ASSESSMENT OF FUEL INJECTION STRATEGIES FOR DIRECT INJECTION SPARK IGNITION ENGINES

1. 1.1ms 305.6CA 1.1ms 305.6CA 1.3ms 306.5CA 1.3ms 306.5CA 1.4ms 306.9CA 1.4ms 306.9CA 1.6ms 307.7CA 1.6ms 307.7CA ASSESSMENT OF FUEL INJECTION STRATEGIES FOR DIRECT INJECTION SPARK IGNITION ENGINES Julian Kashdan and John S. Shrimpton Thermofluids Section, Department of Mechanical Engineering, South Kensington Campus, Imperial College London, SW7 2AZ RESULTS ABSTRACT The characteristics of the hollow-cone fuel spray produced by a centrally-located pressure swirl atomizer were investigated in a constant-volume pressure chamber and in a motored single-cylinder direct-injection spark ignition (DISI) research engine. The aim is to describe the effect of elevated chamber pressure and in-cylinder bulk air motion on the spray development process. In addition to ambient conditions, chamber pressures of 5 and 7 bar absolute, corresponding to air densities of 5.82kg/m3 and 8.14kg/m3, respectively, at atmospheric temperature (300K), were investigated as these conditions are representative of the range of in-cylinder pressure and densities corresponding to ‘early’ through ‘late’ injection strategies. Spray dynamics in a constant-volume chamber, under near quiescent flow conditions, are shown to be representative of in-cylinder sprays and therefore provide appropriate means for evaluating the relative effect of the intake air flow and in-cylinder density on the fuel spray development process. A wide range of operating conditions of a four-valve DISI engine with a centrally located pressure swirl atomizer were examined at engine speeds of 700 and 1500 rpm and for four start-of-injection (SOI) timings of 90, 180, 270 and 300 after top-dead-centre (aTDC) of intake. The results revealed that fuel spray impingement on the flat piston only occurred with injection at 300ATDC, and that larger droplets are produced by pressure swirl atomizers operating at higher gas pressures which suggests that achieving consistent late injection strategies for low load operation with spray-guided systems using such injector designs may be an insurmountable problem. High chamber pressure under quiescent conditions was found to have a marked effect in suppressing both the axial and radial spray penetration (Fig. 5) but negligible effect in terms of spray symmetry. An increase in chamber pressure from 1 to 5 bars led to reduced spray penetration by about 25% by the end of the injection pulse and decreased cone angle from approximately 60 to 49. Elevated chamber pressures also appeared to reduce the tendency of droplets being re-entrained at the leading edge of the spray for t>1ms aSOI. Phase Doppler anemometry (Fig. 6) revealed a wide spectrum of droplet velocities associated with the pre-spray for Pch=1 bar, with peaks of about 0.75Uo at t=0.7ms aSOI at Z=45do from the nozzle orifice. Measurements within the main spray cone showed much lower velocities, with peaks of about 0.35Uo at t=1ms aSOI followed by a rapid decrease. An increase in the chamber pressure caused a significant reduction in the droplet velocities; for example at t=1.7ms aSOI the mean velocity reduced by a factor of 2 from U=0.6Uo to U=0.3Uo as Pch was increased from 1 to 7 bar. Atomization quality, In terms of the mean droplet diameter was found to deteriorate with increasing chamber pressure. For Pch=1 bar the mean droplet diameter generally remained constant at about 10m for t>1.8ms aSOI whilst for Pch=7 bar peak mean droplet diameters of approximately 35m were measured decreasing down to about 18m in the tail of the spray (t>3ms aSOI). These findings have implications for stratified engine operation where fuel is injected against increasing in-cylinder pressures during the compression stroke coupled with reduced timescales for fuel delivery, atomization and evaporation. In-cylinder spray visualisation studies have shown that, for central positioning of the injector, the intake airflow has relatively little effect on the generated spray structure in terms of spray dispersion, repeatability and symmetry for injection during the intake stroke (90aTDC). With the adoption of the ‘late’ injection strategy (Fig.7) where injection started