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. Iíll also try to keep this on the lighter end of the technical scale, but itís hard not to get deep into it with this subject.
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 conversationÖ
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 itís the reason why you see turbochargers spinning such high RPM when compared to their centrifugal supercharger cousins. Itís all about necessity.
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 housingís scroll. If you understood the flow of air through the centrifugal compressor design discussed earlier, here itís 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 systemís 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 whatís 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 ďbasicĒ 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:
2- exhaust manifold for turbo
4- blow-off valve (aka bypass valve)
5- lines for oil supply and return
6- intercooler (optional)
Weíve 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 itís a ďbest of both worldsĒ 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 wonít 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 thereís 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 thereís 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 donít 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.
In the most basic of terms, a turbo system is self-feeding. That is, as the system creates more boost, it also creates more exhaust flow. This exhaust flow is what powers the turbocharger, so if left unchecked the turbo system will quickly spiral out of control. Now it takes time and a specific amount exhaust flow to start creating boost, but once this point is reached (called boost threshold), either exhaust flow to the turbine is regulated, or the system keeps building pressure until something gives, usually a hard part in the engine. Which is where the wastegate comes in.
Controlled by vacuum signal from the manifold (or more correctly, positive pressure in the manifold), the wastegateís job is to re-route exhaust flow around the turbine wheel to control boost levels. Remember, the turbo creates boost by extracting energy from exhaust gas flow, so this is the prime location to regulate turbocharger RPM, and therefore boost levels. What a wastegate does is provide an alternate path for exhaust gasses to flow through that doesnít cause them to contact the turbine wheel. This prevents the exhaust gasses from contributing to boost production, thus regulating boost to preset levels.
There are also two main types of wastegates, internal & external. Both are there to perform the same task, the only difference is location and effectiveness. Internal wastegates are located inside the turbine housing itself, and although effective at re-routing exhaust gasses around the turbine wheel, they can impart a good bit of turbulence to the exhaust flow path. This increases exhaust system pressure and hurts performance. The external wastegate, the true performance choice, has provisions made for itís mounting before the turbo on the exhaust manifold. An entirely alternate flow path is created where exhaust gasses skip going through the turbine housing altogether, contributing much less to turbulence in the system. They also tend to be more accurate at controlling exhaust flow and turbo boost; combine these two attributes and you have a recipe for superior performance.
This is both the insurance policy of the turbo system, and itís protector. Two things are governed by the blow-off valve; maximum boost levels and pressure spikes in the intake tract. While the first job is primarily handled by the wastegate, in the event of a big enough overboost, the blow-off valve will vent excess pressures to help maintain safe levels of boost. Basically, the blow-off valve is a springloaded poppet valve contraption that will bleed off and excess pressure that builds up in the intake system. This can occur due to either boost creep or a sudden closing of the throttle body when boosting (such as during full throttle, high RPM shifts), but either way itís the blow-off valveís job to prevent pressure spikes in the intake tract. This serves two functions: one, to prevent serious engine damaging overboosts, and two, to prevent airflow from reversing direction into the turbocharger itself. The second one is itís principle job, to keep the intake tract from building up large pressures during sudden lift throttle situations (such as shifting). When the engine is at full boost and full song, the turbo is spinning madly to supply air to the intake system. The momentum of air and turbocharger are not easily stopped on a dime, so when the throttle body is suddenly slam shut, things tend to get interesting in the intake system. There is an immediate pressure spike between the turbo and throttle body, putting great stress on the compressor wheel which is still trying to pump air into a closed system. To keep the turboís RPM up and the pressures in the intake tract down, the blow-off valve vents this excess pressure for maximum performance and reliability.
This is the most important performance part you can add to a forced induction system, and is well worth its price if boost numbers rise beyond 8 psi. Compression equals heat, and blowing hot air into the engine is neither efficient or reliable. An 8 psi forced induction system can produce air inlet temps over 200 degrees farenheit, making the engine a detonation machine. The greater amount of space between the air molecules also lowers charge air density, meaning the 8 psi of air isnít as potent as it could be. The solution lies in cooling the air charge before it enters the engine, and thatís precisely what the intercooler does. Two types of these are in production, air-to-air and air-to-water intercoolers. Air to air intercoolers are inexpensive and easy to maintain, but they can be very large and must be in a good airflow path to be effective. They are also rarely over 80% efficient, meaning the charge temps only get to within 80% of ambient during engine operation. Air to water systems are more compact but also more complex, their biggest advantages lie in placement freedom and efficiency. An air to water intercooler does not need a supply of fresh air and can be well over 100% efficient (when filled with a cooler than ambient liquid), but they do need an external reservoir of coolant and some means to extract heat from that coolant. Traditionally, air to air units are preferred for simplicity, reliability and effectiveness in street cars, while the superior cooling and placement possibilities of air to water systems are most at home in drag vehicles (or ones that only see occasional boosting, where heat soak isnít an issue). There are of course exceptions, and in fact the Jaguar XJR uses air to water intercoolers, but these are few and far between. At any rate, either system is universally a good thing if you plan on running even moderate levels of boost.
A lot of people pass this part up when explaining a turbo system, and yet itís one of the main things you will have to deal with on any turbo install. The turbo needs both a supply and return line, where the supply line is generally in the form of a sandwich adapter mounted between the oil filter and engine block. The return line is usually the pain in the ass, since the oil pan of the engine needs to be removed and fitted with provisions for this line to connect to. Some aftermarket oil pans have NPT bungs on them ready for this type of use; I highly recommend you think about buying one of these (which is always a good investment even without the turbo) if you are planning on a serious turbo buildup
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