May-June 2008

Internal Combustion Engines and Their Emission Controls

The first of two articles on the science and mechanics of reducing air pollutants

Prior to our 20th and 21st century concerns over automobile emissions, air pollution, and global warming, personal transportation presented urban dwellers of the 19th century with a much more immediate and serious health threat. “While the nineteenth century American city faced many forms of environmental pollution, none was as all encompassing as that produced by the horse. The most severe problem was that caused by horses defecating and urinating in the streets. ... The normal city horse produced between fifteen and thirty-five pounds of manure a day, and about a quart of urine, usually distributed along the course of its route or deposited in the stable. ... In 1866, the Citizen’s Association Report on the Sanitary Condition of the City observed that, “The stench arising from these accumulations of filth is intolerable” (“The Centrality of the Horse to the Nineteenth-Century American City,” by Joel Tarr and Clay McShane).

The health and aesthetic problems created by horse-drawn carriages, inflicted cities of all sizes. At the turn of the (last) century, Milwaukee, WI, had a human population of 350,000, and a horse population of 12,500, that produced 13 tons of manure daily. The 15,000 horses of Rochester, NY, produced enough manure each year to create a pile 175-feet high, covering an entire acre. Aside from the odor, manure was a breeding ground for illnesses and the disease vectors that carried them. Now, imagine what it was like in major cities such as New York, NY; Chicago, IL; or London, England. In fact, The Times, of London, predicted, in 1894, that if their present manure creation trends continued, every street in London would be buried in 9 feet of manure by 1950.

Fortunately for London and every other city in the world, the automobile replaced the horse as the primary means of personal transportation. When the automobile arrived, it was hailed by sanitation and health officials as an environmental savior, and with good reason. Yet if the automobile has taught us anything, it is that each environmental solution usually creates new problems of its own. So, today we seek to control and further reduce the air pollutants generated by the internal combustion engine and vented out in tailpipe emissions. Great strides have been made since the passing of the Clean Air Act and the installation of the first catalytic converters. More needs to be done, and more can be done with emerging emission control technologies and improvements in
engine-operating efficiencies.

Internal Combustion Engine Exhaust
An “internal combustion engine” is defined as, “an engine, such as an automotive gasoline piston engine or a diesel, in which fuel is burned within the engine proper, rather than in an external furnace.” Since its development in the 19th century and application to the invention of the automobile, it has become the primary energy source for personal transport. One of the reasons for this dominance has been the availability of cheap, easily extracted oil, that can be readily refined into gasoline and diesel fuels. These fuels are mixtures of various hydrocarbons—molecules consisting of hydrogen and carbon atoms. Theoretically, a 100% efficient combustion of these fuels would yield pure water and carbon dioxide, as the oxygen content of our oxygen-nitrogen atmosphere combines with the carbon and hydrogen atoms in the fuel, leaving the nitrogen content of the air unaffected. This ideal combustion process can be represented by the following formula:
Air (O2 and N2) + Hydrocarbon Fuel (C and H atoms) = Carbon Dioxide (CO2) + Water (H2O) + Unaffected Nitrogen (N2)

However, the laws of thermodynamics ensure that a 100%-efficient process never occurs. As a result of the theses, inherent inefficiencies, pollutants, and other byproducts are created by internal combustion. A more realistic example of fuel combustion is given by the formula below:
Air (O2 and N2) + Hydrocarbon Fuel (C and H atoms) = Carbon Dioxide (CO2) + Water (H2O) + Carbon Monoxide (CO) + Trace Pollutants + Particles + Unburned Hydrocarbon Fuel (C and H atoms) +
Nitrogen Oxides (NOx)

Automobile emission controls are designed to minimize the amount of pollutants generated during combustion, and emitted in the engine exhaust. By doing so, the adverse health effects and environmental impacts created by these pollutants are greatly reduced.

Pollutants of Concern
Pollutants emitted by automobiles are classified as major components and secondary pollutants whose toxicity is out of proportion to the actual amount of the contaminants produced. Major components include unburned hydrocarbons, nitrogen oxides, carbon monoxide, and carbon dioxide.

