Onboard determination of fuel stability in plug-in hybrid diesel cars by dielectric relaxation spectroscopy

Transcription

1 DOI /senso215/C7.1 Onboad detemination of fuel stability in plug-in hybid diesel cas by dielectic elaxation spectoscopy M. Eskine 1, F. Bä 1, M. Rossne 1, A. Munack 2,3, J. Kahl 1,3 1 Univesity of Applied Sciences and Ats, Fiedich-Steib-Staße 2, 9645 Cobug, Gemany mustafa.eskine@tac-cobug.com 2 Thünen Institute of Agicultual Technology (TI), Bundesallee 5, Baunschweig, Gemany 3 Fuels Joint Reseach Goup, Abstact A method based on dielectic elaxation spectoscopy (DRS) is intoduced fo detemination of fuel quality of biodiesel containing fuels in plug-in hybid electic vehicles (PHEV). Dielectic paametes such as the eal and imaginay pat of pemittivity wee ecoded in a boad fequency ange. As capacitive cell fo detemining the eal and imaginay pat of the pemittivity, a small-sized intedigital capacito was developed, which could be integated into PHEV in the futue. Especially in plug-in hybid cas the stoage time of fuel in the fuel tank is expected to be significantly longe than fo cas with conventional combustion engines. Theefoe the fuel long-tem stability is of paticula impotance. By means of DRS, high-molecula oxidation poducts, such as oligomes fom biodiesel that ae consideed as pecusos fo deposits can be detected easily. Key wods: dielectic elaxation spectoscopy, pemittivity, plug-in hybid electic vehicles, diesel fuel, biodiesel Motivation To decease the dependency on fossil fuels the use of electic vehicles will be focused on by many automotive manufactues. Electical vehicles such as PHEVs ae additionally equipped with a combustion engine which combines the advantages of an electical vehicle and a conventional vehicle with combustion engine. Seies-poduction PHEV equipped with diesel engines ae aleady available in the maket (e.g. Volkswagen XL1 o Volvo V6). In the futue moe diesel equipped PHEV will be available. Today, a quota of biodiesel in fossil diesel fuel is established in seveal counties all ove the wold. Consideing the tanspot secto in Gemany, a biodiesel popotion of max. 7 %v/v in fossil diesel fuel is statutoy [1]. Since 215, the convesion to a climate quota to educe geenhouse gas (GHG) emissions fom biofuels is established. Accoding to the Geman fedeal govenment a GHG eduction commitment of 3.5% (215), 4.% (217) and 6.% (22) must be satisfied e.g. by fuel industy, tades etc. [2]. Thee ae some facts of blending fossil diesel fuel with biodiesel: saving of fossil diesel fuels, independence of global enegy supplies thus stengthening the local economy and the eduction of climate-affecting emissions especially CO 2. Woks fom [3] showed an impoved GHG balance, when using biodiesel as fuel. In case of 1% ape seed oil methyleste (RME), which is commonly used as a biodiesel admixtue in fossil diesel fuels in Gemany, CO 2 savings amounts to 38%. In fossil diesel fuel with 7% biodiesel popotion this would mean a saving potential of 2.63%. Ealy investigations on aged diesel fuel/biodiesel blends showed a cetain tendency to fom sediments, which can patly be consideed as oligomes fom biodiesel [4, 5]. Unsatuated fatty acid methylestes ae especially esponsible fo this undesied fuel instability and lead significantly to the fomation of sediments. Geneally, unsatuated hydocabons undego autoxidation foming alkyl and peoxy adicals [6, 7], which can eact with oxygen to fom peoxides o hydopeoxides. These peoxides can lead to epoxides, which fom acids, aldehydes, ketones o eact with othe alkyl and peoxy adicals to fom dimes o oligomes [4]. In PHEV, the combustion engine is only used when the powe supply fom the battey is too low. On shot distance ides, the combustion engine is not necessay, because of fequent battey echaging. Consequently, the stoage time of the fuel in the tank is much longe than AMA Confeences 215 SENSOR 215 and IRS

