Understanding Forced Induction Systems: Turbochargers and Superchargers Explained
Introduction:
Internal combustion engines face a fundamental limitation when operating at high speeds: they simply cannot draw in enough air to produce their maximum potential power. This airflow restriction is precisely why many race cars feature hood scoops these devices deliver cool air under pressure to the intake manifold while providing an open source for incoming air. However, modern vehicle body styles have made hood scoops less desirable because they increase aerodynamic drag. As a result, automotive engineers have developed alternative methods to increase the volume of intake air and compression, including variable intake manifolds and advanced valve timing systems. The relationship between engine power and air compression is direct and unmistakable. The power generated by an internal combustion engine is directly related to the amount of air that is compressed in the cylinders. In simple terms, greater compression within reasonable limits produces greater engine output. Two primary approaches can increase an engine's effective compression. The first involves physically modifying the engine to raise its compression ratio through methods such as using domed or high top pistons, altering crankshaft strokes, or changing the shape and structure of the combustion chamber.The second approach, which is less expensive and does not require physically changing the combustion chamber, involves increasing the intake charge. By pressurizing the intake mixture before it enters the cylinder, more air and fuel molecules can be packed into the combustion chamber. This is achieved through two methods known as turbocharging and supercharging. Both systems force additional air into the intake manifold by compressing it before it reaches the manifold. Turbocharging accomplishes this using exhaust gases heat that would otherwise be wasted while supercharging relies on the engine's rotation to compress the air. Both systems offer significant benefits but also have certain limitations. The most notable disadvantage of either system relates to the compression of air itself; when air is compressed, its temperature increases substantially. High temperature air is less dense, meaning it contains less oxygen per unit volume. To address this issue, most turbocharged or supercharged systems incorporate an intercooler to increase the density of the air.
Forced induction systems both turbochargers and superchargers represent sophisticated engineering solutions to the fundamental challenge of increasing internal combustion engine power output. By compressing intake air before it enters the cylinders, these systems effectively increase the engine's volumetric efficiency and allow more fuel to be burned, producing greater power from a given engine displacement.
Turbochargers harness otherwise wasted exhaust energy to drive a compressor, offering the advantage of improved fuel economy during normal driving conditions. They are particularly effective on both diesel and gasoline engines and have become increasingly sophisticated with variable geometry designs and twin-turbo arrangements that minimize turbo lag and optimize boost across the engine speed range. Superchargers, driven directly by the engine crankshaft, provide immediate boost response proportional to engine speed. Their positive displacement design delivers consistent performance, and modern systems incorporate bypass circuits and magnetic clutches to improve efficiency during light load conditions. The helical rotor design has largely replaced straight-lobe configurations, offering quieter operation and improved airflow characteristics. The choice between turbocharging and supercharging depends on the specific application and performance requirements. Turbochargers offer better fuel economy and high end power potential, while superchargers provide instant throttle response and predictable power delivery.
Intercoolers:
Intercoolers play a vital role in forced induction systems by cooling the turbocharged or supercharged air before it reaches the combustion chamber. When intake air is compressed, its temperature increases dramatically, reducing air density and limiting the amount of fresh air that reaches the cylinders. Additionally, the elevated temperature increases the likelihood of engine knock, a destructive condition where fuel detonates prematurely. To counteract these problems, many turbocharger and supercharger systems are equipped with an intercooler, also known as a charge air cooler. The removal of heat from pressurized air as it passes through the intercooler increases air density, which improves efficiency, engine horsepower, and torque. Intercoolers function essentially as radiators for the intake air, removing heat from the compressed air and dissipating it to the atmosphere. A typical intercooler system consists of an additional radiator positioned in the grille area or above the engine, a coolant reservoir separate from the engine's cooling system, a pump, and hoses and tubes to connect the components. The intercooler is always positioned after the turbocharger or supercharger and before the intake manifold. As heated air flows through the intercooler, heat is transferred to the coolant circulating through the system. This coolant is subsequently cooled by the air passing through the intercooler. The amount of coolant moving through the intercooler is normally controlled by the Powertrain Control Module (PCM), which effectively manages the temperature of the incoming air.Turbochargers:
