Environ. Sci. Technol. 2001, 35, 1343-1349
Measurements of NOx Emissions and In-Service Duty Cycle from a Towboat Operating on the Inland River System JAMES J. CORBETT† AND ALLEN L. ROBINSON* Departments of Mechanical Engineering and Engineering and Public Policy, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213
This paper describes measurements of NOx emissions from one engine on a commercial towboat operating on the Upper Ohio River system around the Port of Pittsburgh. Continuous measurements were made over a one-week period to characterize emissions during normal operations. The average NOx emission factor is 70 ( 4.2 kg of NOx per t of fuel, similar to that of larger marine engines. A vesselspecific duty cycle is derived to characterize the towboat’s operations; more than 50% of the time the vessel engines are at idle. Although recently promulgated EPA regulations apply only to new marine engines, these data provide insight into inland-river operations, which can be used to evaluate these regulations within the inland river context. This vessel operates as a courier service, scheduling pickups and deliveries of single- or multiple-barge loads per customers’ requests; as many as 30% of the 277 towboats in the Pittsburgh region operate in this fashion. The EPAprescribed ISO E3 duty cycle does not accurately describe inland-river operations of this towboat: its application overestimates actual NOx emissions by 14%. Only 41% of this vessel’s operations fall within the Not-To-Exceed Zone defined by the EPA regulations, which limits the effectiveness of this component of the regulations to limit emissions from vessels that operate in a similar fashion.
Introduction Boats and ships are an increasingly important source category in mobile air pollution source inventories as regulations continue to reduce emissions from traditional sources such as automobiles (1). Several national inventories of ship emissions have been produced to help quantify the impact of ships on air quality (2-4). Ship emissions contribute to air pollution in large ports; for example, marine diesel engines may contribute 17% of the oxides of nitrogen (NOx) on a summer day in San Diego, CA (2, 5). More recent research suggests that more than 90% of the NOx emissions in U.S. waters occur in the navigable waters outside of ports (3). The paucity of data on actual ship emissions has made it difficult to assess the impact of ship emissions on air quality. The existing ship emission factors are derived from data * Corresponding author phone: (412) 268-3657; fax: (412) 2683348; e-mail:
[email protected]. † Present address: Graduate College of Marine Studies, University of Delaware, Newark, DE 19716. 10.1021/es0016102 CCC: $20.00 Published on Web 03/06/2001
2001 American Chemical Society
collected from a limited set of oceangoing vessels (6). Lloyd’s Marine Exhaust Emissions Program produced the most comprehensive set of emission factors to date, measuring emissions on several dozen oceangoing ships (7). More recent research has measured engine emissions on an integrated tug-barge vessel in ocean and coastal service (8), and on a passenger ferry (9). The small number of vessels characterized creates a significant source of uncertainty in existing emission inventories (10, 11). More measurements are needed to reduce this uncertainty and enable better characterization of different engines and ship types in the fleet. Recent research suggests that in the top twenty states with waterborne commerce, between 54% and 87% of ship NOx emissions come from commercial marine engines operating on inland waterways (3). These estimates are based on emission factors measured on oceangoing vessels because emission factors do not exist for vessels operating on the inland waterways. The existing ship emission factors for oceangoing vessels likely do not accurately characterize emissions from vessels operating on the inland waterways because of differences in engine size, fuel type, vessel type, and mode of operation. Inland-river vessels typically use high-speed compression-ignition marine engines, which have performance characteristics different from those of than medium- and slow-speed engines typically installed on oceangoing ships. Vessels operating on the inland waterways typically burn distillate (i.e., diesel) fuels, whereas most oceangoing vessels often burn residual fuel oil. Towboats and barges are the most common commercial vessels on the inland waterways versus oceangoing cargo ships. Inland river towboats passing through locks with multiple barges in tow are likely to operate on duty cycles very different from those of oceangoing vessels. Recognizing the potential impact of ship emissions on local and regional air quality, the U.S. EPA recently promulgated regulations to limit emissions from new marine engines (5). These regulations apply to U.S.-flagged vessels that primarily operate in U.S. waters, and apply to new vessels or engines deployed after 2004. Assuming no dramatic changes in the U.S.-flagged fleet, towboats will be a significant fraction of the vessels impacted by the new regulations. Towboats make up more than 50% of the current U.S. fleet (12), the majority of which operate on the inland waterways. The lack of operations and emissions data from inland-river vessels makes it difficult to evaluate the applicability of these regulations to inland-river context. For example, the EPAprescribed duty cycles for new marine engines are based on oceangoing vessels and may not accurately characterize inland-river vessel operations. To improve our understanding of the operations of and emissions from inland-river vessels, we have initiated an investigation of NOx emissions from towboats operating in the Pittsburgh region, which includes the Upper Ohio, Monongahela, and Allegheny Rivers. Pittsburgh ranks as the 11th largest port in the United States, and the largest inland river port in the world based on annual tons of cargo moved (13). A recent emission inventory indicates that NOx emissions from waterborne commerce in the Pittsburgh region are between 14 and 28 tons of NOx per mile of waterway, roughly equivalent to a major highway in the region (3). Like most inland waterways, towboats and barges are used to transport waterborne commerce through the Pittsburgh region. This paper describes measurements of NOx emissions from a towboat operating in and around the Port of Pittsburgh. The goals of this work are to characterize the emissions and operation of this vessel and to evaluate the VOL. 35, NO. 7, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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new EPA regulations in the context of inland-river operations. Measurements of engine performance and exhaust composition using a continuous emissions monitor were made during normal vessel operation to determine an emission factor and the vessel operating load profile. On the basis of our review of the literature, we believe that these are the first reported measurements of emissions from a towboat operating on an inland river system.
