Lean Burn -vs- Cylinder Drop
 

Just brainstorming here. I'm about to move my '89 BMW 325i into the modern era with a Megasquirt 3/Expanded, which, amongst many other things, will allow me to run sequential fuel injection and wasted-spark (or COP) ignition. I also noticed while I was setting up the initial tuning, that there are options to drop random cylinders per event (i.e. remove fuel pulse from 2 cylinders per revolution), as well as dual-mapping (i.e. one "normal" fuel map, hot-switchable with another map tuned to whatever AFR I choose).

Assuming no cost difference between the two, and ignoring the detrimental effects of a lean-burning engine on a catalytic converter (because this is a "theoretical car" that I drive 40 miles a day in with no catalytic converters), What's the general consensus on Lean Burn vs Dropping random cylinders?

and... I just noticed that I've never posted here before (????)

Hey guys! I'm Jeremy. I fix (and build) BMW's for a living. I should say, for the sake of "full disclosure" that I drive fast and hard. I'm not a hypermiler by any means, but if I can make 500 horsepower and still get 25-30 MPG, I'm a happy guy. If I can get it to 50 mpg, I'll be a rich guy. ;)

Welcome. My understanding is that the cylinder cut out thing gains milage by reducing pumping losses. It makes the displacement of the engine smaller, therefore the throttle has to open more, thus your pumping loss goes down. Since pumping losses are not a huge loss to begin with, you are not likely to realize a very dramatic gain.

If you can physically deactivate the intake and exhaust valves on the cylinders to be cut out, then cutting out cylinders is viable. Otherwise, stick to just doing lean burn.

we cut cylinder activation by removing fuel pulse, so if we're removing 2 cylinders out of 6, we gain 33% economy (excluding the pumping loss of compressing the cut cylinders) right off the bat, plus the lower pumping loss of the "more open" throttle body.

In theory yes, but in practice no, you wouldn't see a 33% improvement but cutting only fuel to 2 cylinders (and not closing keeping both valves shut). The amount of energy required to pump air into the "dead" cylinders, compress the air, and then pump it back out through the exhaust negates a lot of the gain. You may still see a small improvement, but the biggest bang-for-your-buck would be lean burn.

When factory engines de-activate cylinders, not only do they cut fuel, the also close both the intake and exhaust valves creating an 'air spring' inside the cylinder. It takes energy to compress the air during the upstroke of the piston, but some of that is regained on the down stroke. This is the most efficient way to do it, and even in these applications, cutting 2 cylinders from a V8 does not result in 25% more fuel economy, because there are energy losses in the system.

I KNEW THIS WAS COMING!!! :eek: :rolleyes:

...but I'm gonna be super nice and refrain from posting what I really think.

Nice?

How the hell is that nice?

Stop teasing us Frank and tell us what you think! :D

Okay... Who are you? And what have you done with Frank?

Yes, like piston and ring friction losses and so on.

If you check out engines that have used cylinder cutout, the economy gain probably less than 10%. And this is for a V-8 switching to four cylinders on the highway.

I don't typically follow this closely, because the gains are not as large as lean burn, which I have in the Insight.

The lean burn gain is more like 20% or so.

Jim.