Parametric Study of Injection Rates With Solenoid Injectors Stephen Busch1 Mem. ASME Sandia National Laboratories, in an Injection Quantity and Rate P.O. Box 969, MS 9053, Livermore, CA 94551-0969 Measuring Device e-mail: [email protected] Paul C. Miles A Moehwald HDA (HDA is a German acronym: Hydraulischer Druckanstieg: hydraulic pressure increase) injection quantity and rate measuring unit is used to investigate injec- Downloaded from http://asmedigitalcollection.asme.org/gasturbinespower/article-pdf/137/10/101503/6395620/gtp_137_10_101503.pdf by guest on 30 September 2021 Sandia National Laboratories, tion rates obtained with a fast-acting, preproduction diesel solenoid injector. Experimen- P.O. Box 969, MS 9053, tal parametric variations are performed to determine their impact on measured injection Livermore, CA 94551-0969 rate traces. A pilot–main injection strategy is investigated for various dwell times; these e-mail: [email protected] preproduction injectors can operate with very short dwell times with distinct pilot and main injection events. Dwell influences the main injection rate shape. A comparison between a diesel-like fuel and a gasoline-like fuel shows that injection rates are compara- ble for a single injection but dramatically different for multiple injections with short dwells. [DOI: 10.1115/1.4030095]

Introduction (isentropic) injection into the chamber, the injection mass flow rate can be expressed as follows: Development and optimization of diesel engines require detailed understanding and control of the fuel injection and mix- dP ture preparation processes. Advances in injector hardware such as dm V V dP direct acting piezo actuators or pressure-balanced, fast-acting sol- ¼ dt ¼ (1) dP 2 enoid valves create new possibilities for advanced fuel injection dt c dt strategies, particularly those with close coupling between pilot dq and main injections. In order to understand how these strategies affect mixture preparation and combustion, and to support model- where m is the mass of fluid in the chamber, V is the volume of ing work, it is of great interest to measure the rate of injection. the chamber, t is the time, P is the chamber pressure q is the fluid This work is concerned with the measurement of several prepro- density, and c is the speed of sound in the fluid. duction solenoid injectors using a Moehwald HDA injection quan- Integration of this equation between two points in time (and tity and rate measuring unit. It is of interest to characterize the thus two discrete pressures) yields a cumulative mass injector rate shapes in terms of their important features and repeat- ð ð ability, and also to understand the effects of some simple but pos- s2 dm P2 1 dP sibly important parameters such as digital signal filtering, m12 ¼ dt ¼ V 2 dP (2) s dt P cPtðÞðÞ dt temperature, axial clamping force, high pressure line length, and 1 1 pull-up time. Also, the variance between injectors is of interest, as is the injection rate behavior in multiple injection strategies. A So, for a given chamber volume, the mass flow rate and cumula- detailed analysis of the causes for the observed trends is not tive mass can be determined if the time-dependent pressure and within the scope of this work. pressure-dependent speed of sound are known. Whereas Zeuch assumed a constant value for the compressibility of his fuel [1], in the HDA, the speed of sound is measured directly. Hence, uncer- tainties in fluid properties, such as density and bulk modulus, do Experimental Setup not adversely affect the accuracy of the results. A schematic diagram of the HDA is shown in Fig. 1. The cylin- Moehwald HDA. The Moehwald HDA is a commercially drical chamber has a volume of 128 ml, and its walls are tempera- available injection rate and mass measurement device that utilizes ture controlled via an external heated recirculator that pumps heat the change in hydraulic pressure that results from injecting fluid transfer fluid through the chamber walls. A piezoresistive pressure into a closed, fluid-filled chamber. It is essentially an evolution of sensor is mounted on the cylinder wall halfway between the top Zeuch’s original measurement device [1]. The underlying mea- and the bottom of the chamber. The pressure signal is passed surement principle relies on both the measured pressure in a through an analog anti-aliasing filter with a cutoff frequency of constant-volume chamber and the speed of sound in of the fluid 25 kHz before being digitized with 16-bit resolution at a rate of within the chamber. With the assumption of a reversible, adiabatic 100 kHz. Additionally, the digital processing of the pressure sig- nal accounts for the temperature dependence of the pressure sen- 1Corresponding author. sor sensitivity, as well as nonlinearities in the sensor output. Contributed by the Combustion and Fuels Committee of ASME for publication in Finally, the pressure signal is digitally filtered using one of several the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received February 28, 2015; final manuscript received March 4, 2015; published online March raised-cosine, finite-impulse-response (FIR) filters; several combi- 31, 2015. Editor: David Wisler. nations of cutoff frequency and roll-off factor can be selected in The United States Government retains, and by accepting the article for the Moehwald software. Available cutoff frequencies range from publication, the publisher acknowledges that the United States Government retains, a 3 kHz to 10 kHz. nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for United States government To measure the speed of sound, an electronic pulse is sent to a purposes. piezoceramic sensor, which creates a pressure wave that travels

