外文翻译--一些中速柴油发动机研究的实验经验

中文4730字

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学院：机电与汽车学院

专业：热能与动力工程

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2014年 5月 5日

一些中速柴油发动机研究的实验经验

文摘:这篇文章的目的是探究一种中速发动机的一些实验结果。这里事实上是进行了单缸共轨柴油机的研究, 这是研究的方向。其主要特征是气体交换阀门调节时间系统是完全可调的电液系统,利用发动机润滑油在250帕时打开气体交换阀门。此外这个引擎没有涡轮增压器, 但是一个单独的空气压缩机供应系统可以改变进气管空气的状况; 在引擎后部、有一个蝶状的改变排气背压的管子。燃油系统是一种常见的轨道类型：轨道的压力、喷射时刻、喷射时间是开始完全可调。这些研究进行了所有的EVE发动机可能性状况研究: 不同的负载、管道压力,开始喷射时刻和边界条件都是可调的。此外，该实验对气体交换阀门的液压系统的不同时间响应性能进行了检测、评价。

介绍

由于相关部门日益严格的限制，高油价和减少排放控制带来的挑战成为内燃机的发展必须面对的越来越富有挑战性的课题。在这些方面，如何研究使得在配置最优化基础上得到的完美结果成为了一种大挑战。

极限引擎是一个四冲程单缸大孔径中速研究的发动机，这种柴油机与

W?rtsil? W20气缸尺寸相似，它是由芬兰的阿尔托大学的国际内燃机研究小组设计的。引擎框架、曲轴和主要轴承能承受缸内最大压力为400 帕[1]。EVE有电液阀执行机构(EHVA) [2] 代替传统的凸轮轴机制，该系统允许改变高气体交换的弹性阀参数。事实上，它可能改变开启和关闭时间的气体交换阀，以及改变其最大气门升程以及它们的开启和关闭提升斜率。阀门执行机构受作用于润滑系统的加压油(250 bar)的控制.EVE连接有一个电动马达，这样可以运行在机动模式。燃油喷射系统是可调的共轨式轨道的压力, 以及喷射时间和持续时间。根据边界条件, 如进气压力、温度以及排气压力，发动机可以被一个空气供给体制和一个反馈压力节流阀所控制，这样就允许运行的涡轮增压器具有无限配置的状况下运行。迄今为止,除了提到的部件的数量外，同时所有的压力和温度的辅助系统(LT水、HT水和润滑油)都是远程调节的。所有这些特点使本发动机的研究成为出色的研究平台。

本文的研究重点是对EVE提出一些可以利用的可行性，不同的气体交换配气相位，喷雾参数和边界条件都被测试。用先前的测试运行, 在那里分析比较已经生产出的相应商业引擎,记录下相应的负荷变化。

首要工作描述的是米勒的一个应用技术。米勒循环[3]可以用来降低NOx: 它是通过适时改变阀门定时减少发动机的压缩比的。

这可以通过在比较早的进气行程的时候关闭进气门 (EIVC)来实现,可以通过在压缩行程关闭进气阀很晚(LIVC),也可以通过在进气结束后的压缩行程开启简要的排气阀实现。本文使用第一种方法:事实上引用Ivc的气门定时时非常接近上止点时刻, 关闭阀门等40多个CAD BBDC是很先进的。米勒技术首先被用在EVE上。根据以前的经验(见例如[4 ~ 7),

一项新的策略在这里将展开讨论。这些保持在气缸的气团时间与气门的保持时间是一致的。质量不能被测量，但是却能得到一维模拟模型，在这项工作中用三种不同载荷进行试验。

表1 EVE 规格

缸径200mm 行程 280mm 连杆 610mm 排量8796 cc

发动机转速900r/min 理论压缩比15.0 喷油器提示(米勒测试)8孔x

0.34毫米喷油器提示(喷射试验)×0.30毫米9洞所用燃料LFO (43 MJ /公斤)

研究1:米勒测试

这些测试的主要目的是测试使发动机在不同的负荷下的阀门作用时间导致

的NOx很大的降低量。在实验装置中,具有流体模拟功能的发动机仿真已经制作出来了，需要通过找到这些边界条件来进行仿真。沿比较以前的W20也被制作出来了,作用 100%, 75% 和 50% 负载进行试验。针对每种情况密封的空气质量也一直不断沿着参考的时刻而变化。同时油耗一直不断, 因为引擎额定功率视为平均有效值，并且油量在每一个测试中都是不变的。此外，充气温度和模拟涡轮增压器的效率没有改变。

