Understanding Valve Spring Bounce in Motor Vehicle Engines

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SUMMARY

The discussion centers on valve spring bounce in 4-stroke, 4-cylinder motor vehicle engines, specifically addressing the natural frequency of valve springs calculated at 5500 rev/min. It is established that valve bounce can occur when engine speeds exceed this frequency, leading to concerns about operating at 8000 rpm. The conversation highlights that the camshaft operates at half the engine speed, yet emphasizes that other factors such as rod stretch, valve timing, and ignition timing also play critical roles in determining the safe RPM limits, often resulting in red lines set at 6500 rpm for safety.

PREREQUISITES
  • Understanding of valve spring dynamics in internal combustion engines
  • Knowledge of engine RPM and its relationship with camshaft operation
  • Familiarity with factors affecting engine performance, including volumetric efficiency
  • Basic principles of mechanical stress and fatigue in engine components
NEXT STEPS
  • Research the effects of valve timing on engine performance and RPM limits
  • Explore the relationship between rod stretch and engine speed in high-performance applications
  • Learn about volumetric efficiency and its impact on engine power output
  • Investigate the mechanics of valve float and its implications for engine longevity
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Engineers, automotive technicians, and performance enthusiasts seeking to understand the complexities of valve spring behavior and its implications on engine performance and reliability.

enkiduz190
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Hi guys, i am doing some calculations on the valve spring of a 4 stroke, 4 cylinder motor vehicle engine.

To my understanding, valve bounce will occur once the valve spring is operated at a speed over their natural frequency (which i calculated to be 5500 rev/min). If i were to push the engine speed to 8000 rpm, does it mean that the valve spring won't 'bounce' since the camshaft only runs at half the engine speed? But if that was the case, why do most vehicle have red lines at 6500rpm on the rev counter? Is it really necessary to have such large safety factor?

Thanks
 
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Valve float is not by any means the only factor determining rpm limits. You get into things like rod stretch, valve closing impact, ring wear, cam lobe wear, and the ever-popular exploding flywheel that cuts your legs off.
 
Also I would think that things like volumetric efficiency, valve timing and ignition timing will do the limiting for you.
 

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