Hydrocarbons are the unburned or only partially burned portions of the engine’s gasoline or diesel fuel. By themselves, they are not strictly a pollutant. The problem occurs when they combine with atmospheric nitrogen to form ground-level ozone. This isn’t the beneficial kind of ozone that forms the protective zone layer, but forms a significant portion of smog. Ozone also acts as an irritant to sensitive tissue, such as eyes and lungs. Nitrogen oxides are also a also a constituent of ozone. In addition to forming ozone, certain types of nitrogen oxides help produce acd rain. Ozone interferes with lung function and can cause pain and discomfort (coughing, wheezing, chest pain, and congestion). Carbon monoxide occurs when the carbon in the hydrocarbon fuel is only partially combusted. In relatively low concentrations, carbon monoxide can reduce the blood’s ability to transport oxygen, since it binds with hemoglobin in red blood cells more efficiently than ordinary oxygen. In higher concentrations, it can pose an immediate health hazard to people with heart conditions, since carbon monoxide poisoning is similar to suffocation in its effects. Carbon dioxide has not been traditionally viewed as a serious pollutant, since it does not directly harm human health. However, since concerns over “global warming” have entered the political discourse, carbon dioxide has come to be seen as a pollutant worthy of serious regulation.

Secondary toxic pollutants include sulfur dioxide, benzene, formaldehyde, polycyclic hydrocarbons, lead, and particulates. None of these are present in high quantities in typical automobile exhaust, but can linger in the environment and lead to long-term health impairment and environmental damage. Sulfur dioxide, in addition to having a foul “rotten egg” odor, irritates the lung and throat, and enhances the harmful effects of ozone. It can cause bronchitis, asthmatic attacks, and respiratory infections. Benzene causes depression of the central nervous system, marked by drowsiness, dizziness, headache, nausea, and (in sufficiently high concentrations) loss of coordination, confusion, and unconsciousness. Benzene is also a known carcinogen and long-term exposure can cause leukemia. Formaldehyde can cause acute health effects including watery eyes; burning sensations in the eyes, nose, and throat; nausea; coughing; chest tightness; wheezing; skin rashes; and other irritating effects. Polycyclic hydrocarbons exposure can cause immunosuppression. Contact exposure is also damaging to the skin. Lead has demonstrative negative effects on the brain and central nervous system after long-term exposure, eventually causing behavioral changes. Young children are especially vulnerable to lead’s effects. Lead can also interfere with the formation of red blood cells, leading to anemia. Particulates are small particles of various substances that are light enough to be suspended in air. When inhaled, they can lodge themselves in the membranes of the respiratory system, leading to emphysema, bronchitis, and asthma.

So, how serious are these health effects? These are toxic substances that are inhaled, enter the bloodstream, and get transported to all parts of the body. From cancer to brain damage, long-term exposure—even to relatively low concentrations—can result in serious health damage. Immediate damage to the respiratory system can occur from even short-term exposure. Whether it is mild shortness of breath or heart attacks, exposure to automobile exhausts can severely degrade quality of life and life expectancy.

Indirect Approaches
Reducing or eliminating automobile emissions can be achieved by either indirect approaches or direct applications of cleanup technologies. Indirect approaches include improvement in both engine and vehicle efficiencies. Efficiency by definition means “doing more with less.” When it comes to improving vehicle efficiency, this means doing more with less “car.” In this case, that means less weight and less air resistance. Utilizing lightweight composite materials as much as possible, instead of heavier metals in the body and framework of the car, can significantly reduce weight. This reduction in weight means less work is required to move an automobile a given distance. The less work required means that less fuel energy is needed to move the car, resulting in greater fuel efficiencies, as measured by the vehicle’s mile-per-gallon rating. When a vehicle requires less fuel per mile, it necessarily emits less exhaust per distance. This indirect approach can result in significant reductions in pollutant results. Too often, we focus on interesting new technical gadgets that directly attack a problem (and grab the headlines); when, in fact, simple, incremental increases in operating efficiency over time can be the most cost-effective solution.

