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BAE Turbo Systems Inc.
Article
 
Article by Motion Performance 2002
 
 
 
 
 
Turbocharging 101

Turbo history
A History Lesson
In the early 1900s, when Swiss engineer Dr. Alfred J. Buchi first presented the concept of an exhaust turbine-driven compressor, it was promptly rejected by his colleagues. In the United States, Dr. Sanford A. Moss also proposed a turbocharger concept in the early 1910s and actually equipped an aircraft engine with a handmade “turbo-supercharger” that set an altitude record in excess of 33,000 feet in 1920. Although there were some attempts in aircraft turbo charging during World War 1, it was not until World War 11 where turbo charging took off, literally, as thousands of units were employed on high-altitude airplanes such as the B-17 Flying Fortress, the B-24 Liberator, the P-38, and the P-47. Between 1940 and 1950, turbos were used extensively on large marine, industrial and locomotive diesel engines. By 1950, there were more than 20,000 turbochargers in use in the United States.
In the early 1950s, in anticipation of relatively high-horsepower requirements for future heavy-duty earth moving equipment, Caterpillar Tractor Co. began experimenting with turbo chargers. CAT designed and built a turbo, which they sent for testing to Garrett, a small heat exchanger company in Los Angeles. The first unit failed miserably during testing and CAT decided that it should let someone else develop and produce turbos for them, someone with experience and expertise in turbo charging, including the difficult demands of metallurgy, seal and bearing technology, etc, because of their aircraft and heat exchanger experience. CAT chose Garrett and a team was put together to explore the feasibility to developing a turbo suitable to CAT’s requirements. One of the members of that original team was Hugh Macinnes, who subsequently left his mark on the turbo world.



A Better Design
In 1953, a prototype turbo, designated the TO2 (not to be confused with today’s automotive T2 series), was tested with enormous success. It did everything it was supposed to do and did it for more then 1800 hours without a glitch. Based this success, a more viable and less complex unit, the T15, was designed. CAT placed an order for 5,000 unites for their now famous D9 tractor, the forerunner of the world’s most successful production crawler tractor. As a result, the AiResearch Industrial Division of Garrett, commonly referred to as Garrett, AiResearch, was formed in 1954, the first corporation solely dedicated to the design and manufacture of turbo chargers. Other companies experimented with turbo chargers during this period as well. Cummins Engine applied turbos, made by Elliot and Schmitzer, to their popular truck engines with limited success in the late 1950s began producing its own units.
It wasn’t until the 1960s that turbos would become reliable essential components of diesel engine designs and finally gain acceptance in the trucking industry. By the end of the late 1960s, AiResearch became the undisputed leader in turbo chargers by virtue of a broad range of well accepted products that covered the full gamut of commercial applications. Their position in the industrial sector remains extremely strong to this date, shared with Schwitzer and several European “copy-cat” producers such as Holset, KKK, etc.

Detroit Steps Up
Relative to automotive turbo charging, the first beast “out of the barn” was the 1962 Oldsmobile Jetfire, a 215 cid aluminum V-8 with a Garret-AiResearch T5 turbo using the most complicated “belts and suspenders” application one could imagine. The engine was high compression (10.25:1). Highly under-carbureted (single-throat side draft) and employed a complex version of a water-injection system (to prevent detonation) utilizing “Rocket Fluid), a basic 50-50 mix of water and alcohol. So complex was this system and so unprepared was the Oldsmobile dealer service network for this technology that Oldsmobile offered a program to its customers that for a fee of $50 they would remove the turbo system and replace it with a conventional carburetor. At the same time, Chevrolet introduced the turbocharged Corvair, a fairly conservative application utilizing a turbo designed and built by TRW under the guiding hand machines. If nothing else, the Corvair proved the viability of turbo charging for mass-produced gasoline engines as well as diesels. During the 1960s, a number of entrepreneurial individuals began to “dabble” in the mystical world of turbo charging and a number of them actually ventured into business. Most of these small operations didn’t make a sufficient dent in the automotive marketplace, but their initiative, imagination, and recognition of the merits of the “black art” of turbo charging are largely responsible for the progress this technology has made to date and the impact and influence it has had on current automotive technology.

