Abstract
The electric hybridization of vehicles with an internal combustion engine is an effective measure to reduce CO2 emissions. However, the identification of the dimension and the sufficient complexity of the powertrain parts such as the engine, electric machine, and battery is not trivial. This paper investigates the influence of the technological advancement of an internal combustion engine and the sizing of all propulsion components on the optimal degree of hybridization and the corresponding fuel consumption reduction. Thus, a turbocharged and a naturally aspirated engine are both modeled with the additional option of either a fixed camshaft or a fully variable valve train. All models are based on data obtained from measurements on engine test benches. We apply dynamic programming to find the globally optimal operating strategy for the driving cycle chosen. Depending on the engine type, a reduction in fuel consumption by up to 32% is achieved with a degree of hybridization of 45%. Depending on the degree of hybridization, a fully variable valve train reduces the fuel consumption additionally by up to 9% and advances the optimal degree of hybridization to 50%. Furthermore, a sufficiently high degree of hybridization renders the gearbox obsolete, which permits simpler vehicle concepts to be derived. A degree of hybridization of 65% is found to be fuel optimal for a vehicle with a fixed transmission ratio. Its fuel economy diverges less than 4% from the optimal fuel economy of a hybrid electric vehicle equipped with a gearbox.
Highlights
The mobility concepts with a high potential to effectively decrease greenhouse gas (GHG) emissions are plug-in hybrid electric vehicles (PHEV), battery EVs (BEVs), and fuel cell EVs (FCVs) [3]
The bottom row shows histograms of the duration of specific small degree of hybridization allows the operating points to be shifted towards peak efficiency, which for the modeled naturally aspirated (NA) engine is approximately 2% higher than for the TC engine
With a sufficiently high degree of hybridization (DOH), the transmission of an HEV can be simplified to one fixed gear ratio, which leads to a fuel consumption that is close to that achieved with the conventional HEV
Summary
In 2020, the European parliament set the CO2 emission limit for new passenger cars to g CO2 /km. A conventional hybrid electric vehicle (HEV) can only be operated in a chargesustaining mode, while a PHEV is capable of driving in a charge-depleting mode. The remaining architecture variants allow the EM to be disengaged from the ICE, which enables pure electric operation and efficient recuperation of braking energy [18,19,20,21,22]. The electric hybridization does allow the recuperation of braking energy, and enables the ICE to run at an efficient operating point [33,34]. The operating points with low power demand, where the ICE exhibits poor efficiency, are driven with the EM in pure electric mode [35,36]. Due to the high range of feasible input variables, it is difficult to implement a causal operating strategy in an on-road application [42,43], which performs closely to the optimal strategy found by noncausal methods
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