Why do engines wear out?
02-13-2019, 01:48 PM
#16
Rennlist Member
Heat-Friction
02-13-2019, 03:34 PM
#17
Rennlist Member
That is an interesting chart for mechanical stress cycling for those materials but of limited value in comparing materials for use in engines. Additional relevant data would be needed such as temperature cycling, chemical exposure ( e.g. water, oxygen. oil, coolant) as well as part design.
02-13-2019, 03:38 PM
#18
Rennlist Member
By the way, those talking about million-mile-engines know that it’s the cycling of on/off, combined with other factors such as run time, that make more of a difference on longevity. I mean a million miles is not that hard to achieve in a relatively short period of time if there’s some sort of focused goal/objective at hand. Taxi cabs and truck drivers know this. When fully automated cars arrive, I predict cars might easily be able to get to 2 or 3 million miles rather easily.
02-13-2019, 06:54 PM
#19
I completely agree. It would be silly to believe that one page of text would be sufficient to have the knowledge to design an engine’s structure. I wanted to give a quick overview of how typical raw metals degrade in life span only due to cyclic loading.
That should answer the question of why motors wear out. Additional factors do come into play and are not as well documented in their relationship to the expected life span when failure occurs.
Bore scoring is a surface defect that is the starting point for fatigue failure, creep and fracture propagation, which ends in what the forums call d chunk failure. Properly oiled components will prevent the chance of scoring and in turn, mitigate fatigue failure due to crack propagation.
One can ask why m97 motors wear out quicker than others (sometimes) and that’s a whole new question.
The modes of failure that you are discussing aren't quite applicable for this situation.
When I was in college, I raced bicycles and really light Aluminum bikes were just coming into the market. These were experiencing true fatigue failures. Felt and Cannondale in particular were pushing the boundaries of large diameter and extremely thin walls to build really light and stiff Aluminum race bikes. They were amazing for the time but they also tended to last only a season or two. The problem was that typically the bottom bracket was experiencing enough elastic deformation with the stress applied in pedaling to engage fatigue failure. For scale, we are talking about deflection around 1mm - 3mm repeated at 80rpm for somewhere in the neighborhood of 400 - 1000 hours per year or 3 to 10 million cycles to failure. Ultimately you see a very specific signature of crack propagation in fatigue failure which bear some similarity to tree rings ending in final failure being ductile or brittle depending on the material. But this is not the only failure mode that can create that type of signature especially where corrosives are involved. If the wall thickness of the tubing at that area were greater (which is the case now), the psi would have been significantly lower and this wouldn't have been an issue at the loads placed on the frame. Fatigue failure comes into play when repeated stress cycles the material above a critical threshold are exceeded. Steel also shows fatigue failure modes, but they don't begin until you enter plastic deformation. What your graph isn't showing is that during elastic deformation work hardening is also occurring in the steel. Think about bending a paperclip back and forth until it snaps.
Bicycles are built around systems designed to balance weight and strength with a pretty heavy emphasis on reduced weight. Steel bicycle chains are made of some very hard steels, but they still experience plastic deformation. I wear out a chain in about 3,000 miles. Or I should say I stretch a chain sufficiently in 3,000 miles such that it has a sufficient mismatch to the tooth spacing in the chainring and cassette cogs to cause those more expensive parts to wear excessively.
If the engine block were seeing elastic deformation on the scale of a bicycle bottom bracket, the entire system would break down due to misalignment issues long before fatigue failure in the block became an issue. You do see these types of failures in certain parts like bolts or connecting rods etc. These are the components that take the dynamic stresses of the engine. The failures in the engine block are catastrophic brittle fracture, possibly related to a casting fault or multi-modal such as cylinder sleeve failures shown above where temperature related expansion, corrosion and chemical interface issues are all combining to result in failure.
The moving parts and contact points in an engine wear due to well, wear. Even materials of the equal hardness wear each other when moved against each other. While the goal of oiling is to create a friction-less layer between the parts, it isn't perfect. Carmakers aren't conspiring against you to force you to buy new cars due to wear. Moving parts are simply going to wear over time. And yes, I do certainly recognize that carmakers do make design compromises that result in premature vehicle failure, but they don't do this intentionally on wide scale because it is a competitive market where reputation matters.
But don't get me started on planned tech obsolescence in new cars.
02-14-2019, 10:20 AM
#20
My degree is in Metallurgical Engineering and Materials Science though I did not go into that profession.
The modes of failure that you are discussing aren't quite applicable for this situation.
