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Characterization of Particulate Matter Emissions from a Current Technology Natural Gas Engine Arvind Thiruvengadam, Marc Besch, Seungju Yoon, John Francis Collins, Hemanth Kappanna, Daniel Carder, Alberto Ayala, Jorn Herner, and Mridul Gautam Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es5005973 • Publication Date (Web): 24 Jun 2014 Downloaded from http://pubs.acs.org on June 30, 2014
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Environmental Science & Technology
Lubrication oil
Stoichiometric CNG with Three‐way Catalyst
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Characterization of Particulate Matter Emissions
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from a Current Technology Natural Gas Engine
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Arvind Thiruvengadam*1, Marc C. Besch1, Yoon Seungju2, John Collins2, Hemanth Kappanna1,
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Daniel K Carder1, Alberto Ayala2, Jorn Herner2, Mridul Gautam1
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1
Mechanical and Aerospace Department, West Virginia University, Morgantown, West Virginia
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26505, Phone (304) 293 0805, Fax (304) 293 6689 2
California Air Resources Board, 1001 I Street, Sacramento, CA 95812, Phone (916) 327 8097,
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Fax (916) 322 4357 Corresponding Author Email:
[email protected] 10
RECEIVED DATE (to be automatically inserted after your manuscript is accepted if
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required according to the journal that you are submitting your paper to)
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ABSTRACT
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Experiments were conducted to characterize the particulate matter (PM) size distribution, number
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concentration, and chemical composition emitted from transit buses powered by a USEPA 2010
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compliant, stoichiometric heavy-duty natural gas engine equipped with a three-way catalyst
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(TWC). Results of the particle size distribution showed a predominant nucleation mode centered
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close to 10 nm. PM mass in the size range of 6.04 to 25.5 nm correlated strongly with mass of
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lubrication oil derived elemental species detected in the gravimetric PM sample. Results from oil
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analysis indicated an elemental composition that was similar to that detected in the PM samples.
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The source of elemental species in the oil sample can be attributed to additives and engine wear.
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Chemical speciation of particulate matter (PM) showed that lubrication oil based additives and
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wear metals were a major fraction of the PM mass emitted from the buses. The results of the study
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indicate the possible existence of nanoparticles below 25 nm formed as a result of lubrication oil
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passage through the combustion chamber. Furthermore, the results of oxidative stress (OS)
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analysis on the PM samples indicated strong correlations with both the PM mass calculated in the
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nanoparticle size bin and the mass of elemental species that can be linked to lubrication oil as the
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source.
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INTRODUCTION
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Engine manufacturers have adopted advanced combustion strategies and after-treatment
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technologies to meet current USEPA 2010 emissions regulations. The diesel segment of the heavy-
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duty population incorporates exhaust gas recirculation (EGR), variable geometry turbochargers
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(VGT), diesel particulate filters (DPF), and selective catalytic reduction (SCR) systems for
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emissions reduction. The natural gas fueled engines incorporate stoichiometric fueling, cooled
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EGR, and a TWC. The soot free combustion of natural gas enables Original Equipment
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Manufacturers (OEM) to meet particulate matter (PM) standards without the use of a particulate
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filter. Exhaust from both natural gas vehicles and DPF-equipped diesel vehicles are characterized
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by low concentrations of accumulation mode particles.1 Diesel engines have exhibited significant
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concentrations of nucleation mode particles linked to catalytic oxidation of fuel and lube oil
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derived sulfur at elevated exhaust gas temperature operating conditions.1,2 The discussions of
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sulfuric acid based particles found in the literature bring to light the importance of lubrication oil
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consumption as a possible precursor to nanoparticle formation.1,2 Many authors attribute the sulfur
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in temperature dependent nucleation mode particle emissions to be primarily from lubrication oil
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rather than from ultralow sulfur diesel fuel.3,4,5
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Components of lubrication oil can manifest themselves as both organic and inorganic emissions
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in the exhaust of an internal combustion engine. The process of lubrication oil based metallic
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compounds to undergo volatilization during combustion and condensation during exhaust gas
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cooling was documented by Khalek et al. The study also showed that engines with lower soot
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emissions such as spark-ignited engines favored the re-nucleation of lubrication oil additives in
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the exhaust.6 The contribution of lubrication oil to PM emissions from uncontrolled diesel engines
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has not been a significant factor because the soot produced from in-cylinder combustion of diesel