Experimental Methods Emissions monitoring was performed on one main engine of an inland-river towboat owned and operated by a river towing company in Pittsburgh, PA. The vessel, referred to in this paper as Small Tow, is one of more than 277 towboats owned by twelve regional and seven long-haul transportation companies operating on the approximately 250 miles of waterway in Pittsburgh’s three-river system (14). Small Tow is a 55-foot by 22-foot river towboat, built in 1979, which operates 24 h a day with a six-member crew. The vessel (including age and engine design) is typical of smaller-sized towboats operated by regional and intra-port carriers, pushing between 1 and 10 barges in a tow, depending on barge loads. Instead of operating on a scheduled route with regular cargoes, Small Tow operates more like a courier service, scheduling pickups and deliveries of single or multiple barge-loads per customers’ requests; as many as 30% of vessels in the Pittsburgh region operate in this fashion. The two main engines on Small Tow are 1979 Detroit Diesel Corporation (DDC) model 12V-71 engines, which are 2-cycle, 12-cylinder, V-formation, marine diesel engines. Each engine is rated to produce a maximum brake horsepower of 343 kW (463 bhp), and a maximum shaft horsepower of 327 kW (438 shp), at 2100 rpm. The engines are mechanically controlled and not turbocharged; approximately 90% of the engines in the existing fleet are mechanically controlled, and as many as 50% of these engines may not be turbocharged. The last major engine overhaul on Small Tow (new short block) occurred in March 1999. These engines are considered high-speed marine diesel engines (max-rated RPM greater than 1200); most oceangoing vessels have marine engines that are medium-speed (500-1200 max-rated RPM) or slowspeed diesels (less than 500 max-rated RPM). The cylinder displacement of this type of engine is approximately 1.16 L, and therefore, this engine would be classified as an EPA Category 1 engine (less than 5 L per cylinder) (5). Small Tow has a fixed pitch propeller; the propeller shaft is connected to the engine with a 6:1 reduction gear. Time-resolved measurements were made on the port main engine to determine engine operating and emissions profiles during normal vessel operations. Approximately 36 h of emissions and load data were collected over a one-week period during November 1999. Bridge logs were reviewed to correlate vessel operations with engine load and emissions profiles; a review of similar logs for previous weeks indicated that the vessel activity during the sampling period was representative of typical Small Tow operations. In addition to characterizing normal operations, measurements were also performed under steady-state conditions at specified throttle settings (idle, 25%, 50%, and 100% full load) for 10minute periods. Instrumentation and Experimental Setup. Combustion products from one main engine were sampled through a port installed in the exhaust duct approximately 3 m downstream of the engine exhaust manifold. The sample was passed through an insulated stainless steel filter, a 4-mlong heated Teflon sampling line, and an ice-bath dehumidification unit. The conditioned sample was characterized using a continuous emissions monitor (CEM) that included a paramagnetic oxygen analyzer (Rosemount Analytical, model 755R), a non-dispersive infrared (NDIR) carbon dioxide 1344
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analyzer (Horiba, model VIA-510), and a chemiluminescent NOx analyzer (Thermo Electron Instruments, model 10). This equipment was installed in the engine room, which had a relatively high (∼35 °C), but stable, temperature. The gas analyzers were calibrated approximately every 6 h using certified standard and zero gases. The temperature and pressure of the air in the intake manifold were monitored continuously using a thermocouple and a pressure transducer (Setra, model 207). A tachometer was installed to measure engine speed. A thermocouple was used to monitor the ambient temperature. A portable data acquisition system was used to record the measurements approximately every 8 s; each data point is the average of 10 samples taken within that 8-s interval. A gravimetric tank setup was used to directly measure the fuel flow rate over a range of steady load conditions. This setup consisted of a 5-gal fuel tank mounted on an electronic scale (Ohaus, model DSL-10L). A valve tree was installed in the fuel line such that the main fuel tanks could be bypassed for short periods of time and the engine could be operated off the small fuel tank. The fuel mass flow rate was determined by measuring the mass change in the fuel tank