阿特金森发动机实现之可行性分析.doc

精品文档摘要：本文介绍了在普通奥托循环发动机基础上改造成为阿特金森循环发动机的可行性。由于阿特金森循环的膨胀行程比压缩行程要长，这样就充分发挥了发动机的爆发压力，提高了燃油效率。关键字：阿特金森循环（Atkinson cycle）发动机 可变气门相位（VVT-Variable Valve Timing）一 背景1 阿特金森循环（Atkinson cycle）发动机在常规奥托循环发动机的做功冲程完成后，封闭在汽缸内的气体压力仍然有35个大气压。在排气冲程中，这部分气体的热量白白地排放到大气中。如果提高做功行程的做功量，在膨胀行程末，汽缸内的压力降为稍高于大气压，再将排气气门打开，则会提高燃油效率，这种工作循环被称之为阿特金森循环，具有这种循环的发动机被称之为阿特金森循环发动机。目前油电混合动力汽车中，基本上对于发动机进行了重新设计或重大改进。如丰田Prius的1.5L汽油机(1NZ-FXE)采用了阿特金森循环（Atkinson cycle），它是在1NZ-FE的基础上改造得到的。这种循环发动机具有高热效率、高膨胀比、紧凑型倾斜挤气燃烧室（以形成有利于燃烧的挤气涡流）以及铝合金缸体，其主要目的是追求高的热效率而不是高功率。由于电机承担了功率调峰的作用，发动机可以舍弃非经济工作区的动力性能而追求经济工作区的高效率。如，日本丰田Prius所用的发动机的工作区域设定在10004500rpm。阿特金森循环发动机的热效率较之传统的奥托循环发动机的提高有赖于控制泵气损失和在保持压缩比不变的前提下增大了膨胀比。在1885年，阿特金森循环的实现是通过曲柄和气门等机构，其燃烧室的容积用以保持固定的压缩比，而膨胀比是随着载荷变化而变动以此来优化燃油效率。在二十世纪初，工程师试图通过复杂的连杆机构以期实现不同的冲程，事实证明这种做法并不实用。后随着电子技术的发展，可变气门配气相位（VVT）使得阿特金森循环真正成为可能。福特和丰田公司已经将阿特金森循环发动机商品化，应用于其混合动力汽车上。图 1 1885年阿特金森循环发动机机构示意图2可变气门相位（VVT）由于常规发动机的凸轮设计是固定的，因此气门叠开角和进气门行程只能在理想的最大值与最小值之间选一个折中值。于是发动机在低速时，由于气门叠开角比理想值大，新鲜混合气就可能被废气带走，造成油耗增加；而在高速时，由于气门叠开角和进气门行程比理想值小，从而限制了发动机所能达到的最大功率。 所以理想的配气相位角应随着发动机转速、负荷及其它工况而改变。为了使发动机在高转速时能够提供较大的功率，在低转速时又能够提供足够的转矩，现代轿车发动机采用了可变气门相位系统（VVT系统），它可以根据发动机的运行情况而改变配气相位或气门升程，同时兼顾低转速和高转速时对于配气相位的要求，以实现发动机在低速区的扭矩需求和高速区的功率响应，并且能够直接地在全转速范围提高发动机的性能。在具体的实现手段上，各个汽车公司各有千秋。但是具体的方式不外乎以下四种：气门中止开启方式、凸轮轮廓改变方式、凸轮相位改变方式和多模式变换方式。世界上第一个实用的VVT系统是意大利菲亚特汽车公司在20世纪70年代开发成功的，该系统使用液压装置，随着发动机的转速和进气压力的不同实现了对于凸轮从动件的支点位置的控制。最简单的VVT系统是通过提前和延迟进气和排气气门正时来实现，如马自达的S-SV系统；还有通过在低速和高速工况下变换配气系统的凸轮来实现，如本田的VTEC系统；此外还有通过改变气门的开闭时间和气门升程来实现，如丰田的VVTL-i。表1为各汽车公司VVT方案。表1 各汽车公司可变气门相位（VVT）方案序号公司名称VVT方案代号说明1BMW宝马VANOS通过凸轮轴从动件的运动控制进气和排气配气相位2FORD福特Variable Cam Timing通过凸轮轴旋转实现可变配气相位3GM通用DCVCP(Double Continuous Variable Phasing）液力马达控制实现可变配气相位4HONDA本田VTEC可变气门升程5HONDA本田i-VTEC在传统VTEC基础上增加了凸轮相位控制6HYUNDAI/KIA现代/起亚CVTT7MAZDA马自达S-VT通过凸轮轴的转动实现配气相位改变8MITSUBISHI三菱MIVTC可变气门升程9NISSAN日产VVL通过两套凸轮实现可变进气配气相位和气门升程10PORSCHE保时捷Vario Cam通过凸轮型线调整气门升程控制进气气门配气正时11PORSCHE保时捷Vario Cam Plus通过凸轮型线调整气门升程和通过调整正时链条的涨紧度控制进气配气正时12ROVER陆虎VVC通过一个偏心盘调整配气相位13SUZUKI铃木VVT14SUBARU斯巴鲁AVCS通过液压调整配气相位15TOYOTA丰田VVT-i进气气门配气相位可变16TOYOTA丰田VVTL-i通过两套凸轮实现可变进气配气相位和气门升程17GEELY吉利进气气门配气相位可变VVT是发动机技术术语，在实现VVT功能上，即便是基于相同的原理，不同的汽车公司采用了不同形式的代号来表示。所以吉利也应为其VVT系统命名并申请专利。3对立统一一般说来，发动机扭矩和功率的提高与燃油经济性的提高是矛盾的。但是通过可变气门相位既可以实现更高燃油经济性（阿特金森循环），也可以有效地改善发动机在低转速区的扭矩响应和高转速区的功率表现。所以说两者是对立统一的。二重点理论1发动机性能（1）评价发动机性能的指标发动机性能，是指发动机的动力性、燃油经济性、轻量化等方面的性能。这些性能，直接影响到整车的有关性能。评价发动机性能的主要指标有：转速、功率、扭矩和燃油消耗率等。另外，还有一些相对性的指标，如：升功率、升扭矩、升质量、比质量等等，以便在不同的发动机之间进行定性的比较。（2）影响发动机性能的因素进气效率、燃烧效率和摩擦损失是影响发动机性能的三个主要因素。相应地，通过增加进气量、提高燃烧效率和降低摩擦损失，可提高发动机的性能。而阿特金森循环发动机的最主要目的便是提高燃烧效率。2理论压缩比和实际压缩比压缩比指的是发动机混合气体被压缩的程度，用压缩前的汽缸总容积与压缩后汽缸容积（即燃烧室容积）之比来表示。汽油机的压缩比在610之间。而实际的压缩比取决于气门相位，因为实际的压缩行程开始于进气气门关闭时刻。类似地，做功行程的膨胀比取决于排气气门相位。奥托循环发动机的压缩比和膨胀比近乎相等，阿特金森循环发动机是保持压缩比不变，增大膨胀比。在最新款的丰田Prius中，膨胀比为13.0；应用米勒循环发动机（米勒循环可以看作增压的阿特金森循环）的马自达 MILLENIA车上，其压缩比设定为8。由于阿特金森循环在压缩行程中，进气气门关闭延迟，使得部分混合气体被推回到进气岐管中，这样每次进入燃烧室的理论空燃比的混合气体量便相对减少了，而做功行程又相对增加了做功量，所以燃油经济性得到了提高。