What Is The Difference Between Normal And Abnormal Combustion In A Motorcycle Engine?
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- Опубликовано: 22 май 2018
- Pre-ignition and detonation are not the same, Kevin Cameron notes, which is why you must not use one term to describe the other phenomenon.
In normal combustion, the piston is on its way up the cylinder toward Top Dead Center. It’s compressing a mixture of fuel and air above it. At a point, before TDC, a spark passes across the gap of the spark plug electrodes and that sets the mixture to burning, creating a tiny flame kernel. If the fuel-air mixture is adequately turbulent, the flame kernel is broken up, shredded, and distributed rapidly so that it has the effect of the flame spreading from the spark plug toward the cylinder walls at rates of 50 feet per second and up.
Bear in mind that an actual explosion would be thousands of feet per second, so normal combustion is simply burning, like a forest fire, by a process chemists call deflagration. As in the case of a forest fire, the heat of the flame front ignites the unburned fuel. This desirable outcome of a spark igniting normal combustion proceeds to the cylinder wall in an orderly fashion-as the Supreme Court would say, with all deliberate speed-consuming the entire charge.
There are other possibilities. Pre-ignition is just what you would think from the name: ignition of the fuel-air mixture before the spark by some hot object in the cylinder or combustion chamber. It may be overheated spark plug electrodes or glowing carbon deposits in the combustion chamber, but the result is always the same: After one or two cycles, the part of the piston farthest from the cool cylinder wall, namely the center of the dome, becomes hot very quickly. When aluminum becomes hot very quickly, it loses strength and eventually sags. And then it punches through. That is pre-ignition.
There is another phenomenon, entirely different from pre-ignition, known as detonation. Pre-ignition and detonation are not the same, so you must not use one term to describe the other. In detonation, ignition is normal. It takes place at the spark plug and the flame front moves normally outward. But if the air-fuel mixture gets hot enough as that flame front expands against and compresses the unburned mixture near the cylinder wall, chemical changes occur that transform it into a sensitive explosive.
If the process goes far enough, at some point around the edge of the piston, a small volume of unburned mixture explodes. It burns at the local speed of sound, creating a shock wave, which strikes the inside of the cylinder head, causing the sound we call “knock.” Modern cars and motorcycles have knock detection systems, so we don’t hear them knocking. To have encountered detonation, you might have to be an older person who has ridden in a three-speed taxi running on cheap gas. When the engine is lugging at low rpm, it provides the extra time necessary for detonation conditions to develop.
What you will see on a piston that is lightly detonated is the appearance of having been sandblasted at some point around its edge. That sandblast effect is the aluminum itself being blown off the surface by the waves from detonation. When this particular piston seized, the whole piston was so weak that the connecting rod tore the wrist pin and the wrist-pin bosses right out of the piston and left it as a pitiable wreckage. Pre-ignition and detonation: Both conditions are well worth avoiding.
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Clear and succinct.... You do not need to be a mechanic to get the point. Kevin is the master of clarity.
"Pre-ignition. Detonation. Bad."
That made my day. Love the videos, they're so informative
At last, someone on RUclips who knows the difference between Pre-ignition; Detonation & Deflagration. Well done to you -- I was beginning to lose the will to live.
Deflagration: originally from a Greek word, then From Latin meaning ‘to burn down’. In an engine, it’s referring to a combustion process that propagates due to the absorption of adjacent heat from the flame front. The flame front travels across the combustion chamber between 35mph & to around 130mph (50ft/sec to about 190ft/sec). It’s subsonic combustion, and it’s what we want inside an engine.
Detonation: originally from a Greek word, then From Latin meaning ‘to thunder down’. In an engine, it’s referring to a combustion process that starts due to heat being generated within a portion of the unburnt fuel/air mixture, at some point after the initial start of combustion. Perhaps due to part of the (as yet) unburnt fuel/air being compressed by the expanding burning mixture. It's a post-ignition issue -- not to be confused with pre-ignition. The flame front of detonation, I believe, can travel at 3000mph (4500ft/sec), & therefore it's supersonic combustion. This supersonic combustion produces a supersonic shockwave. The shockwave then drives the combustion process, as the head of the shockwave them compresses more of any unburnt fuel, raising its temperature and causing the detonation process to continue until there is no unburnt fuel left. The shockwave produces the metallic pinging sound (or pinking as we call it here in the UK). In part, some of the sound we hear is that shockwave hitting some of the metallic components inside the engine. Causing those components to ring out for the length of time that the shockwave exists.
I love these videos. A real hidden gem in the swarf that is so often available. Good stuff
Love the explanation about what knock actually is.
