Tuesday, June 10, 2008

The Car of Yesterday Here Tomorrow

Saddle up auto technoids for an extended ramble. Gonna be long one so settle in. A while back I ruminated on the current and possible future design of hybrid vehicles. Key to the my suggested concept was a newish piece of technology known as the "wheel motor". Sometimes called the in-wheel motor, the tech has more than a little promise for several reasons. First is the fact that due to its basic design the need for a reduction geartrain, otherwise known as a transmission, is eliminated. The design stems from a technology that has been percolating for a couple of decades generically referred to as the brushless motor. A brushless motor is controlled entirely by electronic means and does not rely on brushes to switch current as do most regular electric motors. This reduces friction, enhances efficiency and extends motor service life. In addition for the last few years this technology has been added to a new type of motor design.

Typically the center portion of an electric motor is what rotates and sends torque to whatever device is connected. This design goes back to the infancy of motor technology and is about as "mature" as any technology can be. In the past decade a design was introduced that reversed the normal architecture of the electric motor. In the new design, sometimes called an "outrunner", the portion of the motor that rotates is the entire outside case which is then attached to the driven mechanism. This design does not increase the overall power output of the motor per se but it does drastically increase the torque that the motor can apply to a mechanism. Horsepower is torque times RPM so if the HP of an outrunner is the same but the torque is much higher then the RPM will be concommitantly lower. So what you say? Well this much increased torque in many applications is better matched to the real world needs of mechanical devices. A good example of the difference can be found in the world of electric model airplanes. Before the high-torque outrunner design became common electric models often required a reduction gearbox so that the engine could turn a larger propellor than it could otherwise and slow turning large props are considerably more efficient than fast turning small ones. The new design allows the motor to turn larger propellors at a slower speed thereby matching the powerplant far better to larger more efficient props. The motor is attached to the airframe at the stationary backpiece and the entire outer case and prop mounting assembly turn as a single unit.

The reverse approach of this idea can work very nicely as well An extreme example is the the small electric motor used to drive the main drum in a clothes dryer. The motor pulley is only an inch or two across but the thin rubber drive belt goes all the way around the two or three foot wide drum meaning that the "final drive ratio" is very large. In this way a small fast turning motor can properly drive a large drum that can be very heavy with 20 or 30 pounds of wet clothes. The large gear ratio turns the drum at the slow and deliberate speed needed and is not very sensitive to how much weight is loaded into it. A dryer could no doubt be built that used a "wheel motor" that was the same size as the drum and would be quite efficient but few are in the market for a clothes dryer that costs several thousand dollars. The object in either, or any really, case is to maximize utility and energy while minimizing cost. Advantage goes to the automoble in this case because the wheel motor would eliminate as much as several thousand dollars in mechanical parts.

Virtually all electrically driven car designs, whether pure or hybrid use conventional motors that work through more or less conventional geartrains. In the case of hyrids this is necessary because the electric motor must work in tandem with the internal combustion engine. Pure electrics have been few and far between but in general they have used some sort of transmission between the motor and the wheels because of the use of conventional motors either AC or DC. The new Chevy Volt design along with the ultra expensive Tesla Roadster eschew the use of a multi-speed transmission but do use a reduction gear train to take avantage of their motors' power characteristics.

The wheel motor designs that have been in developement for a while now are versions of the brushless outrunner design. The rotor of the motor, catchy huh, is essentially the inner surface of the wheel rim. Like the small outrunner designs this means that the wheel motor can develop far more torque than a conventional motor. This much greater torque and consequent lower RPM happily matches up with the real-world rotational speeds of a typical wheel/tire unit. The most salutary result is that no gearing is required anywhere in the system which means a very substantial weight reduction in the vehicle's design. No clutches, transmissions, driveshafts or final-drive gearsets needed--a savings of several hundred pounds at least with a healthy reduction in gear friction as a bonus.

Several manufacturers have explored such designs with varying degrees of success. One company that has attracted the attention of outfits as mainstream as Volvo and as quirky as the ZAP company is PML Flightlink which has a very promising product which appears to work well. There are inevitably downsides. One is the fact that no matter how efficiently the system is designed it will always add a significant amount of weight to a wheel. The weight of wheels, tires, and half of all the moving parts of a car's suspension is known as unsprung weight and too much of it can seriously affect ride quality. Wheel motors can easily double the unsprung weight of a given installation. Not so good. Another problem is the service life of the power cables that tranfer electricity to the wheel. They clearly are under a lot more bending and general mechanical stress than electrical wiring is normally. A third problem is how braking is handled. Some designs incorporate a small disc brake into the motor which works okay but complicates things and increases unsprung weight even further. Other designs dispense with a conventional brake altogether and use the car's computer controlled electrical system to provide braking. This supposedly works fine but would I think make the typical engineer quite nervous about just how to make this setup fail-safe. Two more factors are weatherproofing problems and cost effectiveness. Mitsubishi attempted to develop this technology a few years back but dropped it for reasons that likely include at least one of the above mentioned. Others are pursuing it though because of its obvious potential.

