Modeling and Experimental Testing of the Hondamatic Inline Hydromechanical Transmission (iHMT)1

A hydromechanical transmission (HMT) is a continuously variable transmission that transmits power both mechanically and hydraulically. A typical HMT consists of a pair of hydraulic pump/motors and a mechanical transmission in parallel, making it bulky and costly. The Hondamatic transmission is a compact alternative HMT design that uses an inline configuration such that the rotation of the piston barrels of the pump and motor is dual-used for mechanical transmission. This is achieved using a two-shafted pump that plays the role of a planetary gear (PG) and a distributor valve mechanism that replaces the valve plates. This paper provides the operating principle of this inline HMT (iHMT) and analyzes its performance through a combination of modeling and experimentation. Specifically, ideal and lossy average models are developed, and the performance of the Hondamatic is characterized experimentally. The lossy model, fitted with seven empirically determined parameters, is capable of predicting the mechanical and volumetric losses at different ratios and operating conditions. The dominant losses are found to be compressibility losses and no-load viscous friction losses, especially on the motor side. These losses are attributed to be the main causes for the unity transmission ratio to be less efficient than expected. The overall efficiency is between 74 and 86% at the conditions tested experimentally and is predicted to be over 70% under most operating conditions and transmission ratios. This analytical and experimental study is the first study in the open literature on this innovative compact inline HMT configuration.