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fuel in legacy engines overwhelms the PM emissions attributable to lubrication oil by several
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orders of magnitude.7 Results of Liu et al. show that metallic emissions account for only a small
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fraction of PM in an uncontrolled diesel engine, while the use of DPF reduces this fraction by over
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90%. The study also showed a higher metal content with the use of SCR compared to a DPF only
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configuration.8 The contribution of lubrication oil to tailpipe PM emissions from DPF-equipped
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engines is also not likely to be significant. In modern diesel engines, the oxidation catalyst controls
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the organic fraction of lubrication oil PM, and the DPF controls the inorganic fraction (soot and
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metals) of lubrication oil PM. A study by McGeehan et al has shown that all of the ash deposited
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in the DPF of diesel engine is predominantly inorganic and dominated by lubrication oil additives.9
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In the case for natural gas engines, the contribution of lubrication oil to tailpipe PM could be
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significant because, while the TWC will oxidize the organic fraction of PM, there is no DPF to
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remove the inorganic fraction of PM consisting of soot and trace metals.
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Current natural gas engines operate on a spark-ignited (SI) engine platform similar to gasoline
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fueled engines. Zielinska et al. show that the composition of polycyclic aromatic hydrocarbon
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(PAH) emissions in gasoline engine exhaust more closely resembles the composition of PAH in
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used lubrication oil rather than in fuel.10 Recent measurement in the Caldecot tunnel indicate that
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the lubrication oil could be the dominant contributor to primary organic aerosol (POA) emitted by
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heavy and light-duty vehicles.11 The study presents evidence of POA compositions being
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influenced more by lubrication oil based organic compounds rather than byproducts of incomplete
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combustion of gasoline or diesel fuel.11 Lubrication oil also consists of a range of additives that
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provide detergents, dispersant, acid neutralizers, anti-oxidants, corrosion inhibitors, and anti-wear
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properties to the lubricant. Consequently, lubrication oil combustion results in inorganic
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constituents such as calcium, zinc, phosphorus, and sulfur in the exhaust.12
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Factors that could affect the health impacts of heavy duty PM emissions include particle
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composition, particle number concentration, size distribution, surface area distribution, physical
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characteristics, and the penetration fraction into the human lungs. Very few studies have attempted
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to relate PM oxidative stress (OS) characterization with composition characteristics of PM
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emissions from heavy-duty engines. Verma et al. observed a relationship between transition metals
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present in PM samples and oxidative stress in alveolar macrophage cells using a reactive oxidative
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species (ROS) assay.13 Biswas et al. observed strong statistical correlations between oxidative
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potential measured using a dithiothreitol (DTT) assay and the organic acid and water soluble
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organic carbon concentrations of PM extracts.14 These and most other studies on the OS potential
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of heavy duty PM emissions have reported data from legacy or retro-fitted diesel engines. There
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is little literature on the OS potential of PM emissions from modern natural gas engines. The
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increasing population of natural gas engines in urban applications such as transit buses, refuse
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trucks, and goods movement has raised concern about the potential PM related health implications
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of natural gas engine emissions. Current knowledge of PM size distribution, number concentration,
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and chemical characteristics is limited to older technology lean-burn natural gas engines. Lean-
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burn natural gas engines are SI internal combustion engines that operate with excess air. Since
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newer natural gas engines are subject to stricter NOx regulations, engine manufacturers have
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adopted a stoichiometric fueling (chemically correct air to fuel ratio to achieve complete
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combustion of fuel) strategy to leverage the use of TWC for NOx reductions. Lean-burn natural
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gas engines typically operate with a simple oxidation catalyst for CO and hydrocarbon reduction
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as opposed to TWC that simultaneously reduce NOx, CO and hydrocarbons. Stoichiometric
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natural gas engines operate at higher exhaust gas temperatures when compared to lean-burn
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engines due to the higher fueling rate relative to mass of air. The findings presented in this work
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describe the chemical composition, size distribution, and number concentration of PM emissions
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from two modern stoichiometric natural gas vehicles. The results also show the correlation of
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oxidative potential with engine wear and lubrication oil derived elements.
