Why idle before shut off?

[whodat526]

how does the combustion not reach the cylinder walls if it does why do they need such big piston and cylinders and why is there always carbon build up along the side of the top of the cylinders

 

[SPYDERLK]

how does the combustion not reach the cylinder walls if it does why do they need such big piston and cylinders and why is there always carbon build up along the side of the top of the cylinders

Pyrolyzed oil. This is the hottest area on the walls, and the annular area between the cyl and the piston above the top ring captures a small amt of the hot power stroke gases. When crossing TDC [or BDC] there is long dwell in the position compared to other stroke points. The long period spent in the hot area near the head causes pyrolysis of the minute oil/gas there. Since the rings never scrape there it builds up til it almost touches the piston. At that point the area becomes violently scavenged on each stroke enforcing an equilibrium between growth/loss.

Size of piston/cyl is there to give force from the combustion pressure. The extremely thin area not combusted in a warmed engine makes little difference in force and power.
larry

 

[whodat526]

Pyrolyzed oil. This is the hottest area on the walls, and the annular area between the cyl and the piston above the top ring captures a small amt of the hot power stroke gases. When crossing TDC [or BDC] there is long dwell in the position compared to other stroke points. The long period spent in the hot area near the head causes pyrolysis of the minute oil/gas there. Since the rings never scrape there it builds up til it almost touches the piston. At that point the area becomes violently scavenged on each stroke enforcing an equilibrium between growth/loss.

Size of piston/cyl is there to give force from the combustion pressure. The extremely thin area not combusted in a warmed engine makes little difference in force and power.
larry

from what u just stated how i read it is that combustion does use the complete cylinder. And since diesel is sprayed all over the cylinders diamater it use the whole cylinder to fire not just the middle right or does the diesel just drip into the cylinder when it opens

 

[RoyJackson]

from what u just stated how i read it is that combustion does use the complete cylinder. And since diesel is sprayed all over the cylinders diamater it use the whole cylinder to fire not just the middle right or does the diesel just drip into the cylinder when it opens

It's sprayed in (the pattern of the spray is one way to determine if the injector is working properly....but that's really a different topic) and then compressed (along with air from the intake) to a very small volume (depends on the compression ratio) in the top of the cylinder. The design of that "top" can be different configurations (hemispheric is one, wedge is another...but those aren't the only ones...just the one I knew off the top of my head).

 

[Mudfarmer]

All said is well and good, but perhaps idling before shutting down is just tradition. Uncle Vernie, now long dead, had a Fordson diesel conversion. It started by a crank with gasoline, then switched to deisel after the engine was warmed up. You never shut it down while the job was going on (perhaps the folklore about keeping your diesel engine running). When you did shut it down you used the switches to change compression and fuel and ran it on gasoline for a couple of minutes to get the diesel out of the cylinders. N'er-do-well uncle Keith would always forget to do the latter. Next time it had to be started the crank had to be turned and turned until the diesel was replaced by gasoline and the engine would start - a good fifteen minute frustrating job. Uncle Vernie never forgot to idle it on gasoline. It had two fuel tanks obviously. Probably not necesssary to do any thing like that now - but the behaviors remain. Heck, I don't even need to preheat the 3005 to start it - but I always do!
Mf

 

[SPYDERLK]

from what u just stated how i read it is that combustion does use the complete cylinder. And since diesel is sprayed all over the cylinders diamater it use the whole cylinder to fire not just the middle right or does the diesel just drip into the cylinder when it opens

The diesel is sprayed in at near TDC into the highly compression heated air in the cyl. That air is so hot the fine spray injection ignites and burns. I think injection continues for a small part of the power stroke. This can happen because injection pumps deliver higher pressure than the pressures reached in the cyl at full throttle. So ... no dripping -- a fine spray to maximize ignition and burning. A diesel is superior this way because the fuel is injected during the burn. The part of the fuel that doesnt burn [if any] is the portion that ends up very close to the cyl wall before it is totally burned. This tiny thickness of poor to no burn is not enuf to really matter -- 99&44/100ths % of the cyl is used. The point I was making is that the flame stops just short of touching the cylinder wall.
larry

 

[MossRoad]

Its a matter of degree as to remanent oil film left by rings. And it may be hard to accept, but the combustion does not reach the cyl walls. They are "cold" and the mixture near them is poor due to being stagnant. In a truly cold engine the flamefront stops well short of the walls [a few thousandths "]. At operating temperature it gets closer, but the combustion cycle is so short the flame still cannot breach the proximal growingly stagnant area. The best way I know to see a demo is to wave your propane torch across a piece of hot oiled metal. It wont burn the oil unless you expose it to the torch much longer than a power stroke takes.
larry

That makes sense. It just seems that if there is oil left on the cylinder walls above the rings, it would get consumed pretty fast. Your example of the torch is a good analogy. I was always led to believe the oil lubed the bottom end and the fuel lubed the top end of the cylinder. Maybe I spent too much time working on two strokes. :laughing: