Slashdot Mirror
Inventor Demonstrates Infinitely Variable Transmission
ElectricSteve writes with this excerpt from Gizmag:
"Ready for a bit of a mental mechanical challenge? Try your hand at understanding how the D-Drive works. Steve Durnin's ingenious new gearbox design is infinitely variable — that is, with your motor running at a constant speed, the D-Drive transmission can smoothly transition from top gear all the way through neutral and into reverse. It doesn't need a clutch, it doesn't use any friction drive components, and the power is always transmitted through strong, reliable gear teeth. In fact, it's a potential revolution in transmission technology."
Several tractors I have owned have hydrostatic transmissions. These are also infinitely variable, but they use a hydraulic pump and motor to achieve it. They provide very high torque and excellent power transmission. I always wondered why they were never used in cars.
Actually electric motors have a pretty good efficiency over a wide range of power levels. It is ICEs that have a small band of optimal efficiency around a certain rotational speed. So, conventional combustion engines do profit most from this. Besides, electric motors have a rather flat torque curve, so you usually do not need a gearbox for them at all.
Ubi solitudinem faciunt, pacem appellant.
By "frictionless" I assume they're talking about something to do with the clutch, where you have two plates that you can jam against each other to transmit power via friction (and if you take them a little distance apart you they have a little bit of slip to them, so that during a gear change can the engine's speed will be smoothly met by the friction until it matches the drive-shaft's speed without any terrible lurch which would damage everything). This thing still has normal mechanical friction, as any set of gears would, but doesn't have any component explicitly designed for friction.
The World Wide Web is dying. Soon, we shall have only the Internet.
Reality is exactly opposite. Induction motors are very efficient through most of their operating range, while internal combustion is really only efficient along a narrow band of RPM, which is typically optimized to be highway cruise speed in high gear. With induction motors, they would merely allow for a much simpler controller, one that does not have to provide variable frequency power output.
Hate to reply to my own post, but here is a fairly detailed explanation of John Deere's IVT: http://salesmanual.deere.com/sales/salesmanual/en_NA/tractors/2006/feature/transmissions/8030_option_code_1127_1137_ivt_trans.html . The relevant part is "The John Deere IVT uses a hydromechanical, power-splitting design where a portion of the power is transmitted mechanically and a portion hydrostatically. A hydromechanical transmission is more efficient than a purely hydrostatic transmission because gears carry power more efficiently than a hydraulic pump and motor. By careful selection of the gearing, the John Deere IVT carries a maximum of the power mechanically both at normal field working speeds and at transport speeds, taking maximum advantage of the higher mechanical efficiency while providing the control and versatility of a hydrostatic." And of course this power-splitting is done via a planetary gear system.
I say this not to take away from the D-Drive's awesomeness (John Deere doesn't do reverse without shifting a gear), but to help offer explanations of how it actually works.
Yes, the behavior of this "transmission" should look familiar to anyone who has ever played with a differential while experimenting with Lego gears.
With a classic differential (the piece pictured here: http://en.wikipedia.org/wiki/Differential_(mechanics) ), there are four different things rotating, and their speeds are related. The equation is something like (A-B) = (C-D). The problem is that one of these rotating things is very hard to access mechanically - the inner bevel gear, whose axis of rotation moves as the casing of the differential rotates.
It seems like this device is equivalent to a single differential, with one small bonus which explains the additional mechanical complexity: all four rotating parts are easily accessible. There is a shaft coming out of each end, and two shafts exposed in the middle, whose axes of rotation are not moving and therefore motors can easily be attached.
This is a clever re-arrangement of a differential, but I don't really think it will lead to a super-efficient transmission because you still need a secondary motor which needs to be variable speed, and which will be subjected to a potentially wide range of torques. So it just introduces a new problem.
BTW, couldn't you do this sort of thing with a differential?
Yes. That's basically what the Prius does: it uses a differential (actually an epicyclic, which is a flattened differential) as a mixer, and drives one input with a gas engine and the other with an electric motor, giving not only an infinite number of speeds but also a way to use the engine to charge the motor with excess power, or use the motor for braking. But then you need both an engine and a motor. Managing an infinite drive from a single input is pretty cool.
Nostalgia's not what it used to be.
