Solid and metallic ash particle number (PN) and particulate matter (PM) mass emission measurements were performed on a heavy-duty (HD) on-highway diesel engine and a compressed natural gas (CNG) engine. Measurements were conducted under transient engine operation that included the FTP, WHTC and RMC. Both engines were calibrated to meet CARB ultra low NOX emission target of 0.02 g/hp-hr, a 90% reduction from current emissions limit. The HD diesel engine final exhaust configuration included a number of aftertreatement sub-systems in addition to a selective catalytic reduction filter (SCRF). The stoichiometric CNG engine final configuration included a closed coupled Three Way Catalyst (ccTWC) and an under floor TWC (ufTWC). The aftertreatment systems for both engines were aged for a full useful life (FUL) of 435,000 miles, prior to emissions testing.
PM mass emissions from both engines were comparable and well below the US EPA emissions standard. However, the CNG engine emitted a substantially higher number of solid particles, larger and smaller than 25 nm in diameter, compared to the number of particles emitted from the HD diesel engine for each of the three transient cycles tested. The CNG engine metallic ash particle number emission was also much higher than that of the diesel. The stringent solid particle number regulation in the EU and China will address the CNG particle number emission problem. However, in the USA there is no such regulation to specifically address particle number emissions, which is a short coming.
Ultrafine PN emissions from engines is a health concern. Reducing solid particle number emissions from old and new CNG engines to a level comparable to that of a diesel with DPF is an important task that needs to be addressed by policy makers around the globe.