Crevice and Blowby Model Development and Application

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1 Crevice and Blowby Model Development and Application Randy P. Hessel University of Wisconsin - Madison Salvador M. Aceves and Dan L. Flowers - Lawrence Livermore National Lab ABSTRACT This paper describes new crevice and blowby models for use in CFD modeling of combustion chambers in reciprocating-piston, internal combustion engines. For reasons described later in the paper, the geometries of the CFD mesh and the physical engine s combustion chamber are usually not identical. As a result, the CFD mesh, model initial conditions and/or boundary conditions are manipulated to account for geometric differences so that model results are representative of engine results. Although numerous methods are available to account for the geometric differences, this paper introduces a crevice model to account for them. Also, a rather traditional blowby model is discussed; it represents mass flow through the ring pack, which is modeled by a series of volumes and resistances. The crevice model adjusts the gaseous state in each computational cell to account for both geometric differences and for gas exchange between the mesh and the modeled through source terms in the energy equation. It also adjusts cell and crevice specie concentrations, estimates crevice heat release using the same reaction mechanism as that used in the mesh and estimates crevice heat transfer via a user supplied input parameter. INTRODUCTION CFD meshed combustion chambers typically omit some volumetric regions that exist in physical engines. Omitted regions are called in this paper. Examples include regions between the piston and liner, valve pockets, volumes around spark plugs, etc. These regions are often omitted to simplify mesh generation, avoid small cells which can reduce time steps, improve model stability, etc. The crevice model described herein accounts for those volumes by altering computational cell gas temperatures through source terms in the energy. Cell pressure is updated through the ideal gas law. Crevice pressure is assumed equal to the average combustion chamber pressure at each time step. This crevice model is different than most crevice models in that it accounts for all, where many others only represent the piston/liner crevice. Depending on the engine, the piston/liner crevice might be less than half of the total crevice volume. Therefore, even with these crevice models other steps must still be taken to fully account for. It is the job of the blowby model to approximate combustion chamber mass lost or gained through the ring pack. MODELED ENGINE Table 1 lists engine specifications of the modeled engine. The engine is based on the Cummins B-series, a typical medium-duty diesel engine with a displacement of 0.98 liters/cylinder and a flat firedeck. Detailed information on the engine can be found in [1]. 1

2 Table 1 Engine Specifications, length in cm. Bore 10.2 Stroke 12.0 Fueling motored Swirl ratio 0.9 Cycle 4-stroke Connecting rod length 19.2 Table 2 lists experimental operating conditions for all motored cases. Three cases use a nominal intake pressure of 120 kpa and engine speed is varied. Two cases are at 1200 RPM and intake pressure is varied. Trapped mass is estimated from mass flow measurements. Table 2 Motored operating conditions. Case name Engine based on speed, RPM and RPM intake press kpa Nominal intake pressure, Measured intake temperature, C / K Estimated trapped mass, g / / / / Table 3 lists various measured dimensions and calculated compression ratios. Values are given both with and without. The simple relation is: value with = value without + total crevice volume Table 3 Measured volumes, cm3, compression ratios, and squish height, cm. Valve Head pockets gasket volume Extended Total top land BDC with TDC with CR with Swept volume BDC without TDC without 17.7 CR without Squish height Table 4 lists ring pack data. Data is extracted from detailed prints of the ring pack. Table 4 values are used in the blowby model. 2

