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McDougall's Power Equipment


34 Brisbane Street

HOBART TAS 7000

03 6231 0900

 

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Fuel and Oil 

Frequently Asked Questions

What is the fuel requirement difference between 2-stroke machines and 4-stroke machines?

 

As this is a fuel/oil related question we wont go into the mechanical side of things too much.  All you need to know is a 4-stroke engine takes plain fuel without oil mixed with it.  The engine gets its lubrication (to protect moving parts) from oil stored in the 'sump'.  This oil is the only source of lubrication and is constantly cycled through the engine, which is why it needs to be drained and replaced periodically, just like your car.

 

A 2-stroke engine has less moving parts and intakes it's fuel a different way that ends up surrounding all the engine components before it is burned and ejected.  Therefore, the oil can be mixed with the fuel, as it ends up lubricating all the engine.  The advantages of 2-strokes over 4-strokes are many, one being the lack of oil needing to be in a sump results in a machine that is lighter and requires less maintenance.

 

If you are not sure which style engine you have, you can check to see if you have a dipstick or bung to check on the oil in a sump (4-stroke) where as a 2-stroke engine will not have this.

 

 

Are 2-stroke oils different to 4-stroke oils?

 

Yes.  4-stroke oils need to be good enough to repeatedly be cycled through an engine for many hours of work, (usually 25) before an oil change.  It is not designed to blend with fuel easily, or burn in the combustion cycle. Which is why modern 2-strokes can potentially run cleaner than 4-stroke engines.

 

2-stroke oil needs to mix with the fuel and burn cleanly to avoid carbon build up on the spark plug, top of piston and exhaust system.  

 

You can not substitute one for the other - both oils have very different roles to play.

 

 

What happens if I mix up my fuel cans up and put 2-stroke fuel mix in my 4-stroke engine or 4-stroke plain fuel in my 2-stroke engine?

 

One way, you are safe, the other is catastrophic.  2-stroke mix in a 4-stroke engine is completely fine.  The engine will still lubricate in its usual way, and the oil in the fuel will be burnt the same way a 2-stroke engine would have burnt it.

If you are running a high quality 2-stroke oil, like the Husqvarna LS+ oil,  that doesn't smoke when running in a 2-stroke engine, it wont even make your 4-stroke engine smoke.

 

If you put 4-stroke plain unmixed petrol into your 2-stroke machine, the result is major damage.  Because the components rely on the oil being in the fuel, there is nothing to lubricate the engine and the metal parts build up friction and start to weld together.  The piston and cylinder in any engine is a highly precise bit of engineering, and are under a huge amount of heat, stress and load.   The piston moves up and down the cylinder at an average of 13,000 rpm - that's 217 times a second!  Imagine how much it relies on being lubricated as it rubs against the cylinder wall.  Without that lubrication, it soon starts to heat up and literally start to try and weld itself together.

 

 The following picture shows the end result.  What was once a precision made piece of aluminium is reduced to a scored and melted mess.  The usual repair job is a piston and cylinder, gasket set, labour etc which is costly a exercise!

 

 

 

 

 

 

 

 

 

 

How important is the quality of 2-stroke oil I use?

 

Lets try and put this in perspective.  

 

A mid size chainsaw has a stroke of 36mm - that is how far the piston travels in one direction, which would be twice that figure for one revolution (up and down) - 72mm. If a chainsaw revs to an RPM of around 13,500, that would mean the piston has traveled 72mm x 13,500 = 972,000mm or 972 meters - nearly a kilometre of distance a minute!!  If you only used your chainsaw for an hour, you have sent that piston 60km in accumulated distance all whilst under huge loads from expanding combustion gases, high temperatures and vibration.  

 

High quality 2-stroke oil is not a luxury, but a must, to stand up to the lubrication requirements such a hard working engine needs.  If you want many years, if not decades of service out of your machine, you can not afford to skimp on this important liquid.

 

 

What ratio do I mix my 2-stroke oil at?

 

This depends on which oil you are using.  You may of heard of 50:1, 25:1 and now 33:1 as a mixing ratio.  The figure on the left of the equation is the quantity of fuel and the figure on the right is the quantity of oil.  So at 50:1, 50 litres of petrol would require 1 litre of oil etc.  

 

Generally the better quality oil, normally priced at the higher end, means you can use less oil with the fuel.  This gives better starting, crisper throttle response and less chance of carbon deposits from burnt oil forming on the spark plug, piston, exhaust etc. 

 

Husqvarna Australia recommends all machines under 70cc to run at a 50:1 ratio, and over 70cc to run a 33:1 ratio, nothing we sell takes 25:1 anymore.

 

If you run out of 'good quality' oil on a Sunday and only generic oil is available, then you may only get a choice of 25:1 mix oils (cheaper, but lower quality).  If this is the case you can run your machinery at 25:1, but don't expect the same clean running and performance as 50:1 fuel mix.  Remember you have to use twice the oil in the fuel to lubricate as good as a high quality 50:1 mix oil would, so there is an excess of oil that does not burn as cleanly and long term, can be more detrimental as large chunks of carbon can break loose and damage the piston and cylinder.  Using twice the amount of oil usually means that to mix the same amount of petrol with a good 50:1 mix oil will normally work out a cheaper option anyway.

 

 

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