Reducing NOx-ious Emissions
The Exhaust Gas Recirculation, or EGR system is one of the major pollution control devices on modern automotive engines. This system allows exhaust gas to re-enter the combustion chamber in order to reduce combustion temperatures. This reduction in temperature limits the amount of oxides of nitrogen, or NOx , that is formed in the combustion chamber. The key component in this system is the EGR valve. This valve, whether it is vacuum- or electrically-activated, regulates the amount of exhaust gas that is recirculated back into the engine.
The Major Pollutants
There are three major pollutants emitted from an internal combustion engine—hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NOx). HC is basically unburned gasoline that results from incomplete combustion. CO usually results from an incorrect air/fuel ratio. But NOx is a bit trickier. It is the result of the heat of combustion.
When the air/fuel mixture is burned in the combustion chamber, the fuel releases energy to force the piston downward to drive the crankshaft. Chemically, the fuel contains no nitrogen and nitrogen does not aid the combustion process in any way. So, why is it there in the first place? Well, 78 percent of the atmosphere is nitrogen and when air is inducted into the engine, the nitrogen is in that air.
Nitrogen in the atmosphere is a stable compound consisting of two nitrogen atoms (N2) and at ambient temperature it doesn’t combine with anything. But at the elevated temperatures found in the combustion chamber, the N2 splits into two separate atoms that can now combine with another element. Herein lies the problem. If the air inducted into the engine contained no nitrogen, NOx would not be a problem. But that is not the case. To make matters worse, NOx is the most difficult of the three major pollutants to control.
The single nitrogen atom combines with oxygen in the combustion chamber to form oxides of nitrogen (NOx). When this compound is released into the atmosphere, it combines with ozone to form smog, that nasty brown haze that you see along the horizon in major metropolitan areas. In order to prevent the emission of NOx into the atmosphere, the oxygen and nitrogen atoms have to be split apart. This is a very difficult task. Enter the EGR valve.
The Exhaust Gas Recirculation (EGR) Valve
NOx is almost as stable as N2 and over the years, two methods have been employed to reduce the emission of this compound. One method, the 3-way catalytic converter, breaks down NOx into its elemental components, nitrogen and oxygen. This method is expensive as a rare metal catalyst is required. Also, contamination of the catalyst by leaded gasoline renders the device useless (this led to the banning of leaded gasoline). The other method is the use of recirculated exhaust gas that prevents the formation of the compound.
Under normal operating conditions, engine cylinder temperatures can reach 3,000°F. The formation of NOx is proportional to the cylinder temperature. In other words, as the cylinder temperature increases, the amount of NOx formed increases. And today’s automotive engine runs much hotter today than it did 40 years ago. It should be obvious that if you can lower the cylinder temperature, you can reduce the amount of NOx that is formed. This is exactly what the EGR valve does.
While recirculating exhaust gas does reduce cylinder temperatures, it also can have adverse effect on the combustion process. Think about it. You are adding exhaust to the air/fuel mixture. Of course there are going to be some problems. So a delicate balance has to be found between the proper air/fuel mixture and the amount of recirculated exhaust gas.
In the early days of EGR valves, the valve was vacuum operated. But with the use of more sophisticated electronics in the early 1990s, an electric solenoid, controlled by the Powertrain Control Module (PCM), varied the amount of vacuum supplied to the EGR valve, making its operation much easier and more efficient. This design, shown in Figure 1, was used until several years ago. At that time, the vacuum diaphragm valve was replaced by an electric valve.
Let’s take a closer look at how the vacuum EGR valve in Figure 1 works. Even though this design has been updated, there are still many vehicles on the road with this style.
The EGR transducer contains an electrically operated solenoid and back-pressure transducer. The PCM operates the solenoid and determines when to energize it. Exhaust system back pressure controls the transducer.
When the exhaust system back pressure reaches a pre-determined level, it closes the bleed valve in the transducer. When the PCM de-energizes the solenoid, and the bleed valve is closed, vacuum flows through the transducer to operate the EGR valve. This vacuum allows the diaphragm in the EGR to move, raising the poppet valve, or pintle, to let exhaust gas to flow into the engine. Varying the strength of the vacuum changes the amount of recirculated exhaust gas to provide the correct amount for different driving conditions. This system does not allow the EGR valve to operate at idle.
Now let’s look at the current EGR valve, such as that shown in Figure 2. This particular design is used on the 2.7L V6. As you can see it consists of an EGR valve assembly (1), an upper tube (2) and a lower tube (3). Exhaust gas is supplied to the EGR valve through the lower tube. Then, this exhaust gas is proportionately fed to the engine through the upper tube.
The EGR valve consists of three major components. The first component is the valve assembly, which consists of the pintle, valve seat and housing that regulates the exhaust gas flow. The second component is the armature assembly. It consists of the armature, return spring and solenoid coil. This assembly provides the operating force to change the pintle position in order to regulate the exhaust gas flow. The third component is a pintle position sensor.
The amount of exhaust gas recirculation flow is determined by the PCM. For a given set of conditions, based on several inputs from the engine, the PCM knows what the ideal exhaust gas flow should be. This flow will optimize NOx and fuel economy as a function of the pintle position. The pintle position is obtained from the position sensor. Based on this information, the PCM adjusts the duty cycle power supplied to the solenoid coil to move the pintle to the correct position.
EGR Service
EGR service is pretty straightforward. Disconnect an electrical connector, remove a few mounting bolts, and replace the valve and the gasket. But there are several points that must be kept in mind. First of all, regardless of the design, it is important to clean the gasket surfaces of the EGR valve. Any loose dirt can enter the valve and lodge between the pintle valve and its seat. This will cause valve leakage that will be seen as rough engine idle and decreased manifold vacuum.
Also, be careful with the silicone rubber seals that are used on the newer design. You don’t want to drop one of these seals into the intake manifold. And speaking of intake manifolds, never use power tools to remove the tube mounting bolts. Many intake manifolds are made from plastic and using power tools can damage the threads. Be smart and use hand tools when removing and installing these bolts.
Whenever servicing the EGR valve system on newer cars, disconnect the battery for at least 2 minutes. The PCM stores learned values related to powertrain operation. A malfunctioning EGR system can cause bad values to be stored that will cause an erroneous fault to occur after the system is repaired. Disconnecting the battery will reset the addresses in the PCM to normal default values.