at 300 aTDC and lasted for 0.93ms, the higher in-cylinder pressures caused significant contraction of the spray which resembled more a solid rather than a hollow cone type. Spray images revealed that impingement on the piston did occur in this case forming a liquid film, whilst droplets were deflected upwards but retained a symmetric ‘ring’ pattern. INTRODUCTION Despite the continual improvements that have been made to reduce the pollutant emissions and fuel economy of port fuel injected (PFI) gasoline spark ignition (SI) engines, the emergence of electronically-controlled, high pressure fuel injection systems has been instrumental in precipitating a reconsideration of the direct injection spark ignition (DISI) concept as a means of meeting the increasingly stringent world-wide emissions legislation while, at the same time, reducing fuel consumption. With PFI there is the additional mixture preparation stage where fuel is injected onto the intake valve and port walls and the subsequent long residence time of the droplets within the relatively hot intake port prior to valve opening ensures that the charge entering the cylinder is essentially a pre-vaporised mixture with few large droplets. The mechanisms for atomization and evaporation have been attributed to a combination of droplet secondary break-up following fuel impingement on the valves, heat transfer from the surrounding hot wall surfaces and stripping of the fuel film by the air stream. In contrast, in DISI engines, due to the much shorter timescales involved for fuel atomization, evaporation and air/fuel mixing, generation of an ignitable mixture at the time of ignition requires much greater attention to the mixture preparation stage. Load control with direct injection is achieved by means of varying the fuel quantity so that, under high load conditions, the engine is operated with an air/fuel ratio close to stoichiometric. As shown in Fig. 1 this is achieved by injecting fuel ‘early’ in the cycle during the intake stroke thus allowing sufficient time to produce a homogeneous mixture. In order to exploit the full potential of the DISI concept, the more complex modes of operation of ‘mid’ or ‘late’ injection under part load or idling conditions must be established. The ‘mid’ and ‘late’ injection timing strategies take place around bottom dead centre and during the compression stroke, respectively. Under these operating regimes, a smaller quantity of fuel is injected early or later in the compression stroke with the aim of producing an overall lean but ignitable mixture through charge stratification. Figure 1 : piston speed versus time (not crank angle) shows reduction in time between injection and ignition Figure 5: Spray images showing effect of chamber pressure on spray development for (a) Pch=1 bar, (b) Pch=5 bar and (c) Pch=7 bar at (i) 0.5ms, (ii) 1ms and (iii) 1.5ms aSOI where tinj=1.5ms, Pinj=80bar. Figure 6:Centrelinetemporal (a) velocity, (b) size and (c) sample number distributions for Pch= 1 bar --, 5 bar -∆-and 7 bar -- at Z=45do where tinj=1.5ms, Pinj=80 bar, Uo=69m/s and No=2219. Figure 3 : Injector position Figure 2 : piston speed versus crank angle (not time) shows reduction in crank between injection and ignition for different injection modes Figure 4 : Timing control for in-cylinder imaging Figure 7: Mie scattering images of fuel spray development in a motored single cylinder optical DISI engine for (a) 700rpm, (b) 1500rpm and injection at 300ATDC, Pinj=80bar, tinj=0.93ms, minj=9mg. EXPERIMENTAL CONTROLS The in-cylinder fuel spray visualisation studies were performed on a single-cylinder optical engine built by the Advanced Powertrain Department of the BMW Group. As shown in Fig. 3, the four-valve cylinder head accommodated a centrally located injector adjacent to the spark plug. The engine was equipped with a 150mm long quartz cylinder liner, a flat quartz piston and a 45 mirror providing full optical access. Fuel spray development was visualised with the single-cylinder DISI optical engine operating under motored conditions. The effect of injection timing was examined for four cases corresponding to different loads and two engine speeds of 750 and 1500rpm. Injection