Turbos and Superchargers, How do they work? by RocketSled ( copied )
02-05-2009, 09:15 AM
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Turbos and Superchargers, How do they work? by RocketSled ( copied )
I got this off another forum. I found it to be interesting and that some others here might do so as well:
RocketSled has laid out the basic workings of the Turbo, Roots blower, and Centrfigal blower.
Superchargers: The most significant advantage to a supercharger is the simplicity of install. You don't need to plumb them into the exhaust. This also improves the reliability of the compressor, since it doesn't have to operate in the extremely hot exhaust stream. It also results in cooler intake charges since the compressor isn't soaking up heat from the exhaust-side *hot* impeller.
Because superchargers are driven from the crank, the boost they produce is proportional to engine RPM. But superchargers come in two basic varieties, centrifugal and roots. The boost curves are different for these two types.
Roots compressors are constant displacement pumps. They move the same volume of air for each rotation of the compressor assembly and airflow through the compressor varies linearly with RPM. The engine is also a constant displacement pump, it consumes the same volume of air for each rotation of the assembly. As a result, Roots compressors present the same "additional" air volume to the engine across all RPMs, which makes them good at making boost at low RPMs.
Centrifugal compressors accelerate air axially to achieve boost (which is why they're also sometimes called "axial flow compressors"). The volume of air they move is proportional to the *square* of the input RPM (it's controlled by the mV^2/r equation). This characteristic presents design challenges that don't exist for a Roots type. The centrifugal compressor has to be designed so that it won't exceed the maximum acceptable boost level at the maximum intended operating RPM. Because the boost is governed by a square-law, this means that they produces little or no boost at low RPMs.
Centrifugal compressors are more compact than Roots type. Centrifugal compressors also present less parasitic loss to the engine at low RPMs. But Roots types make better boost at low RPMs.
The "correct" choice is a function of your priorities. Roots types like Magnusson are straighforward, simple installations but you need a high-rise hood. Vortech and ATI are more complex (Vortech needs a hole in the oil pan and ATI needs plumbing for the air/air intercooler) but they fit under the factory hood.
Turbochargers: These are also axial flow compressors. They differ from Centrifugal Superchargers in that they're not driven off the crank, they use exhaust gas to spin. This makes them *much* more efficient than crank driven compressors.
Think of it in terms of "thermodynamics". The thermal energy represented by the hot exhaust is "captured" by the turbo and converted in to mechanical energy to spin the compressor. If the turbo wasn't there the exhaust energy would just flow out the tailpipe, so the boost you get from a turbo is essentially "free" (there are some parasitic losses, exhaust backpressure goes up some, for example).
But Turbo's live in the exhaust, and the exhaust get's *HOT*. This can create problems for reliability. Failures principally manifest themselves as bearing/seal failure due to buildup of abrasive "coke" on the bearings ("coke" is what's left of your oil after you cook off all the volatile components).
Besides the advantages Turbos have over Superchargers with respect to nearly nonexistent parasitic losses, they also have another key advantage. Turbos are "load sensitive". The boost the generate is proportional to the engine load NOT the engine RPM. This means that a Turbo will generally be able to produce good boost at low RPMs like a Roots, even though it's an axial flow compressor like a centrifugal type.
Intercooling: Intercooling addresses the effects that compression has on air temperature (as defined by Boyle's ideal gas law: pv=nRT). Having an intercooler is a function of the "completeness" of the design and not a function of the particular type of supercharger (Turbo or otherwise). The only thing that matters is that boost makes the intake air *hot*, and you definitely want to cool it down as much as possible before it gets to the motor.
Intercoolers come in two basic types, air-to-air and air-to-water. The debates about which type is more efficient have raged for years and will continue to rage. Assuming equal "efficiency", what it all comes down to is this... Air/Air require a large radiator up front, with long ducts/plenums to route the charge to/from the intercooler. Fitment can be a challenge, and the longer the duct work, the more boost is lost to parasitic drag. Air/water intercoolers don't need the long ductwork and they can get by with smaller radiators, but they do need plumbing for the water and a water pump.
Boost Control: When you slam the throttle closed at high RPM/Boost levels (like when you lift during a shift), all that air being pushed through the compressor (be it turbo or otherwise) suddenly has no place to go. This can cause a "compressor stall". That doesn't necessarily mean that the compressor stops spinning, it means the compressor stops compressing (think of a cavitating motor boat propeller and you'll get the idea). This is a bad thing, since the system has "inertia" and when you crack the throttle open again it takes a little while for the boost to "bounce back". Good supercharger (mechanical or turbo) designs use Blow Off Valves to address this issue. Usually just simple vacuum operated butterfly valve, BOVs provide a path to the external/ambient environment for the compressed charge air to "dump" to when the throttle is closed. This prevents a stall and improves boost response time when the throttle is next opened.
