In the 1970’s, Exhaust Gas Recirculation Systems were mandated as a pollution control device to limit the production of oxides of Nitrogen or NOx. The acronym of EGR has stuck with the industry ever since. The basic principle is that exhaust gases have very little oxygen and can be used to cool and control the speed of combustion. Since NOx is directly related to combustion temperature, adding a little exhaust gas to the intake mixture will reduce the overall NOx production.
The flip side is that the higher the amount of EGR volume, the lower the combustion efficiency and to a degree, the slower the combustion burn. EGR does not occur at when the engine is cold or at idle, as the lowered efficiency would cause misfiring and elevated hydrocarbons (HC). Besides, in those conditions, the combustion does not really require any cooling to reduce NOx. Generally, EGR does not occur at full throttle, since EGR would reduce the power output and raise hydrocarbons. So EGR is pretty much a mid-range power function; the rpm, load and volume of which varies widely with different engine designs and fuels, and whether the engine has a charger or is normally aspirated. As a loose generality, gasoline engines tolerate between 3% and 15% of the intake volume as EGR gases. Diesels can use up to 3 times that level of EGR gases. The highest EGR volume is during medium load midrange rpm driving.
In short, NOx and SOx are oxides of nitrogen and sulfur, and respectively cause nitric acid, nitrous acid and sulfuric acid in the atmosphere. Together these acids are known as “Acid Rain” a type of pollution that damages and kills plant, fish and animal wildlife. Acid rain also corrodes buildings, bridges, statues, and other infrastructures. EGR systems typically reduce the NOx output of an engine by a factor of between twenty and a hundred or more. Converters in the exhaust further reduce NOx and SOx to near negligible amounts.
The original EGR system was just a hole between the intake and exhaust ports. There was nothing good about how the engine ran, and they could be really hard to start. When the system first appeared in our fleet in the early 1970’s, it had a valve operated by ported vacuum so it would not function at idle or full throttle, and it had an EGR filter to keep carbon and grit out of the EGR valve and intake manifold. Because one gets 7 gallons of water for every gallon of gasoline burned, the filters would only last a few years before they rusted out in short hop, daily driver cars. A vacuum thermoswitch was used to prevent the EGR from operating when the engine was cold. Better control of the EGR valve was accomplished with a vacuum amplifier with an octopus of vacuum hoses. Increasingly stringent EPA requirements led to higher precision systems. EGR Filters were abandoned due to restrictions and holes causing EGR volume failures.
The first way to measure EGR flow and assure that the system was working appeared in the late 80’s. Temperature sensors located in the intake manifold directly after the EGR valve could detect EGR activity and estimate the EGR flow volume. It was soon obvious that temperature was not accurately correlated with flow. In the early 90’s, the method changed to monitoring the intake air volume by using the change in the Mass Air Flow Sensor signal (MAF) and assuming that the EGR volume would be indicated by the reduction of the measured intake air when the EGR system was engaged. That do-the-math method was further refined and is still in use today.
It is commonly thought that EGR causes excessive carbon and that the EGR system should be disabled for the best mileage. Reality is a bit different. First one needs to differentiate between the carbon formed inside an EGR system, and the carbon formed inside the intake manifold and the cylinder head intake ports and valves. Exhaust carbon is generally dry and thin, as evidenced by the coating on the inside of most exhaust pipes. This is the normal carbon inside the EGR system of an engine in good tune and condition. Oil consumption and misfiring will definitely cause way worse carbon to form. Combining EGR with the vapors from the crankcase (PCV) results in the gooey black tar type of carbon that builds up in the EGR valve area and on the intake ports and intake valves. The worse the oil and debris in the PCV, the greater the tendency to build up this gooey carbon. It is quite common that the amount of engine wear and damaged oil caused by the extended factory oil change intervals leads to excessive, acrid and oily blowby vapors in the PCV and hence to large carbon buildups.
When EGR is active on a spark ignition gasoline, the throttle plate is open further for the same power output and fuel usage than when the EGR is off. This reduces intake manifold vacuum, which actually increases efficiency. Think of intake vacuum as a force working to slow the downward movement of the pistons during the intake cycle, as the suction creates a backwards force. That is one reason that diesel engines are more efficient than their gasoline counterparts, as diesels have no intake manifold vacuum.
Basically, all the energy of fuel combustion is liberated as either heat or mechanical energy. The normal energy balance is approximately 20% as mechanical and 80% as thermal, with the thermal energy divided about in half between exhaust gas heat and the heat of engine parts, coolant and oil. EGR also contributes to efficiency by cooling the combustion temperature, which results in less thermal heat lost through the piston and combustion chamber surfaces, making more of that combustion energy available as mechanical energy.
