Question 6 MODE01 - Chief MODU Engineer

The Correct Answer is D. **Why Option D is Correct:** The primary function of plating the wearing surface of a diesel engine cylinder liner with porous chromium (often called "hard chrome" or "chrome plating") is directly related to lubrication. Chrome plating itself is extremely hard and wear-resistant, reducing the rate of wear on the liner. However, smooth, non-porous chrome can lead to poor oil retention and possible scuffing. The special characteristic of *porous* chromium plating is the intentional creation of microscopic pits, channels, or pores on the surface. These pores act as tiny reservoirs, trapping and holding lubricating oil. As the piston moves up and down, the trapped oil ensures a continuous and stable oil film (hydrodynamic lubrication) is maintained between the piston rings and the liner wall, especially under high load and temperature conditions. This retention of the oil film is crucial for effective lubrication and preventing metal-to-metal contact. **Why Options A, B, and C are Incorrect:** **A) The chromium will not wear out the piston rings.** * **Incorrect:** While hard chromium plating significantly reduces wear on the *liner*, it is a very hard material. If not properly finished or if the lubrication fails, hard chromium can actually increase the wear rate on the relatively softer piston rings (especially cast iron rings) compared to a standard, softer cast iron liner. The goal is to reduce liner wear, but it doesn't eliminate ring wear; proper ring material selection is necessary to manage compatibility. **B) The chromium strengthens the liners in the way of the scavenging air ports.** * **Incorrect:** Chromium plating is a very thin surface treatment (measured in hundredths of a millimeter or thousandths of an inch). It provides excellent surface hardness and wear resistance, but it does not contribute meaningfully to the structural, mechanical strength of the thick, heavy cast iron or steel cylinder liner itself, especially in areas like the scavenging air ports where stress concentration or structural failure might be a concern. **C) Chromium eliminates the need for oil scraper rings.** * **Incorrect:** Oil scraper rings (or oil control rings) are essential components of the piston ring pack. Their purpose is to precisely meter the amount of oil left on the cylinder wall (ensuring lubrication) and to scrape excess oil back down into the crankcase, preventing it from migrating into the combustion chamber where it would burn and cause fouling or excessive exhaust emissions. Chromium plating, porous or otherwise, is merely a wear surface treatment and cannot perform the metering and scraping functions of the specialized oil control rings.