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Super charging adds cylinder pressure,and then some!

captinjohn

Some really hopped up engines have just enought compression to start....from then the Super Charger takes over!!!!!!!!!!!!!!!!!

Tired Old Man

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I think I misunderstood you as well. No big deal on my side. Hope I didn't catch you on a one off thing as well.

There's a subtle hint in that article for people that have engine bearing issues, and it's just as subtle for the companies that perform warranty work. Evidence of detonation is usually visible, and someone that returned an engine for warranty work with that evidence present might want to reconsider asking for free warranty repairs This is one of the reasons I don't get excited too early when I read about people new to gas engines experiencing a bearing failure. I figure about 30% of them forced it to happen.

jedijody

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It wasn't subtle to me! The rod bearingangle of detonationis something that I've thought about for quite some time. Detonation is something I'm very familiar with, it most certainly can have severe effects on caged roller bearings. I hear engines run at various clubs, I can hear an engine detonating on an up line and my mind instantly visualizes this chemical and mechanical chain of events (I'm weird that way)that results in what equates to a sledge hammer impact on the top of the piston. If the impact is at atime in the engines rotation that forces the clearanceof the rod bearing to be slammed in the opposite direction, the result is ahorrendous metal to metal collision with the rod bearing as the filling in a connecting rod/crank pin sandwich. If you can visualize this happening several times a secondit'll send shivers down your spine, better yet, visualize it the next time you come around and enter your hover 2 feet off the paved runway. [:@] Try to help someone by enlightening them to what is going on in their engine and you get the deer in the headlights stare or the "you don't know your butt from a bike rack" speech. Really?

I look for clear evidence of detonation in every bearing failure I see, it has been present in several instances but it's never what you're used to seeing in say a motorcycle engine, outboard, or even an automotive engine where the edge of the piston is blown away down to the ring or a hole clear through thepiston half way between the edge and the center of the dome. It's very subtle and looks like a gray dusting ina localized area at theedge of the piston with a corresponding area of similar appearance in the squish band. What it actually is, is an area where the carbon has been blasted off ofthe metal of the piston and at the squish band, using a magnifying glass you can actually see very small pits in the aluminum.

When it comes to engine failures possibly caused by detonation, in the engines I've worked on it would bea very thin line to walk on. The evidence ofdetonationhas beenso subtle it's hard to label it as the precursor to a disintegratedbearing, mainly because almost NOBODY understands exactly what detonation is let alone the forces that are actually involved and how those forcesrelate to parts of the engine far away from the epicenter of the actual event. It would definitely be a very hard sell to most peoplefacing paying for a crankshaft replacement.

BTW Pat, that is a jewel of an article and this is a very good place to post it, kudos for you!

Super08

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I watch out for the sound of marbles rattling. Sometimes it is faint.

av8tor1977

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Several times a second?? At 6000 rpms, those "sledge hammer hits" are ocurring at 100 times a second! Rotax two stroke engines were having problems trashing the cages on the wrist pin roller bearings. Their solution was to do away with the cages and run just rollers. It is a bit tricky to put them together, but no more disintegrated cages...

AV8TOR

jedijody

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Detonation does not happen even close to every revolution, it's somewhat random with an unequal number of revolutions between each detonation "event".

As far as the bearings go, eliminating the cages allows the use of more rollers, more rollers allows the load of the forces to be dispersed over a larger area and a greater number of individual rollers rather than just one or two. Only the most intelligent designers use this bearing method, it's not difficult to install a full roller bearing but in manufacturing it takes less time to slap a caged roller assembly on a shaft than using grease to hold the individual needles in place for assembly. Considering the skill level of most of the Chinese assemblers I'm sure the caged bearing may actually cut down on the number of warranty problems that could be caused by not having enough needles in a bearing surface.

NM2K

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There is no such term as predetonation. There is preignition and then there is detonation. One leads to the other, but no predetonation.


