EGR

By Glen Beanard technical contributor

So Why Do We Have EGR?

The EGR system was created in the early 70 for the same number one ranking motivation that drives every other engine management system design; to reduce emissions. The EGR valve targets one specific pollutant, oxides of nitrogen (NOx).

Under normal combustion, the Oxygen (O2) in the air plus Hydrocarbons(HC) in the fuel combine into water(H2O), Carbon dioxide(CO2) and the Nitrogen remain unchanged. However, when things heat up in the combustion chambers to temperatures around 1300C or 2500F that all changes. At those temperatures, oxygen and nitrogen start combining with each other and form Oxides of Nitrogen (NOx). This is a big problem because when the sunlight hits those oxides of nitrogen, they combine with hydrogen in the atmosphere. When that happens, we now have smog. So how do we combat that? By cooling the combustion chambers down to where the normal combustion can take place.

There are several ways to lower the combustion chamber temps. Enrichening the fuel mixture, lowering the compression ratio, retarding the ignition timing, lowering the overall temperature of the engine, and reducing the amount of pure air can each lower the temperature in the cylinders. Problem is, all of those except for the last one will also reduce fuel economy. Also, each of those except for one will also raise the Hydrocarbon (HC) emissions. Watering down the intake air is the best choice. What is surprising though, is that this not only fixes a problem, it also brings efficiency improvements with it. It is a win/win situation.

Ok, so how are we going to water down the air mass? Obviously, we arent using actual water, that is only a figure of speech. The air coming into the engine contains oxygen, so it supports combustion, and must have a certain amount of fuel mixed with it. If we are going to dilute that down, well need an inert gas to blend in with it, right? Something that doesnt support combustion and doesnt have to be met in the cylinder with a certain amount of fuel is what we are looking for. Carbon dioxide and carbon monoxide are both inert gasses right? So, lets just add a pressurized cylinder under the hood, and shoot a little of either of those gasses into the intake to water down the fresh air entering the cylinders. Thatll work, right? Yes it will, so wouldnt a lot of other gasses, as long as they are inert. However, now wed have a new problem. Wed now have this pressurized tank that wed have to keep refilling. Good thing we have an endless supply of what we need coming out of the tail pipe already. All that has to happen now is come up with some kind of way to divert some of that back into the intake manifold. So now we have the Exhaust Gas Recirculation (EGR) system.

Theory and Operation

Gains in efficiency were mentioned earlier. How do does EGR help efficiency? There are two main sources of the efficiency benefits, reduced pumping loss and the ability to advance the ignition timing.

The addition of the EGR gasses slows the burn time down. Since the fuel burns slower in the combustion chamber, the ignition timing can now be pushed to where to the chemical energy in the fuel can be made better use of.

What about the reduction pumping losses? Notice the illustration provided. The EGR valve actually becomes a second breathing source for the piston. On top of that, the EGR gasses actually choke the combustion action some. In this case, it causes the driver to have to open the throttle plate a little further than compared to without EGR at all. When this happens, the air flows past the throttle plate with less restriction. As a result, the efforts used by the engine to pump the air into the cylinder are reduced. The intake stroke becomes that much less work to achieve. If its a little hard to imagine that way, think of how some engines may benefit from dual exhaust systems. Depending on the engine, dual exhaust is know for increasing engine performance and efficiency. That is because dual exhaust can reduce pumping loss through a less restrictive exhaust stroke. That is no secret. EGR does the same thing on intake stroke because the driver opens the throttle plate further, this effectively reduces the intake restriction. In turn, lowering the pumping losses on the intake stroke.

EGR gasses are hotter than fresh intake air. So how does it cool the cylinder when it actually heats the intake air charge? This isnt about the temperature of the air charge. The cooling action doesnt take place on the intake stroke. The cooling action takes place on the power stroke by controlling the combustion action. Think about this for a minute. Picture a bon fire with five logs burning. The logs are stacked three on the bottom and two over the three in a pyramid formation. The logs are making contact with each other. With a fire burning in this manner, the logs are sharing heat energy with each other. The fire will have a tendency to grow at its own rate and the heat energy between the logs will be very intense. Now, imagine the same logs again, only separate them and pump a fog between them. What would happen to the fire? The second fire wont be as intense between the logs as the first one. Also, most importantly, the fire can now be controlled. The first fire example had no control. The fire was allowed to grow at its own rate. The second one could be controlled by varying the amounts of fog injected into it, and therefore it is not allowed to just run wild. That is the basic principle behind how the EGR gasses cool the combustion chamber. The EGR gasses spread the air and fuel molecules apart so that they arent allowed to just burn as fast and hot as they otherwise could.

Now that the burning action in the cylinders is being controlled, the ignition timing can be advanced to take better advantage of the energy being generated. This is where another efficiency gain comes into place.

Notice the tightly nit relationship between EGR flow, Timing Advance, and Engine Load in this data captures.

Though the overall purpose and operation are the same across the board, there are some details that change from vehicle to vehicle. For example, an EGR can be opened by vacuum control or an electrical signal. The EGR valve might dump all of its gasses into the intake plenum at one port, or it may distribute a small amount into each of the individual intake runners feeding the cylinders. The measuring and controlling of this valve likewise varies from vehicle to vehicle. For this part of the discussion, we will be using Ford EGR systems to reference. Much of this discussion will be only applicable to Ford, yet still some may apply similarly to other makes. Be sure to always review information specific to your vehicles make and model before any diagnostics or repair are attempted.