气门定时

实验对不同的一些气门作用时刻进行了测试,以便验证EHVA运行情况。除了IVC外所有的参数都保持不变。由于EHVA[2]的基础上的液压技术，正在作用的IVC升程的幅度的平滑误差小于1mm. 虽然不同的进气定时被设置，但事实上,在这两个米勒测试周期内其结果看起来几乎相同, 因为这个原因, 进气门的关闭可进行计算机模拟，而当升程是1毫米是,就叫做IVC1。当提升进气阀关闭后,被系统获得的最大升程就减小了。因此, 对测试时间(视为IVC1)没有比BBDC 42计算机辅助设计(CAD) 更先进的了。图1给出了来自这三个时间的研究:

1。参考时间，有5 CAD BBDC的IVC1 和最大值为17.1的进气升程;

2、高级计时，有35 CAD BBDC的IVC1 和最大值为16.5的进气升程

3。最先进的米勒计时,IVC1 42 CAD BBDC和最大进气阀抬的15.8毫米。

排气时间和IVO在每个运行顶点是恒定的；最大的排气气门升程为17.1毫米。排气关闭和打开斜度是不变的：这是因为在EVE中，出于安全的原因，在每一个曲轴转角中气门和活塞距离保持大于或等于4毫米。

图1——米勒测试：测试的气门定时

仿真模型

仿真采用发动机仿真进行，一维的流体动力的程序用于预测发动机的性能。由于EVE系统没有涡轮增压器, 发动机仿真模型的一个基本用处就是找出发动机装置使EVE边界条件可以相当类似于一个真正的发动机。对涡轮增压器的数学模型进行了模拟,分别模拟涡轮和压缩机。该模型是需要找到充入的空气压力、排气压力;此外,负责设定空气温度在每个试验中是相同的。而用于计算压缩机和涡轮动力吸入的大气和废气压力的方程式如下 [8]:

压缩功率（KW）压缩效率进气流量（kg/s）

进气比热[kJ /公斤K] 环境温度[K] 空气压缩比

空气比热汽轮功率[kW) 涡轮效率

尾气流[公斤/ s]. 废气比热[kJ /公斤K]

废气温度[K] 尾气膨胀率废气比热

这台机器效率总TC效率为0.65。该值是根据以前的测试数据计算出的，在这项工作中，这是在每一个普通负载理想的平均估计效率。此外，另一个在计算模型的假设是:

?环境条件: 298 K, 是1.005 kJ /kgk、是1.4。

?上游压缩机压力和下游涡轮压力1bar;因此，和也是测试值空气压力、废气的排气压力,用帕作单位。

?压缩机模型装置设在进气管的面前，而涡轮模型在汽车排气管道靠近调节阀门处。当阀门定时改变时，控制装置可以在发动机运作中得到相应流量运作情况。特别是:

o因为同样数量的燃料注入，SOI被调整以便适应同样的发动机功率匹配；

o进气压力也被调整，以便达到相同的收集到的气体质量；

o 根据施加的效率改变排气压力以平衡模拟TC；

模拟的结果绘出了如下面的图所示。特别的,进气管空气压力、泵气损失和喷射也被记录了。

为了保持相同的空气质量在气缸盖内, 当进气时间减少时充气气压需要增加,也就是说IVC是先进的。在图2中可以

看出, 在高负载时他的数值超过了来自

相关情况下的1.5帕。

图2——米勒模拟结果：进气压力图3——米勒模拟结果：泵送平均有效压力

图四——米勒模拟结果：喷射时刻图5—米勒模拟结果：指示的氮氧化物值

试验结果

测量试验进行了快速和慢速两个试验。下面的图表有关工作的主要成果进行了说明。由于EVE具有比多缸发动机较高的机械损失[9], 结果也表明出来了这一点。在每一个测试负荷下NOx目标值均降低了(图5)。最好的结果来自部分荷载：事实上,50%的减少量是3.5克/千瓦时的ISNOx, 这意味着40%的相应值，这是通过标准时间获得的(图6)。这些大幅降低实现是由于利用了早期的IVC(米勒技术)和以后SOI技术。在之前的EVE运行测试中相比组合使用的相同时间米勒技术。

唯一可能延迟的是SOI会指示氮氧化物的减少相比较使用组合米勒技术在相同的时间内，这仅仅使用的低速SOI能有效减少NOx 的产生。另一方面，实现同样的功率输出需要更多的燃料——即要求长时间喷射燃油。这样导致的一个后果是非常高的废气温度以及可能导致的不稳定燃烧。这些效果对整体经济的发动机而言太消极了，因此，其他方式——例如使用米勒循环——都必须经过研究,就是为了让它达到在实际应用的结果中可以接受。

