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EGR Valve 101

EGR valve

In the early 1970s, as pollution became a big concern and the oil crisis began, exhaust gas recirculation (EGR) systems began to enter the automotive space. Nearly all engines today have an EGR system and many use an EGR valve as their primary component.

The EGR system works by simply diverting a portion of the exhaust gasses back into the manifold where it's injected into the active (firing) cylinder with the air:fuel mix. This re-burns the exhaust, cleaning some of the emissions gasses in the process. Primarily, an EGR system is meant to lower nitrogen oxides (NOx) emissions from the tailpipe. Decades of research have shown that NOx gas is the top contributor to smog and similar exhaust-driven air pollution problems.

EGR is a common item found in most combustion engines today. Diesel engines use it to reduce the amount of soot (hydrocarbon) emissions in their exhaust, using it as a pre-treatment before an after-treatment (chemical binding or scrub) is used before exhaust leaves the tailpipe.

Why Recirculate Exhaust?

There are two things at work in the EGR system to clean emissions. First, it re-burns some of the exhaust gases, as we've already mentioned. Second, and more importantly, is the cooling effect that the gas going into the intake has.

This cooling effect is the primary reason EGR works so well, despite only re-using a small portion of the exhaust gases from the engine. It's a trick that comes to us from racing technology.

The air:fuel mix will include a portion of that air being exhaust gases instead of ambient air. Usually no more than 10 percent, often more in the 4-6 percent range. That isn't much, but it causes the burn in the cylinder to be slightly slower and thus a little bit cooler. Cooler burning means less power output lost to heat and less heat management is required for the engine block. It also means that the burn temperature is below 1500F, which is the point at which nitrogen and oxygen can combine to form NOx gases.

In most modern engines, any performance lost to the recirculating exhaust in the air mix is regained by the fuel efficiency and temperature management gains made.

How Do Most EGR Systems Work?

The EGR valve is basically a controlled leak of exhaust gases into the intake manifold. The valve operates through either vacuum or electronic control. Nearly all newer systems (10 years and newer) are electronic and far more reliable.

During idle and low engine load, the EGR valve will be closed or very near to it. The position of the valve's entry into the intake will be at a vacuum point where air is moving towards the valves so that the positive side of the exhaust, where the valve draws from, finishes a circuit that forces the exhaust being pulled into the EGR to be shoved in with the air intake system. This is important to understand because most EGR valve problems are not the valve itself, but a loss in that vacuum and positive pressure. Most often due to leaks.

Most vehicles increase the amount of exhaust (open the EGR) more as the load on the engine increases. In modern engines, this is electronically controlled through RPM measurement. In older engines, it's controlled via engine vacuum.

Types of EGR Valve

There are six general types of EGR valve. Four of these are in common use today. We'll start with the older valves that are now generally defunct.

Ported EGR Valve - These were among the first in use and were used until the mid-1980s. These use a vacuum diaphram and tapered stem flow control valve. They were generally unreliable and often became non-functional due to carbon buildup.

Digital Electronic EGR - Appearing as the first electronically-controlled EGR, these were introduced in the mid-1980s and were used through the 1990s. These were a sort of mashup between a ported valve and digital valve, using vacuum to open and close the valve, but with a modicum of control through the engine's computer. They were not very reliable by today's standards.

Positive Backpressure EGR - These were among the first to appear on the market and are still used in some applications today, though they've largely been relegated to only small engines with natural aspiration. These use exhaust backpressure to open the valve and to control the flow rate of exhaust into the intake. They are simple and usually very reliable, but not exceptionally efficient.

Negative Backpressure EGR - Similar to a PB-EGR, the negative works in the opposite way, using the exhaust's pressure as the regulator for opening and closing the valve. The lower the exhaust pressure, the more the valve closes. Like the PB-EGR, the NB-EGR is still in use on some small engine applications.

Pulse-width Modulated Electronic EGR - These appeared in the mid-1980s and were intiially used by General Motors as a replacement for ported valves. Using a control solenoid which is itself controlled by the engine's computer. The pulse-width in the name comes from the solenoid's ability to cycle on and off rapidly, with the "on off" time determining how open the valve is. This controls flow not only to the intake, but to individual cylinders if need be. This type of EGR is commonly used today.

Linear Electronic EGR - Like the Pulse-width EGR, the linear has a stepped control of the valve. In this case, it controls it through a stepped opening process that opens and closes in stages rather than all at once. Each step is a larger or smaller opening. This is the most commonly used EGR today as it's completely independent of engine vacuum and is fully electronically controlled.