Turbochargers also have a Waste Gate. The WG is a valve that sits in the exhaust flow between the manifold and the tubro's turbine (aka "impeller"). It's pressure actuated off of the compresed-air side of the turbo. When the boost coming out of the turbo exceeds a given level (controlled by a calibrated spring inside the waste gate actuator), the waste gate valve opens and exhaust bypasses the turbine, the compressor slows and boost levels go down. This makes a Turbo self regulating. Even if it's capable of 100Lbs of boost pressure, the turbo will never boost more than the waste gate allows. This allows a turbo to be designed to produce good boost at lower loads (which are also generally lower RPMs) and not blow the top off the motor at high loads/RPMs.
However... there are tricks you can play with the waste gate and most modern turbo installations do.
An Electronic Boost Controller inserts a solenoid operated valve between the waste gate actuator and the boost-pressure side of the turbo. The valve is normally closed, so the waste gate actuator never sees any "boost" pressure at all. The boost pressure is instead sensed by an electronic transducer attached to the EBC. The EBC only opens it's solenoid valve to activate the waste gate when the boost exceeds the level set on an adjustable **** on the EBC. So long as the waste gate actuator spring is set to a level that's lower than the boost pressure selected by the EBC (a requirement of the design), the EBC is able to regulate boost pressure to any level it wants across the entire operating range. This lets you configure a Turbo (which remember is a centrifugal compressor) so that it can produce higher boosts at lower RPMs (load, actually) without having to worry that the Turbo will produce too much boost at higher RPMs (load, actually).
A mechanically driven supercharger can also use an EBC. But with a Supercharged application, the EBC regulates the opening/closing of the Blow Off Valve not the waste gate. This allows control of boost in a fashion that's similar to the Turbo EBC (except the turbo EBC regulates the source of the energy that spins the compressor, whereas a Supercharger EBC would regulate the compressed charge that goes to the motor). This allows the selection of a centrifugal compressor that could provide better boost at low RPM, without over boosting at high RPM.
Unfortunately, while there are plenty of Turbo EBCs available, Supercharger variants are much harder to come by.
RocketSled has laid out the basic workings of the Turbo, Roots blower, and Centrfigal blower.
Superchargers: The most significant advantage to a supercharger is the simplicity of install. You don't need to plumb them into the exhaust. This also improves the reliability of the compressor, since it doesn't have to operate in the extremely hot exhaust stream. It also results in cooler intake charges since the compressor isn't soaking up heat from the exhaust-side *hot* impeller.
Because superchargers are driven from the crank, the boost they produce is proportional to engine RPM. But superchargers come in two basic varieties, centrifugal and roots. The boost curves are different for these two types.
Roots compressors are constant displacement pumps. They move the same volume of air for each rotation of the compressor assembly and airflow through the compressor varies linearly with RPM. The engine is also a constant displacement pump, it consumes the same volume of air for each rotation of the assembly. As a result, Roots compressors present the same "additional" air volume to the engine across all RPMs, which makes them good at making boost at low RPMs.
Centrifugal compressors accelerate air axially to achieve boost (which is why they're also sometimes called "axial flow compressors"). The volume of air they move is proportional to the *square* of the input RPM (it's controlled by the mV^2/r equation). This characteristic presents design challenges that don't exist for a Roots type. The centrifugal compressor has to be designed so that it won't exceed the maximum acceptable boost level at the maximum intended operating RPM. Because the boost is governed by a square-law, this means that they produces little or no boost at low RPMs.
Centrifugal compressors are more compact than Roots type. Centrifugal compressors also present less parasitic loss to the engine at low RPMs. But Roots types make better boost at low RPMs.
The "correct" choice is a function of your priorities. Roots types like Magnusson are straighforward, simple installations but you need a high-rise hood. Vortech and ATI are more complex (Vortech needs a hole in the oil pan and ATI needs plumbing for the air/air intercooler) but they fit under the factory hood.
Turbochargers: These are also axial flow compressors. They differ from Centrifugal Superchargers in that they're not driven off the crank, they use exhaust gas to spin. This makes them *much* more efficient than crank driven compressors.
