EGR question
EGR question
(OP)
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..
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..





RE: EGR question
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)
RE: EGR question
P.S. Yes, I failed my emissions test with high NOx. Carbureted car too.
RE: EGR question
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 ...
RE: EGR question
RE: EGR question
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.
RE: EGR question
RE: EGR 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.
RE: EGR question
RE: EGR question
RE: EGR question
Closed-loop carbs were a cheap solution to a tough problem, and were used extensively.
RE: EGR question