The influence of selected adjustment parameters on the operation of LPG vapor phase pulse injectors

Journal of Natural Gas Science and Engineering 34 (2016) 1127e1136

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The influence of selected adjustment parameters on the operation of LPG vapor phase pulse injectors Dariusz Szpica Bialystok University of Technology, Faculty of Mechanical Engineering, 45 Wiejska Str., 15-351 Bialystok, Poland

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 May 2016 Received in revised form 24 July 2016 Accepted 1 August 2016 Available online 3 August 2016

The investigations aimed at proving the influence of selected parameters on the flow properties of LPG vapor phase injectors. The author mainly assessed the feed pressure, the injector piston lift, the diameter of the outlet nozzle and the realization of the current-modulating signal. Functional relations have been determined and qualitative assessment of the compatibility has been performed. Additionally, an evaluation of the influence of the analyzed parameters has been performed on the injector full opening and closing times that confirms the response to the impulse and may be decisive in the case of short injection times. It has been confirmed that the injector piston lift significantly influences the response times (for full opening - 1.00 ms and full closing e 1.08 ms) and the PWM duty cycle below 50% may lead to a closure of the injector due to insufficient counter force (difference for full closing by 7.53 ms at the 10 ms opening times). In order to measure the response times, an outflow meter has been proposed. A new parameter has been introduced to the analysis e opening timing that may be used in simplified flow analysis. The investigations are a response to the market demands for this type of analyses and to an increasing demand for multipoint LPG vapor phase injection systems. The investigations may turn out useful in calculations or validations of simulation models. © 2016 Elsevier B.V. All rights reserved.

Keywords: Combustion engines Fuel supply Liquefied petroleum gas Research

1. Introduction Despite the fact the recently prices of crude oil-based fuels are rather unstable, while the crude oil extraction level is constantly changing, engineers are still seeking alternative fuels for transport applications. In the case of transport as well as other cases involving a combustion of fuels, the entire process of conversion of chemical into mechanical energy is determined by the emission of CO2 (Bleischwitz and Bader, 2010; Litschke and Knitschky, 2012). The first step that would reduce the emission was downsizing. For an economy car fitted with a 0.8 l engine instead of a 1.6 l engine the CO2 reduction was 18% under stationary conditions. The torque at 1250 rpm increases by 50%. BMEP is 1.7 MPa, the unit power output is 83 kW/l while BSFC oscillates around 300 gm/kWh (Leduc et al., 2003). Quite often such engines operate on a very lean (A/F) mixture at small and medium loads (GDI). Under full loads the engine still operates on stoichiometric mixture. The operation on very lean mixture results in an increased emission of NOx (Ye and Li, 2010), while direct injected engines (GDI) only contribute to

E-mail address: [email protected]. http://dx.doi.org/10.1016/j.jngse.2016.08.014 1875-5100/© 2016 Elsevier B.V. All rights reserved.

the overall emission of PM from the vehicle (Gordon et al., 2013). In order to reduce the emission of CO2 one may optimize the process of combustion and exhaust gas after treatment or, alternatively, use fuels of reduced carbon content (Hunicz and Kordos, 2011; Kenihan, 1999). Fuels of lower carbon content used in transport are: liquefied petroleum gas (LPG) (Mockus, 2007; MacLean and Lave, 2003; Johnson, 2003; Streimikiene et al., 2013; Masi, 2012; Wendeker et al., 2007; Szpica and Czaban, 2014a,b; Myung et al., 2014; Erkus¸ et al., 2013; Puławski and Szpica, 2015; Ashok et al., 2015), compressed natural gas (CNG) (MacLean and Lave, 2003; Streimikiene et al., 2013; Frick et al., 2007; Hekkert et al., 2005; Aslam et al., 2006; De Carvalho, 1985; Suurs et al., 2010) and liquefied natural gas (LNG) (MacLean and Lave, 2003; Arteconi et al., 2010; Kumar et al., 2011). For availability reasons and lower competitiveness of poorly developed technologies of liquid biofuels, fuel cells and hydrogen they can only replace traditional fuels to a very limited extent compared to LPG (Gula et al., 2009; Anandarajah et al., 2013; Autogas in Europe, 2013). In Europe LPG is used to power 7 million passenger vehicles, which makes it the most commonplace alternative fuel (Raslavi
cius et al., 2014). The basis for a proper operation of any fueling system is fuel

