The process of reducing emissions in an internal combustion engine is a complex endeavor with numerous variables contributing to the content of undesirable emissions. It will be noted that certain emission numbers are actually higher in the dual-fuel engine than the unmodified engine. This can be attributed to the following concepts.
Comparison of the two test engines
When comparing the emissions of the dual-fuel engine with the unmodified diesel, there are some differences between the two test engines which should be recognized.
The ECI converted engine has certain mechanical components which are distinctly different from the unmodified version. In both dual-fuel and 100% diesel modes, a deeper piston bowl design, lower compression ratio and enhanced air cooling have all been employed. Standard timing has been maintained, resulting in no significant fuel penalty. These changes were made to improve engine efficiency and performance as well as to control NOx levels, under both natural gas and diesel operation. However, these measures also contribute to the higher numbers of PM and hydrocarbons (HCs), though still within compliance of the new EPA standards.
Furthermore, in the case of the dual-fuel mode, the engine switches to an Otto-cycle environment, where the air and fuel is mixed prior to combustion. Otto-cycle engines typically show higher levels of HC and CO than in diesel cycle engines, and thus a portion of the increase in these components may be attributed to this fundamental change.
The notable rise in Total Hydrocarbons (THC) (7.7 g/hp-hr) in the dual-fuel mode is due to the use of natural gas (methane) as the primary fuel. Any unburned methane fuel is counted as part of the THC. An exception for THC limits is noted in the EPA Standards for Locomotive Emissions, in which THC for alternative fueled vehicles is represented as Non-Methane Hydrocarbons (NMHC). Under this exception as noted below, the ECI converted engine complies with this requirement.
In the case of SO2, sulfur content of the exhaust is entirely dependent upon the sulfur content of the fuel supplied. The increase in the SO2 level from the unmodified engine to the converted engine under 100% diesel operation is attributed completely to fuel quality, not the mechanical abilities of the engines. Moreover, the notable improvement in SO2 from diesel mode to dual-fuel mode (from 1.50 to 0.24 g/hp-hr) is again due to the lack of sulfur in the natural gas, the primary component. The remaining sulfur in the dual-fuel mode is attributed directly to the diesel pilot fuel.
Emissions with respect to duty cycles
A primary reason for higher readings in PM, particularly in the dual-fuel mode, is the locomotive duty-cycle itself. Due to the nature of engine use in railroad situations, idling constitutes a major sample of the engines overall duty. Since the ECI system in dual-fuel mode runs on 100% diesel at the lower notch speeds (idle-notch 3), idling on diesel constitutes a major portion of the emissions produced by the ECI converted engine. As the engine approaches higher speeds and loads under the natural gas operation, emissions are vastly improved. Thus in applications such as power generation, marine vessels, and commuter trains, where idling is reduced to a minimum and higher notch speeds/loads are maintained, emissions reduction with the ECI system is further improved over the numbers stated in the locomotive test table.
The relationship between NOx and CO
The formation of NOx is caused by a complex combination of factors involved in the combustion event. Time of fuel dispensation, peak pressures, and combustion and exhaust temperatures all affect NOx formation. There typically is an inverse relationship between the formation of NOx and CO. Higher combustion temperature and pressure levels which are often conducive to NOx formation tend to be out of the range of ideal carbon monoxide forming conditions. If conditions within the chamber cool, NOx emissions come down, but CO and Hydrocarbons may rise in the form of an incomplete burn.
The Goal of the EPA and meeting air quality
Although the ideal is to reduce all emissions across the board, the EPA's primary focus with regard to diesel engines over the past decade has been to reduce NOx levels. The EPA has stated in their Locomotive Regulations, under the heading Health and Environmental Concerns, the following:
NOx is a major component of smog and acid rain. NOx emissions combine with HC in the atmosphere to form ground-level ozone, the primary constituent of smog. Ozone is a highly reactive pollutant that damages lung tissue, causes congestion, and reduces vital lung capacity, in addition to damaging vegetation. Acid rain damages buildings and crops, and degrades lakes and streams. NOx also contributes to the formation of secondary PM, which causes headaches, eye and nasal irritation, chest pain, and lung inflammation. Environmental impacts of PM include reduced visibility and deterioration of buildings.
The EPA has looked favorably on alternative fuel projects for their potential to solve the NOx issue. Consequently, this has been a primary focus during the development of the ECI system from an emissions standpoint.
The EPAs alternative fuel exception
The trade off between NOx and CO has been recognized by the EPA, resulting in an alternate standards governing CO and PM emissions for alternatively fueled locomotives. This exception, C.3 Alternate Standards* allows locomotives using alternative fuel a larger output of CO, as long as NOx limits are met and PM is brought to a specifically lower level. This trade off reinforces the EPA's priority for NOx reduction as well as their support and encouragement of alternative fuel development and use in locomotives.
*C.3 Alternate Standard, Federal Register, Vol. 63, No.73, Thursday April16, 1998, Rules and Regulations, Environmental Protection Agency, within 40 CFR Parts 85, 89, 92, Emissions Standards for Locomotives and Locomotive Engines; Final Rule.
EPA Locomotive Emissions Standards