This NG has been the only place, anywhere that has been shown to have
correctly predicted Ficht would fail & explain why. Yes we're a messy
abusive bunch but we seem to have got this correct before anyone at all,
even beforte they started to fail in huge numbers, OMC admitted 1 in 5,
but that was probably a fudge like everything else they said.

Hopefully I can convince anyone who reads these of the rational reasons
why Ficht didn't work & why the latest modifications from the new owners
will not help the situation.

The initial posts on the subject were in 98 & have continued ever
since, even to the same predictions & explanations for the latest
incarnation from Bombardier.

Regrettably the posts & discussions have almost always been in a
combative environment, usually in answer to OMC dealers spamming the NG
with sales spruiking, so this & the posts to follow will be new threads,
hopefully to be reviewed & criticised if anyone has any comments.

I'll start with some very basic understandings of how engines work,
because throughout the years it's apparent even long time OB mechanics
have little or no basic training & not much understanding of the rules.

HOW THE FUEL BURNS

(i) Liquids don't burn, not petrol ( we call gasoline petrol, sorry:-)),
not diesel, not most any liquid. What does burn is the vapour given off
by liquids.

(ii) Temperature & pressure are one & the same, in these premises. So
any "temps" will be different as the pressure the fuel is under at the
time changes. The higher the pressure the higher the temps.

(ii) Various flammable liquids give off vapours at various
temps/pressures, which means some (petrol) will readily ignite at room
temp while others (diesel) will not. The temp at which a liquid gives
off enough vapour at norm atm pressure, to become flammable is usually
called it's "flashpoint"

(iii) The flashpoint for petrol at atm pressure (15psi) is around -40C
(&F) so it will ignite in most places whereas,
diesel has a flashpoint of around 60C (140F) which means in normal
settings it with not ignite, even if you throw a lit match into a bucket
of diesel.

(iv) Next it has to be "ignited", & the only way to do this is expose
the vapour to a heat higher than what is usually called it's
"autoignition" temp, again this varies with lots of things as above but
also, including the type of petrol, octane rating or the type of diesel,
however generally for unleaded petrol this is around 260C (500F) & for
diesel around 220C (430F) So petrol has a low flashoint but a high
autoignition temp & diesel is the otherway around. This is why diesel
engine fires are so dangerous, becasue once vapourised diesel leaks
(cracked injector line?) into contact with even modestly hot (210C)
articles (exhaust or turbo housing?) which are all around a running
diesel, it will autoignite, then any further fuel leaked will easily use
that heat to continue the fire.

(v) So long as the fuel is;
(a) fully vapourised or atomised so oxygen can surround each tiny droplet,
(b) in the correct quantities so the heat from each can create
autoignition in the surrounding droplets (too little or too lean & the
flame will not be sustained or will be very slow OR too much fuel or too
rich & the increasing pressure will force the vapour back into droplets
too large such that it will be a "liquid" & liquids don't burn),
(c) Once any are exposed to a temp higher than their autoignition
temp, then the surface of that droplet will "burn" converting oxygen etc
& in so doing the heat generated will heat the next droplet to above
it's autoigntion temp etc etc &
a flame front will travel away from the point of ignition.

(vi) The flamefront, once established travels at a fairly predictable
speed, again it's variable dependent on many variables but for petrol it
can be talked of as 16m/s (52ft/sec.) & once commenced in a closed
chamber the heat & temp will continue to rise as the burn continues.

HOW THE FUEL IS IGNITED THEN BURNT INSIDE PETROL VS DIESEL ENGINES.

(i) The two engines are totally different in their operation strategy,
although they do appear to be very similar; just fuelled differently;
(a) the petrol engine has to mix the vapourised fuel with the air
inside the combustion chamber BEFORE compression & ignition, which means
the amount of heat generated by the compression or anywhere else has to
be limited, which in turn limits the max compression ratio to around 9
to 1 whereas,
the diesel has no fuel at all in the chamber during compression & only
has fuel injected just before TDC (top dead centre) allowing very high
compression ratios.

(b) the petrol engine has to keep any part of the chamber below the
autoignition temp of the fuel (petrol 260Cor 500F at 1 atm) until the
point when it's required to be ignited (near TDC), whereas,
the diesel engine can allow the chamber temp to be as high as it likes,
because there is no fuel there till the fuel starts to get injected
(near TDC).

(c) the fuel in a petrol engine is ignited by exposure to a spark
which is well above it's autoignition temp so once the fuel /air mix
around the plug are ignited, then the flamefront proceeds out at a
predicatable rate from there whereas,
the fuel in a diesel engine is ignited, after a short delay, by
exposure to the hot compressed air in the chamber, which is above the
low autoignition temp of diesel. In the diesel the burn can be
controlled because as soon as fuel is injected it will autoignite, so at
idle the injection will stop almost immediately, but at higher power the
injection will continue over a longer period of the power stroke.

(d) The petrol engine can only control the power by controlling the
amount of fuel AND air that are in the chamber whereas,
the diesel can control it's power by adjusting the amount of fuel
injected on each power stroke.

(e) The chamber temp in a petrol engine needs to be closely controlled
so the fuel is ignited only by the advancing flamefront, if the chamber
or any pat of it exceeds the auto ignition temp any fuel air mix left
will just ignite starting it's own flamefront & heat pressure rise. This
is usually referred to as detonation & in cylinder readings can go
momentarily as high as 1800psi, which generates extreme temperature rise
very quickly making the detonation self sustaining whereas,
the diesel engine can allow the chamber to be very hot with no risk of
the fuel igniting before intended, because there is no fuel there till
it's deliberately injected.


I'll post more ....

K