Emission Control Devices

Here we're going to talk about PCV, Fuel Evaporative Systems, Thermostatic Air Cleaners, Air Injection, EGR, and Catalytic Converters. What theory do you need to know about how they work? How do they usually fail and cause problems? How do we test them? And we'll have some sample test questions.

PCV: (Positive Crankcase Ventilation) These were our first emission control devices. (Unless you want to call a gas cap a smog device, but the first gas caps were vented, right?) Inside the engine crankcase, where the oil lives and breathes we have gas vapors that got there by sneaking past the piston rings when the piston is compressing the air-fuel ratio or the fuel is burning up. And the more power the engine is developing, the more of these HC vapors get crammed into the crankcase. So, what to do. If we have a crankcase that is totally sealed, this gas under pressure will eventually blow out the seals. So we have to find a way of relieving this pressure. In the old days, when I was a child, we had road draft tubes and open oil filler cap covers. That way the pressure could get out, and as the car drove down the road, suction was created at the road draft tube and it helped pull out the gas vapors. Fresh air could come in through the oil cap and help keep the engine oil from being too contaminated by the gas. This helped the oil last longer.

But, then came pictures of L.A. smog, and the smart scientists realized that we could cut down on about 20% of the smog if we didn't let that gas out that we paid for anyway. So now we have what is called a closed crankcase. The oil filler cap should not let out any fumes. And the oil dip stick should also be sealed, in some newer cars better than some older cars. And we have a fancy PCV valve or orifice that lets the intake manifold vacuum recycle and burn that gas we paid for. Inside the PCV valve is an orifice regulated by a plug and a spring. When manifold vacuum is high, this plug is sucked in hard against the spring and we just have a small calibrated orifice for blowby to flow through. Then, as engine load increases, the vacuum drops and the plug is pushed back by the spring and we have a bigger opening for more crankcase flow. But if we have no vacuum, the plug is pushed against the other end by the spring, and we have no flow. This is a flame arrester in case of a backfire. We wouldn't want the flame in the intake to spread to the crankcase. Also in this system, instead of a road draft tube, we have some sort of vent tube usually running between the crankcase or valve cover and the air intake or air filter area.

Here's how the PCV flow goes. Under idle or light load fumes are sucked through the PCV into the intake and the vent tube lets fresh air into the crankcase. (Otherwise we'd get a big vacuum in the crankcase sometimes.) And this fresh air helps keep the oil cleaner than it ever was before. Under medium load, we have more flow going through the PCV valve because the vacuum is lower. Then under lots of power, when the volume of blowby HC is too much for just the PCV valve to handle, the extra blowby goes out the vent tube and is pulled in with the incoming air. So, under idle, or under full throttle, we keep the fumes from getting out and the air is cleaner. And the oil is a lot cleaner too, and our engines last longer. Good deal!

Fuel Evaporative Systems: We all know gas evaporates very quickly. But it doesn't just disappear, it goes into the air where is helps create smog. So if we can contain the gas in our gas tank, we can limit another 20% of our smog. So instead of an old gas tank that had a vented cap, we now have sealed gas caps. But not totally sealed, because if we contained all that pressure from expanding gas on a hot day we could damage our gas tank. Or on a cold day, the tank might deform inwards from the contracting gas and vacuum created while driving. So we have a vacuum and pressure relief valve built into the system, usually in the cap. But before we get to this point of venting off excess pressure that would release gas vapors, we have a hose that lets the vapors escape to a charcoal canister. Here they are stored, actually trapped in the tiny crevices of this rough carbon stuff that looks like a cross between tiny lava rocks and ashes from your fireplace. If we have a carburetor, we need to be able to route the fumes from the bowl vent out to the canister as well. And we need valves to shut this off or turn it on. But we can't store this gas in the canister forever, so we have hoses and sometimes valves or solenoids that let the fumes into the intake manifold so they can be safely burned. (We call this purging, and it usually happens during closed loop cruising, but some systems do purge at idle while in closed loop.) We purge during closed loop so we can control the air-fuel ratio because we don't know exactly how much fumes we will get. To do the purging, we might connect the canister to ported vacuum on older systems. Or we can have a vacuum valve or solenoid connecting the canister to manifold vacuum. The solenoid can turn the purging on and off exactly when needed. Or it can give us more precise control so the computer can even ground the solenoid at different percentage rates (variable duty cycle) and get different amounts of flow if it wants to.

