Emissions from Internal Combustion Engines

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When automobiles were marketed in the early twentieth century, they were seen by many as a clean form of transportation, doing away with the nuisance of horse feces in large urban cities. In the decades that followed, as the numbers of vehicles increased, the impact of this technology on air quality and health became more pronounced and cars became the main source of environmental pollution. Today, motor vehicles are responsible for roughly four-fifths of all carbon monoxide emissions and nearly half of all hydrocarbons and nitrogen oxides released into the atmosphere globally (Table 1). In addition, motor vehicles produce carbon dioxide, a potent greenhouse gas and a major contributor to the production of ozone in the lower atmosphere. Compared to reciprocating engines, gas turbines produce lower emissions. This is because gas turbines operate under ultra-lean combustion. Furthermore, gas turbines do not respond well to transient operations and must operate in a continuous mode.

Table 1. US Total emission of criteria pollutants in 2002*
Source Total Emission Mobile Stationary Industrial Others**
CO 108 million tons 82% 3.8% 2.3% 12.6%
NOx 20 million tons 55% 38% 3.9% 2.2%
HC (VOC) 16 million tons 42% 6.6% 41.5% 10.2%
PM10 21 million tons 2.2% 6.2% 3.1% 88.5%
PM2.5 7 million tons 6.1% 17.9% 7.1% 69.0%
SO2 14 million tons 4.4% 86.3% 8.9% 0.3%
NH3 7 million tons 6.3% 1.4% 3.4% 88.8%
*To the closest percent
** Waste disposal, recycling, etc.
Source: US EPA, National Emission Inventory Air Pollutant Emission Trends website

The main sources of engine emissions are unburned hydrocarbon from the crankcase, carburetor, and fuel tank, as well as tailpipe emissions. Most emissions come from the exhaust pipes and include carbon monoxide, nitrogen oxides, soot, and various volatile organic compounds. Exhaust pipe emissions are largely a result of incomplete combustion processes and are affected by many factors. Three parameters have profound effects on how combustion proceeds and how much pollution is formed. These parameters are referred to as the three Ts of combustion - temperature, time, and turbulence. Temperature determines the rate of reaction and heat release. If everything else remains the same, higher temperatures increase production of nitric oxides and limit the production of hydrocarbons, carbon monoxide, and soot. Temperature is highest near the stoichiometric conditions and for fuels with high hydrogen to carbon (H/C) ratios. Stoichiometric conditions occur when an air/fuel mixture burns to completion and when, under equilibrium conditions, the only products of combustion are carbon dioxide and water. The time that reactants have in a flame reaction zone determines the extent to which reactions go to completion. Greater residence times generally favor more stable molecules (carbon dioxide, nitrogen, water vapor) and lower overall emissions. Turbulence affects the rate of mixing of fuel and air molecules. Better mixing prevents localized pockets of very rich or very lean mixtures, thus allowing the reaction to complete and produce fewer contaminants.

A brief discussion of major pollutants from automobiles and other internal combustion engines and factors affecting their formation is given below.

Nitric oxide results from a reaction between oxygen and nitrogen heated to combustion temperatures. As we may expect, maximum NOx occurs near stoichiometric conditions where temperature is at its peak and sufficient oxygen is available. Too much air dilutes the mixture and reduces the flame temperature; with too much fuel, little oxygen is left over for reacting with nitrogen. The best way to reduce NOx emission is by lowering the combustion temperature and limiting the availability of air. Recirculating a portion of exhaust gases back to the intake manifold is shown to be effective in reducing NOx emission by lowering the air to fuel ratio and cooling the flame. Unfortunately, exhaust gas recirculation (EGR) valves must operate with a rich mixture, which promotes production of hydrocarbons and carbon monoxides. Another method to reduce NOx emissions is to add some hydrogen to the gasoline/air mixture. The drawback is backfiring, a result of hydrogen’s low ignition temperature and lower power rating.

Carbon monoxide results from incomplete reactions. It is formed when there is insufficient oxygen, temperature, or time to oxidize all carbon to carbon dioxide. The obvious way to reduce carbon monoxide emissions is by increasing the compression ratio (increasing temperatures) or by maintaining a lean fuel mixture. Advancing the spark raises peak temperatures and increases the time available for reaction, both of which inhibit carbon monoxide formation.

