顶置凸轮式汽油机配气机构设计研究设计说明书.docx
顶置凸轮式汽油机配气机构设计研究-四缸发动机【含4张CAD图纸、说明书】【QX系列】
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外文文献及译文 文献题目：THE IMPACT OF VALVE EVENTS UPON ENGINE PERFORMANCE AND EMISSIONS 文献来源： Taylor&Francis 文献发表日期： 2006年12月14日 学生姓名： 学号： 系 别： 专 业： 指导教师： 职称： 2017年5月19日THE IMPACT OF VALVE EVENTS UPON ENGINE PERFORMANCE AND EMISSIONSSummary This paper seeks to provide an overview of the basic parameters used in the specification of valve timing in spark ignition engines. The effect of these parameters on engine performance and emissions will be discussed in general terms rather than with reference to any particular engine or type of engine. Some of the current industry trends will be discussed in terms of their impact on the engine valve timing parameters. General The diagrams above illustrate the conventional 4-stroke cycle of an internal combustion engine. It can be seen that both the intake and exhaust valves remain closed during the compression and ignition phases of the cycle. It is therefore usual for any discussion of valve timing to focus on parts 1&4 of the cycle, that is the valve motion in the periods either side of piston Top Dead Centre (TDC) on the non-firing stroke. The valve motion is controlled by a camshaft(s) that rotates at half the speed of the crankshaft. During the four stroke cycle the crankshaft rotates twice, causing two piston cycles, whilst the camshaft rotates once, causing one cycle of each valve. The different speeds of the crankshaft and camshaft can be the cause of some confusion when describing the timing of valve opening and closing with angles, as 360 of crankshaft rotation is equivalent to 180 of camshaft rotation. It is normal to discuss the parameters of valve timing with reference position of the piston using crankshaft angle measured from piston TDC or BDC (Bottom Dead Centre). This paper will hold to this convention but it should be noted that the duration of valve lift in crankshaft degrees is twice the duration of the profile actually ground onto the camshaft. Duration is defined as the angle of crankshaft rotation between the opening and the closing of the intake or the exhaust valve. The term “valve event” refers to the opening or closing of either the intake or the exhaust valve(s) with reference to piston TDC or BDC. The graph below shows the intake and exhaust valve events as they would typically appear around the end of the exhaust stroke (TDC). The main parameters to be discussed are: - 1. Exhaust Valve Opening Timing EVO 2. Exhaust Valve Closing Timing EVC 3. Intake Valve Opening Timing IVO 4. Intake Valve Closing Timing IVC 5. Peak Valve Lift The overlap region (if present) is the difference between IVO and EVC (if positive) and is thus affected by changes in either IVO or EVC. It can be seen from the above graph that the valve events do not coincide with TDC and BDC as depicted in the “theoretical” four-stroke cycle. The reasons for this and the compromises inherent in the selection of valve event timings will form the basis for discussion in this paper. Effects of Changes to Exhaust Valve Opening Timing - EVO As the exhaust valve opens the pressure inside the cylinder resulting from combustion is allowed to escape into the exhaust system. In order to extract the maximum amount of work (hence efficiency) from the expansion of the gas in the cylinder, it would be desirable not to open the exhaust valve before the piston reaches Bottom Dead Centre (BDC). Unfortunately, it is also desirable for the pressure in the cylinder to drop to the lowest possible value, i.e. exhaust back pressure, before the piston starts to rise. This minimises the work done by the piston in expelling the products of combustion (often referred to as blow down pumping work) prior to the intake of a