melting pistons and lean fuel mixture
melting pistons and lean fuel mixture
(OP)
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. mustben





RE: melting pistons and lean fuel mixture
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.
RE: melting pistons and lean fuel mixture
Correction: a mixture rich of stoichiometric will have a lambda less than 1. This is the inverse of the relative fuel-air ratio phi used by some.
RE: melting pistons and lean fuel mixture
RE: melting pistons and lean fuel mixture
RE: melting pistons and lean fuel mixture
anyway, with a mixture as lean as you describe you may well be outside the range that would normally be the operating range of a three way catalyst (assuming there is one) and then problably the EFI system would try to compensate for the changed air/fuel ratio and try to get it again within the range the threeway catalyst is supposed to operate.
so, even if you succeed in injecting the extra air/fuel you would somehow seem to override the standard engine electronics.
RE: melting pistons and lean fuel mixture
RE: melting pistons and lean fuel mixture
At part throttle it's a different ballgame. Automobile gasoline engines run approximately stoichiometric as that is what the 3-way catalyst requires as feedgas to maintain low emissions. If you run a leaner recipe here emissions will skyrocket.
That's it in a nutshell.
RE: melting pistons and lean fuel mixture
For more in depht info on pre-ignition and detonation, see: http:/
"Burned pistons" are mentioned on page 6
RE: melting pistons and lean fuel mixture
I'm sorry to say it mustben, but the reasoning is flawed from the outset, specifically the statement that, "a combustion engine that runs on fuel in its vapor form uses much less fuel." A combustion engine does indeed run best when the fuel is ultimately burned as completely as possible. The ideal Otto efficiency is approached (but never reached) when heat addition approaches a constant-volume process, that is, in a practically instantaneous amount of time at firing TDC.
However, in a port-fuel injected engine, barring extremely bad design, the fuel is already completely evaporated by the time of spark ignition. I can't think of ANY modern, emissions-controlled implementation that has the mixture preparation so bad as to still have liquid fuel present during the beginning of combustion. Having so would be manifested by large quantities of soot emissions, which we know is not the case for a normally-operating SI engine.
The flawed thesis is the same case that's made by purveyors of "vapour carburetors" and their likenesses. Somehow, the fact is lost that lean combustion cannot be extended without limit, but, these people argue, is dependent only on how good the degree of fuel vapourisation is.
Granted, some improvement in fuel efficiency could be achieved by a reduction of pumping losses by some degree of dethrottling in a fully-vapourised, lean fuel-air charge. Some improvement in indicated efficiency could be achieved through a higher cycle-averaged specific heat ratio.
However, there will be efficiency losses due to the longer combustion duration of homogeneous-lean combustion that would actually cause you to deviate AWAY from a constant-volume heat addition that is defined by the Otto process. As noted above, detonation is a very real threat made more likely by lean mixtures.
RE: melting pistons and lean fuel mixture
RE: melting pistons and lean fuel mixture
Well, methane and hydrogen are pretty much the two endpoints on the spectrum when it comes to spark-ignited fuels, at least in terms of ignitabilty and flame speed. That drives vastly different combustion recipe designs.
RE: melting pistons and lean fuel mixture
You are trying to solve a problem that does not exist.
The inefficiencies in modern engines comes mostly from heat losses, pumping and friction losses to a lesser degree and incomplete combustion to a very small degree. Most of the incomplete combustion is deliberate to keep the catalytic converter hot so as to minimise emmissions.
Regards
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
RE: melting pistons and lean fuel mixture
Methane in natural gas also has a wider flammability range (lambda 2.1 lean limit) and higher stoichiometric AFR (17.2:1) than gasoline, and the laminar flame speed is slightly higher. It also enjoys a RON of 140.
RE: melting pistons and lean fuel mixture
History is full of inventions of vapor carburetors and the like. Plenty of those so-called 100 mpg carburetors were some form of vapor carburetors. If an engine doesn't have a spectacular efficiency improvement by running on propane (gaseous), there is no reason to suspect it will have a spectacular improvement by running on vaporized gasoline as opposed to liquid gasoline.
Plenty of modern engines rely on the cooling effect of the fuel as it evaporates to keep temperature down and control detonation. If you remove that by adding heat to vaporize the fuel, you may find the engine more prone to detonation, and may have to do something (e.g. retard the ignition timing) that is not in the interest of higher efficiency, in order to save the engine.
There are technologies out there that claim to be able to fire leaner mixtures (plasma-jet plugs and similar systems that give much, much higher ignition energy than standard) but the demand has not been there, because modern emission systems rely on stoichiometric air/fuel ratio. No point finding a way to fire a lean mixture if you can't use said lean mixture anyway because it won't pass emission certification.
RE: melting pistons and lean fuel mixture
http://www.cat.com/cda/layout?m=38842&x=7, toward the bottom is the G3616.
And the big big boys are here. http://www
Look at the video on their lean burn engine.
RE: melting pistons and lean fuel mixture
RE: melting pistons and lean fuel mixture
This naturally precludes all indirect injection engines.
RE: melting pistons and lean fuel mixture
Further investigations into adiabatic engines focused on potential efficiency improvements and obviating the need for complex cooling systems. However, recently research in to this area has all but gone by the wayside because the technology will not really hold up now with NOx regulations. But for non-automotive engines (e.g. industrial and marine applications), this might still be worthwhile, what with recent advancements in design methodologies, materials and coatings.
RE: melting pistons and lean fuel mixture
Regards
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
RE: melting pistons and lean fuel mixture
Next gen DI will make port injection obsolete.
RE: melting pistons and lean fuel mixture
RE: melting pistons and lean fuel mixture
Your post is really quite inaccurate, especially with regard to the manner in which water injection works.
If you search this site yo will find half a dozen long threads that explain it properly.
Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers for professional engineers