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Engine Emissions Systems

OSAC (orifice spark advance control) - classic engines only

The OSAC valve has been used since the early 1970s to delay the distibutor's vacuum advance for about 15 seconds during acceleration. The OSAC valve is on the air cleaner (pre-1974 models had it on the firewall!) . One hose runs from the vacuum of the distributor to the valve, the other from the valve to the carb. Running the hose from straight past the valve will eliminate its limitations, and compensate for a bad valve.

EGR (Exhaust Gas Recirculation)

This system was introduced in the early 1970s to all Chrysler Corporation cars, as well as most of those of other automakers. The EGR valve directs exhaust gas from the crossover passage into the intake manifold, and includes a temperature valve (CCEGR) on the radiator, sometimes with a delay timer and a vacuum amplifier. [Webmaster note: EGR is not normally on at idle. Early manuals on emissions control systems noted that EGR burned unused fuel, but this is not an issue in modern cars.] EGR reduces emissions and may increase gas mileage slightly; some vehicles shut off EGR during wide-open throttle in order to avoid having it interfere with power. EGR, when working, is a "good'un."

Bruce Clingerman added:

The purpose of EGR (Exhaust Gas Recirculation) is to reduce the NOx emissions. Air is mainly made of oxygen and nitrogen (O2 and N2). At temperatures above 1300°C (2372°F), these molecules split apart and rejoin with each other to make nitrogen oxides (like NO, NO2, etc...). The nitrogen oxides contribute to smog formation.

EGR puts a portion of the exhaust gas back into the intake manifold, so it mixes with the fuel and air. (Note that the exhaust adds to the fuel and air; it doesn’t replace any of it). The added mass in the cylinder is harder to heat up, so the combustion events have lower peak temperatures. The lower temperatures prevent the O2 and N2 from splitting and combining. Even though the exhaust is hot, about 600°C (or 1112°F), it's much cooler than the 1300°C required to make NOx.

In summary, the exhaust adds mass, increasing the heat capacitance of the mixture (i.e. making it harder to heat up the mixture in the cylinder). Peak temperatures are lower, reducing NOx formation, which ultimately reduces smog in the environment.

The reason EGR improves fuel economy is because it reduces the engine's pumping losses. For the cylinder to move down on the intake stroke, the piston is working against the intake manifold vacuum. Another way to say it is that the vacuum above the piston tries to prevent the piston from going down. EGR increases the mass in the intake; more mass means higher pressure, or less vacuum. Now the piston has less resistance during each intake stroke, which results in better gas mileage.

PCV (Positive Crankcase ventilation system)

This system draws air into the engine through the oil filler cap or the air cleaner and circulates it through the engine. The air combines with vapors in the crankcase and exits the engine through a metering valve mounted in the rocker arm cover. The air vapor mixture than re-enters the engine through the carb or intake manifold and passes the combustion chamber where it is burned. PCV is absolutely a good system and should never be removed or tampered with; but do check the PCV valve and hoses from time to time, and use a good quality (e.g. not Purolator) replacement valve when needed.

Other systems (contributed by the webmaster)

Catalytic converter

The catalytic converter is a fairly simple device, based on the fact that some chemical reactions will take place at lower temperatures when certain materials are around. In short, having exhaust gases pass through a honeycomb of iridium, platinum, and other metals will cause unburned hydrocarbons and carbon monoxide to burn, producing carbon dioxide and water. The catalytic converter is one of the most effective pollution prevention devices, and modern converters cause little power loss thanks to their high flow rates. The converter generally does not "go bad" though it can be rendered ineffective by a dirty exhaust, sudden backfiring, and other events which are foreign to well-maintained, healthy engines.

The main problem with catalytic converters is that they are destroyed by lead, which coats the metals and prevents them from working. (They can also get clogged, though this is fairly unusual). That is one reason why unleaded gas was introduced. The good news about unleaded gas is below.

Unleaded gas

Removing the lead from gasoline has two nasty side-effects: running unleaded gas can be unhealthy for older cars (before about 1973) without hardened valve seats, and it reduces the effective octane of the gas. Over time, new compositions have helped to raise octane, and other additives allow classic car owners to keep their engines healthy.

The problems of unleaded gas are completely obliterated by the incredible health benefits, namely:

  • The amount of lead in the air has been dramatically reduced, particularly in urban areas. Even small amounts of lead can cause mental retardation in children, and physical illness in children and adults.
  • Having unleaded gas allows us to use catalytic converters, which convert all sorts of nasties into carbon dioxide and water.

Fuel injection

One of the great gifts of the emissions era is the widespread use of fuel injection. Though it would have come about sooner or later, fuel injection greatly increases engine efficiency by injecting precise amounts of fuel in well-atomized form just as it is needed. In the most sophisticated systems, used on nearly every modern vehicle (sequential multiport injection), fuel does not wash up against closed valves and enter the combustion chamber as drops or a stream; it is a fine mist ready to explode. This cuts emissions (unburned hydrocarbons), reduces the amount of gas needed to make the same power (increases gas mileage), and makes it possible to build small, high-performance engines - like Chrysler's 2.2 liter Turbo III, which generated 224 horsepower with the help of a single turbocharger.

Oxygenated fuels

A nice idea in theory but not so good in practice, fuels with oxygen deliberately added to the mix (e.g. through MBTE) can have nasty side-effects if the additive is unhealthy (as MBTE seems to be), and may lower gas mileage. There are probably areas where oxygenated fuels are very beneficial, and other areas where they are not. See the gasoline FAQ on Usenet if you can find it.

Additional resources

Bruce Clingerman recommended:
