Adapting laboratory procedures for on-site emission testing

In an engine laboratory, sophisticated equipment is used to control an engine and measure its emissions. Because this equipment is impractical for on-site emission testing, laboratory test methods must be adapted for field use. Laboratory emission results are typically mass-based and normalized with respect to power output (brake-specific) at a given speed and load. This is done so that comparisons may be drawn between different engine models, configurations, and operating conditions. To achieve similar results in the field, engine speed, load, and emission concentrations must be determined. Figure 1 illustrates how 5 practical on-site measurements can be combined with 4 known engine and fuel parameters to calculate brake specific CO emissions at a given speed and load.

Figure 1: Emission measurement flowchart

Emission measurement flowchart

The flowchart in figure 1 is separated into 4 sections. The left-hand section contains the parameters that need to be measured at test time. The two middle sections contain the known and calculated parameters required to calculate the desired quantities at the right. The symbols and calculations are described from top-to-bottom and left-to-right.

COdry: the concentration of CO in the undiluted exhaust measured in parts per million (ppm). Since most portable gas analyzers have water traps that remove most of the water from the exhaust, the measured CO and CO2 concentrations are considered a fraction of dry exhaust.

CO_2dry: the concentration of CO ₂ in the undiluted exhaust measured in percent (%).

P_barom.: the barometric pressure at the test location measured in kilopascals (kPa).

T_amb.: the ambient temperature at the test location measured in degrees Celsius (° C).

Speed: the engine crankshaft speed measured in revolutions per minute (rpm).

BSFC: the brake specific fuel consumption measured in kilograms fuel per kilowatt-hour (kg/kwh). The engine manufacturer can provide this value, which is a function of speed.

H:C_ratio: the molar ratio of hydrogen to carbon in diesel fuel. The fuel supplier may be able to provide this value, but a fuel sample can also be analyzed an appropriate laboratory.

h_v: the volumetric efficiency of the engine. This is the ratio of air aspirated to the volume swept by an engine's pistons. The engine manufacturer can provide this value, which is a function of speed. Although this value is not normally used to describe turbocharged engine performance, the ratio of air aspirated to swept volume may still be calculated. This number may be greater than unity for turbocharged engines.

Displ.: the engine displacement measured in cubic meters (m³). The engine manufacturer can provide this value.

CO_wet: the concentration of CO in the undiluted exhaust as a fraction of the wet exhaust. This calculation corrects for the water removed by an analyzer water trap.

CO_2wet: the concentration of CO₂ in the undiluted exhaust as a fraction of the wet exhaust.

F:A: the mass ratio of fuel to air, assuming complete combustion and an air composition of 21% molecular oxygen (O₂) and 79% molecular nitrogen (N₂) by volume.

m_air: the mass rate of air in kilograms per hour (kg/hr) aspirated by a 4-stroke engine corrected to normal conditions of P_barom=101 kPa and T_amb=20 ° C.

m_fuel: the mass rate of fuel consumed in kg/hr.

m_exh: the mass rate of exhaust produced in kg/hr.

Power: the brake power in kilowatts (kW).

CO_conc: the mass rate of CO produced in grams per hour (g/hr).

BSCO: the mass rate of CO normalized to power output in g/kwh.

Torque: the brake torque in Newton-meters (Nm). Torque is a measurement of engine load.

Simplifying the procedure

The procedure in figure 1 can be simplified by assuming that the test will be performed using a similar fuel, speed, and load and that results will only be compared to results from the same engine or a similar engine model. These assumptions eliminate the need to calculate emissions on a mass basis normalized with respect to power. By making these assumptions, dry CO measurements can be compared directly to determine if there is a need for engine maintenance. Because CO2 is proportional to load at a given speed, dry 2 measurements can be used to verify engine load. Therefore, measuring CO and CO2 and comparing the results to similar tests performed on similar engines can indicate a need for maintenance. Because these assumptions were made to simplify the test procedure, baseline data consisting of normal CO and CO2 concentrations for individual engine models need to be established at the speed and load conditions at which the test will be performed. Results will be compared to baseline data to determine a need for maintenance.

