The Parallel Shift Gearbox (PSG) offers the ideal basis for a drive with high comfort and excellent efficiency. Opportunities to integrate an electric motor are being sought in order to further reduce the fuel consumption. In addition to the quick, noiseless start (start-stop) this should also facilitate the recovery of braking energy (recuperation) and a downsizing of the combustion engine through a booster function. As a first approach, the direct connection of the electric motor to the crankshaft of the combustion engine is being considered. Such a design is also called a 'crankshaft starter alternator (CSA)' or 'Integrated Starter Alternator (ISA)' and has particular application in association with shift gearboxes.
The disadvantage of such a structure is the limited recovery potential through drag losses of the combustion engine when coasting. Another solution is the use of an additional clutch between the electric motor and the crankshaft. If the combustion engine is disconnected when coasting, the full potential can be used for the recovery of deceleration energy. Moreover, the separation of engine and electric motor can also lead to a further reduction in fuel consumption as the vehicle is moving solely under electrical power. LuK's concept of an ESG goes one step further. A proposal has been made to integrate the electric motor into the gearbox. In additional to functional benefits, this structure offers significant space and cost benefits (Figure 1).
In order to be able to clearly explain the individual functions of the ESG, both part gears are shown in parallel (Figure 2). The lower part contains the odd gears as well as the clutch K1 and is described as Part Gear 1. Similarly, Part Gear 2 house the even gears and clutch K2.
During operation in Part Gear 2, the starter alternator (SG) is linked directly to the combustion engine via clutch K2. Figure 2 shows for example driving in 4th gear with the generator function.
When driving in an odd gear, an even gear will be preselected or a switch to neutral depending on the shifting strategy in Part Gear 2. If Part Gear 2 is in neutral, clutch K2 is closed to transmit the alternator torque. With a preselected gear, the drive of the electric motor occurs via the engaged gear. By reversing the direction of torque to the electric motor, a booster function can again be fulfilled.
Purely electrical driving is possible with correspondingly high installed electrical output of the electric motor as well as the battery. If this is the case both clutches are open and, depending on the speed and load as well as the efficiency characteristic, the torque will be transmitted to the drive wheels via 2nd, 4th or 6th gear.
Braking energy recovery (recuperation)
In order to achieve efficient use of the braking energy, the combustion engine is disconnected from the drivetrain during coasting. The electric motor transfers the vehicle's deceleration, which depending on speed, brake pedal position and optimal transmission builds up dynamic torque and the kinetic energy of the vehicle is transformed into electrical energy. By integrating the A/C compressor into Part Gear 2, the kinetic energy of the vehicle can also be harnessed directly for the generation of cooling capacity. By using a suitable coolant such as CO2, the air conditioning system can also generate heat.
In addition to energetic benefits along the lines of the direct generation of cooling capacity with recuperation, the system also promises major benefits for the thermal management of the hybrid vehicle. This is of particular relevance to modern combustion engines with direct injection, fully variable control times etc., which due to their low partial load consumption are already reliant to some extent on auxiliary heaters.