Exhaust question!

[DIY 1985]

It just seems that instead of filling combustion chambers with a proper mix of fresh air and fuel, above idle and below WOT, that a warm motor benefits from adding essentially inert gas ('greenhouse' gasses, or gasses with a higher Specific Heat) to fill combustion chambers.  It is not like you can add greater amounts of fresh air to fill combustion chambers without a damaging lean condition resulting, or, you forfeit torque if you go the route of a just reducing engine rpm by shifting into a higher gear.  EGR helps fill combustion chambers when combustion chambers don't have to deliver their full power potential because they are not under heavy load.  Because the gases that are the product of combustion can hold a lot of heat without as much of a rise in temperature as would fresh air, you can keep spark plugs, exhaust valves, and the overall engine from damaging thermal spikes, not because we are quenching the flame, but because with only a moderate demand of fuel, there is still sufficient O2 for combustion, and the system in 'closed loop' governs it.  Otherwise, we'd just try to run leaner when we were not at WOT or idle because this isn't Los Angeles, and on the trail, it is about having enough gas to get back home, and who cares about smog anyway.

 

I'll use the oxyacetylene cutting torch as an analogy.  First, you light the acetylene, you get an orange flame that produces carbon deposits in the striker, there is suit everywhere, and it isn't hot enough to melt steel; it is akin to a motor's rich condition, wasteful of fuel.  Then, you balance the flame with pure oxygen, it is a balanced combustion process that can turn steel cherry red and is capable of just surface melting it.  However, the actual cutting action only results when you push the oxygen-only lever, displacing combustion-product, aka 'greenhouse' gasses with a jet of pure O2.  In our motors, yes, we want the expansion of air and fuel as it undergoes complete combustion, but with too much fresh air oxygen and nitrogen, it can potentially become thermally destructive, plus a glowing plug is a possible source of pre-ignition.

 

Once again, exhaust gas recycling reduces the consumption of fresh air and fuel at a given rpm, and, according to the article, unlike earlier Detroit hemi motors, EGR in a two-plug firing mode NAPS-Z, EGR allows more work to be performed (mileage) with a given amount of fresh air and fuel.

 

That said, I too smell horseshit regarding the article's specific figures of performance, or design.

 

And, in the era post O2 sensors and air/fuel solenoids, exhaust content is a matter of electronic control and EGR shouldn't have an effect when the system has reached 'closed loop,' and the system is running properly?

[datzenmike]

In the NAPS system weather EGR is present or not the air fuel ratio remains the same... roughly 14.7 to one.  EGR simply displaces some of it. If 20% is EGR then the other 80% is still 14.7 to one. The EGR does not add any oxygen because it is already combined with the carbon in the fuel to make CO_2. Chemically, exhaust is inert and just takes up space in the combustion chamber.

 

Not sure what the cutting torch has to do with this but... The added oxygen in a cutting torch combines with the carbon and iron in the steel being cut and burns them as fuel. This rapid oxidation produces huge amounts of heat.

[Lockleaf]

Yes fully functional egr will improve gas mileage slightly. Since it works the most at low load cruising, when power is not demanded, it reduces the amount of fuel used by the motor by filling space with dead air instead.

 

Its design is much more about emissions than economy in design and upon its removal from MULTIPLE 720s, i have never noticed any significant changes required in my carb tuning, nor any significant losses mpg. So while mathematically, some change in AFR could perhaps be shown, the reality has never really played out to matter.

[datzenmike]

If there are any gains it is mostly the lessening of emissions. Mileage may be better but splitting hairs here.  If EGR is used, the throttle will be open farther than needed to get the same power as without. So the engine doesn't need to suck as hard through the carb throttle plate to draw the fuel and air in because it's open more.  With reduced combustion temperatures there is less pre ignition so timing can be increased or poorer octane fuels used or compression increased. Just remember there is little or no EGR at full throttle so this applies more to highway driving.

[DIY 1985]

Since both the vacuum spark advance and EGR sides of the Thermal Vacuum Valve close at 131 degrees F in 1985, it is reasonable to consider how the two work in conjunction; we may have already suggested it regarding our discussion of timing.  However, only the MPG model of 720 in that year incorporated a knock sensor that could tell the Detonation Control Unit to make the distributor retard as much as 10 crankshaft degrees, so it is reasonable that an EGR delete wouldn't be complete by Nissan engineers without a correspondingly different distributor.  Us 'shade tree mechanics' can't really retard timing beyond changing static, or initial, timing.  But, if you could reduce vacuum advance timing accordingly, I would use that as a proper measure of reduction in MPG by the EGR delete.

