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BorgWarner: Wet Clutch DCT Optimized for Efficiency

The consensus among many transmission specialists is that the superior efficiency of dry clutch dual transmissions will prove to be a critical advantage – especially in small cars − in the intensely CO₂-conscious future that lies ahead. Yet BorgWarner, which was Volkswagen's selected partner on the first-ever volume production DCT system with the wet clutch DQ250 in 2003, is confident that further iterations of its design can optimize both fuel efficiency and cost to remain competitive – and even demonstrate significant advantages – for the B- and C-segment vehicles that are the main candidates for dry clutch systems.

In presenting a paper at the 2010 CTI conference on advanced transmissions in Berlin, BorgWarner Drivetrain Engineering’s head of advanced product design stressed the wet DCT’s advantages of compactness, low weight and low inertia, as well as its significant potential for NVH improvement – the latter an issue of complaint on some dry clutch applications.

Alexander Moser, supervisor of advanced product engineering and clutch systems at BorgWarner’s drivetrain engineering center in Germany, outlined developments which would boost the efficiency of current-generation wet DCTs by more than 4 percent, bringing the overall efficiency gain since the first generation DCT to almost 9 percent.

BorgWarner’s cost- and efficiency-optimized DCT configuration retains the basic layout of the six-speed original, but departs from the format in several important ways. These include the use of a low-pressure hydraulic system, a one-way clutch (OWC) for first gear, and the introduction of controlled slip to improve NVH and allow the elimination of complex flywheel damper arrangements.

Drag Losses
One of the principal efficiency losses of wet clutch DCTs compared to their dry counterparts is the drag loss between the plates on open clutches. When the plates are separated and rotating at different speeds, the presence of oil between the plates induces a drag torque as the fluid is sheared. These conditions, says Moser, arise during idle (when reverse gear is selected but not engaged) and when the vehicle is cruising, with the DCT having already pre-selected the next gear ratio in readiness for the next shift.

"If during cruise there is no gear selected," says Moser, "then there are no drag losses. This can be programmed into the transmission control unit." Figures produced by BorgWarner show first-generation DCTs with a peak drag torque of just under 5 Nm at a lubricant flow rate of 2 liters per minute; for the current generation, the corresponding value is well under 1 Nm. The new solution promises to drop this to zero. Moser did not state whether this option would have any effect on shift speed or shift quality.

Control and Actuation
A second area to come under scrutiny in the quest for improved efficiency is that of the mechanisms actuating the clutches and gear selectors. In the past these have been entirely hydraulic, and combined with the clutch and gear lubrication systems. Because of the necessity of actuating both clutch and gear selection at the same time, hydraulic systems have always had to provide relatively high reserve pressures – up to 14 bar, representing a significant energy drain from the continuous running of the oil pump.

In the efficiency-optimized DCT, this arrangement is replaced by a power pack for shift and clutch actuation and a separate electric low pressure oil pump for on-demand lubrication. This, says Moser, provides the biggest single fuel efficiency improvement. In the first-generation wet DCT, the pump absorbed some 600 kJ of the 950 kJ total losses. On current designs, it is some 350 kJ (out of 600 total), while on the optimized design, it is less than a quarter of the overall 200 kJ losses.

Specifically, the first-generation hydraulic pump consumed an average of 750 W over a 2,500-second test route around Heidelberg in Germany, while its on-demand electric power pack replacement used just 20 W. The difference was most notable at higher road speeds, when the permanently geared hydraulic pump was working unnecessarily fast. At no point on the trace did the electric system absorb more energy than the hydraulic.

Keeping the Lube Cool
Another question BorgWarner engineers asked was whether an oil cooler was strictly necessary in order to keep lube temperatures within acceptable limits. A transmission cooler imposes a back pressure of up to 3 bar, says Moser. This was not a problem when transmissions ran with high-pressure clutch and gear actuation hydraulics, but the transfer of these functions to electric operation opens up the opportunity for truly low pressure lubrication. Eliminating the oil cooler would allow a further drop in pressure – from 3.5 to 1 bar. This would allow further energy savings and a lower cost pump – provided the effect on temperatures was acceptable.

Rigging a vehicle with a transmission oil cooler that could be bypassed – in and out of the heat equation – BorgWarner conducted a severe test of eleven hill launches at 30-second intervals. The experiment revealed a 6-degree sump temperature difference between the cooled and non-cooled modes, leading the engineers to conclude that there was sufficient thermal capacity in both systems to cope with such conditions.

