Cummins 4-cylinder QSB3.3 and QSB4.5 engines offer a simplified Tier 4 installation across a broad 75- to 160-hp range. An air-intake-to-exhaust after-treatment system is specifically designed for flexible machine integration. Proven on Cummins 6-cylinder engines, the exhaust gas recirculation system is scaled down for the 4-cylinder QSB configuration and driven by a variable-flow turbocharger. Using a simple variable-turbine mechanism, the turbo is designed to improve boost across a wider engine speed than a wastegate turbo and avoids the complexity of dual turbochargers.