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barry
Senior Member
Username: barry
Post Number: 53
Registered: 02-2006
| | Posted on Sunday, December 02, 2007 - 07:54 pm: | 
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Instruction information relating to old marine engines, particularly two cycle machines often is action/reaction style rather than a reference to basic principles. For example one very common instruction suggests the following starting procedure.
But if you do not get it to go the first time, open the priming cup and turn the engine over slowly until it spits or explodes through the priming cup.
Rather than trying to remember this action/reaction style folklore, I find it interesting and useful to understand basic principles, to use as a reference during starting, operating and trouble shooting.
I have documented my version of basic information covering such subjects as Principles of Combustion, Principles of Ignition, and Principles of Carburetion relating particularly to two cycle engines, as I find documentation makes the picture clearer and therefore the information more useable.
I welcome the input of others, as my version is neither complete nor accurate.
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First are my thoughts regarding the most basic principle …Principles of Combustion as this relates to two-cycle marine engines.
1 Gasoline is a compound of Carbon and Hydrogen. Combustion occurs when the carbon/hydrogen molecular bond is broken. Then oxygen combines with gasoline vapor to form steam and carbon dioxide. This happens when a combustible mixture is heated above the ignition temperature (approx 220 C).
Gasoline is a liquid; steam and carbon monoxide are gases thus the enormous volume of gases produced during combustion fill and pressurize the combustion chamber causing piston movement in a downward direction.
It is the function of the ignition system to produce a very high temperature in the spark plug gap that rapidly vaporizes, and then heats the air/gasoline vapor within the gap above the ignition temperature. This breaks the molecular bond and starts the combustion process.
2 The combustion process is exothermic, that is it produces heat. This heat of combustion both vaporizes gasoline then heats the unburned layers of air and gasoline in the combustion chamber above the temperature at which it auto ignites (220C). Thus a flame front is established that advances in a conical shape starting at the spark plug then continuing until it reaches the cylinder walls. The spark at the spark plug only starts the combustion process.
Perhaps 30 degrees of engine rotation occurs as the flame front advances to the cylinder wall.
3 Gasoline and air will only ignite if the ratio of air to gasoline vapor is within a range between 12.5/1 and 16.5/1. If the spark plug gap contains a fuel /air mixture outside of these limits, combustion will not start. If the advancing flame front encounters fuel/air mixture less than 12.5/1 (very rich) or greater than 16.5/1(very lean) the flame will extinguish.
4 To combust, gasoline must be present as a vapor. Any liquid present in the spark plug gap and combustion chamber must first change to a vapor. Gasoline that is atomized in the carburetor and subsequently the crankcase is mostly small droplets of liquid. Before combustion can occur, the liquid droplets must change to vapor.
5 Gasoline will vaporize slowly at atmospheric temperature however heat will speed up vaporization. Small droplets will vaporize faster than a puddle.
After a period of engine operation, warming of the crankcase helps vaporization. Heat is produced during compression. Exothermic combustion produces heat. Friction of moving engine components produces heat. However none of these sources of heat to vaporize gasoline are present during cold starting.
6 When a hand start one cylinder two-cycle engine first fires, sufficient energy must be transferred to the flywheel to power one complete revolution so that a second spark can then initiate a second firing. Also there must be a combustible fuel charge in the crankcase before the first firing; otherwise the second firing will not occur as a combustible mixture will not have been transferred to the combustion chamber.
If these two conditions are not present the engine may fire but will not continue to run.
7 A cold engine has no combustible mixture in the combustion chamber or the crankcase so the starting procedure must pre charge both chambers.
8 A stiff or misaligned engine will not be able to complete the first revolution so may fire but stop during the first revolution because the flywheel has insufficient energy to overcome the excessive resistance.
9 Ignition is deliberately retarded at starting to ensure rotation will occur in the desired direction. The more the ignition is retarded the less is the piston force piston force. Therefore less energy is available to power the first critical revolution.