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D. Szpica / Journal of Natural Gas Science and Engineering 34 (2016) 1127e1136

quality. In the case of gasoline this is regulated by legislation (Lekkas et al., 2003), while in the case of LPG this is not quite as  is an imporsimple. Corporate Average Fuel Consumption (CAFE) tant tool in the policy of reduction of Brake Specific Fuel Consumption (BSFC). The Alternative Engines Fuel Act (AMFA) guarantees that alternative fuel vehicles are specially treated when calculating the fuel consumption. It is to secure greater production and use of alter manufacturers. This constitutes an incentive to native fuels by CAFE increase fuel production, thus the alternative fuel share in the market (World LP Gas Association and United Nation Environment Programme, 2008; World LP Gas Association, 2009; Liu and Helfand, 2009). A significant reduction of the emission of HC and CO (65% and 50% respectively) for LPG, at a small drop in the thermal efficiency compared to gasoline makes this fuel potentially useful in engine applications (Bhale et al., 2005; Murilloa et al., 2008). However, greater exhaust emissions are observed at different LPG gasoline proportions in some fueling systems under certain conditions (Gumus, 2011). Early solutions of alternative LPG fueling systems were based on a mixer (principle of operation similar to a carburetor) with the only difference being the carburetor nozzle replaced with an adjustable choke on the LPG fueling line (Raslavi
cius et al., 2014). The adjustment could be performed intermittently, annually or on a continuous basis (oxygen sensor readouts). Unfortunately these types of systems, similarly to gasoline carburetor ones could not precisely control the fuel doses, leading to a increased exhaust emissions, thus failing to fulfill the subsequent stringent emission standards (Mitukiewicz et al., 2015). The external indexes also deviated from those of the base fueling not in the case of LPG vapor phase fueling (Szpica and Czaban, 2011), which is why it is the LPG vapor phase injectors that have dominated the market. Modern gasoline direct injection systems can be partially replaced with LPG vapor phase injection systems (Mitukiewicz et al., 2015). The injection of a vapor phase LPG does entail certain problems due to the specificity of the fuel dosage in the fueling system (Czarnigowski, 2012). The conversion of the liquid state into vapor forces a 300-time increase in the diameter of the flow channels (Duk et al., 2014). The weight of the control components (valves) and the related inertia are decisive of the fuel dosage accuracy, particularly its irregularity in multi cylinder engines (Szpica and Czaban, 2014a,b). The precision of workmanship and premature wear are of significance here as well (Szpica, 2016). LPG vapor phase injection systems utilize the signals from the gasoline system controller where they intercept the gasoline injector control signal of the start of the injection and its duration. The outstanding signals from the sensors are exclusively of informative nature because doubling of the adjustment is impossible the base adjustment is provided by the controller. It is very often the case that the signal from the oxygen sensor is presumed tantamount to the LPG control, as this is the displayed value in the calibration software. In fact, the signal from the oxygen sensor is just information on the excess deviation of the mixture composition from stoichiometric, which allows a quick conversion to base (gasoline) fueling, skipping the response of the gasoline control module. The gasoline module cannot verify whether the engine is fueled with LPG or not as it sends signals to the gasoline injectors. These signals are then intercepted by the LPG module. The engine is working and generates exhaust gas, hence the process continues. LPG vapor phase systems are universal and when injection time multiplier k is applied they can be adapted to any given engine.

tLPG ¼ k$tpetrol

(1)

The multiplier is very often associated with the differences in the calorific values of the base and alternative fuels and their physicochemical differences. However, in general sense, multiplier k indicates the correctness of selection of the components of the LPG fueling system (the system is composed from individually selected elements right from the initial stage of its construction). The manufacturers of LPG systems provide ranges of values in which the multiplier should fall to ensure continuous fuel feed. There are times, however when the universality of the LPG vapor phase systems is not an advantage (Borawski, 2015). Vapor phase LPG dosage is realized by the injectors. There are a variety of design solutions, hence their different operation accuracies (Czarnigowski, 2012; Szpica, 2016). Upon installing and starting of a fuel system, its final calibration consists in determining of the value of multiplier k. If the calibration is unsuccessful, it is usually the fault of the injector nozzles that are responsible for the final mass feed of fuel. There may also be other reasons: * * * * *

fuel system pressure, lift of the working component, parameters of the coil, duty cycle of the modulated signal (PWM), differences in the courses of the opening and closing processes of the gasoline and LPG injectors.

