Recent publications, http://dx.doi.org/10.1021/es5005973, indicate lube oil as a major source.
Article Oxidation Catalyst Effects on Natural Gas Transit Bus Ultraf...
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Abstract:Engine and exhaust control technologies applied to compressed natural gas (CNG) transit buses have advanced from lean-burn, to lean-burn with oxidation catalyst (OxC), to stoichiometric combustion with three-way catalyst (TWC). With this technology advancement, regulated gaseous and particulate matter emissions have been significantly reduced.
Ref: Criteria pollutant and greenhouse gas emissions from CNG transit buses equipped with three-way catalysts compared to lean-burn engines and oxidation catalyst technologies
Journal of the Air & Waste Management Association
Volume 63, Issue 8, 2013;pages 926-933
Contamination of lub oil is one of the major sources to produce particulate emisisons. Use of stoichiometric ratios of fuel with air improves combustion with improved air fuel ratios lowers the formation of particulate emissions. The use of three way catalysts reduces combusiton temperatures thereby avoiding fuel cracking reactions and also lub oi contamination causing reduction of particulate emissions.
Sources of the origin of PM in engine are to be estimated. Maintaining correct theortical air fuel ratios, which reduces lube oil deterioration may significantly lower PM and TWC will reduce them further by oxidation.
I have worked on engine oil and CNG, HCNG closely .Using 3 way catalyst will not affect PM emissions significanlty. CNG is a clean gaseous fuel and therefore it is the lube oil that contributes the most in ultrafine PM emissions. Now, question is how much comparable to diesel engines. Lot many studies have been done by NREL, VTT and other agencies.In my opinion, although CNG is better than diesel in PM10 emissions but CNG should be worse in terms of ultra PM emissions since lube oil burns to emit these utlra fine particles. Since you asked stochiometric, CNG should burn completely leaving behind no PM emissions (ideal case) but in terms of ultrafine particles diesel should be better than a CNG engines. Thats the problem using CNG engines. Good question.
Keeping the air-fuel mixture stoichiometric and having a three-way catalyst both help to reduce particles from CNG engines relative to CNG engines that do not have a catalyst and/or venture into the rich phase of engine operation.
Nanoparticle emissions from CNG engines are generally lower compared to the same technology spark ignition engine running on gasoline.
The comparison with 2010 diesels, with diesel particle filters, depends on the failure rates of DPF, for which I do not have figures. A diesel engine with a DPF has a clear advantage, however, at a modest 1% DPF failure rate, diesel and CNG should be more or less comparable.
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