A Comparison of Cold-Start Behavior and its Impact on Fuel Economy for Advanced Technology Vehicles

Vehicle operation during cold-start powertrain conditions can have a significant impact on drivability, fuel economy and tailpipe emissions in modern passenger vehicles. As efforts continue to maximize fuel economy in passenger vehicles, considerable engineering resources are being spent in order to reduce the consumption penalties incurred shortly after engine start and during powertrain warmup while maintaining suitably low levels of tailpipe emissions. Engine downsizing, advanced transmissions and hybrid-electric architecture can each have an appreciable effect on cold-start strategy and its impact on fuel economy. This work seeks to explore the cold-start strategy of several passenger vehicles with different powertrain architectures and to understand the resulting fuel economy impact relative to warm powertrain operation. To this end, four vehicles were chosen with different powertrain architectures. These include a modern conventional vehicle with a 6-speed automatic transmission equipped with a torque converter, a downsized and turbocharged GDI vehicle with a 7-speed dual-clutch transmission, a modern turbo-diesel with a 6-speed dual-clutch transmission, and a gasoline-electric hybrid with a power split transmission. The vehicles were operated on a chassis dynamometer with instrumentation in place to determine real-time fuel consumption and tailpipe emissions while observing powertrain behavior. The test vehicles were subjected to hot and cold start iterations of the EPA Urban Dynamometer Driving Schedule (UDDS) and US06 drive cycles at 72 degrees F ambient test cell temperature. The vehicles were found to exhibit increased fueling rates, mild changes in shifting behavior, larger levels of tailpipe emissions, and changes to secondary operating strategies such as deceleration fuel cutoff. The duration of cold start behavior varied between the vehicles, and was directly affected by the aggressiveness of the drive cycle. The severity of the cold start penalty was found to vary with vehicle architecture and drive cycle, but was generally smaller for more aggressive vehicle operation. Cold start penalties ranged from a low of 10.5% on the US06 drive cycle to a maximum of 21.8% on the UDDS cycle.