Emission control systems in automotive applications can trace their beginnings to the smoggy skies that were noted over the city of Los Angeles after the end of World War II. There were of course cars before the war and air pollution problems as a result of the activities of steelmaking, oil refining, and coal burning power plants were well known by then. Air Pollution in those days was for the most part written off as the price of progress, but as automobiles streamed onto the nation’s ever-expanding highway system in unprecedented numbers in the 1950s, the pollution problems caused by their use, along with pollution caused by industrial sources could no longer be dismissed as an annoyance.

Tailpipe emissions from automobiles fall into 5 major categories:

Unburned Hydrocarbons
Oxides of Nitrogen
Carbon Monoxide
Sulfur Dioxide
Compounds of Lead

In addition, automobiles also generate several secondary sources of pollution. These Include:

Evaporative Emissions from fuel vapors, which find their way into the air
Water pollution, from fluids that leak from cooling systems, engines, and transmissions.
Hazardous Waste from discarded fluids, tires, batteries, and the like.
Asbestos fibers from brake linings and clutches.

A bit about the nature of automotive air pollutants

Unburned Hydrocarbons are the result of incomplete combustion of fuel in an engine. Theoretically, if an engine burns its fuel perfectly, there should be little or no unburned hydrocarbons. Skipping all the chemistry equations, if there is one part gasoline vapor by volume to 15 parts air, the fuel should burn perfectly, producing only carbon dioxide and water vapor as byproducts. In practice, this is difficult to do with a carbureted engine operating over a wide variety of temperatures, fuel formulations, and load conditions. Blowby, which is combustion gasses escaping past piston rings (or Apex Seals for you Wankel buffs) is also a major source of unburned hydrocarbons.

Carbon Monoxide is formed when combustion takes place when there is a shortage of oxygen. Carbon Monoxide wants to be Carbon Dioxide, but can’t find another oxygen atom combine with. This makes it very reactive, and will bind with Hemoglobin, rendering it useless to transport oxygen in the blood. Small amounts of breathed CO cause headaches and fatigue, but Carbon Monoxide can kill if breathed in large enough amounts. In the Steel industry, Carbon Monoxide is used to pull the oxygen out of Iron Oxide to form pure Iron, but it is bad news in your bloodstream. CO can form in large amounts if too much fuel is allowed into the cylinders, or there are pockets of too rich a fuel-air mixture in the cylinders.

Oxides of Nitrogen form Smog when exposed to sunlight, and form when combustion temperatures exceed 2,300 F, forcing Oxygen and atmospheric Nitrogen (N2) to combine in a sort of chemical shotgun wedding. Like the human kind of shotgun weddings, these types of bonds are unstable and form reactive compounds under the right conditions. They form when preignition occurs, but also form under conditions of normal operation as well, particularly in high performance applications.

Sulfur Dioxide is a byproduct of burning sulfur, a common impurity in many fuels. Sulfur Dioxide can combine with water to form Sulfuric Acid, a very corrosive chemical which can corrode metals, etch paint, and render lakes too acid to support aquatic life. Control primarily consists of removing sulfur from fuel at the refinery.

A Timeline of Automotive Emission Controls

1966: California implements the first emissions standards in the nation.

Early emission controls consisted of a PCV Valve, which provided positive crankcase ventilation while routing blowby gasses back into the air intake to be reburned in the engine. This was a great help in reducing emissions from unburned hydrocarbons, but did little to reduce Nitrogen Oxide emissions or Carbon Monoxide emissions. California cars also got fixed high-speed carburetor jets, and saw other tweaks to engine tuning to reduce other emissions. California, then and now served as a laboratory for nationwide deployment for more advanced emission controls.

1968: Emission Controls were implemented Nationwide

Nationwide crankcase emission controls were implemented in new cars, and were similar to 1966 California cars. Light Trucks started to get emission controls a few years later. For a long time, emission standards for light trucks tended to lag those of automobiles, but the gap has narrowed over time. Air injection was introduced into California vehicles, in an effort to reduce CO and Unburned Hydrocarbons.

1969-1971: A gradual tightening of standards:

Compression ratios were dropped to reduce Nox emissions, and carburetion was leaned out to lower CO and HC emissions. This was the beginning of the end of the muscle car, at least in its original raw form. Air Injection and Exhaust Gas Recirculation made its way into more vehicles.

