Electric Racing

[Admin]

 
Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

Considering how often racing has shown the way in vehicle technology, I was interested to read a bit about auto racing’s Formula E. As in any other battery-powered activity, the battery’s considerable weight is an issue, but Formula E has made that a special area of focus. Instead of just repeating the tired old (30-plus years) phrase, “Battery technology is advancing so fast now…” F-E has turned away from battery technology itself (after all, the li-ion idea dates to 1975 and Sony commercialized it in 1992). So what is it focusing on? Energy recycling.

It’s All About the Brakes

Back in the 1930s, when Germany’s Mercedes and Auto Union fielded roadracing cars powered by 600 hp V-12 and V-16 engines, braking was an infant technology, still based on gigantic drums. It was normal for the GP cars of that time to be pretty much out of brakes by half distance, relying on downshifting and tire scrub for their deceleration. In the final months of World War II some new aircraft designs used greatly superior disc brakes (for example, Grumman’s F8F Bearcat fighter), but it took until 1953 for discs to reach racing cars, with Jaguar at Le Mans, and until 1969 to trickle at last into motorcycling.

Regenerative braking can range to electric vehicles, but we have yet to see it reach its potential in motorcycle racing. (MotoGP/)

Conventional brakes, discs or drums, turn a vehicle’s unwanted kinetic energy into heat, which is then rejected to the air—a complete waste. Ideally, brakes should recover that energy and store it for reuse. In its effect, regenerating braking energy effectively increases vehicle horsepower.

Regenerative Braking

The first two generations of F-E race cars were driven by single motors driving the rear wheels. Because it’s easy to switch electrical machines from motors into generators, those earlier cars could recover some energy from their rear wheels during braking.

But now consider how a motorcycle differs from a car. The bike’s wheelbase is much shorter, a bit over half as long. Its center of mass is quite high up, 20 inches or more above the pavement. During braking, those two numbers add up to such a large rear-to-front weight transfer that if motorcycle designers want to regenerate braking energy, they’ll have a hard time doing so for the same reason it’s so easy to drag the back tire with the rear brake. That is why engine-braking control is essential for today’s fast, capable motorcycles; without it, uncontrolled engine-braking will drag the rear tire, causing the bike to run wide into corners or producing rear-wheel hop.

This tells us that trying to regenerate braking energy from a motorcycle’s rear wheel alone is asking for trouble. And lo, we see that third-generation F-E cars have two motors—one at the front, one at the rear—to allow regeneration of some serious braking energy, namely that collected at the front, which as on motorcycles does most of the stopping.

Crunching the Numbers

When energy is getting used at much-reduced rates around town and at civilized speeds, rear-wheel regeneration can be workable. But now consider a MotoGP bike, braking from its maximum speed of 220 mph, 320 feet per second. Rules require that the bike weigh a minimum of 357 pounds, to which we add full fuel at just over 35 pounds, plus a rider and maybe 30 pounds of protective gear for an all-up weight of maybe 570 pounds. If we look at Brembo’s MotoGP site, they show that these bikes can brake at 1.4 G, or a rate of 45 feet per second of velocity change, per second (1 G is 32 ft./sec./sec.).

Let’s convert that force (1.4 times the all-up weight) times that velocity (320 ft./sec.) to get the maximum rate of braking energy in foot/pounds. Then we can divide that by 550, which is the rate in ft./lb./sec. arbitrarily defined as one horsepower. We get 320 x 570 x 1.4 = 255,000 ft./lb./sec., and dividing by 550 gives us “peak braking horsepower,” which is 464 hp.

Related: Honda’s New 2WD and Regen Braking Designs for Bikes

Hard braking can produce a tremendous amount of power at the front wheel. Harnessing that power is the engineering challenge. (MotoGP/)

This tells us that if we want to regenerate a serious amount of braking power from a motorcycle, we’re going to have to collect that energy from the front wheel, using a device capable of handling 464 hp. That leaves out the dinky No. 35 chain used on the front wheel of the 7 hp, two-wheel-drive Rokon Trail-Breaker. About the only energy converter even remotely capable of handling this at present would be one of the latest high-torque electric hub motors, operating as a generator.

