Abstract:
A fuel injection apparatus with a deterioration detection device that includes a volume changing chamber, the volume of which is determined by fuel pressure inside the injector. After injector nozzle opens, the time for the volume changing chamber to change from an initial volume to a target volume is measured and used for calculating changes in nozzle orifice size. The value of orifice size change can be used for both diagnosing injector deterioration and compensating fuel flow rate in a feedback control. In addition to detecting injector deterioration and failures, the volume changing device also dampens effects of noise in fuel pressure to fuel flow rate control and decreases chances of after-injection and second injection.

Description:
FIELD OF THE INVENTION 
       [0001]    This present application claims priority from U.S. provisional application No. 60/959,859 having the same title as the present invention and filed on Jul. 17, 2007. 
         [0002]    The present invention relates to a fuel injection apparatus that provides and controls fuel flow. 
       BACKGROUND OF THE INVENTION 
       [0003]    Fuel injectors are used to direct fuel pulses into a combustion chamber. Normally in a fuel supply system, fuel pumps deliver and pressurize fuel from a storage tank to fuel injectors or to an accumulator that connects to fuel injectors. Inside a fuel injector, a nozzle assembly including a nozzle valve (nozzle needle valve) is used to control fuel flow through nozzle orifices. At “off” position, the nozzle valve blocks fuel flow. When the nozzle valve moves to “on” position, fuel is presses through the orifices. The overall fueling rate is determined by the injector “on” time. 
         [0004]    Fuel injectors in different applications have different requirements for fuel pressure. For example, in CI (Compression Ignition) engines (diesel engines), high fuel pressure is needed for better fuel atomization and spray penetration, while in SI (Spark Ignition) engines, fuel pressure is lower due to the fuel/air pre-mixing nature. Even in some applications for diesel engines, for example, fuel dosing (used with a DOC in increasing exhaust air temperature) in regenerating DPFs (Diesel Particulate Filters), low pressure fuel systems (&lt;20 bar)are used. When the nozzle valve opens (injector at “on” status), fuel flow rate is a function of fuel pressure for a given injector. Accordingly, in fuel flow control, the injector “on” time is a function of fuel pressure inside the injector. 
         [0005]    To decrease emission, increase combustion efficiency, and improve combustion control performance, fuel flow needs to be controlled accurately. However, due to limitations of sensor accuracy, sensor response time, system complexity, and cost, it is hard to implement a feedback loop in fuel flow control. As a result, accurate nozzle valve timing control and steady fuel pressure inside the injector are required. 
         [0006]    Another important factor that affects fuel flow control accuracy is injector deterioration or aging. Normally fuel injectors (especially diesel fuel injectors) work in an environment with high temperature and high concentration of particulate matter. Consequently, carbon (and other particles) could build up on the nozzle surface, partially blocking nozzle orifices and deteriorating fueling control performance, resulting in poor system performance and more emission. The injector deterioration is more significant in applications with short operation time. For example, in aftertreatment systems using DPFs, high exhaust temperature is generated by burning fuel in a DOC or combustion chamber (burner), and a fuel injector is used in controlling fueling rate. Since the regeneration is only needed when too much soot accumulates in the filter, the interval between regenerations could be hours or days. In this application, there is long period of time for carbon to build up on injector surface without disturbing from fueling flow. Injector deterioration changes fuel spray pattern and causes large error between actual fueling rate and fueling command. These effects result in changes in system gain (ratio of exhaust temperature increase to fueling rate), and temperature control performance deteriorates therewith. 
         [0007]    If the nozzle orifice change is measurable, the value of changed orifice size can be used for compensating fueling control: when orifices become smaller due to deterioration, we can increase injector “on” time to compensate fueling rate change. However, due to limitations of sensor technology and OBD (On-board Diagnostics) technology, it is hard to accurately and reliably measure orifice change in-situ using a compact and economic device. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    It is an objective of the present invention to provide a fuel-injection device with an apparatus that can be used in examining orifice size change, the value of which then is used for adaptively adjusting fueling control. 
         [0009]    Another objective of the present invention is to provide a fuel-injection device with fuel flow insensitive to noise in fuel pressure. 
         [0010]    Yet another objective of the present invention is to provide a fuel-injection device that is resistive to after injection and second injection. 