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EXPERIMENTAL PROCEDURE
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Two heavy-duty natural gas transit buses were tested on West Virginia University’s (WVU)
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transportable heavy-duty chassis dynamometer. The buses were powered by USEPA 2010
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compliant natural gas fueled engines. Table 1 lists the engine and vehicle specifications of the two
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test vehicles. Both buses were identical in their engine and vehicle specification, and in to their
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third year of service in the fleet. One bus had accumulated 84,000 miles, while the other had
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accumulated 77,000 miles prior to this study. Both buses were in the middle of their maintenance
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interval during the testing period. The typical retirement age for a transit bus is 12 years or 500,000
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miles, whichever criteria is satisfied first.
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Table 1. Engine and Vehicle Specifications of the Test Vehicles Engine Manufacturer
Cummins
Engine Model
ISLG 280
Engine Model Year
2007
Displacement (L)
8.9
Power (hp)@ speed (rpm) 280 @ 2000 Fuel
CNG
After-treatment
Three-way catalyst
After-treatment manufacturer
Cummins Solution
Vehicle Manufacturer
Daimler America
Emissions Certification
PM: 0.01 gms/bhp-hr; NOx: 0.20 gms/bhp-hr
Emissions Bus
North
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Both vehicles were exercised over three driving cycles: the urban dynamometer driving schedule
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(UDDS), 45 mph steady-state cruise, and idle operating mode cycles. Extended versions of the
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UDDS (triple length cycle), one hour cruise mode, and one hour idle operation were performed to
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collect adequate PM samples on to the gravimetric filters for chemical speciation analysis and for
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better resolution over the gravimetric weighing system. Prior to beginning of sample collection
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vehicles were pre-conditioned by operating over respective driving cycles prior to the tests that
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involved sample collection. This ensured vehicle engine and after-treatment systems were in
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optimal thermodynamic state for representative gaseous and PM emissions rate. Table S1 in the
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supplemental information lists the different sampling media and flow rates for the respective
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sampling streams. Samples were collected on respective media recommended by the different
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analysis procedures, over three consecutive hot starts with 20 min soak times in between tests. One
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hour duration tunnel blanks were collected for all media before the start of each test day. The test
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matrix also involved the complete chemical speciation of the gas phase and particulate phase
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constituents of exhaust that included analysis for PAH, VOC, carbonyls, metals, ions, elemental
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carbon (EC), and organic carbon (OC).
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Regulated gaseous pollutants were quantified using the WVU transportable emissions
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measurement system (TEMS), and the test results are publicly available.21 Figure S1 in the
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supplemental information represents the schematic of the code of federal regulations (CFR) Title
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40 Part 1065 compliant emissions measurement system contained within the TEMS, as well as the
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unregulated sampling streams employed in this study.
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Particle size distributions and number concentrations were continuously measured at 1 Hz. using
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an Engine Exhaust Particle Sizer (TSI-EEPS Model 3090). The TSI EEPS is designed to measure
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particles in the size range of 5.6 to 560 nm. The Electrical Aerosol Detector (TSI EAD Model
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3070A) was also used to measure a unique parameter called aerosol length (mm/cm3) which is
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indicative of lung deposited particle surface area.15 Particle size measurements were performed by
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directly sampling from the primary dilution tunnel of the constant volume sampling (CVS) system.
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Instantaneous primary CVS dilution ratios were calculated using the CVS tunnel flow and engine
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exhaust flow. The primary CVS dilution air is at ambient condition, filtered using high efficiency
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particulate air (HEPA) filter. Temperature and humidity of the primary dilution air was
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continuously recorded.