To that end, the author has attempted to investigate the influence of selected parameters on the functional properties of the LPG vapor phase injectors. Representatives of the most popular injectors have been selected as a basis. The results of the investigations may constitute a source of characteristics applicable in the injector calibration software, the injector design (fuel system design), calculations or simulation research. The results are to answer the question of what would happen if during the calibration of the LPG fueling system one of the analyzed values changed. It would turn out very helpful before disassembling of the subcomponents and their adjustment or modification (renewal of the injection nozzles). Obviously, not all of the analyzed parameters are applicable instantly; they sometimes require additional adaptive actions. The investigations hint a certain procedure with these actions that will consequently shorten the process of the system calibration. The research methodology and result processing presented in the further part of the work constitute a certain alternative to timeconsuming and costly research procedures utilizing advanced measurement technology (Kakuhou et al., 1998; Park, 2005; ~o and Moreira, 2005; Aleiferis et al., 2010; Zhang Oliveira Pana et al., 2011; Aleiferis and van Romunde, 2013; Movahednejad ~o et al., 2013; Serras-Pereira et al., 2013). et al., 2013; Oliveira Pana The aim of the work was to propose a measurement set allowing a quick assessment of the influence of selected parameters on the operation of LPG vapor phase injectors in the quantitative aspect. Aside from the proprietary LPG vapor phase test stand, instead of a measurement card a fully equipped oscilloscope of the sampling frequency of 2Gs and an original pressure based outflow sensor were used. The sampling of the oscilloscope is sufficient to evaluate the courses at an impulse of the order of several milliseconds. 2. Material and methods 2.1. Preliminary research In the initial phase, an analysis was carried out of the results of investigations performed on a Skoda Fabia vehicle (1.2 l 12 V, model year 2006) fitted with an alternative sequential LPG vapor phase injection system (STAG 2000 by AC LLC). The tests were performed

D. Szpica / Journal of Natural Gas Science and Engineering 34 (2016) 1127e1136

on a chassis dynamometer (LPS 3000 MAHA) utilizing special equipment for normalized driving tests (Czaban and Szpica, 2013). Two trials were performed - one on gasoline and the other on LPG. AC GAS SYNCHRO 1.12.5.0 LPG manufacturer software was used for the data recording. The analysis of the courses (Fig. 1) led to a conclusion that the LPG system maintains constant feed pressure p on the level of approx. 1,105 Pa. This pressure is set to different levels at the input and three such levels are dominant: (1, 1.2 and 1.7) 105 Pa. The most popular are (1 and 1.2) 105 Pa, which is why they can be deemed as representative. As for the injection pressure tinj (2.5, 5 and 10) ms can be deemed as representative. Based on this information input ranges in the main research were set. 2.2. Object of the research The research object was an injection rail by AC LLC. 4 brand new piston AC-W02-4 LPG/CNG injectors were fitted in the rail. The injectors came as a set with a 133140287 tray and components (Fig. 2). Basic technical data of the tested injectors have been presented in Table 1. The author also performed tests on the coil characteristic parameters for different pulse frequencies realized with the LCR Meter CMT 417. The results have been shown in Table 2. From the information in Table 2 we know that at the frequency of 100 Hz the impedance is close to the value of resistance declared by the manufacturer. This frequency (100 Hz) is out of range for the maximum injector pulse frequencies resulting from the engine speed. Determining the parameters for higher frequencies may be of importance when utilizing the pulse-width modulation signal. The PWM signal is used to reduce the current on the coil terminals at the moment when the injector is already open (Fig. 3). High current is required to trigger an abrupt motion of the piston but much lower current is needed to hold the injector open. The PWM duty cycle has impact on the holding current (Fig. 4).

1129

Fig. 2. Object of the research e a system of injectors.

Table 1 Basic technical data of the tested injectors (www.ac.com.pl/en …). Secondary technical data max flow rate at 1.2 bar nozzle size coil resistance opening time closing time max.working pressure operating temperature warranty connector homologation

Nl/min mm

120 min 1.5/max 2.8 1.9 2 1 400 20 ÷ 120 100 000

U

ms ms kPa  C km SuperSeal E8 67R-017064; E8 110R-007085

Table 2 Tests of the injector coil parameters. Frequency f, Hz

Impedance Z, U

Inductance L, mH

100 120 1000 10 000

2.00 ± 1.059 2.10 ± 0.998 6.89 ± 0.442 41.20 ± 0.584

3.08 3.07 2.48 1.12

± ± ± ±

0.949 1.115 2.26 1.711

Capacitance C, mF 824.00 ± 1.055 574.00 ± 0.898 10.28 ± 0.443 0.23 ± 0.584

n, 103 rpm

4 2 0 1.5 p, 105Pa

Fig. 3. Example courses of voltage and current on the injector terminals at the 80% PWM signal duty cycle (tinj ¼ 10 ms, PWM at 2.5 ms, n ¼ 1000 rpm).

MAP p

LPG

1 0.5 0

tinj, ms

20

petrol LPG

10 0 0

200 400 600 800 1000 1200 t, s

Fig. 1. Selected courses recorded during the test on Skoda Fabia 1.2 l 12 V.

