Experimental investigation of direct injection charge cooling in optical GDI engine using tracer-based PLIF technique

Experimental Thermal and Fluid Science 59 (2014) 96–108

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Experimental investigation of direct injection charge cooling in optical GDI engine using tracer-based PLIF technique Mohammadreza Anbari Attar a,⇑, Mohammad Reza Herfatmanesh b, Hua Zhao a, Alasdair Cairns a a b

Centre for Advanced Powertrain and Fuels Research, Brunel University, London, UK School of Engineering and Technology, University of Hertfordshire, Hertfordshire, UK

a r t i c l e

i n f o

Article history: Received 25 May 2014 Received in revised form 22 July 2014 Accepted 28 July 2014 Available online 8 August 2014 Keywords: Planar Laser Induced Fluorescence (PLIF) Thermometry Direct injection charge cooling Gasoline direct injection

a b s t r a c t Investigation of direct injection charge cooling effects is indispensable in design and development of new combustion systems for Gasoline Direct Injection (GDI) engines. The charge cooling can be utilized to increase engine volumetric efficiency or compression ratio. It can be employed to suppress pre-ignition of highly boosted downsized engines or knocking combustion of naturally aspirated engines. The main purpose of this work was to develop an experimental setup for quantitative measurements of charge cooling during fuel injection process inside the combustion chamber of a GDI engine with optical access. For this purpose a tracer-based two-line Planar Laser Induced Fluorescence (PLIF) technique was implemented for the measurements. A specially designed Constant Volume Chamber (CVC) was utilized for quasi in situ calibration measurement so in-cylinder charge temperature measurements can be achieved independent of the photophysical model of dopant tracer. The thermometry technique was evaluated by measurements of average in-cylinder charge temperature during compression stroke for both motoring and firing cycles and comparing the results with temperature values calculated from in-cylinder pressure data assuming a polytropic compression. The PLIF technique was successfully utilized to quantify the extend of global temperature decrease as a result of direct injection charge cooling of two injection timings of 90 and 250 °CA ATDC and two injection quantities of 10 and 30 mg/cycle. Test results demonstrated the capability of the two-line PLIF thermometry technique in quantitative study of direct injection charge cooling effects. Ó 2014 Elsevier Inc. All rights reserved.

1. Introduction Gasoline Spark Ignition (SI) engines are the most dominant power plant for passenger cars throughout the world. This is mainly due to their high specific power and relatively low manufacturing costs compare to diesel engines. Although we have seen a tremendous progress in performance, reliability, fuel-efficiency and exhaust emissions of SI engines since their invention, but still, a huge research and development work is being carried out by automotive manufacturers and research institutes. This is mainly due to ever tightening emission legislations, fuel economy (CO2) targets and customer demands. In recent years, various technologies such as variable valvetrain systems [1–3], friction reduction technologies [4,5] and Gasoline Direct Injection (GDI) technologies [6–10], have been emerged and utilized in SI engines. The GDI is considered as one of the key technologies that has shown a great potential in synergy with other technologies and combustion strat⇑ Corresponding author. Tel.: +44 121 414 4270. E-mail address: [email protected] (M. Anbari Attar). http://dx.doi.org/10.1016/j.expthermflusci.2014.07.020 0894-1777/Ó 2014 Elsevier Inc. All rights reserved.

egies. Introduction of the new generation of solenoid-actuated and piezo-actuated injectors and consequently, precise control of injection timing and quantity, multiple injection strategies and improved spray pattern and fuel atomization have brought new possibilities for further improvements in several areas. These include both homogeneous and stratified operations [11], low temperature combustion modes [12–18] and advanced downsizing [19–22]. The strong synergy between downsizing and direct injection is now reasonably well understood [23–28]. Direct injection charge cooling can be utilized to avoid pre-ignition phenomenon in downsized engines which can lead to catastrophic engine failure [29,30]. For naturally aspirated SI engines, direct injection charge cooling can be employed to increase the volumetric efficiency [31] or the compression ratio [32] which is currently limited to 11:1 by knocking combustion. This is due to the fact that the knocking combustion is most sensitive to the compression temperature, and thus it can be minimized by utilizing the direct injection charge cooling effect [33–35]. Various methods have been exploited to measure amount of charge cooling in SI engines. Anderson et al. [31] suggested

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Nomenclature Roman symbols c speed of light cp specific heat at constant pressure E laser fluence h Planck constant m mass M molecular weight n polytropic constant L latent heat of vaporization P pressure Q energy e R specific gas constant