Journal of Engineering for Gas Turbines and Power OCTOBER 2015, Vol. 137 / 101503-1 Copyright VC 2015 by ASME continuously, a linear interpolation of the two time-averaged val- ues is applied. The resulting interpolated sound speeds are used with Eqs. (1) and (2) to calculate the injection rate and the total injected fuel mass. In the case of multiple, closely spaced injection events (i.e., a pilot–main–post injection strategy), the pressure is averaged between individual injection events to compute the injected mass of each injection event. The second sound speed measurement is made after the completion of the last injection event, and linear interpolation is performed as described above. After the second sound speed measurement, the pressure relief valve is opened and the pressure chamber returns to the desired starting pressure. The HDA is calibrated via a gravimetric procedure under spe- cific testing conditions determined by Moehwald. The calibration Downloaded from http://asmedigitalcollection.asme.org/gasturbinespower/article-pdf/137/10/101503/6395620/gtp_137_10_101503.pdf by guest on 30 September 2021 is then checked with a number of other tests. The total injected mass deviates slightly from the value measured with a precision balance for these measurement points, but Moehwald ensures that each and every HDA has nearly the same deviations from the gravimetrically measured values at each of these points. An evalu- ation of the factory calibration was not within the scope of this project. The analysis of multiple cycles yields statistical information about a particular set of operating conditions. This includes mean injected mass and standard deviation of injected mass, among other quantities. For the current work, no difference was observed Fig. 1 Schematic diagram of the Moehwald HDA injection quantity and rate measuring unit between the mean values and standard deviations when the statis- tics were taken over either 50 or 100 cycles. For this reason, each set of statistical data presented in this paper represents 50 injec- through the fluid in the chamber, is reflected at the top of the tions’ worth of data. The mass flow rate curves shown in this chamber, and detected by the same sensor upon its return. A clock work are ensemble-averaged over these same 50 cycles. speed of 75 MHz yields a timing resolution of 13.33 ns, and quan- In addition to the injection rate, the injection current is meas- tization noise is reported to be on the order of 0.1 m/s [2]. ured using a Pearson Model 411 Current Monitor placed around Neglecting any uncertainty in the chamber height, the total uncer- the injector cable. The resulting solenoid current sense signal is tainty in the speed of sound is estimated to be 0.25 m/s, which is digitized at 100 kHz. typically less than 0.025% of the measured value. Figure 2 depicts the measurement process for a single injection event. Before the injection starts, an average chamber pressure Fuel Injection System. The test bench setup, including the (P1) is taken over a suitable period and the speed of sound is common rail, HDA, and control systems, is shown in Fig. 3.A measured (c1). The desired excitation current is then sent to the in- personal computer (PC) is used to communicate with the injector jector solenoid, and fuel is injected into the chamber. After the driver and the HDA control module. Fuel is supplied by a small injection has completed, a second average pressure (P2) speed of feed pump to the high pressure, three-piston pump, after which it sound measurement is made (c2). The time-averaged pressures P1 flows into a production common rail. Rail pressure is controlled and P2 are used as the limits of integration in Eq. (2) to determine via modulation of the metering valve in the high pressure pump. the total injected fuel mass. The sound speed measurements are The injectors employed in this study are preproduction solenoid averaged with an averaging window of either 20 or 200 individual injectors with minisac nozzles. These are fast-acting injectors injection events (this can be configured in the Moehwald soft- with pressure-balanced nozzle control valves; their advantageous ware). For this work, 20 injection events are used for this averag- rapid dynamic response will be the subject of future studies. For ing, as no significant difference could be observed between the this study, three nominally identical injectors are used; they are two settings. Since the speed of sound cannot be measured