1971: The Clean Air Act was passed

The Clean Air Act mandated drastic reductions in emissions over the next decade. The Clean air Act also mandated the eventual removal of tetraethyl lead from automotive fuels. Detroit’s Big Three howled in protest, complaining that the new standards at best would be hugely expensive, or impossible to meet. In the automaker’s research and development laboratories, a number of approaches were developed, and were the forerunner of modern emission controls. Ford had a program called Programmed Combustion, Honda developed the Stratified Charge Engine, and technologies such as electronically controlled carburetors, electronic ignition, catalytic converters, and the beginnings of computerized engine management systems were developed over the next several years.

1972 to 1975: Emission Controls and the Energy Crisis take a bite out of Detroit

The early to mid ‘70s were the darkest years for Detroit. Work on new technologies was feverishly going on in the lab and on the test track. What found its way into the average car however, was lowered compression ratios to allow operation on 87 octane unleaded gas, lean tuning to reduce unburned hydrocarbons, air injection, and exhaust gas recirculation to reduce peak combustion temperatures. An energy crisis followed the arab oil embargo of 1973, forcing gas prices up and prompted implementation of the much hated 55 mph speed limit in the United States. American cars of this era for the most part ran terribly, hobbled by lean tuning, low compression ratios, and an increasingly crowded engine compartment featuring a rats nest of new plumbing and wiring. They ran even worse when these quickly designed systems became balky. Stalling and hesitation were common problems, and people forced to drive them often resorted to removing emission controls in an attempt to restore some driveability. Emission controls of the time had a negative effect on fuel economy, aggravating an already bad fuel supply situation.

1975 saw the widespread introduction of the Catalytic Converter, which allowed automakers to restore at least some driveability to their offerings, but performance was still a shadow of its late 1960s levels. An informal comparison of vehicles our family owned during the late ‘70s shows how much performance dropped. We owned a 1970 Chrysler Town and Country with a 383 cubic inch engine with a 2-barrel carburetor, which would run on regular gas. It got about 13 mpg around town, 17 on the highway, and had a top speed of about 110 miles per hour, even loaded to the roof. We also had a 1976 Ford LTD Wagon with a 400 cubic inch engine and a 2-barrel carburetor, and in most other respects they were very similar cars. The LTD got 10 mpg around town, 14 on the highway, and had a top speed of barely 100 mph. As performance dropped through the ‘70s, carmakers also limited the top indicated speed on the speedometer to 85 mph on most cars. Performance became a four letter word, and instead automakers chose to emphasise styling and accessories in their large cars, and fuel economy in their smaller models.

1976-1980: Downsizing and the introduction of electronics under the hood

The dark years for Detroit continued, though sales perked up for a while as gas prices stabilized from 1975 through 1978. Detroit engineers also faced the challenge of improving the fuel economy of their fleets, while also meeting stricter safety standards. General Motors trimmed nearly 1,000 pounds and about 100 cubic inches from their full size cars. Ford’s new LTD in 1979 looked suspiciously like their old Ford Fairmont, a much smaller vehicle. The new “premium” LTD Landau was still over a foot shorter, and had an engine 100 cubic inches smaller than the 1978 model. Catalytic Converters, combined with air pumps, exhaust gas recirculation, lean tuning, and electronic ignition on most vehicles allowed most cars to meet increasingly strict emissions standards, but many of the larger engines could not meet the newer standards. The second energy crisis in 1979, combined with emissions problems with many larger V8 engines made engines over 400 cubic inches nearly extinct by the end of the decade, except in the Cadillac Sedan de Ville and 3/4 ton and larger pickups.

Technology by the end of the 1970s for most vehicles made in North America consisted of Catalytic Converters, combined with air pumps, exhaust gas recirculation, lean tuning, and electronic ignition on most vehicles. The end of the decade also saw the beginning of electronic engine management systems. My 1978 Plymouth Horizon had a spark control computer to control ignition timing and my brother's 1979 Plymouth Horizon TC3 had an electronically controlled carburetor in addition to the ignition timing.

Overseas, emissions standards were getting stiffer as well, particularly in Western Europe and Japan. Honda, Mercedes-Benz, and Volkswagen were able to meet US and even California emissions standards without needing a catalytic converter.

