Some emissions systems seem to have some benefit when it comes to MPGs. Like circulating the gas tank fumes into the intake.
But what about things like the EGR system that recirculates the hot exhaust gases?


Asking from strictly a MPG perspective.

 

I think EGR is more for emission control than anything else, but I'm going to bet that the current crop of super-efficient engines is tuned to work with the EGR in such a way as to improve economy.

Interesting reading:
Exhaust gas recirculation - Wikipedia, the free encyclopedia

What else qualifies as an emissions control device on a modern car? Pretty much just the cat and O2 sensor, I guess. The cat doesn't really affect efficiency, as long as it's sized properly and not clogged or broken. The O2 sensor is a feedback device that is intended to keep mixture in a range that is safe for the cat... but, in doing so, it also ensures that the engine is burning the fuel as completely as possible, which results in greater fuel efficiency.

Can you tune a car to be really efficient without any of these things? Probably. I think you'd at least want an O2 sensor (preferably a wideband, so that you can tune "lean burn" under light load) to make it easy to accurately check mixture. But, you could surely do just fine without the EGR or cat.

 

On my truck, there is the EGR system, a system to suck fuel vapors out of the gas tank, and another system that appears to supplement the EGR system. (not 100% sure what it is called, but it does attach the exhaust manifold to the intake manifold - wondering if this is there to get the cat hotter, faster?)

I can see the benefit to getting the fuel vapors into the intake, as those are pretty volatile.
The system seems pretty complicated for what I think the ECU actually needs.
Since it is going to have to be after the MAF to keep from clogging up the MAF sensor, the ECU would have to know when those gases were present. So a pressure switch on the gas tank, and a valve the ECU could open to allow those fumes into the intake manifold. Seems simple enough to me. I would assume that as those fumes get burned, the O2 sensor would have to pick up that there was a difference, and that would tell the ECU to use less fuel at the injectors.

The EGR system seems to do more harm to the actual efficiency of it. It is displacing fresh air with exhaust fumes and while some of those fumes do burn, from what I can tell, they don't burn as well as fresh air does. NOx gases have less energy in them than the air they displace. And they say that the EGR system helps to cool cylinder temps. I know there is a breaking point but I would bet that most stock compression engines aren't anywhere near that threshold of actually needing to lower the cylinder temps. As I understand it, the higher the cylinder temps, the higher the pressure. the higher the pressure, the more power it can create. Am I missing something there? I know the engine's cooling system would stabilize the temps so they won't get a whole lot higher than they are when using the EGR system, but wouldn't a few degrees help?
Or is the EGR system one of those cases where you can have it or not have it and end up with the same result. "They" say that advancing your timing can help make up for the slower burn of the NOx gases. If that is true, then I guess the main question would be, do you gain more from being able to advance a couple of degrees more, or do you gain more by a "cleaner" air/fuel mix and hotter cylinder temps?


I''ll have to find out what that other system is called, and what it does exactly to be a bit clearer on its purpose.

I'm sure there is more under the hood since there are a big handful of vacuum lines that go everywhere and a good number of vacuum solenoids to move that vacuum around.

All of this spurred from a project i'm going to be undertaking soon with my Truck. It is a 2003 F150 supercrew. 4.6 liter. I'm getting pretty crap mileage as compared to what some others are getting in their same generation, same engine f-150s, and even the Explorers with the same engine that weigh significantly more. The real kicker is that I get the same MPG in city as I do on the highway. It really seems like it doesn't matter how I drive it, i get about the same MPG within .5 +/-
I've not been able to troubleshoot it thus far, so I was going to tear it apart, yank the intake manifold off and put it all back together, one piece at a time, cleaning, fixing, removing, re-routing, etc to try and Fix the problem and Squeeze whatever I can out of it MPG wise.

 

For the "how it works" discussion... you're probably on the wrong forum. That's pretty deep automotive engineering discussion there.

But, if your truck isn't getting at least reasonable highway mileage compared to similar trucks of the same vintage, there's probably something wrong. How many miles are on it? It's very possible that it's simply a bad O2 sensor. They can be far enough off to affect MPG without being far enough out of spec to throw a code. I'd start there. Also, silly little things like plug wires. A "bad" wire will cause misfires and throw codes, but a "degraded" wire could just give weak spark and poor combustion. Also spark plugs. Hell, you could even have a damaged or plugged cat.

You may have done some of that stuff already, I'm just thinking out loud. It's easy to overlook the simple things sometimes.

EGR passages are known to get plugged up, too. Carbon build-up in the head or on the pistons can cause problems. So, you're not on an unrealistic path. I'd just do the easy stuff first. Even do a good top-end cleaning with some SeaFoam.