Homogeneous charge compression ignition engine-out emission-does flame propagation occur in homogeneous charge compression ignition?

Abstract

Engine-out emissions data [CO, CO2, speciated hydrocarbons (HC), and particulate matter (size and number density)] were obtained from a single-cylinder, 660 cm3, homogeneous charge compression ignition (HCCI) engine operated on gasoline fuel using direct in-cylinder injection. Data were taken as functions of the air-fuel ratio (A/F) (30–270), r/min, inlet air temperature and fuel injection timing. Three important observations were made A sharp break occurs in the CO and CO2 emissions indices beginning near A/F = 75. Above A/F ∼ 100, CO is the primary carbon oxide while for A/F < 70, CO2 is the major carbon oxide. The HC emissions index increases linearly, beginning near A/F ∼ 30:1. Below this A/F, the HC index is characteristic of crevice emissions (∼ 3.5 per cent). These results do not prove this unequivocally, but can be explained by a mechanism in which, for A/F < 75, flame propagation occurs over relatively short distances between the multiple autoignition sites within the combustion chamber. Adiabatic compression calculations indicate that for A/F < 75, the compression temperature (∼ 1150 K) is sufficiently high to support flame propagation. The linear increase in HC emissions above that expected from crevice storage can be explained by noting that autoignition becomes more difficult as the A/F becomes leaner and fewer ignition sites are likely to exist within the combustion chamber, reducing the amount of fuel combusted. Conventional models of HCCI combustion involving multi-zone autoignition may also explain the data, but the above concept is an alternative combustion mechanism for HCCI, which should be considered. Particulate emissions at moderate load from this HCCI engine, while much lower than from a diesel, are similar to those from early-injection DISI (direct injection spark ignition) engines and should not be assumed to be negligible.