over a specified time period, typically 10 minutes. Relative humidity and barometric pressure data were obtained as hourly averages from National Weather Service stations located at Pittsburgh International and Allegheny County Airports. Data Analysis. The data were analyzed to calculate a time series of the fuel-based NOx emission factor (e.g., kg of NOx per t of fuel), fuel flow rate, and engine load. Before analysis, the data were filtered to account for the time lag in the response of the gas analyzers to changes in load, and the gas analyzer data were corrected for instrument drift. In the worst case, there is a 1.5-min time lag between a step change in throttle setting and the time at which the gas analyzers stabilize. The gas analyzers begin to respond 30 s after a step change in the engine load. It then takes the gas analyzers approximately another 60 s to reach the new steady state value. To account for this lag, we time-shifted the gas data by 30 s and then deleted 1 min of data immediately after each change in engine speed greater than 50 rpm. The gas analyzer data were corrected for instrument drift by assuming linear drift over the measurement period between two calibration checks. The drift in the calibration factors (zero and span) of the NOx and CO2 analyzers was less than 2% between calibrations. The O2 analyzer experienced much larger drift, and was used only to spot-check the results of the carbon balance method. NOx Emission Factor. A fuel-based NOx emission factor can be defined as
EF )
χNOx 46 (0.87) χCO2 12
( )
(1)
where χNOx and χCO2 are the measured mole fractions of NOx and CO2, (46/12) converts the molar exhaust ratio to a mass ratio (grams of NOx emitted per gram of carbon in fuel), and 0.87 is the approximate weight fraction of carbon in diesel fuel. For this analysis we assume CH1.8 is the molar composition of diesel fuel and complete combustion occurs (all fuel C converted to CO2). The assumption of complete combustion is reasonable because the engine operates at lean fuel-to-air ratios: the average and maximum equivalence ratios for the sampling period were 0.38 and 0.64, respectively. The overall experimental uncertainty of the emission factor measurement is (6%. This value represents the uncertainty of the measured emission factor for this engine during this period; it is not an estimate of the potential variability of emissions between engines of the same family or the potential variability of emissions from this engine over long time periods.
Fuel Flow Rate. The fuel mass flow rate can be defined as
m ˘ fuel ) m ˘ air
F A
(2)
The fuel-to-air ratio, (F/A), is determined by assuming complete combustion and applying a carbon balance to the exhaust composition measurements:
9.55χCO2 F ) A 9χCO2 + 20
(3)
The air mass flow rate, m ˘ air, is defined as
m ˘ air ) FSηVd
(4)
where F is the density of the air in the intake manifold (calculated using the ideal gas law and the measured temperature and pressure of the air in the intake manifold), S is the measured engine speed, η is the pumping efficiency (fraction of the cylinder volume of air displaced by each stroke), and Vd is the volumetric displacement of the engine (13.96 L). The pumping efficiency is determined by plotting the indirect measurements of fuel flow rate calculated using equations 2-4 as a function of direct measurements made using the gravimetric tank setup. (A figure containing this plot is contained as part of the Supporting Information for this article). A linear regression of these data yields an r2 value of 0.996; a pumping efficiency of 0.9 makes the slope of the regression line 1. This value is within the range expected for a two-stroke diesel engine. Engine Load. The measured engine speed and the propeller law are used to estimate engine load. The propeller law states that the power required to drive a vessel (engine load) is approximately proportional to the cube of the propeller speed (15). We use this theoretical relationship to define the relative engine load:
Lprop )
( ) P Pmax
3
(5)
where P is propeller speed (1/6th the measured engine speed). In practice, both the power and the proportionality constant of the propeller-power relationship (the propeller curve) can vary. The propeller curve depends on many factors such as displacement, propeller slip, vessel drag, propeller fouling, depth of water, and environmental conditions (wind). The propeller curve does not apply when the propeller is reversed. The fuel flow rate provides a second estimate of engine load if the engine efficiency is relatively constant. This assumption is reasonable for the majority of normal operating conditions of a two-stroke diesel engine (16).
