Internal combustion engines require valves to aspirate and discharge gases. Engine designers require a high degree of flexibility to improve efficiency, reduce pollutants, and optimize the use of renewable fuels. Currently, the gas exchange valves of four-stroke engines are controlled through camshaft drives. Despite some complex additional mechanics, the flexibility of such camshaft-driven systems remains limited. An electrohydraulically actuated valve train — the FlexWork valve train - was developed that enables completely free adjustment of stroke and timing, while at the same time being robust and cost-effective.
The valve train is the “respiratory organ” of combustion engines - it manages the aspiration of fresh air and the discharge of exhaust gases, referred to as gas exchange. Today, only mechanically driven camshafts are used in series production for this purpose and are often equipped with additional mechanisms, some of which are quite complex. This modifies a valve movement pattern given by the camshaft, which is not possible without an increase in friction. At the same time, flexibility is not provided to the desired extent. What is in demand — among other things for adaptation to changing fuel properties — are fast valve movements even at low speeds, stroke adaptations, and cylinder-selective, widely variable valve timing.
The valves are actuated hydraulically and controlled electrically via a solenoid coil. As soon as a control current flows, a specially designed hydraulic valve opens, allowing hydraulic fluid to open the gas exchange valve to the desired extent in milliseconds counter to a spring. When the current is switched off, the gas exchange valve is closed again by the spring force and feeds a large part of the hydraulic energy required for opening back into the hydraulic system. The system achieves a significantly lower energy requirement over a wide operating range compared to camshaft-driven systems. Together with an optimized gas exchange, the fuel consumption of the test spark-ignition engine is about 20 percent lower than with conventional valve control using a throttle in combination with camshafts in the low load range typical for passenger cars.
By selecting the operating parameters, the opening and closing times and the valve lift for each cylinder can be chosen completely unrestricted. This means that each engine operating condition can be varied from cycle to cycle — for example, by intelligent load control — by selecting the residual gas quantity remaining in the cylinder (exhaust gas recirculation) or by deactivating unneeded cylinders without the driver noticing.
This makes the engine highly adaptable to new renewable fuels. Oxygen-containing fuels such as methanol or ethanol, for example, allow more residual gas to remain in the cylinder. Natural gas, biogas, and syngas generated from wind and solar power offer increased anti-knock properties and the valve train can react flexibly to this as well. In addition, alternative combustion concepts can also be implemented comparatively easily.
Another feature of the system is the choice of hydraulic fluid. Instead of using oil, a water-glycol mixture, i.e. engine cooling water, can be used. Due to its physical properties, this medium is suitable for fast-switching hydraulic systems, as it is very stiff and creates fewer hydraulic losses. This makes the cylinder head completely oil-free, which can allow a cheaper engine oil with extended change intervals to be used for the rest of the engine.