5-SPEED AUTOMATIC TRANSAXLE
An advanced 5-speed automatic transaxle was developed specifically
for MDX to meet ambitious performance, driving enjoyment, and comfort
goals. A lock-up torque converter is provided to maximize fuel
efficiency. Torque-converter lock-up and shift timing are both
managed by a 16-bit, 20-MHz transaxle CPU that maintains a
communications link with the engine's CPU. Gear and clutch materials
and the transaxle case itself are all engineered to support towing,
off-road driving, and 4-wheel-drive use.
A notable feature of this unit's design is extra-wide gear ratios. The difference in torque multiplication between first and fifth gears is nearly 5:1 (4.932) to balance low-speed pulling capability against fuel economy and the ability to cruise quietly on the highway.
Creative use of a clutched idler gear permits the transaxle to provide five forward speeds with little more weight or bulk than a typical four-speed automatic. An over-running clutch is provided for first gear to smooth upshift quality. An extra-capacity transmission fluid cooler is offered with the MDX's optional tow package to maintain acceptable lubricant temperatures during heavy-load conditions.
A direct-control strategy is used to provide real-time pressure management of the transmission's clutches. Various safety and control strategies are in place to coordinate engine and transmission operation. For example, driveline shocks during up- or downshifts are minimized by momentarily reducing engine torque during the shift. In neutral and park, engine rpm is automatically limited to 5000 rpm.
For driving through hilly terrain, a Grade Logic Control system monitors throttle position and vehicle speed and acceleration to avoid hunting and excessive shifting. A lower gear is held for a longer-than-normal period to provide better climbing ability hills and more engine braking on downhill grades.
VARIABLE TORQUE MANAGEMENT (VTM-4') 4-WHEEL-DRIVE SYSTEM
After studying various all-wheel- and four-wheel-drive systems
offered by the wide variety of SUVs on the market today, MDX
engineers concluded that virtually every one had functional
shortcomings and was undesirably bulky and heavy. The direct result
of that research was the creation of an innovative system that
automatically distributes torque to all four wheels as needed. Called
Variable Torque Management 4-wheel-drive (VTM-4'), this new system
provides front-wheel drive for dry-pavement cruising conditions and
engages all-wheel drive only when there's a need to do so to improve
mobility, stability, or maneuverability. Unlike many competitive
systems that use an engagement strategy triggered by wheel slippage,
the MDX's VTM-4 system anticipates the need for all-wheel drive and
engages the rear wheels before slippage begins. Another special
feature is a lock button, which temporarily holds engagement of the
rear wheels to aid extraction from a slippery ditch or a snow bank.
Patent applications have been made to safeguard innovative aspects of
this new technology.
To avoid the weight and bulk of a conventional transfer case, VTM-4's torque transfer unit is a compact cast-aluminum housing bolted directly to MDX's transaxle. Since this vehicle is engineered for medium-duty off-road capability, the transfer case is a single-speed permanently-engaged device without the low-range offered by some SUVs. Attached to the front wheel differential's ring gear is a helical gear that provides input torque to the transfer unit. A short horizontal shaft and a hypoid gear set within the case turn the drive ninety degrees, move it to the vehicle center line, and lower its axis by approximately 3.75-inches.
PROPELLER SHAFT AND HALF-SHAFTS
The two-piece propeller shaft that carries drive torque from the
transfer case to the rear-drive unit is made of high-strength steel
tubing to permit a smaller diameter. Minimizing driveline dimensions
improves both ground clearance and interior room. The cross yokes
attached at each end by friction welding are forged steel for high
strength and low weight. The center support bearing is rubber
isolated to block the transmission of driveline noise from the
interior of the vehicle. A low-friction plunger joint located near
the center of the propeller shaft accommodates relative motion
between front- and rear-mounted driveline components. A tuned-mass
damper inside the front portion of the propeller shaft cancels any
bending tendency in response to powertrain vibrations.
Equal-length front-wheel half-shafts have a plunger joint at their inboard end and a ball-type universal joint at the wheel end. Rear half-shafts are similar in design but use a double-offset joint at the inboard end and a ball joint at the outboard end. All universal joints are constant-velocity type.
