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bob miller

4 stroke vs 2 stroke

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why is it that 4 stroke motors do not need oil mixed with fuel but 2 stroke motors do need oil mixed with fuel to last..... thanks ....bob

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Bob,

2 stroke engines dont run a sump to hold oil, so it has to be mixed in the with the fuel, Where as conventional 4 stroke engines has a sump to hold oil, and valves, etc, however model 4 stroke engines like OS effectivley have a dry sump and the oil is still mixed in with the fuel

Edited by Shane William Bastick

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Twp stroke motors have the air fuel charge traveling through the engines crankcase. If the crankcase had oil the same as a conventional four stroke, the air/fuel charge would both dilute the oil with fuel and carry the oil out the exhaust. Some two strokes such as snowmobiles and outboard engines have oil injection systems where there is a sepperate oil tank and oil is injected into the crankcase or air/fuel inlet tract.

 

Early four stroke motors used total loss oil systems.

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2 stroke motors use oil to lubricate piston movement in the sleeve , what lubricates the cylinder walls of a 4 stroke motor ?

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it must be very little amounts of oil , both of my Toyotas burn very little oil if any over 12000 mi . I have been using mobil 1 for years

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Are we talking about model or large engines? Large two strokes don't mix oil in the fuel. They have sealed crankcases containing oil and external blowers for scavenging. Small, simple two strokes use their crankcases for scavenging so anything in the crankcase goes through the whole engine. Model four strokes also mix oil in the fuel as a simpler solution to lubrication. some oil will leak past the piston to lubricate the crankshaft bearings. Model four strokes that use their crankcases as superchargers have the same issues as two strokes.

 

Lohring Miller

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On a four stroke engine, the crankcase is sealed from the combustion chamber by the rings in the piston, and the crankcase is open to the atmosphere (through a breather) so that no pressure is built up from the descending pistons. The engine has an oil pump and oil passages through out so that the oil is distributed to the crankshaft bearings, connecting rod bearings, cylinders, valve train, etc. The bottom ring on the piston is an oil scraper to that prevents the oil from migrating past the rings and into the combustion chamber to be burned.

 

On a conventional two stroke engine, no matter if 3.5cc or 800cc, The cylinder is open to the crankcase via the inlet ports and the motion of piston and hence changing volume of the crankcase is used to pump the air/fuel into the crankcase and through the inlet ports into the combustion chamber. Due to this configuration, the only way to effectively lubricate the crankshaft bearings, connecting rod bearings and cylinder is to either mix oil with the fuel, or inject oil into the air/fuel inlet tract or crankcase.

 

There have been very large two stroke diesels that use the same crankcase lubrication system and superchargers to pump the air/fuel mixture into the cylinder. Such a system would not be practical on a model engine and in terms of power to weight ratio, a conventional two stroke engine can't be beat.

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Two strokes are best for small, inexpensive, light weight engines and really large, slow speed engines. Think models, chain saws and MX motorcycles. Two stroke diesels power most ships these days. Four strokes reign in between.

 

Lohring Miller

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