12
2018.03.30
Hi Paul,
another impressive video, making the combustion process visible ;-)
With regards to processing a homogeneous mixture, I've found a german paper,
dated 1976.
Engine was a NSU KKM 871, 2 rotors.
For preparing an ideal mixture they've used a "boiler" / drum. Fuel was
heated up to 200°C + fed into the boiler / big chamber. Mixture temperature
was 70°C out of the boiler with all fuel evenly vaporized. Intake
temperature at ports was ~ 65°C.
Compared to a conventional downdraft carburetor you can see some advantages.
» pict_1
spez. Kraftstoffverbrauch » specific fuel consumption in g/kWh
KKM » RE
Idealgemisch » ideal mixture
Vergasermotoren » engines with carb
Kraftstoff-Luftverhaeltnis » Relationship of fuel + air » Lambda
Teillastpunkte » part load "points" / adjustments
Without the availability of fuel injection David Garside has found a very
good solution for the Norton motorbike: air gets into the carbs with ~
100°C + will be cooled down because of fuel evaporation to ~ 50°C before
reaching the chambers. He has used SU carburetors (variable venturi /
constant velocity carbs) - they ensure a pretty good fuel spray.
In general there are some rules of thumb, one may pay attention to ...
» duration of evaporation of a gasoline droplet » at 30°C it takes 8 times
longer than at 80°C » higher temperature is preferable
» 8 million droplets with a diameter of 10 µm will evaporate 200 times
faster than 1 droplet with a diameter of 2 mm (same volume) » the smaller
the droplets the better
With fuel injection into the intake manifold or PFI ...
» higher flow velocity in the intake manifold enhances fuel preparation
» lower pressure in the intake pipe (e.g. at WOT) enhances evaporation, too
It's preferable to get fuel droplets as small as possible. That's why the
Renesis engine has primary injectors / nozzles with 12 holes, generating a
relatively fine mist of fuel.
Another solution may be a swirl injector like the one by Mitsubishi, 1996.
» pict_2 + pict_3
Very small SMD (Sauter Mean Diameter)
Once the mixture is inside the engine, it's important to avoid condensation
/ fall-out of the gas. Was a very big challenge for Daimler-Benz with the
DFI system. Therefore the walls should have a temperature of at least ~
40°C. More is better. That's one of the reasons to "heat" the cold lobe a
bit ;-)
(Another one is to avoid deformation of the housing shape)
The disadvantage with warm / hot mixture + walls is, that volumetric
efficiency will decrease. Again, finding the right balance is the target.
Quenching ...
sorry, the next part is a bit "academic" ;-)
Related sketches: t-section.pdf
At all walls of a combustion chamber you will have a wall layer - a thin
film of gas, that will not be burned.
Reason: heat loss to the walls is greater than the amount of energy produced
by combustion » extinction of the flame front.
Wall layer will be smaller / thinner with ...
» higher pressure (exponential effect - good)
» richer mixture (simply more energy available)
» higher wall temperatures (limited by compulsions of lubrication)
The thinner the "chamber" the richer the mixture has to be. But this will -
of course - increase heat losses ...
Near the "cool" walls the disintegration of molecules + atoms of fuel will
be stopped. Termination of chain reactions, free radicals will partly be
bound to the wall + converted into stable species.
Sorry, have forgotten the minimum temperature levels you need for
satisfactory chemical division.
No further chemical reaction will occur - that's the main issue /
challenge. There will be no more conversion, e.g. CO to CO2. That's the
phase, where you will loose a lot of energy / heat / power ...
A simple combustion model may make it more clearly (?) ...
1st step: emergence of short chains (Alcanes ?)
2nd step: appearing of radicals like H, O, OH ... (Alcenes ?)
3rd step: "cold flame", generating ~ 10% of the energy / heat
4th step: "blue flame", you will "see" CO, H, H2O. Oxidation to CO delivers
~ 30% of the heat. This phase may be (partly) not reached in the quenching
areas.
5th step: the main heat / energy producing step. Most of the time this will
not be reached in the spheres / sectors of quenching: conversion of CO to
CO2. This step generates ~ 60% of the heat / usable energy. + only just now
you can measure the big / huge rise in temperature.
Sorry, my English isn't good enough to explain everything correct. We would
need a chemist now ;-)
Another challenge will be to initiate combustion In the mixture + not at the
edge. Flame front is "creeping" along the sliding surface + will be cooled
down unnecessarily. Possible countermeasures ? Perhaps laser or some kind of
"jet" ignition ... ?
After all, picture_4 + picture_5 show the influence of heat loss, crevice volume +
leakage. The last point is the most important ...
But now it's really time to "shut my mouth". So much blah-blah ... ;-)
Best regards
Franky
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