Basic Understanding
A Forced Induction engine uses an externally mounted air pump to force air into the engine. These air pumps are known as Superchargers (blower) or Turbochargers. Again, the only purpose of a super charger or turbo is to pump air into the engine.
Question : What does a super charger or turbo do?
Answer : Force air into an engine's intake. Thats why it's called forced induction.
The difference between a super charger and turbo is how each air pump is driven.
A super charger is driven by the engine's crankshaft. In just about all cases, a belt is used to connect a pulley mounted on the crankshaft to a pulley mounted on the supercharger. Super chargers are often installed in place of an engine's intake manifold. However, centrifugal super chargers (like a pro charger) are mounted off to the side of the engine.
This is a good video for some more info on the different types of super chargers.
A turbo is driven by the flow of your engine's exhaust gas. There are no belts involved with the turbo. A turbo is not driven by the crankshaft of your engine. The flow of exhaust gas is the driving force of your turbo.
Question : Why does that turbo have a belt connected to it?
Answer : That's a centrifugal super charger. You know it's a super charger because it's driven by the crankshaft of the engine.
Naturally, the way air pressure is measured is made way more complicated than necessary. The standard atmospheric pressure is represented as 1 ATM.
Standard measurement of Atmospheric Pressure (1 ATM) Equals
14.7 PSI
101 kPa
1.01 BAR
As discussed in the previous post, a naturally aspirated engine will create a vacuum effect to draw air into the cylinders. This vacuum effect occurs when a piston is moving downward during its intake stroke and just represents an air pressure lower than 1 ATM in the intake manifold. Air will rapidly flow from the high pressure area (atmosphere) to the low pressure area (intake manifold) as soon as the throttle body or carburetor opens to allow air flow. Air will continue to rapidly flow until both are equal. Therefore, you will never see a level of pressure greater than 1 ATM in the intake manifold of a naturally aspirated engine.
The same is true with a forced induction engine. However, when your intake manifold pressure reaches 1 ATM your turbo/supercharger will continue to PUMP AIR and create a pressure greater than 1 ATM in the intake manifold. This level of pressure greater than 1 ATM is referred to as the boost level.
Again, boost is the level of pressure greater than 1 ATM in the intake manifold.
Think about a boost gauge. The boost gauge is measuring the amount of air pressure in the intake manifold. If you watch the boost gauge on a forced induction car while its driving, it goes from a negative value (vacuum) to zero (atmosphere pressure) to a positive value (boost)
It's important to understand that higher boost levels do not always equal more power. Volumetric Efficiency is a measure of how efficiently an engine can flow air and fuel in and out of the combustion chambers.
If you are using an air pump to pump the same amount of air into two engines that have different volumetric efficiencies, the engine with the greater volumetric efficiency will see lower boost levels while typically making more power. Thats because the engine will flow a greater amount of that fixed volume of air into the combustion chamber (because higher VE%) resulting in less air left over in the intake and lower boost levels.
Think about boost as a result of your heads not being able to flow enough air into the combustion chamber.
This Episode of Engine Masters on Motor Trend tests all of this.
Looking at the diagram above, you see there are two sides to the turbo - the turbine (hot) side and the compressor (cold) side. Other than the shaft connecting both wheels, these are two totally separate systems from one another. The turbine and compressor sides each have their own housing with their own wheel inside. The turbine housing is made of cast iron and the compressor housing is made of aluminum. Typically, stainless steel tubing is used to feed the turbine with hot exhaust from the engine, while aluminum tubing is used to feed the engine with cold air from the compressor.
Think about it this way, the purpose of the turbo (air pump) is to feed cold and compressed air into the intake of your engine. The compressor wheel is responsible for drawing air from the atmosphere, compressing it, and sending it to the engine. You want cold air going into the engine. Therefore, everything related to the compressor side of the turbo is referred to as the cold side. The compressor wheel needs to be spinning in order to compress and pump any air. The faster the compressor wheel is spinning, the more air it will compress and pump to the engine.
The turbine wheel's only job is to spin the compressor wheel. The turbine wheel does not spin until there is exhaust gas from your engine flowing through the turbine housing. The higher the velocity of exhaust gas flowing through the turbine housing is, the faster the turbine wheel will spin. Sine both wheels are connected with a shaft, the faster you get that turbine wheel spinning with exhaust gas, the faster you will be driving the compressor wheel, and the more air you will be flowing to the engine.
Question - What is boost lag and why do people say turbo cars are slow off the line?
Answer - The compressor wheel is not pumping enough air to make big power until it is spinning fast. The engine needs to turn some RPMs before it is pumping enough exhaust to really get the turbine wheel spinning, so be patient the boost is loading......
Turbo sizing plays a huge role in how quickly, and how much boost an engine can make.
General rule of thumb - you want the smallest turbo that is capable of making your desired power levels. Everything about a turbo engine is relying on flowing air and building pressure. If components of the system are bigger than what is required to flow the necessary air for your desired power levels, you are wasting time filling unneeded areas before pressures build.
This is a good article on choosing the correct turbos.
An external wastegate is an air valve that is mounted between the engine and the turbine inlet. The wastegates job is divert exhaust gasses away from the turbine inlet. This is how boost control works.
Question - How do you keep boost levels from exceeding your targeted amount? Answer - You control the amount of air your turbo (air pump) is pumping into the engine's intake manifold.
Question - How do you control how much air your turbo is pumping into the intake manifold?
Answer - You control how fast you are spinning the compressor wheel.
Question - How do you control how fast you are controlling the compressor wheel?
Answer - You control how fast you are spinning the turbine wheel
Question - How do you control how fast you are spinning the turbine wheel?
Answer - By limiting the amount of exhaust you allow to flow past the turbine wheel.
Question - How do you limit the amount of air flowing past the turbine wheel? Answer - By opening the wastegate (air valve) that is located between the engine and turbine wheel.
The position a wastegate is mounted is crucial to its effectiveness at diverting exhaust gasses away from the turbine wheel. This video from Motion Raceworks will tell you everything you need to know about mounting your wastegate.
A blow off valve (BOV) is an air valve placed between the compressor outlet and engine intake. When you let off the gas at high rpm you are suddenly slamming the throttle body shut while the compressor wheel is still spinning and pumping air. With nowhere for that air to go, pressure will QUICKLY build in the tubing between the compressor outlet and throttle body. Thats why you have a blowoff valve placed in that path. All it does is open up (air valve) and say "pssshhhh" as it allows that air to escape back to the atmosphere.
The blowoff valve will open up when the throttle body is shut. Shutting the throttle body shuts off air supply from the air pump and will quickly bring intake pressures back below 1 ATM
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