EGR question

[misterbeets]

Hello,

If I have gasoline engine at part throttle, and open the EGR valve, can I assume, holding throttle constant, a rise in manifold absolute pressure?

Does this nonetheless increase the total mass of gas entering the cylinders? while reducing mass of fresh air?

Thanks in advance..

 

[ivymike]

first question seems like the answer would be yes

second question maybe, depending on the density of the (hot) egr gas

third question - yes, based on my answer to your first question (proportion of fresh air goes down, obviously, and if MAP goes up with throttle constant, fresh air mass flow goes down)

 

[misterbeets]

Thanks for the response. Good point about the EGR gas temp. So let's assume the throttle valve is opened enough to restore the original air mass, and torque. Now, with the addition of exhaust gas, the pressure, and so the temperature, inside the cylinder is even greater at the top of the compression stroke. This would tend to raise the combustion temperature, not lower it, for an identical mass of reactants. Yet it is lowered. Any explanation?

P.S. Yes, I failed my emissions test with high NOx. Carbureted car too.

 

[BrianPetersen]

The temperature might be higher at the end of the compression stroke, but when the dilution effect of the extra mass in the cylinder is taken into account (with the energy from combustion being the SAME) the peak temperature during and after combustion, which is when NOx is formed, will be lower.

Lean air/fuel ratio will increase NOx.

Have you done any diagnostics yet? If you give make/model/year, perhaps someone will know the specifics of the vehicle in question. It's guaranteed to be an old vehicle, because carbs disappeared from cars quite a while ago. Usually the EGR system on a carb'ed engine is "open loop" based on a vacuum signal from a designated vacuum port on the carb, with a solenoid valve and thermosensor that switches EGR off (by cutting off and venting the vacuum signal) below a certain coolant temperature.

Common problems: EGR valve itself clogged with soot from the exhaust, EGR valve stuck closed, leaky diaphragm in the EGR actuator, vacuum leaks or blocked vacuum hoses, bad thermosensor or bad EGR solenoid valve. Tampering is not out of the question, either ...

 

[Bribyk]

I don't know if they check for this but you can get your NOx down by retarding the timing a bit; which'll lower your peak temps. You may run into HC trouble though.

 

[Lou Scannon]

All conventional EGR systems I've looked at have the EGR valve located downstream of the throttle. The exhaust gas is an inert diluent and has only a secondary effect on power. The effects of adding EGR while holding power constant will be: the manifold pressure will rise, as will the charge temperature, throttle may have to open slightly to flow the same amount, due to the reduced vacuum; indicated power may go down slightly due to slower combustion, but this will be offset by the reduced pumping loss and can be further offset or even negated with increased spark advance, so thermal efficiency will normally increase with a well calibrated EGR strategy versus non-EGR. This applies to part throttle. At WOT EGR will displace combustible mixture, reducing power. EGR is typically not used at WOT on spark ignited passenger vehicles.

To answer the OP's specific questions, manifold pressure will rise, total mass entering cylinders will rise, fresh air mass will go down slightly due to reduced vacuum. This will cause a drop in power unless throttle is opened and spark timing is advanced as explained above.

Assuming your vehicle is equipped with a 3-way catalyst, far more likely reasons for high NOx are marginal or failing closed loop control, or a failing catalyst.

Retarding timing will actually reduce engine out HC along with NOx. The reason is, with retarded combustion, the effective expansion ratio is lower, which results in higher temperatures late in the combustion cycle; allowing more time for HC to be oxidized; in addition, the higher exhaust temperature provides better oxidation in the exhaust system.

 

[Bribyk]

I don't know too many closed-loop carburetors...

 

[misterbeets]

So the combustion heat is spread out over more mass, and the temperature is lowered as a result. That answers my question.

This is an 88 Accord BTW. It's a feedback carb, set rich internally, with small amounts of air introduced under ECU control downstream of the throttle. EGR is controlled by vacuum, with overrides.

Have not started troubleshooting yet, but will check EGR, then mixture. Thanks again for the replies.

 

[Lou Scannon]

Carburetors with closed loop trim were used from the early '80's to 1990 in certain applications.

 

[Bribyk]

My mistake...carbs are mostly before my time and I try to avoid them in general.

 

[Rob45]

When you don't know about something - carburetors for example - then don't make sweeping statements about them.

Closed-loop carbs were a cheap solution to a tough problem, and were used extensively.

 

[Bribyk]

I apologized. It was true though, I don't know too many closed loop carbs.