measurements of intake airflow rate to study the charge cooling effects. This was due to a fact that the direct injection of the fuel into the combustion chamber alters the in-cylinder charge temperature and hence affects the in-cylinder air density and intake mass airflow rate. Wyszynski et al. [36] used the same technique to measure volumetric efficiency of different fuels on a GDI engine fitted with both Port Fuel Injection (PFI) and Direct Injection (DI) systems. Although this technique has been one of the main methods to quantify the charge cooling in different studies, it is limited to injection timings that end before Intake Valve Closure (IVC) as the fuel evaporation after IVC does not affect the flow rate and hence cannot be measured. Therefore in-cylinder charge temperature calculation based on averaged in-cylinder pressure data can be considered a better approach. Ahn et al. [37] used in-cylinder pressure to evaluate charge cooling effect of ethanol fuel. Sjöberg et al. [38] used the change in the manifold air pressure as a diagnostic which is only suitable for low intake air flow rates. Kasseris et al. [26,27] proposed measurements of knock onset limits as a diagnostic of charge cooling. On a turbocharged SI engine with both PFI and DI systems, they measured how much intake air needed to be heated in the DI mode to cancel out charge cooling and make the engine knock at the same conditions as the PFI mode. In a similar investigation, Stein et al. [28] attempted to identify the benefits of DI and PI and concluded that the ethanol’s cooling effect enhancement to the engine performance was comparable to that of its higher octane number. Fast response in-cylinder temperature measurements were made using cold-wire thermometry [39,40]. Although thermocouples, when bare, can be extremely fine, but under high aerodynamic loads and in presence of particles e.g. fuel droplets and soot, they must be protected. Thus, their spatial resolution is reduced, their temporal response diminished and their accuracy degraded. As a consequence this technique can be only applied to a limited operating condition. Computational and analytical techniques have been also employed in different studies [26,27,36,41–45]. However, the results need to be calibrated against the experimental data. Planar Laser Induced Fluorescence (PLIF) is a well-known powerful diagnostic technique for flow and combustion measurements. It has been utilized for spray imaging, study of charge stratification and measurements of local air–fuel ratio, charge temperature and Exhaust Gas Residuals (EGR) [46]. This paper presents implementation of tracer-based two-line PLIF thermometry technique for study of direct fuel injection charge cooling effect on in-cylinder gas temperature. In this work isooctane was used as a surrogate fuel due to its similar physical and thermodynamic properties to typical gasoline fuels and 3-pentanone was chosen as a seeding tracer. The main physical and photophysical properties required of a fluorescent tracer for in-cylinder LIF studies include: boiling point and transport properties closely match to the carrier fuel,

S T V Xtr

number of photons incident per pixel temperature volume tracer mole fraction

Greek symbols gc transmission efficiency of optics k excitation wavelength r absorption cross section t spatial frequency / fluorescence quantum yield (FQY) Xc collection solid angle of optics

absorption spectrum suitable for available laser wavelengths, satisfactory fluorescence quantum yield, and insensitivity to oxygen quenching [47]. Previous studies of the photophysical behavior of 3-pentanone indicated its advantages over other common tracers [48–51]. Thermometry by two-line PLIF technique using 3-pentanone as a tracer was first demonstrated by Grossmann et al. [50]. They showed after excitation of 3-pentanone by two different wavelengths, the ratio of fluorescence signal intensities reflects the local temperature. As the ratio is independent of local tracer concentrations, it can be used for measurement of temperature distributions in non-homogeneously mixed systems. Einecke et al. [52] reported the first application of the two-line PLIF technique for temperature distribution measurements in an optically accessible two-stroke engine with isooctane doped with 3-pentanone. Rothamer et al. have proposed the combination of 308 nm and 277 nm excitation wavelengths for simultaneous measurements of temperature and EGR of a Homogenous Charge Compression Ignition (HCCI) engine [53]. This work has been followed in similar researches to study the Negative Valve Overlap (NVO) strategy in HCCI operation [54]. However, in these latter works, in-cylinder charge temperature values were calculated using theoretical models of 3-pentanone. And estimated temperatures based on isentropic compression and 1-D engine simulation were used as reference data to modify parameters of the photophysical models and tune them to give the most accurate temperature readings on particular test conditions. The present work was carried out to execute the two-line PLIF thermometry independent of tracer’s models and implement the technique for study of direct injection charge cooling in an optical GDI engine. 2. Principle of the two-line PLIF thermometry technique In the linear regime, the laser induced fluorescence signal is given by

Sf ¼

  E V e X tr P X rðk; TÞ/ðk; T; P; X tr Þ c gc hct kT 4p

ð1Þ

where Sf is the number of photons incident per pixel at the detector or the photocathode of an intensified camera (photons/pixel), E is the laser fluence (J/cm2), t is the spatial frequency of the incident laser radiation (cm1), Ve is the excited volume (cm3), Xtr is the tracer mole fraction, P is the total pressure (MPa), T is the temperature (K), r is the absorption cross section (cm2), / is the fluorescence quantum yield (FQY), Xc is the collection solid angle of the optics used for imaging the florescence, and gc is the transmission efficiency of optics and filters used in the imaging setup. Both the absorption cross section and FQY vary by temperature while the rate of these variations depends on the excitation wavelengths. In

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order to determine temperature, an absorption specie should be chosen so that its fluorescence signal ratio for two given excitation wavelengths exhibits strong temperature dependence. For 3-pentanone, the absorption band extends from 220 to 340 nm with a peak near 280 nm at the room temperature. As temperature increases, the peak intensity red-shifts so e.g. at 1000 K, the peak occurs at 290 nm [50]. This spectral shift of the absorption spectra can be used for temperature measurements by choosing two excitation wavelengths on different wings of this band and taking the ratio of fluorescence signals from these two excitation wavelengths. The 3-pentanone fluorescence signal is emitted between 350 and 550 nm with a maximum at 420 nm. When fluorescence signals from two excitation wavelengths are detected, their ratio is given by

