There’s this apparent trend, a growing demand for automatic dual-clutch transmissions (DCTs). Late in 2008, Germany’s BorgWarner partnered up with the dozen major carmakers behind the China Automobile Development United Investment Company to start manufacturing DCTs in Dalian. Early the next year, BYD Auto, the largest independent carmaker in China and not a participant in the consortium that partnered up with BorgWarner, started developing its combination of a direct-injection turbocharged engine and 6-speed tiptronic DCT—their Ti+DCT technology package.
By April 2012, the BorgWarner-China joint venture was reporting a large and increasing demand for DCTs in the emerging automotive markets of both Europe and Asia. They estimated that the market for DCTs in China alone would grow from 0.6 million in 2012 to 2.7 million units by 2017, growing each year by 35%. Months later, in August 2012, after they had already launched their F5 sedan with the Ti+DCT package that took two years to develop, BYD both admitted that they had problems with supply of the DCTs for their popular vehicles and reported that they had already solved these. Though they didn’t elaborate on their solution, soon afterwards in November 2012 they announced that by the next year, BYD would be putting their own in-house developed DCTs in their vehicles. Then, in 2013, BYD started featuring their in-house developed DCT even on vehicles without high-end turbocharged engines.
The narrative paints a picture of China carmakers, particularly of independent BYD which not only has to compete for market share but also has to defend its position against a consortium of other carmakers, all being keen on putting DCTs on their product, and not only on their higher end offerings. Figuring out why requires a closer look at DCTs, at what distinguishes these from other types, and then an evaluation of its implementation on BYD’s mid-range L3 sedan.
Two where there was one
What makes it a dual-clutch transmission? Why the two clutches? The two smoothen, even simplify, the gear changes. Physically linked, as one clutch disengages from one gear, the other engages the next gear up, or down. Elegant if you think about it. Instead of the sequential steps of lifting the clutch, putting the next gear under it, and then pushing the clutch back in, the DCT does all three in one action.
But what happens at the very start when you roll off? On an automatic DCT, slide the stick into D and what happens? The car being stationary while you keep your foot on the brake (as you normally would on an automatic before rolling off), will one of the clutches immediately press in and engage first gear? You’d think it would if you’ve gotten used to a typical automatic, an AT. But no, it won’t.
Of things mechanical and fluid
On the ATs we’re accustomed to, slide the stick into D and you’ll immediately feel torque on the drive wheels, urging the car forward even as you keep your foot on the brakes. If the AT had a clutch, doing this would’ve stalled the engine, or would otherwise wear down the clutch’s contact surface. But an AT has no clutch.
Instead of a clutch mashed up against a pressure plate, a torque converter replaces that mechanical contact with fluid force. An impeller driven by the engine’s driveshaft pumps automatic transmission fluid (yes, that ATF you need to top-off routinely) against a turbine, turning it and in turn driving the transmission. Instead of the friction of a mechanical link that would wear down the clutch or stall the engine if you kept your foot planted on the brakes, there would be just the turbulence of the fluid as it gets pumped from impeller to turbine and then back again. (By the way, that re-circulation of the ATF from the turbine back to the impeller is achieved by a third component, a stator, but we’ll leave that detail for another story.)
The torque converter transmits significant enough engine power that if you pour on the gas, keeping your foot on the brake pedal won’t keep the car stationary for long, at least not your drive wheels. But yes, you’d know right off that a torque converter can’t possibly transmit all of it—a fluid connection simply can’t match the positive contact of a solid one. Because the force of the engine is transmitted through fluid pumping, the impeller on the driveshaft will spin faster than the turbine on the transmission—what engineers call the “slippage” that’s inherent in torque converters. (In modern automatics there’s a lock-up clutch that engages when the car is at cruising speed, creating a mechanical link with the top gear after it has gone through acceleration, but that detail we’ll also leave for another story.)
Back to clutches
Back again to the DCT which replaces the torque converter with clutches, it isn’t really an evolution of the AT—not a hydraulic automatic—but rather one of a manual gearbox where the clutch-work and gear-shifting has been taken over by electronic actuators.
More efficient with the positive contact of the clutch instead of the hydraulics of a torque converter, a DCT must have the programming to discern when to engage or disengage either of its clutches. Unlike an AT that can rely on slippage to cushion both the torque from and the load on the engine, a DCT needs to actively manage the clutch-work, manage the clutch friction. The DCTs programming triggers when to engage the clutch on the first gear and when to transition to the other clutch for an up-shift, when to go back to the other clutch for another up-shift, and so on until the top gear is reached at cruising speed.
And, at the same time, the DCTs programming needs to make the automated clutch-work and gearshifts familiar and intuitive to motorists. Not a trivial thing since there’s a lot you could get right, or wrong, in the algorithms.
The DCT on the BYD L3
Now, finally, knowing how a DCT works, we ask: how does it stack up on the BYD L3, how good is it on a compact sedan from China’s biggest independent carmaker?
If you’re mainly a stick driver, you’ll appreciate the convenience of an automatic but muscle memory will probably see you reaching out with your left foot for a clutch pedal that isn’t there, or dropping your right hand onto the drive selector for an up-shift you don’t have to do yourself. Seen from this perspective, from the point of view of one used to being enabled, and burdened, by a manual gearbox, the automatic DCT can be a superb alternative.
Before testing the L3’s DCT variant, I tried the one with a 5-speed manual gearbox. The 1.5 liter BYD483QE engine with SOHC over 16 valves featuring variable lift and timing technology delivers peak power and torque of 107hp at 5,800rpm and 107lb-ft at 4,800rpm, respectively. On the 1,210kg sedan, the power is enough for assertive acceleration but you could feel there’s just a slim reserve remaining for when you want things more spirited. But then again it’s tuned for fuel efficiency, the manual variant reportedly reaching 19km/l on the highway. So, given how the L3’s engine is, you’d want all that available power driving you forward. And on the DCT variant, the power is all there.
As advertised, good efficiency
On a highway drive of over 100km, I chalked up 17km/l while insisting for most of the trip on a 100km/h cruise with the engine turning at 2,400rpm. On the occasions when I backed off and settled down at 80km/h, the engine turning at just 2,000rpm, the trip computer had me doing 18km/l.
On a day’s drive in the city, the L3 DCT turned in a figure of 11km/l with frequent prolonged stops burning fuel at a rate of 1ltr per hour being offset by the car’s noteworthy ability to move off again and accelerate decently with the engine never exceeding 2,200rpm—this, even with a full family load of two adults and three kids. With a long enough stretch of open road, that 2,200 rpm cap could bring the L3 up to 60km/h where you could settle back at 2,000rpm and remain in 5th gear. At those readings, at 60 in 5th and with 2,000, the trip computer would show current fuel consumption of just 5ltrs per 100km—the equivalent of 20km/l for those few seconds that you can remain at 60km/h on city streets.
Impressive numbers from a compact sedan weighing in at a ton and a quarter, and close enough to those of the manual variant to verify that the DCT’s clutches do work as advertised. Though more efficient than an AT, a DCT won’t match a manual’s mileage because the latter can be coasted, clutch disengaged, whenever the driver’s inner ear and butt cheeks tell him he can. On a DCT, once the car is moving, one of its clutches is always engaged, unless the driver flips the selector into N. Although, that flipping into N, while already cruising at 100km/h on the highway, shows just how refined the DCT is on the BYD L3.
Smooth clutch-work, smooth shifting
With the car in D and with easy pressure on the gas pedal (pressed just an inch in), the L3 DCT accelerates well enough, shifting up at the aforementioned 2,200rpm as it reaches 10, 30, 45, 60 and then 80km/h when it finally enters sixth gear. The shifts happen so fast and so smoothly there’s no feeling of forward thrust ever letting up, the only sign of a shift being the softening of the engine’s noise as it drops from 2,200rpm down to just under 2,000rpm whenever it takes up the new gear ratio before spooling up again for the next up-shift.
That smooth shifting indicates that the DCTs programming has good routines for toggling between the dual-clutches. But popping the selector out of D and into N, and then back again, while already at cruise, that shows how well the DCT emulates a human driver. Pop her into N to disengage the active clutch, then take your foot off the gas. The car will go free-wheeling and you’ll see the revs settle back down to the 800rpm idling level. All as expected.