Ed Cregger

Tired Old Man

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Ed is correct.

makoman1860

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Just to bring some people "up-to-date" here. Squish is really no longer considered a reliable lever in knock control. Simple elimination of trapped end gasses was the key, not a "jet" of gass coming from squish area as was once thought. Also, by the time detonation is audible its quite well along, there can be long term damaging low-level knock that can only be seen on indicating. So moving top rings up, running tighter piston clearance etc all help. A college ran some testing on a glow 4-stroke engine with indicating, and it was amazing how unstable the glow ignition system really is even on methanol. So like I said before, gasoling has almost no chance, and personally I would think effort would be better spent on spark ignition systems. Maybe using scaled down full size shielded ignition instead of cheap metal boots? Or I wouldnt mind seeing a coil charged system rather than battery powered. Larger engines have had batteryless digital ignition and EFI systems for year, without the problems of CDI.

captinjohn

All high output engines are prone to destructive tendencies as a result of over boost, misfueling, mis-tuning and inadequate cooling. The engine community pushes ever nearer to the limits of power output. As they often learn cylinder chamber combustion processes can quickly gravitate to engine failure. This article defines two types of engine failures, detonation and pre-ignition, that are as insidious in nature to users as they are hard to recognize and detect. This discussion is intended only as a primer about these combustion processes since whole books have been devoted to the subject.

First, let us review normal combustion. It is the burning of a fuel and air mixture charge in the combustion chamber. It should burn in a steady, even fashion across the chamber, originating at the spark plug and progressing across the chamber in a three dimensional fashion. Similar to a pebble in a glass smooth pond with the ripples spreading out, the flame front should progress in an orderly fashion. The burn moves all the way across the chamber and , quenches (cools) against the walls and the piston crown. The burn should be complete with no remaining fuel-air mixture. Note that the mixture does not "explode" but burns in an orderly fashion.

There is another factor that engineers look for to quantify combustion. It is called "location of peak pressure (LPP)." It is measured by an in-cylinder pressure transducer. Ideally, the LPP should occur at 14 degrees after top dead center. Depending on the chamber design and the burn rate, if one would initiate the spark at its optimum timing (20 degrees BTDC, for example) the burn would progress through the chamber and reach LPP, or peak pressure at 14 degrees after top dead center. LPP is a mechanical factor just as an engine is a mechanical device. The piston can only go up and down so fast. If you peak the pressure too soon or too late in the cycle, you won't have optimum work. Therefore, LPP is always 14 degrees ATDC for any engine.

I introduce LPP now to illustrate the idea that there is a characteristic pressure buildup (compression and combustion) and decay (piston downward movement and exhaust valve opening) during the combustion process that can be considered "normal" if it is smooth, controlled and its peak occurs at 14 degrees ATDC.

Our enlarged definition of normal combustion now says that the charge/bum is initiated with the spark plug, a nice even burn moves across the chamber, combustion is completed and peak pressure occurs at at 14 ATDC.

Confusion and a lot of questions exist as to detonation and pre-ignition. Sometimes you hear mistaken terms like "pre-detonation". Detonation is one phenomenon that is abnormal combustion. Pre-ignition is another phenomenon that is abnormal combustion. The two, as we will talk about, are somewhat related but are two distinctly different phenomenon and can induce distinctly different failure modes.

The above is good too read also. Capt,n

captinjohn

If you inspect an engine and find yellow or yellow-orange deposits on the piston dome, that engine has experienced an abnormal combustion condition known as detonation. In this situation, combustion begins normally with ignition from the spark plug but as the flame front moves across the combustion chamber, rather than burning smoothly, the unburned portion of the charge spontaneously ignites. This results in extremely high temperatures and a shock wave heard as "pinging." What has happened is combustion has been completed in about 29 degrees or crankshaft rotation instead of the normal 50.

The oxide of calcium, an element found in 2 - stroke oil additives, is normally near white in colour. At temperature very near that of the melting point of the piston, the oxide will change colour from white to yellow-orange and is an indicator that the engine was overheated. The excessive heat results in rapid piston expansion and possible failure of the lubrication film.