Diesel Engines and other technologies

Mercedes-Benz and Volkswagen were able to meet the standards for emissions by using diesel engines in their cars. Diesel Engines tend to produce pretty low emissions without extra equipment, though they do produce a fair amount of soot which was not a pollutant of concern at the time. Diesel Engines also allowed for greater fuel efficiency, a Volkswagen Rabbit Diesel was able to get nearly 50 miles a gallon, at the time. Mercedes Benz had built diesel vehicles since the ‘30s, and by the late ‘70s, they had developed an almost legendary reputation for design, durability, and fuel economy as well. A late ‘70s 240D got 25 miles per gallon, not bad considering that a similarly sized American car of the time got about 17 mpg.

General Motors got into the diesel act in 1978 with a diesel engine option in full-sized Oldsmobiles, Buicks, and Cadillacs. A 4 cylinder diesel Chevette was also sold for a few years as well, which got nearly 50 mpg. In the fuel-starved days of 1980, they were a popular option, despite their slower acceleration, noise, and $1,000 premium compared to the gasoline powered model. What the buyer got in return was a full-sized car that got nearly 30 miles per gallon on the highway. What the buyer didn't get was an engine designed from the ground up as a diesel, but a converted gasoline engine based on the proven 350 cubic inch small-block V8. Diesel Engines operate at very high compression ratios, up to 22 to 1, and the extreme stresses placed on the engine internals caused many of these engines to self-destruct before they even had 50,000 miles on them. If the engines had managed to get a diet of high-quality fuel, the stiffened internals of the GM Diesels compared to their gasoline counterparts would have been adequate to hold up. The root of the problem turned out to be the fact that the injector pumps corroded from exposure to moisture and acids present in the poor quality of diesel fuel commonly available at the time, combined with an inadequate filtration system to remove these impurities. A damaged injector pump would cause improper timing and amount of fuel to be injected, causing the cylinder pressures to go sky-high eventually blowing the head gasket. Once the head gasket blew, it usually didn't take long for the rest of the engine to self-destruct. General Motors had to replace many of these engines under warranty, and within 5 years the diesel engine option was dropped. Regrettably this gave not only GM a black eye, but gave diesel engines as viable automobile powerplants a black eye as well, at least in the eyes of most Americans. Diesels gained wider acceptance as alternatives to big-block V8 gasoline engines in medium duty trucks, delivery vehicles, and school busses, but have not seen a rebirth in domestic cars, though they outnumber gasoline cars in Western Europe today.

Honda was able to meet 1975 standards by use of a novel gasoline engine called a Stratified Charge Engine, which Honda dubbed the CVCC. The engine featured a cylinder head with 3 valves per cylinder, and a special carburetor. The carburetor featured a main section which provided a lean fuel-air mixture for most of the volume in the cylinder, and a section which provided a richer mixture in the area near the spark plug. The enriched layer of mixture ensured reliable ignition, while the main charge was lean enough to suppress formation of unburned hydrocarbons, oxides of Nitrogen and Carbon Monoxide. The carefully designed combustion chamber promoted swirling combustion, which ensured complete burning of the fuel-air mix. Honda was able to avoid putting Catalytic Converters on their vehicles until well into the 1980s.

The 1980s: The beginnings of Modern Automobile Design

The average American car underwent a fundamental change in design as the 1980s dawned. The old front-engine rear-drive platform was scrapped on all but large cars, trucks, some specialty vehicles for Front Wheel Drive. Front Wheel Drive vehicles as mass-produced vehicles date back to 1936, to the days of the Chrysler Airflow. In the 1970s, the Datsun F-10, VW Rabbit, and the Honda Civic and Accord gained wide acceptance in the 1970s as economical and practical subcompact vehicles, and pointed the way to the future. Front wheel drive had the advantages of a lighter, more compact drivetrain, and allowed more passenger room inside due to the lack of a driveshaft tunnel.

1978 saw the introduction of the Plymouth Horizon, but 1980 was really the watershed year for front wheel drive vehicles. Ford, GM, and Chrysler all introduced new lines of front wheel drive vehicles covering major parts of their market segments. Nissan and Toyota also replaced most of their rear-wheel drive vehicles with front wheel drive replacements, and as the '80s wore on, front wheel drive found its way into larger sedans and an entirely new category of vehicle, the minivan.

Under the hood, things were changing quickly as well. Although fuel-efficiency and emissions standards continued to tighten through the ‘80s, the rate at which standards tightened slowed. In addition to a major reconfiguration of the drivetrain, many familiar underhood parts were being transformed as well. Carburetors were a mainstay of gasoline engines since before the turn of the 20th century, but by the early 1980s, it was clear that their days were numbered, as they became burdened with extra layers of complexity, and practical electronic fuel injection systems were developed.