This translates into the flow properties of the injector. The value of the holding current is one of the values through which the injector efficiency can be adjusted. A reduction of the holding current by reducing the PWM duty cycle may not be sufficient to overcome the force of the injector closing spring. Analyzing the course of the voltage shown in Fig. 4, one may observe that the PWM signal occurs with the frequency of approx. 2000 Hz. The time after which the signal appears, its frequency and the duty cycle vary depending on the manufacturer of the control module or the requirements of the injector manufacturer. The author also determined the values of the inductance L for different piston position h and different fuel feed frequency (Fig. 5). The tests at 1 kHz aimed at evaluating the parameters that may be useful in modeling of the injector operation, in which PWM current limitation was applied. The supply frequency reaches approx. 1 kHz 10 kHz frequency was investigated for exploratory purposes. In order to generate an abrupt movement of the injector piston, a current impulse is necessary, but only in the opening phase. Later, in the holding phase the value of the current can be lower, which is

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Fig. 4. The influence of the PWM duty cycle on the current at the injector terminals. (tinj ¼ 10 ms, PWM at 2.5 ms, n ¼ 1000 rpm): a e 50% PWM, a e 70% PWM, a e 90% PWM.

L, mH

4 3.5

3.5

3

3

2.5 0 3 2.8 L, mH

4

100 Hz

0.5

2.5 0

1

1 kHz

1.16

2.6

1.14

2.4

1.12

2.2 0

0.5 h, mm

1.1 0

1

120 Hz

0.5

1

10 kHz

0.5 h, mm

1

Fig. 5. The influence of piston displacement h on the coil inductance L of different fuel feed frequencies.

why a current limitation with the PWM signal is applied. This allows holding the injector open without excess coil heating. In the control system a transistor key is included with a diode preventing reverse voltage, and the control is realized via a short to ground. Using the BRONKHORST F-113AC-M50-ABD-00-V mass flow meter the author determined the volumetric flow rate (Nl/min) of the investigated injector with its original settings (outlet nozzle 3 mm). The results have been presented in Table 3. 2.3. Research methodology The tests were performed on a test stand where the working medium was compressed air (Fig. 6). The compressed air from source 1 goes to the pressure stabilization system 2 and then to the pulsation damping cylinder 3. On the manometer 5 one can see the feed pressure. From the cylinder 3 the air goes to the tested injector 6 whose opening parameters are adjusted with the dedicated control system 5. The author used the four-channel oscilloscope 13 that enabled a simultaneous recording of four signals. The equipment connected to the oscilloscope was as follows: voltage probe, current lines 8, displacement sensor 9, accelerometer 12 and outflow sensor 11. The author used the flow

Table 3 Results of the flow tests. Parameter

750 rpm tinj ¼ 5 ms PWM ¼ 100%

2000 rpm tinj ¼ 5 ms PWM ¼ 100%

5000 rpm tinj ¼ 5 ms PWM ¼ 100%

MAX

44.40

121.20

p ¼ 1.2$105 Pa, U ¼ 14 V Nl/min

4.64

16.60

meter 10, a voltmeter and the manufacturer's flow meter conversion table to calculate the flow rate. For reference, in the investigations the author used the CL80 ZEPWM displacement sensor. The fitting of the sensor required fixing a special measurement needle to the injector piston, which increased the weight of the moving component by 0.64 gm e 11.26% (ABT-100 KERN scales). The static characteristics of the sensor was determined for which the coefficient of determinance on the level of R2 ¼ 99.97% was obtained. The proprietary outflow sensor was based on the measurement of the pressure at the outlet of the injection nozzle. This type of tests allows an indirect obtainment of information on the process of opening and closing of the injector. This is an alternative for the displacement sensor whose fitting is invasive and requires damaging of the piston or the application of the flow meters (Duk and Czarnigowski, 2012) or any other research equipment. The said displacement sensor also finds limited application in investigating flap, disc or membrane injectors. In the case of the outflow meter (Fig. 7) the air from the injector nozzle gets to the inlet nozzle through the flexible line 8. When the injector is open, the pressure in the inlet nozzle 8 meets the MPXH6400A sensor 5. In order to prevent a constant pressure growth around the sensor 8, in the upper housing 3 blow-by holes were made. The pressure pulses converted into the sensor 8 voltage are sent via the electrical connection 2 to the oscilloscope. The sensor closes the pulsation damper fitted to the housing 6 and the air is channeled via a line of the diameter 10 times greater than the inlet nozzle. Upon plugging of the blow-by holes, the static characteristic of the pressure outflow sensor was obtained, for which the determinance coefficient was R2 ¼ 99.86%. In order to carry out intermittent injector openings it was necessary to develop a pulse inducing system. To this end, a modified STAG AC LLC LPG controller was used along with specialized software by DM VISION allowing the control of the operating parameters (Fig. 8). In the comparative tests the author aimed at utilizing the indirect measurement with the proprietary outflow sensor (Fig. 7), which is why in the beginning of the studies, the readouts of the outflow sensors and the displacement sensors were compared (9 in Fig. 6) to validate the correctness of the indirect method. The author used another injector for the tests similar in design to the one used in the research (VALTEK Rail Type 30 maximum piston lift 0.45 mm) to avoid destroying the base injector. Additionally, voltage (RIGOL), current (HAMEG HZ050) and vibrations of the injector housing (KELAG KAS903-02A) were recorded with the RIGOL MSO4014 oscilloscope. The investigations have shown (Fig. 9) that the outflow sensor correctly reflects the process of injector opening, which was confirmed by the course of current at the injector terminal and the housing vibrations. The divergence occurs in the process of injector closing. The outflow sensor measures the pressure at the outlet of the nozzle and, due to air decompression, responds with certain latency as the injector closes. The closing moment differs by approx. 0.1 ms. This result should be deemed as satisfactory as the manufacturer of the MPXH6400A

