Explaining the EGR (Exhaust Gas Recycle) Valve

Note:

Red text = Jochen Saal's questions
Black text = Lee Janssen's answers

Date: Mon, 06 Jul 1998 17:18:33 -0600
From: "Janssen, Lee K"
Subject: EGR

Jochen Saal Queries:

Can anybody explain the functionality of the E.G.R valve on the late TR6 Carb?

Quoting from Internal Combustion Engine Fundamentals: "Exhaust gas recycle (EGR) is the principal technique used for control of spark ignition (SI) NOx emissions. A fraction of the exhaust gases are recycled through a control valve from the exhaust to the engine intake system. The recycled exhaust gas is usually mixed with the fresh fuel-air mixture just below the throttle valve. EGR acts, at part load, as an additional diluent in the unburned gas mixture, thereby reducing the peak burned gas temperatures and NO formation rates. ...Substantial reductions in NO concentrations are achieved with 10 to 25 percent EGR. However, EGR also reduces the combustion rate which makes stable combustion more difficult to achieve."

From Norman Nocks' Tech Talk:

"This system is fitted to reduce emissions of oxides of nitrogen. The EGR valve is mounted into the exhaust manifold and controls flow of exhaust gases into the intake manifold. The control signal is taken from a throttle edge tapping which gives no recirculation at idle speed or full load, but gives a varying amount of recirculation between these two extremes depending on the vacuum signal and metering profile of the valve, A cut-out valve destroys the vacuum signal to the EGR valve when the choke is operated."

After I suddenly had idle problems I believe I found the problem in this valve.... the piston in the EGR does not fully close.

The valve should be completely closed when no vacuum is applied.

When should it be open and when closed?

The factory intended it to be open during partial throttle operations.

What happens if I simply close the hole with a plug? (I do not have to meet any emission regulation so why not simply remove it?)

If you were to remove it I believe you would join the majority of the late model TR6 owners with EGR. As Gernot points out you have to reset your mixture (make it leaner) after removing it. While you are at it disconnect the vacuum hose to the distributor, the vacuum retard was intended to reduce hydrocarbon emissions.

In '74 the engineers finally threw in that towel and gutted what little performance was still left in the engine to meet US environmental regulations. The anemic compression ratio, EGR and vacuum retard all bear witness to this fact. I should note that EGR should in theory decrease fuel consumption. Internal Combustion Engine Fundamentals states:

"The improvement in fuel consumption with increasing EGR is due to three factors: (1) reduced pumping work as EGR is increased at constant brake load (fuel and air flows remain almost constant; hence intake pressure increases); (2) reduced heat loss to the walls because the burned gas temperature is decreased significantly; and (3) a reduction in the degree of dissociation in the high temperature burned gases which allows more of the fuel's chemical energy to be converted to sensible energy near TC. the first two to these are comparable in magnitude and each is about twice as important as the third."

I actually only believe (1) because the 7.5 compression ratio is too low to generate much heat or disassociation.

[...]

Thanks
Lee Janssen

[Thank you, Lee! -ed]