If this isn't complicated enough, along came OBD II Evap systems and now we have to be able to self test the fuel evaporative system with our engine computer so that if we loose 50% more vapors than our federal standard, we will turn on a MIL (Malfunction Indicator Light--the "check engine" light in the dash.) Now we add some extra pieces to the puzzle. We need a pressure sensor in the tank and a solenoid valve to close off the fresh air inlet to the charcoal canister. Now we can measure whether a closed system builds up some pressure from the evaporating gas like we would expect. Or we might have a flow sensor in the line to the charcoal canister or purging line so we can measure that purging is actually taking place. (We might also check for purging by watching the oxygen sensor go richer than normal.) We need a more sensitive fuel tank level sensor so if we are too full or too empty we know our pressure sensing will not be accurate, so we won't run those monitors then. And most important, make sure the gas cap is on tight! (Many times this has caused the check engine light to come on. The owner pops the hood and sees the engine is still there, so he takes it to the dealer and they tighten the cap for him.)

Thermostatic Air Cleaners (TAC) and other Early Fuel Evaporation (EFE) devices: These systems are pretty simple, so I'm not really going to say much about them. What these devices do is preheat the air going into the intake manifold. They may use heat gathered from the exhaust manifold (TAC). Or exhaust forced to flow under the intake manifold by blocking one side of a V style engine exhaust with a heat riser valve. Or they may use an electric heater grid under the carburetor or throttle body. But the idea is that a warmer intake passage won't have to loose as much gasoline to condensation on the cold metal parts. So the systems don't have to run as rich to start or run as rich when cold. So the CO and HC emissions are lower. Just make sure they are working. Visual inspection will find a lot of these problems. TAC devices can be left on your cool bench and then put on and started to see if the door flaps in the right direction to let in hot air when started cold. Make sure the hot air tubes are in place. When the engine is off, look down the carburetor or throttle body with a flash light and the throttle open to check if the heater grids are still intact. Watch the heat riser control valves for movement as the engines warm up. You know the drill. Just remember some of the TAC's use wax elements, not vacuum. And they go bad over time. So, for instance, don't replace a Volvo mass air flow sensor without making sure the TAC is going into outside air mode when the engine is warm. You don't want to replace another melted mass air flow meter for free.

Air Injection: (Abbreviated AIR for Air Injection Reaction) There are less air injection systems on vehicles now that there used to be, but there are still lots of them around. The general idea here is real simple. If we add more air to the hot exhaust gas, the extra oxygen can make the combustion process complete and clean up the emissions. To the CO that comes out the exhaust valve we add O2 and this completes the process so we get CO2. To the HC that comes out we add the O2 and now some of this HC will oxidize to become H2O and CO2. We can add this air up at the exhaust manifold where the exhaust gases are still very hot; we call this upstream. Or we can add this air down at the catalytic converters to make the CATs work more efficiently. We call this downstream. And there are two ways we can add the air. We can have a pump (driven off a fan belt or electric motor) or we can pulse the air in, using the normal pulsing that takes place in the exhaust stream as the exhaust starts and stops with the closing of the exhaust valve. Air injection is very effective in cleaning up the emissions. I have seen systems that have a very rich mixture, say 6% CO, get cleaned up to under 1% CO when the air injection is fixed.

Switching when the AIR is turned on and where it goes is also important. Some vehicles only turn it on and pump the air upstream when the engine is cold. This is when the engine has the richest mixtures and pollutes the most. The exhaust manifold gets hots first, so air sent there can clean up the exhaust gases. And the added heat generated from this extra oxidation helps to warm up the oxygen sensor and the catalytic converter. Some of the new cars have the electric motors that get turned on to do this, and then when the engine and cat are warm the air injection is turned off and doesn't rob any engine power. Some of the older systems have the belt driven pumps that do this, and then when everything is warmed up, the pump has a valve that lets the air out easily to the atmosphere, so only a little power is taken from the engine now. Once the oxygen sensor warms up, if we sent air upstream, the excess air would pass by the oxygen sensor and the computer would think this is a lean condition and richen up the air-fuel ratio more than it should. So if they don't turn off or go to atmosphere, the air injection has to go downstream when the engine is warm. This is for the more complex systems. And this is one way some folks tell if the system is going into closed loop, they listen or feel for the air injection to turn off or go downstream. What happens downstream at the CAT will be covered in the CAT section. There are also times when the engine is warm and we don't want the air at the CAT because it can overheat it or burn the grassy field we are parked on, so under acceleration, power or extended idle the air will be sent or diverted to the atmosphere.

How the switching is done on these AIR systems is next. Many use vacuum to regulate the Switching valves, which may be called Diverter valves, Bypass valves, or Air Control Valves. Vacuum gets applied to one side of a rubber diaphragm that has a spring on that side. Normally spring pressure pushes the diaphragm in one direction. But if we apply vacuum to the spring side which pulls the air out, the air pressure on the other side can now push against the diaphragm and spring to move it and whatever valve is attached to it. So we can get valves to move back and forth with vacuum. And then to regulate the vacuum older systems have thermal wax vacuum valves that are in engine coolant so the wax expands and switches the vacuum direction when the coolant gets warm. And the newer systems have solenoids that control the vacuum direction depending on how the computer regulates the solenoids. Some may use a solenoid to directly control the air valves.

