In last post, we've seen about BMW's ShiftCam technology. The same concept is also used here. Only the actuation mechanism differs. The main motto is, when the motorbike is running under lower RPM band, normal cam profile will be used which provides limited valve lift and valve lift duration. This normal cam lobe is well and good for lower RPM bands. But when you need more power and torque at higher engine RPM, valve lift and lift duration should be high. For obtaining this, manufacturers use different mechanisms but the need for doing that remains constant. If you need to know some basics about this, check my previous post. In this post, let's directly see about how VVA system works in Yamaha's R15 V3 and in MT-15.
Overview
As we know at lower RPM band, low profile cam will be actuating the valves which is enough to produce low end power and torque. But, after crossing several engine RPM, high profile cam should come to action. For changing the cam lobes at running condition, BMW has used a component named shift gate which pulls the camshaft axially where larger cam profile comes to scene for actuating the valves. But in Yamaha's VVA system, no shift gate will be present. The total system consists of three cam lobes, three rocker arms, sliding pin and the VVA actuator. First cam lobe have normal or low profile. Second cam lobe have high profile for that high lift and more lift duration of inlet valves. Third cam lobe is present for actuating the exhaust valves.
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| VVA system layout |
When the motorbike is running under 7400 RPM, normal cam lobe will be actuating the inlet valves. Thus it provides limited amount of valve lift and lift duration which suits for lower RPM bands. At the same time, high profile cam lobe will also be rotating and actuating its rocker arm. But at this point, both the rocker arms acts as a separate unit and works independently because it is not connected with the sliding pin. When the engine RPM crosses 7400, the inlet valves should be actuated with the help of high profile cam for more lift and duration. This is where the VVA motor or actuator and the sliding pin comes to action.
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Valve assembly
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| Intake camshaft |
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| Actuation of inlet valves by low and high profile cam lobes |
When rider leaves the throttle, ECU senses the engine RPM and sends signal to the VVA motor. Thus automatically, the sliding pin come back to its original position and the normal profile cam starts to actuate the inlet valves. Here, you have to know about several things. At the end of the exhaust stroke, piston will be at TDC and inlet valves starts to open. When the valves are operated with the help of high profile cam, the lift will be higher as we know. At this condition, when the engine runs under high RPM, there are chances for the valves to hit the piston crown and causing severe mechanical damage. To avoid this, valve recess will be present in the piston crown where the valves correctly fit at the given recess of the piston crown.
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| Graphical representation |
In the above model graph, you can able to clearly understand how the power output behaves for the entire RPM band. In first graph, you can able to see the low profile cam is producing a good torque and power in lower RPM band that is below 7000 RPM. After 7000, torque starts to decrease. In second graph, you can able to see the high profile cam is producing a good torque and power after 7400 RPM and producing low power output in lower RPM bands. In third graph you can able to see optimized power and torque output is obtained by using both the low and high profile cams. From this, clearly you can able to understand the necessary of using VVA in motorbikes.
