EmissionsAnalyzers-UniversityofIdaho排放分析仪-爱达荷大学.docxVIP

20Measuring and Comparing Accuracy of Emissions Analyzers for Use with IC Engines(ASME IMECE2009-11295, 2009)1.1 AbstractAutomotive emission analyzers vary in price from under $1000 to well over $100,000. Different analyzers use various technologies to detect exhaust concentrations, and differ in how they condition the sample leading to a difference in price and performance. Manufacturer claims on accuracy from less expensive analyzers are often similar to much more expensive analyzers. With a variety of analyzers available in the Small Engine Research Facility (SmERF) at the University of Idaho, this often leads to confusion in reporting accuracy of exhaust gas measurements. This study benchmarks the performance of three different analyzers: A portable 5-gas analyzer using NDIR and electrochemical cells which costs $5000, a portable 7-gas analyzer using separate sensors for each gas which costs $10,000, and a FTIR spectrometer which costs $100,000. High and low concentrations of single-species calibration gases (methane, carbon monoxide, carbon dioxide, nitrous oxide, hydrogen, and oxygen) were run through each machine. Initial findings showed that all species measured by the 5-gas analyzer were precise around the point of calibration, with CO and CO2 quite accurate across their whole range. The 7-gas analyzer was less accurate than the 5-gas when measuring CO, CO2, and O2, but was far more accurate for THC, NO, and NO2 measurements. The FTIR was very precise provided that water vapor was effectively removed and sample lines were adequately heated. Both of the less expensive analyzers showed reduced accuracy the further away from their calibration points. Because of high setup time, use of the FTIR should be limited to detailed emissions studies, and is not recommended for coarse tuning of an engine.1.2 IntroductionThere is a large variety of technologies available for emissions sampling of engine exhaust gases. Accuracy of results depends on what type of equipment is used, and changes in ambient conditions. The intended use of the analyzer will dictate what it is designed for. Common uses for emissions analyzers are: Tuning engines, local/state emission recertification, engine research, and EPA certification.Emission analyzers are found in many different price brackets. The cheapest portable multi-gas analyzers are commonly found under $5000. Portable units with improved sample conditioning and added program functionality are often found in the $5000 to $25,000 price range. Less portable units like Gas Chromatograph and Fourier Transform Infrared can cost over $100,000. And a full multi-gas rack-mount system is usually well over $100,000. Another thing to consider when selecting emissions sampling equipment is the learning curve necessary for successful operation. Some of the simpler and less expensive analyzers are almost plug and play with no user interaction necessary. For simple units with a few options, often the users manual is sufficient to learn necessary procedures. The more complicated analyzers often require a day of on-site training to set up and familiarize the technician with operation of the equipment. It is not uncommon to have a large portion of an advanced degree becoming proficient with the more advanced emission analyzers. The Small Engine Research Facility (SmERF) at the University of Idaho has three different emissions analyzers: A portable 5-gas emissions analyzer, a portable 7-gas analyzer, and a Fourier Transform Infrared Spectrometer (FTIR). The goal of this research is to help select appropriate emissions sampling equipment, and to measure the accuracy of each of these analyzers over a range of gas concentrations.1.3 Laboratory EnginesThe SmERF lab at the University of Idaho sees a variety of user needs for emissions sampling equipment. As part of the SAE Clean Snowmobile Challenge (CSC), testing of traditional 2-stroke engines is encountered. These engines tend to run rich air/fuel mixtures. Also, due to the consumption of lubricating oil, they often have a lot of soot/particulate/carbon in the exhaust stream. Older versions of the 2-stroke engine often have hydrocarbon (HC) emissions well over 15,000 ppm (Hexane equivalent). Other demands on the emissions equipment come from new technologies being used by the CSC team. Direct injection 2-stroke engines have promise to reduce HC emissions and improve fuel economy. Creating fuel maps for these engines is difficult because of their ability to operate in stratified-charge mode under low loads. Developing ECU maps that smoothly transition between stratified-charge and homogeneous-charge modes is especially complicated. In stratified mode the global air/fuel ratio will be quite lean, but ideally a small zone of combustion will be somewhere near stoichiometric conditions. A global oxygen measurement is not sufficient to make changes to the fuel maps in the ECU. The direct injection 2-strokes produce much less soot/particulate/carbon than traditional 2-stroke engines, but this emission isnt negligible. Research on clean/efficient gasoline 4-stroke engines may be the application many of the less expensive analyzers are best designed for. This is also the application where most emission re-certification is done. Air fuel ratios are typically near stoichiometric, and emissions of carbon monoxide (CO), oxides of nitrogen (NOx), and HC are typically very low. 4-stroke diesel engines are under increasingly stricter emissions standards. Of primary concern are lowering NOx and soot/particulate emissions. However, precise measurement of CO and CO2 help estimate gross thermal efficiency. Few gas analyzers provide any soot/particulate measurement. This often requires separate sensors/equipment. Running piston engines on alternative fuels is also common in the SmERF. Some are mainstream (E85 and