I was absentminded pondering the future of the car industry and reading up on new cars and new car technologies in lieu of me actually having the money to treat myself to a new car. While doing so the engineer part of my brain was assessing the state of current hybrid designs and it seems like a pretty inefficient and clumsy solution that is in place now. One would think that creating 4 independent systems which electrically drive and brake each and every wheel all drawing power from a central gas-electric/battery powerplant would be ideal.
Pros:
[ul][li]Eliminates the weight of an axle, transaxle or driveshaft and all the associated joints and gears. [/li][li]Provides true All Wheel Drive. [/li][li]Opens up the center of the car for other uses[/li][li]Would be a more modular system. [/li][li]Fewer moving parts[/li][li]Less energy lost to friction and rotating heavy steel axles. [/li]
[/ul]
Cons:
[ul][li]Unified drive and regenerative braking systems may not be available yet.[/li][li]Suspensions without axles to anchor to would be new.[/li][li]The weight of 4 electric motors as opposed one or two in a traditional hybrid.[/li][li]Loss of overall rigidity of the frame.[/li][/ul]
So I guess the question I’m asking is if this type of system is in the works anywhere or if it’s been tried yet. Am I overlooking any potential roadblocks to making something like this work? How complicated would it be to make a electric motor that could drive a wheel and offer regenerative braking in one unit? In short, why wouldn’t this be the direction the industry goes?
Since Mr. Rose can only be in one car at a time, I would think that most current hybrids are without Axels.
[quote=“Omniscient, post:1, topic:495624”]
Pros:
[ul][li]Eliminates the weight of an axle, transaxle or driveshaft and all the associated joints and gears. [/li][li]Provides true All Wheel Drive. [/li][li]Opens up the center of the car for other uses[/li][li]Would be a more modular system. [/li][li]Fewer moving parts[/li][li]Less energy lost to friction and rotating heavy steel axles. [/ul][/li][/quote]
[ul]
[li]You are adding the weight of several more motors[/li][li]Yes, so? [/li][li]A FWD car already has the center of the car open[/li][li]Yes, so?[/li][li]I would disagree, it would have more moving parts, 4 electric motors instead of one[/li][li]Are you forgetting the rotating mass of the motor?[/li][/ul]
[ul]
[li]Yes, but this could be overcome[/li][li]Suspensions don’t need axles to anchor them.[/li][li]Huge disadvantage. In racing pounds is horsepower, in street cars pounds is gas mileage. [/li][li]First off most cars don’t have frames, and secondly why would the body be any less rigid?[/li][li]You forgot the biggie. COST[/li][li]Did I mention cost?[/li][/ul]
The three biggest reasons this isn’t practical are cost, cost and cost.
Motor at each wheel? Check. Suspension? How about a double horizontal wishbone with upper and lower torsion bars and a damper unit between the chassis and upper wishbone.
I was careful to get that right in my OP and overlooked the title. Figures.
Well, it seems possible and likely that the weight of 4 small to medium sized motors would be substantially lighter than the combined mass that the transmissions and axles in the car. This is good for gas mileage, which is the whole point of hybrids.
AWD cars seem to be pretty desirable options, especially in the vast majority of the populated US which deals with harsh winters. No other hybrid is likely to be AWD using the systems in place now because it would cost too much in weight. As cars get lighter and lighter, improving winter weather utility will be more important.
A FWD car still has to leave space between the wheels for the transaxle. The rear also has an dead axle which restricts the use of the rear for cargo or extra seating. Axles take up a lot of space, modern cars have gotten good at working around it but it’s contributed to larger, heavier cars.
Modular systems are cheaper to maintain and should be cheaper for manufacturers to port to other platforms. It would reduce platform engineering to component engineering. This happens with most parts of the car already, but drivetrains seem to be less transferable because they are usually too heavily tied into the rest of the cars design.
Electric motors are powerful, small and simple. Adding a regenerative braking system would increase the complexity, but they seem like they’d have fewer moving parts and mass than a 8 foot iron drivetrain, CV joints, rotors, transmissions and god knows what else.
4 small motors might or might not be more efficient than 1 large motor putting out the same work, and when you factor in the structures to distribute the power to two or four wheels it seems like it would certainly be more efficient. This is a question I’d like to see analyzed.
