trainfan1 wrote:
Del Cecchi wrote:



So this is some alloy significantly different from the aluminum alloys
used for conventional pistons?


Not significantly, but to be labeled "high melting point" they'd better
be made to withstand more.




Are you saying that this special oil is necessary to achieve
reliability in etec?


I do not believe this is a requirement, only if you want to commit to
the lower volume setting / longer refill intervals.






OK, here is a calculation. Air has a specific heat of 723
J/kg-degree, and gasoline has a latent heat of vaporization of
300kJ/kg. So at 15 to 1 you get 20kJ or about 30 degrees celsius,
less at leaner mixtures. That about right? Is that really significant
in preventing detonation? Doesn't seem like much help in cooling the
motor to lower the temp by 30 degrees.


The heat of vaporization is not the only critical issue. A two stroke
engine by design has to pass some unburned fuel, as it also must put up
with some exhaust gases in with the new (a trick used in 4 strokes to
lower the combustion temp & oxides of nitrogen for emissions compliance).

Cant pass unburned fuel or EPA and California will have your hide.

2 strokes have always been run richer than "stoichiometricly" required
to provide cooling via unburned fuel.

Again, it's an amazing balancing act that the 2 stroke manufacturers are
attempting in keeping power up, economy up, longevity up, & weight &
failures down. Fast acting processors & high pressure injection systems
are the only way to do it. BRP is in this for the long run.

I thought that unburned hydrocarbons was no longer allowed to any
significant degree. So if you run etec richer than stoiciometric then
you fail epa. Isn't that correct?

Optimax is not high pressure injection. So high pressure injection is
not Only Way. In fact we don't know for sure it is even one way.


And saying what the "properly controlled by ECM injectors *should*
do sort of begs the question of what they actually do.


Well, a 14.7:1 A/F Ratio is ideal, but not attainable in practice, as
there is not enough time in direct injection to vaporize the fuel. A
2 stroke will leave behind some exhaust gasses too. The injector
must be able to atomize the fuel into droplets small enough to
approach vapor, but large enough to absorb some of the heat of
combustion. What E-Tec is trying to do is minimize the losses
associated with overlapping strokes in 2 stroke design theory, while
using as little fuel as possible. The chamber design is very critical.

The above is sort of a truism, eh? Droplet size shouldn't matter in
heat absorbtion because it all evaporates as it burns, right?


Not in a 2 stroke.

So in an etec does it evaporate and burn? I'm not talking about my
dirty merc.

The trick is to get it to evaporate so it can be burned at the right
time.


Only the part you want burned for power...
Again, doesn't it all have to burn? Or does some burn in the exhaust
system?
Just thought of that possibility.


How does running an engine at power on hugely lean mixtures stop
detonation??


This is the balancing trick - and hence the other tweaks - the lean
charge must also be enough to complete the cooling process too.

it caused the base heat buildup that starts detonation.


How does a beefed up huge extra drag gearcase stop detonation??


Keeps the Sidewinder/Switzer kids from picking this model to run
WOT all day long up on a jack plate...



Better not put one on your bass boat. Those guys love their jack
plates. And they love WOT.


I just made that up.



Oh. I have heard that some of the etec motors were having lower unit
problems, not that that would have anything that I can see to do with
the fuel induction system .


I agree on that. I just did not see the correlation, unless the E-Tecs,
once broken in, are putting out more than they are rated for or BRP got
poor leg parts from their supplier.

Or they screwed up the design somehow. Bass and Walleye boats discussed
it some in the shootout article.



The answer is none do: all those things are a lame (is not an MP3
emulator:-)) attempt to make the engines able to withstand the
inevitable detonation consequences when run lean at power on
poorly atomised low pressure injection with very questionable
lubrication.

Thanks for the E-Tec spruik Rob but hey I'm sure you got a
lucrative deal to fit them?? much more dollars than the other
brands???


You REALLY don't know me...

but what is it to your advantage to de-spruik something you don't
understand?



Always looking for technical details. The geekier the better.

del



I can see the dilemma and confusion K. Smith is facing, but the
engines do work. The BRP units are an improvement over the OMC
offerings, which did have isolated issues - as there are thousands
still in use.



Yes, but there was a significant failure rate among at least the
150/175 Fichts. Is more rugged construction, (presumably) higher
pressure injection, and new combustion chamber/piston shapes
sufficient to resolve them?


We'll have to see. BRP gives you a "free ride" for 3 years.

OMC did too, until they didn't.


I have been curious for some time about why the 150/175 was said to
have many more problems than the big blocks. Do you have a theory?
Or even facts?


No facts, but the differences between similar architecture V's are
several when you are dealing w/ different bores/strokes (I'm not sure
which spacing is shared between them) and then the injector volume must
be changed too, it's not like the carby days where you could change the
HP w/ a throttle bore change & an exhaust tuner.

Rob


--
Del Cecchi
"This post is my own and doesn’t necessarily represent IBM’s positions,
strategies or opinions.”