The addition of exhaust gas recirculation (EGR) has demonstrated the potential to significantly improve engine efficiency by allowing high CR operation due to a reduction in knock tendency, heat transfer, and pumping losses. In addition, EGR also reduces the engine-out emission of nitrogen oxides, particulates, and carbon monoxide while further improving efficiency at stoichiometric air/fuel ratios. However, improvements in efficiency through enhanced combustion phasing at high compression ratios can result in a significant increase in cylinder pressure. As cylinder pressure and temperature are both important parameters for estimating the durability requirements of the engine - in effect specifying the material and engineering required for the head and block - the impact of EGR on surface temperatures, when combined with the cylinder pressure data, will provide an important understanding of the design requirements for future cylinder heads. This paper examines the combustion chamber temperatures for several dilution and combustion regimes by using spark plug temperatures as a proxy. The investigation involved a 2.4 L MPI engine with a CR of 11.4 and high levels of cooled EGR. The engine was equipped with a modified ignition system which extended the EGR limit to more than 25% and used instrumented spark plugs for temperature measurements. EGR sweeps were conducted at knock limited and non-knock limited conditions. In addition, potential fuel factors were explored using an E85 ethanol blend. Finally, spark plugs with two different heat ranges were tested at selected load points. The results show combustion phasing is the primary factor effecting spark plug temperatures and, at knock limited conditions, the temperature change was not monotonic with the EGR rate. At constant combustion phasing, i.e. when the engine was not knock limited without EGR, the use of cooled EGR significantly reduced spark plug temperatures. At higher loads, however, spark plug temperatures slightly increased from better combustion phasing and the elimination of fuel enrichment.