captinjohn

This piston is NORMAL in appearance. The deposits on the piston dome are from the oil and ash, fuel contaminants and unburned carbon. As the thickness increases with operating time, the color gets brighter because of higher surface temperatures. Depending on gasoline and oil used, normal deposits may also be brown, from light tan colour to almost black dark brown. Heavy black deposits on the piston dome are unburned carbon accumulated because of too low temperatures resulting from light load operation or too rich a mixture. Brown or black varnish on piston side below rings is baked oil caused by combustion gases blow-by. Improper oil or poor ring sealing is the cause. Very light scoring of the piston skirt is likely from foreign material passing through the engine. It does not suggest a problem in the engine and piston replacement is not required. Always check piston to cylinder wall clearance to make certain the wear limit has not been exceeded. Deposits should be removed from the piston dome and cylinder head using a wood or plastic scraper. Excessive deposits collection will increase the compression ration and reduce heat transfer. This piston is NORMAL in appearance. The deposits on the piston dome are from the oil and ash, fuel contaminants and unburned carbon. As the thickness increases with operating time, the color gets brighter because of higher surface temperatures. Depending on gasoline and oil used, normal deposits may also be brown, from light tan colour to almost black dark brown. Heavy black deposits on the piston dome are unburned carbon accumulated because of too low temperatures resulting from light load operation or too rich a mixture. Brown or black varnish on piston side below rings is baked oil caused by combustion gases blow-by. Improper oil or poor ring sealing is the cause. Very light scoring of the piston skirt is likely from foreign material passing through the engine. It does not suggest a problem in the engine and piston replacement is not required. Always check piston to cylinder wall clearance to make certain the wear limit has not been exceeded. Deposits should be removed from the piston dome and cylinder head using a wood or plastic scraper. Excessive deposits collection will increase the compression ration and reduce heat transfer. Seized Pistons The piston shows yellow spots on the dome and seizure marks on the skirt but there is no sign of metal aluminum. If you inspect an engine and find yellow or yellow-orange deposits on the piston dome, that engine has experienced an abnormal combustion condition known as detonation. In this situation, combustion begins normally with ignition from the spark plug but as the flame front moves across the combustion chamber, rather than burning smoothly, the unburned portion of the charge spontaneously ignites. This results in extremely high temperatures and a shock wave heard as "pinging." What has happened is combustion has been completed in about 29 degrees or crankshaft rotation instead of the normal 50. The oxide of calcium, an element found in 2 - stroke oil additives, is normally near white in colour. At temperature very near that of the melting point of the piston, the oxide will change colour from white to yellow-orange and is an indicator that the engine was overheated. The excessive heat results in rapid piston expansion and possible failure of the lubrication film. Possible causes: · too low an octane fuel used or fuel with too much alcohol used · jetting too lean or failure of the fuel system, i.e., clogged fuel line or filter, fuel tank not venting, problem with fuel pump, carburetor,m air leak into crankcase · spark plug heat range too hot · ignition timing too far advanced, possible failure of the c.d. amplifier box · restricted exhaust system: back pressure too high · overheated, loosely-installed spark plug Seized Pistons The piston shows yellow spots on the dome and seizure marks on the skirt but there is no sign of metal aluminum. If you inspect an engine and find yellow or yellow-orange deposits on the piston dome, that engine has experienced an abnormal combustion condition known as detonation. In this situation, combustion begins normally with ignition from the spark plug but as the flame front moves across the combustion chamber, rather than burning smoothly, the unburned portion of the charge spontaneously ignites. This results in extremely high temperatures and a shock wave heard as "pinging." What has happened is combustion has been completed in about 29 degrees or crankshaft rotation instead of the normal 50. The oxide of calcium, an element found in 2 - stroke oil additives, is normally near white in colour. At temperature very near that of the melting point of the piston, the oxide will change colour from white to yellow-orange and is an indicator that the engine was overheated. The excessive heat results in rapid piston expansion and possible failure of the lubrication film. Possible causes: · too low an octane fuel used or fuel with too much alcohol used · jetting too lean or failure of the fuel system, i.e., clogged fuel line or filter, fuel tank not venting, problem with fuel pump, carburetor,m air leak into crankcase · spark plug heat range too hot · ignition timing too far advanced, possible failure of the c.d. amplifier box · restricted exhaust system: back pressure too high · overheated, loosely-installed spark plug Piston dome damage The piston shows melted areas on the dome and seizure marks on the skirt. Detonation results in extremely high combustion chamber temperatures. That rapid heat rise, if allowed to continue, will heat combustion chamber deposits and spark plug electrodes to the point that they will ignite the charge before the spark at the plug does. This condition is called runaway surface ignition or pre-ignition. Once pre-ignition occurs, the temperature rise is so rapid that melting point of the piston is reached while the engine is still running. Metal will melt away beneath the spark plug or in areas of high heat retention such as at the ring positioning pin. A complete breakdown of the lubrication film also occurs resulting in severe seizure. Detonation always precedes pre-ignition and the causes for this abnormal combustion are the same as those for detonation. Piston dome damage The piston shows melted areas on the dome and seizure marks on the skirt. Detonation results in extremely high combustion chamber temperatures. That rapid heat rise, if allowed to continue, will heat combustion chamber deposits and spark plug electrodes to the point that they will ignite the charge before the spark at the plug does. This condition is called runaway surface ignition or pre-ignition. Once pre-ignition occurs, the temperature rise is so rapid that melting point of the piston is reached while the engine is still running. Metal will melt away beneath the spark plug or in areas of high heat retention such as at the ring positioning pin. A complete breakdown of the lubrication film also occurs resulting in severe seizure. Detonation always precedes pre-ignition and the causes for this abnormal combustion are the same as those for detonation. Piston dome damage The piston shows melted areas on the dome and seizure marks on the skirt. Detonation results in extremely high combustion chamber temperatures. That rapid heat rise, if allowed to continue, will heat combustion chamber deposits and spark plug electrodes to the point that they will ignite the charge before the spark at the plug does. This condition is called runaway surface ignition or pre-ignition. Once pre-ignition occurs, the temperature rise is so rapid that melting point of the piston is reached while the engine is still running. Metal will melt away beneath the spark plug or in areas of high heat retention such as at the ring positioning pin. A complete breakdown of the lubrication film also occurs resulting in severe seizure. Detonation always precedes pre-ignition and the causes for this abnormal combustion are the same as those for detonation.