Barry Millar © December 2007
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jb_castagnos
Senior Member
Username: jb_castagnos
Post Number: 318
Registered: 07-2002
| | Posted on Monday, December 03, 2007 - 08:10 pm: |
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Looks in order to me Barry. On the vaporized fuel, when you prime a dry engine with liquid fuel in the primer cup it will fire three or four times before dying, it's running on the fuel that has vaporized, the liquid will remain there for a few strokes before it is finally vaporized. Another thing to mention is fouled plugs. A high tension ignition will build up voltage untill it is sufficient to jump the plug gap. A fouled plug will allow a short to ground and the voltage will never build high enough to jump the gap. Removing the wire and holding a slight distance from the plug will allow voltage to build and jump the gap at the wire and the plug. It's not uncommon to have a buzz coil buzzing and an engine fire after a few seconds, I think this could be due to the mixture getting into range in the cylinder or a fouled plug starting to fire. On cranking an engine with late timing, there's no reason to go behind TDC, a few degrees above is usually OK if it doesn't kick back. When bounce starting against compression the timing should be far advanced, as soon as it fires it will be in full retard and have to be advanced imediatly. |
barry
Senior Member
Username: barry
Post Number: 54
Registered: 02-2006
| | Posted on Tuesday, December 04, 2007 - 03:32 pm: |
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JB
I would like to better understand your observation regarding a fouled plug.…
“Removing the wire and holding a slight distance from the plug will allow voltage to build and jump the gap at the wire and the plug.”
How do you determine that an operating problem is related to a fouled plug?
Why does removing the plug wire to create an external spark then cause a spark to occur in the spark gap?
How do you know that this happens?
When the plug wire is reconnected does the engine then operate normally?
Thanks for your interest.
Barry |
barry
Senior Member
Username: barry
Post Number: 55
Registered: 02-2006
| | Posted on Tuesday, December 04, 2007 - 04:59 pm: |
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JB,
“It's not uncommon to have a buzz coil buzzing and an engine fire after a few seconds”
I’m very interested to hear more about the circumstances when this occurs. I can’t recall experiencing this, maybe it has happened but I just didn’t recognize it.
I agree that it would seem to be related to the presence of a combustible mixture.
Barry |
doug11k
Advanced Member
Username: doug11k
Post Number: 41
Registered: 12-2003
| | Posted on Tuesday, December 04, 2007 - 06:53 pm: |
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Barry,
Try this link for a simple explanation:
http://www.steamengine.com.au/ic/history/spark-intensifier/ |
barry
Senior Member
Username: barry
Post Number: 56
Registered: 02-2006
| | Posted on Wednesday, December 05, 2007 - 10:49 am: |
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Doug,
Many thanks for the great link to Steam & Engine of Australia Web page that provides excellent information about the Meissner Spark Intensifier .
Meissner patent 1259893 filed Dec14 1914 and issued March 19 1918 shows a clever design where the “booster” spark is contained within a glass enclosure that magnifies to provide an easy to see external spark. Apparently the presence of this external spark is an indicator that an internal spark exists at the same time even though the plug is badly fouled.
A submission by John Culp at the bottom of the Steam & Engine of Australia Web page explains the operation of an Intensifier. I will quote John as his explanation is very clear and forthright.
quote:
"All true enough. However, such a "booster gap" makes it a lot harder for the plug to foul out and stop firing due to carbon on the insulator leaking off the charge on the center electrode. A coil doesn't fire instantaneously when the points open. There's a very brief rise time for the voltage to get high enough to jump the gap. If there's enough current leaking through the conductive carbon on the insulator nose, the voltage won't ever get high enough, and there's no spark. With the booster gap there's no current flow until the voltage has risen high enough to jump both gaps, which it does. Many of these plugs have been used to keep an oil burning, plug fouling engine running."
John
The Web page shows other versions of Intensifiers, one that is integral with the spark plug. The Meissner design is an-add on. I’m sure there are collectors who have examples of the Meeissner and other designs. It’s interesting that plug fouling in 1914 was such a problem as to stimulate the use of intensifiers rather than just routinely cleaning the plug.
Barry
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ernie
Senior Member
Username: ernie
Post Number: 772
Registered: 01-2002
| | Posted on Wednesday, December 05, 2007 - 04:03 pm: |
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Just remember and this is from expeirence that if an engine won't start when it has been runnig ok the first thing to check always is the plug!
Fuel mixture just isn't very important in these old engines. My launch with 3 hp gray 2 stroke runs the same and my seat of the pants dyno indicates no change from the point of almost too lean to run to almost too rich to run.
I have also confirmed this with my hand held GPS. The speed difference is about 1/2 MPH from too lean through best to too rich. All this is within about 1/2 to 3/4 turn of the needle valve.
