With each iteration, and now generation, of its Hybrid Drive system, Lexus continues to push forward its hybrid technology to increase efficiency and performance of its vehicles. Now, nearly four years after the debut of the RX 400h, the world’s first hybrid-powered luxury vehicle, the new Lexus RX 450h brings a broad range of improvements designed raise the bar in the luxury sport utility class once again.
With improved efficiency as their first priority for the new RX, Lexus engineers also actively pursued better acceleration and enhanced drivability. Another key goal was to improve fuel economy in cold temperature driving, an area traditionally difficult for vehicles using batteries as a primary or supplementary power source.
“The Lexus Hybrid Drive in the new RX is a perfect example of how Lexus uses kaizen to improve its vehicles,” said Lexus Vice President and General Manager Mark Templin. “The engineers found ways to improve the efficiency in virtually every aspect of hybrid system. And when you improve efficiency, performance often follows.”
The new RX 450h gains improvements in fuel economy with EPA-estimated ratings of 32 mpg in the city and 28 mpg in highway (30 mpg city on AWD) driving. Gains are even larger in cold weather driving conditions. Plus, there are tangible benefits to drivability, with torque increases where they are most beneficial and a new transmission control strategy that helps keep the engine in the most effective rpm range.
These substantial improvements are courtesy of several important changes to the RX’s Hybrid Drive system, each having prioritized goals (shown in parentheses) including:
Engine of Change
Basic engine improvements (increased power and fuel economy)
Adoption of the Atkinson combustion cycle (improved fuel economy)
Addition of a cooled exhaust gas recirculation (EGR) system (improved fuel economy)
A new exhaust heat recovery system (improved fuel economy)
Reduced friction losses in the hybrid transaxle (improved fuel economy)
Artificial Intelligence (AI) Shift control (improved performance/drivability)
A new power control unit [inverter and DC/DC converter] (improved performance and fuel economy)
Increased motor torque on front (MG2) and rear (MGR) motor/generators (increased performance)
New ECO driving mode (improved fuel economy)
The RX 450h takes advantage of engine improvements shared with the new RX 350, such as roller rocker arms that reduce friction and give designers more freedom with camshaft profiles. These and other changes yield a nearly 20 percent increase in power.
Notable improvements in fuel economy came with several key changes that specifically benefit the hybrid system. The change to the Atkinson cycle and the cooled EGR are two of the keys.
With conventional four-cycle engines, there are times when fuel enrichment becomes necessary to cool the exhaust gases to prevent degradation or destruction of the catalytic converters. With the Atkinson cycle, the expansion/power stroke is longer than the compression stroke so that combustion energy can more effectively used for production of engine power. This results in lower exhaust gas temperatures.
In the process of re-circulating exhaust gas, the cooled EGR system increases the specific heat capacity, also resulting in lower exhaust gas temperature. Regulating the amount of EGR can also control the exhaust gas temperature.
The combination of the Atkinson cycle and cooled EGR minimizes the need for fuel enrichment. The benefit is significant reduction of fuel consumption, especially during high-load driving (e.g.: hill climbs and freeway driving).
The cooled EGR system allows precisely metered and spent exhaust gas to enter the intake system, lowering exhaust gas temperature while also reducing engine pumping losses via decreased intake vacuum. An EGR delivery manifold ensures even distribution of the exhaust gasses to all cylinders. The high-capacity EGR cooling system can reduce the gas temperatures from about 880° C to 150° C, improving volumetric efficiency. The cooled EGR also eliminated the need for exhaust VVT-i.
Calling All BTUs
Another key to improvements in fuel economy comes from the Exhaust Heat Recovery System. An important part of the hybrid control strategy is to stop the engine when it’s not needed, for example in low power demand conditions in city driving. But in winter driving, engine heat is needed to warm the interior cabin and demand on the electric drive motors will be naturally higher in these conditions, especially during warm-up. When temperatures drop below 0° C, battery output is also reduced and drive motor performance suffers accordingly.