         [0011]    In one embodiment of the present invention, these objectives are achieved by providing a fuel injector with a channel communicating to its nozzle pressure chamber. Inside the channel, a sliding piston with one end attached to a spring prevents high pressure fuel from flowing back to fuel tank. The piston moves when there is a difference between the fuel pressure drop (the difference between fuel pressure inside the pressure chamber and combustion chamber) and the stress provided by the spring. The channel together with the piston form an adjustable augmentation to the pressure chamber, and the volume of this chamber augmentation, which is determined by piston displacement, is proportional to the fuel gauge pressure. 
         [0012]    Inside the channel, the piston position is an indication of fuel gauge pressure. Therefore, by using the relation between the fuel pressure drop and piston displacement, we are able to detect changes in nozzle orifice size. In one exemplary application of the present invention, the injector is used in a common rail fuel injection system in which a shutoff valve is used for controlling fuel supply to the injector. In a diagnostic process, the shutoff value is energized first (fuel supply is then shut off). Then the injector is open. The time for the piston moving from a starting position, which is determined by fuel gauge pressure before the injector is open, to a stop position, is a function of orifice area: if piston mass is small, piston moving time is reversely proportional to orifice area. Accordingly, by timing the pressure decaying from an initial value to a target value, we are able to calculate orifice area. Resolution of the measurement is determined by the cross section area of the channel and the spring coefficient for a given pressure decay. 
         [0013]    In another embodiment of the present invention, electronic switches are installed inside the channel for detecting piston movement. This device allows a diagnostic process without using fuel pressure sensors, since pressure decay time can be obtained by detecting pulses generated by electronic switches. Further more, when multiple switches are installed or a position sensor is used, during an injection, the time for fuel pressure drops from an initial value to a value determined by fuel pressure distribution can be detected for calculating orifice area change. By using this method, orifice area can be monitored during normal operations (without a dedicated diagnostic process), and the value can also be used for adjusting fueling rate in real-time feedback control. 
         [0014]    In addition to being used for detecting orifice area change, the piston movement also dampens effects of noise in fuel pressure to fueling rate control. For high pressure injectors, the piston movement makes the channel act as a large orifice to a fast moving injector needle, thereby preventing trapped fuel from building up a high pressure pulse, which results in after-injection and second injection. Decreasing of the high pressure pulse at the end of injections also allows a small nozzle sac design, which reduces emission. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a cross-sectional schematic view illustrating the bottom part of an injector with a deterioration detection apparatus according to a first embodiment of the present invention; 
           [0016]      FIG. 2   a  and  FIG. 2   b  shows curves of fuel pressure drop in pressure decay processes; 
           [0017]      FIG. 3  illustrates an example using a deterioration detection apparatus in accordance with the present invention in a common rail fuel injection system; 
           [0018]      FIG. 4  is a flow chart of an example in which orifice area is measured in a diagnostic process with a deterioration detection apparatus in accordance with the present invention; 
           [0019]      FIG. 5   a  and  FIG. 5   b  are cross-sectional schematic views showing a second embodiment of the present invention; 
           [0020]      FIG. 6   a  and  FIG. 6   b  are timing diagrams of signals obtained from electronic switches installed on a deterioration detection apparatus in accordance with the present invention, injector control signals, and signals acquired from fuel pressure sensors; 
           [0021]      FIG. 7  illustrates changes in fuel flow rate with pulse-like noise in fuel supply in an injector with a deterioration detection apparatus according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    As illustrated in  FIG. 1 , in an embodiment of the proposed injector  100 , high pressure fuel flow through a channel  101  into a pressure chamber  102 . A valve needle  104  controls fuel injection, which starts when the needle leaves orifices  103  and stops when the orifices are obturated. In addition to the channel  101 , there is another channel  105  connecting to the pressure chamber. A sliding piston  106  is inside the channel  105  with one end attached to a spring  107 . Above the spring  107 , there are a retainer  109  and a fuel passage  108 . Fuel leaked from the channel  105  is discharged to fuel tank through the passage  108 , and the pressure inside the passage is approximately ambient pressure. The piston  106  moves when the pressure inside the chamber  102  varies, and the movement is limited by another retainer  110 . 
         [0023]    Assuming the flow through the nozzle  103  is quasi-steady, the flow rate is a function of fuel pressure drop: 
         [0000]        {dot over (m)}   f   =C   D   A   n √{square root over (2 ρ   f   Δp )}  (1) 
         [0000]    , where 
         [0000]    
       
