Exhaust gas recirculation (EGR) is used to control emissions in every automotive engine built since the Clean Air Act of the
1970s. Those early EGR systems were only marginally successful at reducing tailpipe emissions, but they were very successful
at reducing driveability. Even when they worked correctly, they often were disabled to satisfy customer complaints about surging
and stalling.
After 30 years, EGR systems are more effective, completely transparent to the driver and rarely fail. But when there's a light
on the dashboard and EGR codes appear on the scan tool, understanding the strategies behind them can help pinpoint the real
problem.
When a mixture of gasoline and air is compressed and burned, some of the oxygen and nitrogen in the air charge combine chemically
to form nitric oxide (NO) or nitrogen dioxide (NO2), which are abbreviated together as NOx. In the environment, NOx contributes to the formation of ground-level ozone, and when mixed with water in the atmosphere, it becomes nitric acid,
which falls to the ground as acid rain. NOx is truly nasty stuff, and tailpipe emissions of NOx are regulated by law.
When working on the new 6.4L PowerStroke, stay away from the slab-sided EGR intercooler: It's hot.
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To lower those emissions, techs can reduce the amount of NOx generated by the engine or use a catalytic converter to break the NOx molecules apart into nitrogen and oxygen. Both techniques are needed to reach the regulated limit, but engine-out NOx emissions must be as low as possible.
In a spark-ignition engine, in-cylinder NOx formation increases with temperature, combustion pressure and excess air in the air/fuel mixture. NOx formation could be eliminated with an air/fuel mixture that is rich enough to ensure all of the oxygen is used in combustion,
but a rich mixture increases fuel consumption and dramatically increases emissions of hydrocarbon (HC) and carbon monoxide
(CO). Peak combustion pressure can be limited by retarding the ignition timing, especially when the engine is under a heavy
load. But retarding the ignition timing over the whole speed/load range increases CO and HC emissions and reduces power output,
too. Also, compression ratios have increased in recent years in the drive for increased engine efficiency and specific power
output. This means combustion pressures are higher at all speeds and loads.
The last option for reducing in-cylinder NOx formation is to reduce combustion temperature, which is not hard to do in a spark-ignition gasoline engine. By recirculating
exhaust gas back into the combustion chamber, the rate of combustion is slowed, reducing its peak temperature. Up to 15 percent
of the cylinder volume can be filled with exhaust gas to achieve the desired effect. If done properly, exhaust gas recirculation
can also reduce fuel consumption, especially under low-load cruise conditions that naturally produce less NOx. There are two ways to achieve EGR: internally and externally.
Internal EGR
Internal EGR occurs naturally when the intake and exhaust valves both are open at the same time, a condition known as valve
overlap. As the piston rises on the exhaust stroke, a strong flow of exhaust gas is established through the exhaust port.
If the intake valve opens just before the piston stops rising, even though intake manifold pressure is below atmospheric,
the gas flow out of the cylinder is strong enough to pull fresh air from the intake manifold through the combustion chamber.
This is known as scavenging, because it helps evacuate exhaust gases from the cylinder, but it's only effective at the higher
exhaust gas flow rates that come with higher engine speeds.
When the intake valve opens early at lower speeds, there's less flow velocity, so the rising piston has time to push some
of the exhaust gas into the intake port. That exhaust is sucked back into the cylinder during the intake stroke, providing
internal EGR.
