remoer
12-28-2005, 12:51 PM
Turbochargers: Design and Related Parts
By Brian Ferrari
Jul 19, 2004, 17:10
The wise consumer makes a better purchase than the ignorant one, so if you are seriously interested in turbo performance you at least need to know the fundamentals. This article is dedicated to the turbocharger, ultimately the most powerful of all forced induction systems. Here I will try to identify each part in the basic turbo system, what it does and why you need it. I’ll also try to keep this on the lighter end of the technical scale, but it’s hard not to get deep into it with this subject.
TURBOCHARGER BASICS
The turbocharger is one complex little piece of work, at least until you get familiar with it. Although a turbo obviously functions as a single piece, it is commonly broken into these three sections for easy conversation…
COMPRESSOR SECTION:
The compressor section is identical in function to any centrifugal supercharger, the only difference is that the turbine section of the turbo drives it. One thing to know is that turbocharger compressor sections are (generally) significantly smaller than their supercharger cousins. This all has to do with efficiency and the chosen method of powering the compressor, so just know it’s the reason why you see turbochargers spinning such high RPM when compared to their centrifugal supercharger cousins. It’s all about necessity.
TURBINE SECTION:
This section bears a strong resemblance to the compressor section for a reason; it basically functions the same but backwards. The two main parts are the turbine housing and turbine wheel, and if this is an internally wastegated turbo, the wastegate also resides here (there will be more on that later). As exhaust gasses quickly move out of the cylinder and into the exhaust manifold, they are routed into the turbine housing’s scroll. If you understood the flow of air through the centrifugal compressor design discussed earlier, here it’s just the opposite occurring. As the hot and rapidly moving gasses attempt to find an airflow path through the turbine housing (with the ever decreasing scroll area), they come in contact with the turbine wheel on their way to the center outlet of the housing. As they rush through this airflow path and into the exhaust downpipe, they spin the turbine wheel, imparting a portion of their kinetic energy to the turbocharger. Especially notice that with this design comes variable RPM, the turbocharger itself is not physically strapped to any rotating part of the engine. This makes many different turbo shaft speeds possible at a single engine RPM, which is where the system’s basic performance characteristics and tunability are born.
CENTER SECTION (aka bearing section):
The center section is definitely the most complex of the three portions. This is what connects both the compressor and turbine sections, and where all of the cooling and lubrication of the unit occurs. Inside the center section is the main shaft, which is what the compressor and turbine wheels are directly connected to. This main shaft undergoes a great deal of pressure, RPM and heat, so the center section is unsurprisingly very specifically engineered to deal with these. The most common and basic center sections use what’s called thrust bearings to keep the shaft spinning, and oil flow from the engine to both lubricate and cool the unit. Two common updates to this proven design are becoming more affordable and widespread; ball bearing center sections and water cooling in addition to oil. The ball bearing center is both more durable and more efficient at transmitting power to the compressor wheel, making it better for performance and longetivity. The water cooling is more for reliability than anything else, helping to stabilize temperatures and prevent oil coking in the housing. Both are worthwhile additions to your turbo purchase if at all possible.
