"K Smith" wrote in message
...

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)

snip

(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
ok
(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.

This is just the way things are, right?

(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.
no detonation.
(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.

I don't know about this one. It is for sure you don't want it before
tdc. but I don't know that you want it too near after tdc either.

The pursuit of this target over a variety of conditions involves
juggling many variables, including;

That's why the engine designers make the big bucks.


(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)).

yep, definition of octane

snip

(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.

Or the engine has to be designed to cope with the known effects of oil
in the gas.


(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;

snip

(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.

It is not clear it is an EPA thing, or just the way things are. I don't
even know if it is really true.

(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.

This is an interesting theory. You've abandoned the hot piston causes
detonation theory? Or is this complimentary?

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.

Substantive review? wrong kind of engineer

K