Lean burning engine for better MPG?

[SecondMonkey]

I know a little bit about heavy duty backup generators and some of them are set up to run extremely lean on natural gas for the best fuel economy. Instead of using the throttle to control speed, it's left at or near WOT and the air/fuel ratio is adjusted to control the engine.

Anyway...
In my Jeep (4.0L inline 6) I can easily adjust the mixture by modifying the input from the MAP sensor to fool the computer. What if I installed a knob that would let me lean it way out on the highway, and then I could simply switch back to normal for city driving. Would this work? Or am I going to burn up my valves and overheat my engine?

Anyone have any thoughts on this?

 

[chevrofreak]

Those generators almost sound like they have more in common with a diesel engine than a gasoline engine.

 

[Shannow]

When operating lean of stoichiometric the temperatures are lower than those at stoiciometric (think lean cutting torch versus spot on).

Running very lean, you will have better economy, lower temperatures, and an oxidising exhaust.

Best fuel economy (power produced per unit of fuel consumption occurs at around 18:1 Air to fuel (stoichiometric around 14.7:1)....however you have to be able to burn the air fuel mix at such lean conditions...i.e in theory, go about 20% lean.

With any engine, there will be air and fuel flow distribution issues between individual cylinders, meaning that you can only stretch things as far as the worst cylinder, and there's maybe a 10-15% variance between cylinders.

In such a case, Jeep have (probably) tuned the engine to their worst cylinder, and you may only have a percent or two to go...but there's still somewhere to go.

Simplest might be to trick the O2 sensor.

 

[Shannow]

There was a bloke down here (Dave Bennett) who was working on improving distribution, heat flow etc on SBC engines running propane.

He had vehicles fitted with 350s getting 25MPG US on propane, and running excess oxygen in their exhaust due to complete (lean) combustion.

 

[punisher]

Comparing CNG and gasoline engine combustion is tough. In a gasoline engine the heat transfer to the chamber/piston goes up when the mix is leaned due to reduction of the boundry layer on the combustion surfaces. Eventhough the total heat from a lean gasoline mix is less, the amount of heat the engine sees is increased.

NG is a different beastie. Going rich with NG increases heat. Those NG generators run happy leaned out whereas a gasoline engine starts to eat pistons/exhaust valves. The increased NOx emissions won't make the EPA happy either.

 

[Trav]

Running too lean on gasoline increases combustion chamber temps.This can cause lean misfire,pre-ignition,burned piston heads and valves, high NOx readings due to increased chamber temps.Same effect as a vacuum leak.Not good.

 

[Shaman]

Do a search for EIFEs. They play with the O2 sensor. You could also try spraying some water/meth to get the mix lean and safe.

 

[Thermo1223]

Heavy Duty Backup was my first clue as to those generators being diesel in operation which is why they can run soooo lean.

Your Jeep isn't a diesel and therefore cannot safely.

Those generators are overbuilt whereas any production car is built just enough. Now granted there are some shining examples of being built more than that but few.

 

[mechtech2]

The generator is designed to run at a near constant speed.
It really isn't like a direct injection gas or diesel engine.

Anyway, the O2 sensor has the the final say-so in a street car.
Unless you go way to the limit , it will automatically compensate for any tinkering.
The O2 sensor signal is the one to modify, but it is tricky.
The engine will run awkwardly when you get lean - misfires and stumbling will occur.

 

[[RT] ProjUltraZ]

running rich lowers combustion temps, high hp cars are better off south of 14.7:1 to ensure it doesn't lean out too much at WOT and cause dangerous conditions

modern cars can program this with a multitude of factors, like inj duty cycle. most stock cars are programmed pig rich for safety. i had my PCM reprogrammed and i think it's around 13ish:1 at WOT

 

[Shannow]

There's a big difference in leaning out when you are coming back from a rich condition under full load, and extreme lean for fuel economy.

Coming from the rich end (cool), it will get hotter as you approach stoichiometric, then once you get reasonably lean come back down in temp.

Same reason that as you fuel up diesels they get hot, they are running way lean, then you start to bring them up to stoichiometric.

 

[wapacz]

Originally Posted By: Trav
Running too lean on gasoline increases combustion chamber temps.This can cause lean misfire,pre-ignition,burned piston heads and valves, high NOx readings due to increased chamber temps.Same effect as a vacuum leak.Not good.


Its my understanding the higher NOx is not from the extra heat but from the fact you are not using all the oxygen and such there is oxygen left over to react with the nitrogen.

 

[FrozenPilot]

Running a lean engine can only increase combustion temperature to a point, eventually you will run 'lean of peak' and temps decline. Maximum power is made with a richer mixture than you would get at peak cylinder temps, therefore, it takes a significant leaning to get the engine to operate below peak temp, and thus lower peak power. You'll end up needing more throttle while lean to get the same power output you would have running near max power. This isn't anything new. Pilots in piston-engined planes have been talking about this for years. Its somewhat frowned upon if you cannot accurately monitor EGT's as cylinders often don't peak at the same mixture setting.

If you've read any dyno sheets with AFR data, you'd see how the modern engines enrichen the mixture for max power. I believe they could run leaner during closed-loop, but as mentioned before, NOx production would be significant. Some people have been successful in leaning their engines and increasing fuel econ (one such saturn owner has over 50mpg by running heated intake air and an adjustable IAT probe). I'd love to see an emissions report from that car!

 

[TylerL]

Originally Posted By: FrozenPilot
If you've read any dyno sheets with AFR data, you'd see how the modern engines enrichen the mixture for max power. I believe they could run leaner during closed-loop, but as mentioned before, NOx production would be significant. Some people have been successful in leaning their engines and increasing fuel econ (one such saturn owner has over 50mpg by running heated intake air and an adjustable IAT probe). I'd love to see an emissions report from that car!

50 mpg Saturn
What Saturn owners will do to get 50mpg is amazing.

 

[Trav]

[/quote]Its my understanding the higher NOx is not from the extra heat but from the fact you are not using all the oxygen and such there is oxygen left over to react with the nitrogen.[/quote]
High NOx readings are directly related to high combustion chamber temps.This can come from EGR issues,lean mixture,advanced timing,vacuum leaks etc.Anything higher than 14.7-1 means more air and therefore more oxygen is mixed causing dramatic temp increases.Running rich is fine for WOT but undesirable under normal conditions because it not only produces high HC but washes the cylinder walls down with fuel causing excessive wear and contaminating the oil.Todays ECM's do a great job of keeping this balancing act under control.Deceiving it with fixed values always has a trade off somewhere.

 

[mechtech2]

Frozen Pilot is right. Too lean will misfire and run poorly, not get insanely hot. Of course richer is cooler than 14.7/1, but that doesn't mean leaner keeps getting hotter and hotter.

 

[Trav]

I strongly disagree.Too lean a mixture can cause burned valves and burnt piston heads.
Here is just 1 of hundreds supporting this.
http://smogcarsuck.tripod.com/id38.html

 

[Shannow]

Just because you strongly disagree doesn't mean that you are correct.

Note, highest temperatures are at stoichiometric. Anything richer or leaner is cooler.

Interesting curve with nitromethane containing it's own oxidiser.

 

[Trav]

So please explain why vacuum leaks cause burned valves.

 

[edhackett]

Simple chemistry. A lean mixture has more oxygen left after the fuel is combusted. This extra oxygen at combustion temperatures oxidizes(burns) the metal of the valves.

Ed