Factors that affect diesel engine emissions

To develop an effective emissions-assisted maintenance procedure, all of the significant factors that could affect diesel engine emissions were considered.

Expected engine wear

Wear on engine components causes a gradual long-term change in the composition of exhaust emissions, but a properly maintained diesel engine should not demonstrate any significant degradation in emissions quality during the initial 4000 hrs of service(1).

Fuel quality

Care must be taken to avoid contaminating diesel fuel and lubricating oils during their transfer. Fuel contamination can result from dust and moisture in the environment or by simply using the same pump to transfer different fluids.

It has been shown that fuel properties such as aromatic content and Cetane number will also affect emissions(11,17). However, it is expected that these properties will have minor effects on emissions in comparison to the expected changes in emissions resulting from engine faults(17). MSHA requires the use of low-sulfur diesel fuel (<0.05% by weight) in all underground coal mines(3). The small variations of sulfur in low-sulfur diesel fuel should not have a significant effect on emissions either.

Ambient conditions

Conditions such as barometric pressure, temperature, and humidity affect the number of molecules of oxygen present in an engine's intake air, thus affecting the amount of O2 available for complete combustion of the fuel. Particularly dramatic effects on diesel engine emissions have been observed at high altitudes (low barometric pressures)(10).  One study showed that at 1,600 meters elevation, a diesel engine's DPM emission concentration increased by 50-75%(31). While another study showed that, at 1600 meters (simulated), the carbon monoxide (CO) emission concentration increased by 150 – 200%.(10)  MSHA currently addresses this effect by requiring engines to be de-rated based upon the altitude at which they are operated. An engine is derated by reducing its maximum fueling rate. This reduces maximum power output and limits peak emission concentrations to concentrations measured at sea level.

Emissions testing should not be allowed where methane is present in a mine. It has been reported that an engine operated at full load with 1% methane drawn into its intake air emitted over 200% of its normal CO exhaust concentration(4).  Furthermore, the explosion-proof integrity of the engine's exhaust system may have to be compromised to sample the exhaust, and the gas analyzer used to measure emissions may not meet MSHA safety requirements for equipment operated in explosive environments.

Engine design

Specific engine components, such as turbochargers, after-coolers, fuel injection systems, and combustion chambers, determine baseline emissions of a properly operating engine(12).   Therefore, it is not valid to compare test results from one engine model with test results from a different engine model.

Engine speed and load

Any comparison of diesel exhaust emissions should be made only at similar speed and load combinations. Engine speed affects an engine's combustion delay angle, fuel spray characteristics, combustion chamber wall temperature, turbulence, fuel-air mixing, and engine volumetric efficiency(31).  Because all these factors affect exhaust emissions, a diesel engine emission test procedure should ensure that emissions are tested at a repeatable speed.

Engine load affects the engine's fuel-air ratio, which affects exhaust emissions. When a diesel engine is operated at a low load, a significant quantity of the exhaust is excess air that does not contribute to the combustion process. It has been demonstrated that engine faults may go undetected until an engine is operated at a sufficiently high fuel-air ratio(2). The excess air prevents faults such as intake restrictions from triggering a change in emissions because the air provides a surplus of oxygen that, at higher loads, would not be available for combustion (higher fuel-air ratios). Furthermore, a maladjusted maximum fuel setting is impossible to detect unless the engine is tested at, or very near, full load. For these reasons, emission tests should be conducted at greater than 95% full load. This will maximize the likelihood of detecting engine faults.

Engine faults

The most common engine faults and their possible causes include:

• Air intake or exhaust restrictions
  • Dirty air intake filter or flame arrestor
  • Fouled exhaust catalyst, scrubber, or particle filter
• Fuel injection malfunctions
  • Maximum fuel rate setting too high or improperly de-rated
  • Fouled or leaking injectors
  • Injection pump worn or improperly timed

These engine faults are all known to affect emission concentrations(1). Air intake or exhaust restrictions increase an engine's overall fuel-air ratio by decreasing its volumetric efficiency and the number of moles of oxygen available for combustion.  Diesels require excess air for complete combustion. Without enough excess air, unburned and partially oxidized products of combustion are formed and exhausted into the atmosphere. These products include unburned hydrocarbons (HC), DPM, and CO (17,25,32). Fuel injection malfunctions that introduce excess fuel into the combustion chamber affect emissions similarly, by increasing the overall fuel-air ratio.(1) Fouled or leaking injectors will result in insufficient fuel penetration and atomization within the combustion chamber. Even though the overall fuel-to-air ratio may be sufficient for complete combustion, local fuel-rich zones resulting from poor fuel vaporization and mixing will also increase HC, CO, and DPM (16,17). Engine faults that affect an engine's fuel injection timing can affect oxides of nitrogen (NOx), HC, CO, and DPM. When the injection timing is incorrectly advanced, typically an engine will produce higher concentrations of NOx and lower concentrations of HC, CO, and DPM. When the timing is retarded, the opposite is true (16). Variations in engine design, however, will determine how severely emissions will be affected by these common engine faults.

