More Questions on cranking

[Phil Lamovie]

Hi All,

John wrote :-

>I thought one of the injector's jobs was to atomize the fuel though.

atomize =  very small drops   vapor = molecules

Petrol drops burn like a candle. They have essentially two layers of
combustion the  ratio of which  is controlled by chaos. As the drop
surface vaporizes
there are instants of both rich and lean combustion in very pretty
patterns. The resultant gases are all bad news. Keep in mind the
surface area/volume ratio.


> (This is an remanufactured engine turning a propellor on a
hovercraft or aircraft).
> So we have to crack the throttle a little bit at startup.  This has
been the biggest
obstacle to cold starting.  If we open the throttle too far the MAP is

> wrong compared to RPM and mixture is off.

Don't even look at the Map sensor during cranking the signal from the
sensor
is useless as the engine is "cogging"  and slowly to boot. The signal
has both positive and negative excursions. For a high class test fit a
vacuum gauge, and  turn the engine over on the starter  without fuel
or spark. Don't borrow a gauge you can't afford to replace.

Pick an appropriate amount say 7 ms and just squirt. If the engine
fires it may go
from 0 vacuum to 70 kPa  and 600 rpm in 5 or 6 revolution.

If this aircraft is flying above 5000 ft you would be well advised to
reconnect the throttle body water supply. This will prevent freezing
as the aluminum will out shrink the brass and lock it solid. This will
make landings difficult and/or messy.

Check your default for O2 sensor failure in closed loop and make sure
it's not lean.
Aircraft  engines have a habit of operating under much higher average
loads than cars.
Thus a 70% load is not unusual. In most cars the average engine load
is 10-15%
This is not an appropriate load for 14.7:1 A/F in closed loop. If you
have a broad band sensor then you will be able to target 12.5 - 13.0:1
A/F ratio.

Under no circumstance should the engine boost at A/F of more than 13:1
it will
simply melt after a few minutes. Imagine under what circumstance you
would keep the throttle of a turbo car at 3/4 for 3 or 4 hours in
closed loop.

I know that auto makers are "dumb" but they have read the Bosch Blue
Book.

And the cost of the hoses was less important than a throttle stuck
after a long run on the highway in freezing conditions. Remember that
partial vacuum = cooling just as increased pressure = heating

By the way how did you determine the injection amount vs map points
for rpm vs load at 50, 100, 150 etc. rpm ?

> Based on the last dynamometer run,  we found we were at about 12 ms
at
> 6000 RPM and pulling 111 HP and our goal is 125 HP;  We've been able
to
> get over 250 ft/lbs Torque. This, at a little less than 75% duty
cycle.
> As I understand the reason for 'the name of the game' is to provide
a
> smooth unloaded idle.  Since we turn a prop idling at about 900 RPM
we
> have a little bit of a load so a slightly larger injector might keep
me
> under 75%.  (I have a hardware limitation on pulse widths longer
than
>75% if I plan my injection at low rpm for the intake stroke).

If you were firing the injector for 12 ms at 6000 rpm you were
probably
in another dimension.    Lets see 6000 rpm = 100 rps or 1 revolution
every
10 ms. I think you were missing injection cycles because you can't
start the
new event until you finish the old.

The smaller the injector the smaller the spray and the better the
atomization.
etc. etc. as the drop is basically round the cube law applies in time
vs droplet size.

A drop with a diameter of 10% less has a mass of 50% less

This stuff cost millions to learn with horrible cameras and engines
with windows
Take it as a given that the drops can't be too small.

Why not use a single hall and a chopper with a differential width for
the TDC
indicator. Start a timer with the rising edge and stop it with the
falling edge.
The short one is TDC.


> Spraying onto a closed intake valve when the engine is cold or just
> warming up doesn't seem logical for a low pollution fuel efficient
> engine.

You use the words hot and cold like they mean something in an
empirical sense.
Express yourself in kelvin (K)  and you hear a 20% change from "cold"
to "Hot"

Spraying into  a sub atmospheric pressure  gas stream , comprising one
of the best insulators known to man that by the way has a terribly low
specific heat seems like a waste of time unless the air is preheated.
Some cars even use heating elements under the carby for this reason.

Low pressure lowers boiling point but also gives less density and thus
lowers the energy per specific volume of the air.

Do the calculations on the heat that is required to vaporize the fuel
and you will find that you need a generous donor. Heat is the issue
and where to get it from is the problem.

For that matter spray some on your hand. If you don't quantify your
problem in SI
units you won't come up with an SI answer.

Question How many elephants fit in a phone booth ?
Answer    Some.

If you leave out the units you won't pick the core issue

The smoothest idle has the smallest cycle to cycle variation in
combustion and that is hard to control if your target A/F is stoic. If
you vary even a little say 15% leaner you will get a miss fire. This
will decrease the vacuum level and cause the fragile chain to break.
The rich tolerance for a "cold" engine is probably in the 100% range.

The racer has to be separated from the idler. By all means

PS When I say % I mean of course  by mass not by A/F ratio.



 Regards

 Phil Lamovie

 injec at ains.net.au

     cogito ergo zoom