• Contact
• |
• Site Map
• |
• Privacy Policy

• 
• 
• 
• 
• 
• 
• 

Fiat's Mini-Hybrid Uses Small DCT

As predicted earlier this year on DCTfacts.com, Fiat Powertrain Technologies (FPT) has revealed details of a pre-development program for a new downsized DCT transmission, with hybrid capability, to go with its novel downsized two-cylinder TwinAir gasoline engine already on sale in the fashionable 500 city car.

As Fiat engineers explain, the realization of a hybrid drivetrain on a small city car is especially difficult. Not only are the packaging constraints particularly acute, both for the engine itself and for the traction batteries in the trunk area, but the pressure on costs is intense and the benefit-to-cost ratio needs to be much more obvious to the consumer than it is in less price-sensitive segments of the market.

Fortunately, however, Fiat has an important and unique asset at its disposal – the TwinAir engine, just 80 kg and half the length of a conventional four-cylinder unit. Its compactness is all the more obvious following the elimination of the entire front-end ancillary drive and the relocation of these accessories (see below). It is this compactness that will be key to the viability of any downsized hybrid production program.

Just as the 500’s 903 cc engine, complete with throttle-less induction control and optional turbocharging, is derived from the bigger four-cylinder MultiAir units powering medium and larger-sized Alfa Romeos and Fiats, so the new transmission is related to the C635 dual dry clutch gearbox first shown in early 2010 and only recently made available on volume production vehicles.

However, whereas the engines are similar in their overall architecture, FPT’s gearbox designers have taken advantage of the extremely compact two-cylinder engine to adopt a different layout for the smaller DCT unit. With the C635, designed to fit into C- and B-segment cars with four and five-cylinder transverse engines, the priority was to keep the axial length of the transmission as short as possible so as to permit packaging into these relatively narrow platforms. This led to a three-shaft layout and a very tight fit for the dual dry clutch module. With the new two-cylinder engine being so short, however, no such constraints exist and the new transmission, with just five speeds instead of the C635’s six or, later, seven, has moved to a simpler two-shaft layout.

Minimizing Mechanical Losses

This, explains Dr. Ing. Constantinos Vafidis, Director of Transmissions and Hybrids at FPT’s research and technology division in Turin, is a more efficient arrangement in terms of minimizing mechanical losses. It also allows all the synchronizers to be on the same shaft and permits the use of a simpler electromagnetic drum-type sequential gear selector mechanism. Early on in the program, the idea of mounting the electric traction motor concentrically with the engine output shaft was rejected. The chosen solution, that of a motor mounted on a parallel axis, allows the inclusion of an idler gear, thus permitting the fitting of a lighter and cheaper motor capable of multiplying its torque through reduction gearing.

The low weight and generally urban use pattern of the final vehicle helped steer Vafidis and his team toward a five-speed arrangement. The extra torque of the e-motor would allow good low-speed acceleration, even with relatively tall gearing, while a relatively low top speed requirement meant the cruising ratio did not have to be as high as it would have been on a more highway-oriented model.

In keeping with the theme of minimizing energy waste, both the gear shifting and clutch actuation systems are electromechanical. Gear selection takes place via a selector drum, driven by a brushed DC motor, with its three grooves guiding their corresponding selector forks. The complex geometry of these grooves allows for all the combinations of gear engagement to suit hybrid function requirements, says Vafidis.

The concentric dry clutches are of the normally open type. Each is actuated by a separate DC motor via a mechanism converting radial movement into the required axial travel. Again, this departs from the larger C635 DCT’s arrangement, where the actuation of both gear changes and clutches is electrohydraulic.

Architecture

In architectural terms, the electric traction motor is permanently geared to the second gear pinion on the transmission’s layshaft. This means there is no clutch between the motor and gearbox, and that the motor is always spinning. For this reason, says Vafidis, it was important to select a motor type with no parasitic losses. This ruled out permanent magnet motors, at a slight cost to overall efficiency.

The traction motor does much more than just power the car. Under Fiat’s PB, or power-based strategy, it also smoothes out the responses of the drivetrain so that the vehicle experiences precisely the torque requested by the driver. As it is responsible for cranking the engine at each restart, it adds extra torque if it has to restart the engine while also powering the vehicle – so the driver does not feel any reduction in tractive effort – and it even speeds up or slows down the gearbox layshaft so that its inertia does not affect the smoothness or speed of the gearshifts.

The motor itself is of the induction type and is rated at 3 kW continuous, 10 kW for 60 seconds or 20 kW for a 10-second transient, with a peak torque of 65 Nm. Weighing 18 kg and with a maximum rpm value of 12,000, it has its own water cooling circuit that also cools the power electronics.

Keeping Cool

The mechanical air conditioning compressor is driven through an electromagnetic clutch by a short belt off the spindle of the traction motor, ensuring that climate control can be maintained if required during engine-stop periods. This, says Fiat Powertrain, avoids the need to employ a costly and inefficient, electrically-driven air conditioning compressor.

The motor also replaces the conventional 12V alternator and charges the vehicle’s service battery (now smaller in size) via a 1 kW step-down DC/DC converter. The electric power management unit is packaged in an aluminum housing in the underhood space occupied by the standard vehicle’s service battery. The 240V, 3.0 Ah lithium ion Toshiba traction battery weighs 26 kg and is packaged in the spare wheel well in the trunk floor. It is cooled by air drawn in from the exterior and has provision for external recharging, too.

The powertrain weight rises by just 30 kg over the standard TwinAir model, but the other hybrid components add a further 90 kg, bringing the total weight gain to 120 kg. According to the FPT engineers, this has resulted in the vehicle jumping up one inertial class for homologation, though they have not yet suggested measures for further weight reduction to avoid this.

Operational Strategy

As with other hybrid models, the development 500 TwinAir hybrid launches in electric mode when the accelerator pedal load is low or medium. At a suitable speed the engine is cranked via the even clutch, with the motor increasing its torque so the driver does not notice any reduction in tractive effort. Upon release of the accelerator pedal the combustion engine stops, the electric motor provides the effect of engine braking, and any pressure on the brake pedal prompts the motor/generator to absorb further torque.

Higher power demands are met with the boost mode, combining both the electric and combustion power sources; when the traction battery requires recharging, the torque of the combustion engine is increased accordingly.

The development team is confident that the targeted CO₂ emissions reductions targets are feasible. The base vehicle, with its naturally aspirated twin-cylinder engine, already has low overall CO₂ emissions of 95 g/km on the European cycle. The team is aiming at a 12 percent reduction in this figure – to just 85 g/km – in the combined cycle, with an even more spectacular cut of 24 percent in the urban portion of the cycle.

Vafidis points out that the arrangement is equally compatible with the turbocharged version of the MultiAir engine, giving 105 hp, and that, with its already significantly reduced parts count, the transmission would be viable as a non-hybrid DCT through the simple deletion of a handful of its components.

Yet, warn the engineers, though both the investment and variable costs of the hybrid system have been reduced by careful design to an absolute minimum, overall costs will continue to be a critical issue in the context of the price pressures prevailing in the small-car segment.

Component	Characteristics	mass
Gasoline Engine	2-cylinder naturally aspirated, 48 kW @ 6,000 rev/min, 90 Nm @ 3,500 rev/min	80 kg
Transmission and Flywheel	Dual dry clutch and dual mass flywheel; 5-speed sequential	69 kg
Electric Motor / Generator	Asynchronous induction type; 3 kW, 14 Nm continuous, 20 kW, 65 Nm transient for 10 sec	24 kg
Power Electronics	Inverter, DC/DC converter	18 kg
Battery Pack	Toshiba 240 V, 3.0 Ah	26 kg
CO2 Emissions (standard)	95 g/km	-
CO2 Emissions (target)	-12% combined cycle -24% urban cycle	-
Source: Fiat Powertrain Technologies, 9th International CTI Symposium, Berlin 2010

Story Filed: 1/26/2011
By Tony Lewin, managing editor DCTfacts.com