Michael L. Traver

Internal combustion engines have been subject to emission control techniques since the passage of the Clean Air Act in1966. Successive amendments have tightened the allowable levels of emissions emanating from new vehicles and were later extended to cover particulate emissions from diesel engines. The trend towards lower and lower allowable emissions levels appears to be continuing with particular emphasis on diesels.

Hydrocarbons describe the large family of emissions composed of hydrogen and carbon in a variety of chemical bonds. These range from simple non reactive methane molecules (CH₄) to more complex and active chemical chains like benzene (C₆H₆) and butene (C₄H₈). Hydrocarbons (HC) are formed when fuel is not adequately oxidized, or burned. In diesels, incomplete combustion of the fuel results in soot formation, visible as large clouds of black smoke, containing up to 0.5% of the fuel mass. During startup, and subsequent misfire, unburnt fuel may condense and produce clouds of white smoke [Degobert]. Overall, the level of HC emitted as a pollutant is strongly dependent upon the fuel distribution and resulting combustion inside the cylinder.

Hydrocarbon emissions can be split into two major groups: non-reactive and reactive. This grouping stems from the chemical reactivity of the molecules with respect to the formation of smog. Hydrocarbons play a secondary role in ozone formation by accelerating the formation of NO₂, which reacts with O₂ to produce ozone, the basic component of smog. The reactive components include all hydrocarbon chains except methane, which is highly stable and also gives rise to the term "non-methane organic gases" which include all non-methane hydrocarbons and oxygenates. In addition to participating in smog formation, many oxygenates are also irritants to the eyes and lungs. Further many of these molecular chains are not found in the fuel prior to combustion, demonstrating the complex chemical kinetics that occur inside a combustion chamber.

One of the factors in the production of hydrocarbon emissions is the quenching of the flame front as it approaches the relatively colder surfaces of the cylinder walls and piston. These surfaces absorb heat energy to such an extent that combustion cannot sustain itself within the fuel-air mixture. Crevices and gaps such as those seen between the cylinder walls and piston dominate this mechanism as hydrocarbons quenched at the walls are readily oxidized later in the cycle [Heywood]. Cold starting of an engine demonstrates this problem drastically as the relatively cold surfaces of the combustion chamber cause excessive amounts of black smoke. One source unique to direct injection diesels comes from the fuel injector tips. Fuel leftover in the nozzle tips after injection has ceased slowly evaporates and seeps slowly into the combustion chamber where it may or may not be oxidized. The major source, however, contributing to HC emissions are the localized rich or lean conditions found within the combustion zones. As the spray is injected, the air mixes with the outer edges of the fuel producing very lean zones that oxidize in a non self-sustaining manner and seldom to completion. As the spray continues to mix with the air, these lean zones expand outward leaving more combustible mixtures behind in the center of the chamber. The amount of HC left unburned is then a function of the mixing rate (or turbulent swirl) of the engine, the cylinder conditions and because of its association of the prior two, the ignition delay. According to Heywood, there is a non-linear relationship between the ignition delay and the amount of HC produced. Leanness, however, is not the sole condition aiding hydrocarbon emissions. Overly rich mixtures will also result in incomplete combustion, a condition that can be caused by insufficient mixing of the oxygen in the air with the fuel spray. This is especially the case just after the injector nozzles have ceased spraying as the pressure forcing the fuel out has dropped and the remaining fuel enters the combustion chamber at low speed. The low velocity of the fuel causes undermixing of the fuel-air to occur, which of course generates an overly rich region. Desorption of HC from the layer of oil that coats the cylinder walls adds to the overall level found in exhaust gas and is controlled by the characteristics of the fuel being used and its ability to be absorbed by the oil layer.

Engine operating conditions play a role in HC emissions mainly as a function of the load on the engine. Idle and light load conditions generate overall fuel to air ratios of around 100:1 and this causes an excess of over lean regions in the injected fuel spray. Consequently, light load and idle produce substantially more HC emissions than full load [Heywood]. On the other end of the spectrum overfueling of the engine at high loads will produce excessive HC through insufficient oxygen supplies.

The timing of the injection produces an effect on HC as well. If the timing is advanced away from top dead center and away from the optimum timing, the ignition delay lengthens, allowing a higher percentage of the total fuel injected to mix with the air and impinge on the cylinder walls. This also produces more areas of lean mixtures, hindering efficient combustion and raising the amount of unburned HC [Degobert]. On the other hand, retarding the advance produces overly rich regions with insufficient time to combust with the end result being visible smoke. In a similar vein, lengthening the physical time that the injectors are open and spraying fuel into the cylinders reduces HC at low load, but at high load leads to an increase in smoke and particulates [Degobert].

