Independent thesis Advanced level (professional degree), 20 credits / 30 HE credits
An Homogenous Charge Compression Ignition engine is a hybrid between a Diesel and an Otto Engine. It has good fuel efficiency, close to a Diesel engine and also very low emissions of NOX and nearly no particulate soot. Other emissions are higher but can be after treated by a catalyst. The engine has not yet been fully developed so far and lacks among others a good automatic control of the combustion angle which should be held in a small window to achieve the best performance.
The objective in this thesis is to achieve fast control that can hold the combustion angle window for changes in inlet pressure, loads and engine speeds. The challenges are that the system has a delay which limits the bandwidth, the dynamics change with different working conditions and a relatively large noise amplitude.
Using combustion angle as reference, valve timings as control signal and other variables as engine speed, inlet pressure and load changes as disturbances, a closed loop control system can be defined. Two control methods using different kinds of variable valve timings and three controllers for each method were designed to cover most of the working conditions. These were connected by a hybrid automaton which handles all transitions and choice of controller.
The result was a fast control with a 6-8 engine cycles risetime for reference changes and it can suppress inlet pressure and load disturbances well inside the combustion window. The noise showed to be white, that is using all frequencies. To achieve both a fast control and not magnify the noise a non-linear compensation link was designed that uses less gain and bandwidth for small errors. A problem with this kind of dynamic solution is engine speed ramps which needs fast reaction times which was impossible due to the delay. A proposed solution is to use mappings from non-delayed variables to do necessary adjustments of the control signal fast enough.
2006. , 57 p.