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

The goal of the combustion engine related part of the SFB is the study of the real-time multivariable control for controlling thelow temperature combustion in a combustion engine.

 

Both the diesel- as well as the the gasoline low-temperature combustion (HCCI / CAI) react sensitively to external disturbances such as temperature fluctuations of the intake or rapid changes in load or revolution speed. Because this type of combustion is influenced by various parameters, it can only be stabalized by a multivariable control. Studies have shown that conventional control methods can not fulfil the high requirements of this task even in stationary operation.

 

Fast methods of nonlinear Modellgestützten Prädiktiven control (MPC) are therefore explored. With this type of controller an internal model of the regulated process is set up in the so-called state space. Anhand dieses Modells sind auch Aussagen über das zukünftige Streckenverhalten ausgehend vom momentanen Systemzustand möglich. Based on the current system state of this model predictuioins about the future behavior can be calculated. Based on this model the formulation of a cost function is possible, in which the projected deviation from a target value and predited changes of the actuating variable in a future time frame are penalized.

 

In the partner projects in the SFB a gasoline and a diesel combustion engine will be operated with this new combustion in the largest possible part of the load map. The two approaches differ in terms of control primarily by the operating actuators with which this type of combustion can be achieved. For the gasoline engine the amount of residual gas and the end of injection, for the diesel engine boost and rail pressure and additional parameters of the injection ayre the investigated actuators. As controlled variablres, the internal pressure, its rise, the temperature of the cylinder load are possible.

 

The used models are initially determined empirically, and replaced in the further proceeding by those created by the other sub-projects. Condition remains the real-time capability of the models. For each engine a separate controller is set up. The structure of the controller is adapted in the course of the project repeatedly to results from other projects.

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.