Some Experimental Experience Gained With a Medium-Speed Diesel Research Engine Abstract:The objective of this paper is to show some experimental results gained from a medium-speed research engine. The study is in fact carried out with a

single-cylinder common rail diesel engine (EVE), which is used only for research purposes. Its main feature is that the gas exchange valve timing is completely adjustable with an electro-hydraulic system that uses the engine lubrication oil at 250 bars to open the gas exchange valves. In addition the engine does not have a turbocharger, but a separate air compressor supply system that permits to change freely the intake charge air conditions; after the engine, a butterfly valve tunes the exhaust back pressure. The fuel system is a common rail type: rail pressure, start of injection and injection duration are fully adjustable. The studies are carried out exploiting all the possibilities of the EVE engine: different loads, rail pressures, starts of injection and boundary conditions are modified. Nevertheless the hydraulic system of the gas exchange valves is changed to test and evaluate the performance with different timings.

Two studies are described in this paper. The first is an application of the Miller technique, advancing the closure of the intake valve. The purpose of this work is a massive reduction of the NOx emission with no penalties in fuel consumption. The setup of the loads with Miller cycle is found with the help of a simulation model. The results show that high NOx reduction is achievable with the used strategy at every run load but the greatest decrease occurs at partial load. The major drawback is the increase of soot formation in the runs with very advanced intake valve closing.

INTRODUCTION

The development of the internal combustion engines has to face more and more the challenging issues of lowering the fuel consumption because of high oil price and of emissions reduction, due to the increasingly stricter limits imposed by the regulating authorities. In these respects the possibility to optimize the engine operating parameters in order to find the most performing configurations is a big advantage.

The Extreme Value Engine (EVE) is a four-stroke single-cylinder large bore medium-speed research engine. The engine has similar cylinder dimensions with

W?rtsil? W20 engine and it is designed by the Internal Combustion Engine Research Group of Aalto University in Finland. The engine frame, the crankshaft and the main bearings can withstand incylinder maximum pressure of 400 bar [1]. The EVE has electro-hydraulic valve actuators (EHV A) [2] instead of traditional camshaft mechanism. This system permits to have high flexibility of the gas exchange valve parameters. In fact it is possible to modify the opening and closing timing of the gas exchange valves, their maximum valve lift as well as their opening and closing lift

slope. The valve actuators are controlled by pressurized oil (250 bar) that is the same oil used in the lubrication system. The EVE is connected to an electric motor, which also allows running in motored mode. The fuel injection system is common rail type with adjustable rail pressure, injection timing and duration. The engine boundary conditions, such as intake air pressure and temperature as well as exhaust pressure, are controllable by an air supply system and a backpressure throttle valve; this allows running operating points with an infinite configuration of turbochargers. Besides the quantities mentioned so far, also all pressures and temperature of the ancillary systems (LT water, HT water and lubrication oil) are remotely adjustable. All these features make this research engine anoutstanding platform for engine research.

The studies in this paper are focused to present some of the possibilities that can be exploited with EVE. Different gas exchange valve timing, injection parameters and boundary conditions are https://www.360docs.net/doc/b33721987.html,ing previous test runs, where the comparison with its corresponding commercial engine has been carried out, the reference loads are drafted out.

The first work described is an application of the Miller technique. The Miller cycle [3] is used to reduce NOx: it consists in a reduction of the effective engine compression ratio by opportunely changing the valve timing. This can be achieved by closing the intake valve very early in the intake stroke (EIVC), by closing the intake valve very late in the compression stroke (LIVC), by opening briefly the exhaust valve after the intake closure in the compression stroke. In this paper the first method is used: in fact from a reference valve timing with IVC very close to the BDC, the closure of the intake valve is advanced more than 40 CAD BBDC. The Miller technique was tested for the first time in EVE. On the basis of the previous experiences (see for instance [4-7]), a new strategy is here discussed. This consists in keeping the trapped air mass in the cylinder constant with all the valve timings. This quantity is not measured but obtained from the 1-D simulation model. Three different loads are tested in this work.

Table 1 EVE specifications

Cylinder Bore 200 mm Stroke 280 mm Connecting Rod 610 mm Swept V olume 8796 cc

Engine Speed 900 rpm Nominal Compression Ratio 15.0 Injector tip (Miller test) 8 holes x 0.34 mm

Injector tip (Injection test) 9 holes x 0.30 mm Fuel

used LFO (43 MJ/kg)

STUDY I: THE MILLER TESTS

The main purpose of these tests is a massive reduction of NOx using different valve timings at different engine loads. Before the experimental tests, the 1-D simulations with the fluid-dynamic code GT-Power have been carried out. These are needed to find the boundary conditions to be used in the runs.