D. Szpica / Journal of Natural Gas Science and Engineering 34 (2016) 1127e1136

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Fig. 6. Diagram of the test stand: 1 e source of compressed air, 2 e air pressure stabilization system, 3 e cylinder, 4 e manometer, 5 eSTAG AC LLC based pulse induction system controller, 6 e tested vapor phase LPG injector; 7 eRIGOL voltage connector, 8 HAMEG HZ050 current lines, 9 e CL 80 ZEPWM induction displacement sensor, 10 eBRONKHORST F113AC-M50-ABD-00-V mass flow meter, 11 e proprietary outflow sensor, 12 - KELAG KAS903-02A accelerometer, 13 - RIGOL MSO4014 oscilloscope.

Fig. 8. Communication interface of the injector pulse inducer.

3. Results and discussion 3.1. The influence of selected parameters on the mass flow

Fig. 7. Proprietary outflow sensor: 1 e lower housing, 2 e electrical connection, 3 e upper housing with the blow-by holes, 4 e nut I, 5 eMPXH6400A sensor, 6 e pulsation damper fitting element, 7 e nut II, 8 e inlet nozzle.

sensor declares the response time of 1 ms. On this basis the indirect method was considered satisfactory. In the course of the investigations the adjustment of the injector piston lift was necessary. A commercially available tester based on a distance-amplifying indicator was used for this purpose (Fig. 10).

At the initial stage, an analysis of the repeatability of measurements was performed. Based on 30 trials, with the same input parameters it was observed that the standard error of the flow meter was 0.012 Nl/min. The value of the error, hence the repeatability of the measurements, may have been influenced by the control impulse-generating module, yet, based on the 50 (overlain) courses of the impulses, no significant changes were observed (they were identical). In the initial phase, the influence of the feed pressure on the volumetric flow rate (Q) of the injector was analyzed. This parameter is very important (Czarnigowski, 2010, 2015) because LPG vapor phase injection systems use pressure for the calibration of the fuel system. The tests were limited to the range of (0.5 … 2.5) bar. This is slightly out of range for the usual LPG systems of this type but the purpose was to determine the changing trends. The values of constant parameters were as follows: engine speed n ¼ 1000 rpm, injection time tinj ¼ 10 ms, holding signal modulation 90% PWM at 2.5 ms, voltage U ¼ 14 V, piston lift h ¼ 0.6 mm, nozzle diameter d ¼ 3.2 mm with the flow measured with the

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D. Szpica / Journal of Natural Gas Science and Engineering 34 (2016) 1127e1136

Q=Qmin+(-1.1108p2+13.7076p-2.7368) R2=99.93%

Q, Nl/min

30 20 10 0 0

0.5

1 1.5 p 105, Pa

2

2.5

Fig. 11. The relation of the volumetric flow rate and the feed pressure.

Fig. 10. The injector piston inspection and adjustment set: 1 e housing with a vice, 2 e electrical connection with a 12 V adapter, 3 e electrical connection of the injector, 4 e activation button, 5 e distance-amplifying indicator with the scale 0.01 mm, 6 e adjustment wrench.

BRONKHORST F-113AC-M50-ABD-00-V flow meter. The nature of the relation between Q and the feed pressure is close to a multinomial, which is why, when applying non-linear regression and the least squares method, the important equation coefficients were determined (Fig. 11). The minimization of the sum of squares of the deviations was performed in Matlab using the Nelder-Mead simplex method (the required calculation accuracy was obtained 106). The value of the coefficient of determinance R2 ¼ 99.93% indicates high convergence of the model curve with the experimental points. The other parameter that was subject to analysis was the injector outlet nozzle diameter. In this case the diameter was graded from 1.2 mm (a standard value of the set attached to the product) to 3 mm that is the maximum value recommended by the manufacturer. The values of the constant parameters were as follows: feed pressure p ¼ 1.2,105 Pa and continuous opening,

Q=Qmin+Qmaxexp(-0.3593(3.2617-d)3.4693) R2=99.97% 150 Q, Nl/min

Fig. 9. Example courses for the evaluation of the indirect method (p e pressure, h e displacement, I e current, U e voltage, a - acceleration).