Fig. 2 Measurement of an injection event Fig. 3 Test bench setup

101503-2 / Vol. 137, OCTOBER 2015 Transactions of the ASME referred to as injectors A, B, and C. Every injector nozzle has • the length of the high pressure fuel line between the rail and seven evenly spaced holes, each with an outlet diameter of the injector 139 lm and conicity of 1.5. The included angle is 149 deg. A • the pull-up time for the solenoid current Genotec injector driver controls the rail pressure and sends the electric command signals to the injector solenoid. Measured rail Finally, three of these injectors are compared with one another pressures just before the time of injection are typically within 5 for the case of a pilot–main injection strategy for several different bar of the set point and the cycle-to-cycle standard deviation is dwell times. This provides information about injector-to-injector typically less than 5 bar. A mixture of 58 vol. % heptamethylno- variance and highlights some of the capabilities of the injectors nane and 42 vol. % n-hexadecane (DPRF58) is used as the work- used in this study. ing fluid for these investigations unless otherwise stated. In general, it is desired to recreate conditions as close to those Electrical resistance heaters are placed around the line between encountered during engine operation as possible. For all of the the injector and the rail and the portion of the rail surrounding the results shown here, the injection pressure (Prail) is held constant at port to which the injector is connected. The power dissipated by 800 bar. The axial clamping force on the injector is 14.30 kN the heaters is controlled by a variable transformer. Additionally, unless otherwise stated, as this is the clamping force that is used Downloaded from http://asmedigitalcollection.asme.org/gasturbinespower/article-pdf/137/10/101503/6395620/gtp_137_10_101503.pdf by guest on 30 September 2021 the upper portion of the injector body is heated using coils of elas- when the injector is used in our engine. Chamber backpressure (as tic tubing through which the heat transfer fluid used to condition measured before each injection event) is maintained at 5.6 MPa. the HDA is pumped. In this way, the fuel and injector temperature The chamber temperature (Tchamber) values reported are repre- can be varied; these temperatures are monitored using thermocouples sented by the temperature of the heat transfer fluid measured just placed on the fuel line and on the upper portion of the injector body. before it enters the chamber. Injection events (and thus measure- For some experiments, the line between the rail and the injector is ments) are triggered at a rate of 2.5 Hz. The pull-up current and holding current for this injector are nominally 18 A and 12 A, not heated. The rail temperature (Trail) is measured by a thermocou- ple at the high pressure fuel line approximately 50 mm upstream of respectively. Unless otherwise stated, the pull-up time for each the injector. The high pressure line is a rigid steel tube and has a injection event is set to 250 ls. The hold time makes up the bal- length of approximately 250 mm. It is used in each experiment ance of the actuation times reported here. In the case of multiple except for one, in which a flexible high pressure line and its adapter injections, the dwell time dt is defined as the delay between the are used; their combined lengths are approximately 570 mm. point at which the solenoid current begins to drop off at the end of The injector is inserted into an adapter, which is in turn inserted injector actuation and the point at which the solenoid current into a plastic sleeve in the top of the HDA’s measurement chamber. begins to rise for the next injection event; it is an energizing dwell The tip of the injector protrudes through the adapter and into the time. measurement chamber, and is sealed with an O-ring that is squeezed radially against the injector tip and axially against the HDA chamber Results outer wall. Consequently, a large axial force is not required to seal Figure 4 shows the solenoid current and rate of injection the interface between the injector and the HDA, in contrast to the (ensemble averaged over 50 individual injection events) for a sin- typical injector sealing arrangement used in an engine. gle injection with an actuation time of 900 ls in. The rate of injec- To investigate the potential impact of axial force on the injec- tion was determined using a digital filter cutoff frequency of tion rate, the injector is held into the adapter with a special bracket 10 kHz. The lighter shaded region around the mean rate of injec- and clamping mechanism. A capped, slotted hollow sleeve is tion curve represents two standard deviations above and below the placed around the injector body and a screw passing through a mean. It demonstrates that the shape of the injection rate curve, threaded through hole in the bracket applies force to the cap on including its details, is highly repeatable. All injection rate traces the hollow sleeve. By varying the tightening torque on this screw, in this work are similarly repeatable. A time of zero represents the the axial clamping force that normally provides the sealing force start of the actuation current; this convention is maintained for all on the injector can be varied. The threads of the screw are lubri- of the results shown. The total injected mass (averaged over 50 cated with graphite, and the coefficient of friction is assumed to injection events) for this case is 29.24 mg/str and the standard be 0.14. The thrust collar of the screw does not make contact with deviation is 0.17 mg/str. the bracket, so axial force on the injector is calculated from the tightening torque using the following equation [3]:

2s cos a l tan k F ¼ (3) dp l þ tan k cos a

where F is the axial force, s is the tightening torque, dp is the pitch diameter of the screw, a is the radial angle (60 deg), k is the lead angle, and l is the coefficient of friction in the threads (0.14). The injector and its adapter are held together with this clamping mechanism and are isolated from the measurement chamber with a specially designed, mechanically decoupled mounting mecha- nism on the top of the measurement chamber.

Experimental Parameters. The final purpose of this work is to examine and interpret the measured rate shapes for a variety of injection schedules. However, here, we report primarily on how some simple parameter variations affect the measured rate shapes and, in some cases, the measured injected mass. These variations include the following:

• digital FIR filter cutoff frequency Fig. 4 Injector solenoid current (dots) and mean rate of injec- • temperature of the rail, high pressure fuel line, and measure- tion curve (thick line) for a single injection with injector A. ment chamber Prail 5 800 bar; actuation time 5 900 lS; Tchamber 5 Trail 5 90 C. • axial clamping force on the injector Injection rate curve shown with 6 2r scatter bands.

