THE ABCs of EGR

Balance
For example, lowering the compression ratio and retarding ignition timing on a high-performance engine will lower NOx emissions, but also reduce peak horsepower and engine performance. That actually happened to the vaunted powerplants propelling the famous musclecars of the late Sixties and by 1972 they had considerably less muscle.


On the other hand, while leaning-out the fuel mixture and raising the coolant temperature will lower carbon monoxide emissions and improve fuel economy on most engines, driveability may suffer and higher levels of NOx emissions will be produced. The "fuel crisis" in the mid-Seventies placed added importance on the issues of emissions and economy.

Control
The EGR system became the front running candidate for the control of NOx emissions, due to its flexibility in applications and low cost. Early examples of this system were basically just added on to the engine designs existing at the time. With a few exemptions, notably the Honda CVCC design, almost all engines in the later Seventies used this system. However, while NOx emissions were reduced, the vehicle's driveability often suffered. Disconnecting the system and driving the vehicle without it easily confirmed this fact.


As time passed and engineers integrated the EGR system into upgraded or newer engine designs, this drawback was eliminated. Catalytic converter technology improved as well, as new designs now reduced NOx emissions (earlier designs only lowered hydrocarbon and carbon monoxide emissions). As computer-controlled fuel and ignition systems appeared and improved, disconnecting the system actually had an opposite effect on driveability, in contrast to early EGR systems.


On the other hand, on very efficient engine designs with hyper-accurate fuel and ignition management systems, an EGR system was often not even necessary! If your vehicle's engine is one of these, then reducing its NOx emissions will involve other repairs not covered in the following article. But if your vehicle does have this system, which still appears on some new models, then a non-functional EGR system is probably the culprit. This would be especially true if the vehicle is more than five years old and/or has more than 75,000 miles on it.