As to the reality of efficiency and mileage, EGR helps during warmup and local driving, but the efficiency loss shows during prolonged high speed driving. Spark ignition engines loose less mileage than diesel engines. Although manufacturers won’t release data about EGR vs mileage, empirical evidence points to about a 3% loss with diesels and about half that with gasoline engines. A small price to pay to virtually eliminate acid rain.
Diesel Engine EGR Systems
Since diesel motors do not have any significant intake vacuum, EGR must be pushed into the intake manifold. All diesel engines have a vacuum pump for the brake booster, and this vacuum is used to control the EGR system flow in engines from 1996 through 2003. The vacuum is regulated through a solenoid that gets a duty cycle signal from the Engine Control Unit (ECU).
With the introduction of the Pumpe Duse (PD) engine in 2004, vacuum is used to turn the flow on and off using a valve just before the EGR cooler, and the final EGR flow regulation is achieved using a combination of effects. The EGR valve is controlled electronically, and EGR flow at partial boost is assisted by tilting the throttle valve to create a low pressure area for the exhaust gases to enter. The next PD series of the BRM and its 2 liter BHW cousin got another vacuum controlled valve to control hot EGR versus cooled EGR to assist engine warmup.
Starting with the 2009 Common Rail (CR) diesel, the exhaust gas recirculation System used a high pressure EGR system like the early BEW PD system with a throttle valve and an electronic EGR valve, plus a second low pressure EGR system with a filter that put additional EGR gases into the turbo inlet, allowing EGR under high load conditions. This secondary low pressure EGR system included a slide valve in the exhaust to raise the pressure inside the EGR to help control flow.
As the focus of an EGR system is to control NOx, a discussion of catalytic converters is essential to the concept. Two-way oxidation catalytic converters with platinum and palladium for carbon monoxide (CO) and hydrocarbons (HC) were originally installed on US vehicles starting in 1975. Two-way cats had no provision to control NOx. In 1981, the law required three-way catalytic converters which also had rhodium to control NOx.
For rhodium to do its job, a rich/lean cycle is needed. When lean, excess oxygen is captured by the platinum and palladium to convert carbon monoxide (CO) and hydrocarbons into water and carbon dioxide (CO²). Then during the rich cycle when minimal oxygen was available, the rhodium uses the CO², or CO, or H² to liberate and capture the oxygen from NOx to form N², which is the normal form of nitrogen found as ~80% of air.
Diesel exhaust has a typical high level of oxygen (O²) which precludes the use of rhodium to remove the oxygen from NOx. In diesels built before 2008, there really is no way to accomplish the rich/lean cycle with diesel injection to lower the oxygen levels, so those diesels can only use a two-way oxidation catalytic converter. Diesels have their own version aptly called the Diesel Oxidation Catalyst (DOC) which includes aluminum with the platinum and palladium of the original two-way catalytic converter.
As of the 2008 model year, which corresponds to the introduction of the Common Rail diesel injection system (CR), all diesels were required to have a Diesel Particulate Filter (DPF) in addition to the oxidation catalyst plus two new converters for NOx and SOx (oxides of sulfur).
NOx is controlled by an additional converter called an SCR converter, meaning Selective Catalytic Reduction. Using urea in a liquid called AdBlue, the NOx is reduced to nitrogen and water. The generic term for this mixture is Diesel Exhaust Fluid, or DEF. FYI to customers who want to shop for cheap DEF instead of Factory AdBlue version of DEF, make really sure the label states that it passed the ISO 22241 standard. Be aware it could be diluted, contaminated or degraded from age.
Another converter is used in CR diesels for SOx reduction. The biggest reduction in sulfur occurred in two stages when EPA mandates caused sulfur to be removed from diesel fuel. Diesel fuel from last century used to contain up to 5000 parts per million (ppm) of sulfur, primarily as a lubricant. Bunker fuel for ships could be as high as 50,000 ppm sulfur. Near the end of the 1990’s, highway use diesel changed to 500 ppm sulfur, and many diesel fuel pumps started leaking from the change. In 2006, the EPA mandated ultra-low sulfur diesel (ULSD) which lowered the amount of sulfur to 15 ppm. Again, another rash of leaking fuel pumps was the price for cleaner air. The SOx converter is the final solution.
Both the NOx and SOx converters experience an occasional complicated regeneration cycle involving extremely rapid and accurate mixture changes. These converters both appear to be trouble free for the long term.