Experimental evaluation of e-motor engine start in a heavy-duty hybrid vehicle under cold soak conditions

Vehicle hybridization offers the opportunity to eliminate conventional engine starters in favor of e-motor engine start, i.e. using the hybrid´s electric traction motor, `e-motor´, to start the engine. The advantages of e-motor engine start are many: 1) A freight efficiency gain of about 1% can be achieved by eliminating the starter and extra Pb-acid battery weight in a heavy-duty vehicle† 2) Less engine wear via the starter´s crank arm 3) Enhanced engine start/stop performance in terms of start time and smoothness†† More recently, heavy-duty vehicles have been added to the growing list of hybridized vehicles. Heavy-duty engines are much larger than conventional vehicle engines and thus have higher starting torques, especially at lower temperatures as the breakaway torque increases. Regardless of engine size, a common major problem exists in that as temperature decreases, batteries become weaker but must also provide more power to start the engine. This leads to what can be coined a `cold start paradox´ which is amplified by the severe starting demands of a heavy-duty vehicle. This cold start paradox applies not only to conventional 12V starter systems but to hybrid-enabled e-motor engine starter systems as well. This paper explores the theoretical and experimental hybrid-enabled cold start-ability of a heavy-duty parallel hybrid built for the U.S. Department of Energy´s Super Truck program.