汽车专业英语 教学课件 ppt 作者 廖忠诚 杨晓波 主编Chapter1 Engines.ppt

汽车专业英语 教学课件 ppt 作者 廖忠诚 杨晓波 主编Chapter1 Engines.ppt

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When the valves actually open and close can be measured by angles. To make these angles easier to read, let’s use a spiral instead of a circle(see fig.1-4). This intake valve opens 12° before the piston reaches top dead center. And it closes 40° after bottom dead center. The exhaust valve opens 47° before bottom dead center - and stays open - until 21° past top dead center. This gives exhaust gases more time to leave. By the time the piston is at 47° before bottom dead center on the power stroke, combustion pressures have dropped considerably and little power is lost by letting the exhaust gases have more time to exit. When an intake valve opens before top dead center and the exhaust valve opens before bottom dead center, it is called lead. When an intake valve closes after bottom dead center, and the exhaust valve closes after top dead center, it is called lag. On the exhaust stroke, the intake and exhaust valve are open at the same time for a few degrees around top dead center. This is called valve overlap. On this engine, it is 33°. spiral 螺旋线 valve overlap 气门重叠 Fig.1-4 valve timing Different engines use different timings. Manufacturer specifications contain the exact information. 1.3.9 Variable Valve Timing Valve overlap is the amount of time the intake and exhaust valves are both open at once. Less overlap produces a smooth idle and more slow speed torque, but poor high speed performance because there is not enough time for complete scavenging to occur. More valve overlap allows better engine breathing at high speeds, but poor performance at low speeds, rough idling, and higher exhaust emissions. Engines with fixed valve timing can only operate most efficiently at one specific speed. Engines that can vary valve timing and/or valve lift can operate efficiently at a wider range of speeds, and deliver better performance at high speeds, with a flatter torque curve. There are two types of variable valve timing,

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