<p>A hydro-mechanical transmission (HMT) transmits power both mechanically and hydraulically which allows continuously variable transmission ratios. It is more efficient than a hydrostatic transmission since transmitting power via mechanical gears is more efficient than transmitting power hydraulically. However, a conventional HMT tends to be costly and bulky since it requires a hydrostatic transmission in parallel with a mechanical transmission including a planetary gear set. An alternative is a compact inline configuration that utilizes a two-shafted pump that is mechanically and hydraulically connected to a motor. This allows the pair of pump/motors to transmit power hydraulically and mechanically in a compact way without the need for a planetary gear set. In one implementation, the Hondamatic used in some all-terrain vehicles, the flow in the pistons is controlled via radially oriented distributor valves actuated by cams encircling the pump/motor barrel, and the motor displacement is controlled by adjusting the angle of the swash plate. When the pump/motor displacement is zero, all of the power is transmitted mechanically and the transmission ratio is unity, a condition referred to as lock-up that is expected to be very efficient. Previous research however has shown significant losses at this operating condition in both experiments and simulation which are thought to be caused primarily by viscous churning and compressibility losses due to unnecessary opening and closing of the distributor valves. This paper first models the Hondamatic in simulations to confirm that the hypothesized throttling losses contribute to the low efficiency at lock-up. Second, the paper proposes a solution to reduce these compressibility losses by means of a second cam mode that closes the distributor valves to prevent flow between the piston and the alternating high and low pressure sources. The efficiencies of the existing Hondamatic transmission configuration and the proposed solution at lock-up are modeled in simulations and compared. Third, this paper proposes a new valve timing that will achieve partial precompression and decompression, implemented as a novel profile of the actuating cam for the distributor valves. Additionally, by delaying the timing of the distributor valves with respect to the piston movement, the effective displacement of the motor can be changed in a process known as partial stroke piston pressurization (PSPP), and takes the place of the swash plate angle adjustment mechanism as the displacement control method. The efficiency of the transmission with the proposed valve timing mechanism is compared to the baseline efficiency of the Hondamatic over a range of transmission ratios in simulations.</p>

Improving the Efficiency of a Compact Inline Hydro-Mechanical Transmission (i-HMT) at Lock-Up

Paper Proceedings: https://asmedigitalcollection.asme.org/FPST/FPMC2021/volume/85239

2021 ASME/BATH Symposium on Fluid Power and Motion Control (FPMC2021)

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