SH-AWD - Implementation

Implementation

Acura RL

The second generation Acura RL sedan, introduced in late 2004 as a 2005 model, was the first production implementation of SH-AWD. It is the most mechanically complex, and probably the heaviest implementation of SH-AWD. The RL is powered by either a 3.5L 290 hp VTEC variable valve timing and lift V-6 (2005–2008) or a 3.7L 300 hp VTEC V-6 (2009–2010) transverse mounted in the manner of a typical Honda front-wheel-drive car. In fact, the RL uses a highly modified version of the Honda Global Midsize platform, versions of which exist for the Honda Accord, Acura TSX and Acura TL. However, the front transaxle is directly coupled to a torque transfer unit. Within the torque transfer unit is a short horizontal shaft and a hypoid gear set which turn the propeller shaft ninety degrees and moves it to the vehicle center line. An RL unique, two piece lightweight carbon fiber reinforced drive shaft carries power to the rear differential unit. Torque vectoring takes place in the rear differential which contains three planetary gear and clutch sets. Torque from the driveshaft passes through the differential's first clutch/planetary gearset, which as a unit is called the Acceleration Device. Two identical clutch/planetary gearsets power each rear wheel. The rear wheels are always overdriven with respect to the speed of the front wheels. The Acceleration Device can variably overdrive the rear wheel speed up to 5.7%.

2005–2008 RL SH-AWD operating parameters include:

  • During straight-line cruising and moderate cornering below about half throttle, up to 70 percent of the torque is delivered to the front wheels
  • In full-throttle straight line acceleration, up to 40 percent of the power is sent to the rear axle
  • In hard cornering, up to 70 percent of available torque goes to the rear wheels for enhanced chassis balance
  • Up to 100 percent of this torque can be applied to the outside rear wheel if the situation dictates
  • SH-AWD is active in second through fifth forward gears

The 2005 and 2006 system normally sends 70% of power to the front wheels and 30% to the rear under normal conditions but these ratios can reverse according to dynamic driving conditions. The 2007–2008 brochures specification sections indicate that this ratio changed to 80%-20%, and vice versa. The system can work because the clutches can slip under circumstances where the vehicle is turning, thus allowing the rear and front wheels to turn at different rates around a curve. As noted above, the SH-AWD can take 100% of the power shunted through the rear differentials and variably direct power to each wheel, up to 100% of that power to the outside rear wheel. Adding power to the outside rear wheel on a turn has a steering effect on the vehicle which can greatly improve vehicle handling dynamics through the reduction of oversteer and more balanced handling. In limited situations, the vehicle may exhibit power oversteer, characteristic of more sporty rear-wheel-drive vehicles.

While the rear differential also acts like a limited slip differential in spirited driving, SH-AWD is deeply integrated into the Honda Vehicle Stability Assist (VSA) system and various onboard sensors which helps to keep the car under control in a variety of situations.

The 2009 RL was updated to have more aggressive intervention of SH-AWD to give the car more rear-wheel-drive-like handling characteristics. 2009 RL SH-AWD operating parameters include:

  • During straight-line cruising and moderate cornering below half throttle, up to 70 percent of engine torque is delivered to the front wheels.
  • In full-throttle straight line acceleration, up to 40 percent of the power is sent to the rear axle.
  • In hard cornering, up to 70 percent of available torque goes to the rear wheels for enhanced chassis balance. Up to 100 percent of this torque can be applied to the outside rear wheel that can also be overdriven up to 5.7 percent by a built-in acceleration device.
  • SH-AWD is active in all five forward gears

Acura RDX
The Acura RDX was introduced in mid-2006 as a 2007 model year car. The first generation Acura RDX is a small crossover utility vehicle, and features a simpler implementation of SH-AWD than that found in the RL. The RDX does not have the Acceleration Device found in the Acura RL; instead the rear wheels are constantly overdriven at 1.7% higher speed than the front wheels. The RDX is distinguished by a unique 2.3L turbo charged 240 hp in line four-cylinder VTEC engine mounted in what is purported to be a unique version of the Honda Global Small Light Truck Platform, a version different than the platform used for the Honda CR-V and the Honda Element. The engine is also transversely mounted.