Bio-diesel), while others are experimental like ethanol/water blends and HCCI using JP8. In each application there are often research questions about emissions that basic analyzers cannot measure.1.4 Laboratory EquipmentThe University of Idaho SmERF has three emissions analyzers that represent typical analyzers in three cost/learning curve brackets. This section describes each of the analyzers by their cost, usability/learning curve, and unique features. It also covers the type of sensor used for each gas measurement. Portable 5-Gas Emissions AnalyzerThis 5-gas analyzer is available with several different options. The base unit costs $3500 and includes sensors for: O2, CO2, CO, HC, and NOx. The unit can be upgraded to send the display values to Bluetooth compatible units where it can be monitored and recorded. Optional PC interface also allows remote display and recording of the data as well. For lower cost, a 4-gas model (no NOx measurement) is available. Figure 0.1: Portable 5-gas emissions analyzerThis 5-gas unit uses electrochemical sensors for the O2 and NOx measurements. Life of these sensors varies with use, but in general they last 1-2 years. Error codes will flash on the display when the sensors need replacement. The other measurements for HC, CO, and CO2 are done with a NDIR cell. Calibrations are claimed to last up to a year, but periodic checking should be done with a calibration gas. Re-calibrating the unit is done using a Bar 97 gas mixture. All sensors are calibrated at once. There are few programmable parameters on the unit, which makes it very simple to operate for almost any user. Attach 12V power to the lighter-style power plug and after a short warm up period the analyzer will display current exhaust concentrations. The analyzer will turn off after it senses CO levels below 3% for more than 15 minutes. It will also perform an “auto zero” periodically when exhaust gases arent present. The manual for the 5-gas is about 20 pages long, and easy for a non-technical audience to follow. It does not use technical jargon, and does a good job explaining the purpose and meaning behind its features even for users not very familiar with emission sampling equipment. The optional PC software allows recording data, and information about the run. You can save vehicle descriptions and perform a few different kinds of automated tests while connected to a PC. Recorded data can be played back, but pulling the data out to a useful format (text or spreadsheet file) is not a simple task. Data can be exported to a Microsoft Access database, but it comes without column or page descriptions. Recently this 5-gas unit has been interfaced with other engine testing hardware. The serial communications port on the 5-gas can communicate with several of the more common dynamometer data acquisition systems. This allows real-time data streams from the analyzer, so recording emissions along with any other parameter from the dyno data acquisition is seamless. Table 0.1: Claimed range and accuracy of 5-gas analyzer 1SensorRangeAccuracyHC0-2000 ppm (Hexane equivalent)4 ppm (Hexane equivalent)CO0-10%0.06%CO20-20%0.3%O20-25%0.01%NOx0-5000 ppm (Nitric Oxide)25 ppm (Nitric Oxide)Portable 7-Gas Emissions AnalyzerThe 7-gas analyzer was purchased to represents a high quality portable gas analyzer. It can be purchased in four different packages (kits). The kits are set up for their intended usage. The kits are: Boiler, with up to 4 sensors. It includes sensors for O2 and CO. Basic, with up to 6 sensors. It includes O2, CO, NO, and NO2, and a fresh-air purge. Engine, with up to 6 sensors. It includes O2, CO (with dilution options), NO, and NO2, and a fresh-air purge. Turbine, with up to 6 sensors. It includes O2, CO, CO low, CO (with dilution options), NO low, and NO2, and a fresh-air purge.All of the units are expandable to use any six of their drop-in sensor modules. Available sensors are: O2, CO, CO low, NO, NO low, NO2, SO2, CxHy, H2S, and CO2. If more than 6 sensors are desired, multiple units can be daisy-chained together and controlled by a common handheld unit. The University of Idaho SmERF purchased a Kit #3 and added CxHy and CO2 sensors, and touch screen. This brought the price of the unit to a little over $10,000. The 7-gas analyzer uses separate sensors for each gas to be measured. Each unit can hold up to six sensors, however, the CO sensor is hydrogen compensated, and the H2 concentration can be displayed by the analyzer. This gives the possibility of recording seven different gas concentrations. The type of gas being detected determines the sensing technology used. Electrochemical sensors are used for O2, CO, NO, NO2, SO2, and H2S measurements. Each sensor is modular with all other sensors. The calibration is stored on the sensor body itself, so modules can be swapped in the field w/o the need to recalibrate. All of the electrochemical cells are continuously temperature and pressure compensated, and if a fresh-air purge is required, the sensor will be shut down before damage can occur. The CxHy sensor is of a Pellistor type. Pellistor sensors operate by catalytically burning carbon compounds, and comparing temperature on each side of the catalyst. A circuit similar to a hot-wire anemometer is used to determine the temperature. Drying of the sample gas is critical because any water in the system will falsely change this temperature reading. Also, in order to burn the carbon compounds, excess oxygen must be present. For this reason, the CxHy measurement is only available when there is excess oxygen in the sample stream. The HC measurement typically does not work under rich conditions often where the most HCs would be produced. None of the kits for this analyzer come with a CO2 sensor. Instead CO2 is calculated from the O2 measurement, and an input “Max CO2” level that is determined by the fuel type. The calculated CO2 is