PeachParts Mercedes-Benz Forum - View Single Post - Blow-By curioisty questions

Shorebilly · #16 05-19-2007, 08:05 AM

Quote:

Originally Posted by joeblack

Shorebilly, I assume you talk about ship diesels, What causes an explosion and why would it be any different if there is a slight over or under pressure. I would think that with under pressure the crankcase get vented and a possible explosive mixture would be burnt of because it is sucked into the combustion chambers. Overpressure would press the possible explosive mixture to the outside?

Thanks Johan

A crankcase explosion occurs within the engine's crankcase......usually caused by a very hot bearing, or a very worn set of piston rings and liners, heating the lube above it's flash point.....this ignites all of the oil vapors within the engine.....and makes a very loud noise!!!! Occasionally, fuel oil mixed with the lube oil will either cause, and/or exacerbate the explosive mix.....

As the theory (safety/fire prevention) goes .....if you maintain a slight positive pressure (about the pressure that natural gas is in your stove) then....in theory the atmosphere within the crankcase will remain basically static and unmixed with oxygen, thus preventing the formation of a combustible mixture (fuel/air ratio).......if a slight vacuum were within the crankcase it would suck air/atmosphere into the crankcase at various points and allow a combustible mix to form.....and then when a source of ignition presents itself....Boom.....

Trust me, it would make the housekeeping/cleaning of the engines if this were not so.....the slight positive pressure pushes oil out of the engine....

On the side of every industrial engine that I have worked on, there is a Warning Sign.....that reads something like this....."The engine must be stopped for a minimum of 5 minutes before opening the inspection doors"

In the case that I was involved with....MAK engine....we had fuel contamination of the lube oil......this was known, and an oil change was denied by the "powers that be".......I had gone on record "she's gonna blow up, sooner or later"......and she did....when it did, it blew open all of the explosion doors, it blew the gasketing from behind the explosion doors, it also blew the cylinder liner seals from 3 of the cylinders (this allowed the jacket cooling water to flood the crankcase, this interrupted cooling to the rest of the engine).....it also ignited the bilges, FIRE big time!!, and was the only time I ever had to drop CO2 into the engine room......was a very amazing event.....

SB