Controlling engine speed at full load

In an engine laboratory, adjusting the fuel rate and the dynamometer controls engine speed and load. In the field, the fuel rate is adjusted with the fuel pedal, and the engine may be loaded via the transmission. Because emissions are most likely to indicate a need for maintenance at full load, full load is the best condition at which to test an engine. Full load is achieved by operating an engine at its maximum fuel rate. When an engine is operated at partial load with its fuel pedal fully depressed, the speed governor limits the fuel rate to a value less than maximum. To bring an engine to its maximum fuel rate, the load against the engine must be increased while the fuel pedal is fully depressed. Once the load slows the engine to the speed at which rated power is generated, the engine is at full load and operating along its lug curve. Figure 1 illustrates an engine lug curve with several full-load operating points indicated. Because emissions vary as a function of speed, the same speed along the lug curve should be repeated. In an engine laboratory, this is accomplished by applying a constant load with a dynamometer, while the engine is operated at its maximum fuel rate.

Figure 2: Engine lug curve

Torque converter transmissions

In the field, engines on machines equipped with torque converter transmissions can be loaded along the lug curve at a repeatable speed by what is called a "torque converter stall." A torque converter is an enclosed hydraulic fluid coupling that transfers power from the engine shaft (input) to the transmission shaft (output). It consists of three main parts: the pump, turbine, and fixed reactor. The pump is connected to the engine shaft, the turbine is connected to the transmission shaft, and the fixed reactor is mounted inside the torque converter housing. Figure 2 shows a torque converter cross section. Hydraulic fluid enters the inner side of the pump and exits the outer side. The fluid then enters the outer side of the turbine and exits the inner side. The fluid is directed through the fixed reactor and then to the inner side of the pump. This circulation drives the transmission by causing the turbine to rotate in the same direction as the pump.

Figure 3: Torque converter cross section

Torque converter stall is achieved whenever the engine is turning the pump and the transmission is locking the turbine in a stationary position. The torque converter will stall along an engine's lug curve at a repeatable speed when the engine is operated at its maximum fuel rate. Under this condition, all the engine power is dissipated through the pump in the form of heat to the hydraulic fluid. A heat exchanger (not shown) provides cooling under normal conditions, but stalling the torque converter exceeds the capacity of the heat exchanger. To prevent damage to the torque converter, continuous operation at torque converter stall should be limited to less than 30-sec.⁽⁵⁾

The torque converter stall should only be conducted with the vehicle facing a mine rib and by trained personnel. No personnel should be between the front or rear of the vehicle and any ribs. This procedure may be used for loading an engine by the torque converter stall:

1. The engine is started and warmed up for 1-min at idle unless the engine being tested has been in recent service and is already warmed up.
2. The brakes are applied.
3. The transmission is shifted into high gear.
4. If the engine was started cold, the fuel pedal is partially depressed for approximately 2-min to warm up the engine and torque converter hydraulic fluid before the test is repeated.
5. The fuel pedal is then fully depressed for 30-sec while the emissions are sampled. This is the torque converter stall. Because the torque converter oil may overheat during the stall condition, the operator must closely monitor the torque converter oil temperature. The test should be aborted immediately if any equipment operating parameter appears abnormal. If the equipment begins to move, the test should be aborted immediately and the equipment brakes should be serviced.
6. After 30-sec the fuel pedal is released, the transmission is shifted out of gear, the brakes are released, and the engine and torque converter are allowed to cool for a few minutes before the equipment is secured. The torque converter stall may be repeated, but the torque converter oil temperature must be closely monitored.

Hydrostatic transmissions

In the field, engines on some machines equipped with hydrostatic transmissions can be loaded against the transmission. However, engine speed is more difficult to control than with the torque converter stall method. Furthermore, the hydrostatic loading method may damage the equipment, and some hydrostatic equipment may not have an independent braking system, which is required to load the engine.

