共轨技术

随着人们对低油耗、低废气排放、发动机低噪声的需求越来越大，对发动机和燃油喷射系统的要求也越来也高。对柴油发动机燃油喷射系统提出的要求也在不断增加。更高的压力、更快的开关时间，以及根据发动机工况修订的可变的流量速率曲线，已经使得柴油发动机具有良好的经济性、低污染、高动力性，因此柴油发动机甚至进入了豪华高性能轿车领域。达到这些需求的前提是拥有一个可以精确雾化燃油并具有高喷油压力的燃油喷射系统。同时，喷油量必须精确计算，燃油流量速率曲线必须有精确的计算模型，预喷射和二次喷射必须能够完成。一个可以达到以上需求的系统即共轨燃油喷射系统。

共轨系统包括以下几个主要的部分：

①低压部分，包含燃油共轨系统组件。

②高压系统，包含高压泵、油轨、喷油器和高压油管等组件。

电控柴油机系统EDC主要由系统模块，如传感器、电子控制单元和执行机构组成。共轨系统的主要部分即喷油器。它们拥有一个可以快速开关喷嘴的执行阀(电磁阀或压电触发器)，这就允许对每个气缸的喷射进行控制。

所有的喷油器都由一个共同的油轨提供燃油，这就是“共轨”的由来。

在共轨燃油喷射系统中，燃油喷射和压力的产生是分开的。喷油压力的产生与发动机转速和喷油量无关。EDC控制每个组件。

(1) 压力产生。

燃油喷射和压力的产生是通过蓄能器分离开来。将具有压力的燃油提供给为喷射做好准备的共轨系统的蓄能器。

由发动机驱动的连续运转的高压泵提供所需喷油的压力。无论发动机的转速高低，还是燃油喷射量的多少，油轨中的压力均维持在一定值。由于几乎一致的喷油方式，高压泵的设计可以小的多，而且它的驱动转矩可以比传统燃油喷射系统低，这源于高压泵的负载很小。

高压泵是径向活塞泵，在商用车上有时会使用内嵌式喷油泵。

(2) 压力控制

所应用的压力控制方法主要取决于系统。

一种控制油轨压力的方式是通过一个压力控制阀对高压侧进行控制。不需喷射的燃油通过压力控制阀流回到低压回路。这种控制回路允许油轨压力对不同工况(如负载变化时)迅速做出反应。

在第一批共轨系统中采用了对高压侧的控制。压力控制阀安装在燃油轨道上更可取，但是在一些应用中，它被直接安装在高压泵中。

另一种控制轨道压力的方式是进口端控制燃油供给。安装在高压泵的法兰上的计量单元保证了泵提供给油轨精确的燃油量，以维持系统所需要的喷油压力。发生故障时，压力安全阀防止油轨压力超过最大值。

在进口端对燃油供给的控制减少了高压燃油的用量，降低了泵的输入功率。这对燃油消耗起到积极的作用。同时，流回油箱的燃油温度与传统高压侧控制的方法相比得到了降低。

双执行器系统也是一种控制轨道压力的方式，它通过计算单元对压力进行控制，并且通过压力控制阀对高压端进行控制，因此同时具备高压侧控制与进口端燃料供给控制的优势。

(3) 燃油喷射

喷油器直接将燃料喷到发动机的燃烧室。它们由与燃油轨道直接相连的短高压油轨提供燃油。发动机的控制单元通过与喷油器结合在一起的控制阀的开闭控制喷油嘴的开关。

喷油器的开启时间和系统油压决定了燃油供给量。在恒压状态下，燃油供给量与电磁阀的开启时间成正比，因此与发动机或油泵的转速(以时间为计量的燃油喷射)无关。

(4) 液压辅助动力

与传统燃油喷射系统相比，将压力的产生与燃油的喷射分离开来，有利于燃烧室的充分燃烧。燃油喷射压力在系统中基本可以自主选择。目前最高燃油压力为1600巴，将来会达到1800巴。

共轨系统通过引入预喷射或多次喷射可以进一步减少废气排放，也能明显降低燃烧噪声。通过多次触发高速转换阀的开闭可以在每个喷射周期内实现多达5次的喷射。喷油针阀的开闭动作是液压辅助元件助力的，以保证喷射结束的快速性。

