EGR question

[Gsir08]

So it seems like the trend is most all subarus that were 5 speeds had no EGR valve, and all the ones that had auto trans did have EGR. Does anybody know why this is? Is having an EGR valve on a manual transmission car not work out as well or something or did they just not need them on those?

[nipper]

More then you would ever want to know (for simple answer go to the bottom):

 

The exhaust gas, added to the fuel, oxygen, and combustion products, increases the specific heat capacity of the cylinder contents, which lowers the adiabatic flame temperature.

 

 

In a typical automotive spark-ignited (SI) engine, 5 to 15 percent of the exhaust gas is routed back to the intake as EGR. The maximum quantity is limited by the requirement of the mixture to sustain a contiguous flame front during the combustion event; excessive EGR in an SI engine can cause misfires and partial burns. Although EGR does measurably slow combustion, this can largely be compensated for by advancing spark timing. The impact of EGR on engine efficiency largely depends on the specific engine design, and sometimes leads to a compromise between efficiency and NOx emissions. A properly operating EGR can theoretically increase the efficiency of gasoline engines via several mechanisms:

• Reduced throttling losses. The addition of inert exhaust gas into the intake system means that for a given power output, the throttle plate must be opened further, resulting in increased inlet manifold pressure and reduced throttling losses.
  
• Reduced heat rejection. Lowered peak combustion temperatures not only reduces NOx formation, it also reduces the loss of thermal energy to combustion chamber surfaces, leaving more available for conversion to mechanical work during the expansion stroke.
  
• Reduced chemical dissociation. The lower peak temperatures result in more of the released energy remaining as sensible energy near TDC, rather than being bound up (early in the expansion stroke) in the dissociation of combustion products. This effect is minor compared to the first two.
  

It also decreases the efficiency of gasoline engines via at least one more mechanism:

• Reduced specific heat ratio. A lean intake charge has a higher specific heat ratio than an EGR mixture. A reduction of specific heat ratio reduces the amount of energy that can be extracted by the piston.
  

EGR is typically not employed at high loads because it would reduce peak power output. This is because it reduces the intake charge density. EGR is also omitted at idle (low-speed, zero load) because it would cause unstable combustion, resulting in rough idle. The EGR valve also cools the exhaust valves and makes them last far longer (a very important benefit under light cruise conditions)

"

When combustion temperatures exceed 2500 degree F., atmospheric nitrogen begins to react with oxygen during combustion. The result is various compounds called nitrogen oxides (NOX), which play a major role in urban air pollution. To reduce the formation of NOX, combustion temperatures must be kept below the NOX threshold. This is done by recirculating a small amount of exhaust through the "exhaust gas recirculation," or EGR. valve.

The EGR valve controls a small passageway between the intake and exhaust manifolds. When the valve opens, intake vacuum draws exhaust through the valve. This dilutes the incoming air/fuel mixture and has a quenching effect on combustion temperatures which keeps NOX within acceptable limits. As an added benefit, it also reduces the engine's octane requirements which lessens the danger of detonation (spark knock).

The EGR valve consists of a poppet valve and a vacuum diaphragm. When vacuum is applied to the EGR valve diaphragm, it pulls the valve open allowing exhaust to pass from the exhaust manifold into the intake manifold. Some engines have "positive backpressure" EGR valves, while others have "negative backpressure" EGR valves. Both types contain a second diaphragm that modulates the action of the valve. This prevents the valve from opening unless there is a certain level of exhaust backpressure in the system. EGR valves are calibrated for specific engine applications. The wrong valve may flow too much or not enough exhaust and cause emission, driveability and detonation problems.

EGR valves do not normally require maintenance or replacement for preventative maintenance. But the valve can become clogged with carbon deposits that cause it to stick or prevent it from closing properly. Dirty EGR valves can sometimes be cleaned, but replacement is necessary if the valve is defective.

Some newer engines are so clean from a NOX emissions standpoint that no EGR valve is required.

