SUMMARY The way in which position of spark plug affects combustion in spark ignition engine was studied by using the developed quasi-dimensional cycle simulation (using two-zone burning model) program. The purpose of this paper is to model the geometric interaction between the propagating flame and the general cylindrical combustion chamber. Eight different cases were recognized. Appropriate equations to calculate the flame area (Af), the burned and the unburned volume (Vb & Vu) and the heat transfer areas related to the burned and unburned regions were derived and presented for each case. Predicted results for the Paykan, 1600 cc engine are presented and compared qualitatively with the predicted results of the reference [1].

INTRODUCTION In recent years the combined effects of environmental legislation and the energy saving demands have led to a major expansion of research and development work in order to make a better fuel combustion, and reduce noise and pollutant emissions. In this context many codes were developed to simulate internal combustion engines, such as quasi-dimensional models [2-4] and two or three, dimensional codes, which classified as CFD codes [5-7]. Although the CFD codes (like KIVA) permit to simulate very well the physical phenomena involved in engines, but the long time needed for calculation is one of their shortages. In opposition the quasi-dimensional models (like SAPENG used in this research) are fast execution models, which can be used extensively by automotive industry in order to develop engine design and filling and emptying operation very fast. The purpose of this work is to determine the effect of spark plug position on the burning process of disc combustion chamber geometry in SI engines by introducing some algebraic correlations. The approach taken is to use the developed engine cycle simulation to perform illustrative calculations aimed to show the feasibility of using the given correlations in quasi-dimensional cycle simulations. SI ENGINE CYCLE SIMULATION The SI engine cycle is treated as a sequence of four continues process: intake, compression, combustion (including expansion), and exhaust. The combustion process is simulated as a two zone quasi-dimensional model. The combustion chamber is divided into two volumes: the unburned zone (subindex u) composed by air, fuel and residuals and burned zone (subindex b) composed by combustion products. The energy equation for each zone is applied to open system. The Annand correlation is used to calculate the rate of heat transfer from engine. The numerical method for solving differential equations in this work is Range-Kutta fourth order. The detailed of the program is given in Ref. [8].

GEOMETRIC INTRACTIONS If spark plug is located at the center of the disk shape combustion chamber four different cases as shown in Figure1 can be distinguished as follows [9]. rf ≤ B/2 ; rf ≤ hgap rf ≤ B/2 ; rf > hgap rf > B/2 ; rf ≤ hgap rf > B/2 ; rf > hgap (Case 1) (Case 2) (Case 3) (Case 4) (1-a) (1-b) (1-c) (1-d)

Where rf is the flame radius, B is the cylindrical bore and hgap is the height of combustion chamber. If the spark plug is not located at the center, eight possible cases can be distinguished due to different values of eccentricity (e), cylinder bore (B) and combustion chamber height (hgap). These cases showed by the following relations corresponds to the figures 2 (a) to (h). rf ≤ hgap ; rf ≤ B/2 − e rf > hgap ; rf ≤ B/2 − e rf ≤ hgap ; rf > B/2 − e ; rf ≤ B/2 + e rf ≤ hgap ; rf > B/2 + e rf > hgap ; rf > B/2 − e ; rf ≤ B/2 + e ___________________ rf ≤ √( hgap² + (B/2 − e)²) , rf ≤ B/2 + e rf > B/2 − e ___________________________

(Case 1) (Case 2) (Case 3) (Case 4)

(2-a) (2-b) (2-c) (2-d)

(Case 5)

(2-e)

rf > √(...