INTRODUCTION With vehicle fuel economy standards becoming more stringent - in the United States and globally - there has been increased focus across the industry on reducing vehicle fuel consumption. This has led to a focus on improving internal combustion engine efficiency [ 1]. Engines used in hybrid electric vehicles, or as range extenders in electric vehicles, place a high priority on peak engine efficiency [2,3,4,5]. These engines are typically optimized to work synergistically with the specific hybrid electric propulsion system with which they are paired. Several original equipment manufacturers have implemented variants of production engines tailored to their hybrid electric vehicle application over the past decade [ 2,3,4,5]. These engines have typically employed an LIVC strategy combined with a high geometric compression ratio for improved engine efficiency. Collectively, this combination is often referred to as an “Atkinson cycle.” This strategy improves efficiency by reducing engine pumping losses and increasing the engine expansion ratio at the expense of some loss in specific power and torque. The torque loss is generally greatest at low engine speeds due to the use of a long duration intake camshaft profile [ 2,3,4,5]. These engines typically employ port fuel injection (PFI) and fixed exhaust cam timing, which further limits the specific output of the engine. Externally routed exhaust gas recirculation has been an area of renewed study as the industry pushes towards improved fuel efficiency [ 6,7,8,9,10]. EGR improves engine efficiency by reducing in-cylinder heat loss, reducing engine pumping loss, and improving the ratio of specific heat of the unburned mixture. At higher engine loads, where engine efficiency is limited by knock and retarded Development of the Combustion System for General Motors' High-Efficiency Range Extender Ecotec Small Gas Engine Jeffrey Jocsak, David White, Cedric Armand, and Richard S. Davis General Motors Company ABSTRACT General Motors has developed an all-new Ecotec 1.5 L range extender engine for use in the 2016 next generation V oltec propulsion system. This engine is part of a new Ecotec family of small displacement gasoline engines introduced in the 2015 model year. Major enhancements over the range extender engine in the current generation V oltec propulsion system include the adoption of direct injection (DI), cooled external exhaust gas recirculation (EGR), and a high 12.5:1 geometric compression ratio (CR). Additional enhancements include the adoption of high-authority phasers on both the intake and exhaust camshafts, and an integrated exhaust manifold (IEM). The combination of DI with cooled EGR has enabled significant thermal efficiency gains over the 1.4 L range extender engine in the current generation V oltec propulsion system at high engine loads. The addition of a high geometric CR and high-authority camshaft phasers for extended late intake valve closing (LIVC) operation has enabled improved low- and mid-load engine efficiency . The combination of DI and high-authority camshaft phasers has minimized the full-load engine torque loss inherent with traditional high CR and LIVC implementations. The combustion system was developed with extensive use of computational fluid dynamics (CFD) simulation for optimization of in-cylinder mixing and combustion. These tools aided in the sorting and selection of the combustion chamber, ports, piston, and fuel injector. Extensive single- and multi-cylinder engine testing and development was also employed to refine and optimize the engine combustion system. CITATION: Jocsak, J., White, D., Armand, C., and Davis, R., "Development of the Combustion System for General Motors' High- Efficiency Range Extender Ecotec Small Gas Engine," SAE Int. J. Engines 8(4):2015, doi:10.4271/2015-01-1272.2015-01-1272 Published 04/14/2015 Copyright © 2015 SAE International doi:10.4271/2015-01-1272 saeeng.saejournals.org