Emission Controls - What Are Emissions?

 An internal combustion engine uses carbon based fuel, called 'fossil fuel' because it comes from a thick strata of carbon beneath the surface of the earth.  The forests that covered this planet before the Genesis Flood were inundated and buried in mud, compressing millions and millions of trees that ultimately became the fuel we pump out of the ground and use today.

Dinosaur fossils were also buried in the Flood mud, but most of what's down there came from plant life that grew virtually all over the planet before the cataclysmic Flood of Noah that changed the earth from a worldwide tropical wonderland into the world we see today.

What We're Burning

Gasoline and Diesel are both hydrocarbon based fuels - a gasoline engine burns fuel that engineers calculate by weight - a good mix is 14.7 parts of  air to 1 part of  fuel. By volume, an engine burns 9000 gallons of air for each gallon of fuel.

In your engine, the fuel is atomized and carried into the combustion chamber, where the spark triggers a reaction that oxidizes (burns) the fuel - what that means is that each molecule of hydrocarbons wants 2 molecules of oxygen, and when it gets those two, it produces harmless CO2.  That's a good thing, because GREEN things like CO2. 

(Factoid for the curious: For every 100,000 molecules in our atmosphere, 35 are CO2 molecules. Of those 35, only ONE CO2 molecule is produced by human activity ).

As a matter of fact, human beings and animals breathe out CO2 by God's design, and if there was no CO2 being produced, all the plants would eventually die.

Okay, a mixture that is too rich will produce molecules of CO - one hydrocarbon and one oxygen.  CO is Carbon Monoxide and it kills people.  If the engine is running REALLY rich, then there are no molecules of oxygen uniting with some of the hydrocarbons, and the HC leaves the combustion chamber either as soot or a gasoline mist depending on just how much fuel is being delivered.

One other byproduct of combustion is NOx, which is a locking together of the nitrogen and oxygen (78% and 21% respectively in the ambient air) during high combustion chamber temps (>2500 degrees F).

Those are the emissions we're trying to control.  So how do we do it?

Controlling CO 

We control CO by making sure the mix is as near 14.7 to 1 as possible - the Oxygen sensors help with that, but in order for the engine to run as clean as possible, it has to be running pretty warm, with a coolant temperature of just over 200 degrees or so. 

Fuel pressure and injector pulse width are critical, as well as properly functioning injectors.  Mechanical injector leakage or improper injector triggering (the injector is electromagnetically operated by the engine controller that we call a Powertrain Control Module (PCM).

Any CO that is formed in spite of the rigid fuel controls is handled by the catalytic converter, which is an absolute marvel of engineering, but that's another subject altogether.  A catalyst can change other elements without changing itself.  In this case, one molecule of oxygen is added to each CO molecule to make CO2.  Ta DA!  CO problem solved.

Controlling HC

HC is nothing except unburned fuel, and it typically shows up as soot, or in extreme circumstances, as when an injector is stuck open, a mist of gasoline from the exhaust.

 Some HC is bound to be produced, particularly during cold engine operation (which lasts about 90 seconds or so) when the O2 sensors aren't hot enough to work (600 degrees).  The HC from cold engine operation is eliminated on some vehicles by pumping clean fresh air into the exhaust stream just as it leaves the combustion chamber - this maneuver provides oxygen molecules that unite readily with the hot HC molecules, and although there is a low level uncompressed burn going on here, there is no real work being done.

After the engine is warm, any HC that is created is handled by the same part of the catalyst that adds a molecule of oxygen to the CO, only 2 molecules of oxygen are added, thus the HC becomes CO2.  Ta DA!  HC problem solved.  If, however there is TOO MUCH HC going through the catalyst, as in the case of a misfire , the catalyst can overheat and be destroyed - it already runs as high as 2000 degrees while it's doing its work. Ever see a flashing "Check Engine" Light?  Severe misfires will trigger the flashing light.

Controlling NOx 

 Oxides of Nitrogen (there are several different compounds, thus the 'x' in NOx), can be controlled first by keeping the combustion chamber a bit cooler when the engine is loaded.  The Exhaust Gas Recirculation system does that by allowing some inert exhaust gas to mix with the incoming air. Less burnable oxygen means the chamber will be cooler.  Some newer engines started using variable cam timing on the exhaust camshaft to close the valves early and leave some of the exhaust gas in the chamber for the same reason.

Any NOx that makes it out of the chamber is handled by the front catalyst brick, which is slightly different in design than the brick that handles CO and HC - the front brick separates the NOx into nitrogen and oxygen and takes care of the NOx quite handsomely.  Ta DA!  NOx handled.

More on emission controls to come later...

By the way, rate these articles so I'll know if you like what you're reading. 

 R.W.M.