Testing plug-in hybrid vehicles (PHEVs) over standardized and real world drive cycles has shown relatively large fuel consumption differences between ambient cold starts and hot starts. Losses on the order of 25-40% have been observed from ambient 20°Celsius cold starts to optimal hot temperature urban drive cycle operation. This is especially critical for PHEVs, when long durations between engine operating points result in reduced engine temperature. The resulting fuel efficiency loss was not well characterized, nor was the total impact on fuel consumption quantified.
Vehicle researchers at Argonne’s Center for Transportation Research have applied response surface methodology techniques for PHEV experimental test data to characterize the thermal effect on efficiency. Combined with a technique for predicting the engine thermal state from its initial temperature, this unique methodology accurately predicts the fuel efficiency over a drive cycle from ambient cold starts to fully operational temperature. Current work is focused on developing a displacement-independent engine model to be used in vehicle simulation work to account for engine thermal efficiency effects. Analysis from this methodology led to the following conclusions:
- Engine efficiency improves significantly with increasing engine temperature.
- Projected optimal engine temperature is ~25% more efficient than a 22°C ambient cold start.
- The initial enrichment spike encountered during a cold start accounts for a ~3% fueling increase compared to a warm engine. Even greater accumulated losses (~20%) follow this cold start enrichment until the optimum engine temperature is obtained.
- Between the range of 25°C to 60°C, each 5° C increase in initial engine temperature decreases fuel consumption by 3.2%~1.9%, respectively. This fuel usage reduces as temperature increases, ultimately reaching an asymptote.
- For the experimental PHEV, ten-minute soak times may result in ~5 degree variations in temperature (dependent upon power train operating temperature).
- Losses associated with the electric components, rolling losses, and transaxles are minimal relative to engine and transmission thermal losses.