How Does Exhaust Gas Recirculation Work on a Diesel Engine?

closeup powerful diesel engine

What Is Exhaust Gas Recirculation?

Exhaust Gas Recirculation (EGR) is used in diesel engines to reduce harmful emissions of nitrogen oxides (NOx), specifically, nitric oxide, nitrogen dioxide and nitrous oxide. These greenhouse gases contribute to the breakdown of the Earth’s ozone layer and add to increasing levels of unhealthy smog in high-density areas.

Government legislation requires engine emissions of these gases to be below prescribed amounts to meet minimum safety and health standards. As a result, vehicle manufacturers apply exhaust gas recirculation and related technologies to meet these standards.

How Is NOx Formed in Diesel Engines?

NOx is formed mainly at higher engine temperatures, above 1,300C or 2,370F, when nitrogen oxidizes into these harmful gases. Diesel engines use compression to create the substantial temperatures needed to ignite their fuel, and these temperatures can quickly reach the levels required to generate NOx.

Left unchecked, harmful levels of NOx are emitted when nitrogen present in the cylinder air is heated to elevated temperatures on ignition and expelled via the exhaust.

How Does EGR Reduce Levels of NOx?

Exhaust gas recirculation reroutes some of the exhaust gas from the previous ignition cycle back into the engine cylinder. The rerouted gas lowers the amount of combustible air available for ignition and the overall temperature at combustion. The previously ignited exhaust gas has less oxygen and, so, less potential to reach higher temperatures, having been burnt already. As a result, combustion occurs at lower temperatures, and less NOx is created.

Exhaust gas recirculation has dramatically improved since its introduction in 1974 on larger gasoline engines, and it is avalible for smaller commercially available diesel engines in 2010. Originally, processes simply rerouted exhaust gas directly into the cylinder through crude orifice jets—which is basically a small hole that allowed the gas to pass through. These simple solutions resulted in poor engine performance, loss of efficiency, and difficulty in starting because exhaust gases were introduced immediately when the engine was running instead of when needed.

Today’s modern procedures regulate the intake of exhaust gas into the cylinder to ensure it happens at optimized times when the temperature is higher and the maximum impact on NOx emissions achieved. The improved process also provides much better engine performance, giving it maximum power when needed and improving the performance vs. cleanliness balance. Used correctly, EGR enhances fuel and combustion efficiency and knock tolerance, and it reduces the need for fuel enrichment. They all benefit in the elimination of NOx.

isolated engine white

Are There Any Downsides?

Even with modern technology, exhaust gas recirculation has its downsides. These mainly relate to the amount of power the engine produces and its fuel economy.

At lower temperatures with less air in the cylinders, combustion is not as dramatic. As a result, the engine loses some power and some fuel remains unburned, exiting through the exhaust as particulate matter—mainly carbon.

Regulations exist for the amount of particulate matter that can be emitted, and solutions for this need to be discovered, too. Manufacturers remove excess particulate matter using exhaust filters. However, exhaust filters also impact fuel efficiency and engine performance, so the overall solution is far from straightforward.

The right balance between performance, efficiency, and cleanliness is the constant struggle that modern technology is looking to solve. However, today, most engines—both diesel and gas—still need exhaust gas recirculation and its supporting technologies to meet legal emission standards.

Are There Alternatives to EGR?

There are alternatives to exhaust gas recirculation that focus on removing harmful gases from the exhaust outflow. They are expensive and not used in smaller commercially available vehicles.

The most common is called selective catalytic reduction—a process that injects ammonia and urea into the exhaust outflow. These two chemicals react with the NOx gases over a catalyst, converting them into nitrogen and water. Similar processes accomplish the same thing without the catalyst or use other chemicals like sodium hydroxide or hydrogen peroxide to scrub the NOx gases out of the exhaust flow safely.

hand diesel pump gas station


The pursuit of clean, efficient, and high-performing diesel engines continues to advance successful technology today. As a result, diesel engines have shaken off their bad Earth-polluting reputation. They are often greener than gas-powered motors, with the same or better levels of performance.

Diesel engines, in fact, require higher temperatures for operation than gas engines do, and the potential for NOx emissions is more elevated in them than in gas engines. However, advances in diesel engine technology, including exhaust gas recirculation and its sister technologies, combined with other factors like diesel fuels’ higher energy density and proper maintenance and aftercare, have made diesel a better and cleaner choice than gas. Explore the possibilities today!

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