Journal of Engineering for Gas Turbines and Power OCTOBER 2015, Vol. 137 / 101503-3 Examination of the rate of injection curve reveals some small Changing values of fc are observed to significantly affect the negative excursions before the rate of injection increases signifi- rate shape at the beginning and end of the injection event. As fc cantly. This phenomenon has also been observed by other increases, the detected hydraulic start of injection occurs later, but researchers examining rates of injection with solenoid injectors this effect becomes smaller as fc increases. When fc changes from using both Zeuch-based measurement techniques [4,5] and Bosch 7 kHz to 10 kHz, the hydraulic start of injection is delayed by or long tubes [6–8]. Needle lift data taken from a solenoid injector only 10 ls; this corresponds to 0.09 crank angle degrees at an by Kastengren et al. show a small initial needle lift before any sig- engine speed of 1500 r/min. The filter behavior is similar near the nificant lift occurs [9]. We attribute the negative excursion in the end of the injection, except that lower values of fc lead to later injection rate traces the increase in chamber volume (or, alterna- apparent ends of injection. tively, the backflow of fluid from the measurement chamber into Lower values of fc result in significantly different measured the nozzle) created by this initial needle motion before fuel begins rates of injection during the initial and final transients. The result- to flow out from the nozzle and into the chamber. Momentum flux ing combination of over and under prediction during the transient measurements taken by Manin et al. [7] indicate that the hydraulic seems to be the reason that no systematic change in the measured start of injection coincides with the zero-crossing of the injection total injected mass is observed as f is changed. No significant dis- c Downloaded from http://asmedigitalcollection.asme.org/gasturbinespower/article-pdf/137/10/101503/6395620/gtp_137_10_101503.pdf by guest on 30 September 2021 rate signal as it begins to increase significantly. This is assumed to advantages are observed with a cutoff frequency of 10 kHz, so be the case for the current study. The time of the hydraulic start of this value is used for all other experiments reported here (unless injection is determined by linear interpolation around the zero otherwise stated). crossing. The rate of injection increases rapidly at the beginning of the Fuel and Chamber Temperature. As the temperature of the injection, but the rate of increase decreases suddenly as the rate heat transfer fluid is increased and the electrical power delivered approaches 15 g/s. Results reported by Postrioti et al. suggest that to the resistance heaters is increased, the temperature measured at this behavior may change as injection pressure changes [5]. Later, the injector body is typically between 15 and 20 K cooler than the approximately 0.7 ms after the start of actuation and 0.35 ms after temperature measured either at the high pressure line upstream of the hydraulic start of injection, the rate of increase in injection the injector or at the heat transfer fluid before it enters the mea- rate again decreases. The rate of injection then continues to surement chamber wall. (We chose to keep these two tempera- increase nonmonotonically until it reaches its maximum 350 ls tures equal.) The measured injection rate shapes for a single after the end of actuation. The subsequent decrease in injection injection are shown in Fig. 6. rate occurs slowly at first and then more rapidly. After the appa- The initial phase of the injection is only affected slightly by rent hydraulic end of injection, there exist both positive and nega- temperature. The detected hydraulic start of injection changes by tive excursions. It is unknown if these are measurement errors due only 3 ls over the range of temperatures tested here. The maxi- to pressure fluctuations in the measurement chamber, mechanical mum rate of injection increases noticeably with increasing tem- vibrations, injector needle bounce, or some combination of these. perature, and the rate of injection decreases sooner as temperature In the current version of the Moehwald software, no correction is decreases. However, features of the rate shape are largely pre- made to account for the increase in fluid temperature resulting served even as temperature changes. The cumulative effect of this from the dissipation of the kinetic energy that is introduced into manifests itself in a variation of the total injected mass, which is the chamber; this also contributes to uncertainty, especially at the shown in Fig. 7. Error bars indicate one standard deviation in end of the injection event. Future optical spray measurements will either direction of the mean. As expected from the rate shapes, the likely provide more information about whether or not more fuel injected mass tends to increase as temperature increases. The exits the nozzle after the end of the main injection phase. most substantial increase occurs between 40 C and 60 C; the nonlinear decrease in fuel viscosity with increasing temperature is Digital Filter Cutoff Frequency. In order to illustrate the one factor that likely contributes to this behavior. effects of changing the filter cutoff frequency (fc), data are taken with each available combination of fc and filter roll-off factor b; smaller values of b result in a more rapid decrease in the filter fre- Injector Clamping Force. The tests to determine the effects of quency response about fc. Figure 5 shows the rate shapes meas- injector clamping force on rate shape are performed with the rail ured for a single injection for each of these filter settings. and the chamber at room temperature. These temperatures were

Fig. 5 Mean rates of injection with FIR filter cutoff frequency Fig. 6 Mean rates of injection with Tchamber (Tfuel) as a parame- (fc) as a parameter. Single injection with injector A; Prail 5 800 ter. Single injection with injector B; Prail 5 800 bar; actuation bar; actuation time 5 815 ls; Tchamber 5 Trail 5 90 C. time 5 615 ls.

101503-4 / Vol. 137, OCTOBER 2015 Transactions of the ASME Downloaded from http://asmedigitalcollection.asme.org/gasturbinespower/article-pdf/137/10/101503/6395620/gtp_137_10_101503.pdf by guest on 30 September 2021 Fig. 9 Total injected mass as a function of axial clamping force. Single injection with injector A; Prail 5 800 bar; actuation time 5 815 ls. Injected mass values shown with 6 2r scatter bars.

