Experimental Research On The Improvement Of Characteristics Of Homogeneous Charge Compression Ignition (HCCI) Engine By Fuel Additives

1970s, a new combustion mode, Homogeneous Charge Compression Ignition (HCCI), was proposed, the engine working under HCCI combustion mode has higher thermal efficiency and produces very low NOx emissions and gasoline engine-equivalent HC and CO emissions, and has been highlighted and focused by the internal combustion engines researchers around the world. It was considered as an important technology to satisfy the strict emission regulations in the future and was a great progress in combustion technology. However, HCCI engines are prone to misfire at low loads and to knock at high loads, having narrow load and speed ranges.With regard to HCCI misfire at low loads, some small mass fractions of peroxide additives are added to the base fuel, respectively to investigate the effect of additives on HCCI engine ignition and combustion process. Regarding HCCI knock at high loads, some small mass fraction of ferrocene and some small volume fractions of methanol and ethanol are added to the base fuel, and some small volume fraction of CO₂ is inducted into the intake, to explore the effects of fuel antiknocks and CO₂ on HCCI engine knock and high load combustion. Eighteen test fuels were confected at the peroxide mass fractions of 1%, DTBP mass fractions from 1% to 5%, ferrocene mass fractions of 0.025%, 0.035% and 0.05%, methanol and ethanol volume fractions of 1%, 3% and 5% , respectively, and intake air was mixed with CO₂ at volume fractions of 1%, 3% and 5%. HCCI combustion tests were made on the fourth cylinder of a modified 4-cylinder diesel engine.The experimental results showed that under steady speed of 1400 r/min the HCCI engine could only run at narrow load ranges. With a small amount of peroxide additives added, the misfire of HCCI engine was effectively controlled and load range is remarkably extended to lower load. The effect of the DTBP was the most obvious. At the fixed equivalence ratio, with the increase of concentration of DTBP added, the ignition and heat release brought forward, and combustion duration decreased. At the large concentration, however, it resulted in more likely engine knock. The mass fraction of DTBP needs to be about 2%.The knock of HCCI engine was effectively controlled and high load range was enlarged under the same speed by adding antiknocks. At the small concentration, the effect of ferrocene was unobvious and methanol was a little better than ethanol in controlling engine knock. With the increase of concentration of these antiknocks added, the peak values of HCCI combustion cylinder pressures and heat release rates gradually decreased, causing combustion duration increased, the effect of antiknock increased. At the large concentration, however, it resulted in more likely engine misfire. So the mass fraction of ferrocene needs to be about 0.035%, and volume fractions of methanol, ethanol or CO₂ are about 1%, respectively.