Two Stroke Engine Using Reed Valves

Reed valves are commonly used in high-performance versions of the two-stroke engine, where they control the fuel-air mixture admitted to the cylinder. As the piston rises in the cylinder a vacuum is created in the crankcase beneath the piston. This vacuum opens the valve and admits the fuel-air mixture into the crankcase. As the piston descends, it raises the crankcase pressure causing the valve to close to retain the mixture and pressurize it for its eventual transfer through to the combustion chamber.The Swedish motorcycle company Husqvarna produced a two-stroke, 500 cc displacement single cylinder engine with a reed-valve controlled intake, one of the biggest in using this arrangement. Reed valves in two-stroke engines have been placed in the intake ports and also in controlling the intake to the crankshaft space.

Composite materials are preferred in racing engines, especially in kart racing, because the stiffness of the petals can be easily tuned and they are relatively safe in failure. High-speed impact takes its toll on all reed valves, with metal valves suffering in fatigue. The physical inertia of reed valves means that they are not as entirely precise in action as rotary valves, a rotary valve engine may run better than a reed valve engine at a small rpm range but the reed valve engine often runs better over a wider rpm range. More sophisticated designs partly address this by creating multi-stage reeds with smaller, more responsive reeds within larger ones that provide more volume later in the cycle. Nevertheless, current technology favors reed valves almost to the exclusion of rotary valves due to their simplicity and low implementation costs and less rotational mass.

Reed valves are designed considering the pressure gradient and mass flow. The pressure gradient is used to evaluate the valve lift during open condition; the lift and overall component geometry (considering also a pressure loss coefficient) are then used to calculate the mass flow. For high speed applications (compressors and engines) the dynamic response has to be considered. A simple approach consists in the evaluation of first eigenvalue that is compared with exciting frequency. Design of reed valves can be refined using simulations. The dynamic of petals can be studied neglecting the coupling between fluid and structure: in this case the evolution of the structural part are simulated using lumped parameters models or FEM models, discharge coefficients at various valve lift are evaluated with experiments or CFD simulations. The study of the complete system needs an integrated Fluid-structure interaction model.


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