The Gearbox Edition: A Comparison of 2 Transmissions

Conventional Automatic Gearbox (CAT) VS Continuously Variable Transmission (CVT)

While most of us might own cars, or at least have access to one, it is not only necessary but also important for us to at least have the basics around the functionality of a car.

Over 60% of cars manufactured today are automatic, and they sport a variety of variations, but today let us draw all the focus to the Conventional Automatic and Continuously Variable Transmission (forthwith abbreviated with CAT and CVT respectively). We’ll get to the more complex ones over time, like the very fun and quick Dual Clutch Transmissions (DCT).

Conventional Automatic Transmission (CAT)

Nerd fact: The first ever complete Automatic Transmission was designed by Horner Munro in 1921 with four forward gears but with neither reverse nor park gear, a design he later patented in 1923.

Onto the CAT, the principle behind the development of CATs was to eliminate the need for a clutch pedal and allow for more leisurely driving (although I do not subscribe to the idea of driving using one foot for a lifetime, ok 😐). So, unlike the manual transmission where the is a friction clutch, the CAT replaces it with a fluid coupling system which defines a set of gear ranges depending on need – yep, that is why you need to service your automatic gearbox with ATF (Automatic Transmission Fluid), the whole mechanism depends on it.

What happens when I engage park on my CAT then? (Since no motion is not a gear range) In parking gear, all gears in the setup lock up to prevent forward or backward roll.

While we are still on CAT, allow me to throw in a little confusion and give an honorable mention to the Semi-Automatic Transmission. In the semi-automatic transmission, the setup is most closely alike to that of a manual transmission, which allows the driver to shift gears either by themselves or let the car shift itself. In this setup, the clutch is present but is operated by either a hydraulic or electric system (torque converters and planetary gears) that therefore do away with the clutch pedal. Please note, semi-automatics allow user-induced shifts (I will give you an arguable example, the Toyota Mark X gearbox is semi-automatic), and CATs DO NOT. The gearshifts in CATs are autonomous, point home? Great.

Working of the CAT

Now that we know what a CAT is, let us dive a little deeper into its functionality. Jumping right in, let us first de-jargon some terms; torque converter – in the simplest of terms, this is a fluid coupling mechanism that transfers rotational power from the engine to a rotating/driven load (in this case, the driveshaft or the propeller shaft).

Essentially, the CAT system works by combining the torque converter with a set of planetary gears which provide the desired range of gears for the car. Most CATs have five to six speeds but the older versions (surprisingly some new ones too, like the Proton Saga CAT has 4 gears). The presence of the torque converter on the CAT setup does away with the need for a friction clutch (Yay, no clutch pedal!).

As the engine runs, the turning force or rather the mechanical power created by it is transmitted to the pump of the torque converter, channeled through the transmission fluid which creates a power rotation (what nerds call kinetic) that spins the turbine and the attached central shaft. The shaft then transmits this power to the first planetary gear set. Subsequent gear shifts are dependent on sensor readings on engine revolutions, tire revolutions, and load among several other things. See the below illustration of the CAT setup for a better understanding of this works.

This gets me to a popular question most people ask either inwardly or out loud. Can you push start a CAT? I mean we have all seen manual cars being push started to life when they have faulty starter mechanisms. Well, NO. You cannot push start a CAT and here is why; The transmission fluid in the CAT is pressurized by a pump so when the engine is not running, the transmission does not have oil pressure and therefore is not functional. All we are saying is, the CAT only comes to life when the engine is running. Enough of the technical stuff….

Why CAT?

1. No clutch operation
2. Has made it possible for PWDs to operate and use vehicles (the greatest advantage in my opinion).
3. The “Idle Creep” phenomenon – if you have driven a manual, you get this. The hustle of finding where the clutch bite/loading point is when starting especially on hill starts or in traffic. This phenomenon allows for the car to start moving even at idle immediately after you put it in gear and let go of the brakes.

Cons of CAT.

1. Less control as compared to manual cars
2. Expensive to maintain (not worse than CVTs though).

Continuously Variable Transmissions

Nerd fact: The first-ever conceptualization attempt at a CVT transmission was made by Leonardo Da Vinci in the 1490s and the first invention was in 1886.

Every time I talk or write about CVTs, I laugh a little. I do that because while it is the most demonized gearbox to ever exist, it is simply, MISUNDERSTOOD. Try selling a car on the Kenyan market, and you will be met with a lot of resistance once buyers “realize” (after you tell them) it has a CVT gearbox. I say it is misunderstood because unlike what most people assume about CVTs, they were designed and manufactured to deal with the exact issues people have with them. Let me explain how through this write-up.

