ATV Source Article - Foreman Rubicon Transmission

Automatic transmissions have been used in some All Terrain Vehicles (ATVs) for many years, but until the introduction of the Hondamatic hydraulic transmission, ATV automatic transmissions used a traditional belt drive and pulley system. The fully automatic dual-mode Hondamatic transmission featured on the 2001 Honda Fourtrax Foreman Rubicon (TRX500) provides these features, and is compact, quiet, and extremely reliable for use in agricultural, rural, and recreational applications.

The usefulness of Hondamatic transmission is maximized when combined with the computer-controlled, dual-mode continuously variable change program or the Electric Shift Program (ESP), already in use on the Fourtrax Foreman ES (TRX450ES) and Rancher (TRX350FE/TE) ATVs. The ESP allows the operator to select higher or lower output ratios, providing precise manual control of the Hondamatic transmission.

System Overview

In a very simplified description, the Hondamatic hydraulic transmission uses the engine to drive a hydraulic pump that forces hydraulic fluid through sequential pistons. On the other side of these pistons, the pressurized fluid enters a second set of pistons that push against an angled plate (called a swash plate). Because the cylinder body holding the pistons is splined to the output shaft, the pressure exerted on the swash plate causes the cylinder body to rotate. When the angle of the hydraulic motor swash plate is adjusted, the cylinder body (and, therefore, the output shaft) rotates faster or slower, resulting in higher or lower output drive ratios. When either of the two automatic transmission modes (D1-Maximum Performance or D2-Maximum Torque) is selected, the Engine Control Unit (ECU) continuously monitors input from six sensors to position the motor side swash plate at the optimal ratio. Using the ESP mode, the ECU instructs the control motor to move the hydraulic motor plate to preset angles to simulate specific gear selection.

CONSTRUCTION AND OPERATION

Operating Principles

The Hondamatic transmission is a hydromechanical continuously variable transmission with a fixed-volume piston pump and a variable-volume piston motor in opposition on the same shaft. As the Hondamatic is a closed circuit, any excess hydraulic fluid from the Hondamatic system is recirculated to the transmission using a separate charge pump. The Hondamatic transmission uses standard engine oil as hydraulic fluid.

Upon engagement by the centrifugal clutch, the engine turns the HFTs pump-side outer body, causing the fixed-angle pump-side swash plate within to rotate, sequentially stroking the pump side pistons. This action draws low-pressure fluid into the pistons, which discharge high-pressure fluid (corresponding to the input torque). The fluid is distributed to the high-pressure circuit by the distributor valve on the pump side. The distributor valve on the motor side opens and feeds the high-pressure fluid to the piston on the motor side for the suction stroke. Each distributor valve is eccentrically synchronized to the rotation of its respective swash plate, ensuring that fluid is transferred at the proper time. The amount of fluid discharged depends on the angle of the motor side swash plate. The greater the slant, the further the pistons move and the more fluid they transfer. This additional volume transfer makes the motor side less efficient, resulting in a differential based on the volume transferred. As the motor-side pistons travel down the slope of the motor side swash plate, the fluid pressure is drawn through the pistons and rotates the motor-side cylinder body (which houses the pistons). As the cylinder is splined to the output shaft, the output shaft also rotates, transferring power to the drive train.

When the motor-side swash plate is perpendicular to the pump axis, the pistons do not stroke (therefore, oil is not discharged). In this condition, oil cannot flow between the pump and the motor and the motor is hydraulically locked (1:1 gear ratio). An Overdrive is achieved by adjusting the swash plate to an angle beyond perpendicularity to the pump axis, creating a drive ratio of 1:0.84.

As the pistons begin their travel back up the motor-side swash plate, they begin their discharge stroke. The hydraulic fluid is transferred back through the motor-side distributor valve and into the low-pressure circuit of the body. The fluid then passes through the pump-side distributor valve, where it is timed to the suction stroke of the pump pistons.

Distributor Valves

The hydraulic fluid is transferred between the pump and motor piston chambers by the sprue-type distributor valves arranged radially around the shaft. The pump-side and motor-side distributor valves are eccentrically arranged on the shaft and are aligned to time the transfer of low-pressure and high-pressure fluid in tune with the direction and inclination of the pump and motor swash plates. As the cylinder rotates, the eccentrically arranged valves slide in and out of the cylinder body, opening and closing the paths in the body.

Compression Braking and Pressure Control

During normal running, the check valve in the pump-side cylinder feeds hydraulic fluid into the low-pressure circuit. The check valve is open when the hydraulic pressure in the low-pressure circuit drops below a specified level and, upon achieving the proper pressure, closes to prevent backflow. Under engine compression braking conditions (where the rotational force comes from the wheels), the motor side becomes a pump (driven by the input shaft) and the pump side becomes an hydraulic motor. In this situation, the high- and low-pressure circuits in the Hondamatic body are reversed. The Hondamatic transmission uses a separate check valve to feed hydraulic fluid to the engine braking low-pressure circuit.

Pressure control valves vent excessive high-pressure fluid into the low-pressure circuit during both normal running and compression braking conditions.

CHECK VALVES AND PRESSURE CONTROL VALVES

Torque Amplification

The input reaction force torque from the engine is transmitted to the pistons that are fixed within the rotating cylinder. This force is then transferred through the cylinder to the shaft (mechanical power train). When the ratio is 1:1 and the motor side swash plate exerts no reactive force against the motor cylinder, the input torque is transferred directly to output torque with no amplification. When the angle of the swash plate is increased, the reaction force increases, amplifying the output torque. The total output torque is the sum of the input reaction force torque and hydraulic output torque.

Shift Mechanism

As previously discussed, the output shaft speed is controlled by the angle of the motor-side swash plate. The position of this swash plate is determined by the ECU, which uses various pieces of information to send commands to the control motor that moves the swash plate arm. The signals used to determine optimal Hondamatic output are:

Throttle opening (throttle sensor)
Vehicle velocity (speed sensor)
Engine speed (rpm) (ignition pulse generator)
Hondamatic motor swash plate angle (angle sensor)
Gear position (gear position switch)
Control mode and map (mode/map switch)

In the fully automatic modes (non-ESP), the ECU is continuously monitoring these signals. When a change in motor swash plate is deemed necessary, the ECU sends a signal to the control motor, which moves the ball screw (attached to the swash plate arm) via reduction gears. There are two rider-selectable automatic modes: D1-Maximum Performance and D2-Maximum Torque. The D1 mode emphasizes higher engine horsepower output for riding performance, while the D2 mode maximizes torque output performance. Within these rider-selectable modes, there is a choice of standard output ratios (Drive) or lower output ratios (Low) using the automobile-like shift lever. Reverse can also be selected using this gear lever which engages a reverse gear located in the subtransmission.

When the operator selects the ESP feature, the UP and DOWN buttons on the handlebar switch are enabled. When the appropriate button is pressed, the ECU commands the control motor to move the swash plate to the next higher or lower preset position.

SUMMARY

The Hondamatic transmission is the first continuously variable transmission of its kind. Housed within a compact, fully sealed assembly, it uses the principles of hydrostatic drive, mechanical power transfer, and modern electronic controls to create a unique hydromechanic drive. Unlike traditional belt drives, the Hondamatic is quiet, maintenance-free, rugged, impervious to external contaminants, and features true engine braking.