Fuel Atomisation

[Bernd Felsche]

Greg Hermann tapped away at the keyboard with:
> At 9:26 AM 12/10/01, Bernd Felsche wrote:

[insults snipped]

> >> Most carbys are not that good, and port injectors are not even close.

> >What are the droplet sizes? At what airflow rates? From which jets?

> On the same order as what comes from the Bosch air shrouded
> injectors.  eg--approx 8 microns on down. From both main and idle
> circuits, not from the pump jets. Talking Webers here, not typical
> carbys.

Same order... same order as some non-air-shrouded injectors IIRC.
I'll dig out their Bosch PNs and nominal operating pressures later.
You also know that droplet size is related to pressure differential
across the pintle, not the amount of airflow, as it is in a carb.

> >Increased vapourization (due to the greater atomization you say
> >occurs in a carb.) upstream of the inlet valve is going to help how?

> Try reading what I said before , ...

I did read what you said. You're limiting port injection to the
regime of only injecting on a closed valve, which as you well know,
is primarily to reduce HC emissions.

> >> What this does is let the latent heat of the fuel (as well as of
> >> any injected water, SHHHH!!) act as an internal coolant during the
> >> compression stroke. Internal cooling during the compression stroke
> >
> >How many Joules are going to be absorbed in the vapourization
> >process?

> If you are too ignorant to calculate this for yourself, pound salt.

I do know how to work it out; roughly. I knew how to work it out 25
years ago.  It's a small amount compared to the total heat transfer.

>  Unless you're evaporating water, you won't be removing
> >significant heat without running very rich mixtures.

> Any heat removed _during _ the compression stroke is significant.

> >You said something about BSFC?

> Yep. Properly tuned DCOE's at WOT will beat conventional port
> injectors every time. Significantly.

At _WOT_. Which means what? The only situations where you nominally
drive at WOT is in a drag race or on a banked circuit. Oh, and on a
dyno.

> >> reduces the negative work needed during that stroke, reduces the
> >> peak temp of the cycle and the EGT, reduces the peak pressures
> >> during the cycle, and reduces heat rejection to the water jackets
> >> significantly as well.

> >Reduces the pressure by how much to vapourize a liquid droplet into
> >a gas?

> Again, I suggest you go to school if you wish to learn to do such
> calculations. I have done them, ...

I received my B.E. in 1983, having completed studies in 1982,
including a non-significant amount of pratical engine design work at
the Orbital Engine Company. You can check that out. Ask UWA and
Orbital's CEO, Kim Schlunke, who was my supervising engineer at the
time (78/79).  He will probably remember, as do a number of other
Engineers I worked with at the time.

My main work was orbital engine disc valve airflow prediction (by
computer model) and measurement in a test rig I co-engineered. 

I can tell you that much because that work is no longer under NDA.

> >Removing an extra kW of heat from the coolant in a 200kW engine
> >has always been a problem, hasn't it?

> Not at all, ...

> But if that kW of heat stays in the cylinder instead of going into
> the cooling system, a given proprotion of it ends up going out the
> business end of the crankshaft instead of through the cooling
> system.

But the heat doesn't stay in the head. You just said it there was no
problem to remove the additonal heat.

> Again--if you want to know how much, develop some competence, and
> figure it out for yourself!

> >> Good atomization also leads to more uniform distribution of the
> >> fuel throughout the charge, leading to better, faster
> >> combustion (also more power and efficiency here).

> >And increases the tendency to detonate (knock).

> [snip]  of the fact that detonation is a function of the
> residence time of the end gasses at high temp/pressure conditions
> ! Lemme see--if we _reduce_ the peak temp, peak pressure, and time
> of burn, that sure oughtta increase the tendency to detonate,
> oughtn't it???

Detonation is a function of several factors. A high rate of
combustion leads to rapid secondary end-gas compression ahead of the
flame front, increasing the propensity to knock. If the cylinder
volume is not expanding fast enough to ameliorate the secondary
compression, then knock will occur.

> Not to mention the fact that a faster burn increases the basic
> efficiency potential of the Otto cycle !!

Which engines would that be that operate on the ideal Otto cycle?
Peak gasoline efficiency is not by total combustion (pressure rise)
at minimum volume.

The efficiency of converting the cylinder pressure to crankshaft
torque varies as a function of crank angle (and conrod to
crank-throw ratios). You get a great deal more torque with a peak
pressure after TDC.
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