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Tool Talk Discussion Forum

How does a 2 stroke get lubrication?

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Gwiz

12-11-2007 17:47:24




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Can someone please explain to me how a 2 cycle engine (chain saw) gets lubrication to all its bearings and bushings? Does the crank run in the raw fuel?
James




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john18436572

12-14-2007 17:39:32




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 Re: How does a 2 stroke get lubrication? in reply to Gwiz, 12-11-2007 17:47:24  
the oil in the fuel-oil mix deposits on the metal surfaces in the crankcase, forming a coat of oil film that keeps the parts lubed.



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Gwiz

12-13-2007 17:09:31




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 Thanks guys think I finally got it in reply to Gwiz, 12-11-2007 17:47:24  
I see. Bet it's more of a concern to keep the volume of the crankcase down in a two stroke compared to a 4.
James



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onefarmer

12-14-2007 06:35:58




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 Re: Thanks guys think I finally got it in reply to Gwiz, 12-13-2007 17:09:31  
Yes I think you will find that on the 2stroke there is only room for the crankshaft and rod and most of the volume is the underside of the piston. Gota keep it small to get a good squeeze on the mix to charge the combustion chamber when the ports open



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onefarmer

12-12-2007 19:56:17




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 Re: How does a 2 stroke get lubrication? in reply to Gwiz, 12-11-2007 17:47:24  
2 Stroke basics (chainsaw version) as the piston rises a vaccum is formed in the crankcase pulling gas/oil/air mix into the crankcase. This lubes the bearings and cylinder walls. When the piston reaches tdc ignition occurs driving the piston down making power, and pressurizing the mix in the crankcase. As the piston travels down it uncovers the ports allowing the burnt charge to excape out the exhast port. then the intake uncovers allowing the pressurized mix, from the crankcase to move in the cylinder. The piston travels back up and the cycle starts over again.

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buickanddeere

12-12-2007 16:35:16




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 Re: How does a 2 stroke get lubrication? in reply to Gwiz, 12-11-2007 17:47:24  
Roller crank main & rod bearings will survive with the 1-2% oil mixed with the fuel mist swishing around the crankcase. High pressure wear points such as camshaft lobes and lifters are not a 2 stroke lub problem.



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guido

12-12-2007 15:58:56




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 Re: How does a 2 stroke get lubrication? in reply to Gwiz, 12-11-2007 17:47:24  
Hey Gwiz
One reason for the oil to reach the lube points in the engine, on a 2 cycle engine is that the cranckase is pressurized. The oil then migrates into the lube pointS while the fuel vapors get compressed and burned. if you have a heavy oil mix it will get into the combustion chamber and burn with the fuel. Guido.



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Gwiz

12-12-2007 14:48:24




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 Still not 100% clear in reply to Gwiz, 12-11-2007 17:47:24  
Yes I mix oil in my gas, and yes I completely understand 2-stroke, but I'm not real clear on when the fuel/oil is in the crankcase.
Some here have suggested the fuel/oil mix goes through the carburetor and then into the crankcase? Is that correct? Seems there would have to be more to it than just pulling a fuel/oil mix directly into the crankcase and then into the cylinder? When I asked if the crank ran in raw fuel, I meant does the crankcase serve as the fuel tank also? It doesn't seem right that the fuel tank would also serve as the crankcase, because then the crank would have the resistance of hitting the liquid.
James

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Tim B from MA

12-12-2007 20:01:13




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 Re: Still not 100% clear in reply to Gwiz, 12-12-2007 14:48:24  
If you don't understand when fuel is in the crank case, then I don't think you are clear on how most 2-strokes work (the exception being fuel injected diesels and such).

The carburator is connected to the crank case (not an intake manifold to the intake valves as in a 4 stroke). Piston goes up, fuel and air mixture gets drawn into the crank case. Piston goes down, the mixture in the crank gets preassurized and at a certain point in the downward travel, the intake port to the piston is open (often just a hole in the side of, and low in the cylinder). The preassurized mixture then flows to the piston ,ready to be preassurized more and combusted.

There is never an actual pool of liquid fuel in the crank case, just the mixture. As has been said, the mixuture is vaporized gasoline and not-so-vaporized oil.

Try www.howstuffworks.com

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jdemaris

12-12-2007 07:10:55




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 Re: How does a 2 stroke get lubrication? in reply to Gwiz, 12-11-2007 17:47:24  
It all depends on what type of two-stroke-cycle engine. With chain saws, some use piston porting, some use reed valves, some use roto-ports in crankshaft etc. Lubrication depends on the type of system and path the fuel-oil takes on it's way to the combustion chamber. They just-about all rely on fuel between drawn though areas where the moving parts are - so the oil in the fuel lubricates them. But - there are exceptions. Some two-stroke-cycle engines need an oil pump and oil tank to send oil to remote moving parts that do not get lubed by the fuel-oil mix. Some chain saws and motorcycles are this way. And, some two-stroke-cycle engines have conventional oil sumps - e.g. a Detroit Diesel engine.

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El Toro

12-12-2007 04:12:32




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 Re: How does a 2 stroke get lubrication? in reply to Gwiz, 12-11-2007 17:47:24  
When you mix the correct amount of 2-cycle engine
oil in a gallon of gasoline they're designed to get enough lubrication from the oil in the gas mixture. I use 30wt engine oil and most of my engines require 8 ounces of oil per gallon of gas. I just sold an old Lombard chain chain that I bought over 30 years ago. Never had any engine work and started easily. Hal



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Stumpalump

12-11-2007 20:42:45




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 Re: How does a 2 stroke get lubrication? in reply to Gwiz, 12-11-2007 17:47:24  
You never want to run a two stroke out of gas. If you do it will run a short while on fuel vapors from the tank. Fuel vapors have fuel but no oil. Especial hard on an outboard or dirt bike running hard while it runs out. So first sign of bogging back off and check that fuel level.



