Terminology: Pop, Cough, Sneeze, Spit -- All the same thing?

I've seen all four terms used in this forum. Are they all the same thing? What causes it? Where is the detonation? In the crankcase? Is that why the engine may stall (intake mixture turned to exhaust)? Why does a lean mixture at idle lead to this?

A lot of terms are used to describe, and often used synonymously with, two very different phenomenon that can occur in a internal combustion engine. In this case the proper nomenclature is backfire and after fire. Backfire is the ignition and burning of the fresh fuel and air mixture in the intake manifold and after fire is the ignition and burning of unburned fuel in the exhaust system. Both are most commonly caused by improper ignition timing and/or fuel/air mixture ratio. However in the two stroke engine things get a little hazy because the exhaust port and the intake transfer port timing overlap (that is they are open at the same time) to a much higher degree of crank shaft rotation than say the valve overlap of a four stroke. Detonation is something completely different; although it too can result from improper ignition timing it is more commonly associated with a lean fuel/ air mixture and is contained in the combustion chamber. (Note: there are always exceptions to the rules).

“Why does a lean mixture at idle lead to this?” Lean mixtures burn at a slow rate, slow enough that the mixture is still burning when the intake transfer ports open causing the incoming fresh charge to ignite and burn building up pressure in the intake side of the crankcase.

Different reasons to all four depending if coming from carbs/intake or thru exhaust

Actually, lean mixtures burn FASTER--that's why an engine backfires if it doesn't et enough gas.

The charge in the cylinder fires so quickly, a flame runs out the intake port (or valve) to light off the charge in the crankcase (or manifold ).

What we call "Sneezing" is when the idle mixture screw is set too lean. That causes what I described above. THe solution is to add a bit more fuel, which slows the combustion in the cylinders enough so there's no flash back of flames into the crankcase.


Jeff

Just to add to the above comments...I was always told that the fuel octane rating determines the rate of the fuel burn...low octane = fast burn vs. high octane = slow burn. That's why I run high octane fuel in my OB.

Yes, high octane has a lower flashpoint as well which is why it is called for in higher compression engines - you can squeeze the snot out of it and it will ignite properly while a lower octane will not.

However, when talking about a 2 stroke, believe it or not, just about anything over 65 octane (you can get it some places in Asia) will burn without too much trouble - commercial models will burn kerosene (which is only a slight refinement above diesel) if that's all that's around - a little choppy, but will get you from one place to the other. As Jeff notes when the intake and exhaust ports are "both open" at the same time, the motor usually isn't too picky about what it's burning. Many motors are simply "cross flow" where the air/fuel/oil mix simply passes from one side of the cylinder to the other (port to port) over top of the piston - some getting trapped during compression, while others are "loop charged" where they give the top of the piston "some shape" to try and deflect the gas/oil/air that comes in to the dome of the head, keeping more in the cylinder while the exhaust port is exposed.

(personally don't see any performance difference between the two designs - and ALL two strokes are gas pigs - so are 4 strokes when you get about 3500 rpms)

And yes, as to the terminology, I don't think there is an "outboard dictionary" anywhere so you get the same term describing a number of different conditions (which is what makes it confusing sometimes).

If you are speaking "Merc" with a "Canadian accent" (eh!) I usually refer to:

Sneeze - a lean intake condition - usually caused by improper carb adjustment or a bad gasket somewhere in the intake side of things
Pop - usually caused by an exhaust leak or a bad powerhead base gasket
Cough - a timing issue resulting in a (backfire) that either comes back into the intake or out through the exhaust
Spit - that's the water coming out of the tell-tale (pee-hole, pisser, etc) or the exhaust relief holes in the exhaust housing OR gas being thrown out of the mouth of the carb due to broken/chipped reeds

And my all time favourite (see Canadian spelling eh):

Crank - to some it means "turn over" to others it means "start" - as in my motor won't crank which leaves me wondering if the starter isn't turning over or it is, but the motor won't start. For me "crank" is what I do to the jack to get the trailer tongue off the hitch of my truck

And remember, when talking outboards - the only place for a 4 stroke is in the front end of a tow vehicle

@fastjeff:"Actually, lean mixtures burn FASTER--that's why an engine backfires if it doesn't et enough gas."

