Combustion Control and Performance Analysis of CI Modified HCCI Engine Using Lean Combustion Technology

Abstract

Amongst the numerous research papers published over the last decade, the homogenous charge compression ignition (HCCI) has often been considered as a new combustion process in reciprocating internal combustion engines. With increasingly stringent emission legislation and demand for significant reduction in CO2 emission, research and development of cleaner and more efficient combustion engines has been intensified. HCCI combustion has emerged as an effective and viable technology that has the potential of simultaneously reducing pollutant emissions and fuel consumption from internal combustion engines. The investigation focuses on the effect of diesel vapour induction on the engine performance and to try and achieve Homogeneous Charge Compression Ignition (HCCI) mode of combustion in the engine. An existing direct injection CI engine is modified to work as an HCCI engine by using a shell and tube heat exchanger which aids in the production of diesel vapour by utilising heat content of exhaust gases . The external mixture formation is adopted for the preparation of homogenous charge. The diesel vapour coming out of the heat exchanger is mixed with air near the intake manifold. The experimental set up is modified so that the flow of exhaust gas to the heat exchanger and the flow of diesel vapour to the engine can be controlled. A separate fuel tank is provided to measure the amount of diesel vapour utilized. Vapour utilization studies were carried out. It is found that a maximum utilization was limited to 60 percentage at different load conditions. After that knocking occurs and engine stops working. The loading of HCCI mode was limited from 50 to 100 % due to poor vapour quality at lower loads. The performance and emission characteristics of HCCI engine is studied at different injection pressures and injection timings and is compared with conventional engine. It is found that a higher injection pressure of 200 bar and advancing the injection timing to 31.50 bTDC improved the brake thermal efficiency of the engine and reduced NOx emissions with increase in Hydrocarbon and Carbon Monoxide emissions. A maximum of 20.54 % increase in brake thermal efficiency is obtained at 75 % load condition. NOx emissionsare reduced upto a maximum of around 50% at 31.5 deg. bTDC, 200 bar injection pressure and in vapour induction by 37% whereas CO emissions are increased around 31% and HC emissions are increased by 47%. Normally Research engines are fitted with piezoelectric pressure transducer for the measurement of in cylinder pressure. But strong ion concentration is formed in the cylinder by the combustion process. If the ion concentration is detected, it can be used as a combustion diagnosis tool. Here a standard spark plug is converted to work as the ion sensor. It is carefully mounted over the cylinder head without affecting the passage of cooling water. An electrical circuit is designed to measure the ion voltage produced during the combustion process. Then a correlation between ion voltage and cylinder pressure is developed from the measured data so that ion voltage is calibrated in to in cylinder pressure. The results show that there is only 10% difference between the pressure given by the pressure sensor and ion sensor. Therefore the expensive pressure transducer is replaced by cheap and reliable ion sensor for combustion monitoring. Extending the operation range in HCCI mode is a very important factor. For every load condition the amount vapour inducted to the engine is limited. Preheating of intake charge reduces the possibility of condensation of diesel vapour near intake manifold. Therefore the effect of preheating was also studied by heating the inlet air by using a heating coil. Preheating always improved the percentage vapour utilization. A percentage increase of 5.91 %, 7.93 % and 7.3 % in percentage vapour utilization is found for 50 %, 75 % and 100 % load conditions respectively. Preheating improved the brake thermal efficiency and brought down CO and HC emissions however it slightly increased NOx emissions. The maximum efficiency is 33. 5% seen at 75% load condition for preheating temperature of 65 °C and at percentage vapour utilization of 37.95 %. A maximum percentage reduction of 78.33 %, 45.15 % and 57.14 % in CO emissions was attained by preheating of air at 50, 75 and 100 % load conditions respectively. A maximum percentage decrease of 48.3 %, 50 % and 44.82 % in Unburned Hydrocarbon emissions was attained by preheating for 50, 75 and 100 % load conditions respectively. NOx emissions are increased by almost 5% for different load conditions by preheating of theintake air. Preheating of vapour was limited to 65 °C due to continuous increase in NOx emissions. Thus the most suitable operating condition for HCCI mode can be identified as 75 % load coupled with 65 °C preheating. The increase in vapor mass fraction increased the performance of the engine. This was mainly because the HCCI mode of combustion was approached. At the same time the start of combustion was still governed by the injection of vapor fuel. This gave a method of control of combustion which is normally difficult in HCCI engines