Control of CAI and HCCI combustion engines
Especially for individual mobility and transportation the use of hydrocarbon-based liquid fuels appears to be without alternative as this source of energy provides a high energy density.
The combustion of hydrocarbons brings a number of well-known disadvantages. On the one hand these are the resulting emissions of pollutants such as nitrogen oxides (NOx) and soot, which contribute significantly to urban and regional air pollution, on the other hand the emissions of CO2, wich is a greenhouse gas and made responsible for the increase in temperature in the Earth's atmosphere and thus for the change in the global climate.
Therefore, new combustion processes are developed, which meet the requirements regarding low emissions while maintaining the same efficiency. These are the HCCI (Homogeneous Charge Compression Ignition), or CAI (Controlled Auto Ignition) combustions, which avoid high peak temperatures by homogenization and exhaust gas recirculation, and thus lower the emission of NOx and soot significantly. Es stellen sich jedoch Verbrennungsinstabilitäten in Form von räumlich und zeitlich zufällig verteilten Selbstzündungen ein. Combustion instabilities arise in the form of spatially and temporally randomly distributed self-ignitions.
SFB 686 - Model-based Control of the Homogenized Low-Temperature Combustion
CAI gasoline single cylinder research engine
The CAI single cylinder research engine is operarted in the partner project D2 of the SFB by the Institute of Combustion Engines VKA. The engine is embedded into a test bench. It offers direct injection and a fully variable electromechanical valve train allowing an opening and closing of the valves regardless of the position of the piston. These two actors provide interference potential in the quantity and timing of injection(s) and strategy, quantity and homogenization of the exhaust gas recirculation.
The valve train allows various ways of internal exhaust-gas recirculation.
With the so-called Combustion Chamber Recirculation CCR the exhaust is extended only partially. The exhaust valves close before the upper top dead centre (OT) is reached, so that a residual amount of gas remains in the cylinder. This is compressed in an intermediate compression until top dead centre, and relaxed again until the intake valves are opened shortly to suck in the necessary amount of fresh air.
Alternatively, with the Exhaust port Recirculation EPR, the exhaust first is blown out as usual. The exhaust valves remain opened over top dead centre, so the exhaust is sucked back immediately into the cylinder unill they close. Subsequently, the intake valves are opened briefly to aspire the desired fresh air.
These two strategies are only two of many possible approaches. CCR covers an area in the load map below that of EPR. Within the SFB these two strategies are selected as the most promissing ones. The resulting controll problem is to influence the combustion with the exhaust gas recirculation, or the valve train respectively, and the parameters of the injection such that the desired load is achieved optimally in terms of efficiency.
HCCI diesel engine
The HCCI diesel engine of the project D3 of the SFB is operated by the Institute for Combustion Technology ITV. This series engine has an externally cooled exhaust gas recirculation (EGR), a turbocharger with variable geometrie turgine (VGT) and common rail injection.
Hier sind die Stellgrößen für die Verbrennung ebenfalls die Einspritzung und die Abgasrückführung. Here the alternated variables for the combustion are also the direct injection and the exhaust gas recirculation.
Because the engine is turbocharged and equpped with an external EGR, at first a controller for the air path of the engine will be set up, which allows to adjust the EGR flow mostely independent of the boost pressure. This is achieved by the EGR valve and the position of the nozzle guide vane inside the VG turbine, and additionally the position of a throttle located behind the compressor before the induction of the EGR.
Later on the overlay controller for the cobustion is created, which uses the EGR and possibly also the boost pressure as actors.
Other controlled variables for the combustion are the amount of injected fuel and the end of energizing.