Study On Combustion And Emission Characteristics Of An HCCI Engine Fueled With N-heptane/isoamyl Alcohol Blends

The energy crisis and environmental pollution have become two major problems that restrain the sustainable development of human society.Energy consumption and air pollution brought by the increase of the number of vehicles have forced people to search for new methods for energy-saving and emission reduction of internal combustion engines.As a new combustion method,homogeneous charge compression(HCCI)has attracted more and more attention of researchers.HCCI has the advantages of high thermal efficiency,low NO_x and PM emissions.But it also faces challenges,such as unmanageable combustion phase.And the operating range is narrow.In this paper,an HCCI test bench system,which uses n-heptane/isoamyl alcohol mixed fuel,was set up based on an engine bench test.The influence of fuel blending ratio,intake air temperature,excess air coefficient and engine speed on combustion characteristics and emission characteristics were investigated,and the operating range was also studied.The experimental results show that with the increase of isoamyl alcohol ratio,the peak in-cylinder pressure,the peak value of pressure increase rate,the peak temperature and the instantaneous heat release rate all gradually decrease,and the phase corresponding to the peak value is gradually delayed;the start combustion timing and main combustion timing move back and the combustion period becomes longer.The peak in-cylinder pressure and COV become larger and the combustion stability becomes worse.the CO and HC emissions show a slight decrease at first and then increase gradually.With the increase of intake air temperature,the peak in-cylinder pressure,the peak value of pressure increase rate,the peak temperature and the instantaneous heat release rate all gradually increase,and the phase of the peak value gradually advances;the start combustion timing and main combustion timing advance and the combustion period is shortened;the COV is gradually reduced and the combustion stability was improved;CO and HC emissions gradually reduced.With the increase of the excess air coefficient,the peak pressure in the cylinder,the peak value of pressure increase rate,the peak temperature and the instantaneous heat release rate all gradually decrease,the phase of peak value is gradually delayed,and combustion period move backwards.The combustion period becomes longer,the COV becomes larger and the combustion stability becomes worse,CO and HC emissions increase gradually.With the increase of engine speed,the peak pressure and peak temperature in the cylinder increase firstly and then decrease slightly,the COV decreases firstly and then increases slightly,CO and HC emissions decrease firstly and then increase gradually.It can be found from the investigation of the HCCI operating range that increasing the proportion of n-heptane under small loads may help to achieve better low loads,and increasing the ratio of isoamyl alcohol under heavy load conditions will improve the adaptability of heavy load condition and they both help to expand the HCCI operating range.In addition,an appropriate increase of the intake air temperature can also extend the HCCI operating range.