Rajay Spreads the Word
In 1969, Texas-based Rajay Industries purchased the TRW product line and hired Macinnes as its Chief Engineer. Rajay set up a shop in Long Beach, California, and became the pioneer producer of turbo-chargers for general aviation, limited-production industrial, and performance applications. Probably one of the most significant contributors made by Rajay was the availability of its products, which allowed many would-be turbo Einstein’s to get their feet wet. Under Rajay’s wing, companies such as M&W Gear, Spearco, and Daytona Marine appeared on the scene. Rajay’s largest business, by far, was to compete directly with Garrett-AiResearch in the agricultural diesel market.
Most of the heavy-hitting serious professional performance turbo application, however, we’re coming out of AiResearch, undoubtedly because of its relatively prolific product line and its established technology base as the most knowledgeable turbo supplier with international engine manufacturers. Various attempts were made by varying degrees of success from the whole gamut of professional racing, from the original Formula cars to Indianapolis. None of this technology, however, was made available to the general automotive enthusiast. In the early 1970s, the most significant attempt to get turbo- charging into the hands of the enthusiast was made by the creation of the Turbo-Sonic product line of retrofit hardware produced by Accel, the performance marketing arm of Echlin Corporation. (Bob Keller created Turbo-Sonic in 1973.) Accel had been enjoying a very lucrative business in the performance aftermarket and, through a very ambitious campaign of advertising, feature articles, and nationwide product promotion; it was able to move thousand of turbochargers into the hands of energetic consumers who knew nothing at all about what turbos were, how they worked, or how to install them.

The Market Grows
In 1976, shortly after the introduction of the Accel Turbo-Sonic product line (which utilized Garrett-AiResearch turbochargers), Echlin purchased an up and coming industrial aftermarket turbocharger “copy-cat” house by the name of Roto-Master in California. Roto-Master had by then established itself as the principal aftermarket source of remanufactured industrial diesel turbochargers and components in the U.S. and probably the world. This purchase enabled Accel to produce its own turbos for the Turbo-Sonic product line and firmly establish Roto-Master as the leading producer for the aftermarket performance turbochargers. Roto-Master’s Chief Engineer and VP of Engineering was the ubiquitous Macinnes and Keller became Roto-Master’s Chief Engineer of Turbochargers Systems. In the late 1970s, Detroit enthusiastically “discovered” turbo charging, the threat of a serious fuel crunch as 1980 approached as well as the then severe CAFÉ requirements imposed by the government forced automakers to develop small engines that delivered great fuel economy but which were woefully underpowered. In order to restore some resemblance of performance to these small, displacement engines, auto manufacturers applied turbochargers earnest, so much so that by the early 1980s, essentially every auto manufacturer from Ford to Mercedes had multiple families of turbocharged vehicles in their line-up, much to the delight of Garrett-AiResearch, the major supplier at this time. Aftermarket retrofit and performance turbo-charging became more of a political subject than a technical subject when government regulations imposed by the Clean Air Act of 1977 and California’s Air Resources board made it extremely difficult and costly for retrofit turbocharger kit manufactures to produce economically viable products, which could be sold legally. Some houses did take on this challenge, however, and were able to demonstrate that a retrofit turbo could provide that extra desired “kick in the pants” and satisfy emissions requirements at the same time. Spearco, Dina Engineering, Custom Automotive, Gemini Turbo systems, BAE, Advanced Turbo systems, Gale Banks Engineering, Turbonetics (created by Keller after he left Roto-Master in 1978) and others have proved it can be done by obtaining executive orders from the California Air Resources board that declared, after extensive testing and engineering review, that the product was in compliance with all emission requirements.

The Frustrating 1980s
Performance turbo-charging for legitimate competition was a very frustrating exercise in the 1980s. It seemed that any time an adventurous car builder put together a turbo vehicle that showed some degree of success, he found himself strapped with arbitrary weight and/or displacement penalties or even ruled out of competition. The established success of turbos in Europe and at Indy still had a long way to go to overcome the ignorance of many U.S. sanctioning bodies in discouraging turbos because of their “exorbitant” cost and “unfair” advantage.
Rota-Master purchased Rajay in 1982 and in 1986 Garrett purchased Roto-Master, thereby establishing an effective monopoly in the turbocharger aftermarket. However, in the middle 1980s, Japanese manufacturers began to make major inroads with the Big 3 by virtue of being pushed out of aggressive marketing and Garrett’s inability to learn how to play the “game” let to Garrett being pushed out of the American automotive market. Look under the hood of most American turbo cars and you will see name plates that read Mitsubishi, Hitachi, IHI, etc. Garrett has had some success recently in reversing this trend but still has a long way to go to endear itself in Detroit.

