In Detail: Lexus Hybrid Drive
Last year, UK sales were dominated by hybrid technology, with some 77.3% of all cars sold here featuring Lexus Hybrid Drive. Contributing heavily to the carmaker’s 33% sales uplift in 2011 was the CT 200h compact hybrid, which made up 4,220 of the 8,269 vehicles sold in the UK and offers 2.0-litre diesel power with the economy of a smaller unit.
The CT also completes Lexus’s family of performance-tuned hybrid drivetrains, which now covers rear wheel drive, front wheel drive and four wheel drive models. All offer the ability to function on petrol power or electric power, with the ability to combine both for high performance, and have been designed to propel the cars with the refinement Lexus is renowned for. The CT 200h fits the entire system into a conventionally-sized engine bay, here’s how it works.
- Engine – The CT 200h uses a 1.8-litre petrol engine similar to the unit found in the Toyota Prius. This uses variable valve timing and a slightly different combustion cycle to a conventional petrol engine, known as the Atkinson Cycle, which improves efficiency by reducing the energy being wasted as heat. It meets the forthcoming Euro 6 emissions standards, makes 98bhp and runs at specific engine speeds depending on the load required by the car’s electric motors.
- Hybrid battery – Toyota’s nickel metal hydride hybrid batteries have covered more than 38 billion miles in real-world use powering three generations of the Prius and Lexus hybrid models. In the CT 200h, it’s fitted under the boot floor for a low centre of gravity and to avoid reducing cabin space, and is covered by a five year, 60,000 mile warranty.
- Power Control Unit (PCU) – Roughly the same size as a conventional car battery, the PCU manages the flow of electricity through the hybrid drivetrain. It comprises of three components – one to boost the system’s power output, another to convert electricity from the battery to AC for the electric motor and the last to reduce the hybrid battery’s 202V to 14V to power on-board accessories.
- Hybrid transaxle – Lexus houses most of the hybrid components within the space of a conventional gearbox, and this varies in location depending how the engine is mounted and which wheels are being driven. Within the single casing is an electronically controlled continuously variable transmission, which allows the petrol engine to run at its most efficient speed, a set of planetary gears to govern the source of power to the wheels and two electric motors. One works as a generator and starter motor for the petrol engine, while the other 81bhp unit drives the wheels and can also function as a generator for the hybrid battery during braking or deceleration.
How it works:
- EV mode – Depending on battery charge, the CT 200h automatically starts in EV mode. This decouples the petrol engine and allows the car to be powered entirely by its electric motor at speeds up to 28mph for 1.2 miles while the petrol engine stays shut off. EV mode can also be selected manually by the driver. As there is no reverse gear in the car’s gearbox, reversing is also done while driven on the electric motor, supplied with a negative voltage from the battery. When the battery charge is low, the petrol engine starts to power the generator and supply electrical energy to the battery.
- Cruising – For higher speeds and once the battery’s range is exhausted, the CT 200h works similarly to the Toyota Prius, using a combination of both power sources. Two hybrid modes are available – Normal, which is selected automatically, and Eco mode which adjusts on-board systems and softens throttle responses to promote efficient driving.
At low speeds, the engine’s power is used to drive the wheels, but with surplus energy supplied to the hybrid battery via the generator motor. This is then used to drive the traction motor, which provides extra torque to complement the petrol engine.
- Acceleration and high speed cruising – At higher speeds, the system changes to a different mode to boost efficiency. In this mode, the engine provides most of the power, while the two electric motors switch roles to generate power and boost output for acceleration as required. To help drivers use the system to as efficiently as possible, the dashboard shows how economically the hybrid system is being used, and whether the battery is being recharged.
- Sport mode –In Sport mode, the CT 200h prioritises high performance from its available power sources. With this selected, the Power Control Unit increases power from the hybrid battery to boost the output from the electric motor and make the car more responsive. The engine runs at higher revs to provide more assistance, steering is quickened and driver aids are turned down to make the car more involving to drive. In this mode, the dials turn red and the efficiency display becomes a rev counter, hinting at the combined 134bhp output from the hybrid drivetrain.
- Battery regeneration – The CT 200h doesn’t require an external power source to recharge its hybrid battery. Instead, the electric motor can be used as a generator under deceleration and braking, converting energy which would otherwise be wasted as heat into electricity for the battery which can then be used to assist or substitute engine power. This electricity can also be used to run on-board systems when the engine is deactivated in traffic.