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Engine Backfires

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algrajek

Joined: 10 Jan 2006
Posts: 41
Location: Lexington, Ky

Posted: Thu Feb 15, 2007 9:48 am Post subject: Engine Backfires

I think you are confused between BACKfire and AFTERfire.

All(or most) of the engines have afterfire when you pull to idle. All 

engines do it. The ony reason you hear it in the RVs is you dont have a 

muffler. It is just excess fuel burning off at the idle setting.

Al Grajek

RV8

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just rv8in

gerf(at)gerf.com
Guest

Posted: Thu Feb 15, 2007 10:21 am Post subject: Engine Backfires

Agreed that I am confused, if one can be confused having no knowledge whatsoever.



If "afterfire" is the case - where is the excess fuel coming from, assuming:



- fuel is burned to provide power when the throttle is open

- there is little or no fuel when the throttle is closed (or the engine would be developing power)



(BTW I have the muffled exhaust from Vettermans, although I understand its little more than 2 expansion chambers.)



g

 

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Ed Anderson

Joined: 10 Jan 2006
Posts: 475

Posted: Thu Feb 15, 2007 11:32 am Post subject: Engine Backfires

"After fire" occurring in the exhaust system  is the  result of several factors. 

  

  One, when you close the throttle, you have  immediately reduced the amount of air available to cylinders to combust  fuel. But the fuel quantity already in the manifold does not fall off  as quickly.   That means there is generally some unburned fuel ending  up in the exhaust system.  This fuel can be "cooked off" by the  temperatures it encounters in the exhaust - this is promoted in short exhausts  as there are "reversion" waves reflected back from the exhaust pipe opening  which can push some oxygen back into the exhaust system.  When  these two factors combine - "after fire" is a common result.  There is  generally no  damage and nothing to worry about - unless you have  a passenger with a weak heart.  However, it occurred frequently and  prolonged I would check my exhaust valves for cracks.

  

  

  

 "Back firing" (occurring in the intake manifold) is  generally when the combustion process occurs in the cylinder before the intake  valve is completely closed.  This may cause air/fuel mixture in the intake  manifold to ignite - sometimes blowing off components depending on the degree of  air/fuel mixture and other factors.  This can happened due to misfire due  to ignition time set incorrectly or to an overly lean mixture.  Lean  mixtures burn slowly and the process may not be completed if the mixture is very  lean by the time the intake valve again opens.  This can then ignite the  mixture in the intake manifold.  Back firing can result in  damage.

  

 At least that's what I recall about the topic.   

  

 Ed

  

  

 [quote]   ---

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Ed Anderson
Rv-6A N494BW Rotary Powered
Matthews, NC
eanderson@carolina.rr.com

gerf(at)gerf.com
Guest

Posted: Thu Feb 15, 2007 2:55 pm Post subject: Engine Backfires

Thanks, that makes sense to me ... now I'm wondering why I experience the same backfiring when I lean too aggressively looking for the "peak" in cruise flight at altitude.



g



 [quote]--

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skylor4(at)yahoo.com
Guest

Posted: Thu Feb 15, 2007 4:44 pm Post subject: Engine Backfires

Backfires caused by over leaning is probably caused by

the engine missing frequently and the resulting

unburned mixture igniting in the exhaust. It may also

be due to the fact that the combustion rate at very

lean mixtures is so slow that the mixture continues to

burn as it leaves the cylinder and enters the exhaust.



Skylor

RV-8 QB

Under Construction

--- Gerry Filby <gerf(at)gerf.com> wrote:



[quote]  Thanks, that makes sense to me ... now I'm

 wondering why I experience the same backfiring when

 I lean too aggressively looking for the "peak" in

 cruise flight at altitude.

 

 g

 

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Ed Anderson

Joined: 10 Jan 2006
Posts: 475

Posted: Thu Feb 15, 2007 4:54 pm Post subject: Engine Backfires

G,  It would take instrumentation to be certain, but I suspect  that the cause is similar to closing the throttle. 

  

  If you have WOT then your manifold pressure is high meaning your  engine in ingesting high  density air into the cylinders.  The piston  then compresses the mixture and the plug fires it.  However, igniting the  mixture with an electrical spark under high pressure is not as easy with a lean  mixture as it is with a richer mixture.  As a consequence, there is more  "misfire" , as you known the leaner the mixture the more misfire occurs until  ultimately the engine will not run as the mixture is too lean.  At this  point no fuel is being burnt in the cylinders but fuel is still flowing into the  exhaust.   These lean misfires in the cylinder can once again result  in unburned fuel getting into the exhaust  and being "cooked - off".   At least that's my take on it.

  

 Ed

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Ed Anderson
Rv-6A N494BW Rotary Powered
Matthews, NC
eanderson@carolina.rr.com

n616tb(at)btsapps.com
Guest

Posted: Thu Feb 15, 2007 6:06 pm Post subject: Engine Backfires

Your exhaust temps are also quite a bit higher when leaned and will re-burn

any spent fuel in the exhaust.



[quote] --

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skylor4(at)yahoo.com
Guest

Posted: Thu Feb 15, 2007 7:05 pm Post subject: Engine Backfires

Quite a bit higher than what?  When Lean of Peak,

EGT's are lower than peak.  At peak, the exhaust gas

temperatures are, of course, higher than anywhere

else, and backfire sounds don't occur there (unless

something is really wrong).



--- Tim Bryan <n616tb(at)btsapps.com> wrote:



 	  Quote: 	
		   

 <n616tb(at)btsapps.com>

 

 Your exhaust temps are also quite a bit higher when

 leaned and will re-burn

 any spent fuel in the exhaust.

 


	


 

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sportav8r(at)gmail.com
Guest

Posted: Fri Feb 16, 2007 8:08 am Post subject: Engine Backfires

Thanks, Ed.  I feel exonerated and "affirmed" for my earlier post in

this thread.



-Stormy



On 2/15/07, Ed Anderson <eanderson(at)carolina.rr.com> wrote:

[quote]



 "After fire" occurring in the exhaust system  is the result of several

 factors.



  One, when you close the throttle, you have immediately reduced the amount

 of air available to cylinders to combust fuel. But the fuel quantity already

 in the manifold does not fall off as quickly.   That means there is

 generally some unburned fuel ending up in the exhaust system.  This fuel can

 be "cooked off" by the temperatures it encounters in the exhaust - this is

 promoted in short exhausts as there are "reversion" waves reflected back

 from the exhaust pipe opening which can push some oxygen back into the

 exhaust system.  When these two factors combine - "after fire" is a common

 result.  There is generally no  damage and nothing to worry about - unless

 you have a passenger with a weak heart.  However, it occurred frequently and

 prolonged I would check my exhaust valves for cracks.



 "Back firing" (occurring in the intake manifold) is generally when the

 combustion process occurs in the cylinder before the intake valve is

 completely closed.  This may cause air/fuel mixture in the intake manifold

 to ignite - sometimes blowing off components depending on the degree of

 air/fuel mixture and other factors.  This can happened due to misfire due to

 ignition time set incorrectly or to an overly lean mixture.  Lean mixtures

 burn slowly and the process may not be completed if the mixture is very lean

 by the time the intake valve again opens.  This can then ignite the mixture

 in the intake manifold.  Back firing can result in damage.



 At least that's what I recall about the topic.



 Ed



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