TURBO KIT BASICS
Although I say “basic” here, know that this is pretty much an oxymoron when dealing with turbos. There is nothing basic about a turbo system, as many different things concerning engine operation need to be addressed. The basic turbo system should come with a bunch of different things, and few systems effectively address all these unless your car was originally equipped with the system. Here they are, in no particular order (with the little things like vacuum line omitted), and notice I left out engine management from the list, because I want to deal with that separately:
1- turbo
2- exhaust manifold for turbo
3- wastegate
4- blow-off valve (aka bypass valve)
5- lines for oil supply and return
6- intercooler (optional)
TURBOCHARGER:
We’ve already gone through the basic explanation, but one more thing bears mention here. Ever hear the T25, T3/T4, T04e turbo designations? Well, these refer to the size and basic flow potential of the turbocharger. Garret and other manufacturers created turbo families, ones in which all members prescribed to certain physical characteristics. A T3 compressor section is one that prescribes to a specific characteristic set, such as overall size and design features. Generally speaking, larger numbers and higher letters mean a larger (and sometimes newer) family of turbos, meaning a potential increase in flow ability, power production and possibly even efficiency. The T3/T4 designation is an example of a hybrid turbo; one where a T3 turbine section has been mated to a T4 compressor section. This popular hybrid attempts to combine the excellent low RPM spool characteristics of the smaller T3 class turbo with the big flow potential of a sizable T4 compressor. Really it’s a “best of both worlds” attempt, which seems to be very successful on smaller displacement, high RPM engines. Now there are a few other considerations to turbo sizing, such as A/R ratio and wheel trim, but I won’t go into those unless someone really needs to know everything. The point here is simply to get a basic feel for turbo function and sizing, as the experts who designed the turbo kit or upgrade likely have already made an excellent choice in turbo size for your specific application.
TURBO EXHAUST MANIFOLD:
In order to mount the turbo to the engine, the first step is to route exhaust gasses through it. This is where the special manifold comes in, dumping exhaust gasses directly into the turbine housing (provisions for mounting an external wastegate should also be found here). Usually these are fairly crude looking log style cast iron manifolds, instead of the nicely shaped and finished stainless steel header piping. But there’s good reason that virtually every car to come off the production lines with a turbo follows this example: it works. Turbos build up a tremendous amount of heat and pressure in the initial part of the exhaust system, and the thick cast iron manifolds are perfectly suited to reliable performance in this environment. Also, space considerations often prohibit the use of nicely tuned tubular exhaust primaries, so there’s little reason to go to the expense of crafting them. The point here is this: there are possibly some finely crafted tubular manifolds available for your application if you want maximum performance and don’t mind the extra money, but these are largely unnecessary for a typical street setup. Ugly cast iron manifolds are routinely found on 400-500hp cars.
By Brian Ferrari
Jul 19, 2004, 17:10
The wise consumer makes a better purchase than the ignorant one, so if you are seriously interested in turbo performance you at least need to know the fundamentals. This article is dedicated to the turbocharger, ultimately the most powerful of all forced induction systems. Here I will try to identify each part in the basic turbo system, what it does and why you need it. I’ll also try to keep this on the lighter end of the technical scale, but it’s hard not to get deep into it with this subject.
TURBOCHARGER BASICS
The turbocharger is one complex little piece of work, at least until you get familiar with it. Although a turbo obviously functions as a single piece, it is commonly broken into these three sections for easy conversation…
COMPRESSOR SECTION:
The compressor section is identical in function to any centrifugal supercharger, the only difference is that the turbine section of the turbo drives it. One thing to know is that turbocharger compressor sections are (generally) significantly smaller than their supercharger cousins. This all has to do with efficiency and the chosen method of powering the compressor, so just know it’s the reason why you see turbochargers spinning such high RPM when compared to their centrifugal supercharger cousins. It’s all about necessity.
TURBINE SECTION:
This section bears a strong resemblance to the compressor section for a reason; it basically functions the same but backwards. The two main parts are the turbine housing and turbine wheel, and if this is an internally wastegated turbo, the wastegate also resides here (there will be more on that later). As exhaust gasses quickly move out of the cylinder and into the exhaust manifold, they are routed into the turbine housing’s scroll. If you understood the flow of air through the centrifugal compressor design discussed earlier, here it’s just the opposite occurring. As the hot and rapidly moving gasses attempt to find an airflow path through the turbine housing (with the ever decreasing scroll area), they come in contact with the turbine wheel on their way to the center outlet of the housing. As they rush through this airflow path and into the exhaust downpipe, they spin the turbine wheel, imparting a portion of their kinetic energy to the turbocharger. Especially notice that with this design comes variable RPM, the turbocharger itself is not physically strapped to any rotating part of the engine. This makes many different turbo shaft speeds possible at a single engine RPM, which is where the system’s basic performance characteristics and tunability are born.