A hydrostatic transmission consists of a variable-displacement hydraulic pump coupled to the engine. The pump delivers high-pressure hydraulic fluid to one or more hydraulic motors, which drive the equipment wheels. If the displacement of the pump is gradually increased from neutral while the fuel pedal is fully depressed and the wheels are held stationary, the pump discharge pressure will increase and actuate a pressure relief valve. The excessively high discharge pressure will cause the pump to resist engine rotation, and the pump will increase load on the engine. Once the engine is loaded along its lug curve, slight adjustments to the pump displacement will cause large changes in speed, and the engine may stall. If the pump displacement can be controlled properly by the equipment operator, a constant speed along the engine lug curve can be achieved, but this is dependent on the equipment and the operator.

Loading an engine with a hydrostatic transmission should only be conducted with the vehicle facing a rib and by trained personnel. No personnel should be between the front or rear of the vehicle and any ribs. This procedure may be used for loading an engine with a hydrostatic transmission:

1. The engine is started and warmed up for 3-min at idle unless the engine being tested has been in recent service and is already warmed up.
2. The brakes are applied.
3. The fuel pedal is fully depressed.
4. The lever used to control the speed and direction of the vehicle is gradually adjusted until the engine speed falls below the speed at which rated power is generated. If the equipment does not have a tachometer, a portable tachometer may be used to monitor speed.
5. This condition is maintained for 30-sec while the emissions are sampled. Because the hydraulic oil may overheat, the operator must closely monitor the hydraulic oil temperature. The test should be aborted immediately if any operating parameter appears abnormal. If the equipment begins to move, the test should be aborted immediately and the equipment brakes should be serviced before the test is repeated.
6. After 30-sec the equipment speed lever is returned to neutral, the fuel pedal is released, the brakes are released, and the engine and transmission are allowed to cool for a few minutes before the equipment is secured. Loading an engine with a hydrostatic transmission may be repeated, but the hydraulic oil temperature must be closely monitored.

Selecting emissions that indicate a need for maintenance

Consideration was given to which pollutants would make the best surrogate indicators of an engine fault. The following emissions were evaluated as possible candidates: DPM, NO, NO2, CO2, O2 and CO. Exhaust temperature was also considered.

Diesel particulate matter (DPM)

DPM is an emission of concern from a health and safety perspective, but accurate DPM measurements still require sophisticated instruments.(2,40,41) Light extinction techniques, or smoke measurements, can be used to measure the non-volatile portion of diesel exhaust.(20,40,41) Smoke measurements, however, require either sampling exhaust DPM onto a filter, or using a dedicated piece of instrumentation to measure the amount of light extinction across an exhaust stream.

Nitric oxide (NO)

Exhaust concentrations of NO can change with fuel injection timing faults. It is a good indicator of peak combustion temperature, and is not difficult to measure. However, it does not necessarily change rapidly with faults that affect DPM concentrations. It is a good secondary emission to monitor, but if only one pollutant can be selected, NO is not be the best choice.

Nitrogen dioxide (NO₂)

NO2 has a lower permissible exposure limit than either CO or NO, but NO2 is not a good early warning indicator of the most common engine faults. It is not normally emitted in sufficient concentrations in the exhaust to be a reliable fault indicator. NO2 would make a good secondary pollutant to monitor if the option is available.

Carbon dioxide (CO₂)

CO2 should be measured as part of any emission test procedure. CO2 is proportional to engine load at a given speed, and consistent, high CO2 verifies that a steady-state, full-load condition has been achieved. Similar CO2 exhaust concentrations from test to test verify engine load.

Oxygen (O₂)

O2 can be used as a substitute for monitoring CO2 to verify full load. Most portable CO2 analyzers actually calculate CO2 based upon measured O2 and a fuel type selected by the user. Selecting a fuel type specifies the hydrogen-to-carbon molar ratio (H:C ratio) of the fuel. The instrument then calculates the stoichiometric fuel-to-air ratio and CO2 concentration by measuring O2 and assuming complete combustion of the specified fuel. For diesel engines assuming complete combustion is a reasonable assumption.