      
       Δp=p 
       i 
       −p 
       c  
      
     
         [0000]    ; p i  is the fuel pressure inside the injector pressure chamber, p c  the fuel pressure in combustion chamber, {dot over (m)} f  the fuel mass flow rate, C D  the discharge coefficient, A n  the overall cross section area of orifices (orifice area), and ρ f  the fuel density. 
         [0024]    In the channel  105 , fuel volume change can be described using the following equation: 
         [0000]        m{umlaut over (v)}+k   f   {dot over (v)}+k   e   v=− ( p   i   −p   0 ) A   2    (2) 
         [0000]    where m is the mass of the piston  106 , k f  the friction coefficient of the channel  105  to the piston  106 , k e  the spring coefficient, A the cross section area of the channel  105 , p 0  the pressure in the fuel passage  108 , and the volume v is defined by the following equation: 
         [0000]      v=xA   (3) 
         [0000]    In equation (3), x is the distance between the position when the spring  107  is in equilibrium status, and the position when a pressure (p i −p 0 ) is applied. 
         [0025]    When the weight of the piston  106  is very small and thus negligible, and the friction coefficient or moving speed of the piston  106  is very low, equation (2) can be approximated using the following equation: 
         [0000]        k   e   v=− ( p   i   −p   0 ) A   2    (4) 
         [0000]    Equations (1) and (2) or (4) describes the relations among the pressures p i , p 0 , and p c , fuel mass flow rate {dot over (m)} f , and the movement of the piston  106 . When fuel flow to the injector is cut off, e.g., when a shut-off valve between the fuel supply to the injector is energized, if the injector nozzle is open, the fuel mass flow will be solely provided by the movement of the piston  106 : 
         [0000]      {dot over (m)} f ={dot over (v)}ρ f    (5) 
         [0000]    In this situation, according to equation (1), (4) and (5), the relations among fuel pressure, combustion chamber pressure, and ambient pressure can be described using the following equation: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           - 
                           
                             A 
                             2 
                           
                         
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                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
         [0000]    Assuming during injection, ambient pressure and combustion chamber pressure keep constant, i.e., {dot over (p)} 0 =0, and {dot over (p)} c =0, then solving equation (6), we have the solution: 
         [0000]    
       
         
           
             
               
                 
                   
                     Δ 
                      
                     
                         
                     
                      
                     p 
                   
                   = 
                   
                     
                       ( 
                       
                         
                           
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                         - 
                         
                           
                             
                               
                                 c 
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                                 k 
                                 e 
                               
                                
                               
                                 
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                                    
                                   
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                           t 
                         
                       
                       ) 
                     
                     2 
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
         [0000]    In equation (7), 
         [0000]    
       
      
       Δp 
       0 
       =p 
       i 
       0 
       −p 
       c 
       0  
      
     
         [0000]    ; p i   0  and p c   0  are, respectively, the injector fuel pressure and combustion chamber pressure when the nozzle starts to open. According equation (7), if the spring  107  is relaxed when (or before) the piston  106  reaches the retainer  110 , the pressure decay time t d , which is defined as the time from the moment the nozzle starts to open until a zero pressure drop (Δp=0) is detected, is a function of the orifice area: 
         [0000]    
       
         
           
             
               
                 
                   
                     t 
                     d 
                   
                   = 
                   
                     
                       
                         A 
                         2 
                       
                        
                       
                         
                           2 
                            
                           
                             ρ 
                             f 
                           
                            
                           Δ 
                            
                           
                               
                           
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                         C 
                         D 
                       
                        
                       
                         A 
                         n 
                       
                        
                       
                         k 
                         e 
                       
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
         [0000]    Thus, the orifice area A n  can be calculated using the following equation: 
         [0000]    
       
         
           
             
               
                 
                   
                     A 
                     n 
                   
                   = 
                   
                     
                       
                         A 
                         2 
                       
                        
                       
                         
                           2 
                            
                           
                             ρ 
                             f 
                           
                            
                           Δ 
                            
                           
                               
                           
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                         C 
                         D 
                       
                        
                       
                         t 
                         d 
                       
                        
                       
                         k 
                         e 
                       
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
         [0026]      FIG. 2   a  shows decay curves of the fuel pressure drop Δp. In this figure, a curve  201  shows the curve of fuel pressure drop in a healthy injector. When the orifice area decreases, the fuel pressure drop follows a curve  202 . Using equation (9), by measuring the pressure decay time, we can monitor the orifice area change. And this calculated orifice area value can also be used for adaptively adjusting the pressure compensation table (according to equation (1)) in fuel injection pulse control. 
         [0027]    Another method for detecting change in orifice area is measuring fuel pressure drop Δp before the piston  106  reaches the retainer  110 . According to equation (7), if the fuel pressure drop at time t s  (t s &lt;t d ) is Δp s , then, 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       t 
                       s 
                     
