Exhaust and Emissions - Get an online auto repair estimate

RepairPal gives you independent and unbiased repair estimates, user
ratings and reviews, plus advice you can't get anywhere else.

It's easy, accurate, and FREE!

Find out what your auto repair should cost

- Select Make - Acura Audi BMW Buick Cadillac Chevrolet Chrysler Dodge Ford Geo GMC Honda Hummer Hyundai Infiniti Isuzu Jaguar Jeep Kia Land Rover Lexus Lincoln Mazda Mercedes-Benz Mercury Mini Mitsubishi Nissan Oldsmobile Plymouth Pontiac Porsche Saab Saturn Scion Subaru Suzuki Toyota Volkswagen Volvo

	Find the right shop The best shops near you - with ratings and reviews Find a Shop (City and State or Zip)
	Get inside info about your car Common problems and Q&A Recalls, ratings & reviews
	Track your car's service Keep everything online in one place. Get service reminders

Exhaust and Emissions

Share Email Print

Exhaust systems reduce the amount of noise produced by the engine and channel the high-temperature, poisonous exhaust gases from the engine by safely routing them under the vehicle. Then, through a system of manifolds, pipes, and sound-absorbing mufflers, the gases are released out of the back of the vehicle.   

Emission control systems reduce the amount of harmful gases and particles released by vehicle operation. These systems have evolved into complex computer-controlled systems that balance ever-tightening vehicle pollution control regulation by the U.S. government with the constant demand for more power and economy from vehicle owners.  

Related Services & Repairs

Catalytic Converter
EGR Valve
Exhaust Manifold
Exhaust Pipe
Muffler
Oxygen Sensor
PCV Valve
TailpipeWhen an engine operates, it burns fuel. Because no engine can burn 100 percent of the fuel, harmful gas and particulate byproducts (emissions) are released.

The Clean Air Act Extension in 1970 mandated that the Environmental Protection Agency (EPA) "develop and enforce regulations to protect the public against airborne contaminants that are known to be hazardous to human health." The EPA found that motor vehicles produced three main types of harmful gases and particulates:

Hydrocarbons (HCs)
Raw, unburnt fuel particles from the combustion process that are suspended in the exhaust gases or released in a raw state from the fuel storage and crankcase systems.

Carbon monoxide (CO)
A fuel and oxygen gas formed from incomplete combustion.

Nitrogen oxide (NOx)
A nitrogen and oxygen gas compound that is formed when combustion temperatures go above 2500° F.

In conjunction with the EPA, vehicle manufacturers from around the world developed vehicle emission control systems and devices to control (and in some cases eliminate) the production of these harmful emissions. Vehicle emission control systems use a two-tiered approach in the management of these harmful substances.

Upstream Emissions Control
Manages the pre-combustion and combustion process and recycles leaked fuel storage vapors and crankcase vapors into the combustion flow.

Downstream Emissions Control
Monitors the results of the combustion process by constantly testing the oxygen content in the exhaust system before and after the catalytic converter. This information is fed to the engine management computer, which adjusts the combustion process, thereby completing a "closed loop."

The catalytic converter provides a means to "after burn" the three remaining harmful substances (HCs, CO, NOx) and converts them back into the non-poisonous substances of carbon dioxide (CO2), oxygen (02), and water (H20). Some vehicles deploy the use of a secondary air injection system to provide the extra oxygen needed at critical times to complete the combustion process. This secondary air is injected into the exhaust manifold(s) and/or catalytic converter(s) by a computer-controlled electric pump or pulse valve(s) and is distributed by specialized plumbing and check valves.

Common Upstream Emission Control Systems

• Positive Crankcase Ventilation (PCV) System
• Evaporative Control (EVAP) System
• Exhaust Gas Recirculation System
• Fuel Management System
• Spark Control System
• Variable Valve Timing System
• The cooling system and thermostat have recently been included in the upstream emission controls. The cooling system provides accelerated engine warmup by restricting the amount of coolant flow. The cooling system also provides an ever-narrowing window of allowable operating temperature by utilizing more advanced thermostat design and computer-controlled, speed-varied cooling fan(s) operation. This allows the engine to reach and maintain optimal air-to-fuel burning temperature as quickly as possible, thereby minimizing the "dirty" warmup emissions output and maximizing overall fuel economy. A cold (or cool) engine needs a wasteful "rich" air-to-fuel mixture in order to operate and compared to a properly warmed-up engine, its combustion only burns a fraction of this "rich" mixture. An engine that runs too hot will produce nitrogen oxide (NOx) and will misfire, which produces hydrocarbons (HCs) and NOx. The cooling system controls maintain the ideal engine temperature range of 195 to 205° F.
• The comprehensive component monitor is a software program inside the engine management computer that continually monitors the performance of the upstream emission control sensors and actuators. It constantly compares the sensor readings to each other in order to verify that the readings are within "rational" tolerances (e.g. is the intake air temperature sensor within a "rational" range of the engine coolant temperature sensor?). If it is not, then an emissions code will be set and the Check Engine Light will illuminate. The comprehensive component monitor also allows for adaption of the range (within preset limits) of the sensor readings to account for engine and component wear. This adaption is then shared with the other upstream emission systems, which modify their operating parameters to account for engine and component wear.
  

Downstream Emissions Control Systems

• The exhaust manifold is the first downstream emission control device
• The catalytic converter is a sophisticated after-burning device designed to complete the combustion of the exhaust gases that pass through it
• The secondary air system adds oxygen to the exhaust gases under specific operating conditions to aid in the final "burn off" of the emissions
• The oxygen sensor is a device that constantly monitors the oxygen content in the exhaust gases and sends this information back to the engine management computer (ten or more times per second)
  

Emissions systems also gave birth to fuel injection, which is now found on virtually every modern vehicle. It increases engine efficiency by injecting precise amounts of fuel in a fine mist spray for optimal combustion. This greatly reduces unburnt fuel (hydrocarbons) emissions and reduces the amount of fuel required.