(5) 控制和调节

发动机的控制单元通过传感器检测加速踏板的位置以及发动机和车辆的当前工况。采集到的数据包括：

① 曲轴转速和转角；

② 燃油轨道的压力；

③ 进气压力；

④ 进气温度、冷却液温度和燃油温度；

⑤ 进气量；

⑥ 车速等。

电控单元处理输入信号。与燃烧同步，电控单元计算施加给压力控制阀或计算模块、喷油器和其他执行机构(如EGR阀，废气涡轮增压器)的触发信号。

喷油器的开关时间应很短，采用优化的高压开关阀和专业的控制系统即可实现。

根据曲轴和凸轮轴传感器的数据，对照发动机状态(时间控制)，角度/时间系统调节喷油正时。电控柴油机系统(EDC)可以实现对燃油喷射量的精确计算。此外，EDC还拥有额外的功能以进一步提高发动机的响应特性和便利性。

其基本功能包括对柴油燃油喷射正时的精确控制，和在给定压力下对油量的控制。这样，它们就保证了柴油发动机具有能耗低、运行平稳的特点。

其他开环和闭环控制功能用于减少废气排放和燃油消耗，或提供附加的可靠性和便利性，具体例子有：

① 废气在循环控制；

② 增压控制；

③ 巡航控制；

④ 电子防盗控制系统等。

(6) 控制单元结构。

由于发动机控制单元通常最多有8个喷油器输出口，所以超过八缸的发动机需要两个控制单元。它们通过内置高速CAN网络的“主/从”接口进行连接，因此也拥有较高的微控制器处理能力。一些功能被 分配给某个特定的控制单元(如燃料平衡控制)，其功能根据需求情况(如检测传感器信号)可以动态地分配给一个或多个控制单元。

The Common Rail

Calls for lower fuel consumption, reduced exhaust-gas emission, and quiet engines are making greater demands on the engine and fuel-injection system. The demands placed on diesel-engine fuel-injection systems are continuously increasing. Higher pressures, faster switching times, and a variable rate-of-discharge curve modified to the engine operating state have made the diesel engine economical, clean, and powerful. As a result, diesel engines have even entered the realm of luxury-performance sedans. These demands can only be met by a fuel-injection pressure. At the same time the injected fuel quantity must be very precisely metered, and the rate-of-discharge curve must have an exact shape, and pre-injection and secondary injection must be performable. A system that meets these demands is the common-rail fuel-injection system.

The main advantage of the common-rail system is its ability to vary injection pressure and timing over a broad scale. This was achieved by separating pressure generation (in the high-pressure pump) from the fuel-injection system (injection).The rail here acts as a pressure accumulator.

Principle of the Common Rail

The common-rail system consists of the following main component groups:

① The low-pressure stage, comprising the fuel-supply system components;

② The high-pressure system, comprising components such as the high-pressure pump, fuel-rail, injector, and high-pressure fuel lines.

The electronic diesel control (EDC), consisting of system modules, such as sensors, the electronic control unit, and actuators. The key components of the common-rail system are the injectors. They are fitted with a rapid-action valve (solenoid valve or piezo-triggered actuator) which opens and closes the nozzle. This permits control of the injection process for each cylinder.

All the injectors are fed by a common fuel rail, this being the origin of the term “common rail”.

In the common-rail fuel-injection system, the function of pressure generation and fuel injection are separate. The injection pressure is generated independent of the engine speed and the injected fuel quantity. The electronic diesel control (EDC) controls each of the components.

(1) Pressure Generation.

Pressure generation and fuel injection are separated by means of an accumulator volume. Fuel under pressure is supplied to the accumulator volume of the common rail ready for injection.

A continuously operating high-pressure pump driven by the engine produces the desired injection pressure. Pressure in the fuel rail is maintained irrespective of engine speed or injected fuel quantity. Owing to the almost uniform injection pattern, the high-pressure pump design can be much smaller and its drive-system torque can be lower than conventional fuel-injection systems. This results in a much lower load on the pump drive.

The high-pressure pump is a radial-piston pump. On commercial vehicles, an in-line fuel-injection pump is sometimes fitted.

(2) Pressure Control

The pressure control method applied is largely dependent on the system.