fresh charge. These are two conflicting requirements, the first requiring EVO to be after BDC, the second requiring EVO to be before BDC. The choice of EVO timing is therefore a trade-off between the work lost by allowing the combusted gas to escape before it is fully expanded, and the work required to raise the piston whilst the cylinder pressure is still above the exhaust back-pressure. With a conventional valve train, the valve lifts from its seat relatively slowly and provides a significant flow restriction for some time after it begins to lift and so valve lift tends to start some time before BDC. A typical EVO timing is in the region of 50-60 before BDC for a production engine. The ideal timing of EVO to optimise these effects changes with engine speed and load as does the pressure of the gasses inside the cylinder. At part load conditions, it is generally beneficial if EVO moves closer to BDC as the cylinder pressure is much closer to the exhaust back pressure and takes less time to escape through the valve. Conversely, full load operation tends to result in an earlier EVO requirement because of the time taken for the cylinder pressure to drop to the exhaust back-pressure. Summary: Effects of Changes to Exhaust Valve Closing Timing - EVC The timing of EVC has a very significant affect on how much of the Exhaust gas is left in the cylinder at the start of the engines intake stroke. EVC is also one of the parameters defining the valve overlap, which can also have a considerable affect on the contents of the cylinder at the start of the intake stroke. For full load operation, it is desirable for the minimum possible quantity of exhaust gas to be retained in the cylinder as this allows the maximum volume of fresh air & fuel to enter during the Intake stroke. This requires EVC to be at, or shortly after TDC. In engines where the exhaust system is fairly active (i.e. Pressure waves are generated by exhaust gas flow from the different cylinders), the timing of EVC influences whether pressure waves in the exhaust are acting to draw gas out of the cylinder or push gas back into the cylinder. The timing of any pressure waves changes with engine speed and so a fixed EVC timing tends to be optimised for one speed and can be a liability at others. For part load operation, it may be beneficial to retain some of the exhaust gasses, as this will tend to reduce the ability for the cylinder to intake fresh air & fuel. Retained exhaust gas thus reduces the need for the throttle plate to restrict the intake and results in lower pumping losses (see Appendix A) in the intake stroke. Moving EVC Timing further after TDC increases the level of internal EGR (Exhaust Gas Recirculation) with a corresponding reduction in exhaust emissions. There is a limit to how much EGR the cylinder can tolerate before combustion becomes unstable and this limit tends to become lower as engine load and hence charge density reduces. The rate of combustion becomes increasingly slow as the EGR level increases, up to the point where the process is no longer stable. Whilst the ratio of fuel to oxygen may remain constant, EGR reduces the proportion of the cylinder contents as a whole that is made up of these two constituents. It is this reduction in the ratio of combustible to inert cylinder contents which causes combustion instability.Typical EVC timings are in the range of 5-15 after TDC. This timing largely eliminates internal EGR so as not to detrimentally affect full load performance. Summary:Effect of changes to Intake Valve Opening Timing IVO The opening of the intake valve allows air/fuel mixture to enter the cylinder from the intake manifold. (In the case of direct injection engines, only air enters the cylinder through the intake valve). The timing of IVO is the second