I really have no idea who reads this, but a year and a half ago I wrote this post about how a mud dobber caused about a day’s worth of trouble-shooting a non-functional compressor. This morning I got a call from my grandpa asking if I remembered what we did, because it was doing it again. The symptoms were the same, it would run for 2-3 minutes, then it would start knocking, make some groaning noises and coast to a stop.
When it stopped, you could tell something wasn’t right. The engine was under heavy compression, more than usual. It would shake to a violent halt, not coast to an easy stop. The effect is akin to putting a heavy weight on the rim of a bike tire, and spinning while holding it off the ground. It doesn’t coast to a stop, it gets to a point where the weight can’t make it around and yanks that part of the tire to the ground. Likewise, whatever pressure buildup was inside the engine was forcing it to stop in a particular spot.
The problem before turned out to be a pressure relief hole, plugged up by a mud dobber. This is actually kind of a scary thought, since plugging that hole prevented activation of the pressure bypass. Basically it’s the safety to make it stop pumping when the tank gets to the set pressure. So what would happen is the pressure would build up so high that the compressor could not overcome it, and from what I can tell, it was blowing by the rings and pressurizing the crank case.
The reason I know it is pressurizing the crank case is that every time it stopped like this, there was a hissing noise that lasted a few minutes. I thought it would be a good idea to check the oil, and when I pulled the dipstick out, air started blowing out of the dipstick hole. So the engine was actually pressurizing the crank case, which is even more scary since the crank case is not designed to hold that much pressure.
But an interesting phenomenon saves the day. The diesel engine piston shares the crank case and crankshaft with the compressor piston (one piston each) and because of this, as the crankcase pressure increases, the resistance that the engine saw tripled. This is because not only was the engine trying to pump on the compressor side, it was also trying to compress the air inside the crank case with every revolution. This extra work was too much for the single cylinder diesel to handle, so it starts to knock, then shuts down completely.
Still, this is not good. There are a lot of dangers with this, but the good thing is that the engine was probably grossly over-designed to begin with. There was also no risk of tank rupture, because the pressure never got high enough to pop the safety valve on the tank.
I guess where I was going with this is that if I hadn’t wrote that blog about the mud dobber, I don’t think I would have remembered exactly what happened, and my grandpa and the neighbors who help him out would be scratching their heads for another day or so. So regardless of my near-non-existent reader base back then, that blog was worth writing. I’m glad I did it.