Results and Discussion Time-resolved measurements are shown in Figure 1 to illustrate emissions and engine loads during typical Small Tow operations. Figure 1a shows engine speed; Figure 1b shows exhaust concentrations of O2 and CO2; Figure 1c shows exhaust concentrations of NOx; and Figure 1d shows the fuelbased NOx emission factor. During this 4-h period, the vessel maneuvered to take a barge in tow, transited downbound the Ohio River with the barge to a landing in Pittsburgh, and then maneuvered to deliver and tie off the barge at the landing. The break in the measurements between 1 and 1.5 h corresponds to calibration of the gas analyzers. The NOx emission factor varies by a factor of about 3 during this 4-h period; the horizontal dashed line in Figure 1d is the fuelflow-weighted average NOx emission factor for this period, 68.4 kg per t of fuel.
FIGURE 1. Time-resolved measurements of (a) engine speed, (b) exhaust O2 and CO2 mole fraction, (c) exhaust NOx mole fraction, and (d) calculated NOx emission factor for a 4-h period of typical operations. Exhaust composition is reported on a dry basis. The break in the measurements between 1 and 1.5 h corresponds to calibration of the gas analyzers. The horizontal line in (d) is the fuel-flow-weighted average NOx emission factor for this 4-hour period. The breaks in the NOx emission factor are caused by filtering the data to correct for the time lag in the gas analyzer response, as described in the text. Small Tow has three basic operational modes: full throttle, maneuvering, and idle. Two of these three modes are illustrated in Figure 1. The period between 0.5 and 2.5 h shown in Figure 1 is illustrative of full-throttle operations. Full-throttle operations occur during transit between landings or locks on the rivers, typically with a barge or set of barges in tow. During this mode the engines are operated at near steady state, full-load conditions, except for occasional adjustments for navigational reasons. During full-throttle operations NOx emissions are relatively stable. The period after 2.5 h shown in Figure 1 is illustrative of maneuvering operations. Maneuvering operations occur when picking up, dropping off, or rearranging tows of barges. Maneuvering is characterized by periods of idling broken by frequent and rapid changes in throttle setting to provide power and control. During maneuvering NOx emissions are highly variable. It is important to note that during maneuvering the Small Tow engines spend the majority of their time at idle; during these periods the crew secures or releases a barge before the next maneuver. The Small Tow often spends hours maneuvering when rearranging and ordering a fleet of barges at a landing in preparation for a bigger towboat to pick up a tow. Idle operation is the final operational mode. Small Tow often sits VOL. 35, NO. 7, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Comparison of Average NOx Factor for Small Tow with Current In-Use Fleet Averages of Other Diesel-Based Transportation Modes diesel-powered vehicle type Small Tow (this work)a medium-speed marine engineb slow-speed marine engineb truck diesel enginec locomotive enginec non-road vehicle enginec EPA Tier 2 standard for new CI marine enginesd
NOx EF (kg NOx /t fuel)
NOx EF (g/kWh)
70 57 87 33 81 50 33
15.3 12 17 7 18 11 7.2
a Fuel-flow-weighted average. Conversion to g/kWh for Small Tow assumes an engine efficiency of 0.45. b Lloyd’s Marine Exhaust Emissions Program (7). c AP-42: Compilation of Air Pollutant Emission Factors (19). d U.S. EPA (5).