Fig. 7 Total injected mass as a function of Tchamber (Tfuel). Sin- gle injection with injector B; Prail 5 800 bar; actuation time 5 615 ls. Injected mass values shown with 62r bars to indicate scat- ter in measurements. not observed to change as testing progressed. Before each data point was taken, the hold-down screw was loosened and then tor- qued to the desired value. Figure 8 shows the observed differences in rate shape for a single injection case. Dashed lines are used for the zero clamping force and 4.77 kN clamping force traces. All of the other traces lie very close to one another. The fluctuations observed with zero clamping force that are not seen with any other traces are attributed to movement of the injector. This movement is nearly completely attenuated with a clamping force of 4.77 kN, as only a small deviation (near 0.5 ms after the start of actuation) from the expected rate shape is observed for this clamping force. The measured variation in the average injected mass is very small (<1% for clamping forces >0), and no consistent trend in total injected mass is observed, as shown in Fig. 9. Fig. 10 Mean rates of injection for two different line lengths. Single injection with injector B; Prail 5 800 bar; actuation time- High Pressure Fuel Line Length. Figure 10 shows that chang- 5 581 ls; Tchamber 5 90 C; Trail 5 20 C; Tinjector 5 66 C. ing the length of the high pressure line between the rail and the injector leads to very subtle differences in the injection rate shapes after the start of actuation, however, the measured rates of injec- during the initial phase of the injection. However, more significant tion diverge. The total injected masses are 12.37 mg/str with the differences appear after the end of actuation. Beyond 0.62 ms 250 mm line and 13.18 mg/str with the 570 mm line. It is apparent that line length can have a significant effect on the total injected mass for a constant actuation duration. Assuming a speed of sound of 1500 m/s at a pressure of 800 bar, the time required for a pressure wave to travel from the injec- tor to the rail and back through the 250 mm high pressure fuel line is approximately 330 ls. This corresponds to the time between the local minimum in injection rate preceding the main injection event and the point at which the injection rates begin to diverge at approximately 0.62 ms. It is, therefore, conceivable that the pres- sure waves within the high pressure fuel line can influence the rate of injection; this is discussed briefly in Ref. [7] and in detail in Ref. [10]. Time-resolved measurement of the fuel pressure and/ or needle lift measurements would be helpful in interpreting these data. The detailed modeling efforts such as those presented in Ref. [10] could also confirm whether or not pressure wave dynam- ics are responsible for the increase in maximum injection rate with the shorter fuel line. Likewise, more information is needed to explain the differences in the rate shapes at the end of the injec- tion event; both the aforementioned measurements and modeling efforts could provide insight. Fig. 8 Mean rates of injection with axial clamping force as a parameter. Single injection with injector A; Prail 5 800 bar; actuation time 5 815 ls; testing performed at room Solenoid Current Pull-Up Time. In this section, a comparison temperature. is made for a single injection case for two pull-up times: 250 ls

Journal of Engineering for Gas Turbines and Power OCTOBER 2015, Vol. 137 / 101503-5 Downloaded from http://asmedigitalcollection.asme.org/gasturbinespower/article-pdf/137/10/101503/6395620/gtp_137_10_101503.pdf by guest on 30 September 2021

Fig. 11 Mean rates of injection for two different pull-up times. Single injection with injector B; Prail 5 800 bar; actuation time- 5 565 ls; Tchamber 5 Trail 5 90 C; Tinjector 5 68 C.

(the standard pull-up time used in all other tests) and 350 ls. Injection rate data for this comparison are shown in Fig. 11; the total actuation time is held constant. The rate shapes for both cases are nearly identical except for the final phase of the injec- tion, during which the rate for the shorter pull-up time decreases slightly more rapidly than for the longer pull-up time. The total injected masses are 10.93 mg and 11.30 mg for the pull-up times of 250 ls and 350 ls, respectively.