Wet systems in general have the advantage that the oil can be used to dissipate heat generated at the clutch facings, allowing smaller and lighter clutches to be used. BorgWarner figures show that, of the transmission’s 2100 kJ total heat capacity, 550 kJ is accounted for by the transmission fluid, with the remainder split equally between the transmission housing, the shafts, gears and clutch facings. The company also points out that the new low-energy pump will reject less heat into the oil, thus reducing the overall heat generated.

Slip and Smoothness
An additional, and perhaps unexpected, bonus of the optimized transmission is the potential to reduce overall NVH, as well as weight and cost, through the introduction of deliberate slip at the clutches.

By allowing the driving and driven sides of the clutch to rotate with a predefined level of slip, the angular acceleration caused by engine irregularities – especially at low rpm – is reduced. Though clutch slip could be interpreted as a loss of energy in the system, says Moser, any such energy loss is more than compensated for by the fact that it allows the engine to run at lower rpm, where it has higher efficiency. The fact that clutch slip control dampens engine excitations enables an earlier upshift to a higher gear, as well as wider throttle openings at those lower rpms.

The optimum slip speed varies in relation to engine speed and throttle position, with generally higher slip values for low engine speeds and wider throttle openings. This relationship will be programmed into the transmission control unit and shows a beneficial effect on fuel consumption in the NEDC drive cycle.

Further benefits come from the potential elimination of the weight, inertia, cost and complexity of a sophisticated rotational damper system. This is particularly significant for diesel engines and, says BorgWarner, the development could play an important role in drivetrain optimization for new-generation, compact three- or four-cylinder turbocharged engines with high output torques.

The Getrag View

Backing for BorgWarner’s claims comes in a parallel paper presented by transmission manufacturer Getrag at the same conference. Under the heading of The Transformation of Transmissions, three of the supplier’s engineers describe the transmission’s role as having evolved from that of a mere torque and speed converter for the engine to becoming a filter for engine oscillations, too. The development focus will shift from the hardware to the electronics, software and interfaces, with further improvements in fuel economy necessitating new concepts for actuation on demand, says Getrag.

Beginning with the current (2010) 2.0 liter Ford Focus diesel emitting 154 g/km CO₂, Getrag’s timeline shows emissions falling to 146 with the addition of stop-start and assisted direct start. The baseline 134 g/km is reached with the second-generation DCT with assisted direct start. Beyond the initial second generation, an evolution with seven speeds, allowing optimum ratio setting, sees a further drop to 129 g/km – a reduction of 10 percent over the baseline system and impressive when compared to the 144 of the equivalent manual transmission.

The Getrag engineers also outline a slip-assisted cruising strategy to improve not only NEDC and real-world fuel efficiency, but also NVH performance. Step 1 in the process for reaching the steady speed is for the transmission to shift up to the highest possible gear – for example, from third to fifth or sixth. This allows the engine to run at slightly above idle speed, perhaps as low as 850 rpm. Controlled slip can then adjust engine rpm so as to avoid any boom or resonance bands, and at tip-in, when the driver reapplies the throttle to demand acceleration, there is a fast multi-step downshift to provide the required torque. This is Step 2. Overall, says Getrag, the strategy can provide a fuel efficiency improvement of 3 percent.

One-way Clutch
The architecture of the BorgWarner optimized transmission concept differs in only one significant way: the mounting of first gear on a one-way clutch (OWC). This, says the company, enables the elimination of the first gear synchronizer and all the related actuation components. The company says that it has already proven the viability of the OWC with several prototype transmissions in test vehicles.

Here, the improvement is in weight, cost and complexity rather than direct efficiency, but it is nevertheless worthwhile when automakers are placing such emphasis on the price/performance relationship.

Cost Versus Efficiency
Summarizing the results of the team’s findings, Moser stresses the importance of reducing the cost of the transmission system as well as improving its efficiency.

Though the proposed optimized transmission does add a more sophisticated mechatronic power pack to actuate the gears and the clutches, costs are saved elsewhere in the system by the elimination of the oil cooler and first gear synchronizer, shift fork and actuator, and with the replacement of the complex damped flywheel system by NVH control through programmed clutch slip.

Overall, the parts-cost savings more than outweigh the extra cost of the mechatronic unit, though BorgWarner does not provide details. And with the system also promising useful fuel efficiency improvements, BorgWarner will be able to offer automakers the rare prospect of lower consumption and lower cost in the same optimized package.

Story Filed: 3/15/2011
By Tony Lewin, managing editor DCTfacts.com