h ¼ 0.6 mm. The nature of the relation between Q and the diameter of the outlet nozzle is close to the Gauss function (Fig. 12), which is why the author decided on this type of function. In this case, the coefficient of determinance reached R2 ¼ 99.97%, which leads to a conclusion that the function adopted to describe the relation was correct. Other than the outlet nozzle, the adjustment of the injector flow rate can be done through the variation in the piston lift (Czarnigowski, 2013). Hence, the next parameter was the piston lift (displacement). In this case the value was graded from 0.2 mm, at which proper operation was possible, to 1 mm, at which the injector had excess knock. The values of the constant parameters were as follows: feed pressure p ¼ 1.2,105 Pa and continuous opening, d ¼ 3 mm. The nature of the changes Q depending on the maximum piston lift were also close to the Gauss function (Fig. 13). This is confirmed by the coefficient of determinance of R2 ¼ 99.97%. In the final stage of the flow evaluation, the modulating signal duty cycle was analyzed at maximum injector opening. This parameter is sometimes used to adjust the injector flow rate during calibration. This is performed as an alternative to the adjustment of the nozzle diameter or if no smaller nozzle can be applied and the flow rate is still too high. The values of the constant parameters were as follows: p ¼ 1.2,105 Pa, n ¼ 1000 rpm, tinj ¼ 10 ms, modulation after 2.5 ms, U ¼ 14 V, h ¼ 0.6 mm, d ¼ 3 mm. Also, in this case the nature of the changes of Q is close to the Gauss function (Fig. 14). Yet, the value of the coefficient of determinance of R2 ¼ 98.57% indicates a certain imperfection of the function. In the range (0.3 … 0.6) 100% the Gauss function does not reflect the nature of the experimental changes, however, as shown in (Fig. 4) the duty cycle below 50% may result in the injector closure at insufficient electromagnetic field generated by the coil. Only, in the range (70 … 80) % of the PWM duty cycle is a significant adjustment of the volumetric flow rate possible. This is one of the alternative

100 50 0 1

1.5

2 d, mm

2.5

3

Fig. 12. The relation between the volumetric flow rate and the diameter of the outlet nozzle.

D. Szpica / Journal of Natural Gas Science and Engineering 34 (2016) 1127e1136

the piston lift (manufacturers are against it threatening to void the warranty), but for exploratory reasons it is worth analyzing the influence of these parameters.

Q=Qmin+Qmaxexp(-0.0088(1.8655-h)11.8870) R2=99.97%

100

3.2. The influence of selected parameters on the injector opening and closing times

50 0 0.2

0.4

0.6 h, mm

0.8

1

Fig. 13. The relation of the volumetric flow rate and the maximum piston lift.

Q=Qmin+Qmaxexp(-62.9033(1.2938-PWM)8.0015) R2=98.57% 20 15 10 5 0 0

0.2

0.4 0.6 PWM

0.8

1

Fig. 14. The relation between the volumetric flow rate and the PWM signal duty cycle.

methods to correct the fuel dose. This type of procedure, however, is not a standard one (selection on the software level) but it requires modifications in the structure of the LPG controller. The determined parameters of the functions characterizing the influence of the selected parameters on the volumetric flow rate (Q) may find application in configuring sets dedicated to specific engine types. As a principle, LPG vapor phase injection systems are universal, but in practice a variety of issues surface during adaptation. On the packaging of their products manufacturers often declare diameters of the nozzles suitable for engine power outputs, but it is incorrect. In the times of downsizing, small turbocharged engines produce power equivalent to that of engines of older generations of at least twice the displacement. While the power outputs are similar, the fuel consumption is much different, which is why the injectors should not be selected in this way. The values presented in Table 4 may be useful in the development of, as the author calls it, ‘what if … ’ software useful during the calibration process. Upon starting of the calibration procedure in the controller software during engine adaptation for LPG fueling, the most frequent message is ‘nozzles too small’ or ‘nozzles too large’, which does not necessarily have to result in the adjustment of the nozzle diameter. With the application based on the performed tests one can adjust the analyzed parameters to improve the engine operating conditions and only if such adjustments are impossible could the nozzle be replaced. It is however not recommended to modify

