Tracing the recent evolution of *exhaust gas recirculation *(EGR) systems helps technicians learn the operation and diagnosis of each system, an equipment designer said. Plus, more-sophisticated emission tests coming on line will force techs to upgrade their EGR know-how, said Nick Smith, president of Smithtronics Inc., a Fenton, Mich.-based manufacturer of specialized EGR test equipment.
The EGR system controls oxides of nitrogen (NOx) by directing a small percentage of the engine's exhaust gas back into its intake. The inert exhaust gas reduces part-throttle combustion temperatures, thereby limiting formation of NOx, a major component of photochemical smog.
Many techs never think twice about EGR unless a driveability problem exists. If the EGR valve opens too much or opens at the wrong time, the engine may stall, surge or idle roughly.
Some emission programs already measure NOx levels while others, such as California's, require a functional test of the EGR system during every emission inspection.
However, coming emission programs-specifically, the new federal IM240 test-will measure NOx directly. When a vehicle flunks due to excessive NOx levels, the EGR system will be the prime suspect because it exerts the most influence on NOx readings, Mr. Smith explained.
General Motors Corp. and Ford Motor Co. combined have the largest number of vehicles on the road. Arguably, these two makes also share the most sophisticated EGR systems on the market.
Mr. Smith, who also teaches automotive diagnosis, said many techs can learn EGR quicker by mastering Ford and GM systems first. Then, for the sake of simplicity, they should think of other EGR setups as derivatives of those used by these two automakers.
Mr. Smith simplifies system evolution into the following families.
The most basic family of EGR systems uses a vacuum-operated EGR valve controlled by ported vacuum and a thermal vacuum switch (TVS). Ported vacuum means the valve only operates above idle speed. Opening the EGR valve at idle will either stall the engine or create an extremely rough idle. The TVS prevents the valve from opening when the engine is cold, stalling the engine.
In the next EGR family, a computer-controlled solenoid vacuum valve replaces the TVS.
the engine is warmed, the computer opens the solenoid valve, allowing vacuum to operate the valve in the
Replacing the simple on/off, open/closed solenoid vacuum valve with a pulsed solenoid valve was the next refinement. Electrically speaking, a fuel injector also is a pulsed solenoid valve-except it regulates fuel flow instead of vacuum. Just as the computer regulates fuel flow precisely by pulsing injectors on and off hundreds of times per second, it began regulating EGR control vacuum more accurately by rapidly pulsing the solenoid vacuum valve on and off. The end result was more accurate control of EGR valve openings and closings.
Pulse width, which is usually measured in milliseconds, is the actual length of time a solenoid valve-injector or EGR solenoid-is turned on. Because the computer regulates these solenoids by varying their pulse width, they are often called PWM (pulse width-modulated) solenoids.
The vast majority of Ford and GM EGR solenoids that technicians encounter today are the PWM type, Mr. Smith noted.
Meanwhile, duty cycle or duty ratio is the ratio of solenoid on time to solenoid off time. A 50-percent ratio means the solenoid is on half the time, off half the time.
With a PWM solenoid-equipped EGR system, the EGR valve pintle might be halfway open when the solenoid duty cycle is 50 percent. A drawback was that although the computer was issuing EGR commands by controlling the EGR solenoid, it had no feedback mechanism to tell if the system was complying, Mr. Smith noted.
The ensuing refinements improved EGR system feedback to the computer. First, a vacuum-activated signal switch was mounted in series between the solenoid valve and the EGR valve. When vacuum flowed from the solenoid valve to the EGR valve, it closed the electrical switch, signaling the computer that vacuum had reached the EGR valve.
Then the vacuum signal switch was discarded and a position sensor was mounted directly on the EGR valve to monitor EGR valve activity directly. This position sensor is a potentiometer (variable resistor) that works just like the familiar throttle position sensor (TPS). The computer sends the EGR position sensor a 5.0-volt reference voltage and the voltage signal going back to the computer varies according to EGR valve movement.
Now the computer had two reference points for EGR operation. For example, at a solenoid vacuum valve duty ratio of 50 percent, the EGR valve should be halfway open and the position sensor at mid-range, sending the computer a 2.5-volt signal.
At this point in EGR system evolution, GM designers also relocated the solenoid vacuum valve onto the EGR valve. However, Ford still keeps the solenoid vacuum valve separate from the EGR valve assembly.
Many EGR systems match EGR flow to engine load by sensing exhaust backpressure. The backpressure regulates a vacuum valve which, in turn, regulates vacuum flow to the EGR valve itself.
According to Mr. Smith, many popular Fords have replaced this traditional setup with an electronic backpressure sensor. This sensor (called a PFE or DPFE) is the prime computer input determining solenoid vacuum valve duty ratio.
The two latest GM EGR designs use no vacuum whatsoever, Mr. Smith said. On one system, the EGR valve assembly contains three individual, solenoid-operated valves. Each valve is a different size. The computer-controlled solenoids are either on or off, meaning each ``mini-valve'' is either full open or completely closed.
The computer tailors EGR flow by opening these valves in as many as seven different combinations, he said.
The newest EGR valve design contains a single, PWM valve and an integral position sensor. The computer regulates EGR flow by pulsing the valve on and off. When the EGR valve's duty ratio is 50 percent, the valve pintle should be halfway open and its position sensor signal should be mid-range or 2.5 volts.
In conclusion, the components within newer EGR systems may look unfamiliar. But because the terminology and principles of operation are more familiar than unfamiliar, learning these systems won't be as difficult as some techs think, Mr. Smith said.