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Motor development for hybrid vehicles boosts fuel economy

02 November 2015

CPT aims for near full-hybrid capability with new SpeedTorq unit – demonstrating that two low voltage switched-reluctance machines are better than one.

The application by Controlled Power Technologies (CPT) of a switched-reluctance (SR) motor-generator deep into the driveline, as well as being applied directly to a petrol or diesel engine, will enable the technology developer to deliver to car makers and tier 1 driveline system developers an almost full-hybrid functional capability.

The combination of two low voltage SR machines promises to significantly reduce vehicle fuel consumption and exhaust emissions, whilst avoiding the cost and complexity of a high voltage plug-in hybrid.

“Computer simulations based on our LC Super Hybrid technology demonstrator indicate an impressive 26 per cent improvement in fuel economy for a large family saloon when the rear axle is boosted with low voltage electrical power,” says Nick Pascoe chief executive of CPT.  “Moreover, the vehicle retains excellent performance and drivability.”

The significant reduction in fuel consumption and exhaust emissions results from the electric boosting and regenerative braking of CPT’s engine mounted SpeedStart unit combined with its new axle mounted SpeedTorq unit.  This combination is further enhanced by driveline down-speeding without compromising vehicle drivability - made possible by the electric boosting capability of two low voltage SR machines acting in tandem.

The new SpeedTorq unit is a recent addition to CPT’s portfolio of SR machines.  Unlike its engine cranking and generating focused, belt-integrated SpeedStart unit, which replaces an engine alternator, the SpeedTorq unit has the ‘four quadrant’ contra-rotational functionality essential for integration with a gearbox or axle, requiring the machine to provide its motoring and generating capability in both forward and reverse rotations.

“In this proposed dual-motor mild hybrid configuration, the e-motoring is only effected by the SpeedTorq unit,” says Dr Andreas Hubert, technical business development manager CPT.

 “The 48V e-motor installed on the final drive, results in additional efficiency benefits, because there is no drive through the combustion engine and transmission – and hence minimal parasitic losses.  SpeedStart, mounted on the engine in place of the alternator, meanwhile, is focused on providing an extended start-stop capability, while both units are used for energy recuperation.”

“This relatively simple combination of low voltage electrical machines represents a cost-effective intermediate step between conventional vehicles powered solely by internal combustion engines and expensive plug-in hybrids with their larger traction motors and expensive high voltage battery systems,” says Paul Bloore, product validation and functional safety manager for the company’s hybrid product group.

“The use of smaller switched reluctance machines that operate at 48 volts supported by a modest 1-2kWh battery pack, will enable vehicle OEMs to fully realise the benefits of mild hybridisation.”

“The combination facilitates down-speeding, as well as decreasing the load demanded from the combustion engine,” says Bloore.  “Analysis of our data, moreover, suggests that maximising energy recuperation during braking and electric boosting during acceleration has the potential to further reduce tailpipe emissions, depending on the system solution and calibration.”

“One of the most cost effective proposals for reducing CO2 emissions is simply to replace a vehicle’s standard 12V alternator with a 48V belt-integrated starter-generator [B-ISG],” says Hubert.  “The additional measure of integrating a second 48V e-motor with the axle enables extra features such as torque vectoring and further increases the installed electric power throughout the drivetrain to approximately 22kW (30bhp) for torque assist and energy recovery for this specific LC Super Hybrid application.”

“This latest approach to mild hybridisation will enable car makers to achieve future emission targets by significantly reducing the fuel consumption of an internal combustion engine through judicious use of electrical torque assist and kinetic energy recovery,” says Bloore. “The combination of two strategically placed e-motors using a nominal 48V boardnet electrical system, and a small battery with a high rate partial state-of-charge capability, now offers high volume mainstream car makers a low cost 4x4 hybrid solution, with enhanced drivability.”

CPT emphasised the benefits of their innovative mild hybrid architecture in a technical presentation delivered by Dr Andreas Hubert at the 24th Aachen Colloquium Automobile and Engine Technology congress organised by the Institute for Combustion Engines and the Institute for Automotive Engineering at RWTH Aachen University.  The congress was held in Aachen on 5-7 October, and Hubert will again present his technical paper at a sister Aachen Colloquium event held in Beijing on 4-6 November.

The technical paper, co-authored by Hubert and Bloore, focuses on the benefits of CPT's switched reluctance (SR) machine technology for automotive applications and points out the potential CO2 advantages of installing two 10kW e-motors (in P0 and P4 architecture) in a C/D-segment vehicle compared with a single 10kW e-motor (P0-architecture).

The CPT 48V e-motor development relies on robust and highly thermally managed three-phase switched-reluctance (SR) motor-generator technology, for which the key to successful operation is the precise control of current through the coils.   This is achieved using fully integrated low voltage power electronics combined with sophisticated control electronics and software.

SR machines deliver a number of advantages compared to other motor technologies.   Their stator structure has simpler windings compared with a permanent magnet motor, coupled with a simple lightweight low inertia rotor.   Not only does this provide a rapid response, but the elimination of permanent magnets also means that price volatility and the special recycling requirements associated with rare earth materials are no longer a factor.

And whilst the CPT design includes clever and patented mechanical innovation, SR machines generally are of relatively simple design compared with other motor technologies.   They offer consistent high power and high efficiency over a wide speed range; electromagnetic field weakening is not an issue; and the precise torque control enables a swift response within fractions of a second to changes of load on the machine.  In essence, it’s a highly controllable electrical machine that can switch back and forth very quickly from motoring to generating.

CPT’s SR machines have a wide low-voltage operating range between 6V and 60V, which offers the vehicle OEM an optional dual-voltage (48V/12V) capability to decrease system costs by removing or at least by minimising the need for a dc/dc converter.  Future 48V-boardnet hybrids will include a lot of high power devices such as electric fans, water pumps and air-conditioning compressors.  It’s why CPT's SR e-motors are designed for 8kW continuous high power operation, with the ability to handle a transient of 13kW peak power for 30 seconds in 48V applications.

This high performance requires a guaranteed heat transfer thermal management capability, which ducted air cannot easily achieve especially when ambient conditions are extreme.  CPT machines use a patented liquid cooling system (engine or oil coolant depending on location in the drivetrain) that maintains the temperature of both the motor windings as well as the integrated power electronics.   This offers a more stable environment for the power silicon devices and seals the machine against water and dirt ingress. The packaging is also simpler without the need to seek cool air within the vehicle.

This design also eliminates the need for an internal fan, which would otherwise increase efficiency losses and noise. This enhanced thermal management enables high power utilisation for extended, repetitive periods. Combined with its high efficiency over a wide speed range, this is another key factor for low voltage hybrid vehicle applications.

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