parameter that defines the valve overlap and this is normally the dominant factor when considering which timing is appropriate for a given engine. Overlap will be discussed in more detail later in this paper. Opening the intake valve before TDC can result in exhaust gasses flowing into the intake manifold instead of leaving the cylinder through the exhaust valve. The resulting EGR will be detrimental to full load performance as it takes up space that could otherwise be taken by fresh charge. EGR may be beneficial at part load conditions in terms of efficiency and emissions as discussed above. Later intake valve opening can restrict the entry of air/fuel from the manifold and cause in-cylinder pressure to drop as the piston starts to descend after TDC. This can result in EGR if the exhaust valve is still open as gasses may be drawn back into the cylinder with the same implications discussed above. If the exhaust valve is closed, the delay of IVO tends not to be particularly significant, as it does not directly influence the amount of fresh charge trapped in the cylinder. Typical IVO timing is around 0-10 before TDC which results in the valve overlap being fairly symmetrical around TDC. This timing is generally set by full load optimisation and, as such, is intended to avoid internal EGR. Summary: Effect of changes to Intake Valve Closing Timing IVC The volumetric efficiency of any engine is heavily dependent on the timing of IVC at any given speed. The amount of fresh charge trapped in the cylinder is largely dictated by IVC and this will significantly affect engine performance and economy. For maximum torque, the intake valve should close at the point where the greatest mass of fresh air/fuel mixture can be trapped in the cylinder. Pressure waves in the intake system normally result in airflow into the cylinder after BDC and consequently, the optimum IVC timing changes considerably with engine speed. As engine speed increases, the optimum IVC timing moves further after BDC to gain maximum benefit from the intake pressure waves. Closing the intake valve either before or after the optimum timing for maximum torque results in a lower mass of air being trapped in the cylinder. Early intake closing reduces the mass of air able to flow into the cylinder whereas late intake closing allows air inside the cylinder to flow back into the intake manifold. In both cases, the part load efficiency can be improved due to a reduction in intake pumping losses (see Appendix A). A typical timing for IVC is in the range of 50-60 after BDC and results from a compromise between high and low speed requirements. At low engine speeds, there will tend to be some flow back into the intake manifold just prior to IVC whereas at higher speeds, there may still be a positive airflow into the cylinder as the intake valve closes Summary: Effects of Valve Overlap As discussed previously, valve overlap is the time when both intake and exhaust valves are open. In simple terms, this provides an opportunity for the exhaust gas flow and intake flow to influence each other. Overlap can only be meaningfully assessed in conjunction with the pressure waves present in the intake and exhaust systems at any particular engine speed and load. In an ideal situation, the valve overlap should allow the departing exhaust gas to draw the fresh intake charge into the cylinder without any of the intake gas passing straight into the exhaust system. This allows the exhaust gas in the combustion chamber at TDC to be replaced and therefore the amount of intake charge to exceed that which could be drawn into the cylinder by the swept volume of the piston alone. A given amount of overlap unfortunately tends to be ideal for only a portion of engine speed and load conditions. Generally, the torque