On Suzuki’s Celerio CVT (continuously variable transmission) variant, what would seem a drawback is something that’s stock on all continuous variable transmissions. But, rather remarkably, it’s something that can be worked around, particularly on the Celerio’s JATCO CVT7 gearbox.

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jatco-cvt-7-slides-400Transmission trade-off

The usual complaint against CVT’s is that rubber-band feel of the electronics searching for the ideal gear ratio when accelerating with heavy revs. It manifests with the engine momentarily producing audibly strong revs although without transmitting the commensurate torque to the drive wheels. And yet, that effect is actually caused by intentional slippage in what connects engine to CVT.

Just downstream of the engine is a hydraulic torque converter that uses fluid force pumped from an impeller into a rotor (like two propellers face to face, one blowing at the other) to transmit torque to the core of the CVT gearbox. As expected, there’s slippage in the system, the speed of the impeller typically being faster than that of the rotor that it pumps fluid into, but it’s not merely tolerated.

img20161106110002That slippage is also intended to multiply torque, with multiple revolutions of the impeller to a single one for the rotor translating into actual, albeit somewhat abstract, gear reduction. In fact, there’s liberal slippage in some transmissions (indicated as having high stall speeds) to harness this fluid amplification of torque.

On the compact CVT7 gearbox driving the light Celerio hatchback, this slippage does seem to be kept at a minimum, but it’s still there. And, while this is going on in the torque converter, the core of the gearbox is where pulleys connected by steel belt have their diameters automatically expanded or contracted to continuously vary the gear ratios. Varying pulley diameters to drum up ideal gear ratios already takes finite time, with algorithms still having to act through mechanical servos. And, that perpetual hunt for ideal ratios becomes more complex when coping with surging or receding torque caused by slippage in the torque converter.

img20161106111136Other words, that rubber band effect? It’s caused by slippage in the torque converter changing the parameters even while the electronics are in the process of implementing ideal gear ratios. This analysis of the problem is lengthy, sure, but it’s also the exercise that Suzuki must have endured to have chosen the JATCO CVT7 gearbox for the Celerio. Because, as it turns out, when they made the pulleys more compact by adding an auxiliary two-range gearbox on the CVT7, the dynamics of driving smaller discs, and the serendipity of not having to step through discrete gears (as on a conventional AT), gave JATCO a way to eliminate slippage as soon as the driver makes it superfluous.

Slippage in torque converters is eventually eliminated when the impeller and rotor revolutions come close to matching up and a clutch kicks in to lock one to the other, but this lock-up clutch is typically activated late in the cycle—as on conventional automatic transmissions, only when the vehicle has already reached top gear, after slippage has cushioned all those up-shifts. But, on the JATCO CVT7, since the torque converter feeds into a continuously variable system—no distinct gears, no shift shock—the gearbox triggers the lock-up clutch as soon as the revolutions on impeller and rotor come close to matching up. And, on the CVT7, that can happen as early as on roll-out.

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Gaming the gearbox

The solution then, as it’s made available on the CVT7, is to throttle gently until the lock-up clutch kicks in—the gentler you are on the gas, the sooner the lock-up. Use this Rule of Twos: roll out by just stepping off the brake pedal and then easing in the gas to bring revs up to 2000rpm, no higher, and then wait for about 2 seconds. That much gas for that long will get you to 20km/h where you’ll notice revs dropping to 1500rpm. That’s your cue, the lock-up clutch is in and you now have hard, no-slip contact in the torque converter.

2t5c3452After that short 2 second wait, feel free to put in as much gas as you want. Go stronger with 2500rpm after the 2 second wait and you’ll get distinctly smart acceleration, promptly reaching 100km/h and levelling off to 2000rpm for an easy highway cruise. Go higher and the engine starts to get throaty around 3000rpm. Floor it and the Celerio’s drive-by-wire system will intervene, making throttle up assertive but still somewhat gradual, to accelerate well beyond 100km/h with the revs regulated to a 6000rpm maximum.

Of course, the JATCO CVT7 is remarkable not only because of this feature for early clutch lock-up. There’s the option to even come back out of lock-up with a shift from standard down to low range to contrive a fast-twitch kickdown where there shouldn’t be any, not on a CVT. (See our story on gaming the Mitsubishi Mirage CVT and find the parameters for doing this contrived kickdown safely on the larger hatchback that features the same JATCO gearbox.)

Easy economy

But, bringing things back to urban realities, if you settle for bringing revs back to 2000rpm after that 2 second wait (after that drop to 1500rpm with the lock up clutch kicking in), you’ll soon get to a city cruising speed of 60km/h that can then be sustained with just 1500rpm, there with the Celerio delivering max-conserve figures that are off the scale (beyond the instrumentation’s 30km/l max indication).

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Using this Rule of Twos while generally staying under 2500rpm, I got mileage of 12km/l through 100km of moderate traffic in the city, 10km/l when dealing with heavy traffic that slowed travel to an average of 15km/h. This is still consistent with the 29.14km/l that made the Celerio CVT the top scorer on the mass fuel-eco test conducted by the Department of Energy (DoE) and Petron last May over a test course that included a fair stretch of NLEX.

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