at idle for half an hour or more waiting, for example, to pass through a lock. During these periods, NOx emissions are relatively stable. A period of sustained-idle operations is not shown in Figure 1. The fuel-flow-weighted average NOx emission factor for the entire 36-h period of emission monitoring is 70 ( 4.2 kg of NOx per t of fuel, essentially the same as that indicated by the horizontal dashed line in Figure 1d. Emission factors for other diesel-based transportation modes and the recently promulgated EPA regulations provide benchmarks for comparison with Small Tow emissions. Table 1 lists the average Small Tow NOx emission factor and current in-use fleet average NOx emission factors for other diesel-based transportation modes. Small Tow emissions are less than slowspeed marine diesel and locomotive, and greater than truck, nonroad, and medium-speed marine diesel engines. Not unexpectedly, Small Tow significantly exceeds the EPA Tier 2 standards. Although Small Tow is not subject to the new EPA standards, replacing her with a vessel that met the EPA Tier 2 standard would reduce emissions by more than a factor of 2. Generalization from these results is limited because data from more vessels are needed to derive a robust emission factor for vessels operating on the inland waterways. Evaluation of the Small Tow Duty Cycle. The EPA regulations require new marine engine certification using a prescribed duty cycle of speeds and loads. A duty cycle is intended to provide a simplified representation of actual engine operations for the purpose of emissions evaluation. The overall emissions are determined by combining the duty cycle with emission factors measured at specific load points. In this section, we construct a vessel-specific duty cycle to characterize Small Tow operations. The propeller law and the measured propeller speed are used to define the Small Tow duty cycle. Figure 2 plots the fuel flow rate as a function of propeller speed to evaluate the applicability of the propeller law. The average propeller curve is defined by a least-squares fit of a cubic relationship to the data shown in Figure 2a. This fit yields an R2 value of 0.96, which provides confidence that the propeller law provides a reasonable estimate of engine load. The average propeller curve is shown in Figure 2b. The scatter in fuel flow-propeller speed relationship is due to variations in the performance of the Small Tow propeller caused by differences between upbound and downbound, loaded and unloaded, and forward and reverse operations. A portion of the scatter can also be attributed to variations in engine efficiency, which makes fuel flow rate an imperfect measure for engine load. Small Tow operations are characterized by the cumulative distribution functions of relative engine loads shown in Figure 3. These functions represent the fraction of time that the engine was operated at or below a given relative load defined 1346
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FIGURE 2. Fuel flow rate as a function of propeller speed: (a) all of the data collected over the 36 h of monitoring; (b) data averaged over 20 rpm wide bins. The vertical lines in (b) indicate the standard deviation of the fuel flow rate within a bin. The solid line in (b) is the average propeller curve, which is defined by a least-squares fit of a cubic relationship to the data shown in (a). by eq 5. Evaluation of the relative load requires defining a peak propeller speed. Definition of this peak speed is complicated by the scatter in the fuel flow rate-propeller speed relationship shown in Figure 2a. Cumulative distribution functions, shown in Figure 3, are based on three different peak propeller speeds: 325, 308, and 295 rpm. The highest speed, 325 rpm, corresponds to the peak observed propeller speed; 308 rpm corresponds to the average propeller speed from the three steady-state full throttle tests; and 295 rpm corresponds to the minimum propeller speed observed during a steady-state full throttle test. Comparison of these three distribution functions reveals that changing the peak propeller speed has negligible effect on the distribution of the relative engine load below approximately 75% of maximum load. Above 75% maximum load the uncertainty in the peak propeller speed creates significant variation in the cumulative distribution function. For subsequent analysis we use 308 rpm as the peak propeller speed. The Small Tow duty cycle is defined by collapsing the engine load cumulative distribution function into load ranges comparable to those used by the duty cycles prescribed by the new EPA regulations. We define five load ranges: idle throttle is defined by the fraction of time spent at less than 12.5% full load; one-quarter throttle is defined by the fraction
FIGURE 3. Fraction of time that the engine was operated at or below a given relative load defined by eq 5. The three different cumulative distribution functions plotted were calculated using three different peak propeller speeds.