REAR AXLE DRIVE UNIT
A conventional differential mechanism is not necessary in the MDX's
rear final-drive unit. Instead, a hypoid ring-and-pinion gear set
supported by a cast-aluminum housing switches torque from the
propeller shaft's longitudinal orientation to the lateral orientation
necessary to drive the rear wheels. Surface grinding the ring and
pinion gear teeth yields the quiet operation expected of a luxury SUV
wearing an Acura nameplate.
A connection from the ring gear to each wheel's half-shaft is made by left- and right-side clutches. Each drive clutch consists of three elements: an electromagnetic coil, a ball-cam device, and a set of 19 wet clutch plates which are similar in design to clutches used in any automatic transmission. Ten of the plates are splined (mechanically connected) to the ring gear while nine of the plates are splined to a half shaft. Left and right clutches are identical.
When the VTM-4 system's electronic control unit (ECU) determines that torque should be distributed to the rear wheels, an electric current is sent to the two electromagnetic coils. The resulting magnetic field moves a rotating steel plate toward each fixed coil. Friction between that steel plate and an adjoining cam plate causes the cam plate to begin turning. As it does, three balls per clutch roll up curved ramps, creating an axial thrust against a clutch-engagement plate. This thrust force compresses the wet clutch plates, thereby engaging drive to the corresponding rear wheel.
Unlike mechanically actuated four-wheel drive systems, the VTM-4 system is infinitely variable. The amount of torque provided to the rear wheels is directly proportional to the electric current sent from the ECU and can be adjusted from zero to a preset maximum. This current constantly changes to deliver the optimum rear torque calculated by the ECU. An internal gear pump circulates VTM-4 fluid to cool and lubricate the clutches, bearings, and gears within the rear drive unit. Use of high-strength, low-weight materials -- such as die-cast aluminum for the housing -- minimizes the bulk and weight of this hardware. In fact, the weight of the entire all-wheel-drive system is about 212-pounds, only two-thirds the weight of comparable equipped carried by the Mercedes-Benz ML320.
There are three distinct modes of VTM-4 engagement. The first -- called the acceleration torque control (ATC) mode -- is unique to this system. It works even on dry pavement to distribute driving torque to all four wheels as the MDX accelerates from a stop to cruising speed. One notable benefit is that traction is immediately available to move the vehicle from rest through a slippery intersection before slippage occurs. (Once a wheel slips, the traction available for forward propulsion and lateral restraint is significantly diminished.) A second advantage is that apportioning drive torque among all four wheels greatly diminishes the likelihood of torque steer. Handling dynamics are also improved. Reducing the propulsive force carried by the front tires leaves more adhesion for steering the vehicle into a tight bend or for holding cornering arc in the middle of a turn. In other words, the MDX's dynamic balance is greatly enhanced by ATC logic.
Rear wheel torque rises smoothly from zero to the maximum setting in proportion to vehicle acceleration (both forward and reverse). At higher speeds, the front wheels are capable of providing the desired thrust with excellent handling so torque delivered to the rear wheels automatically diminishes with speed. During constant-speed driving, all driving torque is delivered by the front wheels in the interests of smoothness, quietness, and fuel efficiency.
The second engagement mode uses wheel slippage control logic. If the difference in rotational speed between front and rear wheels rises because of a slippery surface or poor traction at the front of the vehicle, that condition is detected by wheel-speed sensors which are monitored by VTM-4's ECU. In response, the ECU commands an increasing amount of torque for the rear wheels. Torque is proportional to both slip rate and the rate at which the slip rate is increasing. This operation is similar to conventional slip-based all-wheel-drive systems already on the market.
The third mode of all-wheel-drive engagement is activated when the driver taps a lock button mounted on the instrument panel. The maximum amount of rear-drive torque is locked in until the vehicle gets moving and exceeds six mph, at which time rear drive torque is gradually diminished. By 18 mph, the lock mode is fully disengaged. The shift lever must be in the first, second, or reverse-gear position to use the lock mode.
The maximum torque delivered to the rear wheels is sufficient to climb the steepest grade observed on any public road in America -- 31 degrees (60-percent slope) -- with a two-passenger load on board. The MDX will also move from rest up a 28-degree (53-percent slope) dirt grade. On a split-mud grade (different amounts of traction at each wheel), VTM-4 automatically provides sufficient rear-wheel torque to help the vehicle climb a steep, slippery driveway to enter a garage.