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凸轮轴理论与设计 Camshaft Theory and Design.pdf

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凸轮轴 理论 设计 CAMSHAFT THEORY AND DESIGN
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Camshaft Design and Theory Camshaft ? The “brain” of the engine ? Controls valvetrain operation ? Rotates at ? crankshaft speed ? Along with the crankshaft it determines firing order ? Along with the induction and exhaust systems it determines the useful rpm range of the engine Camshaft Design ? Features 1. Max lift or nose 2. Flank 3. Opening clearance ramp 4. Closing clearance ramp 5. Base circle 6. Exhaust opening timing figure 7. Exhaust closing timing figure 8. Intake opening timing figure 9. Intake closing timing figure 10. Intake to exhaust lobe separation Camshaft Measurements ? Lift ? Duration ? Lobe separation angle ? Valve overlap ? Intake valve opening (IVO) ? Intake valve closing (IVC) ? Exhaust valve opening (EVO) ? Exhaust valve closing (EVC) Lift ? Lobe lift is the distance the lifter moves in one direction ? Lobe lift is the difference in measurement between the nose of the lobe and the base circle of the lobe ? Valve lift is what most people are taking about when they refer to lift and is simply lobe lift multiplied by the rocker arm ratio Lift ? Increasing the lift opens the valve further. This reduces the restriction to airflow at the valve and allows air to flow more freely into the cylinder. ? At some point the valve can be opened to a point at which the port is the valve is no longer the greatest restriction to airflow, and at that point opening the valve further will not increase airflow. ? The distance a valve can be opened is limited by duration, rocker arm ratio, lifter design, camshaft design and valve to piston clearance. Duration ? Duration is the length of time (measured in degrees of crankshaft rotation) that the valve remains open ? The point at which a valve is considered “open” will be given along with the duration figure (i.e. .004”, .006”, .050”) ? Example (240 degrees @ .050) Duration ? At higher engine speeds the valve opens and shuts in a shorter amount of time. This limits how completely the cylinder can be filled. ? Longer duration camshafts hold the valve open longer, increasing the amount of time the cylinder has to fill. Duration Vs. Time ? At 2000 RPM a camshaft with 200 degrees of intake valve duration, the intake valve is open for 0.0166 seconds/cycle ? At 6500 RPM the valve is only open for .005128 seconds/cycle ? If we substituted a camshaft with 245 degrees of duration this time would increase to .0204 seconds/cycle @ 2000RPM and .00628 seconds/cycle at 6500RPM ? This is a 22% increase in time available to fill the cylinder Lobe Separation Angle ? Lobe separation angle (LSA) is the number of degrees separating the point of peak exhaust lift and peak intake lift. ? Lobe separation angle directly impacts the amount of valve overlap. ? Because of this, production vehicles usually employ a wide LSA to reduce valve overlap and increase idle quality. Valve Overlap ? Valve overlap is the time in which both the intake and exhaust valves are open. ? Valve overlap is affected by LSA and duration. ? Valve overlap is used because of the principle of exhaust scavenging (the exiting exhaust gases help “pull” in the fresh intake charge, especially at higher rpm when fill time is limited). ? At low RPM when intake port speed is low, a long valve overlap period will cause reversion into the intake port (the cylinder pressure exceeds the force of the air in the intake port and exhaust gasses are forced into the intake port). ? This causes the lumpy idle associated with big camshafts. Intake Valve Closing ? Most critical valve opening/closing point ? To early of an IVC and the cylinder may not have time to fill completely ? To late of an IVC and the cylinder pressure will overcome the inertia of the incoming airflow and revert flow back into the intake port ? This causes a serious disruption to flow and destroys any pressure waving tuning Exhaust Valve Opening ? 2 ndmost critical valve opening/closing event ? Determines the balance between power event efficiency and exhaust pumping losses ? To early of an exhaust opening will reduce the amount of energy converted from cylinder pressure to mechanical force on the piston ? To late of an EVO will cause an increase in the amount of power needed to expel the burned exhaust gases from the cylinder Exhaust Valve Closing ? Along with IVO it is the least critical valve timing event ? Along with IVO it determines the amount of valve overlap and exhaust scavenging ? Too early of an EVC will not allow the exhaust gas to be fully purged from the cylinder ? Too late of an EVC will allow fresh air/fuel mix to be purged into the exhaust system Intake Valve Opening ? Along with EVC it is the least critical valve timing event ? Along with EVC it determines the amount of valve overlap and exhaust scavenging ? Too early of an IVO will allow exhaust gas reversion into the intake ports causing major flow disruption and intake charge dilution ? Too late of an IVO will limit the time available for the cylinder to completely fill Cam Phase Graph Cam Card
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