Also the instructions to open the priming cup and turn the engine slowly is to clear a flooded engine. It is far easier to flood one of these old engines than it is to starve one. I also think that operators in years gone by and now too have a tendency to over prime and engine. I think manufacturers learned this and just about all of them published the same instructions to clear a flooded engine. |
jb_castagnos
Senior Member
Username: jb_castagnos
Post Number: 320
Registered: 07-2002
| | Posted on Wednesday, December 05, 2007 - 09:26 pm: |
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Barry, in the days of distributor caps and point ignition it was common to pull one wire at a time on an auto to locate a miss. If you pulled the wire and let the spark jump a fouled plug would start hitting. On an overchoked loaded up engine holding the coil wire out to make a boost gap often got them started. A fouled plug will not start to fire on it's own, but forcing it to fire would often clean it up.
I have had engines fire while listening to the buzz coil, this could be a plug drying out or the mixture coming into range, don't really know.
A cold engine needs a lot of fuel to get a little to vaporize, hot engines flood easily. Opening the primer lets it draw air without fuel, clearing flood. |
richarddurgee
Senior Member
Username: richarddurgee
Post Number: 1584
Registered: 11-2001
| | Posted on Thursday, December 06, 2007 - 07:34 am: |
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This discussion of "Spark Gaps" reminded me of the early 1950's, an old farm truck was brought into my Dads garage it had an inline 6 engine and on three of the spark plugs a potato had been stuck on top with the wire pushed in just far enough to maintain a gap !
For Several yrs afterwards when going to gatherings, fairs etc where the Snake Oil salesmen touted their wares there always seemed to be one selling spark gap kits in many different shapes ,sizes and even colors for your eng-- assured better starting, smoother running and better gas mileage ?
Late 1950's or early 60's Champion Spark Plug Co. came out with their "New" innovation - a spark gap made into the plug !
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barry
Senior Member
Username: barry
Post Number: 57
Registered: 02-2006
| | Posted on Thursday, December 06, 2007 - 12:48 pm: |
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Ernie, JB,
You raise several issues that are interesting and perhaps useful to discuss.
Regarding air/fuel ratios, it has also been my experience that about ¾ turn of the needle valve of a Schebler covers the range from max lean to max rich with very little RPM change. This occurs when using a standard 40 degree needle. I have undertaken several “test bench” investigations using needles of lesser angle in order to provide improved adjustment sensitivity by increasing the turns from max lean to max rich. Preliminary results suggest a 25 degree angle doubles the range.
I am of the opinion that optimum setting is the setting that leads to maximum combustion efficiency, where the least amount of Carbon Monoxide is produced as at max lean and the least amount of unburned hydrocarbon (gasoline) is present in the exhaust gases as at max rich. This “best” ratio and is about half way between max lean and max rich.
I attempt to set the Schebler with ignition at max advanced (about plus 20 degrees for my single cylinder St Lawrence that powers my Dory ) and fully loaded to 850RPM; then set the Schebler needle valve about half position between where misfiring occurs at max rich and four-cycling occurs at maximum lean. This is not a precision method but is perhaps the best available for old two-cycle engines.
This reference to four–cycling might benefit from clarification. Here is my view, as always with an invitation to others to add (or subtract) information.
During the power stroke the transfer port starts to open about ¼” of piston stoke following the exhaust port opening. Thus new charge enters at the same time as exhaust gases exit. There is an unavoidable mixing of the two streams. Thus a quantity of exhaust gas remains and mixes with the new charge for the subsequent power stroke.
If the carburetor needle is closed so that combustion is too lean to occur, exhaust gases will be unburned fuel/air mixture. Then during the next cycle the residual combustible mixture will mix with and enrich the new charge possibly enough to provide a combustible mixture for next power stroke. The engine will “skip cycle”, four stroke or possibly six stroke. When this occurs it very evident since the engine starts to pound. The solution is to enrich the mixture by opening the carburetor needle valve.
Barry |
ernie
Senior Member
Username: ernie
Post Number: 773
Registered: 01-2002
| | Posted on Friday, December 07, 2007 - 03:59 pm: |
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Barry,
Somewhere on OME is a post I wrote about why 2 stroke engines 4 stroke. I will try to find it. However it was part of another thread and several years ago. The quick reason is due to lean misfire as you alude to in your last paragraph above. |
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