crossfiring with LPG but not petrol

[Orson Cart]

I had a car that was prone to going POP and blowing up air filters, when
running on LPG.
Eventually, found the cause was crossfiring, as 3 spark plug cables were bunched
together in a narrow space. After rerouting the cables in a less intimate
fashion,
the problem is gone (for several months now....)
I understand why this happens: if crossfiring occurs when some cylinder is
on
intake stroke, it takes less energy to fire the uncompressed gas. The intake
valve is open, and the flame spreads to the manifold, with lots of fuel to
go bang.
But I wonder does this happen on a multi-point injected engine? The crossfiring
could still occur. It would ignite a smaller amount of fuel, as the intake
manifold
only contains air. I expect it would still be enough to be heard.

[Scott Dorsey]

In article <4f23f809$1...@x-privat.org>, Orson Cart <ex-p...@parts.org> wrote:
>
>I had a car that was prone to going POP and blowing up air filters, when
>running on LPG.

....

>But I wonder does this happen on a multi-point injected engine? The crossfiring
>could still occur. It would ignite a smaller amount of fuel, as the intake
>manifold
>only contains air. I expect it would still be enough to be heard.

Maybe the intake manifold doesn't just contain air... maybe you have some
leakage past valves that is occurring only with LPG.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[jim beam]

absolutely - and the amount depends on the valve timing. most cars have
some degree of timing overlap - it's used to charge intake manifold
resonators.


--
nomina rutrum rutrum

[Bob Flumere]

On 28 Jan 2012 14:28:41 +0100, "Orson Cart" <ex-p...@parts.org>
wrote:

The LPG may have a lower (spark) voltage requirement at the plug.

[AMuzi]

"LPG has a
much higher ignition temperature of 920-1020 degrees vs.
80-300 degrees for gasoline"

http://www.technocarb.com/index2.php?option=com_content&do_pdf=1&id=13

--
Andrew Muzi
<www.yellowjersey.org/>
Open every day since 1 April, 1971

[Clive]

In message <jg1htc$93t$1...@dont-email.me>, AMuzi <a...@yellowjersey.org>
writes

>"LPG has a
>much higher ignition temperature of 920-1020 degrees vs. 80-300 degrees
>for gasoline"
>http://www.technocarb.com/index2.php?option=com_content&do_pdf=1&id=13

I don't want to rain on your parade, but 80-300 degrees for petrol or
"gas" as you call it is not the ignition temperature, but the
temperature for the liquid to turn to a gas, it's boiling point (or
phase change) at one atmosphere and 20 degrees Celsius The ignition
point is MUCH higher and is dependent on compression pressures. Your
link is incorrect in stating that it is.
--
Clive

[Clive]

In message <SCyCHtwt...@yewbank.demon.co.uk>, Clive
<cl...@yewbank.demon.co.uk> writes

> at one atmosphere and 20 degrees Celsius

Should not have been in the text I wrote, sorry.
--
Clive

[AMuzi]

Thanks. Yeah, probably; first result of a search.

Here's a PE's report on that:

http://www.tedfordpond.com/wp-content/uploads/article_1185463566.pdf

[Orson Cart]

klu...@panix.com (Scott Dorsey) wrote:
>
>Maybe the intake manifold doesn't just contain air... maybe you have
>some
>leakage past valves that is occurring only with LPG.
>--scott
>

Perhaps I should clarify - LPG uses a mixer before the throttle,
so manifold is full of fuel. Petrol uses sequential injection,
so do not expect much fuel to be in the manifold.
The crossfiring is an electrical phenomenon, so should happen
for both fuels. Yet did not notice it with petrol.
Petrol requires a lower spark voltage, so there would be less
energy leaking between spark plug leads, but also lower voltage to fire the
cylinder that should not fire, so I thought this factor would kind of cancel
out.

[hls]

"Orson Cart" <ex-p...@parts.org> wrote in message
news:4f2481b7$1...@x-privat.org...

> Petrol requires a lower spark voltage,

Had never really considered this, but please cite your source.

[jim beam]

On 01/28/2012 03:16 PM, Orson Cart wrote:
> klu...@panix.com (Scott Dorsey) wrote:
>>
>> Maybe the intake manifold doesn't just contain air... maybe you have
>> some
>> leakage past valves that is occurring only with LPG.
>> --scott
>>
>
> Perhaps I should clarify - LPG uses a mixer before the throttle,
> so manifold is full of fuel. Petrol uses sequential injection,
> so do not expect much fuel to be in the manifold.

fuel not being in the manifold is nothing to do with whether it's
sequential but whether it's injected into the port or not. most
sequential injection systems are also port injected, so that is why
there's no fuel in the manifold. if you had an f1 manifold, the
injectors would be above the throttle plates and the whole length of the
manifold would be full of fuel despite it being sequential.

> The crossfiring is an electrical phenomenon, so should happen
> for both fuels.

it's not electrical, it's because the valve timing means the intake is
opening when there is still burning exhaust exiting the cylinder. with
aggressively timed engines, this valve overlap becomes more pronounced,
and with an easily flashed mix of lpg/air, any flame still in the
cylinder from an open valve can burn back up through the manifold,
particularly at lower rpms.

> Yet did not notice it with petrol.

that's because the engine is built for and timed for gasoline, not lpg.

> Petrol requires a lower spark voltage, so there would be less
> energy leaking between spark plug leads, but also lower voltage to fire the
> cylinder that should not fire, so I thought this factor would kind of cancel
> out.

you're barking up the wrong tree with that thinking - see above.


--
nomina rutrum rutrum