2 DOI /senso215/C7.1 usual. Possible damages and poblems due to aged fuels can occu at the fuel pump, injection systems o exhaust afteteatment. Fuel pump and injection system ae vey sensitive to an incease in viscosity due to oligomeization. The inceased viscosity leads to pooe nebulization duing injection. This can lead to an incease of soot o unbuned fuel in the exhaust gas afteteatment system due to an incomplete combustion. Geneally, the oxidation stability descibes the ability of a substance to withstand oxidative pocesses. If the oxidation stability of biofuels is exceeded, thei aging begins and oligomes can occu. A common method fo detemining the oxidation stability of biodiesel o fossil diesel fuels blended with biodiesel is the Rancimat method accoding to DIN EN This is a laboatoy test pocedue whee the measuing of incease in conductivity by acid fomation duing an acceleated aging pocess of biodiesel is detemined. The aim of ou eseach was to detemine the aging degee due to oligomes fomed in biodiesel blends with suitable onboad sensos in PHEV. Such a senso could be integated in fuel tanks of PHEV whee oligomes fomed in the ageing pocess of the fuel should be detected at an ealy stage. Hence, damages of the engine should be avoided. The detection pinciple of the newly designed senso is based on DRS. Theoetical Backgound Measuements of dielectic paametes in the fequency domain, the eal- and imaginay pat of the pemittivity ( and ) ae detemined by applying an altenating voltage between the electodes at diffeent fequencies. Inseting a dielectic mateial (e.g. liquid, solid) between electodes leads to a decease of the pimay electical field because of polaization mechanisms like displacement polaization, oientation polaization o intefacial polaization [8]. In atoms o non-pola molecules exposed to an electical field, the displacement of the positive chage and negative chage concentations, espectively, lead to an intamolecula electical field, which is opposed to the pimay electical field. Consequently the displacement polaization leads to an attenuation of the pimay electical field. Oientation polaization descibes the alignment of pola molecules in an electical field. The ability to follow the changing of the applied electic fields depends on the fequency. Pola and high molecula compounds show at highe fequencies less ability to follow these altenating electic field because of inetia. This phenomenon is called dielectic elaxation. It is also clea that the oientation polaization is stongly dependent on the tempeatue which also influences the mobility of these molecules. Afte that, simila to the displacement polaization, adjusted molecules attenuate the pimay electical field. The intefacial polaization is noticeable at low fequencies. In this case, pola molecules acting as fee chage caie in the mixtue ae able to migate to the capacito plates and theefoe conductivity mechanisms occu. The polaization mechanism and consequently the decease of the pimay electical field lead to an incease of the capacitance. Usually, the elative pemittivity is defined as the quotient of the capacitance with a dielectic C ( ω, and the capacitance of the empty cell, (, ) C ω (eq. 1), whee ω is the angula T fequency and T the tempeatue: [9] C( ω, = (1) C The pemittivity is a complex valued function (eq. 2): = i (2) The meaning of the eal pat is aleady mentioned in (1). The imaginay pat descibes the dissipation caused by polaization mechanism and conductivity. Both, the alignment of pola molecules and motion of fee chages in dielectics lead to fiction with vicinal molecules and thus to losses. In fequency anges whee elaxation occus, a decease of the eal pat and an incease of the imaginay pat of the pemittivity is obsevable. The imaginay pat can be futhe subdivided into two tems (eq. 3) [1]. =, pol + σ ω (3), is associated with losses due to pol polaization, wheeas the second tem is diectly popotional to conductivity σ (e.g. due to fee chage caies). is the pemittivity constant of fee space (8.854*1-12 F/m). The elationship between eal and imaginay pat of pemittivity is as follows AMA Confeences 215 SENSOR 215 and IRS

