del cecchi wrote:
"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?

Yes, mostly takes about half the burn time to establish the burn &
consume 10% of the charge, this immediately starts to increase the
chamber's pressure/temp, so the remaining 90% is consumed in the other 1/2.


(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.

A bit controversial really, that last bit of flamefront is oft termed
flashover, because by that time the chamber pressure/heat is at
autoignition temp anyway. As to "when" designed flashover effect becomes
detonation?? hmmm.


(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.

Once the piston starts down the bore the pressure drops, so too the
temp. This would & in old tech 2 strokes does result in a slower
flamefront, such that over heating of the piston can occur. Years ago we
had a NG fight about how the big 2 strokes tended to go out after a long
period trolling?? then were suddenly spooled up & bang. This was because
they slow the idle speed by using a very much retarded spark timing,
which lengthens the time of the burn because by then the pistonb is
retreating, so putting more heat into the piston.


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.

Well there seems to be lots of copying which makes you 'feel" for the
original people, the Japanese started out just copying, but now they do
have a fair bit of their own stuff as seen in the hondas & Yamahas. It's
probably right that the situation is now the other way around.


(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.

Yes absolutely & the older oil mixed in the fuel engines could be, but
the DFIs?? How?? they don't really know when the oil will arrive in the
combustion chamber.


(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.

The early Orbital designs tried to deal with it & the fix seems to have
settled on just using tiny amounts of oil, particularly 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.


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

No we're still sticking with that being the "cause"; hot uncooled
piston caused by lean mix & poor atomisation, but this is something
which is "probably" an unpredictable wild card, as to it's occurrence &
it's effects. The fact E-tech have so severely cut the oil supply back
even further, is a thing worth noting. Given they're going for
reliability first & foremost, given their priors:-) you'd think that
would be the last thing they'd be doing if it was just to have the
dealers spruik about lower oil costs, because even the old DFI's didn't
use much oil compared to carbed or efi anyway. They have never been able
to stop the oil baking the rings etc, tried new oils as you know etc &
now seem to be trying to just have less there??? It's the heat every
time, everything they do is saying it.

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

Well thanks anyway as always:-)

K

K