For these adjustments, i.e. feed pressure, nozzle diameter, maximum piston lift and PWM signal duty cycle the trials were performed for the pulsating flow. The author searched for the influence of individual adjustments on the time needed to reach full injector opening and closing. As before, in the beginning, the analysis of the measurement repeatability was performed. Based on 30 trials with the same input parameters and maximum sampling of the oscilloscope it was observed that the standard error, when measuring the time to full opening, was 0.0089 ms and to full closing e 0.0049 ms. The adjustment of pressure in the range (0.25 … 2.5) bar, with constant parameters: n ¼ 1000 rpm, tinj ¼ 10 ms, without PWM, U ¼ 14 V, h ¼ 0.6 mm, d ¼ 3 mm, resulted in differences of the full opening times of D1 ¼ 0.37 ms and full closing times of D2 ¼ 0.13 ms (Fig. 15). These are small values, as mostly the injector opening times under actual operation are longer than 2.5 ms. The differences reaching approx. (0.1 … 0.2) ms may result from the nonrepeatability of subsequent openings. The nozzle diameter of the range (1.2 … 3) mm influences the opening and closing times a bit less than the fuel feed pressure. For the constant parameters: p ¼ 1.2,105 Pa, n ¼ 1000 rpm, tinj ¼ 5 ms, without PWM, U ¼ 14 V, h ¼ 0.6 mm, differences of D1 ¼ 0.35 ms until full opening and D2 ¼ 0.12 ms until full closing (Fig. 16) were recorded. This should not significantly impact the operation of a fuel system. The maximum piston lift in the range of (0.25 … 1) mm significantly influences the injector opening and closing times. With the constant parameters: p ¼ 1.2,105 Pa, n ¼ 1000 rpm, tinj ¼ 5 ms, without PWM, U ¼ 14 V, d ¼ 3 mm, the difference in the full opening time was D1 ¼ 1.00 ms and the full closing time was D2 ¼ 1.08 ms (Fig. 17). This may influence the proper operation of the fuel system, which is why the manufacturers prohibit any adjustments of the piston lift under warranty. Nevertheless, this adjustment has been performed for exploratory reasons. The greatest differences were obtained when the PWM signal was adjusted (Fig. 18). The constant parameters in this case were:

15 p, Pa

Q, Nl/min

150

Q, Nl/min

1133

x 10

5

10 5 0 2

Δ

4

6

8 t, ms

10

Δ

12

0.25 0.5 0.75 1bar 1.25 1.5 1.75 2 2.5

Fig. 15. Pressure at the nozzle outlet depending on the feed pressure.

Table 4 Parameters of the function describing the variability. Parameter

Function

Pressure

yminþ(Ax2þBx þ C)

Nozzle Stroke PWM

C

ymin þ ymax exp(A(B  x) )

Qmin Nl/min

Qmax Nl/min

A

B

C

R2

4.2000

e

1.1108

13.7076

2.7368

99.93%

40.8000 0 2.0800

121.1333 140.0000 17.0000

0.3593 0.0088 62.9033

3.2617 1.8655 1.2938

3.4694 11.887 8.0015

99.97% 99.97% 98.57%

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D. Szpica / Journal of Natural Gas Science and Engineering 34 (2016) 1127e1136 5

6

x 10

p, Pa

4 2 0 2

3

4

5 t, ms

Δ

6

7

8

1.2 1.5 1.7 1.9 2.1 2.3 2.5 3mm

Δ

Fig. 16. Pressure at the nozzle outlet depending on the diameter of its outlet.

5

x 10

p, Pa

6 4 2 0 2

3

4

5 t, ms

Δ

6

7

8 Δ

0.25 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1mm

Fig. 17. Pressure at the nozzle outlet depending on the piston lift.

5

x 10

p, Pa

6 4 2 0 2

Δ 4

6

8 t, ms

10

Δ

12

10% 20 30 40 50 60 70 80 90 100

Fig. 18. Pressure at the outlet depending on the PWM signal duty cycle.

p ¼ 1.2,105 Pa, n ¼ 1000 rpm, tinj ¼ 10 ms, PWM after 2.5 ms, U ¼ 14 V, h ¼ 0.6 mm, d ¼ 3 mm. The differences in the time needed for full opening were small and amounted to D1 ¼ 0.10 ms, which may have resulted from the non-repeatability of the subsequent openings. The differences of the times until full closing were D2 ¼ 7.53 ms, which confirms the assumption that the electromagnetic force maintaining the piston in the open position is insufficient at low PWM signal duty cycle. Comparing Fig. 18 with Fig. 14 a correlation was confirmed. The range (70 … 80) % of the PWM duty cycle enables a significant adjustment of the injector opening time. 3.3. The influence of selected parameters on the injector opening timing In the further part of the research, the author focused on the

Fig. 19. Determination of the pressure timing.