Sf 2 E2 t1 r2 /2 ¼   Sf 1 E1 t2 r1 /1

ð2Þ

In the two-line PLIF thermometry, high-energy pulsed lasers are employed to generate two laser pulses at specific wavelengths with a very short time delay. The laser pulses are formed into thin (sub millimeter) laser sheets and steered to the sampling area inside the engine by the transmitting optics. The fluorescence signals from both excitations are captured and amplified by an ICCD camera. The time delay between camera gates is typically kept as short as few microseconds. This is to ensure that the images are taken at a frozen condition with minimum change in thermodynamic state of the measurement volume between two images. 3. Experimental setup 3.1. Test engine A single cylinder optical GDI engine, shown in Fig. 1, was used for the experiments. The engine employed a prototype DI head featuring a pent-roof combustion chamber with four valves operating by double overhead camshafts. Table 1 summarizes engine specification. Optical access to the engine was achieved by modifying the cylinder block and installing a sandwich plate between the cylinder block and the head. The sandwich plate housed four windows on its sides which could be used for illumination or side imaging of the combustion chamber. A quartz window installed in the piston crown and a 45° mirror mounted on the cylinder block provided the bottom view of the combustion chamber. The engine could

Table 1 Key engine specifications. Combustion chamber Displaced volume Bore Stroke Inlet valves diameters Exhaust valves diameters Valve lift Valve duration Compression ratio

Pent-roof 450 cc 80 mm 89 mm 29.5 mm 21 mm 4 mm 110 °CA 12.4:1

run on both Spark Ignition (SI) and Controlled Auto Ignition (CAI) or HCCI combustion modes. To achieve the CAI combustion a set of low valve lift camshafts with 110 °CA duration was used for internal Exhaust Gas Recirculation (iEGR) with Negative Valve Overlap (NVO) as demonstrated in [55,56]. In this method, the exhaust valves were closed early (80 °CA BTDC in the exhaust stroke) to trap a large amount of residual gases in the cylinder while the intake valves opening was delayed (120 °CA ATDC on the intake stroke) to avoid unwanted back-flow of the residual gases into the intake port. 3.2. Two-line PLIF setup 3.2.1. Excitation wavelengths Two excitation wavelengths at 308 nm and 277 nm were used for the measurements. The 308 nm pulses were obtained from a Xenon Chloride (XeCl) laser (Lambda Physik COMPexPro 102) output while the 277 nm pulses were generated by Raman shifting a Krypton Fluoride (KrF) laser (Coherent COMPex 102) output at 248 nm to 1st Stoke of H2 using a bespoke Raman Convertor (RC). As the KrF laser output beam was 10 mm  24 mm (horizontal  vertical), and in order to avoid using extra lenses (a reverse Galilean telescope) to resize the laser beam, the RC was designed with an aperture size of 25.4 mm. And two Lithosil lenses with anti-reflection coating at 248–277 nm were used as the convertor’s input and output windows to focus the laser beam into the RC and re-collimate the output. To achieve the maximum pulse energy with the lowest fluctuation, Excimer lasers were operated with fresh gases. This was done by purging lasers’ resonator and refilling it with fresh premix gas prior to each test. Also both lasers were run at constant energy mode. In this mode, lasers’ internal energy

Fig. 1. Left: test engine, right top: spray guided combustion chamber, right bottom: intake and exhaust valve timings for NVO operation and fuel injection events.

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meter monitor the output pulse energy and vary the resonator high voltage to keep the output pulse energy constant. Thus, lasers could provide output pulses with a very low energy fluctuation (<1.8% and <0.8% relative standard deviation for XeCl and KrF lasers respectively). 3.2.2. Beam delivery and shaping system Fig. 2 shows the schematic diagram of the two-line PLIF system. Four High Reflective (HR) mirrors at 308 nm (76.2 mm diameter) and three HR mirrors at 277 nm (76.2 mm diameter) were used to steer, raise and flip the beams into the combustion chamber. The main reason for flipping the beams was the fact the laser beams’ divergence on the vertical axis of the laser output was much higher than on the horizontal axis. By flipping the laser beams, laser sheets in the measurement area would exhibit little divergence in their thickness as they expanded horizontally. To overlap the two beams, a dichroic mirror was used. The dichroic mirror had the same dimensions as the other HR mirrors but with special coating that transmitted the 308 nm beam with >90% efficiency while reflected the 277 nm beam with >97% efficiency. A rectangular cylindrical lens (76.2  38.1 mm2) with focal length of 1500 mm and a broad band anti-reflection coating was used to turn the expanded laser beams to laser sheets. 3.2.3. Imaging system and synchronization The image acquisition was carried out with a Princeton Instrument PI-MAX II intensified CCD camera system. For the two-line PLIF imaging, the overlapped read out mode was exploited. The overlapped operation allows a new exposure to begin while the readout of the previous one is still in progress, thus enabling the camera to capture two images with an extremely short interval. A UV-Nikkor 105 mm f/4.5 macro photography lens was coupled with the camera and a Schott WG360 long pass filter was used to block unwanted wavelengths. Images were captured perpendicular to the light sheet plane via the piston window and 45° mirror. Several different techniques were employed to enhance the fluorescence Signal to Noise Ratio (SNR) and imaging quality. Light reflections from combustion chambers’ roof and cylinder walls were reduced by covering the cylinder head with candle soot and focusing laser sheets behind the sandwich plate. Rayleigh and Mie scattering signals were suppress by the long pass filter, fine adjustment of ICCD gate timing, and exploiting short gate widths. CCD accumulated dark charge was minimized by operating the

