Two Stroke Induction

Category: Snowmobile Engine

2 srtoke piston port engine 

 There are three general types of two cycle induction. The most simple type of air intake induction is called a piston port engine. This type of two stroke engine has holes cut through the cylinder walls in specific areas. The second type of induction, and the most popular type, is reed block induction. This consists of multiple reed petals that allow air into the crankcase and keep it trapped there. The third type of two stroke induction is generally used in rotax two cylinder engines is the rotary valve engine. It does slightly better than reed engine because there is no restriction from reed petals. The rotary valve rotates past the crankcase intake port at a specific time to allow air through then seal the air into the crankcase.


 All 2 stroke engines use their crankcase similar to a supercharger to pack more air into the combustion chamber. First the engine draws air into the crankcase through whatever induction style they have. Then as the piston travels downward the air becomes pressurized and travels through air passages called transfer ports. These ports deliver fresh air to the upper part of the cylinder. Unfortunately the exhaust port is also open which leads to some fuel escaping out of the exhaust port. This is the real reason non direct injection two stroke engines smoke and have poor emissions. Direct injection two strokes wait until the piston is above the exhaust port to inject fuel so none is lost.

 piston port sleeve

 The first type of 2 cycle induction is piston port induction. Piston port is the most simple design and has the carburetor located on the cylinder. As the piston goes up, the bottom part of the piston skirt raises above the intake port. As the piston passes the port there is a vacuum created in the crankcase. The air then rushes into the crankcase where it will then be transferred when the piston comes back down. Aside from porting the engine, some people will enhance the engine with a special piston. The piston has a cutout in the intake side skirt so that air will be able to enter the crankcase sooner. This induction type is generally used in older or very simple engines that are not performance oriented.

reed block 

 The most popular two stroke intake is the crankcase reed induction style that uses reed petals. In this type of an engine, a reed cage is built into the crankcase. A reed block is removable and is where the petals are mounted to. The carburetor is mounted to the outside cover of the reed area. The air enters the reed cage which has reed petals that will only open into the crankcase. The air will fill the crankcase better in this style versus the piston port. The air doesn’t have to wait for the piston bottom to pass instead it can come in as soon as the piston starts to travel upward. When the piston comes back down no air can escape out of the crankcase because the reeds press up against the back of the reed block. The air then rushes through the transfer ports.

 The reed petal is also very important because it has to flex to allow air to pass by it many times. They need to be durable so they don’t brake off after many cycles. If they are made too stiff to be durable they can’t flex well and will cause a air restriction. If the petal is too light it will flutter at high rpm’s and won’t seal well. This fluttering would result in a loss of power although lower rpm’s torque would be better. Standard petal materials are spring steel and composite petals such as fiberglass. Many performance companies produce new high performance reed blocks which have better airflow. Companies also produce reed petals made of carbon fiber that flex sooner but don’t flutter at high speed and increase power at all rpm’s. 

rotary valve 

 The third type of induction is the rotary valve engine. They are generally offered on two cylinder engines due to the design but can be used on multiple cylinder engines. Generally this valve is large enough to rotate past the port area for both cylinders. Multiple odd numbered cylinders would need their own disc and this can become costly. The valve is metal and rotated by a gear between the cylinders that is driven off of the crankshaft. This valve looks like a circle cut in half although the “removed portion” doesn’t need to be half of the total circle. The edges will expose or open and close the ports depending on their angle.

 Another less popular version is a full circle disc with holes cut into it in a fashion that opens and closes the intake ports at a specific time. This is where the rotary valve engine has it’s advantages, there is no air restriction. Unlike the others that wait on a piston or have an air restriction from reed petal tension. Rotary valve engines also have the ability to easily change the opening and closing time of the ports. Aftermarket rotary valves have different angle valves optimized for different rpm ranges. Others also make lightweight material valves such as carbon fiber to replace the aluminum. Aluminum is generally used for best reliability though.

carbon fiber rotary valve