Video: Is Steve Durnin's D-Drive the holy grail of infinitely variable transmissions?
By Loz Blain
May 13, 2010
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 - it could be pretty much the holy grail of gearboxes... if only it wasn't so diabolically hard to explain. We flew to Australia's Gold Coast to take a close look at the D-Drive - and it looks to us like Durnin has pulled a rabbit out of his hat. Check out the video after the jump and see if you can work out if there's a catch.
Geared transmissions - a useful compromise
In basic terms, the idea of a gearbox is to create the ideal level of mechanical advantage between a motor and its output. Motors of all kinds have a speed of maximum efficiency, and a speed of maximum power, and you use a gearbox to decide what engine speed you're running compared to the output speed.
In a car, that means you want a low gear at low speeds or for quick acceleration - because in lower gears, the engine revs harder and produces more power. Cruising on the freeway, you want a high gear that lets you trundle along using the minimum practical engine RPM so you don't waste fuel.
So most gearboxes offer a compromise - manual, semi-auto and auto transmissions offer you a set number of gears you can choose to find one that's close to the ideal ratio for what you're doing. But there's efficiency losses in between gearshifts as you disengage the engine using a clutch - or in the case of an auto, a torque converter. And although some geared transmissions offer lots of gear choices, every set gear ratio is a compromise.
And the clutch itself is a fairly crude device - when you're engaging a clutch, you're basically pushing a set of plates together, some of which are coated in high-friction material, which grab the other plates and force them to spin. This approach is inefficient and prone to slip and wear under large power loads.
Variable transmissions - very good, but not quite
Then there's Continuously Variable Transmissions, or CVTs. The CVT is in theory a much better solution, because it allows a constant range of gear ratios between low and high gears. Scooters use them, as do some cars nowadays - with a CVT, the engine can sit at its most efficient or powerful RPM, while the gear ratio constantly adjusts itself to match wheel speed.
But most CVTs have a limited range of ratios they can work through - so while you can transition all the way from low gear up to high, you can't go all the way down to neutral. So they need to use a friction clutch or torque converter to get them started from a standstill - and what's more, in order to achieve variablilty in the gear ratio, they're almost always built around some sort of friction drive too - like belts pulling on conical rollers, or rollers being mashed against toroid shapes.
All these friction components cause troubles when you start trying to put high power and torque through them - they start to slip and fail, they wear and generally contribute to inefficiencies in the drive train. That's why you tend to go back to gears when you're designing a high-powered machine. Gear teeth are reliable - the bigger the teeth, the more power they can handle.
The D-Drive - infinitely variable, no friction components
If all this gearbox talk seems like a long setup, it's kind of necessary to understand the problem when you're looking at the solution Steve Durnin has come up with.
Because at the heart of it, what Steve has managed to do is create a gearbox that:
But how on Earth do you obtain infinite variability using gears? After all, a cog's a cog - it's not like you can make them magically grow and shrink in size.
The answer is that you've got to stop thinking about gear sizes, or cones and belts, or any familiar transmission picture you have in your head, when you're talking about the D-Drive.
Because when you look at it, the only way to tell what sort of ratio it's in at a given moment is to look at the two spinning shafts in the middle of it. If the bottom shaft is still and the top one's turning, you're in top gear. If the top shaft is still and the bottom one's turning, you're in reverse. If the top and bottom shafts are spinning at the same speed but in opposite directions, you're in neutral. And you can speed up or slow down those shafts as much as you like to vary the gear ratio to any point between full speed reverse and full speed forward.
You really have to watch the attached video to start to understand how this gearbox works - but in essence it's built around planetary gear systems at either side, with sun gears, planet gears and revolving ring gears all interacting with one another.
The energy efficiency equation
In order to control the spinning speeds of the upper and lower shafts, you have to input a certain amount of energy - for instance, to put the D-Drive transmission into neutral, you have to spin the bottom shaft around at a speed that equals the speed of the driven top shaft.
But according to Steve and his engineers' calculations, the energy you put in to do spin that bottom shaft is only a tiny fraction of the energy your main engine is running. All that energy has to do is to spin the planetary gears around one another in such a way as to effect the final ratio.
And you can do that in a number of ways. Steve's current demo prototype uses electric engines both as the input engine and to spin the control shafts as needed.
But, taking the example of using the D-Drive in a car, you could easily use an auxiliary electric motor to control the gear ratios, or a kinetic energy recovery system, or some sort of regenerative braking system. You could even harvest energy directly from the driven shaft and use it to spin the control shaft.
Steve's prototype is only sufficient for demonstration purposes - and you'd have to question how effective a demonstration it is when just about everyone that looks at the thing is left scratching their heads and wondering 'er, so how exactly does that thing work again?'
The next step - building a test rig
Durnin is currently in the process of raising funds to build a test rig - a strong, metallic rendition of the D-Drive with the ability to measure how much energy is going in at the input end, what's coming out at the other end, and how much power is being put into the control shafts - but he and the engineers he's consulted are confident that the D-Drive will be proven to be "an order of magnitude more efficient" than existing gearboxes.
The implications are pretty huge if he's right and the numbers come up looking good; as a geared system, the D-Drive is scalable in the extreme, and could remove the need for friction components or manual gearboxes in everything from cars, motorcycles, trucks, industrial and farm equipment, massive marine applications, wind power generators... basically anything that's got an engine.
Because it's all gears and bearings, reliability should be excellent and servicing or repairing the D-Drive a snap. Because you just need to spin (or lock) those control shafts to come up with your final ratio, you could use anything from a fully computerized smart control system to a manually applied pin through the control shaft to change your gear ratios, making it useful in certain very low-tech situations as well as extremely tunable in an automatic automotive application.
About the inventor
Steve Durnin is a plumbing inspector from Queensland, Australia, who has been tinkering with the D-Drive and several other ideas for more than 20 years. The "D" in D-Drive, incidentally, stands for Durnin.
This is the first invention that Steve has tried to patent and commercialize, so while the D-Drive looks very promising, it's taking him some time to push through the relevant channels. His demonstration prototype and patents were paid for by a small group of private investors, who stand to gain a heck of a lot if the D-Drive cranks out the right numbers on a test dyno and breaks into the market.
We thank Steve for his time and wish him all the best with the D-Drive. It's a diabolically hard invention to understand even when you're looking at the prototype in action - so he's one clever cookie to be able to come up with the concept from scratch, particularly seeing as he claims he had never heard of a planetary gear system before he'd designed one as part of the D-Drive.
Quite an achievement!
Steve can be contacted through his (very embryonic) website.