how is lean fuel ratio good for FE ?
 

hi

i have read on this forum that most fuel saving gadgets that claim to improve completeness of combustion are BS because the combustion is already 97% efficient. therefore, how does using a lean fuel mixture make better fuel efficiency?

i'm thinking of going for a wide band lambda sensor mainly because my current one is broken and needs replacing anyway.

modmonster -

My best understanding is that lean fuel mixtures lead to higher NOx emissions.

Going lean under high-load (performance) driving conditions is bad because it can burn your valves and/or melt your pistons. I call this the Christmas Story Mom retort :

Ralphie: I want my go-cart to run lean!
Mom: You'll burn your valves out!

Under low-load (hypermiling) driving techniques, you can drive lean. This means consuming less fuel. The problem with this is that NOx emissions are increased. Your catalytic converter is *designed* to process exhaust at the 14.7 stoichiometric ratio because that is the sweet-spot for clean emissions and healthy engine performance.

The Fifth generation (1992-1995) Honda Civic VX hatchback and the Honda Insight both have "lean-burn" modes under freeway low-load steady-state driving conditions.

My car has a narrow-band sensor. If I was starting from scratch, I would probably invest in a wide-band sensor/fuel controller (0-5 volt) with enough "support gizmos" to report narrow-band (0-1 volt) signals to my ECU/PCM. Here is an example :

A'PEXi U.S.A. - AFC-neo
http://www.apexi-usa.com/graphics/products/2603.jpg
It's very $$, but it's like getting a Scanguage and a fuel controller all in one.

CarloSW2

thats cool. but how is it like a scan gauge? what data does it show and does it need obd2?

The main problem with lean burn is overheating. Perhaps water injection can be used to counter that? To check temperatures, could a cheap thermocouple be installed in a hole carefully drilled in the exhaust manifold?

Water injection IS used to counteract overheating in lean conditions... see turbo/Nitrous motors.

Yes, you can use a thermal sensor to check exhaust temps at each cylinder... but that's 4 more gauges to watch, and averaging the temps won't help you, b/c engines are mass produced - no two combustion chambers are the same.

There are things you can do to make it easier to run lean though, if you're rebuilding your engine... one of which is cleaning the cast marks and sharp edges off the piston faces. Fuel/air mixtures prefer a clean (not shiny, but clean) piston face. Same w/ the combustion chamber... you can clean the combustion chamber/valve faces up if you ever do a head gasket... do not attempt to clean the pistons unless you can take them out of the block though.

Oh - OP question...

Lean ratios basically burn less fuel for the same engine speed... often lowering engine power output as well, meaning higher throttle angles, meaning less VE losses.

It's a two-prong gain - Less pumping losses will net you better efficiency, while leaner ratios means you're burning less fuel to go just as fast. (provided you're still making enough power to maintain your speed.)

Honda and several other MFG's have designed GDI engines that will run at a/f ratios of ~60:1... that said, I don't think they're produced en masse as yet, for several "great" reasons, I'm sure.

*GDI - Gasoline Direct Injection... think diesel, but with spark.

Christ -

Couldn't you put one high-temp sensor in the exhaust? The idea would be to monitor the combined engine exhaust temp under "normal" operating conditions, and then monitor it under lean burn.

If yes, then I think you could use something like this to keep your engine safe (installed using weld-in bung adapter?!?!?) :

(Jaycar Electronics) High Range Adjustable Temperature Switch with LCD

CarloSW2

Yes and no... you'd have to test very carefully as some engines really DON'T like lean burn conditions.

Also, the average temp, like I said, won't give you an idea of whether or not one cylinder is in danger, which is quite the possibility, due to manufacturing differences. You could technically be at a lean mixture that is best for one cylinder, and deadly for another one, and you wouldn't know that based on a single temp sensor/switch.

If you were testing very carefully, You could use a single switch/sensor to kick on an additional injector or something that would add water/meth to cool down the intake charge, but you'd have to be doing testing in small increments, as you wouldn't get real time data from each cylinder.

Using either way, you still won't be able to detect hot spots, which are a primary concern when considering a/f ratios. (and in general)

I am planning on doing this to my car sometime, and I will add a multi firing spark plug module (MSD) so that I can ensure that the flame completley burns.

Other than that, you have to be sure the air and fuel mix THOROUGHLY (aka homogeneous) otherwise you start developing knock.

Christ -

Thanks. That agrees with something I read recently on saturnfans. A fellow was driving 90 MPH with 93 octane, and he burned a hole in his piston :

Picture of Hole in Piston

This is a response he got from a wise old Saturn Fan :

SaturnFans Forums - View Single Post - What would cause a hole in the piston?

CarloSW2