FIGURE 4. Comparison of Small Tow, ISO E3, and ISO E5 duty cycles. The vertical “error” bars indicate the potential maximum range in the duty cycle due to uncertainty in the peak propeller speed. of time spent between 12.5% and 37.5% full load; half throttle is defined by the fraction of time spent between 37.5% and 62.5% full load; three-quarter throttle is defined by the fraction of time spent between 62.5% and 87.5% full load; and full throttle is defined as the fraction of time spent greater than 87.5% full load. The Small Tow duty cycle is shown in Figure 4. The columns represent the best estimate of the duty cycle while the vertical “error” bars represent the full range of each load point based on the uncertainty of the peak propeller speed. This uncertainty is largely manifested in the partitioning of the duty cycle between the three-quarter and full throttle load settings. Based on observations and conversations with the captain and crew, Small Tow operates primarily at two throttle settings, idle and full throttle. Small Tow requires frequent full-throttle operations because her engines are undersized; undersized engines are common among vessels
of small regional and intra-port carriers. The lack of engine power is exacerbated by the fact that the Small Tow engines are likely not generating their rated power. The maximum observed engine speed was 1947 rpm, which is significantly lower than the maximum rated speed of 2100 rpm indicated in the engine specifications. Comparison with EPA New-Engine Emissions Standards. Although the recently promulgated EPA regulations apply only to new engines, the operational data collected on Small Tow are useful for evaluating these regulations within the inland river context. The EPA regulations require engines to meet two types of standards: (1) a primary standard based on steady-state measurements conducted using a specified duty cycle of engine speeds and loads; and (2) a not-toexceed standard which limits emission over the entire range of possible load and speed conditions within a specified zone (5). The regulations allow manufacturers to perform engine certification using an engine test stand and standard landbased nonroad procedures, and allow for in-use testing to verify engine performance in the field similar to those described here. The EPA regulations specify duty cycles derived by the International Standards Organization (ISO) (5, 17, 18). The ISO E3 duty cycle represents heavy-duty diesel marine engine operation, whereas the ISO E5 duty cycle represents diesel marine engine operation on vessels less than 24 m in length. These cycles are based on oceangoing vessel operations, and focus on operational modes that lie on a propeller curve. The EPA regulations require that engines that operate with a fixed propeller, such as Small Tow, must be certified using the ISO E3 duty cycle. Figure 4 compares the Small Tow duty cycle to the ISO E3 and E5 duty cycles. The data in Figure 4 indicate that neither of the ISO duty cycles accurately characterize Small Tow operations. The ISO E3 duty cycle does not contain an idle load condition, which is the most common Small Tow operational mode. The ISO E5 duty cycle provides a somewhat better representation of Small Tow operation but still significantly underpredicts both idle and full-load operations. These conclusions are not affected by the previously discussed uncertainty in the Small Tow duty cycle. To calculate Small Tow emissions using the ISO duty cycles, Figure 5 presents measured Small Tow emission factors at the five load points corresponding to the ISO E5 duty cycle. These emission factors were determined by averaging the continuous measurements within the load ranges defined to derive the Small Tow duty cycle. Figure 5 also presents emission factors measured during the 10minute steady-state tests at specified throttle settings. These steady-state measurements are comparable to those specified by the EPA regulations for evaluating the primary emission standard. Although the emissions during these steady-state measurements are somewhat lower than the averages of the continuous measurements, they follow the same trend as the averages. Using the load-specific emission factors shown in Figure 5, both the ISO E5 and E3 duty cycles overestimate the actual emissions by 14%. Both of these standard duty cycles systematically overestimate emissions because they do not account for the length of time spent at idle. The not-to-exceed (NTE) component of the EPA regulations limits emissions over a much broader range of operating conditions than the duty-cycle based standard. The NTE standard is intended to limit actual in-use operation, and reflects EPA’s recognition that a standard duty cycle may not accurately characterize actual operations. The boundaries of the NTE zone are shown in Figure 6a. Figure 6b presents Small Tow operations plotted on top of the NTE framework in order to examine the extent to which a vessel that operates in a fashion similar to Small Tow might be impacted by the NTE standard. The propeller curve shown VOL. 35, NO. 7, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 5. Small Tow NOx emission factor as a function of engine load. The square symbols are averages of continuous measurements over specified load ranges, as described in the text. The solid diamond symbols represent measurements made during steadystate operations at specified throttle settings for 10-minute periods. The vertical “error” bars represent 2 standard deviations of the continuous measurements over the specified load ranges. Peak engine load is defined at a propeller speed of 308 rpm. The horizontal line is the fuel-flow-weighted average Small Tow NOx emission