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 3
rd to
6
th) 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.