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The study also included the characterization of the OS potential of PM through dithiothreitol
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(DTT) and alveolar macrophage reactive oxygen species (ROS) assays. The DTT assay is a cell
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free, chemical based assay that is highly selective in response to organic materials and certain
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transition metals such as copper and manganese.16,17 The rate of consumption of the chemical DTT
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is found to be directly proportional to the OS potential of the sample.18 A study by Charrier and
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Anastasio has illustrated the response of the DTT assay towards the presence of transition metals,
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specifically copper and manganese. The study further shows that up to 80% of DTT consumption can
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be attributed to the presence of transition metals in a sample consisting of mixture of transition metals
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and quinones.17
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Alveolar macrophage assay is a cell based OS assay that exposes alveolar macrophage tissues
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extracted from a rat to the PM samples. Alveolar macrophages are commonly found in the inner
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epithelial cells of the airways and lungs and hence by quantifying ROS generation within alveolar
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tissues, would be an indicator of pulmonary inflammatory responses to PM.13 A recent study by
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Landerman et al. has shown that this assay is effective for PM mass loading of less than 100
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micrograms and that it accurately represents pulmonary response to PM exposure, as the lung airways
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are the first line of defense for the human body against particulate matter ingestion19. Recent studies
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by Hu et al. and Verma et al. have also observed the selectivity of this assay towards water soluble
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transition metals.16,20
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High volume sampling and extended test cycles were employed to collect adequate PM mass
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onto T60A40 filter substrates for the respective OS assays. Detailed procedures of PM sample
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extraction and analysis procedures for DTT and alveolar macrophage assay are described in the
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works of Cho et al.18 and Verma et al.20 respectively. This paper will use the results from the OS
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assays to present statistical correlations with chemical composition of the PM. Detailed analysis
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of OS potential of PM will be the subject of a different paper.
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RESULTS AND DISCUSSION
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Figure 1 shows the dilution corrected average particle size distributions and concentrations
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observed during the triple-length UDDS cycle from the two CNG buses. The error bars in the
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figure show the maximum and minimum particle concentration change for each particle diameter
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over three hot start repeats. The particle size distribution from both buses show particle
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concentrations close to 1E7 for particle diameters close to 10 nm size range. The error bars indicate
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a lower variation in particle concentration in the 10 nm size range for both buses over three hot
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start triple length UDDS tests. Particle size distributions from both buses indicate the emission of
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particles in the accumulation mode (50-100 nm). However, observed particle concentrations in the
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accumulation mode were an order of magnitude lower than particles in the nucleation mode (100nm is not
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commonly observed in exhaust of natural gas or DPF-equipped diesel engines.
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Figure 1. Cycle averaged particle size distribution and concentration during UDDS cycle
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Figure 2 shows the average particle size distribution and concentration, corrected for
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instantaneous dilution ratios during the 45 MPH cruise mode operation. The results from both
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buses showed high variability in the measured particle size distribution and concentration. Particle
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concentrations from both buses during the cruise mode operation were found to be two orders of
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magnitude lower compared to the transient UDDS operation. The particle concentrations measured
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over the steady-state cycle were characterized by higher levels of electrometer noise due to the
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low signal to noise ratio attributed by the low PM emissions during cruise mode operation. Very
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low gravimetric PM mass measured over the steady-state cycles from both buses further
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corroborate the low particle concentrations measured using the TSI EEPS. Cruise mode operation
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contributes to a relatively steady thermodynamic state of the engine. This steady-state
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characteristic results in better sealing of piston rings and oil pathways and therefore inhibiting the
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contribution of lubrication oil to tailpipe PM emissions.