Actually, the transmission in the Prius is completely different from this. The Prius takes two full power inputs (the engine and the electric motor), and adjusts the power output from the two (balancing them) to achieve the end ratio. This takes a single full power input (and two factional inputs, perhaps a very small fraction if friction losses are small enough), and produces a variable end ratio. Quite a big difference between them. For the Prius transmission to work, both engines need to be of comparable power (A 100 hp gas engine would need somewhere near a 100hp electric motor). This would likely work with a 100hp engine and a pair of 1/2 hp (or less, depending on precision and friction) electric motors.
And FYI, an OTTO cycle engine is not most efficient at 2000 rpm. It's most efficient at its horse power peak RPM, and at full throttle. Anything less than that (RPM or throttle), and you lose volumetric efficiency. And when I say efficient, I'm saying the power/fuel use is the maximum. It's all about the intake and exhaust design (you can tune them for maximum efficiency at a particular RPM for a particular engine design). That's why hybrids typically use smaller engines. So that you can run it closer to its peak power for longer (40hp at full throttle would be plenty to cruise on the highway and still be able to charge the batteries without needing to be throttled back).
If a man isn't willing to take some risk for his opinions, either his opinions are no good or he's no good
I agree, the motors are not torquing against each other, that would be very inefficient.
But the control motor will be be subjected to torques related to propelling the vehicle. It doesn't just "turn the gear".
Example: Let's say the control shaft is rotating at a rate r. When the control shaft rotates faster, at rate 2r, that would be a higher gear (in other words, the output shaft would have higher speed and lower torque than the input shaft). If it's rotating at rate r/2, that would be an easier gear.
Now, with the control shaft rotating at r, let's say the vehicle experiences a reaction force (e.g. friction, or going up a hill). This torque against the output shaft will be transmitted back to the engine, obviously trying to make it go slower. But the control shaft is equally linked to the drive train. The reactive torque at the output shaft will try to slow down the control shaft (because slower rates, like r/2, are easier gears, and the system is continuous - there's nothing locking it into a gear). So in the same way an outside reactive torque places a load on the main engine, it will also place a load on the control motor.
Then finally at the end they showed the back and surprise, there's another motor there
They mention the electric motor 2 minutes in, and they constantly talk about driving the bottom shaft, implying you are providing some sort of power input. They didn't show the back of the device for a while because looking at an electric motor is less helpful than seeing the output when trying to understand how the thing works.
As they wrapped up the video they did admit that this little kink is going to be the determining factor in whether or not it's a useful design
They spent most of the video trying to explain how the device works, so understandably they get to the application stuff only at the end. He just showed the device working perfectly fine with an electric motor- you don't need to work out a continuously variable input from the main motor unless you really want to. As for the efficiency, the input power is exactly the main concern, but it sounds perfectly plausible for this input to require minimal power. As they mention, the electric motor isn't seeing any of the main motor's power, so the required power for it can be very small.
I agree vibration issues and robustness have yet to be seen, but the device is simple enough it should be feasible. Engineering this from a demo to a working transmission for a full-size motor can be as much work as developing it in the first place, so it may be a while before we see where this goes.
My webcomic
No, the most efficient point is at peak torque. That's where the engine is able to produce the most energy for a given amount of gas. The horsepower peak is where the engine is producing the most power (energy/time). It is not necessarily it's most efficient point unless they coincide which is rare.
Not true. Volumetric efficiency is measured as the the volume of air taken in on each stroke vs the displacement of the cylinder. So if 1 liter of STP (Standard Temperature and Pressure) gets drawn into the cylinder, and the dispacement of that cylinder is 1.2 liters, the total efficiency is 1/1.2 (or about 83%). At 0 RPM and 100% throttle (well, any throttle position that isn't completely closed), volumetric efficiency is always 100%. But as the engine starts turning (at full throttle, otherwise the vacuum drawn by the throttle restriction will reduce efficiency), the actual efficiency will depend on intake design. Considering that OTTO cycle engines use valves, air is only drawn in 25% (about) of the time. So the vacuum drawn trying to draw that air in will cause the efficiency to drop. However, intake runners are designed for this. So basically, when the valve closes, the momentum of the air causes a pressure build up behind the valve. That pressure will cause the air to reverse direction. This leads to a harmonic wave in the intake runner. The frequency of the wave is dependent on the design of the intake runner (cross-sectional area, cylinder volume and length mainly, but curves and other obstructions do play a part). If the valve opening is timed properly with this frequency, the incoming pressure wave from the harmonic will actually force air into the cylinder. That's how some racing engines can actually achieve a higher than unity volumetric efficiency at a specific RPM. It's all relative to the design of the engine. Some engines may be designed for 2000 rpm. And increasing the RPM over that WILL decrease VE. But you cannot say as a general rule that VE is inversely proportional to RPM, because it isn't. And pumping losses are directly proportional to VE (in fact, the pumping losses are DUE to VE below 100%).