In addition to the obvious vehicle improvements, such as reduced weight and a smoother aerodynamic shape to reduce wind resistance, greater vehicle efficiencies can be achieved by reducing rolling resistance, improving power train efficiency, and regenerative braking (hybrid vehicle that recover the energy required to brake the car in the form of stored electrical charge). In addition to reduced weight, reducing the air drag and rolling resistance, that the vehicle struggles against, reduces the need for fuel per distance traveled. And, it is not only the vehicle design itself that affects vehicle efficiency, driving techniques, and operations (reducing traffic obstructions, operating at optimum speeds, reducing cold starts, and even the now familiar “right-hand turn on red”). Too often, we focus on interesting and new, technical gadgets that directly attack a problem (and grab the headlines), when, in fact, simple, incremental increases in vehicle efficiency over time can be the most cost-effective solution.

A more technical approach involves improvements to engine efficiency. The three primary technologies, that have been developed for increasing engine-operating efficiency, are electronic ignition, fuel injection systems, and electronic control units. The use of electronic ignition involves replacing the mechanically timed ignition of the distributor, and its associated spark plugs and ignition coil (whose contact points are subject to wear and tear, and oxidation over time) with optical and rotating magnet angular sensors (which trigger the operation of a thyristor, which switches current flow in the ignition coil). Further refinements came with digital electronic ignitions, which utilize digital electronic ignition modules that store charged energy for the spark in a capacitor within the module.

Fuel injection systems replace the carburetor with an injection pump, that atomizes the fuel by using high pressure to force it through a nozzle with a small discharge opening. Though their design can be customized for specific types of fuel, most fuel-injection systems are designed for gasoline or diesel. Operating at the output end of the power train is an electronic control unit, that is an embedded system regulating the transmission, engine, or power train. Prior to the advent of these control units, engine-operating parameters (amount of fuel injected into the piston cylinder, the timing of the ignition firing and valve synchronization, the pressure applied by the turbocharger, etc.) were fixed. These controls allow for variable-operating characteristics, depending on driving conditions. Additional engine efficiencies are, thereby, achieved by the tailoring of engine operations to specific circumstances.

Direct Technologies, Development and Current Applications
As useful and cost effective as these indirect approaches to reducing automobile emissions can be, they aren’t always sufficient to meet stringent emission control standards mandated by law. What are required are technologies that directly deal with the emissions after ignition and before they exit the vehicle’s tailpipe. Introduced in the 1960s, one of the earliest air emission controls was the “air injection reactor,” which essentially caused a secondary ignition to occur within the tailpipe itself. By injecting fresh air into the exhaust pipe manifold while the exhaust is still at a very high temperature, a secondary ignition occurs, that reduces the bulk of the unburned hydrocarbons which then consumes many other pollutants in the exhaust stream. Cleaner, more efficient engines, and more advanced emission controls, have made this approach obsolete. Further attempts to remove hydrocarbon emissions involved the installation of charcoal canisters to collect hydrocarbon vapors.

The next decade saw the introduction of “exhaust gas recirculation” designed to reduce nitrogen oxide emissions. This system utilizes a gas recirculation valve connecting the exhaust pipe and intake manifolds. By introducing the exhaust back into the carburetor’s air and fuel mixture, the effective peak combustion temperature can be reduced. Since nitrogen oxides are formed when the combustion temperature gets too hot (2,500°F or hotter), lowering the ignition temperature effectively minimizes their formation. Recent developments (some as simple as adjusting the valve timing, so that some of the exhaust is held in the combustion chamber during the subsequent burn cycle) have superseded this approach.

Another direct emission control, introduced at about the same time, but still widely used, is the catalytic converter. In chemical terms, a “catalyst” is a “substance that causes or accelerates a chemical reaction without itself being affected.” There are two types of catalysts—an oxidation catalyst and a reduction catalyst. An automobile catalytic converter relies on a combination of catalysts (palladium, platinum, and rhodium). These catalysts are use to coat a ceramic structure that allows exhaust emissions to pass through, while providing the largest possible surface area for contact with the exhaust gases.

Originally, the ceramic structure consisted of metal coated beads, but most catalytic converters today are designed with a honeycomb structure. Most converters are three-way converters (removing three
primary pollutants: carbon dioxide, unburned hydrocarbons, and carbon monoxide) operating in two stages. The first stage consists of rhodium and platinum, which act as reduction catalysts to reduce carbon dioxide emissions. The second stage consists of an oxidizing catalyst, that eliminates carbon monoxide and unburned hydrocarbon fuel by passing them over a heated catalyst, consisting of platinum and palladium. By the early 1980s catalytic converters came as standard equipment on all new-model American cars.

