The Air You Are Breathing Right Now

emissions outside airThis is the composition of our air around us as well as the air entering our engines.

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Tailpipe Emissions

emissions ideal combustionIn a perfect world, this would be all that came out of a cars tailpipe. Nitrogen, oxygen and fuel would enter the engine and all that would come out the tailpipe would be water, unchanged nitrogen and water.

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emissions realistic combustionIn a perfect world, this would be all that came out of a cars tailpipe. Nitrogen, oxygen and fuel would enter the engine and all that would come out the tailpipe would be water, unchanged nitrogen and water.

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Hydrocarbons (HC)

HC in the exhaust is a result of unburned fuel. High HC levels can be caused by (but not limited to) a misfire, low compression, leaking injector or an excessively rich or lean air fuel ratio. HC being released into the atmosphere contributes to SMOG, forms of cancer, and destruction of the ozone.

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Carbon Monoxide (CO)

Carbon monoxide in the exhaust is a result of incomplete combustion. CO is formed when there is not enough oxygen in the cylinder to burn all the fuel (rich mixture). However, CO is a product of combustion so a misfire will not raise CO levels. High CO levels can be caused by (but not limited to) a dirty air filter or high fuel rail pressure. Carbon monoxide can be toxic to humans, when inhaled it will change the chemistry of a persons hemoglobin and inhibit the body’s ability to transport oxygen through the blood stream.

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Oxides of Nitrogen (NOx)

Oxides of nitrogen are caused by nitrogen combining with oxygen in the combustion process. This is caused by high levels of heat and pressure in the combustion chamber, or a lean air fuel ratio. The “x” in NOx is a variable, the amount of oxygen doesn’t matter they all can be formed and they are all oxides of nitrogen. NOx are only produced under light to moderate load, so no NOx are produced at idle or WOT. High NOx levels can be produced by (but not limited to) faulty EGR (if equipped), overheated engine, or a vacuum leak but most likely the catalyst has failed. NOx is a main ingredient in SMOG and acid rain.

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Oxides of Sulphur (usually SO2)

When sulphur is burned, it produces SO2. Since sulphur can be found in petroleum, it most of it must be removed before it is delivered to gas stations in the form of gasoline and diesel. There are even regulations in some areas as to how much sulphur content is acceptable.

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Carbon Dioxide (CO2)

CO2 is a result of complete combustion. High CO2 levels indicate an engine is running properly, therefore it is desired in the exhaust but it is considered a greenhouse gas.

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Oxygen (02)

Oxygen in the exhaust is not harmful to the environment but if too much oxygen ends up in the tailpipe it could indicate a lean air/fuel ratio.

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Other Emission Sources

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Engine Crankcase

During combustion, some unburned fuel and compression gets past the piston rings and into the crankcase. This is called blow-by which can cause engine sludge as well as pressure in the crankcase which can lead to oil leaks. It can also cause HC to be released into the atmosphere just like HC in the exhaust. To get rid of this, the PCV (positive crankcase ventilation) system, allows manifold vacuum to pull these gasses out of the crankcase and burn them. Some systems have a PCV valve that regulates flow and others simply have a hose going from the crankcase or valve-cover to the intake. On the systems without a valve, the crankcase gasses are like unmetered air entering the engine. The management system must account for this.

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Fuel Tank

The fuel tank itself can emit raw fuel vapours into the atmosphere. These vapours must be controlled and burned. The fuel cap must allow air in to fill the void created when the fuel level goes down, but it must not allow air to escape. To burn off the fuel vapours, the EVAP system is used. Fuel vapours are directed from the highest point in the tank to the charcoal canister which can hold the vapours. At the top of the canister is the vapour separator, which directs fuel droplets back to the tank. When the PCM wants to purge the system, the PCM activates the EVAP purge solenoid to allow manifold vacuum to pull in fuel vapours and burn them during combustion.

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Emission Control Devices

Emission control devices are designed my manufactures to get their engines to pass emission standards. Although the engine management system itself can be considered an emission control system, only devices that were designed solely to control emissions will be listed here. For information on management systems, check out our engine management page.

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Catalytic Converter

emissions catalytic converter Catalytic converters are the most effective emission control device in automobiles today. They are a post-combustion emission control device, meaning that it “cleans-up” the exhaust gasses coming out of the engine, rather than preventing the emissions before they are made. The catalytic converter is a catalyst, or something that causes a chemical reaction without being involved or consumed in/by the reaction. The “cat” consists of 2 pucks with small holes to allow exhaust flow through the converter. The first puck is the reduction catalyst, it is made of rhodium and cerium. Its job is to convert NOx into just nitrogen and oxygen. As exhaust gasses pass through this puck, the puck pulls the nitrogen atom allowing oxygen to continue on in the exhaust. So the reduction catalyst converts NOx into N2 and O2. The second puck is the oxidizing catalyst, it is made of palladium and platinum. Its job is to oxidize HC and CO emissions. This converts HC and CO into H2O (water) and CO2 (carbon dioxide).

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Air Injection

emissions air injectionAir injection systems pump fresh air into the exhaust to help oxidize HC and CO emissions, as well as supply oxygen to the catalytic converter. These have a pump that can be electric or they can be driven by the engine. The pumps that are driven by the engine are driven all the time and are controlled with electronic valves which control air flow to the exhaust when the system is in use or the atmosphere when it is not needed. Since the mechanically driven pumps are run anytime the engine is running, they consume engine power and hurt fuel economy. This is why most modern systems have switched to electronic systems. In the near future, these systems will most likely not be necessary. With variable valve timing the engine can increase overlap time causing more oxygen to end up in the exhaust, also the engine can demand very lean air/fuel ratios for a period of time.

 

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EGR System

The EGR (exhaust gas recirculation) system allows a controlled amount of exhaust gas to enter the intake stream. Since the exhaust gas has little to no oxygen left, this causes combustion to be much cooler, which reduces NOx emissions. It may sound confusing to think that hot exhaust gasses can cool combustion. Keep in mind that even exhaust is much cooler than combustion and there is much more cool intake air than there is hot exhaust gasses in the intake stream, even when the EGR is fully active. These systems have used several control devices over the years. Some are vacuum activated, some use exhaust pressure/vacuum and some are simply controlled by the PCM electronically. These systems will not be active at idle or heavy load conditions. EGR activation at idle can cause a rough idle condition and in heavy load conditions the engine needs as much oxygen as possible, also NOx emissions are low at idle and WOT anyway. These systems are active when the vehicle is at a steady cruising speeds. EGR systems are phasing out because engineers have figured out how to use valve timing to get small amounts of exhaust to remain in the combustion chamber after exhaust stroke.

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Engine Design Changes

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Piston Rings

Engineers have developed better sealing piston rings to help prevent blow-by gasses.

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Combustion Chambers

Placing the spark plug in the centre of the combustion chamber, as well as causing air turbulence to better mix the air and fuel before combustion can greatly decrease emission levels.

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Intake Manifold Design

Modern intake manifolds are designed to deliver equal amounts of air to each cylinder, and the switch from carburetors to port fuel injection has allowed equal amounts of fuel to be delivered to each cylinder.

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