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Cleaner lawnmower? Answered

http://www.sparkbangbuzz.com/els/pplmwr-el.htm Small engines typically burn dirtier than cars do. Can this adaptation make them "greener?"


Jack A Lopez

1 year ago

Yes. The emissions from an engine can be improved (or made worse) by the choice of fuel.

Although, it is kind of unfair to say, "Small engines typically burn dirtier than cars do," without making a distinction between 2-stroke and 4-stroke engines.

The big difference, between 2-stroke engines and 4-stroke engines, is the 2-stroke kind run on fuel and oil (as lubricant) mixed together, and the oil does not burn very completely. So the emissions from a 2-stroke engine are much dirtier. Or, you know, you get more grams of unburned hydrocarbons, per gram of fuel+oil, or per kilojoule of mechanical work.

Getting back to the topic of emissions depending on fuel choice, there is much folklore on the topic of building a fuel+air charge, for to give a clean burn, for every cycle of the piston.

A notable characteristic of a batch made of propane plus air, is the fact that this batch is a homogeneous mixture of gases.

In contrast, a fuel+air charge with fuel that is gasoline, or diesel, this is, I think, mostly aerosol; i.e. a multitude of very tiny liquid droplets suspended in air. Some of it is actual fuel vapor, but a lot of it is liquid droplets.

I mean, if those little droplets have to burn from the outside in, kind of like logs in a fireplace (but much smaller and faster), I can imagine a circumstance where there are drops so large, or so cohesive, that they do not completely burn, in the fraction of a second it takes for that cylinder's power stroke.

I don't know if this mental picture is correct, but I think it makes for a good, like, "hand-waving" explanation.

By the way, while on the topic of combustion folklore, I kind of feel like I have to mention the legend of, "hydrogen boosting".

The way the legend goes, is supposing this analogy of liquid droplets in a fuel+air charge, being like logs in a fireplace, well... adding hydrogen gas (aka H2, aka dihydrogen) is like adding a liquid fuel to the logs in a fireplace. That is to say the hydrogen quickly moves into the gas spaces between the drops, and acts as an "accelerant"


So, continuing with the analogy, the igintion spark to a mixture of air, plus hydrogen, plus fuel aerosol, is like dropping a lit match onto a pile of logs soaked in gasoline. The whole thing just goes FOOM!

Or that is the legend.

Actually, if you want to see a 3+1/2 hour presentation, of the same story, from about 10 years ago, there is a DVD made by these guys, Roy McAlister and Steven Harris, who put a high pressure H2 tank, in the back of a 1991 Geo Metro, and made some mods to the fuel intake plumbing, so that this car's engine could be boosted with hydrogen, in quantities as high as 10% of the total enthalpy in each fuel charge comming from the hydrogen+air combustion (with the other 90% of the heat comming from the liquid fuel + air combustion)


The interesting part of this legend was they claimed they could burn a wide variety of liquid fuels, like gasoline mixed with diesel, or gasoline mixed with vegetable oil, or like whatever, just as long as it was not so thick as to clog the fuel injectors. And this was all made possible through the ability of hydrogen gas in the mix, for to improve the burn-ability of otherwise crappy liquid fuel.

I think this was a fuel injected engine, rather than a carbureted one. I think. Like how far back in time do you have to go to find a car with carburetor in it?

Also, I seem to recall they modified the ignition timing as well. Like, they had this knob on the dashboard, for to directly adjust the phase of the ignition timing.

The only caveat, was that it took significant amounts of hydrogen (the rule of thumb seemed to be approximately 10% hydrogen, by enthalpy) to make this magic happen.

The technical drawback was this demand for large amounts of hydrogen, pretty much necessitated a big tank of compressed hydrogen, rather than something simpler like an electrolysis setup, that could use the car's electrical output to make hydrogen and oxygen from water, which is much easier to store than compressed hydrogen.

I mean there are people that will tell you there is magic in these under-the-hood, water electrolyzers, but it would tricky to build one of those that could supply significant quantities of hydrogen, like in amounts close to this, rule of thumb, 10% of the total fuel enthalpy.