Different Injectors and Multiple Injections. For this section, a simple pilot–main injection strategy is investigated and a varia- tion of the dwell time is performed. These tests are repeated with three injectors to demonstrate possible differences between injec- tors of the same type. The actuation schedules, each of which con- sists of the pilot injection actuation time, a dwell time dt, and a main injection actuation time, are shown in Table 1, as well as the measured pilot injection masses. The main injection energizing times are taken from engine testing with injector A at a constant load and depend strongly on dwell time. The rates of injection for all injectors and all actuation sched- ules are shown in Fig. 12. The injection command signals are shown below each set of injection rate profiles. For all dwell times, the injection rates of the pilot injections are nearly identical. There are slight differences in the maximum rates between the injectors. For a dwell time of 1200 ls, the rates of Fig. 12 Mean rates of injection with a pilot–main injection strategy for various dwell times. Prail 5 800 bar; injection for all three injectors increase at similar rates at the be- ginning of the main injection. However, injector A is character- Tchamber 5 Trail 5 90 C. Top: dwell time dt 5 1200 ls; middle: dt 5 400 ls; and bottom: dt 5 100 ls. ized by lower maximum injection rates during the main injection, whereas injectors B and C are similar to one another. All three injectors behave similarly during the end-of-injection transient. At for the other two injectors, which have maximum rates of injec- a dwell time of 400 ls, the rising edge of the main injection is tion that are similar to each other. For a dwell time of 100 ls, the steeper than for a dwell time of 1200 ls. However, the rate of two injection events are very close to each other, but are indeed injection momentarily stops increasing around 20 g/s, after which still distinct from one another. This very close spacing is made the rate of injection increases toward its maximum. The tops of possible by the injectors’ pressure-balanced control valves. For the rate traces are generally flatter than for 1200 ls, and the maxi- injectors B and C, the rising edge of the main injection is the mum rates of injection are lower. This is an example of how steepest of all the data shown, and the ramp-up in the main injec- changing the dwell time can be used to shape the rate of injection. tion is again interrupted. Injector A’s rate of injection also As before, injector A’s maximum rate of injection is lower than increases very rapidly for a short time, but the rate then begins to

Table 1 Actuation schedule and injected masses for pilot–main injection strategy

Pilot dt Main mpilot,A mpilot,B mpilot,C mmain,A mmain,B mmain,C

325 ls 1200 ls 775 ls 1.40 mg 1.85 mg 1.58 mg 24.3 mg 28.1 mg 26.5 mg 325 ls 400 ls 745 ls 1.39 mg 1.82 mg 1.57 mg 22.9 mg 26.6 mg 24.8 mg 325 ls 100 ls 590 ls 1.38 mg 1.76 mg 1.63 mg 21.0 mg 29.8 mg 27.4 mg

101503-6 / Vol. 137, OCTOBER 2015 Transactions of the ASME increase at a slower rate and a boot-shaped profile is not observed. In fact, this rate of injection decreases slowly during the quasi- steady portion of the injection. The apparent ends of injection are different; the first zero crossings on the falling edge of the main injection lie as far as 150 ls from one another.

Fuel Effects. In order to investigate possible effects of chang- ing fuel properties, experiments were performed with a blend of 75 vol. % n-heptane and 25 vol. % isooctane (PRF25). Properties of the mixtures are not available for the injection pressures employed in this work, but properties of the normal alkanes n-heptane and n-hexadecane are given in Table 2 for a pressure of

800 bar and a temperature of 360 K [11–14]. These properties rep- Downloaded from http://asmedigitalcollection.asme.org/gasturbinespower/article-pdf/137/10/101503/6395620/gtp_137_10_101503.pdf by guest on 30 September 2021 resent conditions in the injector rather than in the nozzle or the measurement chamber. Figure 13 compares rates of injection for a single injection with the reference, DPRF58, and with the PRF25 for a fixed actuation duration. Some slight differences are noticed during the initial ramp-up of the injection rates, but the most significant difference is observed during the ramp-down: the injection rate for PRF25 ramps down slightly later than for DPRF58. These changes are not negligible, but they are comparable in magnitude to those obtained by changing the line length (cf. Fig. 10). An increased amount of fluctuation is present in the PRF25 injection rate signal and is clearly visible after the apparent end of injection. In the case of the pilot–main injection strategy, these large pressure fluctuations complicate the interpretation of the measured rate traces, particularly for the main injection. A lower filter cutoff frequency of 6 kHz was chosen to mitigate these effects with PRF25 at the cost of some dynamic response. Figure 14 shows a comparison between these two fuels for the injection schedules listed in Table 3. The peak pilot injection rates are higher for DPRF58. This may be due in part to the stronger filtering of the PRF25 signal. The main injection events exhibit much more substantial differences:

Table 2 Physical properties of n-heptane and n-hexadecane at 800 bar and 360 K

n-Heptane n-Hexadecane

Density (kg/m3) 699.25 [11] 783.62 [12] Kinematic viscosity (mm2/s) 0.630 [11] 4.05 [12,13] Speed of sound (m/s) 1385.7 [11] 1508.4 [14]

Fig. 14 Mean rates of injection for DPRF58 and PRF25. Pilot–main injection strategy with injector B; Prail 5 800 bar. Tchamber 5 90 C. FIR filter cutoff frequencies: DPRF58: 10 kHz; PRF25: 6 kHz. Top: dwell time dt 5 1200 ls; middle: dt 5 400 ls; and bottom: dt 5 100 ls.

