|LS 600h L.|
At the Paris Auto Show, Lexus provided more details on the powertrain and performance of its luxury hybrid flagship, the LS 600h L, introduced earlier this year at the New York International Auto Show. (Earlier post.)
The Lexus Hybrid Drive in the LS 600h L combines a new 5.0-liter gasoline direct-injection V8 with large, high-output electric motors and a newly-designed battery pack to deliver more than 330 kW (442 hp) of total system power. Fuel consumption is rated at less than 9.5 liters/100km (more than 25 mpg US), and CO2 emissions are less than 220 g/km.
As with the Lexus RX and GS hybrid models, the LS 600h’s “600” suffix refers not to the cubic capacity of the engine but to a power output comparable to that of a 6.0-liter V-12 normally-aspirated engine.
Engine. The longitudinally-mounted 5.0-liter V8 gasoline engine is derived from the 4.6-liter powerplant in the LS 460. To reduce the overall weight of the engine, the cylinder block is die-cast from a lightweight, high-strength aluminium alloy. The block structure and rib reinforcement design have been finalized through the incorporation of cylinder combustion pressure data to minimize both vibration and noise. The engine head cover is also constructed in a lightweight magnesium alloy.
The new V8 features a D-4s (direct injection 4-stroke superior version) stoichiometric direct injection system, the world’s only automotive injection system to adopt two injectors per cylinder—one injector installed in the combustion chamber and a second mounted in the intake port.
The D-4s system’s port injectors employ 2 holes to inject fuel at a maximum pressure of 4 bar, while the in-cylinder injectors feature twin, 0.52 x 0. 3 mm rectangular slits producing a double fan injection pattern to effect the most homogeneous possible air/fuel mix.
Under cold start conditions, D-4s employs port injection during intake and direct injection during compression, producing a lean air/fuel mixture of 15-16:1. By concentrating the richer mixture around the spark plug it is possible to raise the combustion temperature, contributing to a quicker warm up of the Lexus thin-wall catalysts.
At idle, the engine runs on direct injection alone, due to its higher efficiency. In addition, and unique to the LS 600h and GS 450h, the electronic management maintains engine revolutions at an ideal speed to improve warm-up.
When the engine is running under a low- to medium-load at lower speeds, both direct and port injection systems are used during the intake stroke. This creates an homogeneous, 4.5: stoichiometric air/fuel ratio to stabilize combustion, improve fuel efficiency and reduce emissions.
When the engine is running under heavy loads, the direct injection system alone is employed. This achieves an intake cooling effect by injecting fuel directly into the combustion chamber, which improves the efficiency of each charge. It also allows for a higher engine compression ratio of 11.8:1, reducing pre-ignition tendencies and improving engine output and performance. Once again, a 12-15:1 stoichiometric air/fuel ratio is effected during the intake stroke.
D-4s substantially reduces combustion fluctuations in comparison to any conventional, direct or port injection system. D-4s realizes optimum engine efficiency throughout the power band and improving torque by 7.5% across the rev range, while minimizing fuel consumption and emissions.
The engine is also equipped with Lexus’ Dual VVT-i. This optimized, low-pressure loss, variable intake and exhaust valve timing system incorporates VVT-iE, the world’s first electric motor-driven intake camshaft, which operates across the full engine revolution and temperature spectrum.
Hydraulic VVT cannot operate below 1,000 rpm or during engine warmup. However, the Electric Motor Driven VVT system will operate across the full engine revolution and temperature spectrum, with a cam response speed of 50 degrees per second towards the lag phase and 50 degrees per second towards the advance phase.
Due to cam phase shifts when the engine stops, it is difficult to halt the cam at the optimum position for engine re-start using the electric motor alone. For that Lexus engineers have developed a mechanism employing frictional resistance and speed reduction gearing to hold the cam phase in the ideal position for engine start-up.
The new V8 further features a semi-dual exhaust manifold that reduces interference in the flow of exhaust gases, further improving output and combustion efficiency.
Motor and Electronics. The LS 600h Lexus Hybrid Drive employs a three-phase, permanent magnet AC synchronous motor, operating on a 650-Volt current, delivering more than 60 kW.
A change in the magnet distribution enhances operating quietness. Coiling the magnetic alternate, and hence holding the magnetic force, results in a smoother, more stable motor rotation.
The hybrid drive also consists of a generator; a high-performance NiMH battery; a power split device which combines and reallocates power from the engine, electric motor and generator according to operational requirements; and a Power Control Unit (PCU) to govern the high speed interaction of the system components.
All-wheel drive and transmission. The LS 600h features a new, mechanical, all-wheel drive system and a newly developed, dual-stage, electronically controlled continuously variable transmission.
The all-wheel drive transmission relies on a 3-differential configuration and a propeller shaft, coupled directly to the hybrid transmission.A permanently engaged mechanical transfer system distributes drive power with a ratio of 40% to the front wheels and 60% to the rear.
A center limited-slip differential optimizes grip, traction and vehicle handling on all-road conditions. Traction and grip characteristics are further enhanced by the vehicle’s advanced stability control system, Vehicle Dynamics Integrated Management.
As in the GS 450h, the Lexus Hybrid Drive’s electric motor, generator, power split planetary gear mechanism and motor-speed reduction gearing are all housed in one lightweight, highly compact transmission casing.
The combined installation of these components within a single compact casing is fundamental to the successful installation of Lexus’ hybrid drive system in a longitudinal, front-engine sedan platform.
The Lexus Hybrid Drive ECU selectively controls the rpm of the engine and electric motor, and the E-CVT (Electric Continuously Variable Transmission) simulates a continuous variation of the transmission’s current ratio. Similar to that of the GS 450h, the two-stage motor speed reduction gearing generates maximum low-gear torque for significantly enhanced acceleration, as well as extended high-gear performance for high speed cruising with improved fuel efficiency.