3 GENERAL MODEL SETUP Table 4 Measured ring pack data. Ring Ring gap area 1, cm2 Volume above 2, Volume behind 3, cm3 cm3 Top Middle Bottom Area as viewed from above the ring. 2 For top ring, volume= top land volume. For other rings volume= volume between current ring and ring above it. 3 Includes volumes behind ring and between ring top surface and piston ring groove surface. The meshed domain (2D, axisymmetric) is built using measured bore, stroke and squish values. No crevice volumes are meshed, therefore, the model compression ratio is 19.8 (Table 3, CR without ). With this setup the model and physical piston locations should be identical, within measurement accuracy. All cases from table 2 are modeled from intake valve closure (IVC) to exhaust valve opening (EVO). IVC pressures are determined by extracting measured cylinder pressure at -90 atdc and extrapolating back to IVC. This method is chosen because pressure transducer readings are more accurate at -90 atdc than at IVC. Temperature at IVC is calculated using the ideal gas law (T = P*V/(M*R*MW)). All quantities on the right-hand side are found or derived from tables 2-3 and from cylinder pressure measurements as just described. Table 5 summarizes model initial and boundary conditions. Table 5 Model initial and boundary conditions. Case name based on RPM and intake press IVC pressure, kpa IVC temperature, C / K Trapped mass 1, g Surface temperatures, K, liner/head/piston / /396/ / /398/ / /400/ / /398/423 1 Includes mass contained in the meshed domain and modeled. MODEL RELATIONSHIPS The meshed portion of the combustion chamber is simulated with CFD as in any normal ICE CFD combustion simulation. KIVA3V-MZ [2] is the CFD solver used. As mentioned previously, the meshed domain has a CR of 19.8 where the engine CR is Therefore, without some method of compensation, CFD calculated pressures on the meshed domain will be greater than measured pressures. Although the are not meshed, their aggregate volume is a model input. Therefore, the CFD solver, through the crevice model, can compensate for CR differences. The ratio of the physical combustion chamber CR, subscript cc, to the meshed CR is defined as the following correction factor: F correction,cr = CR cc / CR meshed (1) 3

4 Through algebraic manipulation a generalized correction factor takes the following form and is calculated at every time step. V crevice is the aggregate crevice volume, V IVC,meshed is the meshed volume at IVC, V meshed is the meshed volume at any time step. F correction,v = (1.0 + V crevice /V IVC, meshed ) / (1.0 + V crevice /V meshed) (2) In the following it is assumed the in-cylinder heat transfer characteristics can be represented by a polytropic-type equation and this equation is used to scale specific internal energy (SIE) in each cell for geometric differences, i.e., differences in CR. The polytropic-type temperature-volume relationship is given in equation 3, where T and V are temperature and volume respectively at any crank angle, T IVC and V IVC are similar at IVC, n is the polytropic-type coefficient. Note that n deviates from the traditional constant-valued polytropic coefficient. In this case, n is calculated anew internally by the CFD code at each time step without being restricted to a specific constant value. For breviety, details regarding the calculation of n are not included in this paper. T * V (n 1) = T IVC * (V IVC ) (n 1) (3) Applying equation 3 to both the meshed and the volume corrected cases, equating T IVC and manipulating terms yields the following relationship, Eq. 4, from which the volume corrected temperature, T corrected,v, is calculated. is the temperature as calculated on the meshed domain. 4 T meshed T corrected,v = T meshed *(F correction,v ) (n 1) (4) Together, equations 3 and 4 relate temperature on the meshed domain to corrected temperature assuming meshed and corrected cases have similar heat transfer characteristics. In the code this relationship is not actually applied to temperature, but instead to SIE as follows: SIE corrected,v = SIE meshed + C v * T meshed * (F correction,v (n 1) 1) (5) SIE is corrected in each cell and at each time step using Eq. 5. A second SIE correction is also applied, this time for mass flowing into and out of. Derivation for this correction starts with continuity in that the total combustion chamber mass, M, equals the mass in the meshed domain plus crevice masses, Eq. 6, substituting ρv for mass, Eq. 7, substituting the ideal gas law for ρ, Eq. 8, assuming pressures and gas constants are equal in the meshed and crevice regions, Eq. 9, rearranging, Eq. 10, results in a definition for the mass exchange correction factor, F correction,m, Eq. 11. M = M meshed + M crevice (6) ρ *(V + V crevice ) = ρ meshed * V + ρ crevice * V crevice (7) (P/RT) * (V + V crevice ) = (P/RT) meshed * V + (P/RT) crevice * V crevice (8) (1/ T) * (V + V crevice ) = (1/ T) meshed * V + (1/ T) crevice * V crevice (9) T = T meshed *(V + V crevice ) (10) (V + T meshed /T crevice * V crevice ). T / T meshed = F correction,m (11) In the above equations, T meshed is actually the corrected temperature from applying equation 5. That is, the volume correction is applied and temperatures are updated, then the mass-exchange correction is applied.