To counter this challenge, Lexus developed an efficient system that recovers exhaust heat to quickly raise coolant temperature during warm-up. This allows the engine to stop sooner, helping to improve fuel economy during this phase. For example, testing at -5° C on a typical driving pattern (below) showed that the engine could be stopped a full 1,000 seconds (over 15 minutes) earlier than the previous model.
Exhaust heat is recaptured from the exhaust pipe just downstream of the catalyst where a recovery unit is mounted directly on the exhaust system. A thermostatically controlled valve forces part of the exhaust gas to re-circulate in a chamber, heating the surrounding coolant quickly. A spring in the valve allows it to open during full acceleration. When engine coolant warms sufficiently, the valve opens to allow exhaust gas to bypass the heat recovery system.
Hybrid Transaxle Gains Efficiency & Performance, Loses Noise and Vibration
Lexus’ elegantly simple and effective hybrid transaxle incorporates several improvements to support the goals for the new RX 450h. On the mechanical side, the adoption of an oil slinger and tuning/profiling of a number of components reduces drive losses; low friction bearings are installed in key locations; an oil cooler is now fitted. These changes combine to increased torque capacity and overall transaxle efficiency.
An improved flywheel damper employing a two-stage torque absorbing mechanism has been developed for the RX. The damper absorbs torque fluctuations as well as the shock created when the engine starts and stops. The new damper significantly reduces vibration and noise, especially in city driving when the engine starts/stops more frequently.
Artificial Intelligence (AI) shift control
is implemented for the first time on a hybrid vehicle. The system estimates whether the vehicle is travelling uphill or downhill by comparing vehicle speed with accelerator pedal angle. Depending on the estimation, the transmission selects the appropriate shift range (from 3rd
) to maintain engine rpm at an optimum speed for either acceleration uphill or engine braking when travelling downhill. Lubricating Electrons
One of the most important developments that greatly improves the RX 450h’ efficiency is a new power control unit
. Also known as an inverter, the power control unit has been called the heart of the hybrid system as it has a number of important functions and must work quickly and efficiently. Key functions include:
- Boosts hybrid battery voltage from 288 V to 650 V DC
- Converts hybrid battery voltage from DC to AC to power the drive motors
- Controls motor/generator operation
- Converts voltage of the hybrid battery to 14 V to supply accessory power and charge the auxiliary battery
The control unit uses a new, more efficient “stacked” cooling structure to greatly increase power density (40 percent) and contribute to reductions in unit volume (from about 30 liters to 18.2 liters) and weight mass (30 kg to 22 kg). Overall control unit losses are also reduced by 10 percent.
Changes in the operation circuit switching frequency reduce switching losses which also helps to improve fuel economy. Additionally, the higher operation frequencies reduce inverter noise.
Drive Motors Muscle Up
The electric drive motors in a hybrid drive system have the crucial dual roles of providing motive force and charging the battery pack. Both of the RX 450h drive motors have increases in maximum torque and torque range, contributing to improved acceleration and drivability. These improvements are in part facilitated by the new power control unit.
The front motor/generator (MG2) sees a small increase (2 Nm) of maximum torque but more importantly, the range of maximum torque is extended by 15 km/h. The rear motor/generator (MGR) receives a notably larger increase of torque (9 Nm) and extends its range even further—by 35 km/h.
The hybrid battery pack is also modified and although its structure and output is carried over, the total package is smaller and lighter than the previous unit. The more compact design improves packaging and minimizes changes between the RX 450h and the standard RX 350.
Three Modes for Different Moods
Three different driving modes add options for varied driving situations and individual desires:
A new ECO mode provides a modified throttle action and air conditioning control to support efficient driving. This mode changes the relationship between pedal movement and system output to help the driver avoid aggressive acceleration and achieve smoother operation. The A/C load on the engine is also reduced more aggressively upon acceleration. Both these factors provide improved fuel economy.
An EV drive mode, under certain conditions, allows operation of the vehicle with only the electric motors in order to reduce noise when driving in quiet areas and limit exhaust emissions in closed quarters such as parking garages.
A sequential shift mode is also available by moving the transmission shift selector to the “S” position. In this mode, the driver can select any of six speeds to provide more responsive performance or to add engine braking assistance.