On the design of very high RPM camshaft profiles minimizing valve float dangers
Discussion to this video:
and another in which three springs stabilise and holde valve stem so that it does not float at all.
There are many vastly improved camshafts on the market which give you more power, but sometimes you want to reach higher RPM and need to know how. Harder springs is certainly the answer, but one answer you might not heard is camshaft profile which properly handles the returning valve propeled by spring.
Valve float is created when the camshaft recedes so fast that it outruns the valve. You can correct it by using stronger springs or using less steep cam profile. Well the less steep profile is hardly practical, is it? Generally yes, it could make the cam overlap into useless area. So you use harder springs to reach higher RPM instead. But again, you can not do it endlessly. What to do next? Consider the cam profile again. While valve float is hazard, the true bad thing is valve bounce where the valve 'slams close' and then shortly opens. We want it 'close shut', not 'slam'. How about... if we closed on a steep profile first (the valve is propelled by the spring and flies freely), but then in the final part where the valve would have little useable lift in airflow terms we designed the profile as such to catch the valve and decelerate it slowly.
The equation on the linear movement of the valve should be such that the da/dt rises linearly from zero to a certain value and decreases in the same manner. (where acceleration a = dv/dt and speed v = ds/dt). The d(da/dt) /dt value can be zero. In other words: the acceleration should have acceleration too, not just jump to a certain value.
You then must convert the designed valve movement into cam profile, depending on your actual mechanism dimensions, then grind new cam profile.
Ideally, the valve and springs should be at all times touching the rocker arm, pressing hard against it, but in racing we sometimes trade longevity for victory. However, properly designed downslope of a cam does have a mechanical advantage over straight acceleration jump even when no valve float occurs!
Next thing you want to hear the engine head has is: 'hydraulic lifters' (not hydraulic actuators, hydraulic valve pads) . Why? Think of them as dampeners able to absorb the returning shock of the flying bullet - which valve stem propeled by a spring certainly is, so that the valve does not bounce back, but releases the energy into the hydraulic fluid and sits down. This of course requires certain construction of the hydraulic valve lifters, but first thing is you have to have them first.
By the way, if you are aware of CFD software, which is free and simulates the supersonic thermoacoustic effects correctly, let me know. Because in 2-cycle engine this is what you want and need and nothing less is quite satisfactory.
and another in which three springs stabilise and holde valve stem so that it does not float at all.
There are many vastly improved camshafts on the market which give you more power, but sometimes you want to reach higher RPM and need to know how. Harder springs is certainly the answer, but one answer you might not heard is camshaft profile which properly handles the returning valve propeled by spring.
Valve float is created when the camshaft recedes so fast that it outruns the valve. You can correct it by using stronger springs or using less steep cam profile. Well the less steep profile is hardly practical, is it? Generally yes, it could make the cam overlap into useless area. So you use harder springs to reach higher RPM instead. But again, you can not do it endlessly. What to do next? Consider the cam profile again. While valve float is hazard, the true bad thing is valve bounce where the valve 'slams close' and then shortly opens. We want it 'close shut', not 'slam'. How about... if we closed on a steep profile first (the valve is propelled by the spring and flies freely), but then in the final part where the valve would have little useable lift in airflow terms we designed the profile as such to catch the valve and decelerate it slowly.
The equation on the linear movement of the valve should be such that the da/dt rises linearly from zero to a certain value and decreases in the same manner. (where acceleration a = dv/dt and speed v = ds/dt). The d(da/dt) /dt value can be zero. In other words: the acceleration should have acceleration too, not just jump to a certain value.
You then must convert the designed valve movement into cam profile, depending on your actual mechanism dimensions, then grind new cam profile.
Ideally, the valve and springs should be at all times touching the rocker arm, pressing hard against it, but in racing we sometimes trade longevity for victory. However, properly designed downslope of a cam does have a mechanical advantage over straight acceleration jump even when no valve float occurs!
Next thing you want to hear the engine head has is: 'hydraulic lifters' (not hydraulic actuators, hydraulic valve pads) . Why? Think of them as dampeners able to absorb the returning shock of the flying bullet - which valve stem propeled by a spring certainly is, so that the valve does not bounce back, but releases the energy into the hydraulic fluid and sits down. This of course requires certain construction of the hydraulic valve lifters, but first thing is you have to have them first.
By the way, if you are aware of CFD software, which is free and simulates the supersonic thermoacoustic effects correctly, let me know. Because in 2-cycle engine this is what you want and need and nothing less is quite satisfactory.
posted by Mario. at 12:19 PM
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