at higher engine speeds and loads can benefit from increased overlap due to pressure waves in the exhaust manifold aiding the intake of fresh charge. Large amounts of overlap tend to result in poor emissions at lower speeds as fuel from the intake charge can flow directly into the exhaust. High overlap can also result in EGR which, although beneficial to part load economy, reduces full load torque and can cause poor combustion stability especially under low load conditions such as idle. Poor idle quality can therefore result from too much overlap.The valve overlap tends to be fairly symmetrical about TDC on most engines. The further away from TDC that valve overlap is present, the more effect the piston motion will have on the airflow. Early overlap may result in exhaust gasses being expelled into the intake manifold and late overlap may result in exhaust gasses being drawn back into the cylinder. Both of these situations result in internal EGR that can be beneficial to part load emissions and efficiency. As discussed earlier, internal EGR tends to be avoided due to the detrimental effect it has on full load torque. Valve Peak Lift Valve peak lift directly affects the ability for air to flow into the cylinder and exhaust to leave the cylinder and as a result it significantly influences engine performance. There are some practical limitations of peak lift in most engines that are dependent on the design of the particular engine:1. Normally the piston crown is profiled to maximise the clearance adjacent to the valves but there are limitations as to how much valve lift can be accommodated without unduly compromising the piston design.2. The duration of the valve lift imposes a restriction on valve lift due to the acceleration required to achieve a high lift with a short duration. The higher the running speed of the engine, the more the peak lift is restricted for a given valve lift duration.Low values of valve peak lift will clearly restrict the ability of gas to flow into and out of the cylinder, effectively throttling the engine. Maximum engine power output will generally benefit from as much valve lift as possible up to the point where air flow becomes restricted by other features such as the manifold system or cylinder head porting. It does not follow that engines should have the maximum possible valve lift as this can adversely affect low speed performance. Lower intake valve lifts result in higher gas velocity past the valve and this improves fuel mixing and combustion. For maximum torque at a given speed, lift should be kept as low as possible up to the point where the intake of fresh charge becomes restricted. Part load economy will benefit from the intake being restricted by valve lift, as it reduces the need for throttling by the engine throttle and this will reduce intake pumping losses. Lift is not normally dictated by part load considerations, as this would severely limit the potential output of the engine.The chosen value of peak valve lift is therefore a compromise between low speed and high speed full load requirements. A typical value for a production engine is in the range of 8-10mm. Industry Trends There is a constant need for internal combustion engines to become more powerful and efficient with reduced levels of emissions driven both by legislation and increasing customer expectations. For an engine with fixed valve timing, there are inherently compromises made between emissions, high/low speed torque and full/part load efficiency. Ways of avoiding compromise between different engine requirements are constantly being incorporated into new engines and investigated for application to future engines. The following technologies are becoming increasingly common and are discussed in the context of the engine valve timing