These high costs, and doubts surrounding range, could continue to restrict consumer acceptance of hybrid and electric vehicles. Although sales are growing, BEVs and PHEVs still have only a tiny percentage of the total car markets in Europe and the US. The improvements in battery and motor technologies that are currently underway can help improve both cost and range, which could accelerate the transition to electric motoring.
Better batteries
Enhanced lithium-ion technologies can deliver greater energy density, extended driving range without increasing size and weight,. Li-ion battery types are usually named according to the cathode material which, in today's PHEVs and BEVs, may be a phosphate material such as lithium iron phosphate (LiFePO4) or a transition metal oxide (TMO) such as nickel cobalt manganese (LiNiCoMnO2), also known as NCM.
The maximum theoretical specific energy (MTSE) of LiFePO4 technology is around 170Wh/kg, while conventional NCM technology has MTSE of about 260Wh/kg.
Practical limitations mean the batteries themselves can only offer 70 to 75% of the MTSE: around 200Wh/kg for an NCM battery. With more advanced TMO cathodes and higher performing anode materials such as silicon alloy, future Li-ion batteries may achieve practical energy density of 300Wh/kg. This is 50 to 100% better than batteries in use today.
In addition, Li-ion chemistries that produce a higher cell voltage can offer advantages for EVs. Reducing the number of cells required for a given battery voltage should not only help lower cost, but also increase reliability since fewer connections, sensors, wiring and securing features are needed.
George Paterson, business development and marketing manager with Johnson Matthey Battery Systems, which helped develop the power pack for the Rolls Royce Phantom Experimental Vehicle, explains that a high energy density chemistry must satisfy several other important criteria for automotive applications such as cost, life cycle, calendar life, performance at low temperatures, and power capability.
"A suitable high energy density battery will have to allow rapid charge as increased capacity requires longer charge time unless charged rapidly. Ideally, its calendar life will have to be 12 to 15 years – the life of the vehicle – and its lifetime range in the region of 200,000 miles. As well as the focus on energy density, the industry really needs cells and batteries which are less expensive. This could boost the industry more than an increase in energy density."
Improvements in other major systems, such as the electric traction motor, can help to further extend the range and reduce total vehicle cost.
Motor efficiency
The permanent magnet AC (PMAC) motor is the main traction motor in most current EV platforms. Its advantages include greater efficiency across a wide range of operating conditions, compared to an alternative such as an AC induction motor (ACIM, see figs 1a and 1b). For a given torque and power rating, the PMAC can also have a shorter effective length and smaller overall diameter, resulting in a smaller, lighter and more easily packaged unit.
Jay Schultz, business development manager for vehicle electrification with Parker Hannifin, suggests that a PMAC may allow 10 to 30% greater range with a given battery than an induction motor designed for comparable power, torque and operating voltage. He cautions that much depends on the drive cycle. In urban or city centre driving, where the vehicle frequently accelerates and decelerates, the efficiency advantages of the PMAC can mean range improvements closer to the 30% estimate. The behaviour of the driver will also have a key role; as with a conventional car, more aggressive driving returns fewer miles per unit of energy stored.
On the other hand, the cost of materials used to build a PMAC is 20 to 30% higher than for a comparable induction motor, owing partly to the PMAC's need for permanent magnets. The price difference, however, reduces with motor size, tending towards parity. In this context, Schultz explains that the PMAC's greater efficiency allows a smaller and lower-cost battery to deliver comparable driving range. The battery is the more important factor in the whole vehicle cost.
Parker Hannifin has delivered motors for a variety of EV platforms, including small trucks for urban courier duties that deliver sufficient driving range for an entire working shift. A small car likely to be used for domestic trips to shops, school or similar venues can certainly have enough range to satisfy daily usage requirements.
Lowering drivetrain cost
If the motor efficiency and battery energy density available today can provide usable BEV range, reducing the overall vehicle cost is the next important target. The European collaborative project ODIN (Optimised electric Drivetrain by INtegration) emerged as a response to an EU desire to stimulate the EV market by establishing a low-cost EV drivetrain. The project has designed a drivetrain using a switched reluctance motor (SRM), designed for the Renault Zoe.
Barry James, CTO of consortium partner Romax Technology, explains. "The SRM provides a competitive alternative to permanent magnet (PM) machines, as it does not require any rare earth material, whilst achieving acceptable power density." A rare earth free solution is desirable, as available supplies of some elements such as neodymium and dysprosium needed for high-performing magnets are considered inadequate for PM motors alone to satisfy the anticipated total future demand for EVs.
To be suitable for EV traction applications, some characteristics of the SRM, such as high torque ripple and acoustic noise, must be addressed. "The NVH (noise, vibration, harshness) behaviour of the SRM is indeed a key challenge, particularly in a highly integrated design," James agreed. As a part of ODIN, Romax's simulation technology helped identify such issues early, allowing its engineers to propose design changes to mitigate NVH.
Elsewhere in Europe, Belgium based Punch Powertrain has successfully developed motors and control techniques, such as current profiling, that allow SRMs to perform satisfactorily. Current profiling can virtually eliminate torque ripple by adjusting the requested current in relation to rotor position. Punch Powertrain engineer Saphir Faid explains the company has developed a multiphase SRM featuring current profiling and torque sharing for a smooth torque characteristic with high efficiency. In addition, vibro-acoustic simulation contributed to minimising noise. This technology is used in the 100kW SRM of the company's ES2 integrated drive system.
As battery energy density and motor efficiency continue to improve, and work to extend affordability bears fruit, car buyers may begin to envisage electric motoring fitting into their lives.