Despite the problems this technology has great promise for hybrid vehicles. Clearly there would be huge advantage in weight reduction and system complexity if the wheels could be driven directly by electrical current. Instead of the rather high complexity of overlaying an electrical drive system onto an internal combustion drivetrain the whole business could be simplified significantly if a small IC engine could be used to drive a generator which would provide not only power to charge a modestly sized battery but also to drive the vehicle directly. Add a plug-in recharge capability and we're really starting starting to cook.

The weight savings could be used for increased battery capacity and greater range or simply to reduce the weight of hybrid vehicles from their present porky avoirdupois. A much smaller IC engine could be used if its primary function was charging the battery and would also be used as a range extender. Since it only requires about 10 to 15 horsepower to maintain steady highway speeds in modern aerodynamic vehicles the engine, if sized correctly, could power the vehicle at safe interstate speeds even if the battery were completely depleted.

Obviously there would be losses incurred from engine to generator to wheel motors but with proper engineering they might not be much if any worse that what is experienced by typical mechanical drivetrains. The big payoff here would be two-fold. Due to the plug-in design many short trips could be managed without ever having the IC engine engaged. Naturally battery capacity would determine electric only range but with the weight reduction inherent in the use of wheel motors higher capacity battery packs can be used that would give an electric only range of say 20-30 miles which is easily twice what most hybrids can achieve. This would suffice for most commutes. The vehicle could either plug in while at work or the engine could run, in a very efficient range, to recharge the battery for the drive home. Longer commutes, and trips of essentially any length could be accomodated by the engine running to both recharge the battery and drive the vehicle directly for any distance that the fuel tank would allow.

I'm guessing that an engine of no more than 40 horsepower would be needed by for a small four seat sedan. In direct drive mode the top speed would be 80 plus and a steady 70 could be easily be maintained while charging the battery at the same time. I'm convinced that a small two door four-place sedan properly configured could achieve a minumum of 50 mpg on the highway and as much as a hundred effective mpg in city driving. Acceleration, starting, passing, and hill-climbing duties would be abetted by the battery. Long grades could be a problem but something has to give somewhere I suppose.

Even if wheel motors do not deliver on their promise the high-efficency design of the outrunner brushless motor could be engineered into an independent suspension and would power the wheels through small driveshafts. This would allow much lower unsprung weights and the use of conventional brakes. I feel sure that this is the type of hybrid we will see in the mature phase of the technology. What is available now is just interim stuff that manufactures have slapped together out of the parts bins as much as possible to minimize their investments. They will likely soon "go all the way" under the lash of escalating fuel prices. Whether or not wheel motor tech is used driving the car directly from the battery with an IC engine on board to charge the battery this is an architecture too good to pass up especially when combined with plug-in capability.

Pure electrics are going to fail to capture significant market share for a long time to come. They simply are too limited in their ranges and operating parameters for people to spend large amounts of cash on them in any kind of numbers. Pure electrics are unsuitable for almost any climate except the California coast. Both air conditioning and/or heaters will drain any battery system in a fat hurry. A pure electric would have diddly for range on either a below zero morning or a 100 degree afternoon. Having an IC engine on board gets you the AC and heat that is crucial in most parts of the country. The 40 horsepower engine I referred to above should let a small vehicle cruise at highway speeds, charge the battery for passing and hills and at the same time provide heat in the winter and air conditioning in the summer.

It's true of course that ol' Debbil' carbon would still around but it would be dramatically reduced for a high percentage of the vehicle's operation. There's no reason that the IC engine used in this architure should be any less techologically advanced than any other. In a high-tech ultra low emission configuration a 40 horsepower engine should not need to be larger than about .75 liter in displacement and could easily be only a three or even just a two cylinder design. The carbon footprint of such a vehicle is going to be at nearly irreducible levels in terms of real world solutions. The countless alternative fuel proposals and schemes that clutter the news are not only impractical or implausible but even worse they are impractical in economic terms even with gas going for 4 dollars a gallon.

Any alternative fuel proposal has to account for the vast scale of motor fuel usage in this country. Even the most absurdly optimistic proposals for alternative fuels are wildly inadequate to the task of replacing the 190 billion gallons used each year. Combined they are inadequate. Combined and octupled they are inadequate. If fuel usage were to collapse to half its current levels any combination of alternatives would still be hopelessly inadequate. It is desperately unrealistic to consider that a variety of different fuels will help. Fragmenting the motor fuels infrastructure to accomodate several competing fuels will dramatically elevate delivery costs and complicate vehicle design. My guess is that gsoline will have to rocket above 10 dollars a gallon to cause a serious stampede to alternatives. Even if the economics start to work for alternatives the volume and distribution problems will remain.

There is little inherent reason why this proposed architecture could not be scaled to any size vehicle although in the case of big trucks the cost differentials may be so extreme that it will not be commercially viable. The steady speeds and lengthy trip profiles of big cargo trucks are in an operational regime least likely to result in serious benefit from this architecture or any other hybrid design for that matter.

Even as we speak fuel prices are starting to improve overall fleet mileage. Economic incentives are always more powerful than government mandates. It's going to hurt. The economy's adjustment to permanently higher fuel costs will take time and will have turbulent political consequences. It happened in the early 80s and we survived but it wasn't much fun.


Post a Comment

Subscribe to Post Comments [Atom]

<< Home