timings of 90 aTDC (intake), 180, 270 and 300 were investigated. Under high load operation, injection takes place during the intake stroke commencing commencing at 90 aTDC and at bottom-dead-centre (180aTDC). The imaging investigation within the constant-volume chamber made use of a non-intensified 12-bit CCD camera (PCO Sensicam) with a resolution of 1280x1024 pixels and a minimum exposure time of 100ns whilst illumination was provided by an argon-spark source with a flash duration of approximately 1s. Figure 4 shows a schematic diagram of the single cylinder engine set-up. In order to obtain single shot, cycle resolved images of the in-cylinder fuel spray it was necessary to use an intensified CCD camera (Proxitronic HF-1) due to low light levels. The camera provided a 512 x 256 pixel resolution, a maximum frame rate of 50Hz and the shutter was triggered externally by a 5V TTL signal that was referenced to the shaft-encoder signal. The images were then digitised by a frame-grabbing card (Matrox LC). To enable a relatively short exposure time (5s) to be used, the aperture was kept fully open (f 2.8) to maximise the amount of light collected. A 0.6W argon-ion (=514.5nm) laser beam (Spectra-Physics) was passed through a cylindrical lens forming a diverging laser sheet which provided illumination across the vertical plane on the cylinder centreline. The lens was then rotated through 90 such that a horizontal plane at an axial distance of 20mm below TDC was illuminated, thus allowing cross sectional spray images to be taken via the 45 mirror and through the piston window. The crankshaft encoder (Muirhead Vactric) allowed identification of the engine cycle and crank angle position and provided 1440 pulses per revolution resulting in a resolution of 0.5 crank angle degree (CA). Droplet velocity and size measurements were obtained by a phase Doppler anemometer (PDA) which was configured to the specifications given in Table 3. The PDA comprised a Dantec 55X transmitter in which a Bragg cell unit provided a frequency shift of 40MHz to one of the beams. An Aerometrics receiver (RCV100) collected light at a forward scattering angle of 30 where signal quality was found to be superior to that at the preferred Brewster angle condition. Using lenses of focal lengths of 310mm for the transmission and 500mm for the receiving optics, resulted in a phase factor of 0.713/m. Typically each measurement location comprised 30,000 valid samples resulting in statistical uncertainties of less than 2% CONCLUSIONS Overall, the results of this investigation have indicated that spray-guided direct-injection gasoline engines may require a different spray pattern than that generated by existing swirl pressure atomizers, independent of injection pressure. Whether new designs of single-hole swirl atomizers or multi-hole injectors will be able to satisfy the very demanding mixture preparation requirements of closely-spaced injector/spark-plug systems remains to be seen. REFERENCES JT Kashdan – Experiments on intermittent swirl generated sprays, Thesis (Ph.D. and D.I.C.) -- Department of Mechanical Engineering, Imperial College, London 2002 JT Kashdan, JS Shrimpton, CA Arcoumanis, ‘Pulsed fuel sprays for spray guided direct injection SI Engines: Characteristic Dynamics’ Atomization and Sprays, vol 12, no. 4, pp.539-557, 2002. H. Lienemann, J.T. Kashdan, JS Shrimpton “Spray quality at elevated gas densities via swirl and air-assisted atomisation methods”, Proc. ICLASS 2003, 13-17 July 2000, Sorrento, Italy. JT Kashdan, JS Shrimpton, C Arcoumanis, GE Gaade, Wallance, S. “Spray characteristics of high pressure, Non-swirling fuel injectors for direct Injection gasoline engines”, IMechE Fuel Injection Systems conference, 1-2 Dec 1999, IMechE HQ, London, UK. EPSRC project grant GR/M 17969, £212,441, 8/98-8/01 “Advanced Direct-Injection Gasoline Engines” JH Whitelaw, CA Arcoumanis JS Shrimpton. ACKNOWLEDGEMENTS The authors would like to acknowledge the financial support provided by EPSRC (GR/M17969) and the technical support offered by Mr. W. Huebner and Mr. F. Puchner of the BMW Group. The contributions of Dr. J. Nouri and N. Perquis to the research programme are also gratefully acknowledged.