Hydrocarbon emission can be divided into two categories, unburned and partially burned. Roughly half of all hydrocarbon emissions come from unburned vapor during fill ups, evaporation from the carburetor, and from other hot surfaces when the engine is shut off. Other sources of hydrocarbon emission are crankcase blow-by and emission from leaky valves, piston rings, and gaskets. The hydrocarbon release during fueling is reduced by installing vapor recovery nozzles on gas pumps. The crankcase emission is practically eliminated by closing off the vent to the atmosphere and installing a positive ventilation valve (PCV) to recycle blow-by back into the engine’s intake.

Particulates– Besides gaseous pollutants, most internal combustion engines produce some solid carbon particles called soot. Soot particles are large clusters of carbon atoms generated in the combustion chambers and aggregated to sizes ranging from a few nanometers to hundreds of microns. Soot can also be formed from the lubrication of oil next to cold surfaces and from the rapid expansion cooling during the power stroke. Although some soot is generated in gasoline engines under heavy loads, diesels are by far the main source of soot. This is due to the relatively poor mixing of the fuel and air in the combustion chamber. Diesel particulate problem has largely been solved in Europe with superior fuel injection systems and specially manufactured catalytic filters. In the US, improved fuel injection systems have helped remove odor and visible smoke. However, because of the high sulfur content of the US diesel fuel, catalytic filters have yet to be deployed.

Lead was a major source of particulate emission prior to the 1980s, during which time it was added to gasoline as an anti-knocking agent. Knock occurs as a result of premature ignition in engines with high compression ratios. A small amount of lead additive had the effect of increasing the octane number (and ignition temperature), which allowed building engines with higher compression ratios and better efficiencies. Lead, however, is a poisonous metal with detrimental health effects. It also contaminates catalytic converters that are designed to reduce exhaust emissions. Because of this, use of lead additives was gradually phased out and, except for very old cars, all cars operate with lead-free gasoline now.

In addition to carbon particles, many combustion systems such as industrial boilers and kilns produce sulfate. Because sulfur in gasoline and diesel fuels is removed before they are sold, few sulfate particles are formed in automobiles.


Catalytic Converters

Vehicular exhaust emissions not only contain carbon dioxide and water vapor, but also a significant amount of carbon monoxide, nitrogen oxides, and hydrocarbons. Generally speaking, nitric oxides are produced during cruising and acceleration (driving mode) where the mixture burns at its maximum temperature near stoichiometric conditions. On the contrary, during braking and when the engine idles (stopping mode), some exhaust enters the intake manifold, causing carbon monoxide, hydrocarbon, and particulate emissions to be high.

For the best performance - maximum efficiency and minimal environmental pollution - it is ideally desirable to convert all carbon into carbon dioxide, all hydrocarbons into water vapor, and all nitrogen into molecular nitrogen. To convert carbon monoxide and hydrocarbon we need a high temperature and an oxidizing atmosphere. The problem is that these same conditions provide a favorable environment for the oxidation of nitrogen to nitric oxides. Three-way catalytic converters can achieve successful modifications of CO and HC while at the same time reducing NOx to molecular nitrogen. This is achieved in two steps: - Convert all nitric oxides into nitrogen in a reducing atmosphere (fuel rich).

Figure 1 Conversion efficiency of the catalytic reactors as function of the fuel/air ratio. NOx removal efficiency is very low under lean conditions where efficiency is highest for CO and HC. In a narrow region near the stoichiometric ratio the conversion efficiency is high for all three contaminants.
Figure 2 CO+HC+NO Eq.
Figure 2 CO+HC+NO Eq.

NO->Rh N2 + O2 (1)

- Convert all hydrocarbons and carbon monoxide into carbon dioxide and water in an oxidizing atmosphere (air rich).

