Reformer Gas Application in Combustion Onset Control of HCCI Engine

Homogenous charge compression ignition (hcci) combustion is spontaneous multi-site combustion of anominally premixed air/fuel mixture that exhibits high rate of pressure rise and short combustion duration. toavoid excessive pressure rise rate and knocking, hcci engines are fueled with highly diluted mixture using acombination of excess air and/or egr. hcci combustion is attractive due to extremely low nox emission outputand high thermal efficiency but practical engines must overcome de-rating to a part-load power level and high hcand co emissions. more importantly, hcci engines lack a direct method of combustion timing control and thislimits operating flexibility. one method of combustion timing control is to adjust mixture ignitability using a fuelblending agent with differing ignition properties than the base fuel. reformer gas (rg) is a mixture of light gasesdominated by hydrogen and carbon monoxide that can be produced from the base hydrocarbon fuels by severalreforming techniques such as partial oxidation, autothermal or steam reforming. in a series of experimental studies,reformer gas was used to control combustion timing using a cfr engine and various base fuels: compressednatural gas (gas, high octane), iso-octane (liquid, high octane) and n-heptane (liquid, low octane). the effects ofreformer gas on engine operating parameters and combustion characteristics were shown to differ for differentbase fuel. keeping other influential parameters constant, increasing rg mass fraction in a natural gas mixtureadvanced combustion timing and shifted the operating range of the engine toward leaner mixtures. this enabledthe natural gas-fueled hcci engine to operate at leaner mixtures with decreased knock intensity and smoothercombustion behavior. for iso-octane and n-heptane base fuels, combustion timing was retarded significantly asrg blend fraction increased. for the case of lso-octane the operating region did not change. however, for then-heptane base fuet, the operating range was shined toward richer mixtures, enabling higher indicated power andthermal efficiency. a chemical kinetic study of n-heptane / rg ignition showed that the influence of hydrogen wasto suppress the first stage combustion and decrease the radical concentration after first stage combustion, thusdelaying the main stage combustion, (despite faster major reaction rates during the main combustion period).