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Detroit 92 series running backwards
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Posted by NC Wayne on May 24, 2006 at 18:51:35 from (152.163.100.74):
Saw the post from yesterday evening about the Detroit running backwards. I thought I'd start a new thread because I could tell from many of the replys that the guys weren't really sure what was going on when one ran backwards. Many of the replys said shut off the air at the intake or the fuel. Typically shutting off the fuel is the best way to stop a Detroit under normal conditions, the emergency air shutdown is good in case the fuel rack sticks, etc and you can't shut the fuel off. When it's running backwards though even shutting off the fuel is effectively worthless. The reason it doesn't get "out of time" is because it's a two stroke with no intake valves. Instead their are ports in the bottom of the liners that serve as the intake and their open at the same time the exhaust valves are for scavaging purposes. In other words it allows the blower to force a charge of air through the cyliner clearing out all the waste gases and leaving behind all fresh new air. So forward or backwards the intake and exhause are always open together, the only difference is the direction the engine turns. If any two stroke Detroit 53,71,or 92, series is running backwards the exhaust side becomes the intake and vice-versa. At the same time the blower is also turning backwards so instead of "blowing" air through the airbox, the liner ports and out the exhaust valves, it's now "sucking" air through the exhaust valves, the liner ports and airbox and exhausting it through intake. Basically the emergency shutdown doesn't do any good because it's designed to stop a flow of air blowing against it to seat it against a gasket, not a flow being sucked through it. As far as shutting off fuel you can do that but when she's running backwards there's enough suction in the air box to suck oil past the oil control rings on the piston, and often the blower seals, at which point and she'll run on engine oil. That's one reason the smoke is so black when they run backwards. That said in this case if their hadn't been a raincap in the way that doidn't really give a positive seal, and the exhaust piping had been "tight" a board, book, etc over the exhaust should have worked. Otherwise the way they shut it down with a CO2 extinguisher was an excellent idea. Just my .02. Hope it stears some of you guys that really weren't sure what needed to be done or why it could run backwards in the right direction in case this ever happens to you.
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