Question 26 3AE01 - Third Assistant Engineer

The Correct Answer is A. ### Why Option A ("Liquid in the cylinder") is Correct The relief valve (or safety valve) fitted to the cylinder head of a diesel engine is designed to protect the engine structure, particularly the cylinder head, liner, and piston, from catastrophic failure due to dangerously high pressures. Liquid (such as water from a leaky cooler or steam from a damaged jacket, or unburned fuel accumulating due to a misfire) is essentially incompressible. If liquid enters the combustion space and the piston attempts to complete the compression stroke, the resulting pressure spike will far exceed the normal maximum firing pressure. Since the relief valve is set to lift at a pressure slightly higher than the maximum expected firing pressure, the instantaneous and extreme pressure caused by hydraulic lock (hydrostatic pressure) will immediately cause the relief valve to lift, venting the pressure and preventing mechanical damage. ### Why Other Options Are Incorrect **B) Low compression in the cylinder:** Low compression results in reduced pressure and temperature during the compression stroke and subsequently lower firing pressures. This reduction in pressure is the opposite of what is required to lift the relief valve. **C) High exhaust temperature:** High exhaust temperature is an indication of poor combustion (e.g., late or prolonged burning) or high load. While high combustion temperatures are generally associated with high cylinder pressures, an increase in exhaust temperature alone is usually not indicative of a sudden, dangerous overpressure event that would cause the relief valve to lift. The valve is sensitive to peak pressure, not average thermal load reflected in the exhaust gas. **D) Poor fuel penetration:** Poor fuel penetration (e.g., due to low injection pressure or nozzle wear) typically leads to delayed, incomplete, or prolonged combustion, resulting in lower peak cylinder pressures and possibly high exhaust temperatures. It does not cause the dangerously sudden, excessive pressure spike needed to trigger the relief valve.