[Diy_efi] boosted compression ratio

Bernd Felsche bernie at innovative.iinet.net.au
Sun Dec 22 13:48:31 GMT 2002


On Sun, Dec 22, 2002 at 09:09:47PM +0000, Mike wrote:
> At 08:17 PM 22/12/2002 +0800, you wrote:
> >> Well why the heck dont you add a water injection setup and get the
> >> humidty artificially - and you could chill it as well :-)

> >Expense and water is scarce. :-)
> 
> Yeah methanol might soon be cheaper, the way the rains are going ;)

> >Still trying to get a cheap inlet fogger together; that should do
> >the trick. In winter, I'd have to heat the fog so that the air
> >temperature rises as well. At 8 degrees C, air is saturated with
> >only about 8 g of water per kg of air. Heating the air to about 25
> >degrees increases that to about 20 grams. That's worth 3 Octane
> >points; but the temperature rise reduces the effect by 1 to 2
> >points. <sigh>

> But hang on, you dont want water vapour and you dont want too
> small a droplet size otherwise it will become vapour by the time
> it gets to the cylinders. I dont recall what boost you were

Yes I DO want water vapour. Check the Gasoline FAQ. It deals with
humidity - absolute humidity is what appears to be significant in
inhibiting knock.

The mechanisms as I understand it are:
  1. absorption of heat; though a few g/kg aren't going to
     work miracles at most 0.5% in mass so about 1% in heat.
  2. displacing some oxygen molecules resulting in slower burning.
  3. ummm... oh my brain hurts... can't think of it right now.

Liquid water becoming vapour in the cylinder raises cylinder
pressure. Unless the heat of vapourisation is so great that the
temperature drops markedly, that could do more harm. 1 gram of
liquid water displaces about 1 cc. 1 gram of water vapour displaces
vastly more... unless it's compressed lots; and in essence if you
want to remove the heat, you have to let it expand...

BTW: Once you reach 100% RH, the air is saturated and you won't
easily evaporate any more drops.

> running but over a wide range where the droplets will displace
> a little air - you still want to introduce quite a large amount
> of water so the cylinder compression can dump its heat in the
> latent heat of vapourisation. SOmeone called 'Pete' on the

I don't need the cooling... at 37 degrees C with 90% RH; i.e.
stinking hot like a few days ago, there was no ping off idle, under
load with WOT.

That's all I want to do; let the engine management system advance
the timing as far as it wants without encountering knock.

> nissans13 user group on Yahoo did a nice setup and managed to
> run straight ULP at around 15psi or so boost, went up to
> almost 20psi on optimax,  using off the
> shelf items and an atomiser head from a agri company, I think
> it cost him around $200 and most of that was the pump which
> I recall was one the best most reliable marine ones around
> (I'm not on that group anymore - 9 lists are enough <sigh>)

I want my engine to last another 250,000km and liquid water is
not conducive to long engine life!

And I'm from a part of Germany where Scotsmen are considered
generous people; so $200 for a pump is way off - and it'll probably
deliver much more water than I want as vapour. I worked out a
maximum of 10 litres per hour at 6000 rpm starting with 4 g/kg
absolute humidity and elevating that to about 30 g/kg at 40 degrees
ambient air temp.

> >> The time is not the issue, its the fact that at higher revs there
> >> are all sorts of other things that may 'sum' to way too much
> >> noise floor for effective ping detection...

> >Unless you know the noise floor and can compensate; which is what
> >the (defunct) Harris chip did; measure noise floor outside the knock
> >window and "subtract" that from the knock window levels. Well; it
> >was rather more complicated but that's the gist. Hopefully, the
> >valvetrain isn't clattering away just inside the knock window;
> >though it's conceivable that some valves might be opening or closing
> >at the same time.

> mmmm, Bit difficult due to the random nature of noise but fair point
> that 'noise' at different times but at the same rpms should be close
> enough that a signal might get through, especially integrating over
> the window and doing some convolution etc Its all those other
> nasties like occasional valve bounce (minor I know) but changes as
> the valves rotate, that and ringing through the rods, eccentric
> motion around the big ends, twisting and reflections along the crank
> etc - which contribute to making it unreliable to just sub the last
> known noise - then the air con kicks in - another set of varying
> noise or a change in gear propogates back through to change the
> instantaneous torque fluctuations which make everything else 
> twitch in frequency and amplitude ad infinitum etc etc

Like I said; the chip is rather more complicated! The data sheet
should still be floating around.

> I really like the idea of correlating one knock sensor per
> cylinder with those fibre optic pressure sensors (in spark plugs)
> with individual EGT per chamber that might make it good
> for high rpms and high loads - not quite synonomous with my
> general use as a road car, compliments the twin tires I have
> on it at moment - I must be probably the last person in Australia
> with them if not the Southern Hemisphere <sigh>

A UK research team used piezo sensors under the head bolts to get a
measure of cylinder pressure.

-- 
/"\ Bernd Felsche - Innovative Reckoning, Perth, Western Australia
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