The decompression valve: start your petrol chainsaw with ease
The STIHL decompression valve releases compression to make starting your chainsaw significantly easier.
07.07.2023
What is a decompression valve?
The engine cylinder of a chainsaw contains a mixture of gases. When you begin the starting procedure this mixture needs to compressed in order for ignition to succeed – but excessive pressure requires you to put a lot more effort into starting the engine. When the starter cord is pulled, the piston inside the cylinder moves, compressing the fuel-air mix and increasing the internal pressure. Overcoming this pressure on chainsaws with a larger engine capacity takes considerable effort, but a decompression valve reduces the pressure, making the engine easier to turn over. There's a variety of engines fitted with decompression valve systems that are activated in different ways - we'll talk about them later on!
2-stroke engines - a brief overview
Not all combustion engines feature a decompression valve, but all 2-stroke engines work on the same principle: a mixture of fuel and air is drawn through the cylinder and into the combustion chamber, where it is put under significant pressure and then ignited. This process moves the piston, which continues the compression and ignition process while also working with the connecting rod and crankshaft to convert the stroke motion of the piston into a rotation of the crankshaft. This stroke can then drive additional components, e.g. a saw chain on a STIHL chainsaw.
The decompression valve
You may wonder why our chainsaws don't have an electric starter. The reason for this is because the fuel-air mix is compressed to a pressure of several bar, in order to overcome the mass inertia of the crank mechanism. The larger the stroke motion of the engine, the more power needed to do this. To make the starting process faster and easier, we have instead developed a decompression system that is based on a small but effective component: the STIHL decompression valve.
How does the STIHL decompression valve work?
Decompression means easier starting
Opening a decompression valve releases gas to ensure that the peak pressure when starting the engine is reduced. This compression release means less effort is required when starting your chainsaw or other tool. As soon as the engine starts, the decompression valve automatically closes again; compression increases; and the start-up momentum of the cylinder initiates normal combustion operation so you experience full engine performance.
Decompression system types
There are three decompression valve types available, all of which are activated in different ways. Below is an overview of the different systems for compression release.
A decompression lever is a purely manual device that you use to actuate the decompression valve in the cylinder head. This method isn't used in any of STIHL's current range. The decompression lever works by means of spring tensioning, requiring you to pull or push the lever as you start the tool. With the valve open when you activate the starting mechanism of the engine (such as the starter cord on the 2-stroke engine of a chainsaw or boat, or a kick starter or pedal on a moped), compressed gases are diverted out of the cylinder head and the engine can be started with ease. Once the engine has started you must release the lever to close the decompression valve so the engine can function as compression builds up again.
This common decompression method only requires you to manually open the decompression valve before you start the engine. The valve then closes automatically when the cylinder head starts to move. This mechanism ensures optimal fuel mix density as well as the simplest possible starting of the machine. All newer STIHL chainsaws with decompression valves use this semi-automatic system.
Fully automatic STIHL engine decompression systems are used in our 4-MIX® engines and lawn mower engines. When the crank shaft revolutions start, the inlet valve opens automatically, and is then closed again by the action of centrifugal force weights when the set revolution speed is exceeded.
Can decompression valves be retrofitted?
In principle, yes they can. Not all chainsaws are large enough to need a decompression valve, but if your STIHL chainsaw has a blind plug on the cylinder, you can assume it can be used to retrofit a decompression valve. Contact yournearest STIHL Approved dealer to discuss further.
Summary: the STIHL decompression valve:
- The STIHL decompression valve releases gas to reduce peak compression pressure in the cylinder of your chainsaw
- The STIHL decompression valve makes it easier to start engines that use pull cords
- Engines in general may be fitted with manual, semi-automatic or automatic decompression valve systems
- All STIHL decompression valves close automatically after the engine starts
- Decompression valves can be retrofitted in STIHL tools that are appropriately equipped