Ported Vacuum Switch

 

Another component that may be a part of your vehicle's EGR system is a "ported vacuum switch" (PVS), which may also be called a "thermal vacuum valve" or "temperature vacuum valve" (TVS). The switch controls the passage of vacuum that operates the EGR valve.

This device is a heat-sensitive switch that remains closed until the engine's coolant reaches a certain temperature. The PVS screws into the intake manifold, thermostat housing or engine so the heat-sensing element is in contact with the engine's coolant. Inside the switch is a wax plug that pushes a sliding plunger to uncover or block vacuum ports in the switch. As the engine heats up, the wax expands and pushes the plunger up until it uncovers or blocks the vacuum port. At this point, vacuum to the device that the switch controls is either applied or blocked. Severe engine overheating can damage the switch, making replacement necessary.

Vacuum Control Solenoid

 

A solenoid is another device that is often used to control vacuum to the EGR valve. A solenoid is a magnetic coil attached to a plunger that uncovers or blocks a vacuum port. The solenoid vacuum port may be normally open or closed depending on the application. When voltage is applied to the solenoid, the coil moves the plunger which either opens or blocks the vacuum port. Voltage to the solenoid may be routed through a relay or timer and is usually controlled by the engine computer on newer cars.

When used to control the vacuum to an EGR valve on a late model engine, the engine computer will wait to energize the solenoid until the engine is warm and is operating above a certain rpm.

 

http://www.asashop.org/autoinc/dec97/egr.htm - EXCELLENT article on how it works and to diagnose, should be a sticky!

 

Operation

 

Whether it is electronically or mechanically operated, the EGR valve introduces a regulated amount of exhaust gas into the fuel and air mixture, which is then burned by the engine in the combustion chambers. Contrary to conventional wisdom, the introduction of exhaust gases into the fuel-air mixture actually has a cooling effect, which improves overall combustion, resulting in more complete burning of fuel. In this way the EGR valve assists in lowering levels of noxious gases within the exhaust (In other words, the automatic never gets a break, The manual has constant throttle opening and closing, except for at cruise, and the EGR is not used then. An automatic always has throttle application, and needs the cooling effect of the EGR-nipper)

[Gsir08]

Do ALL 5 speed outbacks, up to 99 atleast, have NO EGR?

And basically the reason being because in a MT car you'd have no optimal time to introduce exhaust gasses.

Edited March 14, 2010 by Gsir08

[hankosolder2]

I can tell you with certainty that many of the 2.5L manual vehicles had EGR, including my '98 Outback 5MT.

 

I suspect that the reason Subaru didn't equip some 5MT vehicles with EGR is that the the 5mt vehicles could pass the EPA cert procedure without it (due to lower curb weight, lack of torque converter slippage or different gearing- if the 5MT vehicles produce under the spec of grams NOx /mile in the test procedure w/o EGR, there's no reason for them to install it.)

[Gsir08]

Your 98 2.5 MT HAS an EGR valve?

Edited March 14, 2010 by Gsir08

[987687]

Don't all the 2.5s have EGR?

I thought it was just the 2.2s that didn't have EGR on the manual versions.

[Gsir08]

that could be... 2.5 is likely to produce much more emissions.

[hankosolder2]

Your 98 2.5 MT HAS an EGR valve?

 

Yes, it does.

[86subaru]

my 2000 a/t has no egr and it's a 2.5

[987687]

huh ... I wonder if there is a list somewhere that has this info

[Gsir08]

my 2000 a/t has no egr and it's a 2.5

 

Thats cause its a 2000, I think they did out with all them completely after like 99. not sure though

[Gsir08]

Anyone around here have a 1997 Legacy with a 2.2L and a 5 speed in it? I'm pretty sure the 97 2.2 5 speeds didnt have EGR and that would be the ECU I need.

 

I need to find out if any 97 2.2's had no egr

Edited March 15, 2010 by Gsir08