Drivers may bemoan the CVT gearbox's notorious droning, but the advantages make the headache worthwhile
22 April 2019

Toyota’s umpteenth incarnation of the Corolla is now on sale, mainly in Hybrid form. It’s billed as having an ‘e-CVT’, which at first had our news antennae all a-quiver.

In fact, e-CVT is simply another marketing moniker for essentially the same hybrid driveline concept Toyota came up with in the 1990s for the first Prius and has stuck with ever since. Originally called the Toyota Hybrid System (THS), it then also became Hybrid Synergy Drive (HSD), giving a nod to the fact that it was also used by Lexus and even sold to a couple of other car makers. 

Swapping cogs, gear changing, shifting: whatever your favourite expression, gearboxes and cars go together like sticky toffee pudding and custard – unless it’s a CVT. Some drivers loathe the way a CVT’s soaring engine revs are disconnected from the car’s rate of acceleration – known as the ‘rubber band effect’. 

The CVT was made famous by DAF when it launched the first production version, the Variomatic, in 1958. Instead of a complex box of cogs, it consisted of two pulleys of continuously variable diameter, connected by a belt. To give the lowest ratio (like first in a manual), the engine-driven pulley is at its smallest diameter and the second pulley, driving the wheels, at its largest. 

As speed increases the engine-driven pulley gets bigger and the drive pulley smaller, increasing the ratio – so the car speeds-up. Controlled not by a computer but by a vacuum, it continuously and automatically adjusts for hills and harder acceleration or cruising. The design has been used by many manufacturers over the years, including Audi, Ford and Fiat

CVTs are not all alike, though. Although Toyota offers a CVT in the new Corolla (but not in the UK), its hybrid drive e-CVT is nothing like the original Variomatic and there’s no belt. Instead, it consists of two electric motor-generators (MG1 and MG2) connected to a planetary gearbox. The whole caboodle has the engine at one end and the driven wheels at the other. 

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Planetary gear sets exist aplenty inside conventional automatics. The compact package consists of a sun, planets and an enclosing ring gear and resemble a desk toy of the solar system. There are only a few components, but making the drive take different routes through the mini solar system allows the two motor-generators to perform different roles. 

MG1 can start the engine and at other times act as a generator to charge the hybrid battery. MG2 can act as a drive motor on its own or with the engine and also a generator to perform a regenerative braking role. MG1 can also apply small amounts of torque to the gear set to control the balance between the engine and electric drive from MG2, and there are many more combinations. The system allows electric-only drive by decoupling the engine (without the need for a clutch), and it’s small and compact. 

So not all CVTs are what they seem. This latest one is clever and mega-efficient, and it’s not surprising the basic idea has endured for more than 20 years.

Reverse engineering

Bosch’s electronically controlled version of the original CVT remains mechanically simple. Despite CVTs being scorned by some, Dutch rallycross star Jan de Rooy dominated with his DAF 55 and 555s in the 1970s. DAFs were banished to their own category in the annual Dutch backwards racing championship (yes, really, it used to be a thing) because CVTs enabled them to drive as fast in reverse as forwards. 

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Comments
14

22 April 2019

Two types of drivers in the world: CVT drivers and everybody else. 

The everybody else group puts Isaac Newton in the driver's seat when safe and possible; while the CVT driver merely dawddles from A to B and home again... as uneventful, predictable, and exciting as beige wallpaper paste 

22 April 2019

I once ran a DAF 44, so know the charicteristics. But every magazine going mentions the noise problem of a screaming engine and not much thrust. You would think by now, manufacturers would have dealt with the ambience effects of the system and made them more ameanable to the drivers and passengers. They would remove one of the major (if not the biggest)  complaints if they did so. Sadly, for far east manufacturers in general, "noise" suppresion does not appear to carry much weight, whether road or engine generated.

 

22 April 2019
john386 wrote:

I once ran a DAF 44, so know the charicteristics. But every magazine going mentions the noise problem of a screaming engine and not much thrust. You would think by now, manufacturers would have dealt with the ambience effects of the system and made them more ameanable to the drivers and passengers. They would remove one of the major (if not the biggest)  complaints if they did so. Sadly, for far east manufacturers in general, "noise" suppresion does not appear to carry much weight, whether road or engine generated.

