DieselJeepLuvr wrote:
geordi wrote:
Dent's research is available on the forum here, the summary is that the cylinder temperatures over 1200 degrees (or approaching) are the primary factor in causing metallurgical changes of molecule density in the valve stem. Over a period of roughly 300 billion cycles which roughly corresponds with 200k miles, the valve stems reach a fatigue point...
Just to clarify, the math here is not correct. Let me explain:
I cruise at 75mph. My tach reads 2,200 rpm. Because it's a 4 stroke the exhaust valves open and close every OTHER rotation. So at 2,200 rpm each set of valves are activated 1,100 times (cycles) per minute or 66,000 times per hour. If you are traveling at 75mph , then you divide 75 into 66,000 to get 880 cycles per mile. Now if your valve life cycle is 300 billion, you divide 880 into 300,000,000,000 which is over 340 million miles NOT 200k even if the life cycle was supposed to be 300 million (Only 1%) that equates to 340k.
IMHO the valves fail because of four main reasons.
1. Over revving
2. Towing. Especially when it's hot.
3. Having a working EGR system
4. Not removing the intake and intake horn and removing the gobs of encrusted soot.
I believe that #4 is the worst because when chunks of soot breaks off and gets hit by an exhaust valve on its way out of the engine causes catastrophic damage to the integrity of the exhaust valves. And finally these valves are TINY! Good because that makes the engine more efficient with lighter valves but bad because they are in a diesel and under tremendous stress!
DieselJeepLuvr:
You can count on geordi to screw up the math every time... you should look at his mathematical "calculations" in regards to the fuel heater; LOL!
In this case, I would consider the following variables...
1) ... CRD engines are not running always at 2200 RPM; sometimes they run higher than that, but mostly lower than that. Is there a way to calculate an "average" RPM?
2) ... the speed of the vehicle is almost never at 75 MPH; most certainly it is way, way above average. The average would be lower, even on a vehicle running on the highways 90% of the time. A vehicle puttering along in the city would have a far lower average; perhaps 25 to 30 MPH. My 2022 Colorado's average speed has been calculated to be about 30 KMPH, even after running on the highway a lot recently.
3) ... Idle time during warm-up and cool-down. Idle times, (warm-ups), increase in cold weather.
Even factoring in all of the variables, I suspect geordi's calculations are still way off and that you are closer to the truth.
I also agree with the four factors for valve failure you have listed, but you forgot that cylinder head warpage can also lead to valve failure... in the case of the CRD, I suspect that head warpage is the leading cause for exhaust valve failure because it has only been exhaust valves that have failed. A cylinder head will warp the most where the temperature differentials are the greatest, and temperature differentials are the greatest in the exhaust area of the cylinder head of any internal combustion engine. It does not take much for bad things to happen, and when you get things twisting enough in that area the valves can end up being slightly misaligned on their valve seats and/or the valve guides may be affected.