Battery Types

Now let’s consider the battery required for this kind of vigorous braking-energy regen. We know that the general lithium-ion “umbrella” in fact encompasses a multitude of electrode chemistries, each with its own strengths and weaknesses. It turns out that if we single-mindedly seek the highest possible kilowatt-hours per kilogram of energy-storage density, we find that certain requirements come with it, such as accepting limits on how fast it can be charged or discharged, or that either overcharging or running the battery down below 15 percent charge can reduce cycle life or even cause permanent damage.

Density or discharge? Pick your poison. Ducati has focused on energy-storage density for its 1152-cell pack that has a 18kWh capacity at 800 volts for its V21L MotoE racer. (Ducati/)

If instead we seek tolerance for rapid charge/discharge (as needed for regenerating braking energy during racing or hard sporting use) we find we must give up a considerable amount of energy-storage density.

Coping With Battery Heat

Another question has to do with waste heat generated during charge/discharge. Lithium ions (those that have lost one of their three electrons, thus becoming positively charged) can find a great many sites in the commonly used carbon anode, but getting them to those sites encounters resistance, and resistance causes heating. This resistive heating is proportional to the resistance in ohms times the current squared. An alternative anode construction has an open spinel crystalline structure that offers much less resistance to the in or out movement of Li ions—but since that spinel structure is not a natural conductor, it needs a carbon coating to make it conductive. In 30 years of research, man has discovered no li-ion “free lunch.”

Power tools need to be charged and discharged a great many times. One of the electrode chemistries chosen here is lithium iron phosphate. Yet even though such lower-resistance electrodes generate less heat, the very high currents generated during race-car regenerative braking still push a lot of heat into the battery cells. If not continuously removed, as our lungs constantly reject our excess heat to the air we breathe, enough of that heat will cause the battery to self-destruct.

In F-E racing, the battery cells are therefore given a flat “pouch” design, and are interleaved with thin plates of highly heat-conductive aluminum to make compact stacks of hundreds of cells. The outer edges of the aluminum plates are joined to a sort of “heat bus bar” that can be compactly water-cooled, carrying away heat generated in the rapid charge/discharge regen cycles at very high power.

Why so many cells? In order to limit the amount of resistive heating in the traction motors, the higher the voltage, the lower the current for a given power (wattage) and the lower the resistive heating. So, with each cell giving from 2.5 to just over 4 volts, to produce 800 volts we need a series-connected string of 200 cells or more.

Why not even higher voltage for even lower resistance loss? The wire insulation in motors can only stand so much voltage without breaking down.

Recent Developments

Recent high-torque electric motors are interesting because they are now both lighter and more compact than internal combustion engines of equal power. Much of this change has come from developing very powerful permanent magnets and from designs that produce short, powerful paths for magnetic flux. Conventional motors use a radial-flux design, with the stator magnetic pole-and-windings assembly surrounding a cylindrical rotor. Axial-flux motors make better use of their diameter by giving the flux gap a disc shape that places magnetic flux at a larger radius, thus increasing torque. So-called “transverse flux” motors surround the thin disc of permanent magnets with something like a continuous 360-degree ring of brake caliper—the stator with its circular winding surrounds the outer diameter of the disc rotor.

It is the very high torque of recent motor designs that makes it possible to consider hub motors. On a race car, with its constant accelerating and turning and the ever-changing loads on each wheel, each motor could be continuously made to deliver torque in proportion to the load it is carrying.

Some motor designs lend themselves more easily to cooling than others. Transverse-flux motors have a very simple winding that could be easily cooled by including liquid-cooling lines in the wire bundle.

A lot is going on in electric-vehicle design. Do motorcyclists care about this? That remains to be seen. Some traditionalists want nothing to do with electric, but some electric advocates seem to limit their interest to the ideological side: “We’re saving the planet, but you gearheads are still filling the air with carbon dioxide.” I’m hoping there can be electric hot-rodders and performance enthusiasts, just as there have been for the 137 years of vehicles powered by internal combustion—hands-on enthusiasts with real understanding.

Mea Culpa

Meanwhile, electric traction advocates must temper their ardor with the knowledge that in 2021 about 60 percent of US electricity was generated by fossil-fueled generating stations. Civilization has now and will far into the future have serious environmental costs. None of us can be without sin.

View the full article