CENTER SECTION (aka bearing section):
The center section is definitely the most complex of the three portions. This is what connects both the compressor and turbine sections, and where all of the cooling and lubrication of the unit occurs. Inside the center section is the main shaft, which is what the compressor and turbine wheels are directly connected to. This main shaft undergoes a great deal of pressure, RPM and heat, so the center section is unsurprisingly very specifically engineered to deal with these. The most common and basic center sections use what’s called thrust bearings to keep the shaft spinning, and oil flow from the engine to both lubricate and cool the unit. Two common updates to this proven design are becoming more affordable and widespread; ball bearing center sections and water cooling in addition to oil. The ball bearing center is both more durable and more efficient at transmitting power to the compressor wheel, making it better for performance and longetivity. The water cooling is more for reliability than anything else, helping to stabilize temperatures and prevent oil coking in the housing. Both are worthwhile additions to your turbo purchase if at all possible.
TURBO KIT BASICS
Although I say “basic” here, know that this is pretty much an oxymoron when dealing with turbos. There is nothing basic about a turbo system, as many different things concerning engine operation need to be addressed. The basic turbo system should come with a bunch of different things, and few systems effectively address all these unless your car was originally equipped with the system. Here they are, in no particular order (with the little things like vacuum line omitted), and notice I left out engine management from the list, because I want to deal with that separately:
1- turbo
2- exhaust manifold for turbo
3- wastegate
4- blow-off valve (aka bypass valve)
5- lines for oil supply and return
6- intercooler (optional)
TURBOCHARGER:
We’ve already gone through the basic explanation, but one more thing bears mention here. Ever hear the T25, T3/T4, T04e turbo designations? Well, these refer to the size and basic flow potential of the turbocharger. Garret and other manufacturers created turbo families, ones in which all members prescribed to certain physical characteristics. A T3 compressor section is one that prescribes to a specific characteristic set, such as overall size and design features. Generally speaking, larger numbers and higher letters mean a larger (and sometimes newer) family of turbos, meaning a potential increase in flow ability, power production and possibly even efficiency. The T3/T4 designation is an example of a hybrid turbo; one where a T3 turbine section has been mated to a T4 compressor section. This popular hybrid attempts to combine the excellent low RPM spool characteristics of the smaller T3 class turbo with the big flow potential of a sizable T4 compressor. Really it’s a “best of both worlds” attempt, which seems to be very successful on smaller displacement, high RPM engines. Now there are a few other considerations to turbo sizing, such as A/R ratio and wheel trim, but I won’t go into those unless someone really needs to know everything. The point here is simply to get a basic feel for turbo function and sizing, as the experts who designed the turbo kit or upgrade likely have already made an excellent choice in turbo size for your specific application.
TURBO EXHAUST MANIFOLD:
In order to mount the turbo to the engine, the first step is to route exhaust gasses through it. This is where the special manifold comes in, dumping exhaust gasses directly into the turbine housing (provisions for mounting an external wastegate should also be found here). Usually these are fairly crude looking log style cast iron manifolds, instead of the nicely shaped and finished stainless steel header piping. But there’s good reason that virtually every car to come off the production lines with a turbo follows this example: it works. Turbos build up a tremendous amount of heat and pressure in the initial part of the exhaust system, and the thick cast iron manifolds are perfectly suited to reliable performance in this environment. Also, space considerations often prohibit the use of nicely tuned tubular exhaust primaries, so there’s little reason to go to the expense of crafting them. The point here is this: there are possibly some finely crafted tubular manifolds available for your application if you want maximum performance and don’t mind the extra money, but these are largely unnecessary for a typical street setup. Ugly cast iron manifolds are routinely found on 400-500hp cars.