By Jim Kerr

Auto Tech: 2006 Toyota Highlander Hybrid Synergy Drive  environment auto tech
2006 Toyota Highlander Hybrid. Photo: Paul williams. Click image to enlarge

Every year, a panel of experts from the Automobile Journalists Association of Canada (AJAC) vote for the best new automotive technology presented to them by manufacturers at their annual Test Fest in Shannoville, Ontario.

In previous columns I have described other entries, and the final entry for this year is Toyota’s Hybrid synergy Drive (HSD) system in the 2006 Toyota Highlander. The votes are already in for the Best New Technology award, but the winner won’t be announced until February at the Canadian International Auto Show in Toronto. Let’s see what’s under the sheet metal of the Highlander.

Toyota’s Hybrid Synergy Drive system was first introduced in the Prius sedan. The Lexus RX400h used it next but in an advanced format. Now the Highlander is using this hybrid system. It is very similar to the Lexus application but appears tuned a little more for economy. The HSD
system has had a big impact on how vehicle buyers perceive hybrids, and as Toyota introduces it on more models, it will have a significant impact on the marketplace.

The Highlander HSD is a series-parallel drive system. This means it can run on electricity alone, gasoline alone, or a combination of both of them. Three electric motors are used. Toyota refers to them as Motor/Generator units such as MG1 or MG2. Two of the MG units are located in the transaxle unit under the hood. Like a conventional vehicle, the transaxle is mounted to a gasoline engine, in this case a 3.3 litre DOHC V6 engine. A third electric motor/generator is mounted at the rear of the SUV and provides the driving force for the rear wheels.

Auto Tech: 2006 Toyota Highlander Hybrid Synergy Drive  environment auto tech
2006 Toyota Highlander Hybrid. Photo: Paul williams. Click image to enlarge

Other components include an inverter assembly located under the top cover of the transaxle, an integrated computer controller for the system and a sealed Nickel Metal Hydride battery module. The 68 kg battery pack consists of 240 individual cells packaged in three modules under the vehicle’s rear seat. Each of the modules has its own monitoring and cooling control system. This battery pack provides 40% more power than the Prius battery yet it’s 18% smaller.

Let’s look at the components in more detail. Electric motors are fantastic at providing high torque at low rpm. The Highlander uses this characteristic to produce strong acceleration from a stop. Typically electric motors don’t rev high, so they are limited at higher speeds, but by redesigning the motors, including repositioning the magnet field around the motor armature in a flattened position, they are able to achieve 12,500 rpm out of a front motor and 10,500 rpm out of the rear electric motor. That improves performance at higher speeds.

The two motors in the transaxle are coupled to a planetary gear set so that is also coupled to the gasoline engine. The computer control can operate each motor at variable speeds or reverse the direction to continuously vary the gear ratio of the transaxle. There are no gears to shift in this transaxle. It changes ratios smoothly and continuously as the vehicle drives.

The Motor/Generator units can operate on variable voltage anywhere from 280 volts to 650 volts. The battery pack supplies 288 volts, but the inverter inside the transaxle changes this to a higher voltage. Instead of using a transformer, which would create heat and waste energy, Toyota raises the voltage electronically by rapidly shutting off the current flow and capturing the peak voltage surges. Similar in concept to a water hammer that occurs when a water tap is shut off quickly and the pipes bang due to the sudden pressure increase, the electrical flow is shut off and the peaks are captured for use. There is much Toyota has not described about the details of this operation, but I did observe two huge capacitors about 3 inches in diameter and several inches long under the transaxle cover that could act as storage for the voltage surges.

The battery cells are now connected to each other at two points to reduce internal resistance in the battery pack. The computer can use the generating force from all three MG units to recharge the batteries and is programmed to keep them between 80% and 40% charge. As the Toyota specialist says, this makes for a “happy battery”.

Using the combination of the three electric motors and the V6 gas engine, the Highlander produces 268 horsepower. The strong torque of the electric motors makes it seem like much more. Overall, the HSD system adds only 136 kg to the weight of the Hybrid Highlander compared to a conventional Highlander. That’s a lot of technology per kilogram!




About Paul Williams

Paul Williams is an Ottawa-based freelance automotive writer and senior writer for Autos. He is a member of the Automobile Journalists Association of Canada (AJAC).