I was recently giving a presentation for IEEE at MIT Lincoln Labs here in the Boston area. The topic was one of my favorites, my new playground, Use Case 7 ! The crowd loved the idea of expanding how we access use cases and came up with great examples. and experiences, of their own. They found many areas in their work where the Use Case 7 exercise may yield some interesting insight.
This crowd was so good that I my Q&A session ran for over 30 minutes. It turned into a fun open discussion that should have been at a lab or a engineering bar (is there such a thing?), I would say we verged on workshop. One of the more interesting paths we went down was the cases of high stress from a product not being used. I can’t share where the conversation started because it was a current product an attendee was working on but I can share some of the personal experiences we talked about.
Mine was of course car related. I know I know, why is it always cars? I have no idea. There are people who I met once at a party six months ago, I have no idea what their name is or what they do for work, but I can tell you what kind of car they have. I’m like Rainman but on only two topics, cars, and reliability. Everything else I am marginally average. Anyway… my car UC7 for a product not being used is a strange situation was with my 1978 Datsun 280Z that has 29k miles on it. The care will occasionally burn some oil in only one of the six cylinders. I noticed that if it is cold and I gun it that I get a puff of smoke with a blue tint to it. The blue is a sure sign that the smoke is oil. But generally she burns really clean.
But I knew right away what had caused this. The stress of sitting for years without ever being turned over. That’s a 41 year old car with 29K miles. This is an averages 707 miles/year. It’s possible someone only took it out for a short cruise on the occasional Sunday for every year of it’s life. But another scenario is more likely. It’s much more likely that it had a period in it’s life where it was put away in storage or just left in the garage and shown to friends when they came over for drinks for many years. A long period of storage could do damage to only one cylinder.
This is how. When an engine is at rest there is going to be one intake or exhaust valve that will remain open. This is because the 12 valves for the 6 cylinder motor all run in sequence. When you stop the motor one of them is going to be on it’s open cycle. That open valve permits air and moisture to enter that cylinder while it sits. This is no big deal over short periods of time because it is a long path for fresh air to go through the intake or exhaust to get to the cylinder. It could be left like this for a few months in a moderate climate without issue. The cylinder walls, piston rings, and valves are freshly covered with oil from being run, and it will take a long time for that oil film to totally displace from gravity. But if you leave a motor unturned for years, or maybe even months in a high humidity environment, rust can form on the delicate raw metal sealing surfaces, often the piston rings. So that was the UC7 stress that got my Datsun. The irony of course being that the individual who put it away and only looked at it for so long was trying to preserve it. They should have been a little more of a car guy to know to “fog” the cylinders on that last shutdown before the big sleep. “Fogging” is having the air suck in a thick cloud of a much more viscous/waxy oil that will coat all these surfaces and stay in place for much longer.
I don’t remember all the other stories people shared during the session because they weren’t’ car related. But I remember other people found them interesting.
-Adam
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