5 Applying the correction of equation 11 to SIE instead of temperature is accomplished through equations 12-15, where d(sie) is the difference between the mass-exchange corrected SIE, SIE corrected,m, and volume corrected SIE, SIE corrected,v. SIE corrected,m = SIE corrected,v + d(sie) (12) = SIE corrected,v + C v * ( T T meshed ) (13) = SIE corrected,v + C v * T meshed * (T/T meshed 1) (14) SIE corrected,m = SIE corrected,v + C v * T meshed * (F correction,m 1) (15) Finally, the crevice temperature is updated for motored cases assuming a polytropic crevice process, Eq. 16, where P is the average in-cylinder pressure at the current time step and P IVC and T IVC are average in-cylinder pressure and temperature at IVC. n crevice is the only input parameter used for tuning the crevice model. T Crevice = T IVC * ( P / P IVC ) (n 1) / n crevice crevice (16) Details of the blowby model are not included here, but instead a brief overview follows. The blowby model is bounded below by a constant pressure crank case and above by a variable pressure top-land volume. The top-land volume is simply just another crevice. It is indistinguishable from any other crevice. Therefore, the blowby model adds or subtracts mass from the crevice mass and the crevice model adds or subtracts mass from the mesh portion of the combustion chamber. The blowby model is sub-cycled using ever smaller sub-cycle time steps until convergence criteria are met. Sub-cycle pressure at the blowby model topland interface is interpolated between average in-cylinder pressure after volume correction has been applied and in-cylinder average pressure after mass correction has been applied. Physically these pressures can be interpreted as the pressures applied over the time step during filling or emptying of the. Ring gaps are modeled as flow restrictions using an orifice flow equations, Eq. 17 [3]: m g/s = 0.86 * ρ * c * A gap * η (17) where m g/s is mass flow rate, ρ is upstream density, C is speed of sound, A gap is the ring gap area viewed from above and η is a function of pressure ratio across the gap and the ratio of specific heats. RESULTS AND SUMMARY Figure 1 compares experimental and CFD pressure if the crevice model is not active; it emphasizes that some form of compensation is required. Figure 2 compares experimental and CFD pressure with crevice and blowby models active for the cases of table 2. The curves essentially lie on top of each other. Figure 3 compares experimental to CFD blowby for the 120 kpa intake pressure cases. They compare very well, although the low RPM case is over-predicted. But, even this case is considered satisfactory given measurement uncertainties. The 1200 RPM-120 kpa intake pressure case (not shown) also performed well. 5

6 Fig. 1 Measured [4] and calculated pressure without compensation. Fig. 2 Measured [4] and calculate pressure for motored cases. Fig. 3 Measured [4] and calculated blowby. n crevice values (Eq. 16) for the 120 kpa cases are 1.185, and for 600, 1200 and 1800 RPM respectively, which says that crevice heat transfer increases as piston speed decreases. This suggests that increased time for heat transfer at lower piston speeds dominates compared to potentially higher heat transfer coefficients due to faster piston speeds. The n crevice value for the 1200 RPM-100 kpa case is 1.255, which suggests that crevice heat transfer is rather insensitive to intake pressure, at least over the 100 to 120 kpa range. Making n crevice predictive rather than a user input is currently being investigated. Regardless, using the crevice and blowby models has reduced the number of tuning parameters from about three (squish height, Tivc, Pivc) to one, n crevice. ACKNOWLEDGMENTS This work was sponsored by the DOE, Office of Vehicle Technologies, Gurpreet Singh program manager. Engine data was provided by John Dec and Magnus Sjoberg of Sandia National Laboratories. The following companies have provided software, ANSYS, Inc (ICEM-CFD, mesh generation), CEI (EnSight, post-processing) and Intelligent Light (Fieldview, post-processing). From University of Wisconsin-Madison, Chris Rutland provide technical advice and Joshua Leach computer system support. REFERENCES 1. Hessel, R., P., Aceves, S., M., Davisson, M., L., Espinosa-Loza, F., Flowers, D., L., Pitz, W., J., Dec, J., E., Sjoberg, M., Babjimopoulos, A., Modeling Iso-octane HCCI using CFD with Multi- Zone Detailed Chemistry; Comparison to Detailed Speciation Data over a Range of Lean Equivalence Ratios, SAE Technical Paper , Babajimopoulos, A., Assanis, D.N., Flowers, D L, Aceves, S.M. and Hessel, R.P., A fully coupled computational fluid dynamics and multizone model with detailed chemical kinetics for the simulation of premixed charge compression ignition engines, IJER, Vol. 6, No. 5, p Namazian, M., Heywood, J.B., Flow in the Piston-Cylinder-Ring Crevices of a Spark-Ignition Engine: Effect on Hydrocarbon Emissions, Efficiency and Power, SAE Technical Paper , Personal correspondences with John Dec and Magnus Sjoberg of Sandia National Laboratories. 6