                     = 
                     
                       
                         
                           
                             A 
                             2 
                           
                            
                           
                             
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                                
                               
                                 ρ 
                                 f 
                               
                             
                           
                         
                         
                           
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                             A 
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                   and 
                 
               
               
                 
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                   10 
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                               f 
                             
                           
                         
                       
                       
                         
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                              
                             
                                 
                             
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                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
         [0000]    Therefore, based on the pressure at the moment when the nozzle is open and pressure at time t s , we can calculate the orifice area using equation (11). 
         [0028]    If the length of channel  15  is short, and when the piston  106  reaches the retainer  110 , the spring  107  is still in compression, fuel pressure decay time is shorter than that calculated using equation (9). In this situation, as shown in  FIG. 2   b,  the value of Δp drops sharply when the piston  106  hits the retainer  110  at time t e . The sudden pressure drop is caused by a much faster decay process at t e . Accordingly, based on equation (11), we can monitor the value change of Δp and use the pressure value Δp e  at t e  for calculating the orifice area: 
         [0000]    
       
         
           
             
               
                 
                   
                     A 
                     n 
                   
                   = 
                   
                     
                       
                         
                           A 
                           2 
                         
                          
                         
                           
                             2 
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                               ρ 
                               f 
                             
                           
                         
                       
                       
                         
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                   ( 
                   12 
                   ) 
                 
               
             
           
         
       
     