EGR stands for Exhaust Gas Recirculation. Sounds strange, but we put some hot exhaust gas back into the combustion chamber and it cools the combustion because the same stuff can't burn all over again. And cooling the combustion down keeps our NOx emissions down. Very effective. The only problem is too much at the wrong time makes the engine run poorly, so we have to carefully regulate it. We don't need EGR at idle, there's not enough heat there to cause much NOx. (Unless you are a diesel engine, but we're not talking diesel here.) We don't need EGR when the engine is cold, this lowers the NOx we put out. Under full acceleration, EGR would limit your power, and the Federal test procedures that test a vehicle emissions let you get away with not being tested under full power, so we don't have to turn on EGR then either. (Although we do create a lot of NOx under power.) We only need EGR during light to medium cruising. That's where we spend most of our time driving so that's when we put out the most NOx emissions.

Controls to regulate EGR vary a great deal, from simple but not very effective, to complicated electronics but very exact. The simplest and oldest way is with a ported vacuum signal and a vacuum diaphragm working against spring pressure. As we open the throttle to accelerate, the ported vacuum signal comes on and creates vacuum to work against the spring and open the valve. There is often a thermal vacuum valve that turns off the vacuum when the engine is cold. But with this, we tend to get too much EGR at light cruise and not enough at medium cruise as ported vacuum gets less. Now, pressure in the exhaust pipe increases as we increase our engine load, so if we combine this exhaust 'backpressure' with the ported vacuum signal, we can get more accurate regulation. Those fancy engineers came up with a vacuum transducer to do this. If you let the transducer be a vacuum leak that is gradually shut off by more exhaust back pressure, you could have more signal vacuum to the EGR valve as the load increased. Going from light throttle that has light exhaust back pressure to medium throttle with more exhaust back pressure decreases the vacuum leak so we get more vacuum signal to the EGR valve. "Now we're "cookin'" (with gas... pun intended) By the way, these systems are using what we call "positive" backpressure to do this. We need to know this, because some get real fancy and put the vacuum transducer inside the EGR valve. Now many of them use this positive exhaust backpressure to regulate how much EGR we get depending on the engine load. But sometimes they set up the transducer backwards so that it is not a leak normally. Then vacuum pulses pull it open instead of pressure pushing it closed Then they call it a "negative" backpressure EGR valve. And these backpressure EGR valves are so good at regulating that some manufacturers even run manifold vacuum to them and the EGR won't open until the right amount of engine load is present.

Computer controls to EGR give us even more accuracy. By listening to all the inputs, the computer can accurately decide when to turn on the EGR and how much EGR to turn on. We might see a computer switching a solenoid on or off to control the vacuum to the EGR. Or we might see it pulsing the solenoid duty cycle (%) to send variable vacuum levels to the EGR. Or the computer might bypass the vacuum altogether and use a solenoid directly on the EGR to control it. GM has the Digital EGR that uses three different solenoids controlling three different sized holes. It can switch on one or more solenoids to mix and match different openings for the correct amount of flow it needs for that particular condition. Or there is the GM Linear EGR that uses only one valve, but pulses it at different duty cycles (%) to get an infinite amount of variable EGR flow. Then computer controls can even go beyond that and control ignition spark timing so well that EGR is hardly necessary anymore. They just don't let the timing advance as much as it used to. Some EGR can also be built into the intake and exhaust valve overlap. Or with an engine using computer controlled valve timing, this valve overlap can now be controlled depending on the condition of the engine. And this "EGR valve" won't have the deposits and reliability problems that regular EGR valves have.

Catalytic Converters (CAT): These are really something: simple, effective, yet so technically advanced. Reminds me of UFO technology or something. When you understand them, and there is a lot to understand, you realize how far reaching their technology is. Catalyst technology is also what is used in fuel cells to power space craft and we will soon be seeing motor vehicles powered by fuel cells. All the major manufacturers are working on them, and there are prototypes working now in Europe. Fuel cell vehicles don't put out CO2, so there is no greenhouse gas problem. But I get carried away. Let's get back to CATs.
Basic Theory: A catalyst is something that helps a chemical reaction take place, but it doesn't get used up in the chemical reaction. Kind of like a Matchmaker. She is supposed to find this young man and woman who would make a good couple. But she's not supposed to get too involved when their chemistry really starts to take off, if you get my drift... There are basically three chemical reactions that need to take place in a CAT. We need to have these reactions to clean up our HC, CO, and NOx. So let's review with these simplified equations:

1. HC + O2 = H20 + CO2	In this reaction our gasoline (HC), combines with Oxygen from the air to create water (H2O) and Carbon Dioxide(CO2). Notice that this reaction needs oxygen to burn up gasoline. The chemical term is oxidation. This reaction doesn't work well when the air-fuel ratio is rich. Not enough oxygen around in the leftover exhaust. So we need the exhaust from a lean condition for much of this to occur.
2. CO + O2 = CO2	This reaction is our deadly carbon monoxide (CO) combining with Oxygen from the air (O2) to create more Carbon Dioxide (CO2). This also needs oxygen to take place, so it is an oxidation reaction. It also needs to take place in the exhaust from a lean condition to be very successful.
3. NOx + CO = N2 + CO2	This is our NOx being cleaned up. Carbon Monoxide (CO), which wants to combine with Oxygen (O2) in the worst way, is used to pull the oxygen off the Nitrous Oxides (NOx) so we just get clean nitrogen (N2) and Carbon Dioxide (CO2) as byproducts. But notice that this doesn't take place where oxygen is, but where oxygen is not. If we have much oxygen around, it would turn the Carbon Monoxide (CO) into Carbon Dioxide (CO2) and we would have nothing to clean up our NOx. Since we remove oxygen, not add it, this is called a reduction reaction. (That's the chemical name for the reaction.) And notice that this needs exhaust from a rich condition to work, or the CO could not be there.

The basic concept is the catalysts in the catalytic converter help these reactions to take place so we clean up the pollution in the exhaust. The catalysts that do this are Platinum and/or Palladium, and Rhodium. These are 'noble metals' that don't react much to anything, they just make things happen around them, hence the term 'noble'. (Like rich royalty with the 'stiff upper lip' that don't get upset over much, but rule the country.) Now, it is Platinum or Palladium that makes the oxidation reactions happen. And Rhodium makes the reduction reactions happen.

With this background, we can now describe the basic types of catalytic converters:

Conventional: This is the most simple type, and was used a lot when CATs first came out, but not as much since then. This only cleans up HC and CO. So it only has platinum or palladium in it. Because it only cleans up two of our pollutants, we call it a Two-way catalytic converter.

Three-way CAT: As you can guess, this CAT cleans up all three of our pollutants: HC, CO and NOx. And so it contains either platinum and/or palladium, and also rhodium. This is the basic kind of CAT around, and there are lots of them.

Dual Bed Three-way CAT: This is fancy. This has two sections, a front section and a rear section. In the front we have a Three-way CAT with the capability of cleaning up all three pollutants. (Or we might have just a section with Rhodium that could clean up the NOx) Then in between the two sections, we bring in air from an air injection system. In the back we have a Conventional Two-way CAT. In front we can concentrate on cleaning up our NOx, or we can clean up all three pollutants. Then in the back we can get real good at cleaning up HC and CO. But we can't clean up NOx any more, because we are too lean with all the air we added. There won't be any CO around.

There may be other combinations of CATs around. The car makers do lots of different things, like put one Conventional CAT near the exhaust manifold and then put a Three-way CAT down under the car. But this will give you a basic idea of what's out there. One other type of CAT should be mentioned:

Three-way CAT with HC Absorber: This is real fancy. This CAT has a 'carbon canister' like chamber to absorb the extra HC put out by a cold engine, because when you first start an engine is usually when it runs the dirtiest. Then when the CAT is hot, it gives off the HC and cleans it up so it's ready for the next time. Some books call this an HC trap. (It's technically called adsorbing with a 'd' by some, but let's keep it simple.)

Just one more thing about CATs: Did you realize we have a problem here, a contradiction that we haven't explained yet. If the CAT cleans up HC and CO when the exhaust is lean and then cleans up NOx when the exhaust is rich, how do we clean up all three at the same time? Well, glad you asked...

If we keep the air fuel ratio right at 14.7:1, what we call stoichiometry, the CAT is right in the middle between rich and lean, and the CAT can do both the oxidation and reduction jobs at once. But it doesn't do them really well. There is a better way, a way that allows the CAT to clean up more and be more efficient. That is to vary the air-fuel ratio between being lean and rich. By switching the ratio back and forth a little between lean and rich, the CAT enjoys the benefits of both worlds. It can be rich and clean up the NOx, then it can be lean and clean up the HC and CO. This is why the normal O2 sensor system is designed to cycle back and forth--lean, rich, lean, rich when it is in closed loop. This is the most efficient way for the CAT to work to clean up all three emissions. But it must cycle back and forth at the correct rate and only just a little. If the O2 sensor is too sluggish, this cycling spends too much time at either extreme condition, and the CAT doesn't work well either.