When I was in college, I raced bicycles and really light Aluminum bikes were just coming into the market. These were experiencing true fatigue failures. Felt and Cannondale in particular were pushing the boundaries of large diameter and extremely thin walls to build really light and stiff Aluminum race bikes. They were amazing for the time but they also tended to last only a season or two. The problem was that typically the bottom bracket was experiencing enough elastic deformation with the stress applied in pedaling to engage fatigue failure. For scale, we are talking about deflection around 1mm - 3mm repeated at 80rpm for somewhere in the neighborhood of 400 - 1000 hours per year or 3 to 10 million cycles to failure. Ultimately you see a very specific signature of crack propagation in fatigue failure which bear some similarity to tree rings ending in final failure being ductile or brittle depending on the material. But this is not the only failure mode that can create that type of signature especially where corrosives are involved. If the wall thickness of the tubing at that area were greater (which is the case now), the psi would have been significantly lower and this wouldn't have been an issue at the loads placed on the frame. Fatigue failure comes into play when repeated stress cycles the material above a critical threshold are exceeded. Steel also shows fatigue failure modes, but they don't begin until you enter plastic deformation. What your graph isn't showing is that during elastic deformation work hardening is also occurring in the steel. Think about bending a paperclip back and forth until it snaps.
Bicycles are built around systems designed to balance weight and strength with a pretty heavy emphasis on reduced weight. Steel bicycle chains are made of some very hard steels, but they still experience plastic deformation. I wear out a chain in about 3,000 miles. Or I should say I stretch a chain sufficiently in 3,000 miles such that it has a sufficient mismatch to the tooth spacing in the chainring and cassette cogs to cause those more expensive parts to wear excessively.
If the engine block were seeing elastic deformation on the scale of a bicycle bottom bracket, the entire system would break down due to misalignment issues long before fatigue failure in the block became an issue. You do see these types of failures in certain parts like bolts or connecting rods etc. These are the components that take the dynamic stresses of the engine. The failures in the engine block are catastrophic brittle fracture, possibly related to a casting fault or multi-modal such as cylinder sleeve failures shown above where temperature related expansion, corrosion and chemical interface issues are all combining to result in failure.
The moving parts and contact points in an engine wear due to well, wear. Even materials of the equal hardness wear each other when moved against each other. While the goal of oiling is to create a friction-less layer between the parts, it isn't perfect. Carmakers aren't conspiring against you to force you to buy new cars due to wear. Moving parts are simply going to wear over time. And yes, I do certainly recognize that carmakers do make design compromises that result in premature vehicle failure, but they don't do this intentionally on wide scale because it is a competitive market where reputation matters.
But don't get me started on planned tech obsolescence in new cars.
The modes of failure that you are discussing aren't quite applicable for this situation.
When I was in college, I raced bicycles and really light Aluminum bikes were just coming into the market. These were experiencing true fatigue failures. Felt and Cannondale in particular were pushing the boundaries of large diameter and extremely thin walls to build really light and stiff Aluminum race bikes. They were amazing for the time but they also tended to last only a season or two. The problem was that typically the bottom bracket was experiencing enough elastic deformation with the stress applied in pedaling to engage fatigue failure. For scale, we are talking about deflection around 1mm - 3mm repeated at 80rpm for somewhere in the neighborhood of 400 - 1000 hours per year or 3 to 10 million cycles to failure. Ultimately you see a very specific signature of crack propagation in fatigue failure which bear some similarity to tree rings ending in final failure being ductile or brittle depending on the material. But this is not the only failure mode that can create that type of signature especially where corrosives are involved. If the wall thickness of the tubing at that area were greater (which is the case now), the psi would have been significantly lower and this wouldn't have been an issue at the loads placed on the frame. Fatigue failure comes into play when repeated stress cycles the material above a critical threshold are exceeded. Steel also shows fatigue failure modes, but they don't begin until you enter plastic deformation. What your graph isn't showing is that during elastic deformation work hardening is also occurring in the steel. Think about bending a paperclip back and forth until it snaps.
Bicycles are built around systems designed to balance weight and strength with a pretty heavy emphasis on reduced weight. Steel bicycle chains are made of some very hard steels, but they still experience plastic deformation. I wear out a chain in about 3,000 miles. Or I should say I stretch a chain sufficiently in 3,000 miles such that it has a sufficient mismatch to the tooth spacing in the chainring and cassette cogs to cause those more expensive parts to wear excessively.
If the engine block were seeing elastic deformation on the scale of a bicycle bottom bracket, the entire system would break down due to misalignment issues long before fatigue failure in the block became an issue. You do see these types of failures in certain parts like bolts or connecting rods etc. These are the components that take the dynamic stresses of the engine. The failures in the engine block are catastrophic brittle fracture, possibly related to a casting fault or multi-modal such as cylinder sleeve failures shown above where temperature related expansion, corrosion and chemical interface issues are all combining to result in failure.
The moving parts and contact points in an engine wear due to well, wear. Even materials of the equal hardness wear each other when moved against each other. While the goal of oiling is to create a friction-less layer between the parts, it isn't perfect. Carmakers aren't conspiring against you to force you to buy new cars due to wear. Moving parts are simply going to wear over time. And yes, I do certainly recognize that carmakers do make design compromises that result in premature vehicle failure, but they don't do this intentionally on wide scale because it is a competitive market where reputation matters.