         [0029]    As depicted in  FIG. 2   b,  curves  203  and  204 , respectively, are the fuel pressure drop of a normal nozzle and a deteriorated one. If initial pressure Δp 0  is the same, for a deteriorated injector with a smaller orifice area, the moment when the fast decay starts (t′ e ) comes later than that for a normal injector (t e ). Therefore, the degree of deterioration can also be monitored by simply measuring the slow pressure decay time t e , when keeping the initial pressure constant. 
         [0030]      FIG. 3  shows an example of using the proposed injector in a common rail diesel fuel injection system. In this system, a fuel pump  301  is used for providing high pressure fuel supply to a common rail accumulator  307  through a fuel line  304 . A shutoff valve  305  controls fuel flow in the fuel line  304 , and the fuel pressure inside the accumulator  307  is measured by a pressure sensor  308 . An injector  306  is connected to the accumulator  307 . Low pressure fuel is supplied to the fuel pump  301  through a fuel line  302 , while a fuel return line  303  is connected to both of the pump  301  and the injector  306  for ducting fuel back to a fuel tank (not shown in  FIG. 3 ). The fuel pressure inside the accumulator  307  is controlled by an ECM  309  through the pump  301  with fuel pressure fed-back from the sensor  308 . The injector and the shutoff value are also controlled by the ECM  309 . 
         [0031]    A diagnostic process example for this common rail fuel system is illustrated in  FIG. 4 . The diagnostic process can be realized using an interrupt service routine in the ECM. At the beginning of this process, with the nozzle diagnostic enabled, a shutoff valve status flag is examined. If the status flag is “on”, the shutoff valve  305  is energized and fuel flow in the fuel line  304  is shut off. The shutoff valve status flag is then set to “off” thereafter, and a fuel pressure decay timer is enabled. Before the routine ends, the fuel pressure drop value is read in and saved as Δp 0 . If the status flag is “off”, then fuel pressure drop value is read in and compared with a threshold. The system defines the ending time of pressure decay as the moment when the fuel pressure value is lower than this threshold. If pressure decay ends, the fuel pressure decay timer value is saved as t d , otherwise the routine ends except the decay time is too long. An overly long decay time is caused by a blockage in the nozzle. At this situation, a fault flag is set to report a serious deterioration. After pressure decay ends or a too long decay time is detected, the shutoff valve  305  is de-energized, and the fuel pressure decay timer is cleared. The shutoff valve status flag is reset to “on” for the next diagnosis. 
         [0032]    After the diagnostic process completes, orifice area is calculated using values of Δp 0  and t d  according to equation (9). The orifice area value can be used for adaptively adjusting fueling by correcting fuel pressure, injection pulse duration (according to equation (1)), and/or used directly to report a deterioration when a significant change is detected. 
         [0033]    In a fuel injection system, depending on the location and response time of the fuel pressure sensor, there could exist an uncertainty in detecting the pressure decay ending time. This uncertainty affects the accuracy in measuring orifice area, and according to equation (9), the error is determined by the pressure decay time: smaller error is induced by long pressure decay time and vice versa. In order to decrease this uncertainty, an electronic switch can be used for detecting the pressure decay ending time. As illustrated in  FIG. 5   a,  a cap  502  is connected to the piston  106  through a rod  501 . The retainer  109  limits the position of the piston  106  before fuel pressure drops to combustion chamber pressure. An electric switch  503  is installed inside the channel  105  and above the retainer  109 . When the cap  502  passes by the switch  503  during pressure decay, it presses the switch and thereby an electrical pulse signal is generated. An ECM  504  connecting to the electrical switch  503  detects the pulse signal, and calculates the decay time t d  as t d =t 1 −t 2 , where t 1 is the time when injection starts and t 2  the time when the switch pulse signal is detected. 
         [0034]    Variations in the initial pressure Δp 0  also affect the measurement of orifice area. To decrease this effect, another electrical switch can be used for detecting the initial position of the piston  106 . As depicted in  FIG. 5   b,  in addition to the switch  503 , another switch  601  (also connects to the ECM  504 ) is positioned above the switch  503 . When high pressure fuel enters the channel  05 , the piston  106  together with the cap  502  is pushed upwards. At pressure Δp 0 , the switch  601  is pressed by the cap  502  and an electrical pulse signal is generated. This pulse signal is received by the ECM  504  as an indication of fuel pressure drop reaching Δp 0 . (This feature can also be used for diagnosing fuel pressure sensor or control failures that causes low pressure fuel supply) During a pressure decay process, when fuel pressure drops, the cap  502  moves downwards. After the cap  502  leaves the switch  510 , a pulse is generated. This pulse is detected by the ECM  504 , and a timer is enabled. When the cap  502  reaches at the switch  503 , another pulse is generated, and the timer is disabled. The timer value is the decay time t d  and used for monitoring the injector deterioration. 
         [0035]      FIG. 6  a shows the signal timing chart in such a system. The injection command is a pulse  601 . High level of this pulse enables injection. When the shutoff value is closed during injection, fuel pressure drop Δp (curve  602 ) decreases after the injection starts. When the Δp value drops below Δp 0  at a moment t 1 , a falling edge of a pulse  604  is generated by the switch  510 . As the Δp value is below a level, at a moment t 2 , the cap  502  presses the switch  503 , a rising edge of a pulse  605  is generated. The interval between the moments t 1  and t 2  is the decay time t d . When orifice area becomes smaller, the Δp value drops slower (curve  602 ) according to equation (8). As a result, the moment when the cap  502  reaches at the switch  503  (t′ 2 ) becomes later (pulse  606 ). The difference between t 2 −t 1  and t′ 2 −t 1  is an indication of orifice area variation. 
         [0036]    In an injector shown in  FIG. 5   b,  by employing the two electronic switches, even if there is not a shutoff value in the system, we are still able to monitor the orifice area change, though the relation between the piston moving time and orifice area is more complex that that described by equation (9). 
         [0037]      FIG. 6   b  illustrates the timing chart for an injection system without shutoff value. At the rising edge of the injection command  601 , due to pressure gradient, the piston  106  moves as fuel pressure inside the channel  105  drops (curve  610 ). When the Δp value drops below Δp 0 , the switch  510  is released and a falling edge of pulse  612  (t 1 ) is generated. As the fuel pressure decrease causes the piston  106  press the switch  503 , a rising edge of pulse  614  (t 2 ) is generated. For a deteriorated injector with smaller orifice, due to slower pressure drop (pulse  611 ), under the same operating conditions (same combustion chamber pressure), the moment when the switch  503  is pressed on (t′ 2 ) is later (pulse  613 ). Therefore, the interval between the pulse falling edge generated by the switch  510  and the pulse rising edge generated by the switch  503  is an indication of injector deterioration. 
         [0038]    In the system depicted in  FIG. 5   b,  if cost allows, electrical switches  503  and  510  can also be replaced with a strain/distance/position sensor, e.g. an induction ring, an eddy current sensor, a potentiometer, or an optical sensor. Through the position sensor, more detailed kinetics of piston movement can be monitored, and thereby a more sophisticated model can be used for better identifying injector deterioration and accurately calculating orifice area value during normal operations. 
         [0039]    In addition to monitoring changes of actual orifice area, the apparatus of the present invention can also be used for detecting failures equivalent to orifice area change, such as injector leakage and injector being stuck off, and injector being stuck open. When an injector is stuck open, the ECM cannot command fuel injection off. As a result, extra fueling will be directed into combustion chambers. This extra fuel causes emission issues, and more seriously, if a DOC is installed at the downstream of the injector, the fuel could be burnt inside the DOC, causing safety issues. Using the apparatus in the present invention, if after the shutoff valve is energized, a fuel pressure decay is detected before the injector is commanded open, or when the injector is commanded open, however no pressure decay is detected, then the injector is stuck open. 
         [0040]    The injector of the present invention is resistive to noise in fuel pressure. In the injector depicted in  FIG. 1 , changes in fueling rate can be described using an equation: 
         [0000]        {dot over (v)}   in   ={dot over (v)}   out   +{dot over (v)}   (13) 
         [0000]    where {dot over (v)} in  is the volumetric flow rate of fuel entering the injector, {dot over (v)} out  the volumetric injection fuel flow rate and 
         [0000]        {dot over (v)}   out   ={dot over (m)}   f   /ρ   f    (14) 
         [0000]    , {dot over (v)} the rate of injector volume change caused by the piston ( 106 ) movement. 
         [0041]    According to equations (1), (4), (13), and (14), the fuel pressure drop Δp, which determines fueling rate, follows the relationship: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       Δ 
                        