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camera at the lowest CCD array temperature for test conditions (20 °C) as well as programming the camera to perform CCD clean cycles while the intensifier was waiting for start acquisition command. For synchronization, first a precise time budgeting was carried out by investigating signal delays within and between all the PLIF system’s units. The Excimer lasers maximum repetition rate was 10 Hz, similar to engine reference signal at engine speed of 1200 rpm, but the camera readout frequency at full CCD (1024  1024 pixels) was 4 Hz. Therefore to be able to use the 10 Hz trigger-in signal for the imaging system, either a smaller Region of Interest (ROI) should be assigned on the camera CCD or the imaging pixels were binned together. However, in order to avoid shrinking the actual measurement area and have the maximum spatial resolution neither of these two methods were employed. Instead an external counter/divider unit was used to reduce the engine reference signal frequency. The unit received the 10 Hz engine reference signal and provided a trigger-in signal at lower frequencies. This signal was also used for skip injection operation where the fuel injection took place only at required cycles in order to minimize windows fouling. The output of the counter/divider unit was sent to a delay generator to trigger both the lasers and the camera, while the camera programmable timing generator was utilized for fine adjustment of the ICCD gate width and delay. 4. Measurements of Raman conversion efficiency The Raman conversion is a nonlinear optical effect with relatively low efficiencies. As the maximum pulse energy of the KrF laser was limited to 400 mJ, it was required to increase conversion efficiency of the Raman Convertor (RC) to achieve a satisfactory fluorescence signal to noise ratio. In order to measure conversion efficiency of the RC, a series of tests were carried out. The Raman convertor was filled with grade 5 hydrogen (purity 99.999%) at 55 bar (800 psi). The KrF laser was used to pump the RC at 10 Hz repetition rate with different pulse energies. In order to measure the conversion efficiency of 1st Stoke of H2 (conversion of 248– 277 nm), it was required to eliminate the pump beam as well as the higher Stokes and anti-Stokes from the RC output. The spectral filtering of the unwanted beams was achieved by placing an equilateral dispersive prism in front of the RC exit window and sending the laser beam into a relatively long optical path which allowed the

Fig. 2. Schematic diagram of the two-line PLIF setup.

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angular deviation of the prism to separate all the wavelengths. The H2 pressure was reduced from 55 bar to 7 bar (100 psi) in 3.5 bar (50 psi) increments and at each step the KrF pulse energy was varied from 350 mJ to 150 mJ in 50 mJ increments. The output pulse energy at 277 nm and its standard deviation were measured and recorded using a Coherent FieldMax energy meter. Fig. 3 shows the RC output pulse energy at 277 nm as a function of input pulse energy at 248 nm for different RC fill pressures. It was observed that the RC output pulse energy increases with increase in both pump pulse energy and H2 fill pressure. However, highest conversion efficiency of 13% was achieved at the lowest pump energy (150 mJ) while it dropped to 10% at 350 mJ. Also the minimum required H2 fill pressure was found to be 24 bar (350 psi). Although both the RC optics and the prism exploited anti-reflection coatings at 248 and 277 nm respectively, measurements showed 6% pulse energy reduction at the RC and additional 8% reduction at the prism. In case of the prism (as it is shown in Fig. 4) there was 3% reduction due to reflection from the first surface, 3% internal reflection at the second surface plus 2% reduction due to light absorption and scattering inside the prism. Consequently considering these losses, the actual RC conversion efficiency was calculated to be 12–15%. Furthermore it was required to study and compare the RC output pulse energy variation with the KrF output. An energy sensor was placed right after the RC and energy variation of 200 shots at selected laser output pulse energies and RC fill pressures were measured. It was observed that by increasing the KrF output pulse energy to up to 300 mJ, the RC output pulse energy fluctuation reduces. This was due to a fact that the laser resonator operated in a more stable high voltage range. Also as it can be seen in Fig. 5, increasing RC fill pressure could further drop the RC output energy fluctuation to the laser limit. Consequently, RC was filled with H2 at 55 bar for all tests in this work to:  achieve maximum Raman conversion efficiency,  achieve lowest output pulse energy fluctuation,  suppress unwanted higher Stokes and anti-Stokes [57].

5. Temperature calibration measurements In order to perform quasi in situ calibration measurements, a Constant Volume Chamber (CVC) was designed and manufactured so it can be mounted on the engine block in place of cylinder head. Same optical components (side windows and piston window) were utilized on both the CVC and the engine. This was to eliminate any effect of variation in light transmission efficiency and surface

Fig. 3. Raman convertor’s output pulse energy at 277 nm as a function of pump pulse energy and fill pressure.

Fig. 4. Pulse energy reduction at Raman convertor and prism.

Fig. 5. Raman convertor’s output pulse to pulse energy variation as a function of pump pulse energy and fill pressure.

flatness of different optics. To supply the tracer into the CVC, a metal tank was filled with liquid 3-pentanone, cartridge heaters were used to heat up (to 318 K) the tracer inside the tank and a nitrogen bottle was connected to the bottom of this tank through a second regulator. By setting the bath gas pressure and adjusting the inlet and outlet valve openings on the chamber, continues flow of homogenous mixture of the tracer and the bath gas was introduced into the chamber. A combination of tubular heaters (wrapped around the CVC) and cartridge heaters (placed inside the CVC liner) were used to heat up the premixed charge and achieve high gas temperatures in combination with short residence times to avoid decomposition of the tracer. Three NiCr–Ni thermocouples (with response time of 0.25 s at 300 K and 0.5 s at 800 K), two in contact with the CVC wall and one placed at the center of CVC cavity and isolated from the CVC body, were used to monitor chamber wall and charge temperature respectively. For the PLIF measurements, camera mode was set to Double Image Frame (DIF) with a gate width and burst period of 1 ls and 5 ls respectively and a shutter compensation time of 4 ls. This time was the amount of time inserted between the end of exposure time and the beginning of the array readout which allowed time for phosphor to decay (the lowest residual intensity on the second image for different burst periods was observed when the shutter compensation time was close to the burst period). To obtain temperature calibration curve, a set of 20 background DIFs (20 images at 277 nm excitation and 20 images at 308 nm excitation) was captured prior to seeding the chamber at the room temperature. Then the CVC and tracer tank heating systems were switched on and a set of 20 data DIFs was captured at each selected temperature up to 750 K at 2 bar chamber pressure. Using a Visual Basic image processing code each set of DIFs was first divided into two stacks of