One way of controlling rail pressure is to control the high-pressure side by a pressure-control valve. Fuel not required for injection flows back to the low-pressure circuit via the pressure-control valve. This type of control loop allows rail pressure to react rapidly to changes in operating point ( e. g. in the event of load changes ) .

Control on the high-pressure side was adopted on the first common-rail systems. The pressure-control valve is mounted preferably on the fuel rail. In some applications, however, it is mounted directly on the high-pressure pump.

Another way of controlling rail pressure is to control fuel delivery on the suction side. The metering unit flanged on the high-pressure pump makes sure that the pump delivers exactly the right quantity of fuel rail in order to maintain the injection pressure required by the system. In a fault situation, the pressure-relief valve prevents rail pressure from exceeding a maximum.

Fuel-delivery control on the suction side reduces the quantity of fuel under high pressure and lowers the power input of the pump. This has a positive impact on fuel consumption. At the same time, the temperature of the fuel flowing back to the fuel tank is reduced in contrast to the control method on the high-pressure side.

The two-actuator system is also a way of controlling rail pressure, which combines pressure control on the suction side via the metering unit and control on the high-pressure side via the pressure-control valve, thus marrying the advantages of high-pressure-side control and suction-side fuel-delivery control.

(3) Fuel Injection.

The injectors spray fuel directly into the engine’s combustion chambers. They are supplied by short high-pressure fuel lines connected to the fuel rail. The engine control unit controls the switching valve integrated in the injector to open and close the injector nozzle.

The injector opening times and system pressure determine the quantity of fuel delivered. At a constant pressure, the fuel quantity delivered is proportional to the switching time of the solenoid valve. This is, therefore, independent of engine or pump speed( time-based fuel injection ).

(4) Potential Hydraulic Power.

Separating the functions of pressure generation and fuel injection opens up future degrees of freedom in the combustion process compared with conventional fuel-injection systems; the injection pressure at pressure at present is 160 MPa; in future this will rise to 180 MPa.

The common-rail system allows a future reduction in exhaust-gas emissions by introducing pre-injection events or multiple injection events and also attenuating combustion noise significantly. Multiple injection events of up to five per injection cycle can be generated by triggering the highly rapid-action switching valve several times. The nozzle-needle closing action is hydraulically assisted to ensure that the end of injection is rapid.

(5) Control and Regulation.

The engine control unit detects the accelerator-pedal position and the current operating states of the engine and vehicle by means of sensors. The data collected includes:

① Crankshaft speed and angle;

② Fuel-rail pressure;

③ Charge-air pressure:

④ Intake air, coolant temperature, and fuel temperature:

⑤ Air-mass intake:

⑥ Road speed, etc.

The electronic control unit evaluates the input signals. In sync with combustion, it calculates the triggering signals for the pressure-control valve or the metering unit, the injectors, and the other actuators ( e.g. the EGR valve, exhaust-gas turbocharger actuators, etc. ).

The injector switching times, which need to be short, are achievable using the optimized high-pressure switching valves and a special control system.

The angle/time system compares injection timing, based on data from the crankshaft and camshaft sensors, with the engine state ( time control ). The electronic diesel control (EDC) permits a precise metering of the injected fuel quantity. In addition, EDC offers the potential for additional functions that can improve engine response and convenience.

The basic functions involve the precise control of diesel-fuel injection timing and fuel quantity at the reference pressure. In this way, they ensure that the diesel engine has low consumption and smooth running characteristics.

Additional open-and close-loop control functions perform the tasks of reducing exhaust-gas emissions and fuel consumption, or providing added safely and convenience. Some examples are:

① Control of exhaust-gas recirculation;

② Boost-pressure control;

③ Cruise control;

④ Electronic immobilizer, etc.

(6) Control Unit Configuration.

As the engine control unit normally has a maximum of only eight output stages for the injectors, engines with more than eight cylinders are fitted with two engine control units. They are coupled within the “ master/slave ” network via an internal, high-speed CAN interface. As a result, there is also a high microcontroller processing capacity available. Some functions are permanently allocated to a specific control unit ( e.g. fuel-balancing control ). Other can be dynamically allocated to one or many of the control units as situation demand ( e.g. to detect sensor signals ).

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