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Some Emission Control System Components Explained

This DIY Guide is provided by AutoBodyShop.Org, the largest US Auto Body Repair Directory with 120.000 listings.

There is no doubt about it- the air in and around all the major cities in the world is much cleaner than it was twenty years ago, and it is due in large part because of the efficiency of emission control systems on modern vehicles. Of course, there are exceptions (there always is); in some countries such as South Africa, emission control is not enforced in any way, and as a result, the air in the cities there is as toxic as it has ever been.

However, the fact that emission control is not enforced does not prevent the emission control systems on South African cars from failing, and when they do fail, the cars they are fitted to exhibit the same symptoms as cars anywhere else.  So in this article we will take a closer look at some of the components in modern emission control systems- what they do, how they do it, and what happens when they fail. Let us start with the most important component- the engine.

Modern engines as emission control devices

Modern gasoline engines are only about 30-,to 35% efficient, meaning that a modern gasoline engine can only convert about 35% of the calorific value of its fuel into kinetic energy. The rest is lost in the form of heat, but for all that, modern engines are about as efficient as they can be.

In contrast to older engines, a modern engine that is computer controlled is able to burn all of its fuel, using all of the air that enters the engine, which results in relatively clean emissions compared to older, non-fuel injected engines that did not burn all of their fuel.   However, this is true only if the engine runs at a constant speed and at a constant load; acceleration, deceleration, gear shifts, braking, and a host of other control inputs can upset this balance, and this is where engine design enters the emission control equation.

Modern fuel injection systems are capable of almost infinite adjustment via the ECU (Engine Control Unit), which gathers information about throttle position, engine speed, ambient air temperature, ignition advance, and many other factors. For this capability to be effective though, the overall design of an engine has to allow for a smooth gas flow through it on the one hand, and a way to fully combust the air/fuel mixture and to extract the resulting exhaust gas efficiently, on the other.

All of this accomplished with carefully calculated inlet manifold lengths and diameters to optimize air flow, valve timing/duration/travel/overlap that optimises compression and the efficient distribution of the air/fuel mixture in the cylinders, and improved combustion chamber designs to allow for the full combustion of the fuel. Last but not least, comes exhaust manifolds that are designed to extract the maximum amount of gas in the shortest possible time, without scavenging unburned fuel/air from the inlet manifold.

The long and the short of all of this is that modern engines have become reasonably efficient emission control devices, since they are able to reduce emissions simply by burning their fuel more completely than was possible even ten years ago- thus reducing the amount of unburned hydrocarbons that end up in the atmosphere. Of course, the downside is that there are a great many things that can go wrong with a technologically advanced engine, all of which can have hugely negative effects on the quality of the exhaust gas.

Catalytic converters explained

Assuming that the engine is functioning as designed and the correct type and grade of oil is used, and  that issues like mechanical wear, defective sensors, and low quality fuel do not cause increased emissions, the catalytic converter can quite easily clean up the exhaust gases by converting harmful substances in the gas to oxygen and water.

Catalytic converters use catalysts such platinum and palladium to oxidize carbon monoxide and unburned hydrocarbons by means of the high temperatures that arise as a result of the oxidization process. Therefore, on cold engines, a catalytic converter does nothing to convert harmful gases, and it only becomes effective as the engine and thus the exhaust gas, heats up.

However, there are many other factors besides engine temperature that effects the efficiency of a converter, and chief among them is the use of unsuitable engine oil. Engines that require the use of synthetic oil cannot run on mineral oil because of the smaller engineering clearances in these engines. The molecules of mineral oil vary greatly in size, and some of the smaller molecules can slip under the piston rings into the combustion chamber, where they are partially combusted, leading to increased levels of hydrocarbons that the catalytic converter cannot effectively deal with.

The result is a serious reduction in the life of the converter as the catalysts get overwhelmed, which in turn, leads to a serious loss of performance as the ECU tries to compensate for the increased levels of hydrocarbons by leaning out the air/fuel mixture. Moreover, using leaded fuel coats the catalysts with lead that renders the converter useless- which is the main reason why lead no longer occurs in gasoline in most parts of the world.

Clogged, or overwhelmed catalytic converters cause vastly increased fuel consumption, rough running, serious power losses, stalling, and even engine overheating. So if you experience any of these symptoms, there is an excellent chance that the catalytic converter has stopped working.

Evaporative controls explained

Because gasoline evaporates at high rates at relatively low temperatures, all cars made after the early 1970’s are required to use gas caps that cannot allow fuel vapors to escape in to the atmosphere. To control the vapors, all gas tanks are fitted with charcoal canisters that absorb the fuel vapors until the engine vacuum can be used to suck the vapors into the engine, where it is combusted with the air/fuel mixture.

To regulate the flow of vapors, a purge valve is used to control, the flow of vapor, but when this valve fails, it can happen that raw fuel is sucked into the engine. If this happens, the air/fuel mixture can be enriched to the point where the engine can stall, or fail to start. An additional complication is that the catalytic converter can fail because it cannot cope with the excess hydrocarbons.

Why you need a PCV valve...

 All engines produce volatile, hydrocarbon-laden gases as a result of their normal operation, and the job of the PCV (Positive Crankcase Ventilation) valve is to direct these gases into the inlet tract. However, being less combustible that the air/fuel mixture, these volatile gases can have a serious impact on the combustibility of the air/fuel mixture, which is why at low engine speeds, the PCV valve limits the amount of volatile gases that can enter the engine.

At high engine speeds, and at large throttle openings, when the enrichment (or dilution) of the  air/fuel mixture is no longer as critical, all of the volatile waste gas is directed into the engine, where it is combusted with the air/fuel mixture. However, PCV valves become clogged, or they fail in the ways, and if this happens the waste gases can no longer be controlled.

One effect is that all of the waste gas is directed into the engine all of the time, increasing emissions, or, none is directed into the engine. In the latter case, pressures build up inside the engine that can push out oil seals, or equally bad, push oil into the air filter box, where it will clog the filter, thus increasing emissions because this oil gets sucked into the combustion chambers. PCV valves have finite service lives, and they MUST be replaced in accordance with a vehicle’s maintenance schedule to maintain emission standards. well as an EGR valve

EGR stands for Exhaust Gas Recirculation, and in practice, this valve reroutes a small amount of exhaust back into the combustion chamber to reduce the overall combustion temperature, because the exhaust gas dilutes the  air/fuel mixture. High combustion temperatures cause high levels of nitrogen oxide, which is one of the main constituents of smog; however, modern engines are not designed to run on exhaust gas, and therefore the amount of exhaust gas that is rerouted through the engine at any given time is very carefully controlled with a series of vacuum and electrical  switches, as well as with the Engine Control Unit.

Reducing combustion temperature is a very effective way to control the level of nitrogen oxide in exhaust gas, but the downside is that it causes a significant loss of power. To limit power losses, the EGR system does not function when the engine is cold, or during a high demand for power. 

Nevertheless, EGR valves often fail, and as a result, they reroute more exhaust gas through the engine than the other emission control system components can cope with. When this happens, the engine may run rough (or not at all), there will be excessive amounts of black, blue, or even white smoke from the tail pipe, and the catalytic converter will fail because the catalysts are overwhelmed.

A final thought...

The brief explanations above of how some emission control system components work are meant to be just that- brief explanations. There are many other possible causes of some of the symptoms described here, and professional advice should be sought if any of them appear on your vehicle. In many cases, these symptoms may not be related to the emission control system in any way, but play it safe, and  perform regular preventive maintenance to prevent the possibility of the emission control system on your vehicle being the cause of the symptoms described here.