **Explanation for Option D (Air cranking motor(s)):** The question specifically asks about the starting method used with the system illustrated (MO-0199) on a salvage tug's main propulsion diesel engines, and the correct answer is stated to be **Air cranking motor(s)**. This type of system is characterized by using compressed air to power a pneumatic motor (cranking motor) which engages the engine's flywheel (usually via a Bendix gear or similar mechanism) to turn the engine over and initiate starting. This method is common for medium-sized, high-speed, or auxiliary diesel engines, which are often found on tugs for propulsion or power generation. If the illustration MO-0199 shows a small starter motor unit bolted to the engine block or gearbox housing, this confirms the use of a pneumatic (air) cranking motor. **Explanation for Incorrect Options:** * **A) Hydraulic cranking motor(s) with air over hydraulics:** While hydraulic starting systems are used on some engines, they are separate from pure compressed air starting systems. "Air over hydraulics" typically refers to using air pressure to pressurize a hydraulic accumulator or fluid (often used for braking or control systems), not the primary cranking method for the main engine itself. * **B) Direct air admission with cam actuated air start valves:** This is the standard starting method for **large, slow-speed** main propulsion diesel engines (like those found on large cargo ships). It involves admitting high-pressure air directly into the cylinders in the correct firing sequence, timed by mechanical cams. This system is usually too complex and heavy for the medium-speed engines typically used on tugs, making it unlikely if the system shown is a cranking motor. * **C) Direct air admission with air start distributor:** This is the equivalent of option B but using a rotary distributor to time the air admission, often found on medium-speed engines that are too large for a conventional cranking motor. However, since the specified correct answer is D (Air cranking motor), this direct admission method is incorrect for the system illustrated (MO-0199). The cranking motor method (D) is simpler and lighter than any direct admission method (B or C).