3 DOI /senso215/C7.1 tan δ =. (4) ( ω; δ tan is the so-called dissipation facto. Laboatoy Test Setup The newly designed capacitive senso is a pinted cicuit boad with intedigital stuctue which is used to detemine the dielectic paametes (Fig. 1). Simila senso pinciples wee also mentioned in [11, 12]. To avoid inteaction with fuels, the coppe suface was coated with gold. Fig. 1: Pinted cicuit boad with intedigital stuctue (intedigital capacito) Fig. 2 shows the laboatoy pototype with the integated intedigital capacito. The suface of the capacitive aea can be coveed with fuel. Since the pemittivity has a stong dependency on tempeatue, a constant tempeatue contol of the dielectic is necessay. Fo this eason, a Peltie element, which is fixed below the pinted cicuit boad, a PT1 senso and a coole (fo cooling down the Peltie element) ae connected to a tempeatue contol unit (Belektonig HAT contol). In addition to a constant tempeatue contol, this test setup allows also vaiation of the tempeatue in the ange between and 5 C. Below C condensed wate fom ai humidity in the dielectic could cause distotion duing pemittivity measuement. The senso is connected to an impedance analyze (HP 4162a) that detects capacitance and dissipation facto tan δ. Knowing the capacitance of the empty cell (ai) C ( ω, ) and with dielectic T C( ω,, can be calculated fom eq. 1. Aftewads, with eq. 4 the imaginay pat can be calculated. Fo cleaning puposes, the electodes can be easily washed (eg. isopopanol). Calibation was pefomed using isopopanol (VWR) as efeence liquid. Fig. 2: Laboatoy pototype Sample Pepaation To detemine the fuel stability, the alteation of dielectic paametes fo fesh and aged biodiesel wee measued in the fequency ange of 2 khz to 13 MHz and the tempeatue ange between 1 C and 5 C (±.1 C). The biodiesel used was RME without any additives, which was deliveed by ADM, Hambug, Gemany. It was within the specification fo biodiesel (DIN EN 14214:23). Futhemoe, seven biodiesel samples wee aged with the Rancimat method accoding to DIN EN (11 C and 1 L/h ai supply). Aged RME samples wee taken afte an aging duation of 2, 4, 6, 7, 8, 1 and ous. Fo compaative puposes, a fesh RME sample was used with an induction time of 6.3 hous, measued with the Rancimat method. Size Exclusion Chomatogaphy (SEC) Oligomes fom aged RME wee analyzed via SEC (Agilent 126 Infinity Quatenay LC System) with efactive index detection (Agilent 126 Infinity Refactive Index Detecto). The sepaation was done with 3 x 3 cm PSS SDV (paticle size 5 µm poosity; fist column 5 Å; second column 1 Å; thid column 1 Å) columns. Futhe paametes of the SEC wee: Tempeatue of RI detecto and column: 45 C Injection volume: 1 µl Concentation: 1 mg analyte pe ml THF Flow ate:.5 ml/min Mobile phase: Tetahydofuan (Applichem, GPC gade). A calibation cuve was pefomed with polyethylene glycol (Agilent PEG calibation kit Pat No.: PL27-1), with mola masses between 196 Da and 213 Da. To veify the accuacy of the GPC system, an analytical standad of methyl oleate (Fluka) was measued. With PEG as calibation, the mola mass of methyl oleate was detemined to about AMA Confeences 215 SENSOR 215 and IRS

4 DOI /senso215/C Da, while the eal mola mass of methyl oleate is Da. In ode to make quantitative statements with egad to the molecule size, calibation standads and analytes must be identical. Howeve, since pecise oligome stuctues ae not known so fa, an accuate calibation could not be pefomed. Results Pemittivity measuements at 1 C Figue 3 shows the eal and imaginay pat of pemittivity fo fesh ( h) and aged () RME at 1 C. Fo fesh RME no fequency dependency was obseved in the fequency ange between 2 and 3 khz ( = 3.32 ±.2). Only maginal dissipation could be obseved fo fesh RME ( =.5 ±.3). Pobable elaxation is caused by methyleste and natual antioxidants pesent in RME, which ae visible in the ange between 3 and 13 MHz. In this ange, the eal pat of the pemittivity is deceasing (3.58 ±.1), wheeas the imaginay pat is inceasing (.98 ±.1). Real pat of pemittivity shows a clea incease fo aged RME () most pobably due to pola oxidation poducts (low- and highmolecula). In the fequency ange between 2 khz and 2 MHz, is 4.9 ±.7. Above 