Influence of Oxy-Fuel Combustion on Engine Operating Conditions and Combustion Characteristics in a High Speed Direct Injection (HSDI) Diesel Engine under Homogenous Charge Compression Ignition (HCCI) Mode

Oxyfuel combustion and nitrogen-free combustion coupled with Carbon Capture and Storage (CCS) techniques have been recently proposed as an efficient method to achieve carbon free emissions and to improve the combustion efficiency in diesel engines. In this study, a 3-D computational fluid dynamics model has been used to evaluate the influence of oxyfuel-HCCI combustion on engine operating conditions and combustion characteristics in a HSDI diesel engine. Investigations have conducted using four different diluent strategies based on the volume fraction of pure oxygen and a diluent gas (carbon dioxide). The first series of investigations has performed at a constant fuel injection rating at which 4.4 mg of fuel has injected per cycle. In the second part of analysis, the engine speed was maintained at 1500 rev/min while the engine loads were varied by changing the fuel injection rates in the range of 2.8 to 5.2 mg/cycle. Results showed that oxyfuel-HCCI combustion has brought CO and PM emissions to very ultra-low level while NOx emissions have been completely eliminated. It has found that by increasing the diluent ratio at low engine loads the amount of ISFC is largely increased while it doesn’t bring any remarkable change at high engine loads. In addition, the ITE is reduced from 21.8% to 14% as the carbon dioxide concentration is increased from 77% to 83%. Applying higher percentage of diluent gas will greatly increase the overall specific heat capacity of intake charge which leads to decrease of temperature and consequently, the cylinder pressure during combustion process results in lower indicated power and ITE.