Turbochargers are employed to increase engine power by compressing intake air before it enters the engine. These devices are essentially air pumps driven by the engine's exhaust stream. The heat and pressure of exhaust gases spin the turbine blades of the pump, hence the name turbocharger. The turbine wheel connects to a compressor wheel, so as the turbine spins, the compressor wheel spins as well. The compressor wheel rotates at very high speeds and compresses the intake air, which is then sent to the cylinders. Since exhaust gas is a waste product, the energy developed by the turbine is considered free because it theoretically does not use any of the engine's power to produce the boost. Turbochargers are used on both diesel and gasoline engines. Their primary advantage is that they allow for an increase in power without a substantial decrease in fuel economy. This is because they boost the engine's power output only when extra power is needed. A smaller engine can be used to provide low fuel consumption and emissions levels during normal driving and when increased power is required, the turbocharger is activated.Construction and Components:
A turbocharger is typically located close to the exhaust manifold. An exhaust pipe runs between the exhaust manifold and the turbine housing to carry exhaust flow to the turbine wheel, while another pipe connects the compressor housing intake to the throttle plate assembly or intake manifold. A typical turbocharger, commonly called a turbo, consists of several key components: the turbine wheel or hot wheel, shaft, compressor wheel or cold wheel, center housing and rotating assembly (CHRA), wastegate valve, and actuator. Inside the turbocharger, the turbine wheel and compressor wheel are mounted on the same shaft. The CHRA houses the shaft, shaft bearings, turbine seal assembly, and compressor seal assembly. Each wheel is encased in its own spiral-shaped enclosure within the housing, which serves to control and direct the flow of exhaust and intake air. Because the turbine wheel is positioned in the exhaust path, it becomes extremely hot and spins at very high speeds. Consequently, it is normally made of heat-resistant cast iron.Turbocharger Operation:
The turbocharger typically begins to compress the intake air when the engine's speed exceeds 2,000 rpm. The force of the exhaust flow is directed against the side of the turbine wheel. As the hot gases strike the turbine wheel, causing it to spin, the specially curved turbine fins direct the exhaust gases toward the center of the housing where they exit. This action creates a flow known as a vortex. Once the turbine starts to spin, the compressor wheel shaped like a turbine wheel in reverse also begins to spin. Intake air is drawn into the housing and is caught by the whirling blades of the compressor, then thrown outward by centrifugal force. From there, the air exits under pressure to the intake manifold and the individual cylinders. Normally, air is drawn into cylinders by the difference in pressure between the atmosphere and engine vacuum. A turbocharger, however, is capable of pressurizing the intake charge above normal atmospheric pressure. Turbo boost is the term used to describe the positive pressure increase created by a turbocharger. For example, 10 psi of boost means the air is being fed into the engine at 24.7 psi, which is 14.7 psi atmospheric pressure plus 10 pounds of boost.Turbo Lag:
Increases in horsepower are normally evidenced by an engine's response to a quick opening of the throttle. However, some turbocharged engines experience a lack of immediate throttle response. This delay, known as turbo lag, occurs because exhaust gas requires a little time to build enough energy to spin the wheels fast enough to respond to the engine's speed. This causes the power from the engine to temporarily lag behind what is needed for the driving conditions.Wastegate Valve:
If the pressure of the air from a turbocharger becomes too high, knocking can occur, engine output decreases, and the pressure created by the combustion of the air fuel mixture can become so great that the engine may self destruct. To prevent this, turbochargers are equipped with a wastegate valve. This valve is part of the turbine housing and allows a certain amount of exhaust gas to bypass the turbine when boost pressure exceeds a certain value, thereby reducing pressure. The action of the wastegate can be controlled directly by manifold pressure or by the PCM according to manifold pressure. Most late model systems feature PCM controlled wastegates. In non PCM systems, an actuator that senses air pressure in the induction system opens the wastegate when pressure becomes too high. This action decreases the amount of exhaust reaching the turbine, which in turn reduces turbine and compressor wheel speed and decreases the output pressure from the turbocharger.When the pressure in the intake manifold is not great enough to override the spring in the actuator, the wastegate remains closed and all exhaust flows past the turbine, allowing the turbine to spin accordingly. When boost pressure overcomes the tension of the actuator's spring, the actuator opens the wastegate valve and some exhaust gas is diverted around the turbine wheel. As a result, turbine speed is controlled, as is boost pressure. On late model engines, the wastegate is controlled by the PCM, which directly controls a solenoid that manages vacuum to the wastegate. When vacuum is introduced to the wastegate, it opens to allow exhaust gases to bypass the turbine. The action of the solenoid is controlled by the PCM according to various inputs. The PCM also adjusts ignition timing and air-fuel mixtures according to turbocharger output.
