The Roots supercharger is the oldest type of supercharger and dates back to the early 1900’s when it was first used as an industrial air moving device. In the past 50 years or so however, the roots supercharger has under gone drastic changes and has become so efficient and quiet it is now commonly used as a forced induction system for automotive and racing applications.
An internal combustion engine draws in air which is mixed with fuel. This fuel/air mixture is drawn into the engine as a result of vacuum created by the down stroke of the piston. When the piston travels back up, the mixture is compressed to a fraction of its original volume. If an engine has a 10:1 compression ratio, the mixture will be compressed to 1/10th of its original volume. When the spark plug is fired, the resulting combustion causes an expansion of the charge, forcing the piston back down to create power. Packing more air/fuel mixture into the engine causes combustion pressures to increase, creating more horsepower and torque.
In a naturally aspirated engine, when the piston travels down on the intake stroke, atmospheric pressure (14.7psi at sea level) tries to fill the empty space now present in the cylinder. If the cylinder could fill completely, the engine would be 100% volumetric efficient. Due to restrictions created by air cleaners, cylinder heads, valves, cam specs, cylinder seal, etc, all of the air that should enter the cylinder, can not. Typical stock engine volumetric efficiency is in the range of 75-85%. Improvements can be made by reducing restrictions, but achieving 100% VE with a naturally aspirated engine is extremely difficult.
With a roots supercharger, the amount of air/fuel mixture that can enter the cylinder greatly exceeds 100% VE. Since air is now being forced into the engine, you can put a substantially denser air/fuel charge into the cylinder. With most street tuned blown applications running 6-7psi boost, approximately 45-55% more air/fuel can be packed into the combustion chamber than in a comparable naturally aspirated engine. The reason a large cubic inch engine can make more power than a smaller one is that more air/fuel is available for combustion. Using a supercharger, a small displacement engine can produce similar horsepower and torque to a naturally aspirated larger displacement engine. Using a purpose-built large displacement engine with a supercharger………….The sky is the limit.
The roots supercharger is known for its ability to produce large amounts of boost while spinning at very low speeds. This characteristic has contributed to its success and popularity on the top fuel racing circuit and has made it ideal for use on smaller 2, 4, and 6 cylinder engines that traditionally struggle in the lower half of the rpm range (and is why Thomson Superchargers uses a roots type design). Another advantageous characteristic of the roots type supercharger is its simplicity of design. The roots supercharger has very few moving parts and spins at lower rpms, making it one of the most reliable and durable supercharger designs. The biggest disadvantage to the roots supercharger is its thermal inefficiency – or its nature to produce high discharge temperatures – which robs power from the engine. Additional heat is created by compressed (hot) air that leaks backwards past the rotors and heats up the temperature of the inlet charge. Proper sizing, rotor fitment, and advances in rotor design have allowed the Thomson Supercharger to drastically improve this issue.