Bi-level iterative-driven dynamic matching design for power reflux hydro-mechanical transmission system: A method for improving transmission efficiency in construction machinery powertrains

The low transmission efficiency of torque converters (TCs) has become a critical bottleneck restricting energy conservation, emission reduction, and performance upgrading in construction machinery. Although the power reflux hydro-mechanical transmission system (PRHTS) is regarded as a promising alternative to the TC, the absence of a systematic matching design method for construction machinery applications has prevented the efficiency potential of the PRHTS from being fully realised. To address this gap, this study proposes a bi-level iterative-driven dynamic matching design method (BID-MD). The method accounts for the strong multi-metric coupling and application-specific characteristics of matching design under complex operating conditions. Specifically, at the upper level, PRHTS matching optimisation is formulated under static operating-condition boundaries. At the lower level, dynamic operating-condition excitations are incorporated into an integrated engine–transmission–vehicle evaluation framework, in which a matrix-augmented fast dynamic programming algorithm re-evaluates the upper-level solutions and iteratively updates the candidate designs. Experimental validation is conducted using a 160-hp crawler bulldozer as the case study. The results show that the PRHTS provides an efficiency advantage and load-start capability, as well as exhibiting low-speed torque-multiplication and load-adaptive characteristics. Compared with the target bulldozer, the dynamically matched bulldozer improves operating efficiency by 25.2% and 21.8%, and reduces fuel consumption by 11.9% and 16.4%, under frequently occurring and heavy-load operating conditions, respectively. The results demonstrate the feasibility of replacing the TC with the PRHTS and verify the effectiveness of the BID-MD.