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even images (laser illumination at 277 nm) and odd images (laser illumination at 308 nm). Then average of each data stack was calculated and subtracted by its averaged background image. In order to eliminate the effect of boundary layers, a Region of Interest (ROI), 600  600 pixels (40  40 mm2), located at the center of the CVC cavity (shown in Fig. 6a), was defined on the resultant images and the pixels’ value within the ROI were averaged. Finally the ratio of two fluorescence signals was calculated. The image processing steps are illustrated in Fig. 7. Fig. 8 shows the test result. The red diamond and blue cross markers represent fluorescence signal intensity as a function of CVC temperature for laser illumination at 308 nm (IO) and 277 nm (IE) respectively. The calibration curve, solid black line, was obtained by fitting a linear trend line on the calculated fluorescence signal ratio, R (IO/IE), at different chamber temperature. It was observed that while the fluorescence signal intensity for the excitation at 277 nm drops as the temperature increases, the fluorescence signal intensity for the excitation at 308 nm slightly increases and the ratio of two signals showed a linear increase. The absolute value of coefficient of variation (COV) of the temperature calibration measurements was less than 3%. This was calculated by comparing 20 single DIFs at each selected temperature. This uncertainty was partially due to inaccuracy of ±0.25 K in CVC charge temperature control during measurement at each point. In addition, relative standard deviation of the averaged signal intensity of the ROIs was found to be 1.9% and 1% for excitation at 308 nm and 277 nm respectively. Both these values were close to the relative standard deviation of the lasers’ pulse energy fluctuation, indicating that actual precision of the singleshot thermometry technique could be further enhanced by correcting for the pulse energy fluctuation of lasers. However, as the single-shot thermometry technique was not the focus of this work and temperature values were calculated from averaged DIFs, application of this correction was not beneficial. The choice of nitrogen as the bath gas (instead of air) for calibration measurements was due a fact that the two-line PLIF system was initially designed to be implemented for CAI combustion measurements with 60% internal exhaust gas recirculation. Although ideal condition for the calibration measurements would be a case of having exactly the same charge composition as the engine measurements, but previous studies indicated that the ketones’ fluorescence at atmospheric pressure is relatively insensitive to the bath gas composition and quenching. This is due to dominant non-collision dependent decay rates of the first excited

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Fig. 7. Image processing procedure for the CVC temperature calibration measurements.

singlet state [58,59]. In addition, experimental test results at elevated pressures up to 20 bar, indicated that 3-pentanone shows even less sensitivity to oxygen-related quenching influences than does other ketones [60]. This was associated with the electron density at the carbonyl group [61]. In addition, it should be noted that although temperature calibration up to 750 K does not cover the full range of in-cylinder charge temperature values for spark

Fig. 6. (a) Imaging lens was focused on the measurement plane inside the CVC, dashed square and dashed-dotted circle indicate the ROI and inner diameter of the chamber respectively (b) single-shot background image, (c) single-shot data image.

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Fig. 8. Fluorescence signal intensities of two excitation wavelengths and the PLIF temperature calibration curve.

ignition engine operation, but it was sufficient for this work as the measurement during the compression stroke and up to ignition timing was concerned. 6. Investigation of direct injection charge cooling effects 6.1. Measurements of in-cylinder charge temperature The two-line PLIF experimental setup for in-cylinder charge temperature measurements was identical to the calibration measurements. Isooctane was introduced by direct injection into the combustion chamber using an air-assisted low pressure direct injector while the tracer was injected (with mole fraction of 0.4% in total charge) into the intake port via a port fuel injector to achieve a homogenous tracer seeding. Intake port was insulated with heat resistant fiberglass tape and the intake air was heated to 318 K to facilitate tracer vaporization. In order to ensure stable thermal conditions, engine was warmed up prior to the measurements with both engine oil and coolant temperatures fixed at 363 K and 313 K (±2 K) respectively. Three sets of 40 DIFs were captured at desired crank angles from 94 °CA BTDC to 35 °CA BTDC (ignition timing) during compression stroke at engine speed of 1200 rpm. The first set was background frames (B) with both lasers on and both port and direct injectors off. The second set was motoring frames (M) in which both lasers and PFI injector were on but the DI injector was switched off. Finally, the third set was firing frames (F) in which both lasers and both injectors were switched on. To minimize effects of windows fouling on the measurements, all the three sets of frames at each crank angle were captured following each other. In-cylinder charge temperature for motoring and firing cycles were measured by first calculating normalized fluorescence signal ratios using

