20056083 81 NUMERICAL INVESTIGATION OF PRESSURE REACTIVE PISTON TECHNOLOGY IN A SPARK IGNITION ENGINE Wooheum Cho Hyundai Motor Company, Korea Dohoy Jung, and Dennis Assanis W. E. Lay Automotive Laboratory, The University of Michigan, Ann Arbor, Michigan The novel, two-piece Pressure Reactive Piston technology providing the advantages of Variable Compression Ratio and rapid response to cylinder pressure changes was developed and numerically simulated in order to investigate its dynamic behavior and effects on fuel efficiency and emissions. The analytical model for flame-combustion chamber interaction was newly developed incorporating the VCR feature of the PRP engine and implemented into the engine cycle simulation program. The cycle simulation was subsequently used to explore the potential of the PRP engine over the full operating range and to investigate the effect of the PRP spring set characteristics on engine performance and emissions. Keywords: Variable Compression Ratio, Pressure Reactive Piston, Brake Specific Fuel Consumption Introduction sensitive to disturbances or wear to be used in production applications. Evenmore, many VCR technologies require In a typical Spark Ignition (SI) engine, better new engine architecture to implement, which require efficiency and higher performance can be achieved with significant investment to produce. Brevick patented the higher Compression Ratio (CR). However, higher Pressure Reactive Piston (PRP) technology (Patent cylinder pressure associated with higher CR can increase #5,755,192), which is two-piece piston with a spring set the end gas temperature and cause knocking at high load. between the upper and lower pistons. This mechanism In an effort to achieve higher CR while still avoiding with acceptable cost and complexity effectively limits the knocking, various types of Variable Compression Ratio peak cylinder pressures at high loads, while allowing the (VCR) technologies have been developed. engine to operate at high compression ratios under low In general, compression ratio can be varied by loads. changing both the clearance volume and the displaced In this paper, an analytical method for computing volume or only the clearance volume, and the designs flame front propagation has been newly developed for the achieving VCR can be divided as followings: PRP engine in order to model the geometric interaction Eccentric Crankshaft: A lengthened crank-arm not between the spherically propagating flame and the only increases the stroke length and thus the engine changing boundaries in the PRP combustion chamber. In displacement but also decreases the clearance volume [1]. addition, the upper to lower piston motion of the PRP is Linkage System: An additional linkage on the crank implemented into the cycle simulation program and mechanism can be superimposed in order to adjust proper parametric study of the PRP engine is subsequently CR [2~3]. carried out in order to investigate the effect of engine Axial Engine: The angle of a wobble plate is varied speed, load, and spring preload on the PRP engine with respect to its rotational axis or adjusted performance and NO emission. longitudinally to change the piston stroke or clearance volume [4]. Pressure Reactive Piston Configuration Variable Combustion Volume (VCV): The complete cylinder head or a section is adjustable [5~8]. It is The PRP assembly is separated into an upper piston classified further by the control device; Chamber Tilting, or crown and lower piston or skirt by a spring set as Liner Lifting, and Auxiliary Cylinder. shown in Fig. 1. Both piston crown and piston skirt are Variable Compression Height (VCH): The upper made of an aluminum alloy with the spring seat area portion of piston is adjustable [9~11]. It is classified by anodized. Belleville spring geometry was selected as the the piston configuration; Two-Piece Piston with hydraulic spring set for the PRP because of its compactness and device or load-deflected spring and Flexible Piston ability to carry high load with small deflections. A Crown. retainer ring is inserted into the groove of piston crown Variable Radius Length Engine (VRLE): The inner side and functioned to hold the two-piece piston complete piston is controlled by adjusting the radius of together. the connecting rod big end [12~13]. The cycle simulation program, Spark Ignition Simulation (SIS), is used in this study to examine the On the whole, the early VCR designs for existing maximum CR without knocking and provide the engine architecture were technically too complex or too boundary conditions for spring design. In order to Presentation No. 81 Speaker Name: W. CHO 1/6 20056083 investigate the geometry and configuration of the spring Geometric Model for Flame Propagation set, a spreadsheet program for