Closed-Loop Combustion Control of a Multi Cylinder HCCI Engine using Variable Compression Ratio and Fast Thermal Management
Abstract: The current Spark Ignited (SI) engine equipped with three-way catalyst offers low emissions, but has low efficiency at part load, which results in unnecessarily high CO2 emissions. The Compression Ignited (CI) engines have higher efficiency and hence lower CO2 emissions, but suffer from higher Nitrogen Oxide (NOx) and Particulate Matter (PM) emissions, and no three-way catalyst can be used. Large efforts are made to reduce the emissions of the CI engine and raise the efficiency of the SI engine. A third internal combustion engine type, which combines the principles of the SI and CI engines, has the potential to meet demands from society in terms of current and foreseeable future emissions regulations. This engine type is called Homogeneous Charge Compression Ignition (HCCI) engine. It utilizes a homogeneous premixed air fuel mixture like the SI engine, but the mixture is compressed to auto ignition like in the CI engine. Very dilute mixtures are used to slow down the combustion rate. This results in a low combustion temperature with extremely low NOx emissions. Due to the homogeneous charge the combustion produces no PM. The Carbon Monoxide (CO) and unburned Hydrocarbon (HC) emissions are, however, higher than for the other engine types due to the low combustion temperature, but an oxidizing catalyst can quite easily treat CO and HC emissions. Combustion control of the HCCI engine is a challenge since there is no direct means to control when combustion starts unlike the SI or CI engines. Combustion phasing in an HCCI engine can be achieved by affecting the time history of pressure and temperature in the cylinder. The most common way to do this in the lab has been to control the inlet air temperature and thereby the temperature in the cylinder at the end of the compression stroke. With an electrical heater this is a slow parameter to control. In the present study a multi cylinder engine equipped with Variable Compression Ratio (VCR) and a Fast Thermal Management (FTM) system is used to control the combustion phasing. Both the Closed-Loop Combustion Control (CLCC) using VCR and the CLCC using FTM are the first presented such systems on a multi cylinder engine in the literature. A system identification is made and a Linear Quadratic Gaussian (LQG) controller is designed for the fast thermal management instead of the standard PID controller. CLCC performance of both PID and the state feedback based LQG controllers using fast thermal management are investigated by perfroming step response experiments. A CLCC strategy using variable compression ratio and a cylinder individual fast thermal management is presented and investigated experimentally by running a hot drive cycle test. To this date it is the first experimental drive cycle test using inlet air preheated HCCI, i.e. FTM. It is concluded that the proposed CLCC strategy in terms of fuel consumption is good enough, but in terms of emissions there is still some room for improvement. For a scaled engine to 3.0L an improvement in fuel consumption of 16% is accomplished compared to an SI simulation using mean steady state data from the same engine, with an EC2000 drive cycle calculated for a 1.6L Opel Astra. If doing an imagined mode transfer without scaling the engine a fuel consumption of 6.8L/100km is achieved. Due to very high friction losses of this engine a simulation with lower friction losses of a modern engine is made, which result in a 15% improvement in fuel consumption compared to an ?ideal? simulated HCCI drive cycle test with this engine. With an optimized controller and an optimized engine in terms of friction losses a fuel consumption of 5.8L/100km is realistic for the HCCI-SI approach.
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