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Figure 2. Cycle averaged particle size distribution and concentration during 45 MPH cruise
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Figure 3 shows average particle size distribution and concentration corrected for instantaneous
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dilution ratios during the idle phase. Particle concentrations from bus 1 were found to be close to
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levels in tunnel blanks. However, bus 2 exhibited significantly higher particle concentration over
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a broad size range of the distribution. The highest particle concentrations were detected at both
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nucleation mode and accumulation mode regions of the size distribution for bus 2 during idle
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operation. The nucleation mode particle size distribution was centered on the 10-20 nm size range
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and that in the accumulation mode was centered around 80-100 nm size range. In corroboration
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with particle size distribution results from bus 2, the gravimetric filter samples collected during
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idle mode operation reported higher mass loading compared to bus 1 and discoloration of filter
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materials. The observed particle size distribution could be attributed to in-cylinder combustion of
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lubrication oil. The entry of lubrication oil in to the combustion chamber is dependent on engine
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load. Typically low-load operations result in insufficient sealing of the piston rings which can
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contribute to the combustion of lubrication oil. The observed particle size distribution from bus 2
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could be attributed to worn cylinder linings and hence lower piston ring sealing during the
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extended idle operation. The difference in PM size distribution between the two buses could be
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indicative of the differences in their day-to-day operating characteristics that influences their
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engine wear pattern and consequently the oil consumption characteristics.
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Figure 3. Cycle averaged particle size distribution and concentration during idle operation
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In comparing the results obtained from this study to previous work documented by Holmen and
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Ayala1, the particle size distribution and concentration observed in cruise mode operation is similar
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to the CVS measurements obtained from older technology lean-burn natural gas engines with an
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oxidation catalyst.1 Similar particle concentrations in the range of 1E4 to 1E5 for particle diameters
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less than 10 nm is observed between both studies. Similarity in idle mode PM emissions between
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the two studies is observed only for bus 1. Bus 2 PM emissions are significantly different from
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both bus 1 and results from Holmen and Ayala.1 The similarity in PM emissions between older
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and newer technology natural gas engines suggests that PM emissions could largely be contributed
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only by lubrication oil and not so influenced by changes in combustion of fuel and type of after-
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treatment system.
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Figure 4 shows the mass fraction of different particulate phase chemical species emitted from
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the two CNG buses. The distance-specific emissions rates of the individual species are detailed
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elsewhere.21
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The mass fraction results of PM from stoichiometric operating CNG engines reveal a very
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interesting composition. The fraction of organic carbon (OC) emissions in comparison to other
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species is significantly lower than previously documented for lean-burn natural gas engines.21,22
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The lower OC composition can be attributed to the high post-TWC exhaust gas temperatures that
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were observed to be greater than 500oC for the transient and steady-state operation and close to
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300oC for idle operation. Such high temperatures are conducive for efficient catalytic oxidation of
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higher and lower chain hydrocarbons with the exception of methane.23 VOC analysis indicated all
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higher chain hydrocarbons to be below detection limits with trace levels of lower chain
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hydrocarbons, which typically do not participate in particle formation mechanisms. Also worth
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noting, is that the observed OC fractions could reflect positive sampling artifacts due to gaseous
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adsorption of methane and certain lower chain non-methane hydrocarbons onto the pre-fired quartz
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filters. Table 2 shows the distance-specific EC/OC emissions rate from the two buses over the
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UDDS and 45 MPH cruise driving cycle and the time-specific emissions rate during idle operation.
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EC emissions is observed only over the UDDS and idle mode operation. EC fraction was observed
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in the PM samples from both bus 1 and bus 2 during the UDDS and idle mode operation.
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Gravimetric filters from idle mode operation of bus 2 also showed signs of discoloration, often
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indicative of ash or soot emissions in the exhaust. The EC emissions reported from bus 2 over
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UDDS cycle and idle cycle could be a result of lubrication oil combustion during low load transient
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operation and extended idle periods respectively.
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Table 2 EC/OC emissions rate from two test vehicles over the UDDS, 45 MPH cruise and idle
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operation
Bus 1
Bus 2
mg/mi or mg/sec
UDDS
45 MPH Cruise
Idle
OC
0.489
0.175
0.002
TEC
0.631