SH-AWD operating parameters include:

  • Up to 90 percent of available driveline torque can be transferred to the front wheels during high-speed driving
  • During straight line full-throttle acceleration, up to 45 percent of available torque can be transferred to the rear wheels
  • In hard cornering during acceleration, up to 70 percent of available torque can be directed to the rear wheels for enhanced vehicle dynamics
  • Up to 100 percent of the torque sent to the rear axle can be applied to either rear wheel as the conditions dictate

Acura MDX
The second generation Acura MDX, introduced in late 2006 as a 2007 model, uses a simpler implementation of SH-AWD than that found in the RL, the main difference is that unlike the RL 70 front 30 rear distribution, the MDX offers the opposite having a 30 front 70 rear distribution. The MDX configuration is similar to that in the RDX in that there is no Acceleration Device as found in the RL. The rear wheels are constantly overdriven at 1.7% faster speed than the front wheels. The MDX is powered by a 3.7L 300 hp VTEC V-6 engine tranversely mounted in a version of the Honda Global Large Light Truck Platform, versions of which are used for the Honda Pilot, Honda Ridgeline, Honda Odyssey, and the 2010 Acura ZDX. As can be seen below, the vehicle dynamics of the Acura MDX require different operating parameters.

SH-AWD operating parameters include:

  • Direct Yaw Control promotes precise line trace in all conditions
  • Full-time "active and intelligent" all-wheel drive requires no driver interaction
  • SH-AWD direct control electromagnetic clutches provide ultra-quick response
  • Grade logic control allows worry-free starting on steep grades with up to 70 percent of available torque being sent to the rear wheels
  • Up to 70 percent of available torque can be directed to the rear wheels during hill climb acceleration for maximum traction
  • Up to 50 percent of available torque can be transferred to the front wheels during cruising to help maximize fuel efficiency
  • In hard cornering, up to 50 percent of available torque can be directed to the rear axle for enhanced chassis balance and increased cornering capability
  • Up to 100 percent of the torque sent to the rear axle can be applied to either rear wheel as the traction conditions dictate
  • 37 percent increase in rear torque capacity as compared to the previous MDX VTM-4 system
  • 7 percent reduction in weight compared to the previous VTM-4 system

Acura TL
The fourth generation Acura TL, introduced in late 2008 as a 2009 model has two major trim levels, a front-wheel-drive version, and an upmarket SH-AWD version. Similar in layout to the Acura RL, the Acura SH-AWD TL features a 3.7L 305 hp VTEC V-6, transversely mounted front engine in the Global Midsize Platform. The 2009 model uses a five-speed automatic transmission, but Acura has also promised a new six-speed manual transmission for the 2010 model year. It remains to be seen if the 2010 model will also feature the newly announced six-speed automatic transmission featured in the prototype Acura ZDX announced at the April 2009 New York Auto Show. Unlike the Acura RL, the SH-AWD TL does not have the Acceleration Device.

SH-AWD operating parameters include:

  • Up to 90 percent of available torque can be transferred to the front wheels during normal cruising.
  • In hard cornering and under acceleration, up to 70 percent of available torque can be directed to the rear wheels to enhance vehicle dynamics.
  • Up to 100 percent of the torque sent to the rear axle can be applied to either rear wheel, depending on conditions.

Acura ZDX
The first generation Acura ZDX was announced on April 8, 2009 at the New York Auto Show as a 2010 model, slated for introduction in late Fall 2009 as an all new prototype concept car, billed as a luxury four-door sports coupe. The Acura ZDX went on sale December 15, 2010. The car is powered by an all-aluminum 3.7L 300 hp, 270 lbs-ft torque VTEC V-6, mated to an all-new 6-speed automatic transmission with Sequential SportShift and Super Handling All-Wheel Drive. The V-6 appears to be mechanically similar to the Acura MDX V-6. The Acura ZDX SH-AWD implementation description appears to be very similar to the Acura MDX SH-AWD implementation.

Read more about this topic:  SH-AWD