Investigation of Unregulated Exhaust Emissions from Diesel Engines Operating Under Alternate Combustion Modes and With Advanced Aftertreatment Schemes, 03-9511

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Principal Investigators
Patrick Merritt
Yiqun Huang
Stefan Simescu

Inclusive Dates:  10/01/04 – 09/30/05

Background - Alternative diesel combustion modes are being explored as means to reduce exhaust emissions of particulate matter and oxides of nitrogen in diesel engines. Because these technologies alter factors such as combustion temperature, rate, and air-fuel ratio (A/F) in the combustion chamber, along with peak combustion chamber temperature and pressure, it follows that the makeup of the exhaust gas will vary from that of traditional diesel combustion processes. In particular, there is concern that increased levels of harmful compounds may result.

Approach - The exhaust emissions from light-duty diesel engines operating in three alternate combustion modes [homogeneous charge compression ignition (HCCI), premixed charge compression ignition (PCCI), and low-temperature combustion (LTC)] were characterized for regulated and unregulated emissions. Carbonyl compounds (aldehydes and ketones), polynuclear aromatic hydrocarbons (PAH) and nitro-PAH compounds were measured. Untreated exhaust was sampled for standard diesel operation and for the three alternate combustion modes, including HCCI, PCCI, and LTC. The engine used for LTC and PCCI operation, manufactured by PSA, was equipped with a diesel oxidation catalyst, diesel particulate filter, and lean NOx trap (in series configuration). Limited sampling was performed at intermediate points between these devices and at the final treated, or tailpipe, exhaust. Sampling HCCI operation on diesel fuel, as requested by the project mentors, required a second engine, which was a single-cylinder, variable compression, research engine.

Accomplishments - For the speciated exhaust hydrocarbons, ethylene, propylene, and acetylene dominated every mixture except HCCI. For HCCI, a very large mass of unidentified material was in the C6 range. Overall, hydrocarbon concentrations in PCCI operation were roughly on par or lower than those observed in diesel combustion. LTC, however, produced substantially higher concentrations than diesel, with LTC Rich tending to be two to three times higher than LTC Lean. Compared to baseline diesel operation, engine-out exhaust emissions of carbonyl compounds were significantly increased for all LTC modes sampled and for PCCI Lean conditions. PCCI Rich produced much lower carbonyl emissions than diesel operation. The carbonyl compounds detected and the levels measured can explain the foul, choking odors that some researchers have observed while operating engines in alternate combustion modes. For PAH compounds, LTC Lean and LTC Rich produced substantially higher emissions rates as compared to baseline diesel operation. PCCI Lean PAH emissions were substantially higher than PCCI Rich operation, which were comparable to emission rates from diesel operation. For total NPAH compound emissions, both LTC Lean and LTC Rich emissions at Mode 3 were approximately 2.5 times higher than diesel. PCCI operations did not present any problems for NPAH emissions. While much of these harmful compounds were effectively mitigated by the aftertreatment systems in place, LTC Rich remains a problem for carbonyl and PAH compounds. This preliminary investigation of unregulated emissions from alternate combustion modes has indicated clearly that hazardous emissions must be considered as these modes are deployed, even if they are to be used briefly as regeneration cycles for aftertreatment devices. 

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