Retarding spark timing is an often used method of controlling detonation on turbocharged engines. However, any time the ignition is retarded, power is lost, fuel economy suffers, and the engine tends to run hotter. Most systems use knock sensor signals to retard timing only when detonation is detected. These sensors are also used to limit boost pressure according to the octane rating of the fuel being used. This maximizes engine performance and reduces the chances of engine knocking during all conditions regardless of the fuel's octane rating. When the PCM detects excessive manifold pressure, it opens the wastegate and also enriches the mixture. This rich mixture reduces combustion temperature, which helps to cool the turbocharger and combustion chamber.
Various Turbocharger Designs:
In an effort to increase the efficiency of turbocharged engines, manufacturers have developed various designs of turbochargers and their control systems. Common alternative designs are called variable nozzle turbine (VNT) and variable geometry turbochargers. In these designs, the cross-sectional area through which the exhaust flows is variable. This area is adjusted via movable vanes that change their angles according to turbine speed. At lower engine speeds, the vanes restrict exhaust flow, thereby increasing boost pressure. At higher engine speeds, the vanes open wider and exhaust backpressure decreases. This allows the turbocharger to provide more boost at lower engine speeds without producing too much boost at higher speeds. It is claimed that the use of a variable turbocharger can reduce a gasoline engine's fuel consumption by 20 percent. Variable turbos do not have a wastegate. They provide higher boost at lower engine speeds and are more responsive to changes in engine load. They also help reduce the effects of turbo lag.Twin Turbochargers:
Some engines feature two turbochargers, and the action of the two depends on the specific application. Some engines have a turbo for one-half the cylinders and another for the other half. Certain V-type engines have a turbocharger for each bank of cylinders, with the turbos using exhaust from specific cylinders to compress air for those same cylinders. Other engines utilize two different sized turbos, with each designed for specific conditions. Normally, the smaller of the two spools up to speed very quickly, which reduces turbo lag. The larger one is slower to get up to speed but adds boost at higher engine speeds. This is a two-stage design: one for lower engine speeds and immediate increase of speed, and one for sustained power.In a typical twin turbocharger system, the function of the two turbochargers is controlled by the operating mode of the larger turbocharger. Its operation is controlled by control valves that regulate exhaust gases to it and the amount of air from it. During low engine speeds and loads, boost is only provided by the smaller unit, with the control valves of the other turbo keeping it disabled. When boost pressure from the first turbo reaches a predetermined level, exhaust gas is allowed to flow to the second turbo. At this time it is spinning but not providing any boost, serving as a preparation step where the second turbine spins before it is needed.
Once the load or speed conditions demand more power, a valve opens and allows the boost pressure from the second turbo to enter the intake manifold. At this time, boost from both turbos is sent to the intake manifold. In some systems, the higher pressure from the larger turbo causes the wastegate valve, which is an integral part of the smaller turbo, to open and control the maximum boost. When the engine moves from high to low speed, a control valve stops the flow from the second turbo to the intake, and another control valve blocks off exhaust flow to the turbo. These actions prevent high boost during deceleration.
Superchargers:
Superchargers are positive displacement air pumps driven directly by the engine's crankshaft via a V-ribbed belt. They improve horsepower and torque by increasing air pressure and density in the intake manifold. The pressure boost is proportional to engine speed. A typical supercharger is normally made up of a magnetic clutch, two rotors, two shafts, two rotor gears, housing, and a rear plate and cover. Some superchargers feature four rotor assemblies. The drive belt connects the engine to the magnetic clutch connected to one rotor shaft. Gears connect the two aluminum rotor shafts and drive them in opposite directions. The rotors have three helical lobes and are press-fit onto the rotor shafts, then held in position by pins and serrations.The rotors turn within a sealed housing and pressurize the air as they rotate. The rotor shafts are supported by ball or needle bearings in the rear plate and are lubricated by oil in the supercharger unit. The magnetic clutch allows the PCM to engage and disengage the drive power to the supercharger. The PCM also can control the amount of fuel injected and the ignition timing during boost. When the clutch is energized, the engine drives the supercharger. The PCM de-energizes the clutch when the engine is running under a light load.