The Enthusiastic 1990s and Beyond
The 1900s brought a new consumer to the marketplace and with it an enthusiastic recognition of turbo-charging. Unlike the U.S., Japan continued to develop turbocharged vehicles at a great rate and these cars began appearing in the States. The cars performed well (primarily because of the decent electronic engine management systems, the lack of which was a significant factor in the lackluster performance of most of the Detroit products in the 19980s) and a demand was created for retrofit turbos for popular vehicles such as the Honda.
The success of these applications and the popularity of Japanese imports sparked an interest in turbo-charging that has yet to see its peak. A new breed of aftermarket performance companies (“tuners”) evolved which were comfortable with the intricacies of not only turbo-charging, but of total engine management and all that it entails. Racing venues were established which recognized turbo-charging as an integral part of performance and new companies were founded daily to take advantage of this enthusiasm.  There are more entrepreneurial ventures involving performance turbo-charging now than existed in the hot-rod them selves in a demand market that has is almost impossible to satisfy.

What is a turbo?
Quite simply, a turbo is merely an exhaust-driven compressor. Imagine a small shaft about the size and length of a new pencil. Now rigidly attach a pinwheel to each end of the pencil. One pinwheel (called the turbine) is placed in the path of the exhaust gases which are exiting the engine. These gasses are ‘caught’ in the turbine, causing it to spin. This in turn spins the whole shaft, along with the pinwheel on the other end (called the compressor). The compressor is placed in the intake air’s path; once it begins spinning, it actually compresses the air on its way into the engine.
Why is this beneficial? Well, normally aspirated engines have to work to draw in their intake air. In other words, as the intake valves open, the piston’s downward movement creates a vacuum which ‘sucks in’ some air through the intake system. Ideally, the piston’s movement would suck in 100% of the air that could fill the combustion chamber. In the real world this is not the case; the typical engine will draw in only about 80% of the total volume of the combustion chamber. There are many reasons for this, intake restrictions, valve timing, camshaft design, and much more.
Now imagine that the engine mentioned above has a turbocharger. When the turbo compresses the air it builds up pressure in the intake manifold. So now when the intake valves open, air is actually forced into the combustion chamber. (This is one reason why turbocharged engines are sometimes referred to as ‘forced-induction’ engines.) As you might imagine, this allows for more air to fill the chamber; the amount will still not equal 100% of the entire volume but nonetheless it is closer to this elusive goal.
Okay, so now we have more air entering the engine. To benefit from this, we need more fuel to match. On computerized cars, various sensors will see this amount of boost pressure and increase the amount of fuel accordingly. Now that we have more fuel entering the engine, more power is made. (Yes, when you get right down to is, the only way to make more power-on any engine, is to burn more fuel.)

How Do Turbochargers and Superchargers Differ?
While they perform the same function, turbochargers and superchargers go about it in completely different ways. As has already been mentioned, a turbo is driven by the exhaust gasses which are already being expelled from the engine. So, in effect, turbos add ‘free’ power since their compression is created by what was already discarded.
Superchargers, however, are different” they are belt-driven. They feature a pulley whose belt is directly attached to the crankshaft, this allowing them to spin in direct proportion to the engine itself. The upside is a near absence of lag (see below); at least some boost is typically available the instant you crack the throttle. The primary drawback to a supercharger, however, is that they take power to make power. The overall result is more power then there would be without the supercharger; it’s just that they aren’t as efficient as a turbocharger from an energy standpoint. Other drawbacks include lower mid-range power than a turbo, lower thermal efficiency than a turbo, (sometimes) much harder to incorporate inter-cooling, etc.

What is Turbo Lag (And How Do I Avoid It)?
The majority of turbochargers feature a wastegate, a valve which allows some of the exhaust gas to be directed around the turbine. This allows the turbo’s shaft to spin at a reduced speed, promoting increased turbo life (among other things). Think of it as a ‘stand by’ mode. Since the turbo isn’t needed during relaxed driving anyway, this effect is harmless…
…until you suddenly want to accelerate. Let’s say that you are loafing along, engine spinning 1500 rpm or so, you instantly floor the throttle, the exhaust gas flows through the turbo and causes it to spool (spin up to speed and create boost). However, at this engine speed there isn’t very much exhaust gas coming out. Worse still, the turbo needs to really get spinning to create a lot of boost. (Some turbos will spin at 150,000 rpm and beyond!) So you, the driver, need to wait for engine revs to raise and create enough exhaust gas to flow to spool the turbo. This wait time, the period between hitting the throttle at low engine speed and the creation of appreciable boost, is properly called boost response. Many people incorrectly call it lag, which is really something different. Lag actually refers to how long it takes to spool the turbo when you’re already at a sufficient engine speed to create boost. For example, let’s say your engine spinning 4000 rpm, and now you floor it. How long it takes to achieve your usual 12 psi is your turbo’s lag time. Between the two, slow boost response usually causes the most complaints.
There are two aspects to consider when dealing with boost response: engine factors and driver factors. As far as engine factors go, there are many things which affect turbo lag, although most are directly related to the design of the turbo itself. Turbos can be designed to minimize lag but this usually comes at the expense of top-end flow. In other words, you can barter for instant boost response by giving up gobs of horsepower in the upper third of your rpm range. (Behold the catch-22 in designing one turbo for all uses.)
Driver factors are another matter. You basically need to understand how a turbo works and modify your driving style accordingly. To sum it up, don’t get caught with your pants down! If you feel that there may soon be a sudden need for serious thrust, downshift until your engine speed is at least 3000 rpm. This way there will be noticeable boost almost as soon as you hit WOT. If you are going up a hill at WOT around, say 1800 rpm and your speed is dropping, you’ll need to downshift just like any other car in the same situation. Remember: turbos need exhaust gas in order to spin. Let them have some when they need it.

