Electric issues01 November 2011
Analysts now agree that the level of vehicle electrification will increase sharply in the years to come, confronting engineers with an entirely new set of challenges
The Chevrolet Volt is a familiar sight and a common topic of conversation in North America; start-up automakers Fisker and Tesla are making headlines with their battery-powered sedans and sports cars, and, in the US, Europe and Asia, the Nissan Leaf is on ordinary buyers' agendas as a plausible small car choice. There can no longer be any doubt about it: electric cars have now made that crucial transition from speculative motor show stand candy to serious showroom propositions. The engineers' promises and the designers' visions are being put to the test in the real world by real consumers.
It could prove to be the biggest technology shift the auto industry has ever experienced and its ramifications will be felt at all levels of the business.
Take the professional engineering community, for instance: the lead debate in this year's SAE Global Drivetrain Virtual Summit on December 7 (www.sae.org/ events/globvirt) is billed as "adjusting drivetrain design for EVs and hybrids," with further topics such as understanding environmental performance also high up the running order.
At least half a dozen major automakers have committed themselves to selling battery-powered models in volume from next year onwards. The industrial muscle is clearly in place and the Renault-Nissan alliance, intent on leading the global market for electric cars, is installing manufacturing capacity for 500,000 traction batteries a year.
Suppliers, too, including Getrag, ZF, GKN, Schaeffler, Zytek and many others have extensive ranges of dedicated electric drivelines, and electronics specialists have evolved increasingly sophisticated energy management control systems for this new genre of vehicle. It was originally thought that battery electric vehicles would be electronically complex, but mechanically simple; the motor, with full torque available from zero rpm, would drive the wheels directly, without the need for a clutch or gearbox.
Yet, says Lubrizol's driveline strategic technology manager Richard Vickerman, this impression is misleading: "In fact, there are a lot of different technologies involved in transmitting the motor torque to the wheels and those technologies seem to be changing rapidly."
"The way I see things now," he explains, "many automakers use current transmission technology, but with the addition of the electric motors inside the unit. Others, Toyota for example, are using specialised transmissions. In both cases, they are using the fluid to cool and to lubricate the electric motors."
So far, he observes, it has worked out well—but future developments may pose new challenges, in terms of lubrication and cooling. "As time goes by, the nature of the electric motors will change. For greater efficiency, they soon will run at higher voltages and rev up to around 14,000 rpm, from about 6000 rpm now – and then will require more than one gear ratio between the prime mover and the wheels."
Just how many gears will be needed is another interesting question. "It's great that you can get torque from almost zero rpm from an electric motor, but they, too, have their operating envelope where they're at their most efficient.
"Combustion engines are getting more gears or an infinitely variable ratio and we're going to see more of that with electrically driven cars."
In recent months, two key electric-car transmission developments have been revealed. UK specialist Vocis has installed its prototype two-motor, multi-speed EV transmission
on the high performance French Exagon supercar concept, and Getrag has shown a two-speed, twin-clutch, electric design that allows seamless shifting – just like a combustion engined,
The move to multiple speeds may, says Vickerman, allow manufacturers to make motors smaller and to keep them at the optimum point in their power curve. His view is echoed by Mike Everitt of Vocis, who says that though the Exagon transmission is a four-speed, the concept is "eminently scalable."
"It's set for four speeds at the moment, but it could easily be three or six," he says. "The smaller electric motors are very helpful, in terms of packaging."
New component families that will require attention of lubrication specialists include:
•In-wheel motors, as developed by Michelin and Continental. These combine suspension, steering, braking and traction, and require only electrical and cooling connections to the chassis.
•Electric axles will soon become popular as a quick route to hybridisation and all-wheel drive (AWD). A combined electric motor and differential are housed in the rear axle, while a conventional engine powers the front.
•Range-extender powerpacks are
off-the-shelf modules, integrating a combustion engine, possibly even a Wankel rotary, with a generator. They allow battery-electric vehicles to perform as plug-in hybrids.
•Electric drive modules, such as those fitted to the Mercedes SLR AMG e-cell electric supercar, are more sophisticated. One motor powers each wheel, allowing differential and torque vectoring functions to be carried out by regulating the speed and torque of each motor.
•Crankshaft-mounted motor-generators are axially compact motors that replace the flywheel or torque converter within the transmission envelope and provide full hybrid functionality.
•Belt-driven externally mounted starter generators replace the standard alternator, harvesting energy under braking and restarting the engine in stop-start cycles. Some can provide a limited degree of launch assist.
•Electric motors pose no real issues, but newer designs running at higher rpm will require integrated step-down gearboxes to suit desired wheel speeds.
•Single-speed gearboxes are fitted to most EVs, but the chosen ratio is a compromise between low-speed drivability, efficiency and maximum speed capability.
•Multi-speed or variable-speed transmissions may become necessary, in order to raise maximum speeds, reduce motor rpm for greater range and improve energy efficiency.
Looking into conductivity
If there is one issue that could be a future concern when designing fluids for use with electrified vehicles, it could be that of electrical conductivity. Naturally, Lubrizol is already on the case, as Richard Vickerman explains:
"Clearly, from the point of view of oil technology, as transmissions become more and more sophisticated, the demands on the lubricant increase. If clutches or push-belt CVTs come back into the mix, the performance required from the fluid needs to adjust accordingly and the value received by the customer from a high performance fluid will continue to be significant."
Yet, adds Vickerman, the one big difference between a combustion-engined vehicle's driveline and the equivalent in an EV or hybrid is the presence of high voltages. "What happens in a vehicle, if the voltage goes way up? It might cause a problem, as conductivities aren't zero. That's why we're looking into the fluid's conductivity properties and compatibility with insulation materials to see if there could be a problem in this kind of environment."
Efficient vehicles are coming, be they high-efficiency internal combustion engines or some kind of electric powerplant, concludes Vickerman. "There's no one technology that will cover all— automakers will ultimately do their own thing. There will always be lubricant challenges as they design new types of systems. In ten years' time, we may have completely different technologies that we wouldn't recognise today. It's all going to get more complicated. The numbers of different fluids will increase—and we are already on the case."
To examine this topic further, join Lubrizol at the 2011 Global Virtual Summit.
A Free Online Event
Live: December 7, 2011
Can't Attend? Watch On Demand, starting December 9, 2011
Register Now! http://www.sae.org/events/globvirt/
The Lubrizol Corporation, a Berkshire Hathaway company, is an innovative specialty chemical company that produces and supplies technologies to customers in the global transportation, industrial and consumer markets. Technologies offered through our Lubrizol Additives business segment include lubricant additives for engine oils, other transportation-related fluids and industrial lubricants, as well as fuel additives for gasoline and diesel fuel. Lubrizol's industry-leading technologies in additives, ingredients and compounds enhance the quality, performance and value of customers' products, while reducing their environmental impact.
With headquarters in Wickliffe, Ohio, The Lubrizol Corporation owns and operates manufacturing facilities in 17 countries, as well as sales and technical offices around the world. Founded in 1928, Lubrizol has approximately 7,000 employees worldwide. Revenues for 2010 were $5.4 billion.
The Lubrizol Corporation
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