I am designing a gasoline vaporizer that will run a stream of air thru liquid gasoline then inject the the mixture into the intake manifold of a 6 cylinder 98 nissan maxima. this will be in addition the gasoline injected via the EFIs. i hope to be able to run a lean mixture perhaps up to a air fuel ratio of 20:1. i have been told to be careful about melting the pistons because  the lean operating mixture can reach very high temperatures.  i am a retired chemical engineer without any automotive experience. all feedback will be gtreatly appreciated.

I suspect that designing the fuel delivery system will be the least of your worries.  An AFR of 20:1 equates to a Lambda of about 1.4, and that's coming close to the lean ignitability limit of gasoline.  No amount of vaporization or fancy pre-treatment will help your cause.

The part and cause of melting pistons is poorly understood by most.  The adiabatic flame temperature is a property of the fuel.  It peaks slightly rich of stoichiometric at around lambda 1.1 for most hydrocarbon fuels and falls off on either side of this peak.  The laminar flame velocity also falls off on either side of a peak near this region of AFR.  The bit that causes melted pistons is that lean mixtures extend the combustion duration due to the slow flame speed, and therefore the mean cycle temperature rises.  Peak flame temperatures for most hydrocarbons could be in the region of 2500 K and doesn't cause any damage to pistons, but it takes far lower mean cycle temperatures to damage them, usually first from lubrication failure before outright melting.

The way in which Diesel- and lean-burn gasoline engines designed operation run lean without incident is by stratification (spatially heterogeneous distribution of AFR) of the charge.  That is, the global AFR in the combustion chamber is appreciably greater than stoichiometric, but there are that has AFR near stoichiometric for robust combustion to initiate.  Aside from designs like the Honda CVCC of the late 70s and 80s, the most common way to achieve charge stratification is by direct injection of fuel into the combustion chamber with late timing close to the spark plug firing.  This may require special spray geometry or air motion to ensure an ignitable mixture is present near the spark plug at the time of ignition.

When you do get to running mixtures that lean in a homogeneous, port mixture preparation, ignitability will be a major problem, usually further requiring a higher-energy ignition system(filter).

An idea off the wall here is that gasoline is a oil and depending on your geographical location there are many diffrent blends of gasoline diffrent times of the year hot locations have a more oily fuel and cold locations have a "dryer" fuel. With hot weather fuel being more oily then cold weather fuel we can assume that not 100% is able to evaporate as fast as it is injected and burned on a day to day changing temperature unless this is a perfect world with perfect blended gasoline. By the addition of water into the combustion process at correct ammounts. If it was "fogged" into the intake stream before fuel is added allowes the fuel to float on the water droplets giving a larger surface area for evaporation and for the fuel to burn off the surface of the water particles. The extra heat from a lean burn would be used to make steam giving a higher peak pressure after TDC and controling detionation. I know with diesel applications this has been used alot wiht great sucess in pro stock tractor pulling, tho these engines have turbochargers and the water is also used to control EGT so as not to damage turbos and EX valves.

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