assessment of the opening timings, i.e. the area below the pressure course taken from the outflow sensor reproducing the injector opening (Fig. 19). To this end, a special application in Matlab was developed. Standard error in this case for 30 trials was 4.2091 Pa s. By determining the timings for the presented adjustments for the pulsating flow in the above-described manner, the author sought correlation between the timings and the volumetric flow rate. In the case of a variable feed pressure small differences occur in the comparison (Fig. 20). The relation obtained from the timings is linear. In the rest of the analyzed cases there is convergence in terms of the character of the volumetric flow rate (Q) and pressure timing (T) variability. The comparisons have been shown in Figs. 21e23. Unfortunately the description of the coefficient of proportionality that could transfer the results from the timings to volumetric flow rate under this research conditions was unsuccessful. 4. Conclusions Alternative systems based on multipoint injection of vapor phase LPG are very popular in many countries (South Korea, Italy, Poland to name only a few). The chief factor decisive of its application is the price of conventional fuels. The variety of design solutions as well as the assumption that these systems are rather universal also contributed to their popularity. Unfortunately, in the course of adaptation, only general guidelines of the LPG system manufacturers are applied and the system is often retrofitted into the vehicle without in-depth analysis. Despite advanced software designed for the LPG system maintenance, the negative conclusions in the final stage of calibration usually indicate issues with the injector nozzles. It is noteworthy that LPG vapor phase injection systems are often composed of randomly chosen subassemblies where the main criterion is the price, which is why the research was performed aiming at determining the influence of selected parameters on the injector operation in the said randomly composed systems. The author kept in mind that the injector nozzle is the key component of the system. Following the performed measurements and calculations the following conclusions have been drawn: 1 The adjustment of the feed pressure during the trials of constant injector opening influences the volumetric flow rate (close to multinomial). The coefficient of determinance in this case exceeded 99.9%. 2 The adjustment of the maximum injector piston lift and the nozzle diameter during the continuous opening trials indicate a functional relation with the volumetric flow rate close to the Gauss function. The coefficient of determinance for each of these cases exceeded 99.9%. 3 The adjustment of the PWM signal duty cycle for the pulsating flow also indicates a functional relation with the volumetric flow

Fig. 20. Comparison of the volumetric flow rate (Q) and timing (T) as a function of feed pressure.

D. Szpica / Journal of Natural Gas Science and Engineering 34 (2016) 1127e1136

Fig. 21. Volumetric flow rate (Q) and timing (T) as a function of nozzle diameter.

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translate into a change in the opening times of the LPG injectors that utilize the signals of the gasoline injector control module. The second case mainly results from the change in the resistance of the wire used in the coils following a change of temperature. This influences the parameters of the magnetic circuit. The temperature however does not change abruptly, which the LPG control module can easily handle in the adaptive system. The results of the performed investigations may turn out useful in the calibration of fueling systems when an adjustment of a given parameter is necessary to obtain the desired fuel feed range. Acknowledgments The investigations described in this paper are a part of the research project No S/WM/2/13 realized at Bialystok University of Technology. References

Fig. 22. Volumetric flow rate (Q) and timing (T) as a function of maximum piston lift.

Fig. 23. Volumetric flow rate (Q) and timing (T) as a function of PWM signal duty cycle.

rate close to the Gauss function. The coefficient of determinance exceeded 98.5%. By utilizing a special outflow sensor based on the pressure sensor, an evaluation was carried out of differences in the times needed for full injector opening and closing for the pulsating flow. The author confirmed that: 1 The adjustment of the feed pressures resulted in differences of times needed until full opening of 0.37 ms, and full closing 0.13 ms. 2 The diameter of the outflow nozzle changed the time needed until full opening by 0.35 ms and full closing - 0.12 ms. 3 The maximum injector piston lift resulted in a difference in the time needed until full opening of 1.00 ms and full closing  1.08 ms 4 The adjustment of the PWM signal duty cycle changed the time needed until full injector opening only by 0.10 ms and for full closing by 7.53 ms. The author also carried out an assessment of the opening timings, i.e. the area under the course of pressure from the outflow sensor reproducing the injector opening for the above-described adjustments. It has been proven that the nature of the changes is close to the results obtained in the volumetric flow rate measurements. From the outstanding research that could be performed related to the impact on the LPG vapor phase injector parameter, there is still the voltage. The gasoline injector control module adjusts the opening times in relation to the supply voltage. It will, thus

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Nomenclature MAP: manifold absolute pressure  corporate average fuel economy CAFE: AMFA: alternative motor fuels act BMEP: brake mean effective pressure BSFC: brake specific fuel consumption A/F: air fuel ratio GDi: gasoline direct injection LPG: liquefied petroleum gas CNG: compressed natural gas LNG: liquefied natural gas PWM: pulse-width modulation CO2: carbon dioxide CO: carbon monoxide HC: hydrocarbons NOx: nitrogen oxides PM: particulate matter Glossary and units t: injection time, ms T: period, ms k: multiplier, - f frequency, Hz Z: impedance, U L: inductance, mH C: capacitance, F p: pressure, Pa h: displacement, mm d: diameter, mm I: current, A U: voltage, V a: acceleration, m/s2 A, B, C: coefficient, - n rotational speed, rpm R2: coefficient of determination, - Q volumetric flow rate, Nl/min D: deviation, ms PT: pressure timing, Pa s