Restricting the flow rate of air to the intake manifold is a convenient and popular method used by several motor sport disciplines to regulate engine performance. This principle is applied in the Formula SAE and Formula Student competitions, the rules of which stipulate that all the air entering the engine must pass though a 20mm diameter orifice. The restriction acts as a partially closed throttle which generates a vacuum in the inlet plenum. During the valve overlap period of the cycle, which may be as much as 100 degrees crank angle in the motorcycle engines used by most FSAE competitors, this vacuum causes reverse flow of exhaust gas into the intake runners. This, in turn, reduces the amount of fresh air entering the cylinder during the subsequent intake stroke and therefore reduces the torque produced. This effect is particularly noticeable at medium engine speeds when the time available for reverse flow is greater than at the peak torque speed.
The objective of the study described in this paper was to mitigate the reverse flow effect by reducing the duration of the valve overlap period. A thermodynamic model of the Yamaha YZF R6 engine was developed for this purpose and validated using cycle-averaged and crank-angle-resolved test data. The resulting model was then used to find the optimum values of lift, duration and timing for both the intake and exhaust valves. The camshafts required to give these valve lift profiles were designed using valve train analysis software. This process included a consideration of the dynamic forces encountered by the valve train and ensured that the resulting stresses remained within safe limits.
The new camshafts increased the torque output by up to 30% at medium engine speeds, without reducing the high-speed torque, and therefore significantly improved the vehicle drivability.