Table 3 Actuation schedule and injected masses for pilot-main injection strategy with PRF25 and DPRF5

mpilot, mmain, mpilot, mmain, Pilot dt Main PRF25 PRF25 DPRF58 DPRF58

310 ls 1200 ls 470 ls 1.38 mg 15.5 mg 1.82 mg 26.2 mg 310 ls 400 ls 448 ls 1.37 mg 14.9 mg 1.76 mg 24.8 mg 310 ls 100 ls 278 ls 1.37 mg 16.6 mg 1.76 mg 27.8 mg

for PRF25, peak rates of injection are approximately 50% lower and injection durations are noticeably shorter. The rate of injec- Fig. 13 Mean rates of injection for DPRF58 and PRF25. Single tion decreases for PRF25 after reaching its peak early during the injection with injector B; Prail 5 800 bar. Actuation time 5 581 main injection; the opposite is typically true for DPRF58. The ls; Tchamber 5 90 C. mass of fuel injected during the main injection is approximately

Journal of Engineering for Gas Turbines and Power OCTOBER 2015, Vol. 137 / 101503-7 40% higher for DPRF58 than for PRF25 for these injection Acknowledgment schedules. The authors thank Kan Zha and Jeremie Dernotte for their help in reviewing the manuscript and Julien Manin for some very help- Discussion and Conclusions ful discussions. Support for this work was provided by the United States The Moehwald HDA has demonstrated its ability to measure Department of Energy (Office of Vehicle Technologies) and by the rate of injection of individual injection events with very good General Motors Corporation (Agreement No. FI083070326). This repeatability. The parameter variations performed here are of work was performed at the Combustion Research Facility of San- practical importance when considering the measurement of rates dia National Laboratories in Livermore, California. Sandia is a of injection, mounting the injector, and how the injector is to be multiprogram laboratory operated by Sandia Corporation, a controlled. Raised cosine filters, although typically intended for Lockheed Martin Company, for the United States Department of shaping of digital data pulses [15], have been applied here for the Energy’s National Nuclear Security Administration under purpose of analog signal conditioning. Varying the cutoff fre- Contract No. DE-AC04- 94AL85000. quency primarily affects the detected hydraulic start and apparent Downloaded from http://asmedigitalcollection.asme.org/gasturbinespower/article-pdf/137/10/101503/6395620/gtp_137_10_101503.pdf by guest on 30 September 2021 end of injection, and this should be taken into account when selecting a cutoff frequency. Nomenclature Changing the chamber and fuel temperatures does not affect the c ¼ speed of sound of the fluid in the HDA chamber details of the injection rate shape to any significant extent, and for dp ¼ pitch diameter of screw used to apply axial clamping intermediate temperatures, the mass is not sensitive to tempera- force on the injector ture. It may be more practical to perform testing at an intermediate dm=dt ¼ injection rate; rate at which mass is added to the HDA temperature, although a noticeable difference has been observed chamber in the measured total injected mass between 40 and 60 C. Some dP=dt ¼ time derivative of HDA chamber pressure axial clamping force is necessary to hold the injector tightly in its dP=dq ¼ derivative of HDA chamber pressure with respect to adapter, but once this is ensured, the measured injection rate and density of fluid in the chamber total injected mass do not change to any significant extent with DPRF58 ¼ mixture of 58 vol. % heptamethylnonane and 42 vol. changing clamping force. High pressure line length does affect % n-hexadecane total injected mass and rate shape for a given actuation time. Care F ¼ axial injector clamping force was taken with the 250 mm line to use the same line that is used FIR ¼ finite-impulse-response when the injector is installed in the engine, so as to ensure trans- HDA ¼ German acronym for hydraulic pressure increase: ferability of the results when analyzing behavior observed within Hydraulischer Druckanstieg; name for injection the engine. The limited data and scope of this preliminary work quantity and rate measuring device do not provide a detailed explanation of the reasons for the m ¼ mass of fluid in the HDA chamber changes in rate shape observed with changing line length. A P ¼ HDA chamber pressure change in pull-up time from 250 ls to 350 ls does not dramati- PC ¼ personal computer cally alter the rate shape, but it does change the behavior of the PRF25 ¼ mixture of 75 vol. % n-heptane and 25 vol. % transient at the end of the injection event and slightly affects the isooctane total injected mass. t ¼ time With a pilot and main injection strategy, changing the dwell V ¼ HDA chamber volume time can have a dramatic effect on the shape of the main injection a ¼ radial angle of the screw used to apply axial clamping event, including the maximum rate of injection. With a fast acting force on the injector solenoid injector like the ones used in this study, dwell times as k ¼ lead angle of the screw used to apply axial clamping short as 100 ls can be employed while maintaining two separate force on the injector injection events. This