Think of it in terms of "thermodynamics". The thermal energy represented by the hot exhaust is "captured" by the turbo and converted in to mechanical energy to spin the compressor. If the turbo wasn't there the exhaust energy would just flow out the tailpipe, so the boost you get from a turbo is essentially "free" (there are some parasitic losses, exhaust backpressure goes up some, for example).
But Turbo's live in the exhaust, and the exhaust get's *HOT*. This can create problems for reliability. Failures principally manifest themselves as bearing/seal failure due to buildup of abrasive "coke" on the bearings ("coke" is what's left of your oil after you cook off all the volatile components).
Besides the advantages Turbos have over Superchargers with respect to nearly nonexistent parasitic losses, they also have another key advantage. Turbos are "load sensitive". The boost the generate is proportional to the engine load NOT the engine RPM. This means that a Turbo will generally be able to produce good boost at low RPMs like a Roots, even though it's an axial flow compressor like a centrifugal type.
Intercooling: Intercooling addresses the effects that compression has on air temperature (as defined by Boyle's ideal gas law: pv=nRT). Having an intercooler is a function of the "completeness" of the design and not a function of the particular type of supercharger (Turbo or otherwise). The only thing that matters is that boost makes the intake air *hot*, and you definitely want to cool it down as much as possible before it gets to the motor.
Intercoolers come in two basic types, air-to-air and air-to-water. The debates about which type is more efficient have raged for years and will continue to rage. Assuming equal "efficiency", what it all comes down to is this... Air/Air require a large radiator up front, with long ducts/plenums to route the charge to/from the intercooler. Fitment can be a challenge, and the longer the duct work, the more boost is lost to parasitic drag. Air/water intercoolers don't need the long ductwork and they can get by with smaller radiators, but they do need plumbing for the water and a water pump.
Boost Control: When you slam the throttle closed at high RPM/Boost levels (like when you lift during a shift), all that air being pushed through the compressor (be it turbo or otherwise) suddenly has no place to go. This can cause a "compressor stall". That doesn't necessarily mean that the compressor stops spinning, it means the compressor stops compressing (think of a cavitating motor boat propeller and you'll get the idea). This is a bad thing, since the system has "inertia" and when you crack the throttle open again it takes a little while for the boost to "bounce back". Good supercharger (mechanical or turbo) designs use Blow Off Valves to address this issue. Usually just simple vacuum operated butterfly valve, BOVs provide a path to the external/ambient environment for the compressed charge air to "dump" to when the throttle is closed. This prevents a stall and improves boost response time when the throttle is next opened.
Turbochargers also have a Waste Gate. The WG is a valve that sits in the exhaust flow between the manifold and the tubro's turbine (aka "impeller"). It's pressure actuated off of the compresed-air side of the turbo. When the boost coming out of the turbo exceeds a given level (controlled by a calibrated spring inside the waste gate actuator), the waste gate valve opens and exhaust bypasses the turbine, the compressor slows and boost levels go down. This makes a Turbo self regulating. Even if it's capable of 100Lbs of boost pressure, the turbo will never boost more than the waste gate allows. This allows a turbo to be designed to produce good boost at lower loads (which are also generally lower RPMs) and not blow the top off the motor at high loads/RPMs.
However... there are tricks you can play with the waste gate and most modern turbo installations do.
An Electronic Boost Controller inserts a solenoid operated valve between the waste gate actuator and the boost-pressure side of the turbo. The valve is normally closed, so the waste gate actuator never sees any "boost" pressure at all. The boost pressure is instead sensed by an electronic transducer attached to the EBC. The EBC only opens it's solenoid valve to activate the waste gate when the boost exceeds the level set on an adjustable **** on the EBC. So long as the waste gate actuator spring is set to a level that's lower than the boost pressure selected by the EBC (a requirement of the design), the EBC is able to regulate boost pressure to any level it wants across the entire operating range. This lets you configure a Turbo (which remember is a centrifugal compressor) so that it can produce higher boosts at lower RPMs (load, actually) without having to worry that the Turbo will produce too much boost at higher RPMs (load, actually).
A mechanically driven supercharger can also use an EBC. But with a Supercharged application, the EBC regulates the opening/closing of the Blow Off Valve not the waste gate. This allows control of boost in a fashion that's similar to the Turbo EBC (except the turbo EBC regulates the source of the energy that spins the compressor, whereas a Supercharger EBC would regulate the compressed charge that goes to the motor). This allows the selection of a centrifugal compressor that could provide better boost at low RPM, without over boosting at high RPM.
Unfortunately, while there are plenty of Turbo EBCs available, Supercharger variants are much harder to come by.