factor. in Figure 6b is the average Small Tow propeller curve. Only 39% of Small Tow operations fall within the NTE zone. To cover a greater fraction of operations the NTE zone would need to be extended to cover idle operations. Policy Insights. This paper makes several important research contributions toward understanding emissions from vessels operating on inland rivers and the national policy framework to regulate them. The results indicate that the EPA-prescribed ISO standard marine duty cycles derived for larger oceangoing commercial marine engines do not characterize the operations of Small Tow, and that emissions factors derived using the prescribed duty cycles overestimate the emissions of this vessel. To our knowledge these are the first reported measurements of in-use duty cycle and emissions for a vessel operating on inland waterways. The failure of the ISO duty cycles to characterize Small Tow operations is not surprising considering that both of these duty cycles were derived for oceangoing vessels. Inlandriver operations are inherently different from oceangoing operations because of the locks and navigational conditions. This is especially true around the Port of Pittsburgh where locks are closely spaced. Small Tow also likely represents one extreme of inland-river vessel operations because it operates in courier mode, whereas most of the inland-river fleet may be in long-haul service. However, it is important to note that a significant fraction of towboats (∼30% of those around Pittsburgh) operate in a fashion similar to that of Small Tow. More research is needed to characterize the differences between oceangoing and inland-river vessel operations and to determine whether the ISO standard marine duty cycles characterize typical inland-river vessel operations. The failure of the standard marine duty cycles to characterize Small Tow operations underscores the importance of the NTE component of the regulations. However, the effectiveness of the NTE standard to limit emissions from 1348
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FIGURE 6. (a) Boundaries of EPA Not-To-Exceed (NTE) Zone for Category 1 E3 engines (5). (b) Small Tow operations plotted on top of the NTE framework. There are two regions of the NTE zone: emissions in the upper region above 45% power must not exceed 1.2 times the federal emission limit (FEL); and emissions in the lower region between 25% and 45% power must not exceed 1.5 times the FEL. Engine power is estimated using measured fuel flow rate normalized by the peak fuel flow rate. a vessel that operates in a manner such as that of Small Tow is limited because the majority of Small Tow operations lay outside of the NTE zone. This work demonstrates the importance of making inuse emission measurements to understand vessel emissions. The new regulations specify in-use testing to certify that realworld emissions meet the standard. In-use measurements are especially important for vessels operating on the inland waterways, which must frequently pass through locks and make load changes for navigational and/or shift barge tows.
Acknowledgments The authors thank an anonymous local marine transport company which provided assess and support on the Small Tow, and Jim McCarville, Executive Director of the Port of Pittsburgh Commission, for helping to arrange the study. Project funds were provided by the Faculty Development Fund and the Center for the Study and Improvement of Regulation (CSIR) at Carnegie Mellon University. J. Corbett was supported by the Center for Integrated Study of the Human Dimensions of Global Change at Carnegie Mellon University. This center is funded by the National Science Foundation (SBR-9521914), Carnegie Mellon University, U.S. DOE, the U.S. EPA, NOAA, EPRI, Exxon, and API. Mark
Freeman at the DOE National Energy Technology Laboratory supplied the CEM and provided other critical in-kind support.
(10) Corbett, J. J.; Fischbeck, P. S. Science 1997, 278, 823-824.
Supporting Information Available
(11) Corbett, J. J.; Fischbeck, P. S.; Pandis, S. N. J. Geophys. Res. 1999, 104, 3457-3470.
Figure comparing measured fuel flow rate to that calculated from exhaust measurement during operating conditions. This material is available free of charge via the Internet at http://pubs.acs.org.
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(12) MARAD ’99: Annual Report of the Maritime Administration (MARAD). U.S. Department of Transportation; U.S. Government Printing Office: Washington, DC, 2000. (13) NDC Publications and U.S. Waterway Data CD. CEWRC-NDC; Water Resources Support Center, Navigation Data Center: Alexandria, VA, 1999. (14) The Southwestern Pennsylvania Freight Transportation Guidebook, 3rd ed.; Southwestern Pennsylvania Regional Planning Commission, Port of Pittsburgh Commission, Southwestern Pennsylvania Corporation: Pittsburgh, PA, 1998. (15) Principles of Naval Engineering. Bureau of Naval Personnel, U.S. Government Printing Office: Washington, DC, 1970. (16) Heywood, J. B. Internal Combustion Engine Fundamentals; McGraw-Hill: New York, 1988. (17) Reciprocating Internal Combustion Engines - Exhaust Emission Measurement - Part 4: Test Cycles for Different Engine Applications, ISO 8178-4: 1996(E); International Organization for Standardization: Geneva, Switzerland, 1996. (18) Annex VI of MARPOL 73/78 and NOx Technical Code, IMO-664E ed.; International Maritime Organization: London, UK, 1998. (19) AP-42: Compilation of Air Pollutant Emission Factors. U. S. Environmental Protection Agency: Research Triangle Park, NC, 1997.
Received for review August 22, 2000. Revised manuscript received January 8, 2001. Accepted January 16, 2001. ES0016102
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