530 F. Zhao et al. / Progress in Energy and Combustion Science 25 (1999) 437–562 diameters of less than 10 mm. The new EPA “fine particle”, or PM2.5, standard includes all particles having equivalent diameters of less than 2.5 mm. This indicates the heightened interest in smaller particles that is reflected in the evolution of PM emissions standards. For particulates from diesel <strong>engines</strong>, only particles in the size range above 100 nm in diameter were found to have any significant effect on the mass-weighted PM10 and PM2.5 mean values, and nuclei mode particles were found to have little or no effect on the mass-weighted distribution regardless of the number density [233]. Historically, <strong>gasoline</strong> <strong>engines</strong> have been exempt from the requirement to meet the particulate emissions standard for diesel <strong>engines</strong>. The justification for this has been that <strong>gasoline</strong> <strong>engines</strong> produced particulate emissions that were on the order of only 1% of those diesel <strong>engines</strong>. This was certainly the case prior to the advent of recent diesel particulate legislation [235]. Recent studies indicate that current <strong>gasoline</strong> SI <strong>engines</strong> often emit an increased fraction of nanoparticles even though steady-state particulate number emissions are generally several orders of magnitude lower than those from modern diesel <strong>engines</strong> [234,236]. Particulate number emissions from <strong>gasoline</strong> <strong>engines</strong> have also been shown to increase significantly when operated under high-load, transient and cold-start conditions. It was reported by Graskow et al. [236] that, unlike the PM emissions from diesel <strong>engines</strong>, the particulate emissions from PFI <strong>engines</strong> are quite unstable. Typically a stable baseline concentration of engine-out PM emissions is on the order of × 10 5 particles/ cm 3 ; however, a “spike” in the PM emissions is occasionally observed. These spikes are found to be composed of nearly 100% volatile particles of less than 30 nm in diameter, and can exhibit number densities exceeding 100 times that of the baseline concentration. An analysis of particulates from PFI <strong>engines</strong> by Andrews et al. [235] revealed that the bulk of the mass is ash, with the second largest fraction being unburned lubricating oil. Carbon emissions were found to be significant only at high load with mixture enrichment, whereas, at other operating conditions, carbon was reported to comprise less than 10% of the total PM mass. The large ash fraction of the <strong>gasoline</strong> PM emissions were found to include a large fraction of metal compounds, with calcium and sodium evident for operation at low load without EGR, and copper and magnesium predominant for operation with EGR. Preliminary research indicate that GDI <strong>engines</strong>, as evolving powerplants for automotive applications, may emit a larger amount of particulates than do conventional PFI <strong>engines</strong>, especially during stratified-charge operation. Depending on the degree of combustion system optimization, smoke emissions from prototype GDI <strong>engines</strong> could be as high as 1.2 BSU [100,101,210]. A comparison of the particulate emissions for a current PFI SI engine, a current production GDI engine and a 1995 European IDI diesel engine for the US FTP cycle is illustrated in Fig. 94(a) These data represent mass measurements of particulate matter collected on filter media. It may be seen that the level of PM emissions for the GDI engine is between those of the diesel and PFI SI <strong>engines</strong>. As reported by Maricq et al. [237], the PM emissions from a vehicle powered by a GDI engine are on the order of 10 mg/mile. In comparison, the PM emissions from a comparable dieselpowered vehicle are on the order of 100 mg/mile. As compared to the 1–3 mg/mile PM emissions from a model PFI engine [235,237], the PM emissions from current production GDI <strong>engines</strong> are relatively high, even though they are well below the current US standard of 80 mg/ mile as measured on the FTP test cycle. In interpreting this published comparison, it should be noted that both the PFI and GDI <strong>engines</strong> have been developed and massproduced without major specific efforts being <strong>direct</strong>ed towards the minimization of particulate emissions. Graskow et al. [238] measured the particulate emissions from a 1998 Mitsubishi GDI engine using a chassis-dynamometer test. It was reported that the average polydisperse number concentration was on the order of × 10 8 particles/ cm 3 and that the number-weighted, geometric mean particle diameter was from 68 to 88 nm. In contrast, modern PFI <strong>engines</strong> tested by the same authors [234,236] emit average particulate number concentrations ranging from 10 5 particles/cm 3 at light load to 10 7 particles/cm 3 at high load. Older PFI <strong>engines</strong> have been shown to emit number concentrations in excess of 10 8 particles/cm 3 for conditions corresponding to highway cruise operations. It was also reported that, for the operating conditions tested, the numberweighted, geometric mean diameters of the particulate matter emitted from the GDI engine are larger as compared to the PFI <strong>engines</strong> tested. This relatively large mean particle size is likely to increase the particulate mass emissions from the GDI engine, but could also lead to a decrease in the relative fraction of particles emitted in the nanoparticle size range. Maricq et al. [237] investigated the PM emissions from a 1.83L, four-cylinder, four-valve production GDI engine for a range of operating conditions. As illustrated in Fig. 94(b), the particle number emissions are found to increase by a factor of 10–40 when the operating mode of the GDI engine is stratified-charge instead of homogenous-charge. The emissions of particulate matter exhibit a strong dependence on the GDI <strong>injection</strong> timing, and it was observed that the particle number and volume concentrations increase markedly as the <strong>injection</strong> timing is retarded. Advancing the <strong>spark</strong> timing generally yields an increase in both the particle number concentration and the mean particle size for both homogeneous and stratified-charge operation. An increase in the engine speed and load generally leads to an increase in PM emissions; however, this trend is dependent on the <strong>injection</strong> timing for stratified-charge operation. Based upon a chassis-dynamometer test that emulated the FTP test cycle, it was found that the PM emissions exhibit very substantial fluctuations, with a strong correlation existing between vehicle acceleration and the observed increases in PM emissions. These increases are theorized to occur
F. Zhao et al. / Progress in Energy and Combustion Science 25 (1999) 437–562 531 Fig. 94. PM emissions from GDI <strong>engines</strong> [237]: (a) comparison of FTP PM emission rates for a diesel, GDI and PFI vehicles; (b) PM, CO, UBHC, and NOx emissions as a function of fuel <strong>injection</strong> timing (EOI); (c) particle size distributions as a function of fuel <strong>injection</strong> timing (EOI); and (d) particle size distribution as a function of <strong>spark</strong> timing.
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