 

The Hayne's manual describes burnt spark plugs as a result of over-advanced timing, a lean mix, or a plug with the incorrect heat range.  In tenth grade chemistry, the Ideal Gas Law proved that you could get more pressure and/or volume out of a gas body without an increase in temperature if you increase its mass (i.e. running EGR).  It takes more heat energy to get exhaust to the same measurable temperature as a corresponding body (pressure and volume) of fresh air.  So when you are just cruising, the body of burnt gasses in the cylinder is great in volume relative to its pressure, and because the amount of throttle (which is supplying a 14.7 to 1 mix) is not correspondingly high, temperature will be, in a non-EGR situation.

[datzenmike]

The Detonation Control was purely for reduction of timing in response to detonation detected by the knock sensor and has nothing to do with EGR. (although EGR may reduce detonation in some cases) This was a 9 to 1 compression 2 liter engine running on cheap gas that only came with a 5 speed and a 3.364 differential. An almost lethal combination for detonation under load.

 

EGR was required on all engines so there was never an EGR delete nor a corresponding distributor change by Nissan engineers.

 

 

The thermal vacuum valve is an air bleed to the two vacuum sources one for advance and one for EGR. When the air bleed is open the vacuum signal is destroyed. When closed the vacuum signal is allowed past.

 

The TVV for vacuum advance is open but closes to pass vacuum at 59-140F but opens again above 140F. So full vacuum advance is allowed from 59 to 140 F only.

The EGR vacuum is open below 140 but closed above. EGR is only allowed above 140F.

 

 

 

I'm not seeing how adding EGR increases the mass of a cylinder's contents. It enters the engine at the same pressure as the fuel air but is warmer so less mass. EGR is just inert vapor taking up space and diluting the 14.7 to 1 fuel air mix without disturbing the volume or compression. With up to 15% to 20% of your fuel and air missing, you will get a reduction peak cylinder temperatures and pressures produced during combustion.

[DIY 1985]

The TVV functio for spark advance is actually open between 59 to 140F (95 to 131F for Z24 Non-Cali '85). An open TVV is an air bleed connected to the air cleaner, so full vacuum won't reach the distributor until the thermostat housing is above 140F (131F Z24 for non-Cali), or, below 59F (95F Z24 for non-Cali) which is when you first start the motor. Hence, a warmed up motor was designed to run both EGR and vacuum spark advance together, when of course the Back Pressure Transducer and the Vacuum Control Valve had their respective bleeds closed as well.

[datzenmike]

Yup, I had it right and then changed it around. :blush: It's now corrected.

[DIY 1985]

Thinking outside of the box, back to the 'inert' property, again.  Well, EGR is inert, but fresh air is mostly inert too, containing only 21% oxidizer.  So the cutting torch is akin to the concept of enriched fresh air, or mixing fresh air with more oxygen.  If you were to use pure oxygen in your carb, you would quickly have a molten motor.  Here is a company that actually uses cheap petrol instead of expensive acetylene: http://www.petrogen.com/petrogen-innovation.html#savings

Gasoline burnt that way is over 5,000 degrees F.  So you may see that inert gas present in the combustion chamber is not really a benefit, it is absolutely essential, thermally.

 

So based on the factory manual for my rig, I believe that full vacuum advance and EGR are synchronous operations.  I know that people modify the centrifugal advance in distributors, but is there a way to back off the vacuum advance?  My Weber DGV only came with a single ported vacuum source making it impossible to run EGR and vacuum advance off of different TTV air bleeds.  In fact, the instructions from Redline describe merely disabling the EGR by just removing the vacuum lines and capping.

[datzenmike]

The cutting torch gasses are mixed to get a blue flame (most complete efficient burning) that is just enough to form a small pool of melted metal. The added blast of oxygen 'burns' the carbon and the iron in the steel oxidizing it and releasing the extra heat that melts it. The steel provides the extra fuel to melt itself. You can do this experiment. Hold a lighter to some steel wool till it lights up and then blow on it. It will burst into flame and melt to a blob of slag.

 

 

Engines mix gas and air in the correct mix (like the cutting torch) but adding outside air or oxygen to a combustion chamber would just lean out the mixture and it won't burn. If the inside surfaces were hot enough, they're not or gas would auto ignite, it might burn off any carbon.

 

Only way I can see to lower the vacuum advance operation (less advance) is to add a spring to the diaphragm so it takes more vacuum to move it. California vacuum lines often have a restriction that slows the signal but doesn't reduce the advance.

 

EGR can easily be deactivated by removing the hose to it or the BPT valve.