strategies outlined in this paper: Exhaust Gas Recirculation (EGR) Systems The presence of exhaust gas in the combustion chamber can be beneficial for emissions, yielding reductions in NOx, HC and CO. The timing of EVC and IVO can cause exhaust gas to be retained in or re-introduced into the cylinder as discussed earlier in this paper. This is known as “internal” EGR and is generally avoided due to its impact upon full load torque. “External” EGR systems are now becoming more common where gas from the exhaust system is pumped back into the intake manifold at part load conditions. This provides benefits in part load emissions and improved efficiency due to a reduction in intake pumping losses (see Appendix A). As the quantity of EGR can be changed to suit engine speed and load conditions, there need not be any detriment to full load torque. Internal EGR however, does have two significant benefits over external EGR: 1. External systems are expensive and are prone to durability problems due to their continual exposure to hot, dirty gasses. The intricate components within EGR control systems are susceptible to the build up of deposits causing leakage or blockage. 2. The recirculated gas in the case of internal EGR is the last portion to have left the cylinder. This portion generally contains the gasses from any crevice volumes in the cylinder and therefore contains a significant portion of the unburned hydrocarbons from the combustion process. External EGR takes a portion of all the exhaust gasses once they are mixed and so has much less ability to reduce hydrocarbon emissions. Variable Valve Timing An increasing number of engines are using variable valve timing systems to avoid some of the compromises of fixed valve timing. A fully flexible system, which could vary valve lift and intake and exhaust valve event timings independently for different engine speed and load conditions, could in principal overcome all of the compromises inherent in a conventional valve train system. In practice however, the more flexible a variable valve timing system becomes, the more complex and hence expensive it tends to become.There are a number of different variable valve timing systems, currently available and under development, to control different valve timing parameters. Although there are many different designs for achieving such variations, these systems can be grouped in terms of their operation. 1. Phase Changing Systems These systems change the timing of the camshaft in relation to the crankshaft in order to advance or retard the timing of the engine valve events. If applied to an engine with a single camshaft, all of the valve events are shifted by the same amount i.e. if IVC is to be retarded by 10 IVO, EVO and EVC will also be retarded by 10. On engines with separate Intake and Exhaust camshafts, a phase change system can be used to change the timing of the intake valve events or the exhaust valve events. The use of two phasing devices can permit independent control over Intake and Exhaust timing changes. Phase change systems have no effect on peak valve lift and cannot change the duration of the valve events i.e. IVO and IVC cannot be moved independently. Phase changing systems have been available on production engines for a number of years but have tended to be applied only to the highest specification engine in a particular range. Phasing of the intake camshaft to gain increased performance with a mechanism that can be moved between two fixed camshaft timings is the most common application with the change in timing normally occurring at a particular engine speed. More recently, there has been a move towards more flexible control systems that allow the camshaft phasing to be maintained at any point between two fixed limits. This has facilitated