But then you’d worry, will putting it back in D cause a sudden, maybe catastrophic re-engagement of the clutch at too low a gear? No, it won’t. Pop the selector back into D and the clutch doesn’t reengage immediately. The DCT first pre-selects the proper gear based on current speed—6th for 100km/h in my case—and then eases in the clutch. Keep off the gas and you’ll see the tachymeter needle gently go back up to around 2,000rpm as the clutch is finessed back in and the engine revs get picked up by the turning of the drive wheels.
Smooth roll outs with a DCT
Now finally, an answer to the question: what happens differently on the L3’s DCT when you pop it into D to roll out? Like that popping out and back into D at cruising speed, the clutch won’t engage immediately. Pop into D to roll off while still stepping on the brakes and you’ll feel nothing. There’s no jerking forward against the braked wheels. Reason is that the clutch won’t engage until you take your foot off the brakes.
Do release the brakes and a moment later you’ll feel the clutch pushing in, again gently, to start the car rolling. I’ve heard reports of how engines with DCTs typically shudder at this point. For my money, it wasn’t any worse than what I’ve felt on ATs, but putting it that way would have me still missing the point. The shuddering is no different from what I’d feel on a manual transmission if I was too slow in giving it some gas as I let off the clutch pedal. In other words, the vibration is merely a sign of the engine approaching stall, prompting you to be more lively on the gas.
The best way I found for making roll-outs silky smooth on the L3 is to take my foot of the brake then immediately imagine myself easing my left foot off the non-existent clutch pedal. I’d let that visualization trigger muscle memory, let it cue me to gently press on the gas pedal. On a manual, it’s a simultaneous thing: ease off the clutch pedal as you ease in the gas. On a DCT, still with a clutch though it’s controlled by computer, the routine isn’t really different. And, as it happens, it’s a good safety feature. Better to not have the DCT intervene on the gas just for the sake of smoothing out your roll off. That part, you can take care of yourself.
Uphill, not a problem
This said, there is one situation when the DCT will partly engage the clutch and throttle up the engine even while you’re stopped and stepping on the brakes. Recall that a DCT won’t engage the clutch when you put it in D if your foot is still pushing down on the brake. This won’t do if you’re hanging on an uphill incline. That fraction of a second delay will see you sliding back before the clutch engages, compelling you to throttle up quickly to stop the slide and jerk the car into forward motion … a crude and nerve-wracking way to tackle a hanging situation.
No wonder that the L3’s DCT has an uphill assist feature—it’s absolutely necessary after all, not just a gimmick. What is surprising is the sheer elegance of how the designers will have you activate it. If you’re stopped on an incline and needing to climb uphill, a tried and tested technique on manual gearboxes is to pull up the hand-brake to keep you stationary while you balance you’re clutch and throttle to put torque on the wheels and make them start pushing against the brake. This way, when you release the hand-brake, the car will immediately start inching up instead of down the incline.
So how would they have you do something similar on the L3’s DCT? Why, by also pulling on the handbrake of course. Say you’re stopped, hanging on an incline, and wanting a smooth roll out. Keep stepping on the brake, slide the selector into D, then pull on the handbrake to instruct the DCT to set you up with enough clutch and throttle to have the L3 chomping at the bit, ready to move forward as soon as you set the handbrake back down and ease off the brake pedal—very natural for those who are accustomed to manual gearboxes.
Advantages on many levels
Discard notions of it being like a traditional automatic and instead treat the DCT as a manual gearbox on which you’ve delegated clutch and gearshift control to a computer. Do this and the efficiency benefits of this alternative and new automatic, particularly on the BYD L3, will come with no trade-off in terms of drivability. In fact, it could be more enjoyable than either a traditional AT or an old-school manual.
By their very nature, a DCT’s clutch and gearshift servos need to be computer controlled. What’s noteworthy on the BYD L3 is how the engineers have shaped this automation to both rely on and to channel the competence of the driver. With sensors to monitor the energy state of the vehicle, its velocity, its load, its momentum, the DCT makes full use of its controller suite to both emulate and respond to what any experienced driver would do if he were in control of the clutch and gearshift.
And finally, also because of its dependence on electronic control of clutch and gearshift servos, making a particular DCT distinct from those of other carmakers is as much a function of the heavy industry behind it as it is of the programming talent that goes into it. So, with DCTs and while they are still working at becoming major global players, Chinese carmakers can leverage the country’s deep programming talent pool into an immediate competitive advantage.