2 MHz a clea decease fo the eal pat of pemittivity is obsevable (4.22 ±.5) due to the elaxation pocess of high-molecula oxidation poducts (oligomes). In compaison to the eal pat of the pemittivity, the imaginay pat of the pemittivity is moe suitable fo the detection of oligomes, because it is moe sensitive [13]. Hee, the imaginay pat is aleady inceasing at 5 khz fom.5 (±.1) to.295 (±.9) at 13 MHz. Accoding to eq. 3, conductivity losses ae obsevable in the low fequency ange. At 2 khz the imaginay pat of the pemittivity fo fesh RME is.4 (±.1). Afte an aging duation of ous the imaginay pat inceases to.25 (±.9). [-] h h Fequency [khz] Fig. 3: Pemittivity measuements fo fesh ( h) and aged () RME at 1 C SEC measuements Figue 4 shows SEC measuements fo all tested RME samples ( ous). The mola mass anges between 35 and 425 g/mol ae those fom fatty acid methylestes of the RME. Fo fesh RME an unexpected peak occus in the highe mola mass ange (about 74 g/mol). It is pobably caused by glyceides as esidual poducts of an incomplete tansesteification eaction duing the biodiesel poduction. The decease of the RME peak duing aging is associated with an incease of the oligome ange (>425 g/mol) due to its eaction to highmolecula oxidation poducts. The esults indicate that in efeence to thei mola masses thee wee no pefeable oligomes poduced duing the aging pocess. Hence, it is difficult to detect a valid peak maximum with inceasing aging time. The highest detectable mola mass was about 16 g/mol afte an aging duation of ous. el W (log M) [-] RME Oligomes h 2 h 4 h 6 h 7 h 8 h 1 h [-] Relative mola mass [g/mol] Fig. 4 Mola mass distibution fo fesh and aged ( ) RME measued by SEC AMA Confeences 215 SENSOR 215 and IRS

5 DOI /senso215/C7.1 Compaison of the imaginay pat of pemittivity (1 C) and SEC Due to its highe sensitivity, in the following only the imaginay pat of pemittivity will be discussed. Theefoe, Figue 5 shows the imaginay pat of the pemittivity (1 khz 13 MHz) fo all investigated samples ( ). The esults fo the fesh ( h) and aged RME () samples wee aleady discussed at Figue 2. Significant effect occus fo samples afte an aging duation of 7 hous. Until an aging duation of 7 hous ( 7 h) the imaginay pat of pemittivity is about.111 (±.2) at 13 MHz. Afte 8 hous it inceases to.26 (±.42) h 2 h 4 h 6 h 7 h 8 h 1 h As aleady discussed in Figue 3, the low fequency ange of the imaginay pat of pemittivity is dominated by conductivity losses pesent in aged RME. An enhanced conductivity is obsevable when tempeatue is inceasing due to lowe viscosity and thus highe mobility of ions. Fo aged RME () the imaginay pat of pemittivity was.25 (±.9) at 1 C and 2 khz (see also Fig. 3). In compaison to that the imaginay pat of pemittivity is now.159 ±.6 at 5 C and 2 khz (Fig. 7). [-] h 2 h 4 h 6 h 7 h 8 h 1 h [-] Fequency [khz] Fequency [khz] Fig. 5. High fequency ange (1 khz 13 MHz) of imaginay pat of pemittivity at 1 C fo fesh and aged RME ( ) In Figue 6 the incease of the integated aea of the oligome ange (425 to 16 g/mol) fom SEC measuements (see also Fig. 4) is compaed with the incease of the imaginay pat of pemittivity at 13 MHz and 1 C (see also Fig. 5). Using an intesection of tangents applied to both cuves an induction time of 6.7 hous fo both cuves was found. Aea ( g/mol) [-] 4 Integated aea between 425 to 16 g/mol (OIigome ange) Imaginay pat of pemittivity at 1 C and 13 MHz Aging [h] [-] Fig. 7: Imaginay pat of pemittivity at 5 C The incease of the imaginay pat of the pemittivity measuements (5 C and 2 khz) is compaed to the incease of conductivity measued with the Rancimat method (Fig. 8). Via Rancimat an induction time of 6.3 hous was measued. Using an intesection of tangents applied to the imaginay pat cuve an induction time of 7.1 h is obsevable. Thus, DRS esults slightly deviate fom the Rancimat induction time. As mentioned befoe, the samples used in the pemittivity measuements wee aged with the Rancimat method. The outcome of this is that these samples ae without volatile low-molecula caboxylic acids that ente the conductivity measuing cell