如图2 所示（a）奥托循环 （b）阿特金森循环 图 2 奥托循环和阿特金森循环发动机每个工作循环消耗燃料对比（定性地）以丰田Prius为例，其发动机为1NZ-FXE，其进气门和排气门相位数值如表2所示表 2 Prius发动机的配气相位进气门正时开启，上止点前，018至25关闭，下止点后，072至105排气门正时开启，下止点前，034关闭，上止点后，02进一步地，参见图3图 3 阿特金森循环原理图上图体现了阿特金森循环的理念，汽油发动机的燃料能量作为驱动力的只有全部的1/3，其他的2/3的变成了排气损失、冷却损失和摩擦损失等。汽缸内膨胀行程的距离越大，则做功行程后温度越低，冷却损失减少，输出功率增加。但是正常工况的发动机膨胀比和压缩比相同，膨胀比增加导致压缩比增加，受爆燃影响膨胀比不易增大。为使压缩比小于膨胀比，开发了非正常工况的发动机（阿特金森循环发动机）。当活塞从下止点开始向上运动时，让气门关闭得更晚，这样就可以得到更高的燃油经济性。三 阿特金森循环发动机实现之可行性分析在保证发动机性能的基础上，对JL4G10发动机进行改造，使之成为阿特金森循环发动机。为此需首先了解JL4G10发动机。其主要参数如表3所示表3 JL4G10发动机的主要技术参数序号项目名称技术参数1型号JL4G102型式直列四缸、四冲程、水冷、爽顶置凸轮轴（DOHC）、16气门、智能连续可变进气相位（VVT-i）、链条传动电控燃油喷射汽油机3燃烧室形式交叉气流、单斜顶面平面型燃烧室4电控系统形式无分电器、分组点火、闭环、多点顺序喷射5缸径（mm） 行程（mm）69.066.5（属于短行程发动机）6总排量 （L）0.9957压缩比10：18额定功率（ kW/rpm）50/6009最大扭矩（Nm/rpm）87/4100430010全负荷最低燃油消耗率（g/kWh）25511最低空载稳定转速（怠速）rpm（空调A/C ON时 ）12排放（g/km）CO满足EU3排放要求HC13汽缸压缩压力（MPa /rpm）1.8/25014点火次序1342（14缸和23缸分组点火）15火化塞电极间隙（mm）16燃油牌号 GB17930-199993号及以上无铅车用汽油17机油容量（L）（干式充满）4.018冷却液容量（L）（带储液罐）6.019起动方式（12V）电启动20排气温度 （摄氏度）85021汽油机干质量（ kg ）8722外形尺寸mm （长宽高）56058060023冷却方式强制循环水冷24润滑方式压力与飞溅25机油消耗（g/kWh）1.826冷却水温度（摄氏度）885（最高）27机油牌号SAE10W-30,，API质量等级SG以上（冬季寒冷地区SAE 5W-30）28机油压力（kPa）怠速（rpm）29453929气门间隙（冷态）进气门 （mm）排气门 （mm） 在奥托循环发动机的基础上将其改造为阿特金森循环发动机，需要兼顾技术可行性和生产可行性。而且成本是一个很重要的约束条件。结合吉利汽车混合动力车上拟采用原型发动机-JL4G10发动机（属于低速型发动机），进行可行性分析。图 4 JL4G10发动机1提高实际膨胀比，可采取的措施减小燃烧室的容积。调整气门正时，使进气气门延迟关闭，可通过改变进气凸轮的型线。同时仍可适当减少压缩比。注：目前JL4G10发动机的压缩比为10。2减少摩擦和磨损，可采取的措施降低经济转速：燃料消耗率最小时对应的发动机转速，称为经济转速。一般处于最大扭矩和最大功率对应的转速之间。获得更高的燃油经济性的同时，最高功率和最大扭矩值均有所减小。可利用混合动力系统的电动机进行补偿。由此带来的另一个益处是降低了高转速运行时零部件的强度要求,即轻量化得以实现。具体说来：可以采用曲轴轻量化，活塞环低张力化，减少气门弹簧的刚度等措施使得摩擦损耗降低。曲轴偏置曲轴偏置是将曲轴中心偏离缸径线，其结果是连杆可以保证垂直而减少了侧压力，从而减少了摩擦如图5所示。JL4G10的曲轴偏置为8mm。（注：该技术并非混合动力所特有。）图 5 曲轴偏置示意图 活塞偏置活塞偏置是为了减免在上止点换向时“敲缸”现象的发生，如图6所示。活塞销中心线偏离活塞中心平面，向做功行程中受侧压力的一方偏移了12mm。这种结构可使活塞在压缩行程到做功行程中较为柔和地从压向汽缸的一侧到压向汽缸的另一面，以减小“敲缸”的声音。（注：JL4G10发动机活塞是否偏置待确认。由于将JL4G10改造为阿特金森发动机后，其运行在燃油效率高的区间而不再接近或达到峰值转速，所以即便不采用活塞偏置，“敲缸”的矛盾应不会突出。）图 6 “敲缸”现象示意图3排气系统 在油电混合动力车上，在电动机的辅助作用下，发动机的负载变动受到抑制，因此催化剂的暖热也被抑制在低水平上。如果我们的混合动力车的设计方案为完全混合动力，意味着发动机是间歇工作的频率会相对更高些，即频繁地启动和停止。在目前排放标准日益严厉的条件下，当混合动力的发动机在冷起动时，如何抑制排放就成为关键问题。4冷却系统由于混合动力车发动机的峰值功率降低，燃油效率提高。这减轻了冷却系统的压力。但另一方面，大功率拖动电机需要共用车体前端的散热器和风扇，因此又在一定程度上增大了冷却系统的压力，因此需要重新设计冷却系统的总布置及风扇、散热器等部件的参数选择。由于完全混合动力车的发动机间歇运转，当发动机停止运转、电机运转模式时，电机需要依赖水泵带动循环水实现冷却。所以，传统发动机中由曲轴通过皮带带动水泵的方式不可行，混合动力车上的水泵由电力常时拖动。5起动系统由于ISG（集成起动机/发电机）具有兼有起动机功用，因此取消了传统意义上的直流启动电机。6耗能设备常规汽车上，发电机时主要电源，而且通常安装在发动机上，由于全混合动力车的发动机为间歇工作，因此常规发动机上由发动机直接驱动的功能部件需要变更为电力拖动。（1）空调A/C如图4所示。JL4G10发动机上由发动机曲轴通过皮带带动空调压缩机工作，混合动力车中变更为电动空调。（2）助力转向如图4所示。JL4G10上的助力转向装置为常时保持油压的油压助力转向，是由发动机曲轴通过皮带带动工作，混合动力车中变更为电动助力转向。（3）制动辅助装置常规汽车在汽车制动时利用真空制动辅助装置减轻驾驶员操作力，它是利用发动机进气歧管的真空负压为力源，对液压制动装置进行加力。混合动力车中要变更为电动制动辅助。四小结在我们应用混合动力技术时，辨证思辨是有必要的。比如发动机紧凑化和整车整备质量之间是对立统一的。在发动机燃油效率最佳点，发动机的输出功率值大约为其额定功率的3248之间（平均40左右）。以JL4G10为例，其额定功率为50kW，则发动机最佳燃油经济性时的输出功率为1624kW。