Mass fuel (g)
Modeling PHEV Thermal Eects on Engine Eciency
Efficiency losses of 25-40 percent are seen in plug-in hybrid vehicles (PHEVs) between ambient 20° C cold starts to optimal hot temperature urban drive cycle operation. These losses are especially critical for PHEVs, when long durations between running the engine result in reduced engine temperature.
4 The initial enrichment spike during a cold start accounted for a ~3 percent fueling increase compared to a warm engine. Even greater accumulated losses (~20 percent) followed this cold start enrichment until the optimal engine temperature was reached.
“Variations in ambient temperatures and driving styles have a significant impact on fuel consumption by advanced powertrain vehicles. Understanding these efficiency losses will ultimately result in engineering more fuel-efficient vehicles,” said Argonne’s Forrest Jehlik, principal mechanical engineer.
To learn more, Jehlik and his team used response surface methodology (a type of statistical modeling) to characterize the thermal effect on PHEV engine efficiency. Combined with a technique for predicting the engine’s thermal state from its initial temperature, this unique method accurately predicts the fuel efficiency over a drive cycle from engine cold start to its fully operational temperature.
Their modeling experiments showed that
4 Engine efficiency improved significantly with increasing engine temperature.
4 Projected optimal engine temperature was ~25 percent more efficient than a 20° C ambient cold start.
4 Between the range of 25-60° C, each 5° C increase in initial engine temperature decreased fuel consumption by 3.2 percent and 1.9 percent.
4 Losses associated with the electric components, rolling losses and transaxles were minimal relative to engine and transmission thermal losses.
Fuel ow (g/s)
5 4 3 2 1
150 145 140 135 130 125 120 115 110 105 100
30 40 50 60 Initial engine oil temperature (° C)
Fuel flow rate response surface shown at engine oil temperature of 22° C. Fuel flow rates and surface changes as temperature increases.
Jehlik and his colleagues are working to develop a displacement- independent engine model to be used in vehicle simulation work that will account for engine thermal efficiency effects.
Funding for this project was provided by the U.S. Department of Energy, Vehicle Technologies Program under the direction of Lee Slezak.
For more information, contact Forrest Jehlik
Modeled urban dynamometer driving schedule (UDDS) fuel consumption as a function of initial engine temperature. Background photo is a test vehicle on chassis dynanometer.