HS46C With Engine View

The 2-stroke engine has been part of the power tool industry as well as the motorbike industry for many years and in that time, huge advances in combustion efficiency and emissions have been made. STIHL has invested greatly in the combustion engine since 1929 when Andreas Stihl developed the world’s first portable chainsaw and for many years, we have continued to push the boundaries of technology with the introduction of 2-MIX, 4-MIX, hand-held fuel injection, M-Tronic, variable ignition timing and even our own fuels.

BR350 With STIHL 2-Mix Engine On ViewIn this blog, we’ll explain the difference between standard 2-stroke engines and our own STIHL 2-MIX engines. Many STIHL hand-held power-tools use 2-stroke combustion engines but some chainsaws, hedge trimmers, grass trimmers and other tools use engines with 2-MIX technology – so what’s the difference and what does it mean for you when you are using the tools?

Actually, a 2-MIX engine is still a 2-stroke that has a slightly different engine design that makes it much more fuel-efficient, and more powerful too.

2-Stroke Engines

The operation of a 2-stroke engine involves two-piston movements (strokes) and one crankshaft revolution, hence the name; 2-stroke.  Combustion occurs in the same way as any internal combustion engine (such as a car engine) with the “suck, squeeze, bang, blow” principle which is explained a little further below but the mechanical components of a 2-stroke engine are slightly different as:

  • The piston also controls the opening and closing of the inlet and exhaust ports instead of valves and
  • The crankcase (bottom of the engine where the rotation of the engine occurs) is used during the induction process
  • There is no oil reservoir, hence the need to add 2-stroke lubricating oil to the fuel.

The combustion process of a 2-stroke engine

The combustion process starts when the rising piston creates a pressure drop in the crankcase. This allows atmospheric pressure to push fuel/oil/air mix from the Carburettor into the Crankcase via the open inlet port. At the same time, the rising piston is compressing the previously transferred gas ready for ignition.

The gaseous fuel mix is ignited as the piston approaches the top of its stroke and the pressure pushes the piston back down. The piston covers the inlet port on the way down beginning the crankcase pressurisation process. As the piston approaches the bottom of its stroke the exhaust port is uncovered allowing the high-pressure exhaust gas to escape.

Just before the piston reaches the bottom of its stroke the transfer ports are uncovered allowing the now pressurised fresh fuel mix to be transferred from the crankcase into the combustion chamber and the process starts again.

The STIHL 2-MIX engine

A 2-MIX engine is a 2-stroke engine that replaces the unburned fuel, which would have left via the exhaust during the transfer process, with fresh air.

This is achieved by the addition of an extra ‘fresh air’ port either side of the inlet manifold between it and the transfer ports. Fresh, filtered air is drawn into the transfer ports during the induction process (piston moving up) via the shallow depression on each side of the piston that momentarily connects the fresh air and transfer ports together.

Fresh air from the transfer ports then enters the combustion chamber first when the ports are uncovered by the downward moving piston and it’s this gas that accompanies the remainder of the exhaust gas leaving via the exhaust port. Adding air (oxygen) to the exhaust lowers the NOX value as well as reducing the hydrocarbon content quite considerably – typically 60 – 80%. A reasonable fuel saving can also be achieved.

So, overall, 2-MIX engines set new standards by increasing the overall efficiency of the engine and reducing emissions – better for the user’s wallet, health and the environment!

To help reduce emissions further, STIHL recommends using MotoMix fuel which has almost no olefins or aromatic compounds and uses a cleaner-burning lubricating oil, STIHL HP Ultra. Better still, the fuel contains no ethanol which is well known to degrade the engine components that can be found in carburettors, such as diaphragms and gaskets, so using MotoMix can prolong service life too.

Paul Hicks
Paul has worked for STIHL for six years and is responsible for the range of STIHL products that is sold here in Great Britain, which means he definitely knows his stuff when it comes to all things STIHL.