理论上可通过使用更小规格的发动机来解决储备功率过大带来的问题。较小规格的发动机不是为了满足急加速和爬陡坡的要求而设计，而是满足缓加速和爬缓坡的要求。当要求急加速的时间很短或爬较长的陡坡时，利用储存在蓄电池中的电能驱动电动机提供辅助动力。当蓄电池能量不足时，需要利用发动机充电以保证其充电容量。但是发动机紧凑化存在着实际的限制。由于混合动力车上发动机质量的减少的同时却增加了电池和马达的质量。汽车需要能够以合理的速度爬上较长的斜坡，即使电池放电耗尽电能的情形下。这些都约束了发动机的规格不能过小。再比如，提高燃油经济性要综合考虑，不仅依靠发动机的贡献度，还应包括电动机、整车零部件轻量化及空气动力性能等的提升，这将牵涉到新技术、新工艺和新材料的应用。这些都要受控于成本分析和价值工程（VA/VE）。参考资料1 汽车构造图册细川武志（著） 魏朗译 人民交通出版社 20042 The inside story on the Miller-cycle engine download from 3 Efficiency of Atkinson engine maximum power densityElsevier Science Ltd Vol. 39 No. 3/4,pp.337-341 19984 汽车构造与原理（上册发动机）蔡兴旺编 机械工业出版社 20045 混合动力电动汽车专题（九）-Prius的发动机 吉利汽车研究院 20056 混合动力电动汽车专题（八）-Prius的动力系统的组成部件 吉利汽车研究院 20057 混合动力电动汽车专题（三）-Prius的参数 吉利汽车研究院 20058 Performance analysis and comparison of an Atkinson cycle coupled to variable temperature heat reservoir under maximum power and maximum power density conditionsEnergy Conversion and Management 462637-2655 20059 汽车环保新技术松本廉平（著） 曹秉钢等译 西安交通大学出版社 200410内燃机的控制方法专利号：02819896.4 中华人民共和国知识产权局 200511奥托循环、狄塞尔循环、阿特金森循环的改进方法专利号：200410090895.9中华人民共和国知识产权局 2005附文： Variable valve timingVariable valve timing, or VVT, is a generic term for an automobile piston engine technology. VVT allows the lift or duration and timing (or all) of the intake or exhaust valves (or both) to be changed while the engine is in operation. OverviewPiston engines normally use poppet valves for intake and exhaust. These are driven (directly or indirectly) by cams on a camshaft. The cams open the valves (lift) for a certain amount of time (duration) during each intake and exhaust cycle. The timing of the valve opening and closing is also important. The profile of these cams is optimized for a certain engine rpm, and this tradeoff normally limits low-end torque or high-end power. VVT allows the cam profile to change, which results in greater efficiency and power. Most simple VVT systems (like Mazdas S-VT) advance or retard the timing of the intake or exhaust valves. Others (like Hondas VTEC) switch between two sets of cams at a certain engine rpm. Still others can alter duration and lift continuously. HistoryThe first experimentation with variable valve timing and lift was performed by General Motors. GM was actually interested in throttling the intake valves in order to reduce emissions. This was done by minimizing the amount of lift at low load to keep the intake velocity higher, thereby atomizing the intake charge. GM encountered problems running at very low lift, and abandoned the project. The first functional variable valve timing system, including variable lift, was developed at Fiat. Developed by Giovanni Torazza in the 1970s, the system used hydraulic pressure to vary the fulcrum of the cam followers. The hydraulic pressure changed according to engine speed and intake pressure. The typical opening variation was 37%. The next big step was taken by