Today's Message Index:
----------------------
1. 05:34 AM - Re: Pressure altitude and short sparks (Trampas)
2. 06:30 AM - Re: Pressure altitude and short sparks (Gary Casey)
3. 08:16 AM - Re: Re: Pressure altitude and short sparks (Brian Lloyd)
4. 10:11 AM - Re: Pressure altitude and short sparks (Ed Anderson)
5. 09:01 PM - Internal Regulated Alternator converted to External? (Bobby Hester)
6. 10:01 PM - Off topic - tracing house wiring (Ron Koyich)
7. 10:39 PM - Re: Off topic - tracing house wiring (richard@riley.net)
Message 1
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| Subject: | Pressure altitude and short sparks |
--> AeroElectric-List message posted by: "Trampas" <tstern@nc.rr.com>
To get back to the original topic, which related to altitude and ignition
system.
The cross firing that happens at higher altitudes is usually due to the air
density in the distributor. That is the rotor inside the distributor is
moving between tabs on the distributor cap, so the air acts as the
dielectric between the plug that you want to fire and the neighboring plug
or tab in the distributor. As the altitude increases it is easier for the
spark to jump to the neighboring distributor tab.
As far as the ignition system goes, basically the spark plug is has two
electrodes and the voltage potential required to jump an arc between the two
electrodes is proportional to the number of air particles between the two
electrodes. That is again the dielectric strength of the air, with cylinder
pressure this dielectric strength increases. So when the cylinder has the
most pressure, the spark plug requires the highest voltage to start the
spark. After the spark is started the voltage required to maintain the spark
decreases.
So what happens is that when the engine is under a load, like when
accelerating in a car, the cylinder pressure is very high. Thus it takes a
higher voltage to start the spark in the plug. In most engines with good
ignition components it takes about 8-12KV to start the spark on the plug.
Now in the distributor cap, at sea level it takes over 20KV to have the
spark jump to neighboring plug. Now for example imagine you go up in
altitude and now it takes about 15KV to fire neighboring plug. Now also
imagine that your ignition wires are old (higher resistance) and maybe your
plugs are worn such the gap is a bit wider and now instead of 8-12KV to fire
the plug you are looking at 15-18KV. Then at this point it is easier for the
arc to jump to the neighboring plug than your plug. Keep in mind the
neighboring plug has little compression in it's cylinder, thus the 15KV is
mainly used to jump gap in distributor cap. Thus the engine will run perfect
at low altitudes and miss or cross fire under a load at high altitudes.
Now if we want to get further into the spark plug and ignition system we
would notice that a coil is charged with energy and then that energy is
released as a spark to the plug. Thus there is only X amount of energy and
when your plugs or wires wear such that it takes higher voltages it start
the plug arcing, well this basically means it takes more energy to start the
arc, thus there is less energy left to maintain the arc. That is the spark
duration, amount of time plug is arcing, will decrease. This means that the
spark plug may not have enough time to completely ignite the fuel in the
cylinder and again you get a miss. The reason being that there is fuel and
air in the cylinder, ideally these particles are mixed really well. However
the spark plug has to ignite enough fuel particles such that they can in
turn ignite other. Thus the more fuel particles the arching of the spark
plug can hit the more likely combustion will propagate. Thus on newer car
engines they are going to wider spark plug gaps, the problem here is again
the coil has to be charged with more energy. Here again if you have one coil
for eight cylinders you can only charge it for a short time. Thus automotive
manufactures have started going to coil on plug. That is there is one coil
per plug. This means they can put more energy into the spark plug and
increase the spark plug gap, thus making it more likely you can ignite the
fuel, getting fast combustion propagation and also run a leaner mixture, one
closer to 14.7:1, and some times even leaner.
Trampas
www.sterntech.com
Message 2
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| Subject: | Re: Pressure altitude and short sparks |
--> AeroElectric-List message posted by: "Gary Casey" <glcasey@adelphia.net>
Lots of interesting comments on this subject, but there were some that
played a little loose on the terminology and technical details, possibly
contributing to confusion:
You can't assume that if the compression ratio is 10:1 that the pressure is
10 times the manifold pressure. The pressure actually rises as the 1.35 to
1.4 power of the volume ratio. The difference depends on leakage and the
heat rejection of the air to the cylinder walls and there is less heat
rejection at higher speed. Also, volumetric efficiency will generally rise
with engine speed, at least from cranking speeds to a mid speed range.
Therefore, the peak compression pressure will rise with engine speed. For
instance, with a 10:1 compression ratio the peak compression press will be
22 to 25 times the manifold pressure.
You can't mix the terms "conductivity" and "dielectric strength." All
material is conductive to some degree, but dry air is almost non-conductive.
However, if you apply a large voltage gradient across air it will ionize and
become very conductive. The voltage gradient at which this happens is
dielectric strength and that voltage goes up with pressure. Once the air is
ionized it takes much less voltage to maintain current flow, probably less
than 1/10 the voltage required for the initial breakdown. If you look at
the spark plug voltage with a good scope you will see the initial rise in
voltage to a very high value (maybe 10,000 to 20,000 volts) and then it will
drop to a very low value of 1,000 to 3,000 volts for the remainder of the
"burn." In order to cause this ionization, individual molecules must be
ionized. If there is a high air velocity through the gap these ions will be
blown away, inhibiting a breakdown. For this reason the required ignition
voltage does go up with engine speed as the air velocity in the combustion
chamber is essentially directly proportional to engine speed. This effect
is not the dominate one, though.