 

A lot of cvt equiped cars have a manual override with paddles, this eliminates the constant high revs under hard acceleration as when you pull the change up paddle the revs drop, the jazz and insight I had both did this and worked well, I only ever really needed to do this when accelerating hard on a sliproad to join motorway traffic, other times like over taking the revs would only rise momentarily until you slightly throttle off and it quietens down. The latest civic cvt has a stepped change in normal driving and behaves like a normal auto. I've found cvt geaeboxes to be excellent, very quick to pull away, very smooth, economical to use in 90%+ of driving, quiet.

22 April 2019

But in my experience using a CVT gearbox is perfectly acceptable and in fact I always feel quite satisfied knowing I'm always using the engine at exactly the most efficient and or powerful revs, given the circumstances.

Don't forget F1 banned CVT after Williams started testing one and it made their car too fast, albeit when Williams were a decent team because they hadn't succumbed to nepotism at that time.

22 April 2019

On the basis that you need a minimum 2 litre engine for an autofor it to be good, this little car of my mum's is not that bad ,semi auto they call them,but in the real world is auto ,the only thing I do not like with it is it can feel like being held in suspension ,if driving in a normal way ,if you thrash it a little this dissipates.

22 April 2019
Ski Kid wrote:

On the basis that you need a minimum 2 litre engine for an autofor it to be good, this little car of my mum's is not that bad ,semi auto they call them,but in the real world is auto ,the only thing I do not like with it is it can feel like being held in suspension ,if driving in a normal way ,if you thrash it a little this dissipates.

A semi auto is something entirely different.

XXXX just went POP.

22 April 2019

This fiesta is a cvt gearbox and I call it semi auto due to it rolling backwards if brake is not on onsay a hill ,so not fully auto in theory ,so do not know if there are grey areas ,but to be honest if you do not have a clutch it really is an auto but technically I am told they are semi auto .can some of you engineers enlighten me if ALL CVT IS THE SAME AS THE FORD FIESTA .

22 April 2019
Ski Kid wrote:

This fiesta is a cvt gearbox and I call it semi auto due to it rolling backwards if brake is not on onsay a hill ,so not fully auto in theory ,so do not know if there are grey areas ,but to be honest if you do not have a clutch it really is an auto but technically I am told they are semi auto .can some of you engineers enlighten me if ALL CVT IS THE SAME AS THE FORD FIESTA .

I thought fiestas has automated manuals like Citroen had and BMW's SMG auto. Not sure though.
I've had an automated manual which had no creep and wouldn't hold on a hill, a C2, also had a proper torque converter auto which would creep forward when you release the brake and would hold on some hills depending on how steep and I've had 2 CVT autos, both had the creep function but neither would hold on a steep hill intact they'd struggle to hold on any hill really.

22 April 2019

I have a Mitsubishi Outlander PHEV, which has CVT. I asked the dealer how it works - but he'd no idea. It seemed unlikely to be via rubber bands. I guess now from the article that it uses the planetary gear system but the annoying thing is that I actually invented the bloody system when I was at school over 60 years ago! A mechanic friend made a model of the gears for me but I never got round to installing the electric motor to drive the outer ring, never mind applying for the patent. Ah well!

22 April 2019

I do believe that the Mitsubishi Outlander (and the latest honda Hybrids) have a single fixed gear between engine and driven wheels, and the CVT effect is as a result of the series hybrid configuration ie at low speeds, the engine drives a generator which in turn drives the wheels via an electric motor. Only at higher speeds when direct drive is engaged is there a mechanical connection between the engine and the wheels. It's a very simple arrangement, but I suspect not quite as efficient as the Toyota system.

Well done Jesse Crosse for explaining how the eCVT works and why there is a disconnect between engine and wheels. I accept that this this system may not applease some enthusiasts / traditionalists, but there is no denying its cleverness and efficiency. And Ford clearly recognises the system's virtue since it uses a license built copy in the current Mondeo hybrid!   

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