Honda in the 1980s and 90s, where Honda began by experimenting with variable valve lift. Pleased with the results, engineers took the knowledge and applied it to the B16A engine, fitted to the 1989 EF9 Honda Civic. From there it has been used in a variety of applications, from sport to utility, by many different auto makers. VVT Implementations BMW VANOS - Varies intake and exhaust timing by moving the fulcrum of the camshaft Ford Variable Cam Timing - Varies valve timing by rotating the camshaft GM DCVCP (Double Continuous Variable Cam Phasing) - Varies timing with hydraulic vane type phaser Honda VTEC - Varies intake, duration, and lift by using two different sets of cam lobes Honda i-VTEC - Adds cam phasing (timing) to traditional VTEC Hyundai/Kia CVTT Mazda S-VT - Varies timing by rotating the camshaft Mitsubishi MIVEC - Varies valve lift Nissan VVL - Varies intake, duration, and lift by using two different sets of cam lobes Porsche VarioCam - Varies intake timing by adjusting tension of a cam chain Porsche VarioCam Plus - Varies intake timing by adjusting tension of a cam chain as well as valve lift by different cam profiles Rover VVC - Varies timing with an eccentric disc Suzuki VVT Subaru AVCS - Varies timing (phase) with hydraulic pressure Toyota VVT-i - Varies intake timing by advancing the cam chain Toyota VVTL-i - Varies timing by advancing the cam chain and switching between two sets of cam lobes The inside story on the Miller-cycle engine The name for the Miller-cycle engine comes from an American engineer, Mr Ralph Miller who patented his version of the forced induction Otto-cycle in the 1940s. Until now his principle had only been used in low engine speed applications - such as driving big ships and also for power generation by stationary engines. The engine in the Mazda Millenia is a 2.3 litre, quad cam V6 which is designed to perform better than a larger 3.0 litre engine but with the efficiency of a smaller (2.0L) unit. It provides the driver with high performance coupled with between 10 and 15 percent less fuel consumption. Power and torque figures are: 164kW of power 5.500rpm and 294Nm of torque 3,500 rpm. This compares with 149kW 6500 and 223Nm4800 for the base 2.5 litre vehicle equipped with the conventional Otto-cycle engine. From the outside, the Miller engine looks similar to other hi-tech units. Aluminium block, lots of belts, 24 valves, four camshafts, except for the two intercoolers and a belt driven Lysholm compressor tucked neatly into the Vee between the cylinder banks. So, how does this 2.3 litre engine produce more power and torque using less fuel than a larger engine, without many of the expected disadvantages; such as high emissions and engine knock?In simple terms, the compression stroke of the Miller-cycle engine is shortened with results in a low compression