Rmotoring ¼

MðOÞX  MðEÞX 

Rfiring ¼

 BðOÞX CA CA  BðEÞX  CA CA

MðEÞ94 MðOÞ94

FðOÞX  FðEÞX 

 BðEÞ94 CA BTDC CA BTDC  BðOÞ94 CA BTDC CA BTDC

 BðOÞX  CA MðEÞ94  MðOÞ94 CA  BðEÞX  CA CA

 BðEÞ94 CA BTDC CA BTDC  BðOÞ94 CA BTDC CA BTDC

ð3Þ

ð4Þ

In these equations, background corrected averaged (of 40 images) motoring image ðMX  CA  BX CA Þ and background corrected averaged firing image ðF X CA  BX CA Þ of both 308 nm (O) and 277 nm (E) excitation wavelengths at each crank angle (X °CA) were divided by background corrected averaged motoring image taken at 94 °CA BTDC ðM94 CA BTDC  B94 CA BTDC Þ. This was to remove lasers’ profile and effect of laser sheet attenuation across the imaged area as

illustrated in Fig. 9. Then the calibration curve obtained from constant volume chamber was used to convert the calculated signal ratios to the temperature readings. In-cylinder charge temperature was also calculated from the pressure data to compare with the PLIF temperature measurements. This was done by considering the engine as a closed thermodynamic system between Intake Valve Closure (IVC) to the ignition timing and taking the IVC timing as a reference point for the calculation. During the operation with normal valve timing, fresh charge temperature slightly increases between the Intake Valve Opening (IVO) and IVC due to heat transfer from the cylinder walls. In case of Negative Valve Overlap (NVO) operation, the charge temperature at IVC is significantly higher than intake air temperature due to presence of a large amount of hot residuals. Therefore for the NVO operation, one cannot simply take intake port temperature as in-cylinder charge temperature at the IVC. The charge temperature at the start of compression (TIVC) was calculated by enthalpy balance equation [62]:

T IVC ¼

ms  cp;a  T in þ mr cp;r  T ex ms  cp;a þ mr  cp;r

ð5Þ

where Tin, is intake gas temperature measured at 5 cm above the intake port, Tex, is exhaust gas temperature measured at 5 cm above the exhaust port, ms is mass of scavenging charge (ms = mair + mfuel + mtracer) calculated by measuring air flow rate and fuel and tracer injection quantities, mr is mass of residual gas and cp,a (air) and cp,r (residual gas) are specific heat at constant pressure. The mass of residual gas mr, is calculated by

mr ¼

PVM e RT

ð6Þ

where P, V, T are at the exhaust valves closure (T from exhaust there ¼ 8:3144 J and M is the molecular weight of residmocouple), R kg mol ual gas. Having calculated temperature at IVC, average in-cylinder charge temperature from start of compression to ignition timing was calculated using averaged in-cylinder pressure data (of 100 cycles) and Eq. (7), assuming a polytropic compression

T 2 ¼ T IVC

 n1 PIVC n P2

ð7Þ

where n is the polytropic constant calculated from log P–log V diagram between the IVC to the ignition timing. Comparison of the PLIF temperature values obtained from optical measurements with the values calculated from in-cylinder pressure data (Fig. 10) showed a very good agreement between two methods with average percent difference of 3% and 2% and standard deviation of 2% and 1% for the motoring and firing cycles respectively. The percent difference at each imaged crank angle was calculated by

Percent difference ¼

Absolute Difference  100 Av erage Value

ð8Þ

where the Absolute Difference was calculated by subtracting temperature value obtained from pressure data by the value obtained from the PLIF images and the Average Value was calculated by taking the average of the two temperature values. The standard deviation was then calculated to measure how widely percent difference value at different crank angles were dispersed from average percent difference. 6.2. Quantitative measurements of charge cooling For quantitative study of direct injection charge cooling effects, experiments were carried out for two injection timings at 90 and 250 °CA ATDC (intake) and two injection quantities of 10 and 30 mg/cycle. Fig. 11 shows in-cylinder peak temperature calculated from averaged (of 100 cycles) pressure data for four engine

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Fig. 9. Two-line PLIF thermometry image processing steps.

runs. For motoring tests with no fuel injection, and tests with only port injection, the variation in measured averaged peak pressure was less than 0.07 bar. Switching on the port injector (injecting 4.5 mg/cycle of the tracer) dropped the peak pressure by 0.15 bar which is equivalent to 6 K drop in calculated in-cylinder peak

temperature. This was associated with a decreased engine volumetric efficiency as a result of port injection. Although injecting liquid 3-pentanone into intake manifold is followed by evaporation of the liquid tracer droplets, charge cooling effect and enhanced air induction, but in this case, the cooling effect was offset by reduc-

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Fig. 10. Averaged in-cylinder motoring and firing charge temperature calculated from both pressure data and two-line PLIF images.