of the Rancimat. Consequently, only those oxidation poducts, which could not escape in the oiginal sample wee measued by means of DRS. Accoding to [14, 15], losses in the low fequency ange ae caused due to poton tansfe fom one hydoxyl oxygen to anothe. Consideing the oxidation poducts in RME, which wee found in [4, 5], this could be, on one hand, low-molecula oxidation poducts like aldehydes, ketones, caboxyl acids and, on the othe hand, high-molecula oxidation poducts like peoxides, hydopeoxides and oligomes. Fig. 6: Compaison of oligome fomation with SEC (integated aea between 425 and 16 g/mol) and pemittivity measuements (13 MHz and 1 C) Pemittivity measuements at 5 C AMA Confeences 215 SENSOR 215 and IRS

6 DOI /senso215/C7.1 [-] Imaginay pat of pemittivity at 5 C and 2 khz Conductivity (Rancimat method) Aging [h] Fig. 8: Compaison of conductivity incease (Rancimat method) and incease of the imaginay pat of the pemittivity at 5 C and 2 khz Conclusion By means of DRS, it is possible to detect oxidation poducts in aged biodiesel. As a capacity cell fo DRS measuements a pinted cicuit boad with intedigital stuctue (intedigital capacito) was developed. The esults showed that the imaginay pat of pemittivity is suitable to detect oligomes in the high fequency ange due to dielectic elaxation. Oligomes ae consideed as pecusos of undesied deposits and theefoe thei detemination enables a fuel quality contol. Futhemoe a good coelation with the standad test pocedue fo oxidation stability measuements via the Rancimat method (DIN EN 15751) is shown in the low fequency ange. The small size of the newly developed intedigital capacity senso is an option fo onboad contol in PHEV. In the long tem, the senso pevents engine damages since fuel degadation is detectable at an ealy stage and the combustion engine can be stated befoe pecipitations occu. In this way, the senso enables the futue use of commonly used diesel fuel even in plug-in hybid diesel vehicles. So the DRS senso suppots and enables electomobility Conductivity [µs/cm] [3] Union zu Födeung von Oel- und Poteinpflanzen (UFOP), Eneuebaen-Enegien- Richtlinie RED Biokaftstoffnachhaltigkeitsveodnung Klimaschutzquote NUTS2 THG-Wete, (213) [4] H. L. Fang, R. L. McComick, Spectoscopic Study of Biodiesel Degadation Pathways, SAE Technical Pape Seies , (26) [5] L. Schmidt, Wechselwikungen zwischen Kaftstoffkomponenten und biodieselbasieten Mischkaftstoffen unte besondee Beücksichtigung de Alteungspodukte von Fettsäuemethylesten, Cuvillie Velag, ISBN: , Göttingen, (214) [6] W. J. Batz, Additive fü Schmiestoffe, Expet- Vel., Renningen-Malmsheim, ISBN: , (1994) [7] R. M. Motie, M. F. Fox, S. T. Oszulik, Chemisty and technology of lubicants, Spinge, New Yok, ISBN: , (21) [8] A. R. von Hippel, Dielectic mateials and applications, Atech House, Boston, ISBN: , (1995) [9] V. Castan, Elektische Enegievesogung, Spinge, Belin, ISBN: , (27) [1] S. Romano, P. Soichetti, Dielectic Spectoscopy in Biodiesel Poduction and Chaacteization Spinge, ISBN: , (21) [11] M. Golio, The RF and Micowave Handbook, ISBN: , (21) [12] A. S. Zolfaka, A. Shah, N. H., Mahzan, A. Manut, M. F. Idos, Design, fabication and chaacteization of ph tansduce using pinted cicuit boad (PCB), Electonic Devices, Systems and Applications (ICEDSA), 21 Intenational Confeence on Electonic Devices, Systems and Applications, Kuala Lumpu, (21) [13] C. J. Colliste, Electical measuement of oil quality, Patent US B1, (1998) [14] J. D. Hoffman, C. P. Smyth, Molecula Rotation in the Solid Foms of Some Long-Chain Alcohols, Jounal of the Ameican Chemical Society, Vol. 71, (1949) [15] Y. Kakiuchi, H. Komatsu, S. Kyoya, Spectoscopic Evidence fo the Poton Conductivity though Solid Alcohols, Jounal of Chemical Physics, Vol. 19, (1951) Refeences [1] DIN EN 59:214-4, Kaftstoffe fü Kaftfahzeuge Dieselkaftstoff Anfodeungen und Püfvefahen, (214) [2] Gesetz zum Schutz vo schädlichen Umwelteinwikungen duch Luftveuneinigungen, Geäusche, Eschütteungen und ähnliche Vogänge (Bundes-Immissionsschutzgesetz BimSchG), , (214) AMA Confeences 215 SENSOR 215 and IRS