                       
                           
                       
                        
                       p 
                     
                   
                   = 
                   
                     
                       
                         
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         [0000]    When the system is at steady status, i.e., {dot over (Δ)}{dot over (p)}=0, equation (15) shows that the rate of feeding volumetric flow {dot over (v)} in , , equals to the injection fueling rate {dot over (v)} out  (equation (14)), meaning that the injection fueling is fully provided by the fuel supply to the injector. When the volumetric flow rate {dot over (v)} in  varies, the fuel pressure drop Δp will change with it. However, with the movement of the piston  106 , a term 
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         [0000]    is created and this term is deducted from the volumetric flow rate {dot over (v)} in . As a result, the overall variation is damped. 
         [0042]      FIG. 7  shows injection fueling rate change when there is a pulse-like sudden variation (curve  701 ) in the feeding flow rate {dot over (v)} in . At the rising edge of the pulse (curve  701 ), the injection fueling rate {dot over (v)} out  (curve  702 ) immediately follows {dot over (v)} in . Then, with the movement of the piston  106 , a flow to the channel  105  decreases the change in injection fueling rate. When piston moving speed slows down, injection fueling rate increases, and when the piston  106  eventually stops moving, the injection fueling rate {dot over (v)} out  equals to {dot over (v)} in  again. Similar change of fueling rate can be found at the falling edge of the feeding flow pulse. According to equation (15), and as shown in  FIG. 7 , piston movement acts like a band-rejection filter in damping variations of injection fueling rate. The channel area A and the spring coefficient k e  are the two most important parameters of the filter. 
         [0043]    In addition to damping variations of injection, the piston movement can also help in decreasing after-injection and secondary injection. At the end of an injection, when the needle valve is catapulted back towards to its seat, without the piston  106 , the high pressure fuel trapped inside the pressure chamber may cause a reflecting pulse that “bounces” the needle and generates after-injection. Coupling with the movement of return spring, the reflecting pulse may further travel back and forth inside the high pressure chamber, unseating the needle and leading to a secondary injection. The after-injection and secondary injection spill unexpected extra fuel during the latter portion of an expansion stroke causing extra HC (Hydrocarbon), CO (Carbon Monoxide), and PM (Particulate Matter) emission. 
         [0044]    In an injection system with the piston  106 , the piston movement releases part of the high fuel pressure built up during the fast moving of injector needle. According to equation (15), when there is a sudden change of Δp value, the large value of pressure changing rate {dot over (Δ)}{dot over (p)} causes large value of the term {dot over (p)}A 2 /k e , which is deducted from the pressure Δp, thereby the change of the pressure Δp is decreased. Consequently, after-injection and secondary injection due to the fast movement of injector needle are decreased. Decrease of high fuel pressure pulses also allows small sac nozzle design, which helps in decreasing emission.