Options for Influencing the Charge Cycle

When operating a quickly switching valve with a freely controllable drive in the induction pipe, various functions can influence the charge. These concern the regulation of the air mass in the combustion chamber, and thermodynamic effects for specifically increasing the temperature of the inflowing fresh air.

Dynamic Supercharging in Induction Engines (Pulse Charge). Dynamic supercharging increases the air mass in the combustion chamber by opening the air stroke valve once or twice during the opening phase of the intake valve. Using the above-described process, the air column in the intake tract is accelerated at specific times and then delayed to excite vibration. For this to work, the pressure waves induced by the vibration must be enclosed in the combustion chamber with the engine intake valves so that the resulting increase in density improves the cylinder charge (see Fig. 10-87). The dual operation of the air stroke valve during the intake stroke poses great demands for the dynamics of the valve and the reproducibility of the switching procedures.

Fig. 10-87. Circuit diagram of dual air stroke valve timing in the intake phase to increase torque

Basically, the effect of the air stroke valve immediately starts upon load changes such as accelerations. Other advantages of this method are the usefulness of the dynamic supercharging when the engine starts. In addition, mixing is supported by faster charging because of the high inflow speed of the fresh air in the opening phases of the air stroke valve. The resulting homogenization of the mixture and the acceleration of combustion can potentially reduce the raw HC emissions of the engines during starting and regular operation.

Supporting and Recharging Supercharged Engines. When using mechanical supercharger or exhaust turbochargers, the air stroke valve can offer additional possibilities for enhancing the properties of the engine. Particularly in the case of turbochargers, the increased mass flow rate can greatly accelerate the response behavior of the supercharger. Especially when starting the engine, additional air mass is available in the combustion chamber that generates higher final compression temperatures and thereby improves cold start properties. Since the air mass flow is already accelerated at low rpm by the air stroke valve, the charging pressure can be lowered, which thereby reduces the specific load of the compressor. It is, therefore, conceivable to use smaller superchargers and dispense with turbochargers with a variable geometry and involved vane adjustments during partial loads.

From the perspective of energy, it makes sense to use the procedural variant where supercharged engines are “recharged.” First, uncompressed air is aspirated by the piston. At bottom dead center, compressed air is supplied from the supercharger through a parallel tract of the air stroke valve system. The air mass in the combustion chamber rises by approximately 50% from one cycle to the next. Only a part of the combustion air flows through the compressor, and dynamic supercharging effects can additionally reduce the compression in the supercharger. This allows the size of the supercharger to be reduced and lowers the energy required for the supercharger since only the air mass flow required for recharging must be compressed.

Throttle-Free Load Control. Throttle-free load control is an important step toward substantially reducing the consumption under partial loads in spark-ignition engines by minimizing the charge cycle loss. The opening time of the air stroke valve is adapted in relation to the required air of the engine. To achieve minimal air mass flow, a phase shift can be set between the air stroke valve opening and the intake valve opening so that both opening times only slightly overlap. This reduces the importance of a fast valve operating time in this method. In contrast, the valve leakage and the volume between the air stroke valve and intake valve gains in importance since the air mass enclosed there during the intake process is available in the combustion chamber.

When a valve is used that can be operated independent of the valve control time of the engine, different methods such as an early inlet close (EIC) or late inlet open (LIO) can easily be used in connection with a conventional mechanical valve gear to optimize engine operation under a partial load. The valve operation must be very precise to limit the aspirated air mass to that required by idling.

EGR Control. Similar to recharging supercharged engines, the exhaust gas recirculation cylinder can be selectively controlled so that exhaust is first aspirated, with a change to the aspiration of fresh air during the induction stroke. This yields a specified charge stratification in the combustion chamber that opens up additional possibilities for variation in terms of the inflow speed and charge movement.

Hot Charging. With hot charging, both an increase in the air mass and a rise in temperature of the aspirated air in the combustion chamber are sought to positively influence the mixing process in spark-ignition and diesel engines during cold starts and the warm-up phase. This process can be used by the starter during the first engine revolutions. If the air temperature is increased enough, the glow plugs in diesel engines can be dispensed with. In addition, the exhaust after treatment system starts much more quickly, which makes it easier to meet the D4 exhaust standard for diesel engines and to start the cabin heating system. Another possibility is to reduce the compression ratio in diesel engines for reasons of consumption without influencing their ability to start cold.

The increase in temperature results from a change in state in the aspirated fresh air. First, the pressure in the cylinder strongly drops when the air stroke valve is closed and the intake valve is opened by the movement of the piston. The enclosed mixture consisting of residual gas and fresh air is expanded. Since sufficient heat can be supplied through the cylinder walls, this process can be viewed as isothermic. After the air stroke valve opens, the cylinder fills very rapidly.

The air is accelerated extremely fast by the strong vacuum. Thermodynamically, this process corresponds to the compression of the aspirated air in the cylinder that necessarily results in an increase in temperature. The amount of the attained temperature difference depends on the moment of opening and the length of opening of the air stroke valve, as well as the leakage of the closed valve.

The arising strong air expenditure, the high inflow speeds, and the higher final compression temperature allow more fuel to be injected with improved combustion quality. This leads to higher exhaust temperatures in a cold start, less combustion noise, faster warm-up, improved load assumption by the engine, and reduced cold start emissions. A prerequisite is also a very quickly operating valve in this method that allows a sufficiently fast intake of aspirated air into the cylinder and prevents backflow at the end of the intake phase.

Cold Charging Supercharged Engines. Like hot charging, the air stroke valve can also be used to specifically reduce temperature of the air in the combustion chamber of supercharged engines. By means of EIC, charging air compressed by the supercharger and precooled by the heat exchanger is enclosed in the combustion chamber, expanded by the piston movement, and thereby further cooled. This reduction of temperature lowers the temperature and pressure at the end of compression in the combustion chamber that reduces NO_x formation in spark-ignition and diesel engines and decreases the knocking tendency in spark-ignition engines. Alternately, a high charging pressure can be achieved with the resulting higher final compression temperature, which can be used to further increase the torque and output of the engine.

Cylinder Shutoff. Another variation is to alternately shut off the intakes to individual cylinders during partial-load operation, which is easily done by closing the air stroke valve during the intake process. The air supply of the shutoff cylinder can be periodically connected to prevent the combustion chamber from cooling as, for example, is possible with electromechanical valve gears. This process can be easily realized in connection with a conventional mechanical valve gear. Primarily, the shift of the working cylinder to a higher load can be exploited. In contrast to electromagnetic valve actuation with the possibility of completely closing all valves, additional shifting work must be done by the engine since the exhaust valve remains open.