World's first geared CVT: A 'holy grail' transmission in prototype form

The efficiency of a tooth-on-tooth geared transmission, with the smoothly variable gear ratios of a CVT – minus the belt friction and capable of going all the way down to a zero ratio without needing a clutch. That's a 'holy grail' level promise!

Italian company Alter Ego, led by mechanical and automation engineer Edyson Pavilcu and assisted by kinematic studies undertaken at the Milan Technical University, says it's prototyped a beautifully complex new mechanism – the world's first geared CVT – that's 10% more efficient than a regular belted CVT.

Indeed, the efficiency is on par with some manual transmissions, while offering the ability to smoothly adjust gear ratios from zero all the way up to overdrive, whether the wheels and/or crank are turning or not.

A quick manual vs CVT transmission primer

Why is this a big deal? Well, transmissions are designed to let a drive system – be that an engine, an electric motor or a set of cyclists' legs turning a crank – operate at its optimal speed for efficient torque delivery, while adjusting the gear ratio between the drive system and the wheels to give just the right leverage for the current speed and situation.

Manual transmissions are very efficient, meshing metal gears together to deliver between 95-98% of input power to the other side. But you can only have as many gears as you can fit in the box, and multi-clutch systems aside, you have gaps in the power delivery where you need to friction-clutch out of gear then back in to the next one. When in gear, your ratio is locked, so your engine speed has to vary as you accelerate or decelerate. You can stay close to your ideal engine speed for the conditions, but not dead on.

CVTs, on the other hand, sacrifice some transmission efficiency for increased engine efficiency and super-smooth shifting. Many operate using a v-shaped friction belt running between two pulleys that can vary their diameter at the same time, making one larger as the other becomes smaller to maintain tension on the belt.

So you can smoothly adjust the gear ratios between the lowest ratio and the highest as you drive; the engine speed sits exactly where it needs to for maximum power or efficiency, and the transmission continually adjusts to match the wheel speed or power requirement.

They need to disengage a friction clutch to come to a complete stop, and without that benefit of metal teeth on metal teeth, most CVTs only deliver between 70-86% efficiency.

These are Alter Ego's figures, and certainly a simplification of the transmission world, but they'll serve as enough of an introduction. Check out the video below from The Engineers Post for a more visual backgrounder!

So each has its pros and cons (ignoring the visceral driving experience, which is key over all for enthusiasts) – and that's why a super-efficient, continuously variable, geared transmission capable of going all the way down to zero wheel speed is such a big deal.

The RatioZero geared CVT: How it works

Enter the RatioZero transmission, riding high on the white horse of two separate patents – yet travelling curiously humble in the form of a crowd-financing program ... Let's bookmark that and take a look at the device itself!

So how do you take solid, unyielding metal gears meshed together, and create a variable gear ratio? Well, as you can imagine, it's complicated. But the key is that you never have a single cog driving the output ring gear – you have three. And those three don't drive it right through the rotation; each one gives it a little push before rolling back a few teeth and pushing again.

So essentially, the torque from the input is delivered to the output constantly, but only by one of these three small gears at a time. A kind of torque relay, if you like.

If you're following so far, well done! So how do you modify your gear ratios? Well, those three inner gears are attached to curved arms. Each of these arms is connected to a free-spinning axle mounted in its own planetary drive gear. The main input ring gear turns all three of these planetary drive gears, so they're always spinning when the input shaft is.

When the axles connected to those curved arms are right in the center of the planetary drive gears, the drive gears can spin all they like; the axles free-spin with them and the arms don't move. This is your zero gear ratio; the output ring gear stays where it is.

But the axles can be offset from the center of the drive gear – there's a sliding actuator that gradually pushes all three axles out and away from the center. Now, as the planetary gears spin, these axles start tracing circles, and the arms start moving back and forth. And that's how you get that torque relay to the drive ring; the free-spinning gear on the end of each arm pulls the output gear through part of its rotation, then moves back as the other arms take over, before doing it again.

The further offset those axles are in the planetary drive gears, the higher your gear ratio becomes and the faster the output gear will turn per rotation of the input shaft.

The mechanically minded may say: wait, there's no way those arm movements deliver a constant rotation speed to the output drive! And they'd be correct – but the Alter Ego team has corrected for this using elliptical gears that constantly vary the rotation speed of the planetary drive gears, so that a constant input speed leads to a more constant output speed.

In applications where you need the output to freewheel – say, a bicycle – you can use a one-way ratcheting mechanism on the main drive gear so your pedals don't jump like mad when you're coasting down a hill. And for heavy-duty applications ... Well, there's a more complex and robust version using four gears instead of three.

What are the next steps?

Alter Ego has built several working prototypes. One's now mounted in a bicycle, with a joystick controlling a small electric motor that varies the gear ratio on demand through a range that takes the output drive from 0% to 600% of the input drive speed. Ratio selection will of course eventually become an automated process.

The next step will be attaching one to a combustion engine; the team is looking to fit the transmission to a 530cc Yamaha T-Max scooter as a further proof of concept, and will then shop it around to motorcycle manufacturers.

But it could also be integrated into a slightly wider ebike motor unit as a one-piece variable speed drive unit, or stuck into a wind turbine generator, or a car, truck, tractor, snowmobile, industrial machine, whatever.

Alter Ego says the main issues that still need to be solved are the remaining 'ripples' in torque delivery, which likely still manifest as small variations in the output speed, and noise. To this we'd add reliability; today's CVTs may be nowhere near as efficient as this transmission, but Toyota, for example, sticks a five-year warranty on its car CVTs and they're generally expected to last well over 100,000 miles (161,000 km), if not double that.

If a consumer product is ever released with a gear ratio that goes all the way down to zero, well, the potential for mechanical stress here is as infinite as the torque you can theoretically put through this system. At a tiny axle offset, those arms won't move the output drive much, but the torque they could apply to a stuck wheel could get pretty epic.

It'll be interesting to see what manufacturers do about that – it seems some RatioZero prototypes are actually locked such that they can't go down too close to a zero ratio.

The RatioZero geared CVT is a mechanically fascinating device, and while it's pretty complex, it certainly seems to do what it says on the tin – so it seems to have plenty of potential to get beyond what Alter Ego describes as its current Technology Readiness Level (TRL) of 5 – that meaning "laboratory testing of an integrated/semi-integrated system" with "component and/or process validation in a relevant environment."

But there's a long way to go yet – particularly once you start talking about getting these things through the grueling process of being verified and tested for automotive or motorcycle OEM use.

And so here's where we end up: the Alter Ego team has a few working prototypes, but they're now trying to raise the half a million-odd dollars they'll need to do another year of R&D, integrate a prototype into that Yamaha T-Max, and take it around the expo circuit fishing for OEMs that might pick the idea up and push it through into a consumer product.

The current plan is to raise this US$500,000 by passing the cap around in a crowdfunding push, which will begin "soon." We're fascinated to see how far this idea goes. If you'd like to see the mechanism explained in further detail – including more on the gear shift actuator than we included – check out this wonderfully patient explanation by YouTuber 'driving4answers,' who got his hands on some prototypes in the video below.

Source: RatioZero