General Motors introduced Throttle Body Injection on their X-Body and J-Body cars in the early ‘80s as a replacement for carburetors. Mechanical fuel injection systems had been available since the late '50s, mostly on expensive exotics and high performance cars. In the 1970s, the Datsun 240Z and some Volkswagen models had fuel injection, but the complexity and cost ruled it out for all but premium and high performance vehicles. The GM system was electronically controlled with the injectors controlled by the Electronic Control Unit or ECU. Using the throttle body as the point for injecting fuel, the system was mechanically simpler than the carburetors of the day. Although the early systems had their problems, such as hardware failures which occasionally left the motorist stranded, or not quite perfect programming which caused some driveability glitches, even the early systems were a big improvement over the old carburetors in terms of driveability. The new systems also helped engines to last longer, since raw gas wasn’t just dumped into the engine on a cold start.

Modern Engine Management Systems

Despite early problems, within 7 or 8 years EFI almost completely displaced carburetors from under the hood of most gasoline powered vehicles and became the heart of the modern engine management system. As the systems became more sophisticated, drivability, performance, and fuel economy were no longer seen as mutually exclusive goals as they seemed during the ‘70s. Since the late 1980s, improvements have consisted mostly of refinements to the basic engine management systems introduced in the early to mid '80s. Improvements in sensors, catalysts, and computerized engine management have resulted in a steady improvement to emissions, performance, and reliability of modern automobiles. Port injection, variable valve timing, and other features once seen only on racing engines, have found their way into basic transportation vehicles such as econoboxes and mini-pickups. Some of the newer vehicles sport performance statistics not seen since the late '60s, while keeping emissions low. Here are the main components of a modern Electronic Engine Management System.

Engine Control Unit: Also know as “the computer” the ECU takes inputs from various sensors to control ignition timing, transmission gear changes, fuel delivery, and even valve timing. The ECU uses software routines and algorithms to provide the correct amount of fuel and a properly timed spark to the engine based on the sensor inputs. The ECU also has on board diagnostic routines to detect failed sensors, and to compensate for their loss as much as possible. Currently, most newer cars use a system called OBD-II, but many manufacturers have extensions to the standard codes.

Oxygen Sensor: Detects the presence of free oxygen in the exhaust gasses. Oxygen Sensors need to be heated to about 800F to operate properly, so most modern oxygen sensors are preheated, rather than relying on hot exhaust gasses to get them to proper operating temperature. Most vehicles have an oxygen sensor somewhere on the exhaust manifold, and many have a second sensor at the exit of the catalytic converter. Information from the oxygen sensor is used to adjust the amount of fuel reaching the engine. Oxygen Sensors do eventually wear out, and usually need replacing after about 100,000 miles.

Mass Airflow sensor Mass Airflow sensors detect the amount of air entering the throttle body, and is basically a strain gauge mounted in the incoming airstream.

MAP Sensor: The MAP sensor stands for Manifold Absolute Pressure. Compares the air pressure inside the intake manifold to an absolute standard, rather than the difference between ambient air pressure and the pressure inside the manifold. This compensates for weather and altitude.

Throttle Position Sensor: The Throttle Position Sensor senses how far the throttle has been opened. This may eventually be replaced by a “drive by wire” system where the accelerator pedal controls a sensor, and the computer sends signals which open the throttle.

Temperature Sensors; Temperature sensors sense the temperature of incoming air, exhaust, coolant, and oil in some cases. Used to help fine-tune the fuel mixture, and also to monitor engine health.

Knock Sensor: An acoustical sensor that senses when fuel is is exploding in the cylinder or detonation, rather than burning smoothly. Detonation causes increases in Nitrogen Oxide emissions, and can also damage the engine. Allows ignition timing and fuel mixture to be adjusted for best performance and emissions, without risking damaging detonation. If detonation is detected, the ignition timing is retarded and the fuel mixture is adjusted to eliminate the knock


Speed Sensors: Engine RPM and vehicle speed is monitored

Other Sensors: Sensors which detect engine misfire, transmission gear position, oil pressure, fuel pressure, and other functions to verify proper operation, monitor engine health, and to provide feedback to controlling systems.

