Determination of a heat transfer correlation for small internal combustion engines

Abstract

Small internal combustion engines (ICEs) with displacements < 100 cm3 have greater surface area-to-volume ratios that promote heat transfer through the combustion chamber walls, subsequently lowering the fuel conversion efficiency. As a result, employing common heat transfer correlations developed for larger engines to estimate wall heat transfer yields inaccurate results. Therefore, it is necessary to review literature correlations and determine their applicability for small ICEs. To this end, this paper first presents a literature review highlighting the characteristic length scales, characteristic velocities, and the properties of the working fluid for ICE convective heat transfer correlations. This information is used within a zero-dimensional ICE model to determine an appropriate heat transfer expression for a relatively small two-stroke ICE (53.2 cm3). Through parameter optimization, by matching the gross indicated power over 85 experimental data points using MATLAB’s fmincon routine, the developed expression (Nu=0.449Re3/4Pr1/3) shows good confidence with respect to the power (R2 = 0.924, RMS = 0.257) and an in-cylinder pressure data trace (R2 = 0.991, RMS = 1.52). A following parametric study varying the bore-to-stroke (BtS) ratio for small engines (0.15 cm3 to 100 cm3) finds a localized choice of the BtS and ignition timing at each displacement volume based on the minimum heat transfer due to the combination of a changing surface area and heat transfer coefficient. In the future, including the effects of volumetric efficiency and tuned exhaust pipes can enhance the scope of the proposed model.