Detroit was not all about super fuel savers... But is 28 mpgUS combined enough?
Wayne Gerdes – CleanMPG
– Jan. 19, 2009
2010 Lexus RX 450h FWD -- 28 mpgUS combined on the 08 EPA.
With each new iteration of Hybrid Synergy Drive (HSD), Lexus takes advantage by offering its customers the very latest in fuel efficient hybrid automobiles. Nearly four years after the debut of the RX 400h, the all-new Lexus RX 450h improves upon its predecessor in almost every attribute and Fuel Economy rises to the top of the list.
Like the previously reported 2010 Prius-III and 2010 Lexus HS 250h, Lexus' first priority was higher fuel economy. A key to that goal was to improve fuel economy in cold temperature driving, an area traditionally difficult for hybrid electric vehicles.
“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 28 mpg in the city and 27 mpg in highway (26 mpg on AWD) driving.
Several important changes to the RXh’s HSD are responsible for the increased fuel economy and performance including the following:
- 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 larger 3.5L gas engine produces 245 HP, up from 208 HP and torque is 234 lb.-ft. at 4,800 rpm as compared to 212 at 4,400 rpm in the outgoing 3.3L equipped 400h.
Like all HSD equipped hybrids, The RX 450h front-wheel-drive model’s hybrid powertrain employs two motor-generators: MG1, the engine-driven generator, is a starter and can charge the battery pack or power other electric motors as needed; MG2 is the front-drive motor. Power from the gas engine and MG2 is distributed to the drive wheels via a planetary gear-type continuously variable transmission. MG1 controls the output speed of the transaxle through the planetary gear set, without clutches or a viscous coupling. MG2 delivers similar maximum torque as in the previous model, but over a wider range for better response and smoother performance. When the RX 450h is coasting or braking, the electric motors function as generators, capturing energy that normally would be lost as heat through the brakes and transforming it into electricity to recharge the batteries.
The RX 450h takes advantage of engine advances including roller rocker arms that reduce friction and specifically the change to an Atkinson cycle intake and external cooled EGR.
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 be used more effectively 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).
Another key to improvements in fuel economy comes from the Exhaust Heat Recovery System. Toyota developed a system that recovers exhaust heat to quickly raise coolant temperature during the critical warm-up phase so many HSD owners have learned about in an all too painful way. EHRS allows the engine to Auto stop sooner, helping to improve fuel economy during this phase.
Real world testing at 23˚F on a typical driving cycle shows the engine Autostops a full 1,000 seconds (over 15 minutes) earlier than the outgoing model.
The RX 450h is rated as a Super Ultra Low Emission Vehicle (SULEV) certification in California (Tier 2-Bin 3 in other states), producing nearly 70 percent fewer smog-forming emissions than conventionally powered vehicles.
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 increase torque capacity and overall transaxle efficiency.
An improved flywheel damper employing a two-stage torque absorbing mechanism has been developed for the RX 450h. 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 algorithm's output, the transmission selects the appropriate shift range to maintain engine rpm at an optimum speed for either acceleration uphill or engine braking when travelling downhill.
An all-new power control unit (also known as an inverter/transverter) has been improved in a number of areas. The control unit uses a new, more efficient “stacked” cooling structure to greatly increase power density and reduce volume (both by 40 percent) while weight decreases by almost 30 percent. PCU losses have also been reduced by 10 percent. Additionally, the higher operation frequencies reduce inverter noise.
More powerful Drive Motors
The electric drive motors in a hybrid drive system have the crucial dual roles of providing motive force and charging the battery pack.
The large propulsion motor/generator (MG2) sees a small increase (2 Nm) of maximum torque but the range of maximum torque is extended by 9 mph.
Three Driving Modes
Just like the Prius-III and HS250h, the 450h also allows the driver to select 3 different driving modes.
- 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 improve 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. Use it as a FAS switch or to turn your vehicle around in a parking lot. Otherwise, DO NOT USE IT!
- 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.
Just like its predecessor, we are all still waiting for Lexus (or Toyota in the HiHy for that matter), to place the 2.4L 4-Cylinder TCH motor into their hybrid SUV’s. Until then, the 2009 FEH still takes the small SUV crown with a 32 mpgUS combined rating on the 08 EPA, a full 14% higher than the overly powerful RX 450h. Fortunately, the ECO button allows the 450h to achieve upwards of a 7% increase in real world Fuel Economy (close to 30 mpgUS) which reduces the mpg delta to something any CleanMPG member would like to explore in further detail later
The Lexus 450h will be available in dealership showrooms later this spring.