Along the comparison with W20 previously carried out, 100%, 75% and 50% load have been run. For each case the trapped air mass has been kept constant along the reference timing. Also the fuel consumption has been constant because the engine power – considered as IMEP - and the fuel quantity are the same for every case. In addition, the charge air temperature and the efficiency of the simulated turbocharger have not been changed.

The Valve Timing

Several valve timings have been tested to validate the operation of EHV A. All the parameters are kept constant except the IVC. Due to the hydraulics of the EHV A [2], advancing the IVC the slope of the lift becomes very flat at lifts lower than 1 mm. In fact in the two tested Miller cycles the IVC looks to be almost the same, although different intake timing is set. For this reason, the intake valve closing is considered as the CAD when the lift is 1 mm and it is called IVC1. When advancing the closure of the intake valves the maximum lift attained by the system is reduced. Thus, tests with timings (considered as IVC1) more advanced than 42 CAD BBDC have not been run. In figure 1 there are presented the three timings chosen for this study:

1. the reference timing, with IVC1 of 5 CAD

BBDC and maximum intake valve lift of 17.1 mm;

2. an advanced timing, with IVC1 of 35 CAD BBDC and maximum intake valve lift of 16.5 mm;

3. the most advanced Miller timing, with IVC1 of 42 CAD BBDC and maximum intake valve lift of 15.8 mm.

The exhaust timings and the IVO are constant for every run point; the maximum exhaust valve lift is 17.1 mm. The exhaust closing and the intake opening slope are not constant: this is because in EVE, for safety reasons, the distance between the gas exchange valves and the piston is kept greater or equal to 4 mm at every crank angle.

The Simulation Model

The simulations are carried out with GT-Power, a one-dimensional fluid-dynamic program used for predicting engine performance. Since the EVE system has not any turbocharger, the GT-Power simulation model is a fundamental tool to find out the engine set-up so that the EVE boundary conditions can be quite similar to a real engine’s. The turbocharger is simulated with a mathematical model that treats separately the action of the compressor and of the turbine. This model is needed to find the charge air pressure and the exhaust backpressure; besides, the charge air temperature is set the same in every case. The formulas used to calculate the charge air and exhaust gas pressure are the equations of the compressor and turbine power [8]:

The machine efficiencies are so that the total TC efficiency is 0.65. This value is calculated from data of the previous tests and it is an average estimation of the efficiency at every load considered in this work. Furthermore, the other assumptions in the calculation model are:

?The ambient conditions: is 298 K, , is 1.005 kJ/kg K, is 1.4. ?The upstream compressor pressure and the downstream turbine pressure are 1

bar;therefore and represent also the values of the charge air pressure and of the exhaust backpressure, expressed in bar.

? The compressor model is set before the intake pipe and the turbine’s model after the exhaust pipe nearby the regulating valve. The controls permit to find the engine set-up to use in the test runs when the valve timing is modified. In particular:

o the SOI is adjusted to match the same engine power, since the same amount of fuel is injected;

o the charge air pressure is adjusted to achieve the same air trapped mass;

o the exhaust pressure is changed to balance the simulated TC along the imposed efficiency.

The results of the simulations are plotted below. In particular, the intake charge air pressure, the pumping losses and the start of injection are reported.

To maintain the same air mass trapped in the cylinder, the charge air pressure needs to be increased when the intake duration is reduced, i.e. IVC is advanced. In figure 2 it can be seen that at high load its value is raised more than 1.5 bar from the reference case.

The Test Results

Both fast and slow measurements have been taken during the tests. Here below the charts concerning the main outcomes of the work are illustrated. Since the EVE has

higher mechanical losses than a multi-cylinder engine [9], the results are here referred to the indicated power. The aimed NOx reduction is achieved at every tested load (figure 5). The best results come from the partial loads: in fact at 50% the reduction is 3.5 g/kWh of ISNOx, which means 40% of the reference value, which is obtained with standard timing (figure 6). The high reduction achieved is due to both the use of the advanced IVC (Miller technique) and the later SOI. Along the previous tests run with EVE, the only usage of retarded SOI may imply even greater NOx reduction than the combined use of the same timing with Miller technique. On the other hand, to achieve the same output power more fuel would be required –i.e. longer injection duration. As a consequence very high exhaust gas temperature as well as unstable combustion may result. These effects are too negative in the overall economy of the engine and, therefore, other means – e.g. the usage of the Miller cycle - have to be investigated in order to achieve outcomes that can be accepted also in a real application.