I suppose suspending a car with the weight of 4 modest motors at the corners would require a different system than currently in use. Independent suspensions probably wouldn’t need to change much, but dependent ones like a leaf springs would certainly be deprecated. This is probably a good scenario since independent suspensions seem to be preferred anyways.
You seem to be presuming that 4 small electric motors is dramatically heavier than one big one with axles and all that goes into distributing power to the various wheels. If this is necessarily the case then my idea is moot since the biggest presumed benefit of my premise is that my solution would be significantly lighter. If the two end up being similar in weight then that changes the debate somewhat.
That’s not a very nuanced answer. Cost always plays a role and new technologies always have a steep cost-benefit hill to overcome, but they do. Hybrids today aren’t cheaper to make or own than a traditional car of comparable class. The green revolution is driving many people to pay a higher price for a smaller carbon footprint and the tax code is trying to help them do so.
The question I’m asking is not if this type of system would be cheaper than the existing systems. The question is if it would, in the end, be more efficient. If the system is lighter and uses the available power more efficiently then the cost equation will eventually work in it’s favor. The cost of developing it now is of secondary concern, the Volt is a perfect example, economically that thing is a disaster in the short term.
**Would a system like I describe be lighter than a traditional hybrid?
Would a system like I describe deliver power more efficiently than a traditional hybrid?
Would a system like I describe allow cars to perform better on the track than a traditional hybrid?
Would a system like I describe allow designers to use the space of a car more aerodynamically and flexibly? **
All different questions that I think are valid and worth asking.
[ul]
[li]Is there still a big weight penalty with 4 small motors compared with 1 large one?[/li][li]All wheel drive is a big selling point, particularly in areas that get snow. If power could be transferred to different wheels without using the brakes or clutches it would be an advantage. Also in some cars stability control works by transferring power to certain wheels.[/li][/ul]
I am not sure exactly how a hub motor is mounted but from what I have seen having more unsprung weight is bad for performance. Unsprung weight is what the shocks/springs have to control, the rims, tires and brake rotors on most cars.
I wasn’t really picturing a motor inside the hub. I was envisioning a motor with a direct linkage to the wheel but was inside the body of the car. Though, if a hub motor were to provide a advantage over that without increasing the complexity to a unreasonable degree it’d be an option.
I had a beautiful point by point rebuttal all typed up and the hamsters ate it. GRRR.
Any here is the Reader’s digest Condensed Version:
Rick’s iron clad laws of the auto business:
[ul]
[li]The way to make a million $ in the car business is make $1/ car and build a million of them.[/li][li]The greatest technology in the world is worthless if it costs so much nobody buys it.[/li][li]If the incremental cost of an improvement far exceeds the benefit it will not be a success.[/li][/ul]
Right now hybrids are nailed to the showroom floor, they are not selling. Raising their price will not help this situation.
Oh, and one more thing. Leaf Springs? are you serious? Stop it you are killing me. Leaf Springs? Dude, go look under some modern cars and find one single car designed in say the last 20-30 years that has leaf springs. (truck based SUVs excepted)
Leaf springs are so 1940.
go look under a modern car.
To the OP: Biggest problem is that you actually need a transmission no matter what. You need to cut the motor’s high speed down to what’s the best rpm (and torque) for the wheels. So now instead of zero or one transmissions, you have four. (You could technically do it with no transmissions, like on the moon buggy, but it’s inefficient… like a 1-speed bike.)
Well, leaf springs were just one type that came to mind, I suspect there are other common dependent ones out there that attach to the axle…and if SUVs and trucks are using them is it really that baffling? It’s a small point in regards to my OP but the snark seems kinda silly.
You seem to be missing the point. Hybrids are on the market and every auto company has plans for one soon. The new ones will gradually become cheaper to buy and cheaper to operate. Right now, with gas reasonably priced, they aren’t really much cheaper to consumers than a conventional car. Are you really claiming that the entire hybrid market is a boondoggle?
Oh, and just about every car is nailed to the showroom floor right now. Hybrids aren’t an exception to that rule.
Again, I’m confining my question to this: Why would my design be less effective/efficient than the current one for hybrids? I’m not particularly interested in turning the thread into a GD discussion on the viability and sustainability of the hybrid and electric car markets.
