Can someone explain why operating these engines lean will cause damage.
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From my pilot days...we would operate lean of peak all the time.
It has always been my understanding that TOO lean causes pre-ignition and detonation which can quickly cause damage. In the extreme a hole in the top of the piston and/or hammering the valve train and/or lower end to the point of failure. The most technically knowledgeable members can add to (or correct) my understanding.
OK, here goes...
Pretty much, yeah. The EPA wants engines tuned to burn the fuel most efficiently, to reduce the amount of pollutants and unburned fuel coming out of the tailpipe. To achieve this, the MoCo has to calibrate the ECM to run very lean mixtures that have a high air-to-fuel ratio, and the intake and exhaust are relatively restrictive to keep everything in check. Everything about a stock air-cooled Harley is to please the EPA... not to make max power. While this achieves cleaner exhaust (also helped by the additional burning of gases in the exhaust by the VERY HOT catalytic converter by your and your BSR's right feet), it generates a LOT of heat.
And if the AFR is too lean because you've opened up your intake and exhaust, it can lead to:
1. Pre-ignition - which is ignition of the fuel charge before the spark plug ignites it at the proper time. This is often caused by carbon buildup on the piston or in the combustion chamber components. Any little piece of hot carbon can provide a nucleation site for the atomized droplets of fuel to cling to and begin burning too soon... before the spark plug ignites the charge properly.
2. Detonation (a.k.a. knocking or pinging) - which is a very fast and often very destructive explosion of the fuel charge, instead of a normal burning of the charge with a predictable flame front across the combustion area. This is VERY bad. If the fuel charge ignites and explodes rapidly, instead of igniting and burning normally across the combustion area over the designed period of time, it can damage or even destroy the engine. If the charge explodes in the cylinder while the piston is still coming up and compressing the charge, and before the crank has turned enough to allow the piston to come back down, the burning and rapidly-expanding charge can blow a hole in the piston or in the cylinder head. Those expanding gases have to go somewhere, and going through the top of the piston is typically the point of least resistance.
When any substance is compressed, it heats up. If the AFR is too lean, or if the octane in the fuel is too low, or if the timing is too far advanced, the highly compressed air-fuel mixture in the cylinder can spontaneously combust before the spark plug ignites it at the proper time, and/or it can explode rapidly instead of burning evenly through the combustion chamber at the designed speed. That's why engines with high compression need a higher octane fuel, a richer AFR, and likely later (retarded) ignition timing... to prevent the fuel charge from igniting before it's supposed to, and to help ensure that the charge burns normally across the combustion chamber instead of just exploding all at once and blasting the engine apart.
There are many articles out on the net that explain all of this in great detail... but that's the nuts and bolts of it.
Detonation is more destructive, and if you hear your engine knocking, it won't be long before it is damaged - perhaps severely. Properly-tuned engines, running the proper fuel, with proper ignition timing, do not knock. Your engine can be knocking before you can even hear it, so an essential part of a proper tune is ensuring that no knocking is occurring after the VE tables, EGR, etc. have been calibrated. The Delphi ECM has ion-sensing knock detection and automatic timing retard, to help prevent engine damage... but a good tune should have the ignition timing set to a couple of degrees before knocking occurs. When the ECM senses knock, it will pull several degrees out of the ignition timing, and then slowly add it back in until it senses knock again. This is extra work for the ECM to do, and it's not desirable. It's better to have the ignition timing set a couple of degrees before knock occurs in the various cells of the timing tables. This also helps compensate for when you occasionally fill up with "bad gas" that doesn't have the octane that it's advertised to have.
Engines are air pumps. When you remove restriction from the intake or exhaust, you are in effect allowing the air pump to pump more air through the engine. This will produce more heat with a too-lean mixture, as well as not producing the maximum amount of TQ/HP of which the engine is capable. The ECM needs to be re-calibrated to provide more fuel in the mixture to compensate for the increased amount of air flowing through the engine. Changing cams, which also changes the airflow through the engine, requires the ECM to be re-calibrated as well... to ensure that the proper AFR is maintained at all RPM and manifold pressures (kPa for lambda cals or throttle position for non-lambda cals)
Cat Eye - regarding your flying experience... I am a private pilot as well. Aircraft engines operate in a very different environment than do surface vehicles. With altitude, the air gets progressively less dense than it is at sea level. Less air density means that less fuel is needed to maintain the optimum AFR for complete combustion and maximum power production. So, you have to lean out the AFR to what it needs to be at altitude, to avoid essentially flooding the engine with a too-rich mixture that would reduce the power produced.
Ken
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