You do have a point that thermal and mechanical losses do increase with RPM (Mechanical due to friction, thermal due to the increased movement of air around the parts). However, your reasoning behind diesels being more previlent is flawed. It's not because they operate at a lower RPM. It's because of a few reasons. First off, diesel is denser (energy/volume) than Gasoline while still having a similar stoichiometric ratio with air. Secondly, diesels are typically built without a throttle blade. That means that even at idle or lower power settings, there is no restrictive plate to draw a vacuum (and hence harm VE). Since diesel doesn't behave as bad as gasoline when run lean, they typically control power output by controlling the fuel flow. Third, diesel engines tend to burn much hotter than gas engines (the flame front is significantly hotter), so there is a more complete burn. You combine these effects, and you can see why they are more efficient (and it's not because they run slower). The reason that diesel engines typically run "slower" is two fold. First, since diesel engines don't use spark plugs, timing is controlled by the mechanical fuel injectors (direct injection). They were simply not fast and accurate enough to time at high rpm. The second reason, is that diesel is slower burning than gasoline. So at higher RPMs, there's a large chance that combustion won't be complete when the exhaust stroke starts (resulting is a large drop in efficiency and a large increase in mechanical stress).
If a man isn't willing to take some risk for his opinions, either his opinions are no good or he's no good
It's quite different from the HSD in that it has three inputs, contrary to what GP said - one power input, and two control inputs, both of which ought to require just a fraction of the input power to control the input/output gear ratio.
Real engineers disagree on the "inventor's" website:
http://infinitelyvariabletransmission.com.au/wp-content/uploads/2010/05/dDrive-Transmission-Report.pdf
"The torque provided by the Control shaft will typically be of the same magnitude as the torque provided by the Input shaft."
"The Control shaft (and associated mechanical elements) should be sized to this torque requirement
accordingly – the Input and Control should be considered as parallel power paths rather than as ‘power’
and a ‘control’ elements respectively."
So this whole thing isn't very useful. To add this as a transmission to a power motor, you need
one ore two additional motors of same power with variable speed and enough torque at any speed.
Excuse me? There plenty criticisms you can make of electric motors and the exotic materials needed for supermagnets, but radioactive is not one of them.
manuals [...] are a small percentage of cars
Nitpicking: that applies in the US. In a great part of the world it is the contrary. As an example, in France, driving school and driving tests are by default on manuals. If you take the test on an automatic, you get a license saying it is limited to automatics. The times I've bought cars, the dealers never even asked if I'd prefer automatics.
In other news, automatics have reputation of being less fuel-efficient and slow to kick in when you quick acceleration. Maybe that is no longer true, but the reputation sticks.
it may increase the life of the car/machine.
Sadly, this may kill the project.
It seems that manufacturers don't want to build things that last forever. Planned obsolescence is the current fashion.
Planned obsolescence because of new safety measures, new gadgets on-board and new designs yes. Because of breakdowns, no. Perception in the eyes of buyers about how reliable your brand of cars is, can kill the sales of any car if people think your car breaks down faster than other cars. Toyota sells so many cars precisely because they don't break down (or are perceived as more robust) as US cars. In fact, it's incredibly difficult to sell US made cars in Europe because of exactly this problem. So any car manufacturer who can make his cars more reliable, in whatever way, *while not heavily impacting the manufacturing cost*, will do so.
Therefore, by the (faulty) logic you're using, you're just a cow with a keyboard - osu-neko (2604)
Not for windmills, generators and the like, keeping those at a constant RPM can be more useful independant of input power. It may also prove useful in electric cars, as if you can keep everything always working at it's peak efficiency you can go further on less power. There are many situations where it would be better to have a constant RPM in and a variable out (or vice versa), the question then becomes are these as easy to work with and reliable as existing gearboxes. Planetary gears, work at approximately the maximum efficiency most of the time, these theoretically can work at maximum efficiency ALL of the time.
If they can be a drop in replacement for existing tech, then I can see this taking off, if not or they require complex setups/electronics it might be a more niche product. Also how efficient this tech is, in comparison with existing systems, as if the gearbox itself is only 75% efficient, this would rule out the benefit of having an infinietly variable ratio. Pulling some numbers out of my ass I believe gearboxes are some where in the 95-99% efficient