Recycling center: Making sense of EGR

Exhaust gas recirculation (EGR) is a topic that I always cover in any engine seminar I deliver to the agricultural community. After identifying what the three letters represent, I ask the farmer participants at the workshop these two questions:

1. Why put hot exhaust gas back into the cylinder?

2. How can hot exhaust gas cool the cylinder?

Undeniably there are not many hands going up anxious to provide the answers. The truth be known, the room is usually eerily quiet until I say, “OK, I will explain it to you.”

Understanding EGR

The theory of introducing exhaust gas into the cylinder during the intake stroke was first brought to the market by Buick in 1972.

The federal government was cracking down on an emission identified as oxides of nitrogen (NOx), and by 1973 almost every gas engine in a production vehicle was equipped with EGR.

In the early 1980s, EGR was fitted to light-duty diesel engines found in pick-up trucks. In just the last number of years, the practice of recycling exhaust is now found on most diesel farm engines.

With ever stricter emission standards, it appears that EGR is here to stay, at least for now.

How hot makes cold

To understand EGR, you need to have a basic knowledge of how NOx is formed during the combustion event in either a gas or diesel engine. This byproduct of combustion is responsible for photochemical smog.

During combustion, when the leading edge of the flame exceeds 2,500 F degrees, oxides of nitrogen are produced at an exponential rate.

Three components are required to create NOx in the cylinder. They are pressure, heat (above 2,500 F degrees), and exposure time, or how long it takes for the flame to expand across the bore.

Once the flame reaches the far side of the cylinder bore, it goes out since there is no more fuel and oxygen to support it.

As with any equation, manipulation of the components can be employed to either enhance the production of NOx or decrease it.

Since cylinder pressure equates to engine power, little can be done there. The burn speed of the flame can be improved by designs that support in-cylinder mixture motion (that is why most if not all diesel engines are now turbocharged). Still, beyond that, not much more can be done to control NOx. Yet an answer needed to be found.

Gasoline has a much quicker burn speed than diesel fuel. By its nature, the cylinder pressure during the combustion event does not remain as high as the piston moves toward bottom dead center as with diesel.

The more consistent cylinder pressure from diesel fuel is what is responsible for the engine’s torque bias, but the slow flame speed limits engine operating speed.

NOx production is highest when the mixture is lean and then again under extreme load; the output resembles an inverted bell curve.

It was discovered that if inert exhaust gas (will not combust) is introduced to the cylinder, it takes the place of the combustible mixture.

The exhaust being used as filler, creates a lower flame temperature, keeping the leading edge below the critical 2,500 F degrees.

In simple terms, the exhaust gas introduced via the EGR valve works as stuffing, so a lower volume of the combustible mixture comes into the bore.

A properly designed and functioning EGR system does not impact engine power, no matter what the guys in the coffee shop say!

Cooling it down

The modern EGR-fitted diesel engine is usually equipped with a cooler that is a small heat exchanger with anti-freeze in it. The goal is to pull heat from the exhaust gas and make it denser and at a lower temperature before it enters the cylinder. In most, if not all applications, the EGR cooler employs a dedicated coolant supply and is not the same fluid that goes through the radiator and into the engine.

Keep this in mind when servicing the system. Due to manufacturer-specific designs, always reference the shop manual to determine if the engine(s) in your equipment employ a cooled EGR system and if the coolant is a dedicated supply.

Problems caused by EGR

The engine needs the means to control when and how much EGR is introduced to the cylinder. In the older systems, this was accomplished with a simple pintle valve, while the newest designs incorporate electronics and solenoids for more finite control.

When diagnosing an engine, it must be recognized that the valve needs to stop exhaust flow to the cylinder under certain conditions. If this does not occur, then the mixture will be diluted with inert gas causing stalling, rough running, a lack of power, or all these conditions.

It is widespread for the pintle to build carbon deposits, especially with Tier 3 and Tier 4 engines that employ cooled EGR. When this happens, the valve leaks internally, never shutting off flow, much like a failed washer in a sink faucet.

Another issue is the plugging of the EGR feed passages with carbon. In early EGR applications, you would probably never know this happened since the engine would run fine. Newer diesel and gas engines monitor EGR flow. If it becomes reduced, the engine controller is programmed to cause it to run very poorly to force the operator to get the problem fixed. Due to this strategy, the EGR gets blamed for a calibration protocol.

Simple steps to keep the EGR system on your diesel engines trouble-free are the following.

If the engine is responding as if it has coolant entering the combustion chamber (white smoke), keep in mind that an EGR cooler can fail and create that symptom.

As designed, the EGR system on your engines can be trouble-free and help all of God’s creation breathe a little easier.

Alphabet soup

Many farmers are rightfully confused with the varied approaches taken by the agricultural and truck manufacturing industry in controlling NOx emissions.

This is a simplistic look at different theories:

EGR only: These engines entirely rely on EGR to control NOx emissions. In some instances, the industry refers to these designs as extreme EGR. All the control takes place in the cylinder with the dilution of the charge with inert exhaust gas.

SCR: Selective catalytic reduction uses a device akin to a catalytic converter on a gasoline engine in conjunction with the introduction of diesel exhaust fluid (DEF) atomized with an injector in the exhaust. The chemical reaction of the DEF, which is a formula of urea and distilled water, neutralizes the NOx emissions to an acceptable level.

EGR + SCR: Some, if not all, Tier IV Final (strictest current standard) engines employ both EGR and SCR. The system is rooted in the theory that with both technologies working in tandem, an excellent running and efficient engine can be produced while meeting the necessary emission standards.

When considering any late model used heavy-duty truck or agricultural equipment, be mindful of the system employed for NOx control. As the engine gets older, it will determine the type of service it will require, and the cost involved.