Current methods of on-site emissions testing

Four on-site test methods were reviewed to determine if they could be used to comply with the MSHA regulation.

Length-of-Stain Tubes

One method utilizes length-of-stain tubes to test engine emissions, but there are significant problems associated with this method. A stain tube is a vacuum-sealed glass tube about the size of a pen, which contains indicators that change color when exposed to concentrations of specific gases. Length of stain tubes were designed for testing ambient-levels of CO, carbon dioxide (CO2), nitric oxide (NO), nitrogen dioxide (NO2), and sulfur dioxide (SO2). The engine emission test procedure is performed by breaking the tip of a stain tube 7 to 8 meters downwind of piece of operating diesel equipment. An emission concentration is determined by observing the stain against a scale printed on the tube. Advantages of this test include simplicity and the ability to obtain immediate results, but the method is inaccurate. A NIOSH-certified length-of-stain tube is accurate to ± 25% of the measured concentration, but in actual use these tubes are often only accurate to ± 35% of the measured concentration due to diesel exhaust interference (13). In addition, the ventilation in underground mines unpredictably dilutes emissions between the engine exhaust pipe and the stain tube. Furthermore, interpreting a change in color in the low light environment of a mine is a challenge, and individuals may interpret color changes differently (13).

Emissions Measurement Apparatus

An emissions measurement apparatus (EMA) was developed to measure DPM and other emission concentrations directly from an engine's exhaust and to identify diesel engines that emit excessive pollutant concentrations in underground mines (2).  The accuracy, precision, and reproducibility attained using the apparatus and test method were excellent. The engine loading method used with the EMA (2) is the same method that is now used in the EAMP. The EMA measured DPM under controlled dilution conditions so that undiluted exhaust concentrations could be determined accurately. The dilution system also provided the ability to use ambient level instruments for measuring CO, CO2, NO and NO2. The EMA test method established certain engine parameters to verify that the engine was loaded consistently, and the method also specified deterioration criteria to determine whether or not maintenance was needed.(19)

The EMA is not needed to comply with the MSHA regulation. To date the present EMA has only been designed as a research prototype. Furthermore, the proposed commercial design of the EMA appears to be too cumbersome to use in underground mines. Because the portable apparatus was designed for measuring DPM, the system includes compressed nitrogen bottles, heated sample lines, sample bags, sample filters, gas analyzers, and a dilution system housed on a large pushcart. Since the test procedure involves weighing DPM filters with a laboratory microbalance, results are not available immediately after the test. The system is ideal for applications where a high degree of accuracy is required, and it is the only field test method that has been developed to measure dilute exhaust DPM gravimetrically. The EMA's capability to measure undiluted CO meets the MSHA regulation for periodic monitoring of emissions, however, the system as a whole requires a high degree of expertise to operate properly.

Short inspection / maintenance program for buses

An on-site test method was developed to check emissions from diesel buses against the Environmental Protection Agency's (EPA) database of transient engine emissions. The method measured CO, NOx, oxygen (02) and DPM. The prototype apparatus included a water trap, filter, and pump to facilitate bag sampling of exhaust. A portable gas analyzer was used to measure the integrated gas concentrations from the bag (18). A portable dilution apparatus was also set up to sample controlled, diluted DPM using a filter, which was measured immediately using a portable light extinction measurement instrument (18). This test apparatus is less complex than the EMA apparatus because it measures controlled, diluted DPM without gravimetric analysis. Results are available after a minimal amount of data reduction, and the results can be readily correlated to an existing database of expected engine emissions. However, this test method still requires development of commercial instruments. Furthermore, simulating a transient test cycle involves training an operator to actuate the fuel pedal and transmission in a timed and repeatable manner.

Australian mining regulations

Australian regulations require coal mine operators to perform monthly tests of undiluted emissions from certain types of diesel-powered equipment for CO, NOx, and CO2 under full load and idling no load conditions (27). The torque converter stall and hydrostatic loading methods are used to load an engine. Full load is determined by monitoring engine speed to ensure a decrease of 200 to 300 rpm from an engine's high idling speed (full speed, no load). Emissions are not accepted unless a minimum CO2 concentration is reached; this concentration is 6% for direct injection engines, 8% for indirect injection engines, and 9% for turbocharged indirect injection engines. If emission concentrations exceed 1,500 ppm CO or 750 ppm NOx at any operating condition, the engine must be removed from service for maintenance (27,28). The instruments used for sampling are approved by Australian authorities. Use of length-of-stain tubes is specifically excluded by the regulation (27). Every 6 months, an independent laboratory audits the diesel-powered equipment emissions. Engine loading and emissions sampling procedures are similar to those used in the monthly tests. Bag sampling is allowed. In some mines, a mobile laboratory is used to audit engine emissions.

The Australian method of on-site emissions testing provided the best model for the development of the EAMP. The sampling of undiluted exhaust, exclusion of direct DPM measurements, and simple pass-or-fail emission criteria make this method simple enough for frequent use. Furthermore, it has already been implemented successfully in underground coal mines.