The distinction between particulate matter and hydrocarbon emissions is a matter of condensation temperature. Generally, heated probes in a dilution tunnel are maintained at 190°C and any hydrocarbon chain that condenses is filtered out and lumped with the soot and ash accumulations as particulate matter, which is gathered by filtering the diluted exhaust stream at a constant 52°C. Particulate formation is a major concern in diesel engine combustion and consists mainly of carbonaceous conglomerations. These clumps are formed mostly through incomplete combustion of fuel with small contributions from the lubricating oil [Heywood]. As the fuel in the advancing flame plume combusts, pyrolytic reactions crack the hydrocarbons that have yet to pass through the flame. As these reactions occur, particulate masses form and are passed through the flame. A side effect of this process is the radiation heat transfer that is given off by the heated particulates which increases the pyrolytic reactions in the unburned fuel. If the fuel mixing is poor within the cylinder, large quantities of particulates can form [Edwards]. Typically, above temperatures of 500 ° C, the particles are composed solely of clusters of carbon, while at temperatures below this, higher molecular weight hydrocarbons condense onto the clumps. As the particulates travel through the flame front and into the more heavily oxygen populated areas, the clumps tend to oxidize and for this reason concentrations are reduced in the leaner regions of combustion. Additionally, inorganic compounds in the fuel can form small clumps of material known as ash.

The main source of nitrogen in the chemical formation of NO_X is atmospheric, and a very small portion is caused by nitrogen compounds found in some fuels. The fuel source is more pronounced in diesel combustion, however. The basic kinetic equations for the transformation of atmospheric nitrogen are known as the Zeldovich mechanism. These two equations have been rigorously tested and a third equation has been generally accepted to contribute significantly and as such the three are sometimes referred to as the "extended" Zeldovich mechanism.

The third equation is usually found in rich mixtures where OH is readily available. As the burned gas region behind the flame front absorbs energy from the combusting mixture, the pressure and temperature both rise significantly. It is this region's high temperature which spurs the formation of nitric oxide (NO) and in most cases, the flame front production is simply ignored. The flame front does, however, play two significant roles by providing the thermal energy required to dissociate the N₂ into N radicals and by providing the reactions which lead to the NO producing chains. The main controlling factors are the amounts of oxygen and nitrogen radicals available and the temperature of the mixture. The temperature of the mixture is especially important as there is a non-linear relation between it and the rate of formation of NO (insert figure here). Due to this, the formation kinetics of NO "freeze" below a given temperature inside the cylinder as the piston continues downward on the expansion stroke. It is also this kinetic freeze which causes diesels to produce a significant amount of nitrogen dioxide (NO₂ ). At light load, there is a significantly large portion of the cylinder charge containing unused and relatively cool amounts of air mixing with the burning fuel. NO₂ is primarily formed in the flame front and can only be conserved by quenching, a process made easy by the generous amounts of cooler air at light load. For this reason, concentrations of NO₂ can approach 10-30% of the overall oxides of nitrogen in a diesel at light load [Heywood]. Speed also plays a small role in NO₂ formation as lower speeds increase the residence time of NO with O₂[Degobert].

Fuel-air ratio also plays a significant role in the production of NO_X, with the peak formation rate occurring at a point just lean of stoichiometric. This peak can be explained by the still fairly high combustion temperatures coinciding with the high availability of nitrogen and oxygen, which is why the peak does not occur at a point slightly rich of stoichiometric where combustion temperatures are highest. As an engine strays farther and farther into the lean region, the combustion temperatures plummet and this effect dominates the kinetics of NO_X formation. However, diesels operate primarily in the lean region (when overall fuel to air ratios are considered) where high gas availability dominates.

Since diesel engines operate at such lean overall air to fuel ratios, and since carbon monoxide formation is generally a fuel rich combustion phenomenon, this pollutant is not significant in diesel engine exhaust. Although there are regions of very rich combustion that do produce detectable quantities of carbon monoxide, the gas is oxidized later in the cycle and reduced to negligible amounts in the exhaust stream [Schafer and van Basshuysen] .

Degobert, P., Automobiles and Pollution, Society of Automotive Engineers, Warrendale, Penn., 1995.