A glossary of Emissions Related Components:

Here are a few of the other components of emission control systems, in rough order in which they were developed:

PCV Valve: A simple check valve which allows excess crankcase pressure to vent. Vented gasses rich in unburned hydrocarbons are fed back into the engine, where they are reburned, rather than vented directly into the atmosphere.

Air Preheater: Introduced in the late ‘60s, it is used to help improve combustion in a cold engine by pulling intake air from around the exhaust manifold while the engine is cold. Still used today, but been partially supplanted by more sophisticated engine management systems.

Exhaust Gas Recirculation: EGR is used to dilute incoming air of oxygen by introducing a controlled amount of exhaust gasses. Used to lower peak combustion temperatures, to reduce Nitrous Oxide emissions. Also robs the engine of power, its role is reduced or eliminated by more sophisticated engine controls. Introduced in the early ‘70s

Air Injection: Also known as the notorious “smog pump”, it uses an engine driven air compressor to inject air into the exhaust manifold to reduce exhaust temperatures, and provide additional oxygen to help burn off excess CO and HC gasses. Air pumps became more effective when catalytic converters took advantage of the extra air to burn off excess CO, HC, and NOx gasses more efficiently than just using air injection alone. First seen in California in the late ‘60s, it became a noisy and bulky addition under the hoods of most vehicles by the early '70s.

Catalytic Converter: A container holding a ceramic honeycomb or other matrix with a very thin plating of a catalyst, such as platinum or palladium. A secondary cataylst of Rhodium is used to break up nitrogen oxides back into nitrogen and oxygen. Exhaust gasses containing unburned hydrocarbons or Carbon Monoxide are converted to water vapor and Carbon Dioxide, which are much more benign than HC and CO. It is easily poisoned by burning leaded gas, and can also be ruined if it is overloaded with unburned hydrocarbons due to engine misfire, which can cause it to overheat. Became standard in most cars around 1975.

Evaporative Emission Controls: When a 15 gallon fuel tank is filled with gasoline, the fuel will displace about 2 cubic feet of vapor-laden air. Gasoline pumps in areas with high air pollution have a system that draws off this vapor when it is filled, and the vapors are trapped in activated charcoal. Under the hood, a similar system is used to catch vapors, which vent from the tank under pressure on a hot day. The vapors run through a charcoal canister before being vented. The canister is purged when the engine is started by routing clean air through the canister and into the engine, where the vapors are burned. Developed in the early ‘70s.

The Future:

For the most directly malevolent of air pollutants, modern automobiles have pretty much reached the practical limit for what can be done with a gasoline engine, and are 99 percent cleaner than cars of the 1950s. Offsetting this somewhat is the fact that people drive a lot more than they used to, and people in the second world and third world are driving in increasing numbers as well. No matter how well emission controls work, burning of petroleum products in automotive powerplants, or of oil or coal to generate electricity for electric vehicles generates large amounts of Carbon Dioxide. While CO2 is not directly harmful to most life at even several times current concentrations (and arguably beneficial to some types of plant life), it has the potential to cause global warming, and cause major and unpredictable shifts in weather patterns, sea levels, and climate. How much disruption by our use of fossil fuels to date is unknown, but as we tap deeper into our reserves of fossil fuels, the risk of climatic disruption increases. Environmental damage due to the exploitation of fossil fuel resources is also inevitable as long as we choose to exploit them for energy.

There is no easy answer to these challenges, and to the eventuality that we will exhaust the limited supplies of fossil fuels within a few hundred years, even if we solve the other problems. In the short run, we can make our use of fuels more efficient. Hybrid Vehicles are a small step forward by recapturing lost energy normally wasted in braking. Fuel Cells also offer promise, but are not practical yet. Electric Cars are not much good without batteries that can provide a reasonable range, good durability, safety, and ability to be recycled. Liquid fuel may end up being the fuel of the future given the known limitations of alternate energy storage mediums, such as batteries, hydrogen, and other alternative fuels. Gasoline, Fuel Oil, and Alcohol can store energy more efficiently and safely than any known battery or hydrogen storage medium.

Humanity’s long-term salvation will probably boil down to nuclear and possibly solar energy in the medium term, though nuclear power is somewhat taboo today. Nuclear fuels properly utilized can supply our energy needs for thousands of years, which will buy humanity time to develop truly renewable energy resources, or to perfect nuclear fusion as a practical way of generating electricity. However it is generated, electricity can be transformed into whatever form we need to store energy in. It can even be used to decompose CO2 and water vapor and build hydrocarbon molecules to run our cars in the future.

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