HCCI Diesel Engines - Engine Combustion - Lecture Notes, Study notes of Sustainability Management

Download HCCI Diesel Engines - Engine Combustion - Lecture Notes and more Study notes Sustainability Management in PDF only on Docsity! Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture34/34_1.htm[6/15/2012 3:09:10 PM] Module 7:Advanced Combustion Systems and Alternative Powerplants Lecture 34:HCCI Diesel Engines The Lecture Contains: HCCI DIESEL ENGINES Emissions with HCCI Operation Regimes of HCCI and Conventional CI Engine Operation Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture34/34_2.htm[6/15/2012 3:09:10 PM] Module7:Advanced Combustion Systems and Alternative Powerplants Lecture 34:HCCI Diesel Engines HCCI DIESEL ENGINES The objective of application of HCCI concept to the diesel engines is control of NOx and particulates simultaneously which otherwise is difficult to achieve in the conventional CI engines. For HCCI combustion the efforts are directed mainly towards: Creation of premixed, lean homogeneous mixture, and Compression ignition and control of rate of combustion to obtain low emissions and optimum engine performance. The basic objective is to increase the formation of premixed charge prior to ignition so that a large fraction of the fuel per cycle burns as premixed. Subsequent to ignition the rate of combustion is to be controlled to keep it within acceptable limits. The different methods which may be adopted to achieve the above two objectives are: Premixed Charge Formation: Fuel introduction in Intake manifold or port Early (much before the conventional injection timing) and multiple direct fuel injection in the cylinder Late direct fuel injection (at TDC or later) in the cylinder The different fuel injection timings that may be adopted in a diesel engine to obtain a long ignition delay and consequently a high fraction of premixed charge before start of combustion are shown schematically in Fig 7.10. For HCCI mode of operation a high fraction of premixed charge is essentially required before start of compression ignition . Ignition and Combustion Control: Multiple/split injection High amount of EGR Reduction in compression ratio Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture34/34_4.htm[6/15/2012 3:09:11 PM] Module7:Advanced Combustion Systems and Alternative Powerplants Lecture 34:HCCI Diesel Engines contd... The engine compression ratio is reduced to13.5 - 14:1 so that ignition delay is long to allow formation of a homogeneous mixture. In split/multiple injection approach, the first injection is made at least 50 to 60 º btdc. Cooled EGR is used to retard start of combustion of the premixed charge so that a higher engine power is obtained before knock combustion results. The low temperature autoignition is obtained near tdc just prior to main injection. The main fuel injection is made slightly after tdc. This strategy shows improved combustion efficiency without excessive CO and HC emissions. Figure 7.11 Schematic of multiple injection strategy for HCCI operation of a diesel engine Late In-Cylinder Injection: If the injection is made very close to or after tdc the fuel encounters a gradual decrease in gas temperature and pressure. It results in a long ignition delay and allows more time for mixture formation and favourable conditions for HCCI combustion are obtained. As most of the fuel now burns as premixed, a drastic reduction in soot formation is observed and high rates of EGR can be used to obtain very low NOx. The late injection strategy has the advantage in that the start of combustion is better related to the injection timing and hence the combustion process can be controlled by injection timing. Also, very little changes are necessary in the injection equipment. Nissan Motor Corporation developed an HCCI diesel engine using late injection strategy with low compression ratio. They called it MK (modulated Kinetics) combustion system. Late fuel injection and low compression ratio were combined with 30 – 45 % EGR to prolong ignition delay beyond end of injection. Injection timing was around 3-6 degrees btdc. Injection duration was reduced by use of high injection pressures and larger nozzle orifices such that the injection duration was 4-10º CA shorter than the delay period. The fraction of fuel that burns premixed increases substantially giving very low Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture34/34_4.htm[6/15/2012 3:09:11 PM] soot formation. Use of EGR and low compression ratio gave low NOx emissions. NOx obtained were less than 1/10 th and PM emissions less than half at 6 bar bmep and 2000 rpm compared to conventional diesel engine. Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture34/34_5.htm[6/15/2012 3:09:11 PM] Module7:Advanced Combustion Systems and Alternative Powerplants Lecture 34:HCCI Diesel Engines Emissions with HCCI Operation Emission potential of HCCI operation relative to conventional diesel engines is illustrated in Fig 7.12. The HCCI engine employed multiple early injection strategy with compression ratio equal to 13.4:1 and high rates of EGR. The NOx emissions were just 0.074 times (7.4%) and soot emissions were only 5.5% of the conventional CI engine operation. NOx emissions with HCCI operation ranged from 0.01 to 0.06 g/kWh and the soot emissions were below 0.02 g/kWh. Nearly 95% reduction in soot and NOx emissions are obtained compared to the conventional diesel operation. However, due to quenching of combustion in excessively lean mixture HC increased to 348%. CO increased to 487% of the conventional CI engine. As the mixture is not entirely homogeneous and some over rich zones exist more CO is produced. Although excess air is present in burnt gases but the combustion temperatures are low to promote oxidation reactions of HC and CO in the post combustion gases. Fig 7.12 Comparative emissions with HCCI and conventional CI engine operation; multiple early injection strategy of HCCI operation. Regimes of HCCI and Conventional CI Engine Operation Developments in application of HCCI combustion to conventional diesel engines may be summarized as below: Most of the strategies for HCCI operation essentially attempt to increase the fraction of fuel burned as premixed charge. The premixed charge obtained with early and more so with late in-cylinder injection actually may not