The EGR system of a heavy-duty vehicle makes a big difference in its performance and safety. In this guide, discover how these systems work. Then, learn how to make your truck's EGR work for you.
What Is An EGR System?
EGRs are pollution-reduction systems in diesel vehicles. Specifically, they reduce the amount of nitrogen oxides, a byproduct of diesel combustion, that are released into the air.
Nitrogen oxides are particularly dangerous pollutants: they damage the human respiratory system, cause smog and acid rain, and are potent greenhouse gases contributing to climate change.
As Motor Trend senior editor Jason Gonderman explains:
“When Tier 2 emissions standards went into effect in 2004, diesel engine manufacturers were tasked with finding a way to lower nitrogen oxide (NOx) output. Their solution was found in the use of exhaust gas recirculation (EGR).”
“The basic function of an EGR system is to route a portion of spent exhaust gases back into the engine's intake tract. This oxygen-deprived air is used to limit peak in-cylinder combustion temperatures, which in turn lowers the engine's NOx production.”
Today, EGRs are standard systems in all trucks and heavy-duty vehicles. They're also mandatory components to comply with EPA standards.
How Does An EGR System Work?
ERGs have been at least partially effective. Emissions have decreased by 82% since 2005, with 3 million fewer tons of NOx released. However, this isn’t quite enough of a reduction to reach goals for human and environmental health. So, in recent diesel vehicles, the EGR works in tandem with three other parts / systems:
The Diesel Particulate Filter (DPF)
The Selective Catalytic Reduction (SCR) system
The Diesel Oxidation Catalyst (DOC)
These pollution-reduction systems are controlled by the Engine Control Unit (ECU). An ECU is also called an Engine Control Module (ECM) or a Powertrain Control Module (PCM).
Regardless of the acronym used, the unit is a monitoring and regulating device composed of a computerized “brain” and sensors.
To learn more about the role DPFs and DOCs play in anti-pollution systems, read, “Diesel Oxidation Catalysts & Diesel Particulate Filters.”
To troubleshoot potential problems with these parts, check out our quick guides by automotive pros: “Common Diesel Particulate Filter Problems and What to Do About Them,” and “5 Reasons for Diesel Oxidation Catalyst Failure.”
Basic Components of an EGR System
A truck’s EGR system is composed of six parts: the EGR valve, the intake throttle valve, an EGR cooler, a turbocharger, and an actuation turbine.
Optionally, the system might also include a cooler bypass, secondary valves and turbines, intercoolers, and an exhaust throttle. These parts are integrated with the ECU and its sensors.
EGR Valve
The EGR valve directs byproduct gases, collected in the exhaust manifold from each of the engine's cylinders, into the EGR system.
Older systems used analog, pneumatic EGR valves, which directed air using a mechanical pressure-response mechanism. While inexpensive and streamlined, these valves are prone to leaking or getting stuck in the “open” position, making them less than ideal.
Vacuum-controlled EGR valves are a step up. These semi-automatic components utilize the vacuum pressure created by the intake stroke of the engine’s pistons, directing exhaust gases at regular, predetermined intervals. Intervals are determined either by the engine's speed and load, or by solenoid valves.
These EGR valves are less likely to leak or get stuck than pneumatic valves. But they are prone to clogs, as their shape makes it easy for soot to build up inside. They're also less precise than electronically controlled valves.
Electronically-controlled valves are the most widely used valves in EGR systems today. Their integration with an ECU allows for more precise, responsive valve positioning and direction. The precision allows for a design that reduces soot buildup without reducing its effectiveness, resulting in a component that requires far less frequent cleaning than previous iterations.
Intake Throttle Valve
Electronic EGR valves are typically combined with intake valves further along the circuit in the system. Intake valves create vacuum pressure by locking out external air from the system. This helps boost the force pushing the gases through the system.
EGR Cooler
The EGR cooler is a reverse heat-exchanger. It’s a metal tube divided by plates into chambers, receiving hot exhaust gases in one chamber and coolant fluid in the other. The exhaust gases cool due to indirect exposure to the coolant in the neighboring chamber.
This increases the density of those gases. It also (somewhat) alters the relative composition and ratios of gaseous compounds within the exhaust gas as a whole. Both of these effects lower the gas's combustion temperature.
Thus, when the gas is recycled into the engine cylinders as fuel, the engine can burn that fuel in a way that generates fewer NOx byproducts. As a result, each cycle generates fewer NOx emissions than the one before it, while simultaneously increasing the lifespan of a gallon of fuel itself—directly increasing the truck's fuel efficiency.
Turbocharger & Actuation Turbine
The turbocharger and actuation turbine function as a unit. These ECU-controlled components speed the flow of air and gases through the ECR, activating a compressor to push more air into the engine's cylinders when needed. It helps mitigate the risk of sub-optimal engine performance in situations demanding high torque—an otherwise fairly common side effect of EGR processes. Fortunately, with these components, it's a side effect that heavy-duty trucks can avoid.
Optional: Intercoolers, Cooler Bypass Valves, & Exhaust Throttle Valves
The three optional components offer considerable benefits to the EGR systems of heavy-duty vehicles.
Intercooler
Intercoolers are essentially additional cooling chambers. They cool gases further into the EGR circuit (after bypassing the initial cooler), or they cool gases a second time—reducing them to an even lower temperature.
Cooler Bypass Valve
Cooler bypass valves are ECU-controlled components that let exhaust gas circulate directly into the engine, skippi