Save 20 Percent Fuel

The electro-hydraulically actuated valve train developed at Empa enables completely free adjustment of stroke and timing, is robust and cost-effective at the same time and has been running successfully in test bench operation for several months.

The valve train is the "breathing organ" of combustion engines: it controls the supply of fresh air and the discharge of exhaust gases, which is referred to as "gas change". Today, only mechanically driven camshafts are used in series production for this purpose, often equipped with an additional mechanism, some of which is quite complex. This allows a valve movement pattern specified by the camshaft to be modified, which is not possible without increased friction. Likewise, the flexibility is not given to the desired extent. Fast valve movements even at low speeds, stroke adaptations and cylinder-selective broadly variable valve timing are sought after - among other things also for adaptation to changing fuel properties.

Patrik Soltic and his team at Empa's Vehicle Drive Systems department invented and developed an electrohydraulic valve train together with hydraulics specialist Wolfgang Schneider that is significantly more flexible than today's series production technology. The valves are actuated hydraulically and individually 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 stroke in milliseconds against a spring. When the current is switched off, the gas change valve closes again by the spring force and feeds most 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 than camshaft-driven systems. Together with an optimized gas exchange, the fuel consumption of the test spark-ignition engine in the low load range typical for passenger cars is around 20 percent lower than with conventional load control using a throttle valve in combination with valve control via camshafts.

By selecting the operating parameters, the opening and closing times as well as the valve lift for each cylinder can be freely adjusted. This means that every engine operating condition can be changed from duty cycle to duty 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 that is not noticeable to the driver. 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 have increased anti-knock properties: the valve train can react flexibly to these too. In addition, alternative combustion concepts can also be implemented comparatively easily, such as homogeneous auto-ignition: a fuel-air mixture is ignited at the right moment without ignition sparks by setting the correct conditions towards the end of compression and burns practically pollutant-free.

Another specialty of the system set up at Empa is the choice of hydraulic fluid: instead of using an oil as usual, a water-glycol mixture, i.e. engine cooling water, is used. Due to its physical properties, this medium is very suitable for fast-switching hydraulic systems. This means that the cylinder head is completely oil-free, which can mean that a simpler engine oil with longer change intervals can be used for the rest of the engine.

As part of the "FlexWork" project sponsored by the SFOE, the new valve train was put into operation in a passenger car engine powered by natural gas and derived from a VW 1.4l TSI engine. The necessary components were manufactured by Empa's test workshop. The control system for the test engine was developed by the Empa researchers themselves. The valve train has been running on an Empa engine test bench since October 2018 and has already survived many millions of operations in fired engine operation without any problems.

The FlexWork valve control requires only low-cost components. No expensive, very fast switching valves and no complex sensors are required. The valve train system was presented to experts in the Motor Technical Magazine (MTZ) on 16 August. Empa is in talks with engine manufacturers for the transfer of this technology, which is suitable not only for combustion engines but also for compressors.