Fig. 11. Charge cooling effect of port and direct injection on the in-cylinder peak temperature calculated from averaged pressure data.

tion in intake air partial pressure due to the increased amount of tracer vapor. This in turn indicates that a portion of injected tracer was evaporated by the heat absorption from the intake manifold. Switching on the direct injector significantly dropped the peak temperature where extent of the cooling effect was depended on both injection quantity and timing. If we assume that the injected fuel (via direct injector) is vaporized by absorbing heat from in-cylinder charge only, the equivalent charge temperature drop at a constant pressure can be calculated using

DT ¼

Q mc c

ð9Þ

temperature drop using abovementioned equations were significantly higher than the values calculated from in-cylinder pressure data. The main reason for the overestimated values was presumed to be the simple assumption that the direct injected fuel is vaporized by heat transfer from in-cylinder charge only. While in practice, a portion of the injected fuel is vaporized by absorbing heat from injector tip or piston surface and liner in case of fuel impingement. This could be confirmed by visualizing the injection and mixture formation events. Although a number of optical techniques have been developed for study of direct injection fuel spray and impingement [64–68], but in this work, the single-line PLIF technique was chosen as it utilized the same optical setup as the PLIF thermometry technique. For the single-line PLIF imaging, 3-pentanone was premixed with iso-octane with a volume ratio of 1% and injected via the direct injector. Only the XeCl laser was used and the cylindrical lens was removed in order to have an expanded laser beam. As it is shown in Fig. 12, the expanded laser beam (thickness of 21 mm) illuminated a larger volume inside the cylinder and resulted in a better visualization of fuel distribution compare to a focused (sub millimeter) laser sheet. At selected engine crank angles, 40 single shot images were captured and averaged to give a mean fuel distribution. For the background correction, the intensity of the averaged background images (collected during motoring with no fuel injection) was increased by a fraction before subtracting them from the data images. This was due to a fact that for the background images, only a very weak Rayleigh scattering (from the gas molecules) and laser beam internal reflections were captured, while for the data images, high concentration of liquid droplets increased the noise level by increasing secondary reflections and Mie scattering signal. The PLIF images indicated a 1.3 ms delay from Start of Injection (SOI) signal to the actual injection event. Fig. 13 shows ensemble average images of 40 cycles of the spatial and temporal evolution of the in-cylinder fuel (isooctane mixed with 3-pentanone) distribution. Due to a significant variation in fuel spatial concentration between different crank angles and for a better visualization, a dynamic intensity scale was applied. By investigating the PLIF images, two mechanisms were linked with the piston wetting fuel evaporation. The first one was direct spray impingement on the piston surface which was observed during the injection process. The second mechanism was witnessed in images taken later during the compression stroke indicating formation of a liquid fuel film on the piston as a result of poor atomization and impact of the rising piston with the airborne fuel droplets. The fuel impingement on the liner could not be directly explored as the field of view through the piston glass did not extend all the way to the liner. However, liner wetting can also occur through different mechanisms including direct fuel impingement on the wall, fuel droplets bouncing back from the piston surface and the liquid fuel film formed on the piston surface which is pushed to the liner due to fuel spray momentum. In addition, the qualitative assessment showed that the

where mc is the mass of in-cylinder charge, c is the specific heat capacity of in-cylinder charge, Q is energy required for vaporizing the injected fuel and is given by

Q ¼ mf Lv

ð10Þ

where mf is the mass of injected fuel and Lv is latent heat of vaporization which can be calculated by [63]

 L ¼ LTbn

T er  T T er  T bn

0:38 ð11Þ

where Tbn is the normal boiling temperature and Ter is the critical temperature of the fuel and LTbn is the latent enthalpy of vaporization at Tbn. However, the calculated values of in-cylinder charge

Fig. 12. Comparison of focused and expanded laser beams.

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105

Fig. 13. Single-line PLIF of fuel distribution for direct injection of isooctane premixed with 3-pentanone.

injection timing at 250 °CA ATDC could not provide a homogenous mixture. The charge inhomogeneity was associated with poor atomization, insufficient mixing time and insufficient in-cylinder charge motion for the late injection timing. These are required to disturb injection cone and mix the fuel droplets in order to reduce injection depth of penetration and fuel wetting. In order to quantify the direct injection charge cooling effect on in-cylinder charge temperature during compression stroke and particularly at the ignition timing (35 °CA ATDC), in addition to temperature calculations from pressure data, the two-line PLIF thermometry technique was applied. Fig. 14 shows the PLIF results for direct fuel injection at 90 °CA ATDC during intake stroke with injection quantity of 10 mg/cycle. The blue cross markers and the green triangles show the O/E ratio (fluorescence intensity ratio of the 308–277 nm excitation wavelengths) and the in-cylinder charge temperature respectively when only the tracer was injected into the cylinder through the port injector (PFI case). The red dash markers and the purple diamonds indicate the O/E ratio and the charge temperature respectively, when the fuel was also injected through the direct injector (PFI + DI case). It was noted that the O/E ratios for the PFI + DI case at 160 and 180 °CA ATDC were higher than the ones without direct fuel injection, which was contrary to what was expected of the charge cooling effect. By examining the PLIF fuel images, it was realized that this was due to interference from strong Mie scattering signal of the fuel droplets. The liquid fuel droplets lifetime for this injection timing was estimated to be more than 90 °CA from the PLIF fuel spray images. Consequently, PLIF images taken at 90 °CA BTDC in the compres-

Fig. 14. Fluorescence signals’ ratio and in-cylinder charge temperature for10 mg/ cycle direct fuel injection at 90 °CA ATDC during intake stroke.

sion stroke were used to normalize PLIF images of the following crank angles and calculate in-cylinder average temperature by reference to the CVC calibration curve. Fig. 15 shows two-line PLIF results for 10 mg/cycle injection at 250 °CA ATDC in the compression stroke. It can be seen that the anomaly of PLIF O/E ratios at 160 and 180 °CA ATDC seen in

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Fig. 15. Fluorescence signals’ ratio and in-cylinder charge temperature for 10 mg/ cycle direct fuel injection at 250 °CA ATDC during compression stroke.

Fig. 17. Fluorescence signals’ ratio and in-cylinder charge temperature for 30 mg/ cycle direct fuel injection at 250 °CA ATDC during compression stroke.

