Advanced compression-ignition engines-understanding the in-cylinder processes

Advanced compression-ignition (CI) engines call deliver both high efficiencies and very low NO(X) and particulate (PM) emissions. Efficiencies are comparable to conventional diesel engines, but unlike conventional diesel engines, the charge is highly dilute and premixed (or partially premixed) to achieve low emissions. Dilution is accomplished by operating either lean or with large amounts of EGR. The development of these advanced Cl engines has evolved mainly along two lines. First, for fuels other than diesel, a combustion process commonly known as homogeneous charge compression-ignition (HCCI) is generally used. in which the charge is premixed before being compression ignited. Although termed "homogeneous." there are always some thermal or mixture inhomogencities in real HCCI engines, and it is sometimes desirable to introduce additional stratification. Second, for diesel fuel (which autoignites easily but has low volatility) an alternative low-temperature combustion (LTC) approach is used, ill which the autoignition IS Closely coupled to the fuel-injection event to provide control over ignition timing. To obtain dilute LTC, this approach relies on high levels of EGR, and injection timing is typically shifted 10-15 degrees CA earlier or later than for conventional diesel combustion so temperatures are lower, which delays ignition and provides more time for premixing. Although these advanced CI combustion modes have important advantages, there are difficulties to implementing them in practical engines. In this article, the principles of HCCI and diesel LTC engines are reviewed along with the results of research oil the in-cylinder processes. This research has resulted in substantial progress toward overcoming the main challenges facing these engines, including: improving low-load combustion efficiency, increasing the high-load limit. understanding fuel effects, and maintaining low NO(X) and PM emissions over the operating range. (c) 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.