ration, yet a high expansion ratio. In order to grasp this and other aspects of the Miller-cycle, one has to go back and have a look at some of the basic principles of internal combustion engine operation. There are four areas worth reviewing. Engine Size vs Frictional Losses When the displacement of an engine is reduced, there is a substantial reduction in frictional losses. For example, 25 percent less friction is produced rotating a particular engine that has its displacement reduced by 30 percent. An automatic offshoot of such downsizing is an improvement in fuel efficiency of around 10-15 percent. Theoretical vs Actual Compression Ratio The theoretical compression ratio is simply a comparison of the volume above the piston when it is at bottom dead centre (BDC), to the volume above it at top dead centre (TDC). However, in practice, the actual compression ratio is determined by the valve timing, since the real compression stroke does not begin until the intake valve closes. Similarly, the length of the power (expansion) stroke is also determined by the opening point of the exhaust valve. With the fairly symmetrical valve timing being found in most engines these days, these two strokes are approximately the same. This means that the actual compression stroke is roughly equal to the expansion stroke. Thermal Efficiency By increasing the compression ratio, the thermal efficiency of an engine is also increased. However, along with this efficiency gain comes higher combustion pressures and temperatures. These characteristics are usually accompanied by two well known bad guys Oxide of Nitrogen (NOx) emissions and knock. NOx is produced as a result of combustion pressures and temperatures greater than 1,300 deg C. At these temperatures the normally inert Nitrogen (78 percent by volume of intake air), reacts with oxygen to form oxides (nitrogen dioxide and nitrogen monoxide). Knock is caused when part of the air/fuel charge is ignited spontaneously by the effect of heat and pressure and not the spark plug as Otto intended. This produces two flame fronts in the combustion chamber which can result in serious engine damage. There are two important things to note here. Firstly, knock is affected by the gas temperature at TDC of the compression stroke. Secondly, most of the gain in thermal efficiency from increases in compression comes mainly from the events that occur on the expansion stroke (more push on the piston). Only a little is gained from the actual increase in compression ratio. Pumping Losses This refers to the energy required to rotate an engine during two of the three non-power producing strokes - pumping air in and pumping exhaust gases out (but does not include frictional losses). It is a term that describes the efficiency of intaking and exhausting the charge. If the piston does