STIHL 4-MIX® engines offer excellent torque and fuel efficiency, all while still using the same 50:1 ratio fuel used in 2-stroke engines
Unlike other 4-stroke engines, the STIHL 4-MIX® engine uses the same fuel mix as a 2-stroke engine. This is petrol and oil in a 50:1 ratio, or a pre-mixed product such as STIHL MotoMix. This means that unlike a lawn mower engine, the machine can be used in any orientation as there's no seperate oil sump lubricating the engine. The STIHL 4-MIX® engine achieves high torque with a comparatively low amount of fuel, making it a great low-emissions option. It’s also surprisingly quiet to use too!
The STIHL 4-MIX® engine works by means of four actions, commonly referred to as suck, squeeze, bang and blow.
As the crankshaft rotates, the piston moves down the cylinder, the inlet valve is opened and some fresh fuel mixture is drawn into the combustion chamber. Here, what distinguishes the STIHL 4-MIX® engine is the bypass channel that connects the valve train and crankcase to the intake channel. This innovative engine layout moves the fuel and air mixture to the engine components with each piston movement, meaning that the 4-MIX® engine is always perfectly lubricated and the need for regular oil changes is eliminated.
When the piston moves down, excess pressure develops in the crankcase. This conveys fuel mix from the crankcase, through the cam wheel housing and into the intake channel and then the combustion chamber, meaning all lubrication points are covered. The piston in the STIHL 4-MIX® engine moves up again and the mixture in the combustion chamber is compressed.
The mixture in the STIHL 4-MIX® engine is most compressed when the piston is at its topmost position – the upper dead centre. Shortly before this is reached, the spark plug starts the process of burning the mixture with a spark. This early ignition means that the mixture in the combustion chamber is burned efficiently and with minimal residues. Ignition also forces the piston down again, and the energy is transferred to the drive shaft of your STIHL product.
Before the piston reaches its lowermost position – the lower dead centre – the cam wheel opens the exhaust valve inside the STIHL 4-MIX® engine. The next time the piston moves up, the resulting emissions are forced out of the combustion chamber through the exhaust channel, and the process can start all over again.
Every STIHL 4-MIX® engine features an integrated automatic decompression system to simplify the process of starting up the engine. When you start the 4-MIX® engine, centrifugal weights are used to open a small valve in the engine cylinder. Some of the pressure escapes through this tiny opening, meaning that less force is required to pull the starter cord. As soon as the engine speed reaches a set point, the decompression system is automatically deactivated so the 4-MIX® engine runs efficiently and powerfully with full compression.
A STIHL 4-MIX® engine is incredibly light. Because the engine is lubricated via the fuel-oil mix during operation, certain components that feature in conventional 4-stroke engine have been eliminated, such as oil pumps, oil filters and sumps. In addition to making the engine lighter, the 4-MIX® design means that oil checks, oil changes and disposing of old oil are all a thing of the past.
Both of our flagship engine systems are innovative, powerful and extremely efficient. We’ve summarised the key features of the two systems in the table below.
|STIHL 2-MIX engine
|STIHL 4-MIX® engine
|Low scavenging losses
|Extremely fuel efficient