However, there was one problem with the early development of the catalytic converters: They were adversely affected by tetraethyl lead. This additive was a standard octane booster put into gasoline sold during the 1970s. Since catalytic converters could not work in the presence of lead emissions, lead additives were phased out, so that the use of catalytic converter equipped cars increase. This had an additional side benefit—the removal of lead contaminants from automobile emissions. Lead contaminants have serious health and mental impact on humans who inhale them. The resultant dramatic reductions in ambient lead levels and alleviated many serious environmental and human health concerns associated with lead pollution. By removing lead at the source, a successful elimination of a serious automotive pollutant was achieved without the addition of any new control technology.

Speculative Projections
One radical approach to the reduction of automobile emission is to do away with the internal combustion engine itself, either completely or partially. There has been much talk about recent technological breakthroughs in fuel cell and electric battery-driven cars. The showpiece of the much heralded, “hydrogen economy” fuel cells are electro-mechanical devices that store hydrogen as its fuel. A fuel cell is basically a sandwich consisting of a plastic membrane between two carbon plates, which are, in turn, set between two endplates that act like electrodes in a standard battery. Electricity is generated as the stored hydrogen, and atmospheric oxygen mix to form water, with protons flowing through the membrane and electrons flowing from the anode to the cathode plate. Though it is currently difficult to extract hydrogen (the process can require more fossil fuel energy to extract the hydrogen, that then is later produced by the fuel cell itself), fuel cells can operate at efficiencies two to three times greater than an internal combustion engine. A fuel cell is a radical departure from other engines, in that it requires no moving parts and produces virtually no exhaust. Its one “waste” product is water. With wide acceptance of fuel cell vehicles, the issue of automobile exhaust becomes moot.

Battery-driven cars have similar operating characteristics, requiring no moving parts and emitting no exhaust. They can be used in various configurations, ranging from pure battery power, and hybrid electrical vehicles (HEV)—that store the energy needed to brake a vehicle as electrical charge in an auxiliary battery—to “plug-in” hybrids, that can also draw electrical charge off the power grid. The most common version on the market is the HEV, whose main power source remains the standard internal combustion engine. The vehicle’s stored electricity provides the energy needed, in order to drive an auxiliary motor to drive the wheels. The source of this additional electrical power is the regenerative brakes, whose energy is normally wasted as excess heat. When baking at slow speeds, the electric motor operates in a reverse direction, creating a counter torque that brings the vehicle to a stop. This creates a counter-torque that stops the vehicle. However, whenever an electric motor reverses its rotation, it becomes a generator. It is this electricity that is stored in the battery or capacity for later use to power the vehicle.

Short of radically changing the actual engine, the fuel used to power the engine can be replaced or modified. Already on the market is E85 fuel, a mixture of 85% ethanol and 15% gasoline. Ethanol is a bio-fuel that can be produced from a wide variety of biochemical feedstocks (corn, sugar beets, sugar cane, cellulose, switch grass, municipal solid waste, and algae with a high lipid levels), which either already contain large amounts of sugars or starches that can be converted to sugars prior to distillation. Compared to gasoline, E85 has a higher octane but lower energy content per gallon. Ethanol burns more completely than gasoline, because its molecules contain oxygen. Ethanol reduces nitrogen oxide, carbon monoxide, and carbon dioxide emissions, which are 100% lower than fossil-fueled engines. The only products of ethanol combustion are carbon dioxide, water, and heat.

A century ago, the automobile replaced the horse as the primary means of personal transportation. Its widespread use eliminated the very real health threat and aesthetic damage caused by horse droppings in urban areas. The switch to the automobile resulted in a significant improvement in quality of life for millions of city dwellers, though that was never the intention of the inventors and manufacturers of the automobile. The health and sanitation improvements were unforeseen side benefits. Today, we a desperately searching for a replacement to the internal combustion engine, or at least the fuels that drive it, in response to political instability and the ever increasing costs of extracting oil from “peaking” oil fields.

The cleaner air will be a side benefit.