Carbon monoxide (CO)

CO can indicate engine faults that also cause high DPM. As shown in this report, CO can increase sharply as an engine's fuel-air ratio increases beyond its smoke limit (smoke limit: the maximum fuel-to-air ratio of a diesel engine at which a small increase in the fuel-to-air ratio will cause a rapid increase in DPM). DPM may be the ultimate pollutant to monitor as it may become the limiting ambient pollutant in underground mines,(38) but CO is a reasonable surrogate pollutant that is much simpler to measure. Furthermore, CO must be measured to comply with the MSHA regulation.

Exhaust temperature

Exhaust temperature was also considered as a factor to monitor during emission sampling because it changes with some engine faults as well as engine load. If a thermocouple is positioned improperly, however, results may be very inconsistent. Furthermore, a water-jacketed exhaust manifold may mask changes in the uncooled exhaust temperature.

Selecting an emissions analyzer

This report describes the evaluation of an analyzer with CO, CO2, NO, NO2, and O2 detecting capabilities. The CO2, O2, NO and NO2 capabilities enhanced results, but determining these concentrations is not required by the MSHA regulation. For analyzing diesel exhaust, an analyzer should be capable of detecting CO in the 0 to ~3000-ppm range and CO2 in the 5% to 15% range, and with a minimum accuracy of ± 10%. The instrument should also have a sampling probe and sample line suitable for sampling undiluted diesel exhaust. The instrument must be capable of surviving rugged mine environments, and the probe itself must be capable of withstanding 650 °C exhaust temperatures. If the sample probe is not designed to sample gases up to 650 ° C, the exhaust gas must be cooled upstream of the probe. The instrument should have the additional capabilities of continuously storing data at 1 to 2-sec intervals, displaying the data, and downloading the data at a later time. This will facilitate a rapid review of emissions data, but it is not necessary for meeting MSHA requirements.

A number of commercially available emission analyzers are available that meet these requirements. These instruments utilize nondispersive infrared (NDIR), Fourier transform infrared (FTIR), or electrochemical gas sensor (EGS) technologies. NDIR and FTIR are highly accurate gas sensing methods, but typically they are more expensive and complex than needed for this application. EGS instruments are available in several portable, rugged instruments, and if maintained and calibrated according to specifications, EGS technology is capable of accuracy within ± 5% of the measured value. Laboratory tests have verified EGS performance. Furthermore, EGS technology offers the additional capability of monitoring NO, NO2, CO, CO2, and O2 using a single instrument. These additional sensing capabilities can enhance the EAMP by providing additional information that will assist engine maintenance personnel in narrowing the number of possible engine faults.

Selecting a sampling location within the exhaust system

It is important that sampling takes place at a proper location within an engine exhaust system. Emissions should be sampled before the exhaust enters any after-treatment devices. Many exhaust systems are fitted with diesel oxidation catalysts (DOC's), which oxidize CO to CO2 and convert unburned hydrocarbons to water vapor and CO2. Modern DOCs can oxidize 80% to 90% of the CO produced by a diesel engine,(6,10) thus masking engine faults that increase CO concentrations. Sampling upstream of a DOC is facilitated by a back-pressure measurement port. These connections are suitable for sampling exhaust emissions with a portable emissions analyzer.

Water scrubbers are used on permissible mining equipment to eliminate flames and sparks and to cool the exhaust. Cooling is accomplished through evaporation, and the exhaust leaving the scrubber typically is saturated with water vapor. Although the CO present in the exhaust is essentially unchanged by a water scrubber, the additional water vapor may break through an analyzer water trap and cause condensation in the sampling instrument, resulting in erroneous readings.(7) MSHA equipment approval regulations indicate that a pressure gage connection must be provided upstream of a water scrubber at a point suitable for measuring the total back-pressure of the exhaust system. MSHA also specifies that this connection must be suitable for attaching gas-sampling equipment to the exhaust system temporarily.(3) This is the point at which emissions from a permissible engine should be sampled. It may be inconvenient to remove and replace the pipe plug that seals this port, but there are MSHA-approved, flame-proof adapters that can adapt a quick disconnect fitting to this port. Paas Technologies, Inc., markets one such flame-proof port through Goodman Technologies.