likely has significant implications for the l ¼ coefficient of friction in the threads of the screw used mixture preparation and combustion processes, but it remains to to apply axial clamping force on the injector be seen if the behavior of the injector in the engine is the same as q ¼ density of fluid in the HDA chamber in the HDA. With these preproduction injectors, substantial s ¼ tightening torque that determines axial clamping force injector-to-injector differences have been observed in the rates of injection. These differences become more pronounced, particu- larly for very short dwell times. This could potentially be an issue References when considering the application of very short dwells in produc- [1] Zeuch, W., 1961, “Neue Verfahren zur Messung des Einspritzgesetzes und der tion engines, and understanding the reasons for the discrepancies Einspritz-Regelm€aßigkeit von Diesel-Einspritzpumpen,” Motortech. Z., 22(9), between the injectors will require a better understanding of the pp. 344–349. [2] Moehwald HDA Instruction Manual, 2008, Injection Analysis Using the dynamic effects inside the injector. Method of Hydraulic Increase of Pressure, Moehwald GmbH, The two fuels used in this work are different in terms of their Germany. physical properties (one can infer this from Table 2), but for a sin- [3] Norton, R. L., 2000, Machine Design: An Integrated Approach, 2nd ed., gle injection, the differences in the rate shapes are modest. This Prentice-Hall, Upper Saddle River, NJ. [4] Ikeda, T., Yukimitsu, O., Takamura, A., Yoshio, S., Jun, L. I., and Kamimoto, changes dramatically for multiple injection strategies, and is likely T., “Measurement of the Rate of Multiple Fuel Injection With Diesel Fuel and related to hydrodynamics within the injector. If these phenomena DME,” SAE Paper No. 2001-01-0527. are similar in nature to fluid hammer (as discussed in Ref. [10]), [5] Postrioti, L., Grimaldi, C. N., Ceccobello, M., and Di Gioia, R., “Diesel Com- then the dynamic pressure is proportional to the product of sound mon Rail Injection System Behavior With Different Fuels,” SAE Paper No. 2004-01-0029. speed and fluid density. For the normal alkanes in Table 2, this [6] Payri, R., Salvador, F. J., Gimeno, J., and Bracho, G., 2008, “A New Methodol- product is approximately 33% higher for n-hexadecane. It is there- ogy for Correcting the Signal Cumulative Phenomenon on Injection Rate Meas- fore hypothesized that for DPRF58, the amplitude of pressure urements,” Exp. Tech., 32(1), pp. 46–49. waves within the injector is higher, which may force the needle [7] Manin, J., Kastengren, A., and Payri, R., 2012, “Understanding the Acous- tic Oscillations Observed in the Injection Rate of a Common-Rail Direct open to a greater extent than with PRF25, thus leading to Injection Diesel Injector,” ASME J. Eng. Gas Turbines Power, 134(12), p. increased rates of injection. Because the maximum needle lift is 122801. higher, the needle takes longer to close, and thus the injection du- [8] Dernotte, J., Hespel, C., Foucher, F., Houille, S., and Mouna€ım-Rousselle, C., ration is longer. The combination of these two effects is attributed 2012, “Influence of Physical Fuel Properties on the Injection Rate in a Diesel Injector,” Fuel, 96, pp. 153–160. to the 40–50% increase in main injection quantity for a given [9] Kastengren, A. L., Tilocco, F. Z., Powell, C. F., and Fezzaa, K., 2011, “Initial actuation time for the DPRF58 fuel. 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101503-8 / Vol. 137, OCTOBER 2015 Transactions of the ASME 23rd Annual Conference on Liquid Atomization and Spray Systems, Ventura, Response Functions and Description Using Monte Carlo Molecular Simu- CA, May 15–18. lation,” J. Chem. Phys., 133(7), p. 074507. [10] Catania, A. E., Ferrari, A., Manno, M., and Spessa, E., “Experimental [13] Baled, H. O., Xing, D., Xu, Y., Bamgbade, B. A., McHugh, M. A., Downey, P., Investigation of Dynamic Effects on Multiple-Injection Common Rail Sys- Tapriyal, D., Morerale, B. D., and Enick, R. M., 2013, “Viscosity of Long- tem Performance,” ASME J. Eng. Gas Turbines Power, 130(3), p. Chain n-Alkanes at Pressures up to 243 MPa and Temperatures up to 520 K,” J. 032806. Chem. Eng. Data (submitted). [11] NIST Chemistry Webbook, 2013, “Thermophysical Properties of Fluid Sys- [14] Ball, S. J., and Trusler, J. P. M., 2001, “Speed of Sound of n-Hexane and n- tems,” National Institute of Standards and Technology, Gaithersburg, MD, Hexadecane at Temperatures Between 298 and 373 K and Pressures up to 100 http://webbook.nist.gov/chemistry/fluid/ MPa,” Int. J. Thermophys., 22(2), pp. 427–443. [12] Bessie`res, D., Pineiro,~ M. M., De Ferron, G., and Plantier, F., 2010, “Analysis [15] Gentile, K., 2002, “The Care and Feeding of Digital, Pulse-Shaping Filters,” of the Orientational Order Effect on n-Alkanes: Evidences on Experimental RF Design, April, pp. 50–61. Downloaded from http://asmedigitalcollection.asme.org/gasturbinespower/article-pdf/137/10/101503/6395620/gtp_137_10_101503.pdf by guest on 30 September 2021

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