Blown Engine Pics: What went wrong?
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Looks like something got really hot (missing plug center electrode).
Why it got hot could be mixture, timing ??
Why it got hot could be mixture, timing ??
Lean mixtures burn hotter.
This is related to why some fuel injected engines are set slightly rich in WOT conditions, it cools the combustion chamber and provides a margin of safety. Although, your neighborhood dyno tuners usually depense with this built in safety feature.
You would think you'd be able to hear that level of detonation...but maybe not?
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(imagine poor lip sinc audio on low budget kung fu movie)
Ah, tpi, we meet again. (we apparently also belong to the same "list").
This is odd. Made more odd by the fact that you don't have an EGR valve to clog and go limp. Your cam overlap should have left enough spent combustion material in the cylinder to cool the process.
I hate it when that happens (getting spanked).
btw-tpi ..are you rebuilding or swapping out??
Ah, tpi, we meet again. (we apparently also belong to the same "list").
This is odd. Made more odd by the fact that you don't have an EGR valve to clog and go limp. Your cam overlap should have left enough spent combustion material in the cylinder to cool the process.
I got spanked here by someone here on this. The leaner mixture apparently ends up burning off the insulating boundery layer of combustable material that clings to the interior of the cylinder/piston/combustion chamber. That allegedly exposes and damages those surfaces (mainly pistons and valves).
I hate it when that happens (getting spanked).
btw-tpi ..are you rebuilding or swapping out??
air leak increases the amount of air versus fuel. Some efi systems tweak individual cylinders, but some dont. I bet this system since its older only tweaks all cylinders. So, add air to one cylinder thru a leak and you get lean mixture which drives cylinder temps very high resulting in a burned piston.
Dan
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If this was a leak somewhere centered on the plenum ..the engine would just either regulate it by the MAP output ..or the entire sensor array. If the leak (since this is a inline engine) is on one runner ..that cylinder will be lean. The O2 sensor would balance the entire mass of exhaust gases ..still leaving that lean.
The DC MPFI uses two "batch fire" pulses to control injector firing in the 2.5 (three in the 4.0). This, I believe, is the current system in use (it was in the 99 FSM). So this engine should have had three somewhat rich cylinders ..to compensate for the lean one. This is a reliable ..but fairly numb FI system.
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I once did the same thing to a 8hp Tecumseh engine in my snow-blower, it was about 8 yo at the time. I used it hard for about 3 hrs, laboring it for long periods pushing it hard and suddenly it lost power. Found a hole right through the piston.
That probably is a combinations of abuse, engine tune etc. Abuse meaning it was being used as it was never designed to be using over-size tires, he admits it.
That probably is a combinations of abuse, engine tune etc. Abuse meaning it was being used as it was never designed to be using over-size tires, he admits it.
I have to disagree strongly with the "conventional wisdom" here. Having studied combustion and mixtures on aircraft engines, the idea that "leaner is hotter" does not explain destructive detonation at all. Detonation is excessive peak cylinder pressure usually caused by preignition. Preignition is caused by glowing carbon deposits, damaged or too hot spark plugs, low fuel octane, etc.
Modern EFI systems are designed to run lean for economy, and due to a slight excess of air, actually cooler than a slightly rich mixture. WOT is calibrated rich because that is where maximum power (and heat) is. What was considered "too lean" with old carbed engines with poor fuel distribution and weak ignition systems is actually close to the WOT mixtures on modern EFI engines. If you instrumented an engine with EGT and CHT gauges and a manual mixture control, you would see that CHT temperatures peak rich, not lean of peak EGT, which is the point of "ideal" air/fuel ratio.
Modern EFI systems are designed to run lean for economy, and due to a slight excess of air, actually cooler than a slightly rich mixture. WOT is calibrated rich because that is where maximum power (and heat) is. What was considered "too lean" with old carbed engines with poor fuel distribution and weak ignition systems is actually close to the WOT mixtures on modern EFI engines. If you instrumented an engine with EGT and CHT gauges and a manual mixture control, you would see that CHT temperatures peak rich, not lean of peak EGT, which is the point of "ideal" air/fuel ratio.
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Why wouldn't the colliding shock waves of a lean mixture ..and subsequent fragmented flame front, have the same effect? Since it's lean ..it would also allegedly tend to burn off the insulating layer of combustion material that clings to the internals of the cylinder.
Like I said ...(it was an aero geek like yourself that did it)...I got spanked on this earlier in my BITOG career. Sure the xxxxoteric 14.7:1 fuel mixture yeilds the most thermal output ...but not necessarily the highest piston temp.
Like I said ...(it was an aero geek like yourself that did it)...I got spanked on this earlier in my BITOG career. Sure the xxxxoteric 14.7:1 fuel mixture yeilds the most thermal output ...but not necessarily the highest piston temp.
Somewhere there is a good pic to demonstrate why detonation does damage and normal ignition does not. Basically detonation happens much quicker and on several fronts.
With modern FI we are edging closer and closer to the edge of detonation to get the most power. Any little problem and it can occur. Engines without knock sensors are just asking for trouble.
-T
With modern FI we are edging closer and closer to the edge of detonation to get the most power. Any little problem and it can occur. Engines without knock sensors are just asking for trouble.
-T
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I'm well aware of the severe effects of detonation. I would find it hard to think of the condtions that typically cause it in this engine ..and only in one cylinder.
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