Optimum Start-Stop Solutions for an Efficient Future
Engine start-stop systems are the point of entry into drive train electrification and even today, their function extends far beyond that of simply starting the engine. Schaeffler considers itself a partner for different start-stop concepts with its broad technology portfolio. The most efficient possible use of the electric power available from the start-stop system installed in the vehicle is one of the engineers’ development goals. An increase in the internal combustion engine’s load point for operation in the ideal operating range, requirement-based operation of the accessory units, regenerative braking, boosting, and sailing are all indispensible when it comes to fulfilling future emissions regulations.
“Over the past few years, Schaeffler has developed a series of innovative products that contribute to the optimization of start-stop systems. These range from the general optimization of the components in order to accommodate the significant increase in the number of start procedures to new Schaeffler solutions for start-stop systems. The number of engine starts is increased from approximately 35,000 in the case of conventional vehicles without start-stop systems to more than one million in the case of vehicles with a sailing function or hybrid operating modes”, emphasizes Schaeffler CTO Prof. Peter Gutzmer. For example, the crankshaft bearings now run much more frequently with mixed friction before a stable hydrodynamic lubricant film is formed. This phenomenon also affects the camshafts and tapping elements of the valve train. The situation is similar for many components in the drive train, such as the dual mass flywheel, which is subjected significantly more often to the resonance that occurs when the engine is started.
Permanently engaged starters
With conventional systems, drivers often find the engine start procedure uncomfortable, especially in so-called “change of mind” situations where the driver decides that he/she wishes to accelerate again while switching off the engine.
As a solution to this problem, Schaeffler has developed a range of concepts for permanently engaged starters. The first concept is for vehicles with automatic transmissions and places the starter motor above the torque converter in the transmission housing. The starter pinion is permanently engaged with the starter ring, which is supported by back-up rollers and can be rotated in relation to the torque converter housing. A wrap spring mechanism connects the starter ring to the torque converter housing as soon as the starter motor begins to turn. When the engine “overtakes” the starter as it starts up, the wrap spring disengages from the torque converter housing and the return spring ensures that a defined disconnection takes place. When the engine is running, the starter is decoupled from the torque converter, thus minimizing wear and friction. This system allows fast and quiet restarting and rapid re-acceleration of the crankshaft while the engine is being switched off.
A second concept involves the integration of a locking roller clutch into the conventional starter ring. The locking roller clutch is connected to the engine oil circuit and thus operated “wet”. This concept also offers rapid restarts and cold starts, even while the internal combustion engine is being switched off and irrespective of the transmission type. The one-way clutch also completely disconnects the starter ring when activated by centrifugal force and thus does not cause any drag torques during normal engine operation.
Belt-driven starter generator
“A problem that occurs when a belt-driven starter generator is additionally used for boosting and energy recuperation is that rapid switching between boosting and energy recuperation can cause vibrations in the belt drive. This is prevented by a special tensioner on the belt drive, which assists the load reversal”, explains Dr. Martin Scheidt, Vice President Development Engine Systems at Schaeffler. The tensioner uses an oscillating movement to decouple the starter generator from the vibrations caused by the internal combustion engine and thus prevents peak stressing in the belt drive. The tensioning element also ensures a constant optimum tension in the belt and compensates for the lengthening that it undergoes due to aging. The decoupling tensioner is mounted on the starter generator in a space-saving way and replaces the two tensioning elements that would otherwise be required in the belt drive. A further reduction of rotational irregularities in the belt drive can also be achieved using a pulley decoupler that is mounted directly on the crankshaft. The decoupler can optionally be equipped with a switchable clutch unit to allow it to perform an independent temperature control function. The belt drive is thus disconnected from the internal combustion engine in a targeted manner.
Quiet engine starting with electronic camshaft phasing units
Another new aspect of start-stop operation is the pre-positioning of the movable components for optimum restarting. Most gasoline engines today have hydraulic phasing units that are automatically moved to the most convenient starting position when the engine is switched off – which is generally a locked position – so that no noise is generated when the engine is started. Schaeffler uses the existing rotational angle sensors, which identify the position of the phasing units with a high degree of precision, for this purpose. Controllable pins ensure that they are locked during the phase in which no oil pressure is present. A disadvantage of hydraulically-activated phasing units, however, is that they cannot be adjusted while the engine is switched off or during the early part of the engine start phase due to the lack of oil pressure. Electric camshaft phasing units offer significant advantages here, as they can be set for slight engine decompression in the start phase even before or during the restarting process. The result: The engine starts significantly more quietly.
Latching valves instead of pumps and batteries
The effects of the engine start-stop system extend all the way into the drive train. Many vehicles feature automatic transmissions with torque converters, which have to be specially prepared for rapid restarting in terms of achieving and maintaining oil pressure. If the oil pressure drops every time the engine is switched off, the driver has to wait for it to be built up again when the engine is restarted before he or she can accelerate again. The solutions currently being used to shorten the waiting times include additional electric oil pumps and hydrostatic pressure accumulators. These provide the transmission with oil pressure for the start-up procedure while the internal combustion engine is at a standstill and therefore the main pump is inactive. Electric pumps and accumulators are not ideal with regard to costs, fuel consumption, and design envelope however. “In choosing to use latching valves, Schaeffler is intentionally choosing to go a different way. This is a valve that was developed by us and is activated by a hydraulic pressure pulse before the engine stops”, explains Jeff Hemphill, Vice President and Chief Technical Officer at Schaeffler North America. This stores a small quantity of oil under pressure for the following start-up procedure. It keeps the switching element at the contact point and assists the rapid initial acceleration of the vehicle.
Non-contact sensors as an enabler
For vehicles with manual transmissions, specific safety strategies and thus special design measures are required for restarting the internal combustion engine. For the engine to be stopped, the transmission must be placed in a neutral position and the clutch pedal must be free of any load. A sensor that is integrated into the pedal mechanism recognizes the actuation of the clutch pedal during the stopping phase as a signal to switch on the internal combustion engine. A further sensor in the transmission’s actuation mechanism monitors the neutral position of the transmission. Both sensors are integrated into existing components – without any sacrifices being made in terms of ease of operation thanks to the non-contact sensors.
Other News:
Schaeffler Offers Comprehensive Solutions for Modern Engine Start-Stop Systems
Demand grows for revolutionary armour steel from Tata
Matrix Shafts announces new OZIK Altus MFS Series of shafts
igus® to Showcase Motion Plastic Solutions at Pack Expo 2013
Redex Invests in Gear Center
NTN Develops ＂High Load Capacity Cylindrical Roller Bearings＂ with Rolling Element Separator
Dillon HPG500 Boundary Lubricant Prevents Metal-to-Metal Contact Under High Loads and Slow Speeds
Alstom to supply $33m wind turbine project