Here is a paper put out by Fernando Salazar at the Department of Aerospace and Mechanical Engineering
University of Notre Dame.

http://www3.nd.edu/~msen/Teaching/DirStudies/Engines.pdf

This should help give you a better understanding of the internal combustion process. In specific reference to the rate of flame speed propagation vs air/fuel mixture read chapter 7, pay particular attention to pages 39-41 and figure 7.3 on page 41. Unfortunately personal opinion and the internet blogs are not always based on scientific facts, and I too have often times been mistaken.

Cool...thanks for the document.

I feel left out - I don't have any scientific paper to post....

@galamb: “Many motors are simply "cross flow" where the air/fuel/oil mix simply passes from one side of the cylinder to the other (port to port) over top of the piston - some getting trapped during compression, while others are "loop charged" where they give the top of the piston "some shape" to try and deflect the gas/oil/air that comes in to the dome of the head, keeping more in the cylinder while the exhaust port is exposed.

I feel left out - I don't have any scientific paper to post....”

[Emphasis added]

Don’t feel left out; “scientific papers” are not always necessary to make a point even a cursory Google search would have revealed this:

Two stroke cross-scavenged (“cross flow”) engines can be identified by the irregular shape of the top of the piston called a deflector.


Pistons in loop scavenged (“loop charged”) engines are generally near flat.

@kimcrwbr1: “There are two conditions that are very similar with the symptom I call it a cough and a stumble. When the idle mix is too rich it will cough and if it is too lean it will stumble.”

Actually a cough is to expel air from the lungs suddenly and noisily. To me a cough ( if we are to anthropomorphize the internal combustion engine in to a living and breathing thing) is a backfire through the induction system. Rich or lean I guess it’s determined by which side of the stoichiometric scale you choose. 14.7 is a number we should all appreciate if we are committed to performance. However, to commit to those temperatures we must always remain cool. You are correct in your stumble analysis, however you must remember that there is a fine line between, idle and trolling “fuel economy”, and “acceleration”. Personally I prefer to go fast, but I respect those who prefer to take life a little slower. To them all I can say is get the hell out of the way.

Here is a paper put out by Fernando Salazar at the Department of Aerospace and Mechanical Engineering
University of Notre Dame.

http://www3.nd.edu/~msen/Teaching/DirStudies/Engines.pdf

Very generous of you to post such a resource. I enjoyed the reading. I'm not sure what to do with all the numbers yet. Actually, I got a little lost, right in the introduction, where Fernando described how four strokes use a mix of fuel and water! Hey, just having fun; I'm sure it was a type-o.

Egg heads!

Hey, Graham, I'm with you on the only good use for 4 strokes is the tow vehicle!

Jeff

Ha, good one H.I. McDunnough, typo we can assume. The mention of water would have been better placed under a different Chapter. The only reason I brought up the paper was for the discussion on the combustion process which is the same regardless of the number of strokes used in the process. Water, Ha, we all know that an internal combustion engine will not run on water right? For those interested read on. This is getting a little off topic pursuant to outboard motors and is only relevant to the four-stroke engine.

During World War II the “War Birds” (military aircraft) were pushing the limits in internal combustion engines. High compression ratios, supercharging and turbocharging were the mainstay in performance. Whoever could fly faster, fly higher and out maneuver the other had an advantage. Detonation was a scourge and a limiting factor in performance so aviation fuels were developed with octane ratings as high as 115/145, the first number being the lean mixture performance number and the second number being the rich mixture performance number. Enter water, a specific fuel consumption booster. Water has two benefits which enhance the high performance combustion process: (1) its high heat of evaporation which is a cooling process and (2) it suppresses detonation. ADI (anti-detonation injection) which was a mixture of water and alcohol (the alcohol was used as antifreeze because of the sub-zero temperatures at altitude) and was introduced into the fuel discharge nozzle.

It also does a good job of steam cleaning the cylinders