But don't get me started on planned tech obsolescence in new cars.
02-14-2019, 10:28 AM
#21
Former Vendor
The failures in the engine block are catastrophic brittle fracture, possibly related to a casting fault or multi-modal such as cylinder sleeve failures shown above where temperature related expansion, corrosion and chemical interface issues are all combining to result in failure.
When you see failure routinely, you learn really quickly, that it's not that easy.
02-14-2019, 04:25 PM
#22
The main reason that engines wear out today, is because they are not designed to last forever. The environment is also a key player in this, from processes like Nikisil that are very harmful to the environment (and therefore expensive) to engine systems that are designed for reduced emissions.
l have a 1954 Chevy truck that my dad bought new. The 235CI straight six engine has never been rebuilt, and has over 1/2 million miles on it. It was designed to live a long, simple life, and it has done just that. It sat for 10 years, and after I filed the points, and cleaned the carburetor, it fired right up and has been running perfectly since. The days of simplicity and over- engineering are gone forever.
Everything mechanical will wear out, unless it fails during the process. Its a guarantee.
l have a 1954 Chevy truck that my dad bought new. The 235CI straight six engine has never been rebuilt, and has over 1/2 million miles on it. It was designed to live a long, simple life, and it has done just that. It sat for 10 years, and after I filed the points, and cleaned the carburetor, it fired right up and has been running perfectly since. The days of simplicity and over- engineering are gone forever.
Everything mechanical will wear out, unless it fails during the process. Its a guarantee.
I would say a 1954 Chevy with 500,000miles and 60+years is a massive outlier. The 5 digits odometer was related to the expected lifespan back then. The 235ci straight six (3.85L) was putting out 93 horsepower, while the porsche 9A1 3.8L in NA form is pushed as high as 430hp stock. And a vehicle with a 235ci would typically consume somewhere between 15-20mpg while only able to produce that power pushing much lighter vehicles. We ask alot more of engines today and apply much larger forces to them while being far more efficient. We require much tighter tolerances than could be produced in 1954 and we've developed technologies that allow the engine to perform at those levels pretty reliably. I think in most cases, modern cars require much less maintenance and downtime. But the other systems of the older vehicle are undeniably easier to repair and reuse, aside from body failure due to rust (which was a bit worse then).
In defense of modern vehicles; a previous employer of mine runs a fleet of vans for a passenger transport service and they have run many modern vehicles to 500,000 miles with proper maintenance and none of them would be retired before 300,000 miles unless involved in a major accident. Most went into the 400k range. That's 4-5 years of operation for 300,000 miles and 6-7 at 500,000 miles. They've been switching over from Ford V-8 E-350 vans over the last 6 years to the Mercedes Sprinter and Ford Transit turbo diesel vans and the new vans are performing fine too with the same or better lifespan and operating expense expectations. They don't retire at a specific mileage target, but at a point where the irregular maintenance costs start to rise quickly and the interior wear is bad for a customer. With 100+ vehicle fleets, you get outliers on both the lemon and over-perform side but the bell curve is undeniable. Today's cars can go the distance.
We normal people do not drive 200-400 miles per day or maintain our vehicles like a fleet does. Age also affects vehicle systems, just as wear does. I travel for my work these days, so I'm in all sorts of different rental cars. Mechanically I think most of the cars I experience are pretty good right now, though a few are really terrible. I think they are mostly incredibly boring to look at and to drive, that most don't handle very well and are overloaded with tech that will be outdated in 3-5 years. But I think the majority of cars from the mid-level and up are mechanically capable of reaching at least 250,000 miles. I think people stop repairing cars because the creature comforts fail and the repair costs start to exceed the value or parts become unobtainable.
I would say the one modern trend I really dislike is the manufacturer using plastic engine covers to signal consumers that they are not qualified to maintain this vehicle along with the warranty culture that is so prevalent. Or even worse, I just saw a review of the McLaren 600LT where they didn't even design a way for the owner to access the engine, you just get to almost see it through a mesh cover. I do think those trends will result in premature disposal of mechanically useful cars in the future.
02-14-2019, 06:21 PM
#23
Rennlist Member
Engine failures are all I deal with everyday, all day long.. In most every case what you've stated above is the norm.. It's always an equation that leads to a failure, yet people here on forums spending all their time trying to "figure out" the one thing that causes a failure.
When you see failure routinely, you learn really quickly, that it's not that easy.
When you see failure routinely, you learn really quickly, that it's not that easy.
EDIT: The above applies to our personal lives as well. People think there is one main reason why a spouce divorces us, why we lost our job, or why our kids hate us. It’s almost always a series of critical failures, one after another, over a period of time, that leads to the demise. Most of the time. There is luck, though, good or bad. This one 9A1 failure seems like bad luck.
