Four-Stroke Diesel Engine Exhaust Valves and Tappet Clearance (a):
Tappet clearance refers to the gap between the valve stem and the rocker arm (or other valve actuation mechanism) when the cam lobe is not pushing on the valve. Here’s how incorrect tappet clearance affects exhaust valves in a four-stroke diesel engine:
(i) Too Large Tappet Clearance:
- Delayed Valve Opening: With excessive clearance, the rocker arm takes longer to contact the valve stem and push it open. This leads to a delayed opening of the exhaust valve, causing hot exhaust gases to remain in the cylinder for a longer duration. This can result in:
- Increased Cylinder Temperatures: Excessive heat can damage pistons, valves, and other engine components.
- Reduced Power Output: Incomplete exhaust gas evacuation can reduce the efficiency of the next intake stroke, leading to lower power output.
- Increased Fuel Consumption: Incomplete combustion due to higher cylinder temperatures and inefficient gas exchange can lead to increased fuel consumption.
- Increased Valve Train Noise: The extra space allows for more play between the valve stem and rocker arm, creating knocking or clattering sounds during engine operation.
(ii) Too Small Tappet Clearance:
- Premature Valve Opening: With minimal clearance, the valve might remain slightly open even when the cam lobe is not pushing on it. This can lead to:
- Loss of Compression: Premature opening allows some compressed air to escape the cylinder during the compression stroke, reducing engine efficiency and power output.
- Burned Exhaust Valve: Hot exhaust gases can leak past a slightly open valve, burning the valve seat and damaging the valve itself.
- Valve Sticking: In extreme cases, tight clearance can cause the valve to stick open due to heat expansion, leading to severe engine damage.
Double (Nested) Valve Springs (b):
Double or nested valve springs are commonly used in diesel engines for several reasons:
- High Exhaust Gas Pressure: Diesel engines experience significantly higher exhaust gas pressures compared to gasoline engines. Double springs provide the necessary force to overcome these high pressures and ensure proper valve closure against the valve seat.
- Control Valve Dynamics: The dual springs allow for a more precise control of valve opening and closing. The spring rates and characteristics can be designed to achieve the desired valve opening profile, optimizing engine performance and reducing valve train noise.
- Safety and Reliability: In case one spring breaks or weakens, the other spring can still provide some level of valve control, preventing catastrophic engine failure due to a completely open exhaust valve.
By utilizing a combination of high spring force and controlled valve dynamics, double valve springs help ensure proper exhaust valve operation under the demanding conditions of a diesel engine.