The clutch is comprised of a stator, pulley, pressure plate, and hub. The clutch pulley rotates with the drive belt and drives the stator. When the stator is energized, the pressure plate is pressed against the clutch pulley, locking them together. The clutch hub is splined to the rotor's shaft and the rotors rotate as a unit. To reduce the shock caused by the quick engagement of the clutch, most clutch assemblies have a rubber damper between the boss of the clutch hub and the pressure plate. Normally, there is a 0.5 mm clearance between the clutch hub and clutch pulley. If this clearance increases because of wear, noise will result. The clearance can be corrected by changing the thickness of the adjusting shim. To handle the higher operating temperatures imposed by supercharging, the engine is typically fitted with an engine oil cooler. This water to oil cooler is generally mounted between the engine front cover and oil filter.
Operation:
The flow of air through a supercharger follows a specific path. Air comes in through the remote-mounted air cleaner and the mass airflow meter. It then moves through the throttle plate assembly and passes through the supercharger inlet plenum assembly, which is bolted to the back of the supercharger. The air enters the supercharger and is pressurized by the spinning rotors. It then exits through the top of the supercharger by way of the air outlet adapter. As the air is compressed, its temperature increases. Because cooler, denser air is desired for increased power, the heated air is routed through an intercooler. An intercooler can decrease the temperature of the air by as much as 66 degrees Celsius. This cooled air then passes through to the intake manifold adapter assembly, which is bolted to the rear of the intake manifold. When the intake valves open, the air is forced into the combustion chambers where it is mixed with fuel delivered by the fuel injectors.Supercharger Bypass System:
Unlike a turbocharger, the supercharger does not require a wastegate to limit boost and prevent a potentially damaging overboost condition. Because the speed of the supercharger is directly linked to the engine speed, its pumping power is limited by the rpm of the engine itself rather than revolutions produced by exhaust gases. Supercharger boost is therefore directly controlled through the opening and closing of the throttle or a bypass system that controls the air leaving the supercharger. The bypass circuit allows the supercharger to idle when extra power is not needed. The bypass routes any excess air in the intake manifold back through the supercharger inlet plenum assembly, allowing the engine to run in effect normally aspirated. This eliminates any boost from the supercharger. The bypass system reduces air handling losses when boost is not needed, resulting in better fuel economy.Some systems use a PCM-controlled stepper motor to control the amount of air that bypasses the supercharger. The PCM determines the required boost pressure based on current engine conditions and controls the operation of the magnetic clutch and bypass valve. Other systems feature a vacuum motor that regulates the amount of air to be bypassed. As power demands from the engine increase, a vacuum motor controls a butterfly valve that routes more or less air to the intake manifold, thereby changing the boost. When the bypass is completely closed, boost can reach about 12 psi. When the actuator is open during high vacuum engine conditions, the air bypasses the supercharger. As the throttle is opened and engine vacuum decreases, the actuator closes and allows more air into the supercharger.
Supercharger Designs:
Although there have been numerous supercharger designs on the market over the years, the most popular is the Roots type. This design uses a pair of three-lobed rotor vanes driven by the crankshaft. The lobes force air into the intake manifold. The key to the supercharger's operation is primarily the design of the rotors.Some Roots type superchargers use straight lobe rotors that result in uneven pressure pulses and consequently relatively high noise levels. Therefore, the supercharger used with most of today's engines employs a helical design for the two rotors. The helical design evens out the pressure pulses in the blower and reduces noise. It was found that a 60-degree helical twist works best for equalizing the inlet and outlet volumes.
Another benefit of the helical rotor design is that it reduces carryback volumes air that is carried back to the inlet side of the supercharger because of the unavoidable spaces between the meshing rotors, which represents a loss of efficiency. Another popular supercharger design, especially in Europe, is the G-Lader spiral supercharger, which is based on a 1905 French design. Spiral ramps in both sides of the rotor intermesh with similar ramps in the housing. Unlike most superchargers, the rotor of the G-Lader does not spin on its axis, rather it moves around an eccentric shaft. This motion draws in air, squeezes it inward through the spiral which compresses it, then forces it through ducts in the center into the engine. Airflow is essentially constant so intake noise is lower than that of a Roots blower. Because there is only a slight wiping motion between the spiral and housing, wear is minimal.
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