To start, we have several types of CVT gearboxes available, they include:

1. Pulley-based
2. Hydro-static
3. Toroidal

For purposes of this blog, I will stick with the first one – as it is the most common one. At the most basic level, CVTs have just THREE major components (please note that I do not mean there are no other components), these are:

• A high-power metal or rubber belt
• A driving pulley (variable input)
• An output pulley

The pulleys are all cone-shaped, this property is important for the functionality of this gearbox system as you will see a little later on.

Working of CVTs.

First off, unlike the CAT and manual gearboxes, the CVT does not have any actual gear. We still refer to it as a gearbox though since “Gears” are ideally the ratio of engine shaft speed to driveshaft speed. Point home? Great. CVTs use the pulley system to allow for a seamless and infinitely variable range between the lowest and the highest gears.

To simulate the gear ratios, (since CATs and Manuals have actual metal gears), the pulleys change based on the needed power, as one pulley gets larger, the other gets smaller allowing for strong and seamless acceleration. As the two pulleys change their radii relative to each other, they create an infinite number of gear ratios. For instance, when the radius is small on the driving pulley, the rotational speed of the driven pulley decreases resulting in a lower gear. When the radius is large on the driving pulley and small on the driven pulley the rotational speed of the driven pulley increases resulting in a higher “gear”.

So how about those fancy-named CVTs that sometimes even come with paddle shifters?
As we had already said CVTs have no gears, so they get a little boring as you drive them. The power delivery might be seamless and smooth, but there also lies the problem. Having gotten used to the progressive engine sound change as the car shifts into a higher gear or downshifts to a lower gear is every driver’s dream, you take away that sensation and it feels like you are driving a generator, so to speak.

So, manufacturers got a little clever and set up preset ratios on the pulleys that simulate actual gears. By so doing, the driver can then “shift” between the available “gears” and liven up the driving experience. Like what Honda did with their L15A gearbox with a simulated shiftable 7-speed experience that comes with paddle shifters.

Why CVT?

1. Quick acceleration – yep, that is right, CVT gearboxes are very quick on acceleration,, this is mainly because they do not have to “hop” through gears like CATs.
2. Provide improved fuel economy
3. Easier drives uphill ( Ideally CVTs are much smoother uphill since unlike the CATs they lock up in the desired gear ratio much easier and quicker and thus provide smooth power delivery).
4. Smooth acceleration

CVT cons

1. Costly to buy and maintain
2. Not feasible for high-performance cars – allow me to camp here for a bit…

I will use Subaru to illustrate. For the longest time, Subaru has made beautiful and powerful cars that have dominated several ranks across the world. Their signature boxer engine setup has been a thing of awe to all car enthusiasts (even the Mitsubishi die-hards 😊).

Nerd fact: The first ever manufacturer to mainstream the CVT gearbox is Subaru in the late 80s.

Moving on, today, Subaru still makes very powerful engines but tragedy seems to have befallen them after they mated some of their most powerful engines to CVT gearboxes, while the gearboxes are quick, they are not meant for the stress associated with performance engines. This explains why the Subaru Forester XT (SJ to SK) or the Subaru Legacy BS9 are known to develop gearbox issues when pushed to the limits. An unforgivable doing though from my POV is Subaru availing a CVT option for the WRX (2018).

Conclusion

To wrap this up, I need to acknowledge that all the gearbox variations we have here have been used effectively depending on the use case. CATs are robust, in that so many variations of it have been made and they are making excellent strides in the car scene. CVTs on the other hand as I stated earlier are just MISUNDERSTOOD and not as unreliable as people may make it seem. At this point I honestly believe that CVTs are here to stay and we had better get on the same page about them:

1. CVTs are unforgiving with maintenance – ensure service at the interval specified by the manufacturer and using the correct fluid (which can be very expensive, Honda CVTF costs Kes. 2000 per liter)
2. Do not push the transmissions to the limits – I can tell you one thing, you will not outrun a Direct Shift Gearbox on a straight or even a hill climb, just give it a rest. If you need performance CVT is not for you.
3. Always remember that CVT is for fuel economy, not power.

Take heed of these and I do not think you will have a problem with your lovely CVT gearbox. That will be all for this blog on CATs and CVTs, feel free to drop a comment or suggestion about the same. See you on the Direct Shift Gearbox VS Dual Clutch Transmission, next time.

Adios.

ww_nab

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Automotive content writer and self-teaching auto-mechanic.
I will mainly discuss and banter about cars.

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