Ok so it seems the basics from thread No 1 have been accepted
without challenge?? Somewhat begrudgingly I might say, but hey this is
rec.boats if it were easy it would be too crowded:-) So now you've all
thrown your various sexist hissy fits, none of which addressed any of
the thread's issues:-) but were more about silencing me, which isn't
going to happen:-) maybe this time you can either comment directly on
the substance of the material of just stay quiet, very very quiet???
(please)

Next, again at this stage lets not bother with Ficht per se; just
see if we can get agreement on the basics of what is needed & "why". I
have thought this might be covered in 3 or 4 threads but it seems the
lack of understanding is even worse than even I imagined (not possible
surely:-)). So I'll take it a bit slower & make sure we all agree in
each step, if that's OK with you. There'll be plenty of later threads to
address Ficht by name & yes spam man Bill don't panic; even E-Tech:-)


(1) The design of petrol engines is dependent on the fuel
burning within the closed combustion chamber in a consistently
predicable way, with the accepted target in petrol engines to;

(a) To have the fuel ignition instigated by the spark plug &
only the spark plug
(b) To have the flamefront propagate at a known speed/time,
reliable throughout the charge, that speed varies with temp/pressure so
it varies as the burn proceeds, starts slow then speeds up as the
chamber temp /pressure rises.
(c) So as the temp/pressure combo in the chamber rises, the
extremities of the charge do not reach their autoignition temp, before
the accelerating flamefront has arrived as intended.
(c) To have the burn all but completed & the highest chamber
pressure occur just at or just after the piston leaves TDC on the power
stroke.
The pursuit of this target over a variety of conditions involves
juggling many variables, including;


(2) the octane rating of the intended petrol
including anything that may dilute/contaminate it (whole books exist
just on this fuel/octane subject so this is very much in a nutshell,
however comments are very much valued),

(a) A lazy way to describe this is the higher the octane
rating of the fuel the higher the temp it autoignites at & usually the
slower the flamefront will propagate. Or the lower the octane rating the
lower the autoignition temp will be & the faster the flamefront will
propagate (flashover, remember the flamefront speed varies as the
chamber's temp/pressure varies)).

(b) Once properly vapourised, (so each drop of vapour has
access to air/oxygen & is close by others) if fuel vapour is heated
beyond it's autoignition temp then the burn/flamefront will auto
commence propagating from that hottest point. This will happen no matter
what, no matter if the spark plug has or hasn't fired nor if other parts
of the charge have already ignited nor where the piston is. The design
depends on the spark plug's spark being the only thing in the chamber
that exceeds the fuel's autoigntion temp. & that nothing within nor any
part of the chamber exceeding the autoignition temp till the
burn/flamefront has arrives.

(c) Old worn engines & new 2 stroke OBs allow lube oil into
the combustion chamber, this has the effect of reducing the fuels octane
rating, again the design has to be aware of & allow for this effect.
Over years this has been recognised as a reliability issue in 2 stroke
OBs, so some effort has gone into reducing the amounts of oil carried up
into the chamber.

(d) So for the burn to be predictable & consistently
controlled over the full range of operating conditions the actual octane
rating arriving within the chamber needs to be as close to the raw fuels
rating as possible with as little oil dilution as possible.


(3) Some side comments?? The DFIs you'd think have solved this
octane dilution problem by not having oil mixed in the fuel?? not so,
what they do do is;

(a) inject really tiny amounts of raw oil into the
crankcase & by design it has nowhere to go.

(b) The DFI people seem to think this is good & market it by
saying the DFI engines "use" no more oil than a 4 stroke does, however
that ignores the fact that a 4 stroke hopefully never burns it, you
drain it into a bucket.

(c) This burning the oil & the way/when it's done are issues
with the DFIs because over say a less than 50 hr season it burns a lot
of oil, certainly much much more than any 4 stroke that is still in use:-)

(d) The very small amounts of oil stay in the crankcase &
get hotter & hotter & hotter,

(e) At lower revs there isn't enough air flowing through the
crankcase to carry any excess oil up the transfer ports & out the at low
revs overlapping exhaust, this is a deliberate "must pass the EPA test"
tactic & it works in stopping raw oil getting out at low revs.

(f) So at low revs the DFI has virtually no oil getting into
the chamber & the fuels octane rating (resistance to autoignition,
flamefront speed(s))) is as placarded however,

(g) When the engine is powered up any excess built up
crankcase oil can escape, but by now the useful overlap between the
transfer & exhaust ports is diminished (again as intended by the design)
so the oil can get into the chamber & dilute the fuel's octane rating.

(h) The thing to be considered is that at low revs the oil
isn't being transfered & burnt at all, so at low to moderate revs the
octane rating of the fuel is "safe", but as the revs rise oil definitely
(what?? 1 ltr ever 12 hrs running??) does get burnt, but because it's no
longer consistently mixed in fuel it goes up in fits & spirts, some not
a risk, others definitely a big risk of severely dilluting the fuels
octane rating.

I hope this gets some genuine substantive review & thanks in
advance to anyone who bothers. I'm sure lots will just say it's No
2's:-) with no actual rejoinders on the issues.


K