What is an Exhaust Gas Recirculation Valve?

One example of a modern computer-controlled exhaust gas recirculation valve.

As their name suggests, exhaust recirculation valves aka EGR valves, are today mostly computer-controlled valves that allow for some exhaust gas to be re-circulated into the air intake systems of vehicles that are equipped with EGR systems.

In the case of light vehicles, EGR systems have a long history as an exhaust emissions control measure, with their first iterations coming into use as long ago as 1977. However, these early implementations of EGR systems had little to no effect on overall exhaust emissions on the light vehicles of the time and as a result, EGR was not considered an effective or breakthrough technology.

Nonetheless, it was only after the adoption of OBD II systems in 1996/97, which was wholly aimed at improving engine management functions as a measure to meet increasingly stringent emissions regulations that EGR systems began to produce measurable reductions of NOx (oxides of nitrogen) in light vehicles’ exhaust emissions.

In simple terms, the theory of exhaust gas recirculation stated that by redirecting small amounts of exhaust gas into the cylinders under some operating conditions, the exhaust gas would have a quenching effect on the temperature of the combustion process. The theory also stated that since nitrogen and oxygen combine to form toxic NOx (oxides of nitrogen) at temperatures above 1350 deg Celsius, it should be relatively easy to introduce sufficient quantities of exhaust gas into engines’ cylinders to prevent combustion temperatures from reaching the critical 1350 deg C threshold.

While this theory was good, the practice was somewhat less so. The problem was this: the engines of the late 1990s were several orders of magnitude less developed, efficient, and refined than the engines we have today, but worse, improvements in EGR control strategies made the engines from the late 1990s even less efficient.

Limited space precludes a comprehensive discussion of the problem in technical terms, but suffice it to say that the Euro 1 and 2 emissions regulations were so lax that many car manufacturers made little or no attempts to refine and develop their engines to meet the (then) generous emissions regulations. Instead, many manufacturers resorted to de-tuning their engines to develop less power as a means to reducing fewer emissions.

However, the practice of de-tuning engines to reduce their power output and emissions, and then adding exhaust gas to the air/fuel mixture through an EGR system had several unexpected and extremely undesirable consequences. These included among others, reduced fuel economy, severe engine oil contamination, rapid engine oil dilution, and accelerated rates of engine wear, all of which produced highly elevated levels of hydrocarbon emissions and reduced engine durability.

The problem was that adding exhaust gas to the air/fuel mixture in a de-tuned engine did not merely reduce the combustion temperature as intended; the exhaust gas effectively smothered the engine, which greatly reduced the efficiency of the combustion process, which in turn, often produced more emissions than the Euro 3 regulations allowed.  

The only viable solution to this problem was to develop a new generation of engines that were more efficient than their predecessors were, and in some ways, this process was greatly speeded up by the introduction of the Euro 4 emissions regulations in 2006.

As a practical matter, the vastly more stringent Euro 4 regulations could only be satisfied by increasing engine efficiencies. This required, among other improvements, increasing both static and dynamic compression ratios, increasing cylinder sealing by reducing dynamic clearances between moving/sliding parts, reducing pumping losses by using "thinner" engine oil and improving the flow dynamics of air intake systems.

The process of improving the overall efficiency of light vehicle engines continues today. However, from an emissions control perspective, the greatest single advantage of more efficient engines is that modern engines can now tolerate relatively large amounts of exhaust gas in air/fuel mixtures. This is almost entirely due to the fact that the combustion process in a modern engine is (as a rule of thumb) at least ten times more efficient than it is in an engine that was made 20 years ago.

So if we look at modern EGR valves and their function against the background described above, the complexity they add to modern engine management systems is largely offset by their efficacy in preventing the formation of toxic and harmful oxides of nitrogen in exhaust gas emitted by modern engines. Therefore, unlike EGR systems of twenty or thirty years ago, modern EGR valves are efficient and effective primary emission control components that improve the efficiency of modern engines, at the same time.

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