Very cool, thanks for the knowledge!
Thank you for your videos!
Learned something new 👍Thanks
However I seem to be the only one that appreciates that background music
andree hammerschmidt You're not the only one that appreciates the music. I quite enjoy it myself! It lends a lighthearted quality that I think goes well with the videos.
Thank you for the video!
Excellent video and super informative!
Excellent content as always! And the ending haha. .Bad.. Very Bad indeed !! I am really surprised that you dont have many subscribers. crazy world huh ?
Here’s a mystery.
New motorcycle runs GREAT on 87 octane and Castrol 10W-40 synthetic oil.
600 mile service, still runs GREAT on 87 octane and Castrol 10W-40 synthetic oil.
1806 miles, 87 octane and Castrol 10W-40 synthetic oil.
ENGINE KNOCKS.
This time a Full tank of 93 octane and Castrol 10W-40 synthetic oil.
STILL KNOCKS.
This is on a Triumph bonneville 1200 engine. Seems to be a common occurrence on these all over the world.
So it can’t be the gas or oil obviously. England had very high octane fuel but they still knock.
We were thinking the cam-chain tensioner but the oil is always the same.
High octane fuel doesn’t eliminate the knocking.
Tried that myself just yesterday.
The only thing I can come up with is a software problem with the ECU during the service at the dealership.
Considering this is the common denominator because the bikes are serviced at different dealers in different countries BUT connected to the same mainframe in London to Triumph headquarters that effect the bikes that get ECU updates.
From the information given in this video if the timing is off, or the mixture and the timing is off, the result is a misfire.
What else could it be ?????
Triumph tells us it’s normal but my these bikes run GREAT other than this knocking.
People have tried valve adjustments, with no difference.
Help please ……..
You just can't 'knock' these videos.
“Bad.” Haha
Love the videos. However: the music is very distracting and ultimately quite annoying. And there is no sound engineering to stop it from interfering with his voice. Please fix this.
It would be a lot better if you just gave us a link to the background music and let us decide ourselves whether or not to play it alongside his narration...
The music is great and adds to the drama and entertainment
@@rdrg7362 The music is for morons who roam RUclips because they got nothing better to do in their pity life.
but, but, sir ....in pre-ignition, why does it get so hot? ..piston rising - hot bit (carbon deposit, plug electrode) ignites mixture - which the spark of the spark plug was just about to ...so what makes that circumstance hotter? .... I'll guess - is it the early ignition of the mixture which isn't as compressed as intended makes a hotter flame/ignition?? ...or maybe the cooling effect of fresh charge is lost when it burns early (that sounds like the best guess) :) ...tia.
Pre-Ignition doesn't occur just a bit before the spark plug, it occurs waaaay before the ignition spark, like down at BDC on the Induction stroke. As such the piston is then trying to compress an EXPANDING gas wave all the way up to TDC rather than just the mixture. This means the heat of the flame is in contact with the piston crown for a very long time (in combustion terms), and the piston crown is oh so very thin, and the hottest part is the around the centre (furtherest from the wall). Hope this helps ;-)
so then the only real difference between preignition and detonation is... preignition happens early, and detonation happens on time, but burns too fast?
Detonation is far more violent, and happens at any time, hence it destroys the edges of the piston with the shockwave. Detonation is certainly not a conflagration environment within the chamber.
@@Force1Com No preignition is compressing the already burning gas and that forces the generated heat into the piston weakening it mostly in the center while generating max pressure BEFORE tdc. There is no explosion.
Detonation is where, during normal burning, some part of the gas is heated and compressed past it's ability to resist exploding so it does. It is always at a place where the flame would be last to reach(the edge of the combustion chamber). The ability of a fuel to resist exploding is what octane measurements measure.
Yes, BAD !
Sir please help me in difference between normal petrol bike engine versus E 20 (ethanol 20) bike engine. And which part is replacing to convert it e20
Wtf happened to test rides, comparos and reviews? I dont remember the last time i saw 1 from any bike account
My motorcycle is an air cooled 2021 and when hot i have bad détonations it drive me crazy. I tink its a lean mixture bug and those motors are F stroke. So, lean mixture is normaly my problem. It will be untroke and remap, will see this summer. Air cooled motor need to be a little rich to keep them cooler. First time i live detonation. Antipolution ginius are realy no mécanics. They F. dont know noting about air cooled motor they just do their job. They dont care that you just bouth a bike with his life shorten with their filty job.
Bad Ass
His voice is cute
Holy shit
I'm first
😁😁