Particulate Emissions from a Gasoline Homogeneous Charge Compression Ignition Engine 2007-01-0209
Particulate Emissions from Homogeneous Charge Compression Ignition (HCCI) combustion are routinely assumed to be negligible. It is shown here that this is not the case when HCCI combustion is implemented in a direct injection gasoline engine.
The conditions needed to sustain HCCI operation were realized using the negative valve overlap method for trapping high levels of residual exhaust gases in the cylinder. Measurements of emitted particle number concentration and electrical mobility diameter were made with a Cambustion DMS500 over the HCCI operating range possible with this hardware. Emissions of oxides of nitrogen, carbon monoxide and unburned hydrocarbons were also measured. These data are presented and compared with similar measurements made under conventional spark ignition (SI) operation in the same engine.
Under both SI and HCCI operation, a significant accumulation mode was detected with particle equivalent diameters between 80 and 100 nm. Where comparisons were possible, the number concentrations from HCCI combustion were slightly higher than from SI. Moreover, the magnitude of the accumulation mode varied inversely with the amount of residual gas trapped, and hence in direct proportion to the nitric oxide emissions. This is the opposite of the well known particulates-NO tradeoff that can be observed in conventional compression ignition engines, and is thought to be because the extra heat in the intake and compression strokes improves the mixture preparation in HCCI. A nuclei mode was also observed with particle equivalent diameters between 10 and 20 nm. The nuclei mode was most concentrated when the levels of residual gases was very high (c55%), correlating to the lowest temperatures in the expansion stroke and highest emissions of unburned hydrocarbons.
In summary, it was shown by experimental measurements that the number concentrations of emitted particles from HCCI combustion in a gasoline direct injection engine are similar in size and concentration to those from conventional direct injection gasoline operation, and thus are non-negligible. The pertinent differences in combustion conditions compared to SI are discussed and the mechanisms contributing to particulate formation in HCCI are put forward.