calculating the relation between spring deflection and force [14] was developed The practical importance of the geometric and stress analysis was performed for reliable and durable interaction between a propagating flame and the spring design. As the result, the CR of the PRP is varied combustion chamber is that the entrained mass rate in the from 9.26 to 13.5, which sets maximum spring deflection quasi-dimensional combustion model is proportional to as 3.3 mm. The spring set is designed to begin to deflect the spherical flame front area and heat transfer between at 22 bar of preload and to be fully defected at 66 bar of the burned gases and the walls is proportional to the peak cylinder pressure. chamber surface area wetted by the burned gases. Most researchers [18~19] modeled the combustion chamber with simple geometry for the calculation of the flame front area and wetted wall area, but this method is not applicable to a combustion chamber with more Upper Piston Lower Piston complex geometry. In particular, for the case of the PRP, it is important to calculate the combustion chamber volume with respect to crankangle, because the PRP has variable compression height depending on engine load. Consequently, Belleville Spring Retainer Ring modeling of the flame front interaction with the combustion chamber of the PRP engine is an important Fig. 1 Pressure Reactive Piston Cross Section pre-requisite for performing simulation studies. The propagating flame in this cycle simulation is assumed to Dynamometer test of PRP Engine develop as a sphere with its center fixed at the spark plug electrodes and truncated by the combustion chamber wall. The experiments of PRP engine were done by using The configuration and geometric notation of the dynamometer at 2000 rpm with various loads. The best combustion chamber used in this research are illustrated improvement in Brake Specific Fuel Consumption in Fig. 2 (a). (BSFC) of PRP engine over baseline was observed as ℓ r 1 s ℓ2 ∆ Spark H 7.8 % at light load. This improvement was less than PRP Z CRH Z 21 11 θ C z θ rf θ1 2 expected because the target spring set load/ deflection tdc Z h characteristics were not achieved with the first spring set. S A A ∆ H The further details of experimental results can be found in base Cho [15]. Therefore, the simulation work was performed to find the full potential of the PRP engine and to investigate the effect of preload on fuel economy. The ℓ 6 R ℓ Ricardo Hydra SI engine used in this research is the 5 ℓ4 ℓ3 r θ γ 11R β baseline of this research and its specifications are shown α θ in Table 1. 22R R r R-rs s Engine Type 4-Stroke, Single Cylinder, PFI Bore x Stroke 80.26 x 88.90 mm (a) Hydra Engine (b) Simple Disk Type Connecting Rod 158.01 mm Fig. 2 Geometric Notation Combustion Chamber Compression Ratio 9.26 IVO/IVC 11 deg. BTDC/49 deg. ABDC Flame Front Area & Entrained Volume EVO/EVC 49 deg. BBDC/11 deg. ATDC Rated Power 20 kW @ 5000 rpm As shown in Fig. 2 (b) the flame front area and Table 1 Specifications - 4 Valve Ricardo Hydra SI Engine entrained volume of a simple disk type combustion chamber are calculated from the following equations. Spark Ignition Simulation π 2 2π h 2π h The quasi-dimensional computer simulation of the SI Ae = ∫∫rf cosθdα ⋅ rf dθ =∫∫rf dαdz =2rf ∫αdz (1) engine working cycle is used due to its overall predictive 0 0 0 0 0 ability, particularly regarding the effects of combustion h ⎛ h ⎞ ⎜ 2 2 ⎟ chamber shape on flame propagation. The basis for this V f = ∫ Ac dz⎜= ∫ (αr + βR − rs Rsinβ )dz⎟ (2) work is the quasi-dimensional SI engine cycle simulation, 0 ⎝ 0 ⎠ SIS, which was developed by Poulos and Heywood [16] where Ae is flame front area, Vf is entrained volume, h is and extended by Filipi and Assanis [17]. The combustion piston crown distance from spark plug, rs is the spark sub-model is based on the turbulent-flame entrainment plug distance from the cylinder centerline, and R is model proposed by Tabaczynski [18] and further refined cylinder radius. The entrained volume and flame front by Poulos and Heywood. The earlier model is also area in Eqs. (1) and (2) are calculated with the angle, α, complemented by a zero-dimensional turbulence model and the cross sectional area, Ac, respectively. which calculates average turbulent flow field parameters Based on this result, the first step to calculate the throughout the whole cycle. entrained volume and flame front is that the flame radius Presentation No. 81 Speaker Name: W. CHO 2/6 20056083 is divided into 11 ranges due to the geometric complexity r + z / tanθ of combustion chamber. Then, for a given crankangle and γ = θ −θ −1 s 1 L d12 d11 θ d11 = cos ( ) flame radius, α and Ac are obtained. Finally, the R integration is done by summing all the α’s and Ac’s over if (R 〈r ),θ = 0 h.