Finally, sampling must always take place upstream of fume diluters or any other device that causes dilution of the raw exhaust gas. In cases where engines are configured with dual exhausts, both exhaust banks should be sampled and recorded separately. Testing both banks can reveal whether an engine fault is present on an individual bank of cylinders or whether there is a more general fault in the engine.

Collecting baseline data

Although MSHA may eventually provide engine model-specific CO and CO2 baseline data, the mine operator must initially establish this data by performing the EAMP on new or recently rebuilt equipment. In some cases, baseline data may be established for a "family" of engine models. For example, the Caterpillar 3304 PCNA and Caterpillar 3306 PCNA engines have the similar air intake, combustion chamber, and fuel system designs. Even though the engines have a different number of cylinders and different power outputs, their emission concentrations should be similar, as long as their maximum fuel rate per cylinder is similar.

In the future, engine manufacturers and mine equipment manufacturers may be able to provide baseline data. If an engine manufacturer collects CO and CO2 data while an engine is loaded at several speed on its lug curve, the data could be presented graphically in an engine service manual. Also, mine equipment manufacturers may be able to perform the EAMP on equipment before the equipment is delivered to the mine. The equipment manufacturer could publish these baseline data in the equipment's service manual.

Keeping records

The MSHA regulation does not specify how the mine is to keep records, but the following information should be recorded to assist in data interpretation.

• Date and time of the test
• Location (elevation or level) of the test
• Engine and equipment identification numbers
• Engine hours
• Torque converter fluid temperature at the end of the test (gages may only have temperature color coding, but recording an approximate gage needle position may be sufficient).
• Ambient temperature
• Engine speed, exhaust CO2, or exhaust O2 concentrations. To verify that the test was conducted under a steady-state, repeatable, full-load condition, one of these parameters should be recorded at the same time that CO is recorded.

A chart of CO emissions plotted over time (weekly) should be created and updated after each test to assist in identifying emission trends and engine faults. Another useful chart to create is CO versus CO2. This chart will simultaneously indicate whether or not a valid loaded engine condition was achieved and whether or not the concentration of CO was acceptable. The "Methods" section of this report contains suggestions for interpreting the CO vs. CO2 chart.

Performing the EAMP

The EAMP can be completed with one person operating the diesel machine and a second person operating the gas analyzer. To ensure safe testing, this procedure should only be conducted in a well-ventilated, methane-free area with the vehicle facing a rib and by personnel thoroughly instructed in the test procedure. No personnel should be between the front or rear of the vehicle and any mine ribs. This procedure may be used to comply with the MSHA requirement.

1. The gas analyzer is turned on and allowed to warm up and, in some instances, self calibrate.
2. An appropriate exhaust port plug is removed from the engine's exhaust, and the analyzer probe is inserted into the port.
3. Exhaust is sampled for 15-sec before torque stall is initiated. Sampling is then performed for 30-sec during torque stall. To conclude, sampling is continued for an additional 15-sec after torque stall has been removed. These consecutive time intervals ensure that emissions are completely drawn through the sample line and analyzed by the instrument.
4. The analyzer is allowed to sample mine air after each test to purge the instrument of raw exhaust.
5. Data are then reviewed to determine the emissions. A strip chart of CO2 versus time can be used to confirm the time interval over which the engine was operated under a steady-state condition. O2 levels or engine rpm can be used in a similar manner to determine this time interval. The CO levels should be fairly constant over this interval, and an average CO value should be recorded to comply with the MSHA regulation.

Figure 4 demonstrates how the EAMP was performed in an underground mine, and figure 5 shows how the EAMP was performed at an equipment manufacturer's facility.

Figure 4: Preparing to perform the EAMP

Figure 5: Performing the EAMP