less work in taking and exhausting, less power robbing pumping losses are produced. One of the reason the original Otto-cycle had the exhaust valve opening brought forward (before BDC) is to allow the residual exhaust gas pressure (which, once the piston is half way down the power stroke is too low to provide much push on the piston) to expel itself and not have to rely on the piston to pump it all out, creating further pumping loss. This modified (Otto) valve timing allows around 50 percent of the exhaust gases to be expelled for free (no pumping losses incurred in getting rid of half of the exhaust gas). A throttled engine (eg cruising with high manifold vacuum) has high pumping losses since a vacuum is not produced for free; energy is consumed in doing so. Some experimental variable displacement engines reduce the number of working cylinders (switching some off by holding the valves open) under partload to reduce manifold vacuum and therefore pumping losses. Volumetric Efficiency The term volumetric efficiency refers to the ability of an engine to fill its cylinders with a volume of air equal to their displacement (100 percent Ve). The greater the Ve then the greater will be the output of that engine. Engine manufacturers go to great lengths to tune their engine design and obtain the greatest Ve. This involves a lot of research into gas flow - including manifold and port design - as well as valve timing and lift, together with multiple valves and combustion chamber design. The easiest way to make dramatic improvements in Ve is to add an external device such as a supercharger or turbocharger. Its job is to force feed as much air as possible into each cylinder. But, as with increased compression ratio, excessively high combustion pressures and temperatures may be produced by forced induction. These can work against our intent to produce a powerful but clean engine. The most common method to overcome this problem is to use an intercooler (as well as lowering the compression ratio). An intercooler is an air-to-air heat exchanger that has the ability to reduce air intake temperature (after the supercharger) by at least 50 deg C. This helps keep combustion temperatures to a safe level. The modern internal combustion engine is a finely balanced mixture of all these (and many more) conflicting requirements. Miller-cycle Technical Details ere are basically four means that the Miller-cycle uses to obtain its increased efficiency. Smaller engine (lower displacement) reduced compression stroke and pumping losses (from late closing of the intake valve) cooler intake charge (intercooled air) combustion improvements Small Engine The graph below indicates the fuel efficiency increase as displacement is decreased. The horizontal axis begins at 1.0 which compares to a 3.3Ls fuel efficiency, whilst 0.7 ind