relatively accurate for common fuels, but is not applicable for some alternative fuels. For better accuracy, the company now sells NDIR CO2 cells that measure the CO2 in the sample stream. This option was purchased so a comparison could be made between the calculated and measured CO2 displayed by the unit. Calibration of sensors is done individually. Calibration is usually only required a few times per year, but it should be checked often with a calibration gas. Single gas mixtures (desired gas concentration in an inert dilution) can be used for calibration of individual sensors. Figure 0.2: 7-gas analyzer control unit and displayBecause of the various setup options and programmability of the unit, it was recommended to purchase some on-site training session to help set up the analyzer for the intended usage. In this training the technician described how to navigate the menu system, and create custom programs. Initial setup time is a few hours, but once set up in a desired configuration the unit is almost plug and play. The unit also has a battery pack that allows sampling for 2-3 hours away from a plug in power source. The manual for the 7-gas analyzer is about 60 pages long, and several supplements are offered covering topics such as calibration and software. Between these resources, there are over 100 pages of documentation. The manufacturer web site also has forums and additional downloads. Because the unit has many programmable options, there is more to discuss in the manual. The level of knowledge necessary to understand the manual is matched for those who have some experience with emissions sampling, though at a basic level. Data can be recorded in the unit, or printed on the built-in thermal printer. The optional PC based software used Microsoft Excel for the interface. Retrieving data from the software is very simple. The 7-gas analyzer has a many features not typically found on less expensive analyzers. The unit has a built-in Peltier condenser to help dry the gas before entering the unit. The programmability also allows monitoring of emissions in an automated mode. For instance, it can be programmed to take a reading once every hour, and store the data string in a single file location. This is useful for monitoring flue gases, or for long-term engine testing. The unit can measure and record delta-pressure, and engine RPM. Table 0.2: Claimed range and accuracy of 7-gas analyzer 2SensorRangeAccuracyHC0-44,000 ppm (Methane equivalent)400 ppm (Methane equivalent)CO0-10%5 ppm (0-99ppm), then 5% m.v. CO low0-500 ppm2 ppm (0-35ppm), then 5% m.v. NO0-3000 ppm 5 ppm (0-99ppm), then 5% m.v.NO low0-300 ppm 2 ppm (0-35ppm), then 5% m.v. NO20-500 ppm 5 ppm (0-99ppm), then 5% m.v. CO2 (calculated)0-max vol %Calculated from O2CO20-50%0.3% plus 1% m.v.O20-25%0.2% of m.v.Fourier Transform Infrared Spectrometer 3Useful for more than just emissions sampling, a FTIR spectrometer uses infrared radiation to detect compounds. For gas sampling, detection is done by absorption. The intensity vs. wavelength of a beam through inert gas (Nitrogen) is compared to that same beam going through the sample gas. The FTIR that is in the SmERF was donated by a local company. In the configuration it was delivered in, the equipment was valued at $150,000. Because there are not individual sensors in a FTIR, the ability to detect species concentrations depends more on the methods reference in the computer. Instead of sensors, the computer uses a spectral database of compounds. The height and frequency of peaks or valleys in the IR signature are compared to compounds in the database for matches. If the desired compound is in this database, the equipment will be able to calculate the percent composition of that species in the sample 4. Setup and use of a FTIR is not trivial. Usually a representative from the supplier will come out for a day or two to set up the equipment and train operators on how to use it. Methods for detecting desired species are usually purchased from the supplier and loaded on the computer during the on-site training.Preparing the FTIR for emission measurement takes more time than the portable analyzers. Liquid nitrogen is used to cool the sensors, and the equipment should be turned on for at least 30 minutes to allow the laser to stabilize and the test cell to reach operating temperature. Also, the system needs to be purged with a low flow of nitrogen during the whole sampling period. The large size and sensitivity of the laser and optics make the FTIR a relatively stationary piece of equipment. It can be moved around in the lab, but is not likely going to be used for any dynamic in-vehicle testing. Because of the volume of the sampling cell, the time response to changes is not as fast as some of the other analyzers. Drastic changes in composition may take a few minutes to reach their true values. Thus, the FTIR is used primarily for steady state testing. One very useful feature of the FTIR is the ability to provide hydrocarbon speciation. Where other analyzers just read an equivalent HC, the FTIR can be used to provide volume fractions of any specific hydrocarbon chain in the methods. In particular, while testing alternative fuels performing a hydrocarbon speciation is highly valuable. In the combustion of ethanol, aldehydes are formed. While not currently regulated, as ethanol-based fuels become more popular having a detailed breakdown of the HC emissions will help target appropriate after treatment systems 5. The FTIR at the SmERF lab did not come with any sample preparation hardware. A high temperature pump is necessary to bring samples in, and filters for particulate, gas dryers, and line heaters need to be installed and maintained. A lot of plumbing and valves were added to allow quick changes from calibration, nitrogen purge, and exhaust sample inputs. The range and accuracy of gases was not provided with the FTIR. Ac