CO + HC ->Pt, PdCO2 + H2O (2)

Generally, catalytic converters are made of a stainless steel canister mounted along the exhaust pipe. Inside the container is either a porous ceramic honeycomb or is filled with ceramic pellets embedded with small particles of alumina with catalytic materials deposited on their surface. Rhodium is the catalyst of choice for reducing, and platinum and palladium work best for oxidizing reactors. Three-way converters employ redox (for reduction/oxidation) catalysts, which are essentially made of two reactors, a reducing reactor followed by an oxidizing one.

Figure 1 shows the conversion efficiencies of the catalytic converter for CO, HC, and NOx at different fuel/air ratios. As can be seen, for redox reactors to work, they must operate within a narrow range of air fuel ratios (around stoichiometric). When in good working conditions, these reactors reduce in excess of 98% of CO and 95% of HC and NOx emissions.

Emission Standards

Table 2. US Federal emission standards for gasoline and diesel passenger cars
Source: 40 CFR 86.000.8 Office of Air and Radiation, US EPA

Prior to 1970, there were no standards regulating automobile emissions. In 1970, the Environmental Protection Agency (EPA) was established and, as its first act, it regulated the emissions of carbon monoxides to less than half their previous level. Complying with this standard was easy; all that was needed was to adjust the carburetors in order to provide a slightly leaner mixture. The result, however, was that the concentration of nitric oxides in the atmosphere climbed to unreasonable levels, forcing the EPA to adopt new emission standards for the control of both nitric oxide and carbon monoxide. As the number of cars grew and air quality deteriorated, the EPA became more stringent. The Clean Air Act Amendments (CAAA) of 1990 defined two sets of standards, Tier-1 and Tier-2, for light-duty vehicles (vehicles under 6,000 pounds such as passenger cars, sport utility vehicles, minivans, and pickup trucks).13 Tier-1 regulations have already been implemented. Tier-2 standards began being phased-in in 2004 and are to be completed by 2007. Because emission standards are expressed in grams of pollutants per mile, light trucks, SUVs, and larger engines will have to utilize more advanced emission control technologies in order to meet the standards. Table 2 shows the minimum EPA standards for vehicles in the United States. California has devised its own standards which have always been more stringent than the Federal standards. Europeans and Japanese impose their own standards, which are generally comparable to the US standards. There is currently no particulate emission standard for passenger cars and light trucks. For big diesel trucks and buses, starting with model year 1994, the EPA has required a reduction in particulate emissions by 90%.


(1) US Environmental Protection Agency website (http://www.epa.gov).

(2) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005

Further Reading

Tillman, D., Fuels of Opportunity: Characteristics and Uses In Combustion Systems, Academic Press, 2004.

Sorensen, K., Hydrogen and Fuel Cells: Emerging Technologies and Applications, Academic Press, 2005.

Dhameia, S., Electric Vehicle Battery Systems, Academic Press, 2001.

Hussain, I., Electric and Hybrid Vehicles: Design Fundamentals, CRC Press, LLC. 2003.

Jefferson, C.M., and Barnard, R. H., Hybrid Vehicle Propulsion, WIT Press, 2002.

Spelberg, D. The Hydrogen Energy Transition: Moving Toward the Post Petroleum Age in Transportation, Academic Press, 2004.

Fuel, Direct Science Elsevier Publishing Company, Fuel focuses on primary research work in the science and technology of fuel and energy fuel science.

Transportation Research Part C: Emerging Technologies, Direct Science Elsevier Publishing Company; this journal focuses on scholarly research on development, application, and implications in the fields of transportation, control systems, and telecommunications, among others.

Fuel Cells Bulletin, Direct Science Elsevier Publishing Company, Fuel Cells Bulletin is the leading source of technical and business news for the fuel cells sector.

International Journal of Hydrogen Energy, Direct Science Elsevier Publishing Company, Quarterly journal covering various aspects of hydrogen energy, including production, storage, transmission, and utilization, as well as economical and environmental aspects.

External Links

US Department of Transportation (http://www.dot.gov).

US Department of Energy (http://www.doe.gov).

US Environmental Protection Agency (http://www.epa.gov).

National Energy Renewable Laboratory Hybrid Electric &Fuel Cell Vehicles (http://www.nrel.gov/vehiclesandfuels/hev).

FreedomCar (http://www.eere.energy.gov/vehiclesandfuels).