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Due to environmental and legislative concerns, less polluting and more efficient vehicle powertrain systems must be developed. A first step is a simple, low cost system such as the presented Engine Stop-Start (ESS) system. A 1.9 liter four-cylinder spark-ignition engine with a four-speed automatic transmission was modified to enable fuel off-on transitions during decelerations and stops. Additional hardware includes a 7 kW electric motor-generator, a power electronics module with an inverter and a DCDC converter, a 36 Volt nominal battery system, and minor modifications to the transmission. A control scheme was developed which takes advantage of the system's fuel saving potential while minimally affecting driveability. Tests have shown EPA City fuel economy gains of approximately 12-14 percent while maintaining the same emissions classification. The EPA Highway fuel economy was increased by approximately 1 percent.