I will say this, your last ironclad rule has some serious issues. There’s a metric shit-ton of stuff on cars these days whose cost far outweighs it’s benefit. Backup cameras, Volvo’s intruder heart monitor thingy, rain sensing wipers, headlamp wipers, headlamps that steer, HUDs, warming and cooling cup holders, automatic liftgate controls, etc. People love their stupid, useless, expensive bells and whistles. And people REALLY love to feel like they are ahead of the times and living in the lap of luxury. These things are at least as esoteric as “going hybrid”.
If it were a pure electric drive, then separate motors can be a good solution. Remember, four separate motors can be smaller than one huge motor, and cost less individually. Electric motors also have a huge amount of torque, and don’t have any specific need for a transmission.
However, packaging is likely to be a problem unless the car is designed from the ground up as an electric or hybrid car from the beginning, and this is where things get really and truly expensive. Automotive platforms are ridiculously expensive and take a long time to develop, and with such a small market demand, there’s the possibility that it’s not cost justifiable. The closer-to-traditional approach employed by all current hybrids means shared platform characteristics, and drastically less up-front cost.
I don’t see this technology coming anytime soon to passenger cars though. Even if all other problems already mentioned are solved, there still remains the problem of increased unsprung weight. To put it simply, the extra weight at the wheels will make the car ride and handle like a wooden horse-drawn buggy. It doesn’t matter if the overall weight of the car is the same or even lower than a car of conventional design, it is the weight at the wheels that counts.
The idea of going without axles (and transmission) has indeed been introduced in several car concepts, such as the Volvo ReCharge plug-in, none though that have production schedules that I know of. The argument for as put out in my link is
The argument against that I’ve heard the most is that the in-wheel motors (which are how that concept is discussed) are unsprung weight - and low unsprung weight is key to good handling.
Most SUVs are car-like bodies screwed onto a light truck chassis. More and more newer SUVs are “de novo” designs and are being built more like oversized cars, as opposed to smallish trucks, so eventually, the leaf springs and fully-boxed ladder frames that are important to trucks will disappear from SUVs.
As for placing a drive motor at each corner of the vehicle, look to a typical subway car for the general mechanical and electrical design. Massive unsprung weight and the handling characteristics of well, a subway.
If the electric motors are inside the body and not hub mounted you would still need the drive shafts and associated CV joints. So at least part of the weight you are trying to save would still be there.
Independant motors at each wheel isn’t necessarily a bad idea (in fact, I’m partial to them myself), but like anything engineering-related, just how good or bad of an idea it is depends on how you weight the trade-offs.
The most important disadvantages have already been mentioned.
To reduce the complexity the farthest, you’d want to mount each motor and associated speed reduction gearing directly on the wheel, which results in unpleasantly high unsprung weight. You can get around that by mounting the motors on the chassis and installing a shaft to the wheel, but now you’re adding back complexity–so much so that you might as well combine each pair of motors and gearing into a single motor and a differential.
As well, motors tend to be more expensive than fairly simple mechanical parts like axles, so I’d wager that four wheel motors would cost more than a single larger motor and the associated shafts. I’m willing to be wrong about this, as production volume costing is not my area of expertise, but I don’t think I am.
That being said, there are also a few advantages of the idea. From an efficiency point of view, reducing the complexity of the coupling and gearing connecting the motor to the wheels would likely increase the efficiency of the system, albeit only slightly. More importantly, being able to switch from two- to four-wheel drive would likely allow you to more optimally distribute the torque requirements among the motors and use them more efficiently. Note I say “likely,” as there would also be some penalty for dragging along unused motors when you’re not actaully using them. The magnitude of the efficiency gain (or loss), I suspect, would depend heavily on what vehicle you’re using it in and how you’re driving it.
The other couple advantages I can think of right away are increased ground clearance and better power delivery on slippery surfaces. And there, I think, lies what might be a practical selling point of a system like this. People who buy Priuses are probably not going to buy a wheel-motor car for it’s fraction-of-a-percent higher efficiency. People who buy Jeeps, though, might be willing to pay more (and put up with some downsides) for what they perceive as greater value.