MTK

Hey Cap'n,

It looked like from one of the descriptions, that the catalytic plug is a single speed device. If one wants to use it in variable speed applications, some type of electronic goodie is needed to make it work. This "Ignition Module" if you will, may require electicity to work, I would guess, and some type of rpm sensor to effect corrected timing

Enya's 180 gas engine will be interesting to watch out for. It appears designed much closer to square than the other 180 gas engines of today.

Nevertheless, interesting stuff

Tired Old Man

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I'll see if I can dig up some pics of detonation combined with exceeding oil design limits. It gets real ugly.

av8tor1977

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Is it now common consensus that squish doesn't work in two strokes?? Because it definitely helps with four stroke auto engines. I made the mistake of building a hot 454 Chevy with open chamber heads and 9.5:1 compression for the street, and it is a "pinging son of a gun" if I don't put some Avgas in it every once in a while and limit the timing advance. The same engine and compression ratio with a quench type head is not nearly as sensitive.

The auto manufacturers tend to try to use other means to limit detonation because the fuel mix trapped in the quench area often does not burn completely and hurts both fuel economy and emissions. Long rod/stroke ratios and the increased piston dwell time they cause seem to lower octane requirements and many engine designs today follow that trend, along of course with fuel injection, knock sensors, computer controlled timing, etc.

AV8TOR

Tired Old Man

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It is not common consensus.

Squish is still quite important in small two strokes. Emphasis on small. When EFI enters into the equation it becomes even more importan, critically importantt-in small two strokes. Then you have squish angle, along with piston dome, combustion chamber shape relationsips to impact things even more.

av8tor1977

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Ok, that sounds more in line with my knowledge of two strokes. I must have misunderstood an earlier post.

Thanks,
AV8TOR

Tired Old Man

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That was Ed saying there was no mention of squish in one post and a later discussion among others about lack of squish importance. Note I stayed clear of that part of the conversation

makoman1860

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I was the one who said that "squish" itself was not what we once thought it was. Basicly the mechanism by which combustion systems would either be knock sensitive or not turned out to not be what we used to call "squish" or the fast movement of gasses before the ignition event. It was really more the elimination of trapped end gasses, or allowing a large enough space for ignition of the end gasses in a timely fashion. So while the same mechanical changes did improve things, now that we have a better understanding of the mechanism causing the problem, we can do an even better job. Not trying to start an argument, just educate. While being a hobbyist, I also design combustion systems for a living, just trying to pass on some learned wisdom.

NM2K

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Not THIS Ed...


Ed Cregger

Tired Old Man

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Mako,

No arguement from me becuase what you say holds very true to form in larger engines. The smaller two strokes have such small bores that squish remains pretty important to them. Seems with the 100cc (twin 50) and under injected group it plays pretty large in the combustion process. For those that thought they saw something they liked in that last sentence, don't go there cuz it ain't happening for RC.

Sorry Ed, thought it was you. I should have gone back to be sure before typing. My bad.

gkamysz

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Yes, I misspoke. Heywood discusses this in his book.

captinjohn

Does that 'Book" have a library number? Thanks Capt,n

Super08

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Library number, I didn't know people knew what that was anymore.