Question 18 3AE01 - Third Assistant Engineer

The Correct Answer is B **Explanation for Option B (Correct):** Option B states that multiple concentric valve springs are used to "operate the valve gear where a larger force is normally required, but space limitations restrict the use of a single large spring." Diesel engines (especially high-performance, high-speed, or heavy-duty engines) typically require significantly stronger valve springs than gasoline engines. This increased force is necessary for two main reasons: 1. **High Pressure/Force:** The forces exerted on the valve by high cylinder pressures during combustion and compression are very high, requiring strong springs to keep the valve seated tightly. 2. **High RPM Control (Valve Float):** To prevent valve float (where the inertia of the valve assembly overcomes the spring force at high engine speeds), a substantial spring force is needed to quickly return the valve and keep the follower in contact with the cam lobe. Designing a single spring strong enough to meet these requirements often necessitates using extremely thick wire or a very large spring diameter. This large size is often physically impossible due to constraints within the cylinder head (such as the size of the valve spring retainer pocket, surrounding rocker arms, or injector placement). By using multiple concentric springs (usually two, or sometimes three, springs nested one inside the other), the required total spring force can be achieved while maintaining a smaller overall diameter, effectively managing space constraints. Additionally, using multiple springs adds a safety factor and helps dampen harmonic vibrations (spring surge) that can occur in a single large spring. **Explanation for Incorrect Options:** **A) enable research and development of cam contour to be simplified:** This is incorrect. Cam contour design is primarily governed by the desired lift, duration, and acceleration/deceleration rates of the valve, which are independent of the number of springs used. Spring force is a requirement based on the cam profile and engine speed, not a factor that simplifies R&D of the profile itself. **C) allow for easier valve replacement:** This is incorrect. Replacing valves requires compressing the spring assembly to remove the keepers and retainer. Using multiple concentric springs generally makes the replacement process slightly more complex (due to the higher total spring force required for compression) or, at best, has no impact on ease of replacement compared to a single spring of equivalent force. **D) enable a total smaller valve spring force to keep the valve tight on its seat:** This is definitively incorrect. Concentric springs are used to achieve a *larger* total spring force within a limited space. If a smaller force were sufficient, a single, less powerful spring would be used, as it is simpler and cheaper. The entire purpose of concentric springs is to handle the *increased* force demands of heavy-duty applications like diesel engines.