I don't know if this clarified the issue or made it more confusing.
Gary Casey
Message 3
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| Subject: | Re: Pressure altitude and short sparks |
--> AeroElectric-List message posted by: Brian Lloyd <brianl@lloyd.com>
Gary Casey wrote:
> You can't assume that if the compression ratio is 10:1 that the pressure is
> 10 times the manifold pressure. The pressure actually rises as the 1.35 to
> 1.4 power of the volume ratio.
I was trying to explain things simply so that people who are not engineers or mathematicians
could quickly and easily grasp the concept.
But as I recall, the ideal gas law states,
PV = nRT
If the compression ratio is 10:1 and you have isothermal compression then the final
pressure will indeed be 10x the inlet pressure as P varies inversely with
V since n, R, and T don't change. I know that this is not totally realistic
but delta-T is going to be a lot smaller than you think because the heat of compression
will quickly transfer to the head and piston crown.
Regardless, you are correct in the details. Sorry I was being too simplistic.
> I don't know if this clarified the issue or made it more confusing.
I guess it depends on who you talk to. You are certainly more correct than I was
in detail.
--
Brian Lloyd 6501 Red Hook Plaza
brianl@lloyd.com Suite 201
http://www.lloyd.com St. Thomas, VI 00802
+1.340.998.9447 (voice) +1.360.838.9669 (fax)
There is a time to laud one's country and a time to protest.
A good citizen is prepared to do either as the need arises.
Message 4
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| Subject: | Re: Pressure altitude and short sparks |
--> AeroElectric-List message posted by: "Ed Anderson" <eanderson@carolina.rr.com>
----- Original Message -----
> --> AeroElectric-List message posted by: "Bruce Gray"
> <Bruce@glasair.org>
>
> Sorry, the engine is just a constant volume air pump. Cylinder pressure
> will not vary, regardless of RPM, MP, or a constant speed prop.
>
> Now cylinder pressure after ignition is a different matter.
>
> Bruce
> www.glasair.org
>
While the engine IS a constant volume air pump (in that the total
displacement of volume over a cycle is constant), the pressure generated
when the piston reaches TDC does depends on the density of the air volume
ingested by the cylinder. At low manifold pressures, the air inside the
manifold and therefore ingested by the engine, is lower in density than when
at high manifold pressures. This difference in density (not volume) is why
your air/fuel mixture needs change as well as the power you produce as you
vary the throttle opening. So taking a the same volume, but with air at say
twice the density at one intake event vs another intake event and compress
the density charge to the same TDC space and the pressure will be greater
than for the less dense charge. The engine is a constant volume machine,
but the cylinder pressure created does differ due to the different air
density in the manifold when the intake valve opens.
FWIW.
Ed Anderson
RV-6A N494BW Rotary Powered
Matthews, NC
eanderson@carolina.rr.com
Message 5
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RV-List <rv-list-digest@matronics.com>
| Subject: | Internal Regulated Alternator converted to External? |
--> AeroElectric-List message posted by: Bobby Hester <bhester@hopkinsville.net>
Has anyone had there Van's internal regulated 60amp alternator converted
to external and used B&C's LR3C-14 regulator?
Does this make since to do?
--
Surfing the Web from Hopkinsville, KY
Visit my web site at: http://www.geocities.com/hester-hoptown/RVSite/
RV7A Slowbuild wings-QB Fuse :-)
Message 6
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| Subject: | Off topic - tracing house wiring |
--> AeroElectric-List message posted by: "Ron Koyich" <Ron@Koyich.com>
Apologies for asking this on the list - but where else can one find
electrical-savy people concentrated in one place?
First - the home in question is in Hong Kong, and I've got no idea who
installed the wiring in the first place, or if I could find them even if
I knew.
Here's the challenge:
We have a concrete ceiling in our living room - with some 'loomex' type
wire imbedded in the concrete, terminating in the middle of the ceiling.
I've mounted a ceiling fan there, but cannot find where the other end of
the wire is located. I'd assumed it was at the large bank of switches
near the entrance door, as some of them did nothing. The wire doesn't go
to those switches, however, so I still cannot put power to the fan.
Somewhere in the dark recesses of my mind I recall seeing a device used
to trace wires. One part of the unit sends some kind of pulses down the
wires, and the other part is a detector for those pulses. So you clip
part a) onto the cable you need to trace, and use part b) to follow the
wire through whatever it's hidden behind.
Do any of you know what this unit might be called - or who makes them
- or where they can be purchased?
Or is this another home brew project coming up?
Thanks - Ron Koyich
Message 7
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| Subject: | Re: Off topic - tracing house wiring |
--> AeroElectric-List message posted by: richard@riley.net
At 09:57 PM 5/4/04, you wrote:
>-
>Do any of you know what this unit might be called - or who makes them
>- or where they can be purchased?
>
>Or is this another home brew project coming up?
This is the one I have
http://bkprecision.com/pressrelease/july27.html
http://www.testequipmentdepot.com/b+k%20precision/262.htm
http://www.tequipment.net/BK262.asp
The label says "made in China." No guarantee, of course, but you might be
able to get one locally.
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