camshaft phase optimisation for different engine speed and load conditions and has allowed exhaust camshaft phasing to be used for internal EGR control. Engines with both intake and exhaust camshaft phase control are now being introduced.2. Profile Switching Systems This type of Variable Valve Timing system is capable of independently changing valve event timing and valve peak lift. The system switches between two different camshaft profiles on either or both of the camshafts and is normally designed to change at a particular engine speed. Due to these systems having an inherently two position operation, they are not suitable for optimising valve timing parameters under different load conditions e.g. EGR control. The ability to change valve event timing, lift and duration ensures that these systems are capable of providing very high power output from a given engine whilst still complying with emissions legislation. 3. Variable Event Timing Systems Variable event timing systems are probably the most flexible type of variable valve timing system to be available on a production engine. Whilst they do not change peak valve lift, they are able to change both the phasing and the duration of valve events. These systems can be controlled to any setting between two extremes and are most effective when optimised for different engine speed and load conditions.Considerable increases in full load torque can be achieved with variable event timing, generally most significant at the extremes of the engine speed range where fixed valve timing is most compromised. Reductions in part load emissions and fuel economy are also achievable through full optimisation of he system. 4. Variable Lift Systems There are two main types of variable lift system currently under investigation, the first “scales” the valve lift such that its opening duration is unchanged whilst the second “truncates” the lift profile such that the valve opening duration reduces as lift reduces. Both of these types of system can only be used effectively when combined with a phase changing device. One major benefit of variable lift systems is the potential of throttling the cylinder by reducing the intake valve lift thus saving the pumping losses associated with the conventional throttle (see Appendix A). This requires very close control of lift to match changes in engine speed and load conditions and has yet to be fully proven. 5. Electro-magnetic Valve Actuation Systems In many ways, this is the system that could provide the greatest potential for optimising the engine valve events. In principal EVO, EVC, IVO, IVC and possibly valve lift could all be directly controlled by an engine management system. EVA systems are still in the early stages of development and have yet to demonstrate whether they can match the performance of a mechanical valve train in terms of durability, power consumption and refinement whilst maintaining an acceptable system price. Gasoline Direct Injection (GDI) EnginesAn increasing number of new gasoline engines inject fuel directly into the combustion chamber. Whilst this technology can be used to generate a homogeneous mixture of air and fuel at all engine speeds and loads, as with most port injected engines, most of these engines create a stratified charge (i.e. a rich mixture surrounded by a far leaner one) at part load conditions. Stratified operation can theoretically remove the need for a throttle in the intake system as small quantities of fuel can be added to an excess of air. This can create a large improvement in part load economy due to the reduction in pumping losses (see Appendix A). Much research is currently being undertaken to improve stratification of the fuel so that part load economy improvements can be achieved within the constraints of current and future emissions regulations. The air motion in the cylinder is taking an increasingly key role in this work and can be significantly affected by the valve events. Currently techniques such as port disablement and variable tumble ports are being investigated but it may be that variable valve timing systems can provide the required control over the combustion process. 