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Now that we have this wonderful Technical forum, I have to ask something that's been on my mind: What is really gained from lean burn, a la VX, Insight MT, etc?

In theory, leaning out the mixture just reduces the energy content of the intake charge. This results in less fuel burned for a given throttle position and RPM but also decreases the amount of power you get. That just causes a driver to get deeper into the throttle to suck in more air and thus bring in the fuel to move the car down the road. My best guess is that the real gain comes from forcing an open-throttle condition which would reduce pumping losses at low load vs. a very closed throttle. Is that it or is something else going on?

I think the point is that it leans out when you don't need the power. If you are cruising down a highway at 55mph why are you going to go open throttle? You don't need much hp to keep a steady speed.

My Insight lean burn experiences is it activates while cruising on near level ground without the A/C cruising on the highway up to about 65mph. Faster than that and the load is too much to sustain lean burn. The only way to tell is looking at the instant mpg bar jump.

You hit on it when you started out saying "In theory, leaning out the mixture just reduces the energy content of the intake charge. This results in less fuel burned for a given throttle position and RPM".

I believe that's where all the gains are coming from. And the vehicle doesn't require any extra power to maintain speed (up to the limits of lean burn).

But don't miss the point that all of the Insight's engine technology allows it use lean burn:

10.8:1 compression ratio (and still uses regular!)
high swirl combustion chamber
indexed iridium spark plugs
narrow included valve angle that centralizes the stratified air-fuel charge around the spark plug for quick light-off and more complete combustion
wide -range O2 sensor
The Insight engine's ratio of gasoline to air can be as low as 22:1, as compared to the normal ratio of 14.7:1. When driving in lean burn, you feel a slight bog in power. Then when a purge cycle hits, you get a little boost in power.
You can accelerate while in lean burn, although it must be done slowly to stay in lean burn.
Another tidbit, a ScanGuage 1 doesn't know about lean burn. It will read 20-25 mpg low. I'm waiting for the fix before I pop for a ScanGuage 2.

Your question was about lean-burn, but there are a bunch of other things contributing to extreme fuel efficiency.
But that's for another time.

Lean burn allows for more efficient combustion, and a reduction in pumping and heat losses. Obviously more efficient combustion results in more torque, and a drop in the amount of fuel required to move at a given speed. Because more air is let into the cylinder, pumping losses decrease, and in order to use lean burn the charge must be combusted as evenly as possible, so hot spots on the cylinder walls don't lead to preignition, which reduces heat losses. New systems are supopsed to allow 25:1 a/f ratios and an increase in combustion efficiency of 20% compared to stoich.

It is my limited understanding that lean burn operation will increase the % of Fuel Burned ... at first you might think higher % of Fuel Burned will = more power .... more accurately it is more power but per Gallon of gas .... since less Fuel is used per Engine Cycel and Per Second than you have two competeing forces... 20% or so more fuel used per volume of fuel , but x% less fuel used per engine Cycle / second .... the net is that power per cycle and second goes down ... engine feels like you have less power... but FE goes up becuase more power per gallon is used.

This is the same thing used in some of the legit Hydrogen injection systems... they use Hydrogen Injection to more completely burn the fuel which will give a lean burn effect as hydrogen has high energy per mass but low energy per volume so you end up with less power per volume but more fuel used per volume.... Honda Went the route of Engine design instead of Hydrogen injection becuase it removes the problems of where to get Hydrogen and the storage of it and how to safely use it etc....

Some of the universities and colleges that build and compete in Hybrid competitions use Hydrogen Injection becuase it not only allows for the lean burn mode the Insight has ... but it also reduces NOx that the Insight Method actually increases the production of NOx... and Hydrogen Injection can also be used in a Desel Engine.... which pushes the Desel Engine Efficiency a little higher than it already is.

But to do Hydrogen Injection correctly is no small task.... and the lack of a Hydrogen infrastructure is also no small thing .... and the problem of asking customers to get two different kinds of fuel is no small issue either.

Check out the Wikapedia article on Lean burn (http://en.wikipedia.org/wiki/Lean_burn).

For people that have never tried MIMA, One can keep the car in lean burn and accelerate(MIMA), one can climb hills in lean burn. Keeping the engine under slight load, say 100-140 MPG, one can cruise at 55-65 MPH, and use MIMA to add power, or to regen to slow down, so the throttle can be held in the lean burn area to produce the base power, while MIMA and the IMA provide the accel and decel power. Of course the speed and terrain, the limited battery capacity, and the percentage of assist to regen one uses will determine if you can sustain this indefinately. Thats where the boost battery comes in. Apply a charge from the boost battery that meets the average IMA energy needed to balance the charge /discharge, and you can run indefinately (as long as the boost battery has charge).:)

I guess it's time to add MIMA to the glossary.

Manual IMA or Manual Integrated Motor Assist.