Studies on the Load Range of an HCCI Engine using In-Cylinder Pressure, Ion Current and Optical Diagnostics
Abstract: Internal combustion engines are continuously developed towards decreased hazardous emissions and lower fuel consumption. The homogeneous charge compression ignition (HCCI) engine is a promising concept which combines the best features of the Diesel engine and the spark ignition (SI) engine. The efficiency is in the range of the Diesel engine and the emissions are low and comparable to the emissions from an SI engine with catalyst. No flame propagation is present in the HCCI engine; instead the whole charge is gradually consumed at several locations simultaneously. The resulting pressure rise rate is therefore high which means that the engine has to be run lean or diluted with burned gases in order not to stress the mechanical parts of the engine or produce excessive noise. Due to the lean and diluted mixtures the combustion temperature is low and fairly uniform. This results in low emissions of NOx and PM but if the temperature is too low emissions of unburned hydrocarbons and carbon monoxide (CO) increase. Furthermore, the pressure rise rate limits HCCI combustion to low or part load and is therefore to be combined with either Diesel or SI operation at higher loads. In this thesis different diagnostic methods have been used to study the HCCI combustion process at different engine loads. Combustion diagnostics have been performed using cylinder pressure and ion current measurement as well as optical techniques such as chemiluminescence imaging. By pressure measurement the overall combustion behaviour in the cylinder can be studied. By ion current measurement local conditions in small volumes can be studied. This is of interest since there are differences in temperature and relative air/fuel ratio between different locations in the combustion chamber. Locations with higher temperature or richer mixtures ignite first and this affects the ongoing combustion cycle. Pressure diagnostics is accurate but expensive as feedback method of the ongoing combustion process. Ion current diagnostics is cheap but due to the low temperature nature of HCCI combustion it is only usable at higher engine loads and thus the method is not an option for low load or idle conditions. Different fuels will provoke ion current differently for the selected compression ratio, initial charge temperature, measuring location and relative air/fuel ratio. If the initial charge temperature is too low, partial or total misfire might occur. At these running conditions spark or laser assistance can be used to increase the charge temperature prior to auto ignition. This advances the combustion phasing, thus minimizing the risk of misfire. This is also interesting for control purposes since it is possible to control the HCCI combustion phasing by changing the spark or laser ignition timing. When running laser assisted HCCI, flame propagation is present from the laser ignition location and, as the pressure and temperature increase, auto ignition occurs. This operation is a mixed mode of flame propagation and auto ignition. Lastly the effect of combustion chamber geometry on HCCI combustion rate was studied. A square bowl in piston geometry compared to Disc geometry showed the possibility to extend the maximum load limit due to decreased pressure rise rates but at the cost of decreased engine efficiency. Chemiluminescence and LIF imaging as well as LES showed stratified combustion behaviour unusual for HCCI operation which normally is a more homogeneously distributed combustion process.
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