气门性能对发动机性能和排放的影响总结本文旨在概述火花点火发动机气门正时规范中使用的基本参数。 这些参数对发动机性能和排放的影响将一般性地讨论，而不是参考任何特定的发动机或类型的发动机。 目前的一些行业趋势将会对发动机气门正时参数的影响进行讨论。一般：上图显示了内燃机的常规四冲程循环。可以看出，在循环的压缩和点火阶段期间，进气门和排气门都保持关闭。因此，任何关于气门正时的讨论通常都集中在循环的第1和第4部分，也就是活塞上止点（TDC）在非点火行程中任一侧的气门运动。气门运动由以曲轴速度的一半旋转的凸轮轴控制。在四冲程循环期间，曲轴旋转两次，导致两个活塞循环，而凸轮轴旋转一次，导致每个气门的一个循环。曲轴和凸轮轴的不同速度可能是在用角度描述气门打开和关闭的时间时引起一些混乱，因为曲轴旋转的360相当于凸轮轴旋转的180。使用从活塞TDC或BDC（底下中心）测量的曲轴角度，讨论活塞参考位置的气门正时参数是正常的。本文将遵循这一惯例，但是应该注意的是，曲轴角度的气门升程持续时间是实际研磨到凸轮轴上的剖面持续时间的两倍。持续时间被定义为进气口或排气气门的开度和关闭之间的曲轴旋转角度。术语“气门事件”是指相对于活塞TDC或BDC打开或关闭进气气门或排气气门。 下图显示进气和排气门事件，因为它们通常出现在排气冲程末端（TDC）的周围。要讨论的主要参数如下： - 1.排气门开启时间 - EVO 2.排气气门关闭时间 - EVC 3.进气气门开启时间 - IVO 4.进气气门关闭时间 - IVC 5.峰值气门升程重叠区域如果存在）是IVO和EVC之间的差异（如果为正）并且因此受IVO或EVC的变化的影响。从上图可以看出，如“理论”四冲程循环所示，气门事件与TDC和BDC不一致。这一点的原因和气门事件时间选择所固有的妥协将成为本文讨论的基础。排气气门打开时间变化的影响 - EVO随着排气气门打开，燃烧过程中汽缸内的压力允许排入排气系统。为了从气缸中的气体膨胀中提取最大的工作量（因此效率），在活塞到达底死点（BDC）之前期望不打开排气门。不幸的是，在活塞开始升高之前，气缸中的压力也希望降至最低可能值，即排气背压。这样可以最大限度地减少活塞排出燃烧产物的工作（通常称之为排放抽油作业），然后再进行新鲜充电。这是两个相互冲突的要求，第一个需要EVO在BDC之后，第二个要求EVO在BDC之前。因此，EVO定时的选择是通过在燃烧气体完全膨胀之前允许燃烧气体逸出而造成的工作之间的折衷，以及在气缸压力仍高于排气背压时升高活塞所需的工作。使用传统的气门机构，气门相对缓慢地升起，并在开始升高之后提供了一段时间的显着的流量限制，因此气门升程趋于在BDC之前开始一段时间。在生产引擎的BDC之前，典型的EVO计时在50-60的范围内。EVO优化这些效果的理想时机随发动机转速和负载而变化，与气缸内气体的压力一致。 在部分负载条件下，当气缸压力更接近排气背压时，EVO更接近BDC是非常有益的，并且花费更少的时间通过气门门逸出。 相反，由于气缸压力下降到排气背压所需的时间，满载操作往往导致较早的EVO要求。 概要：排气门关闭时间变化的影响 - EVCEVC的时间对发动机进气冲程开始时气缸中剩余的废气有很大的影响。 EVC也是限定气门重叠的参数之一，也可以在进气冲程开始时对气缸的内容产生相当大的影响。对于满载操作，期望在气缸中保留尽可能少的废气量，因为这允许在进气行程期间最大量的新鲜空气和燃料进入。这要求EVC在TDC之后或不久之后。在排气系统相当活跃的发动机（即压力波由不同气缸的排气产生压力波）时，EVC的时机影响排气中的压力波作用，将气体从气缸中抽出或将气体推回气瓶。任何压力波的时间随着发动机转速的变化而变化，所以固定的EVC时机往往会针对一个速度进行优化，并可能是其他的责任。对于部分加载操作，保留一些废气可能是有益的，因为这将降低气缸吸入新鲜空气和燃料的能力。因此，保留的废气减少了节流板限制进气的需要，并导致进气冲程中的较低的泵送损耗（见附录A）。在TDC提高内部EGR（排气再循环）水平之后，随着废气排放的相应减少，进一步推动EVC计时。在燃烧变得不稳定之前，气缸容许多少EGR是有限度的，并且由于发动机负载，因此电荷密度降低，该极限倾向于变低。随着EGR水平的增加，燃烧速度变得越来越慢，直到过程不再稳定。虽然燃料与氧气的比率可能保持不变，但EGR减少了由这两种组分构成的整体气缸内容物的比例。这是可燃性与惰性气瓶含量的比例的降低，这导致燃烧不稳定性。典型的EVC定时在TDC后5-15的范围内。 这个时机很大程度上消除了内部EGR，以免不利于影响满载性能。 概要：进气门开启时间变化的影响 - IVO 进气门的打开允许空气/燃料混合物从进气歧管进入气缸。 （在直喷式发动机的情况下，只有空气通过进气气门进入气缸）。 IVO的定时是定义气门重叠的第二个参数，这通常是考虑给定发动机适合哪个时序的主要因素。重叠将在本文后面更详细地讨论。在TDC之前打开进气气门可能会导致废气流入进气歧管，而不会使气缸通过排气气门。所产生的EGR将对全负荷性能造成不利影响，因为它占据了以新鲜电荷为代价的空间。在如上所述的效率和排放方面，EGR在部分负载条件下可能是有益的。后来的进气门打开可以限制空气/燃料从歧管的进入，并且随着活塞在TDC之后开始下降，导致缸内压力下降。如果排气气门仍然打开，这可能导致EGR，因为气体可能被吸回到气缸中，具有上述相同的含义。如果排气气门关闭，则IVO的延迟趋向于不会特别显着，因为它不直接影响汽缸中蓄留的新鲜电量。典型的IVO定时在TDC之前约为0-10，这导致气门重叠在TDC周围相当对称。该定时通常由全负荷优化设定，因此，旨在避免内部EGR。概要：进气门关闭时间变化的影响 - IVC任何发动机的体积效率都严重依赖于IVC在任何给定速度下的时间。汽缸中蓄积的新鲜电量主要由IVC决定，这将显着影响发动机性能和经济性。为了获得最大的扭矩，进气气门应该在最大质量的新鲜空气/燃料混合物可以被捕获在气缸中的位置处关闭。进气系统中的压力波通常会导致在BDC之后气流进入气缸，因此最佳IVC定时随着发动机转速而显着变化。随着发动机转速的增加，最佳IVC定时在BDC之后进一步移动，以从进气压力波获得最大的益处。在最大扭矩的最佳定时之前或之后关闭进气门会导致气缸中的空气质量较低。早期进气闭合减少了能够流入气缸的空气质量，而较晚的进气关闭允许气缸内的空气流回进气歧管。在这两种情况下，由于进气泵送损失减少，部件负载效率可以提高（见附录A）。 IVC的典型时机在BDC之后在50-60的范围内，并且是由于高速和低速要求之间的妥协导致的。在低发动机转速下，在IVC之前会有一些回流到进气歧管中，而在较高的速度下，进气门关闭时仍然可能会有气流进入气缸。气门重叠的影响如前所述，气门重叠是进气门和排气门都打开的时间。 简单来说，这提供了排气流和进气流相互影响的机会。 重叠只能在任何特定的发动机转速和负载下结合进气和排气系统中存在的压力波进行有意义的评估。 在理想情况下，气门重叠应允许排出的废气将新鲜的进气装置吸入气缸中，而不会使任何进气气体直接进入排气系统。 这允许在TDC的燃烧室中的废气被更换，因此吸入量超过单独通过活塞的扫掠体积可以吸入到气缸中的进气量。不幸的是，给定的重叠量仅仅是发动机转速和负载条件的一部分的理想选择。通常，在更高的发动机转速和负载下的扭矩可以受益于由于排气歧管中的压力波辅助新鲜电荷的摄入而增加的重叠。大量的重叠往往导致较低速度的较差排放，因为来自进气装置的燃料可以直接流入排气。高重叠也可能导致EGR，虽然有利于部分负载经济性，但是降低了全负载转矩，并且可能导致差的燃烧稳定性，特别是在诸如空闲的低负载条件下。因此，空闲质量差可能由于太多的重叠而产生。在大多数发动机上，气门重叠倾向于相对于TDC相当对称。存在气门重叠的TDC越远，活塞运动对气流的影响越大。早期的重叠可能导致废气被排放到进气歧管中，并且晚期重叠可能导致废气被吸回气缸。这两种情况都会导致内部EGR，这对部分负载的排放和效率都有好处。
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