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135 Fan

12-11-2007 23:53:28




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 Re: How does a 2 stroke get lubrication? in reply to Stumpalump, 12-11-2007 20:42:45  
I don't think running out of fuel is that big of deal. Dirt bikes, chainsaws and all kinds of other 2 strokes run out all the time. I think it's good to use a quality oil mixed to the right ratio based on the oil. Dave



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Stumpalump

12-12-2007 07:27:34




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 Re: How does a 2 stroke get lubrication? in reply to 135 Fan, 12-11-2007 23:53:28  
Yea it sounds like a wifes tale and I've run out manny times without harm. Guys that would sieze moters were mostly dirt bike racers. They run as little oil as they can and they push them hard. Screaming that hot moter wide open and running out is what did them in. But I still like to pass it on because it can and has happened.



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Davis In SC

12-12-2007 18:55:22




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 Re: How does a 2 stroke get lubrication? in reply to Stumpalump, 12-12-2007 07:27:34  
Yep, Those dirt bikes seemed to run best right before they blew.. Looking back, I think it was due to a lean fuel mixture, with little oil.. The very thing that invites a piston seizure.. I trashed a lot of engines, Hodaka, Yamaha, Suzuki,and more...



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Tim B from MA

12-11-2007 19:54:52




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 Re: How does a 2 stroke get lubrication? in reply to Gwiz, 12-11-2007 17:47:24  
If you are not clear on how two strokes work, and/or the difference between 2-stroke and 4-stroke engines, there are great explainations at

www.howstuffworks.com



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Goose

12-11-2007 18:29:30




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 Re: How does a 2 stroke get lubrication? in reply to Gwiz, 12-11-2007 17:47:24  
Reminds me of years ago when I was in the boat and outboard motor repair business.

Several fishermen customers of mine co-owned a boat with a 75hp Johnson engine. These rocket scientists found out they could troll slower if they idled the engine on pure gasoline, so they had one tank with oil mix and one with pure gasoline for trolling. They'd use the oil mixed tank to go to their fishing hole, switch to pure gasoline for trolling, then switch back to the oil mix to return to the dock.

They got away with it until one day a rain squall came up while they were trolling. They forgot to switch tanks and "firewalled" the throttle for the dock. They made it about a hundred yards before a rod went through the block. Plus, they got drenched.

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Gene Davis (Ga.)

12-11-2007 18:22:52




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 Re: How does a 2 stroke get lubrication? in reply to Gwiz, 12-11-2007 17:47:24  
A 2 stroke cycle engine derives it's lubrication from the oil which is suspended in the fuel/oil mix. the oil in the fuel is held in suspension through the fuel system and into the crankcase where the mixture coats everything. (This is why it is important to have it mixed well and to shake the can before each fill up.) The lighter fuel vapors move into the combustion chamber by way of the transfer ports in the cylinder walls or through the cylinder block ports and some time piston ports also. The microscopic oil particles cling to the surfaces and the lighter fuel vapors migrate to the top of the piston whhere they burn for producing power to the piston and there is still a small amount of oil in the vapor which burns also at this point. The oil in the fuel mix is mostly replacing the oil already on the surfaces so there fore it is a good idea to lubricate all 2 cycle parts well when assembling to give lubrication till the oil begins to collect. The best 2 cycle oils are low ash content oils which leave very little residue in the combustion chamber and smoke less also. These oils also are less likely to clg exhaust ports and muffelers. Some 2 cycle engines have oil injection which meters the oil into the fuel mixture behind the carburetor but it still oils the engine pretty much the same way.

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Gerald J.

12-11-2007 17:58:16




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 Re: How does a 2 stroke get lubrication? in reply to Gwiz, 12-11-2007 17:47:24  
Oil is all in the fuel. The crankcase is supplied directly by the carburetor through a reed check valve while the piston goes up, making suction. When the piston comes down it compresses the mix in the crankcase (and also lubricates the crankshaft) and a transfer port opens near the bottom of the stroke not long after the exhaust port opens and the compressed mixture in the crankcase goes up through the transfer port into the cylinder which gets compressed and fired near the end of the piston stroke up. Hence 2 cycles.

Gerald J.

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Gwiz

12-12-2007 14:57:13




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 Come again in reply to Gerald J., 12-11-2007 17:58:16  
You are suggesting the piston creates a vacuum in the crankcase on the upstroke thereby drawing gas through the carb and into the crankcase? Then as the piston comes down, the crankcase air is compressed forcing it into the chamber? This also requires a check valve to stop fuel from exiting the carb? Does the combustion chamber also have a vacuum at the bottom of the stroke like a four stroke? Seems like the crankcase would be a very large volume for the small piston to effect the pressure much? James

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onefarmer

12-12-2007 20:03:54




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 Re: Come again in reply to Gwiz, 12-12-2007 14:57:13  
Some times there is a check(reed) valve in the intake. Many, maybe even, most just cover/uncover the intake port in the bottom of the cylinder with the piston. The piston acts as the check valve.



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RobMD

12-11-2007 17:52:13




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 Re: How does a 2 stroke get lubrication? in reply to Gwiz, 12-11-2007 17:47:24  
The fuel itself gets sloshed around in the crankcase. That's why you mix 2 cycle oil in the gas. I sure hope you do.



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bo

12-11-2007 17:49:52




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 Re: How does a 2 stroke get lubrication? in reply to Gwiz, 12-11-2007 17:47:24  
that's why you mix oil with the gas. You have been mixing oil with the fuel haven't you?
The oil in the fuel does the lubing.



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