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Exhaust Emissions Formation in Spark Ignited Engines
Christopher J. Tennant
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Introduction
The exhaust emissions of the internal combustion engine are of particular interest because of their contribution to pollution. The allowable amounts of emissions, as regulated by the government, are becoming significantly smaller as the effort to reduce pollution continues. Therefore, it is of great interest to those who study the internal combustion engine to understand the nature of the formation of exhaust emissions. This review summarizes the current understanding of those processes for the spark-ignition (SI) engine. The chief regulated emissions of interest for SI engines are hydrocarbons (HC), oxides of nitrogen (NOX), and carbon monoxide (CO).
Most modern vehicles have aftertreatment devices, typically catalysts, for the reduction of emissions. The subject of this review will be the sources of emissions from the engine prior to any aftertreatment, referred to as "engine-out" emissions. Emissions after the catalyst are commonly referred to as "tailpipe" emissions.
HC emissions consist of unburned fuel and partially reacted fuel components (Cheng,1993). The fuel escapes the normal combustion process through several different mechanisms. Then, some of the fuel is oxidized in the cylinder or in the exhaust port and manifold, leaving the remainder as engine-out HC emissions. The mechanisms by which fuel can escape the normal combustion process include (Cheng,1993, German,1995):
The crevice volumes, the oil layer absorption, and the quench layers are all commonly cited as the most important sources of HC emissions, while the other mechanisms contribute to varying degrees depending on engine operating parameters and specific engine design (German,1995, Harpster,1995, Thompson,1994). The crevice volumes are defined as narrow regions in the combustion chamber into which the flame cannot enter because of significant heat transfer to the walls (Min,1994). Crevice volumes include the piston-ring-liner region, the head gasket, spark plug threads, and valve seat. While the crevice regions are only on the order of 2% of the total clearance volume, their contents are at a higher density due to their lower temperatures, and thus can contribute significantly to the unburnt HC emissions (Jaaskelainen).
On the order of 10 % of the incoming fuel is expected to escape the normal combustion process in a typical engine application. This will of course vary widely based upon the engine and its operating conditions, This fuel is then either oxidized, leaves the engine, or remains in the cylinder as a fraction of the residual gases. The processes by which the first two are accomplished are as follows (Cheng,1993):
The oxidation mechanisms are responsible for reducing the HC levels from the 10 % of total fuel flow that escapes combustion to the fuel that actually leaves the engine as HC emissions, which is on the order of 2 % of the total fuel consumed by the engine (Cheng,1993). This oxidation is a strong function of temperature (higher temperature equals more oxidation), which explains the relation between HC emissions and such engine operating parameters such as coolant temperature, exhaust temperature, and spark timing. (spark timing affects temperature in the following way: a later spark will correspond to a later burn, and therefore the temperatures at the end of the power stroke will be higher, favoring oxidation) (Thompson,1994). Oxygen concentration and fuel structure also affect oxidation (Jaaskelainen). The affect of oxygen concentration is demonstrated by the fact that HC emissions are at a minimum when the equivalence ratio is slightly lean, while combustion temperatures are still fairly high and oxygen concentrations are higher than stoichiometric (Thompson,1994).
NOX emissions from a spark ignited engine consist almost entirely of NO, with typically less than 1% of the total NOX consisting of NO2 (Heywood, 1988). NOX production is a function of peak bulk gas temperature and the oxygen availability (German,1995). NOX production is a kinetically controlled phenomena in the internal combustion engine since the formation of NO is much slower than the main heat release reaction. NOX is primarily formed in the post-flame region where the temperatures are high enough (Harpster,1995). Because it is a kinetically controlled phenomena, it is also secondarily a function of time (Obert,1973). Obert offers methods by which NOX emissions can be reduced, which provides some insight into the nature of the NOX production itself (Obert,1973).
Methods suggested by Obert to reduce NOX by decreasing the combustion pressure include:
Methods suggested by Obert to reduce NOX by decreasing the oxygen available in the flame front include:
Of course some of these methods, such as decreasing the compression ratio or a very-rich air/fuel ratio, are undesirable for their affects on engine performance or emissions. Due to the dependence of NOX production on oxygen availability and temperature, the maximum NOX is produced at an equivalence ratio that is slightly lean, while temperatures are still fairly high and oxygen is relatively abundant (German,1995).
CO production is strongly a function of air/fuel ratio, so much so that all other factors affecting CO production are negligible in comparison. In fact, when the engine is operated at a rich air/fuel ratio, the CO content of the exhaust can increase by a factor of 10 or more (German,1995). In comparison, HC emissions are simply proportional to the excess fueling rate. This indicates that the rise in HC emissions at a rich equivalence ratio is due to the greater percentage of HC stored by the mechanisms previously discussed. The excess fuel not stored away from combustion is successfully converted to CO, which then fails to be oxidized to CO2 due to the lack of available oxygen.
The emissions of HC, NOX, and CO are dependent on many different variables, the dominant ones being gas temperatures at different points in the cycle and air/fuel ratio, two factors which are neither independent nor wholly dependant. Ironically, the conditions for minimizing HC emissions are exactly those for maximizing NOX emissions, requiring any scheme to minimize emissions of both to make some sort of compromise. The emissions can be affected, and thereby minimized, by the parameters used to control the engine such as fueling rate and ignition timing.
Heywood, J.B., Internal Combustion Engine Fundamentals, McGraw-Hill, Inc., 1988.
Obert, E.F., Internal Combustion Engines and Air Pollution, Harper & Row Publishers Inc., 1973.
Cheng, Wai K., Hamrin, Douglas, Heywood, John B., Hochreb, Simone, Min, Kyoungdoug, and Norris, Michael, "An Overview of Hydrocarbon Emissions Mechanisms in Spark Ignition Engines", SAE Publication 932708, 1993
German, John, "Observations Concerning Current Motor Vehicle Emissions", SAE Publication 950812, 1995
Harpster, Michael O., Jr., Matas, Scott E., Fry, Jeffrey H., and Litzinger, Thomas A., "An Experimental Study of Fuel Composition and Combustion Chamber Deposit Effects on Emissions from a Spark Ignition Engine", SAE Publication 950740, 1995
Thompson, Neil D., and Wallace, James S., "Effect of Engine Operating Variables and Piston and Ring Parameters on Crevice Hydrocarbon Emissions", SAE Publication 940480, 1994
Min, Kyoungdoug, Cheng, Wai K., and Heywood, John B., "The Effects of Crevices on the Engine-Out Hydrocarbon Emissions in SI Engines", SAE Publication 940306, 1994
Jaaskelainen, Hannu E., and Wallace, James C., "Performance and Emissions of a Natural Gas-Fueled 16 valve DOHC Four-Cylinder Engine", SAE Publication 930380, 1993
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