🔍 Key Concepts

• Residual fuels and what impurities they commonly contain • Which metallic elements in fuel ash are associated with high-temperature corrosion of exhaust valves and turbochargers • Difference between harmless combustion products (like carbon) and corrosive ash-forming elements

💭 Think About

• Think about which of these substances would remain as part of the ash deposit on hot metal surfaces rather than burning off as a gas. • Consider which element is known in marine engineering for causing hot corrosion and grooving on exhaust valves when present in heavy fuel oil. • Ask yourself: which of these is specifically discussed in marine diesel engine literature as a reason for tight control of fuel quality and sometimes requires additives or operational changes?

✅ Before You Answer

• Verify which element forms low-melting, corrosive compounds (often with sodium and sulfur) that attack exhaust valves. • Eliminate options that are mainly normal combustion products (like soot) and do not typically cause grooving by themselves. • Check your engineering notes: which trace metal in residual fuel ash is closely linked with exhaust valve burning and corrosion?