Fig. 14 is absent due to the absence of interference of direct fuel injection droplets. The shorter droplet lifetime than the one observed with the early injection can be associated with a higher in-cylinder temperature at the retarded injection timing. Fig. 16 shows two-line PLIF results for 30 mg/cycle injection at 90 °CA ATDC. Similar to the engine run with 10 mg/cycle at this timing, a very high O/E ratio was observed at 160 °CA ATDC in the presence of fuel droplets. As the fuel droplets evaporate, the Mie signal reduces and the O/E intensity ratio drops at 180 °CA ATDC. Fig. 17 shows two-line PLIF results for the last injection strategy with 30 mg/cycle injection at 250 °CA ATDC. It can be seen that the O/E ratio at 290 °CA ATDC for the DI run is still higher than the baseline which indicates fuel droplets lifetime of 40 °CA for this injection quantity and timing. The test results indicated that the direct injection charge cooling is a function of both injection timing and quantity. As liquid fuel is injected into the cylinder, fuel droplets absorb heat from gas mixture and evaporate which reduces in-cylinder gas temperature, hence, the larger injection quantity the lower in-cylinder peak temperature. Furthermore, the later injection during compression stroke leads to a lower peak charge temperature than injection during intake stroke. This is due to the fact that early direct injection timings are before or during intake valves opening

and hence the enthalpy of evaporation causes more air in mass to be inducted into the cylinder whereas a later injection during the compression stroke (after intake valves closure) occurs with a constant mass of air trapped in the cylinder. Fig. 18 shows calculated temperature values at the ignition timing (35 °CA BTDC), from both the PLIF and pressure data. The comparison indicated that both methods have a good agreement for the early injection with low injection quantity while both increasing the injection quantity and retarding the injection timing deteriorate this agreement. This was believed to be mainly due to interference of Mie scattering signal from fuel droplets with the fluorescence signal. Which was made worse as the measurements were carried out during motoring operation with lower in-cylinder temperature which resulted in a relatively longer fuel droplets’ lifetime. It was observed that for the low injection quantity, charge cooling increases for about 50% by shifting the injection timing from intake stroke to compression stroke. This was about 10% for the higher injection quantity which indicated the contribution of the fuel impingement. In addition, further analysis of the PLIF images taken at 35 °CA BTDC of consecutive cycles for different engine runs, showed relative standard deviation of 3% in calculated in-cylinder charge temperature from single shot DIFs, indicating suitability of the technique for instantaneous measurements of direct injection charge cooling.

Fig. 16. Fluorescence signals’ ratio and in-cylinder charge temperature for 30 mg/ cycle direct fuel injection at 90 °CA ATDC during intake stroke.

Fig. 18. Calculated in-cylinder charge temperature and cooling effect at the ignition timing from both PLIF images and pressure data.

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7. Conclusion The primary objective of this investigation was to examine application of the tracer-based PLIF technique in quantitative measurement of direct injection charge cooling in combustion chamber of an optical GDI engine. The PLIF setup was constructed for an optimized operation. A bespoke high pressure Raman convertor was utilized and tested under different conditions to achieve maximum Raman conversion efficacy in an attempt to enhance the fluorescence signal to noise ratio. The PLIF imaging was improved by utilizing variety of techniques for noise reduction prior and during image acquisition as well as applying a combination of postprocessing techniques. Calibration curve of the fluorescence signal intensity ratio of the two laser beams and charge temperature was obtained through a quasi in situ measurement, utilizing a specially designed constant volume chamber that could be mounted on the engine cylinder block in place of cylinder head. Unlike the majority of previous studies whereby the two-line PLIF thermometry relies on a photophysical model of a dopant tracer, the calibration process enabled measurements of in-cylinder charge temperature independent of these models. To evaluate the PLIF thermometry results, average in-cylinder charge temperature during compression stroke of the control auto ignition operation was measured for both motoring and firing cycles and the PLIF values were compared with the temperature values calculated from in-cylinder pressure data where both technique showed a very good agreement. A qualitative study of the spatial and temporal evolution of the in-cylinder fuel distribution using the single-line PLIF technique was carried out to investigate fuel wetting. The single-line PLIF images showed the piston fuel impingement via a direct impact of spray on the piston surface. In addition, another mechanism resulting in piston wetting was observed. This was formation of a fuel film on the piston surface due to the impact of the rising piston with airborne fuel droplets during the compression stroke. The relatively long fuel droplets’ life time was associated with a poor atomization and mixing rate due to a lack of in-cylinder charge motion at the injection timing, fuel quantity and insufficient mixing time. For a quantitative study of the charge cooling, the two-line PLIF technique was utilized to measure the global charge temperature drop for two injection quantities of 10 mg/ cycle and 30 mg/cycle and two injection timings of 90 °CA ATDC (during intake stroke) and 250 °CA ATDC (during compression stroke). In the case of low injection quantity, the PLIF results were in a very good agreement with the calculated temperature values from in-cylinder pressure data while for the retarded and significantly increased injection quantity the PLIF values were slightly overestimated. The test results indicated capability of optimized PLIF thermometry technique for quantitative measurements of direct injection charge cooling for typical injection strategies in GDI engines.

Acknowledgment The authors gratefully acknowledge financial support by the Engineering and Physical Sciences Research Council (EPSRC) in the frame of project ‘‘2-ACE’’; reference EP/F058942/1.

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