What’s An Intercooler, &How Does It Help?
 To answer that question, a discussion of thermodynamics is involved. Turbos, as has been mentioned, compress an engine’s intake air. By laws of physics, compressing air also heats it. For an engine, heating the intake air is a bad thing. For one, it raises the combustion chamber temperature and thus increases the chance of detonation (uncontrolled combustion which damages your engine). Another bad thing is that air expands as it is heated. So in other words, it will lose some of the compression effect and the turbo must work harder to maintain the desired level compression.
Thus enters the intercooler into the equation. An intercooler is a heat exchanger, sort of like a small radiator except that it cools the charge (your intake air) rather than the engine coolant. Now that the charge is being cooled, two benefits appear: combustion temperatures decrease (along with the detonation), and the charge becomes denser which allows even more air to be packed into the combustion chamber. Exactly how much heat is removed varies greatly; some what I’ve seen, getting your intake charge temperature within 20 degrees of ambient is excellent; consider this a practical limit for a street-driven car (meaning you might get closer but not without spending tons of money).
There air two types of intercoolers: air-to-air and air-to-water. Air-to-air means that as the charge passes through the intercooler, the intercooler itself is cooled by air flowing through its fins. Picture your car’s radiator but substitute the intake air where the coolant goes and you’ll have a rough idea of how it works. In an air-to-water intercooler, the intercooler is cooled by a liquid rather than air; this liquid has its own radiator placed where it can receive airflow, hoses connect this radiator to the intercooler itself, and the liquid must be circulated through out the entire system.
Each type of intercooler has its strength and weakness. Air-to-air units tend to require longer ducting to route the air from the turbo through the intercooler then back to the engine; this extra tubing might increase lag slightly on some engines and may also present interesting packaging challenges. Air-to-water units, however, can have significantly shorter intake plumbing; the intercooler can be placed in hot under hood areas where no airflow is present since the liquid coolant circulates to its radiator. This allows for simpler installation but at an expense of reduced cooling efficiency. Note that both kinds cool better when air is flowing through the intercooler (air-to-air) or the radiator (air-to-water); both kinds can benefit from the installation of a fan for low-speed operation.
Which type is better, depends on your goal. From where I sit it seems that air-to-water intercoolers are used either for convenience, to eliminate the possible ducting nightmare of the intake, or for drag-only vehicles where a “one shot” setup uses ice to actually drop charge air temps below ambient… for a very short while. I think it is telling that a number of street cars which featured air-to-water intercoolers from the factory, such as the GMC Syclone and the Typhoon, are almost always converted to air-to-air units when upping performance is the goal. Check out an issue of Turbo magazine; you’ll see these cars with huge air-to-air units mounted below the front bumper (or else behind the grill and in front of the radiator).

Advantages of Turbo-charging
Compared with a naturally aspirated engine of identical power output, the fuel consumption of a turbo engine is lower, as some of the normally wasted exhaust energy contributes to the engines efficiency. Due to the lower volumetric displacement of the turbo engine, frictional and thermal losses are less.
The power-to-weight ration, i.e. kilowatt (power output)/kilograms (engine weight) of the exhaust gas turbocharged engine is much better than that of the naturally aspirated engine.
The turbo engine’s installation space requirement is smaller than that of a naturally aspirated engine with the same power output. A turbocharged engines torque characteristic can be improved. Due to the so-called “maxi dyne characteristic” (a very high torque increase at low engine speeds), close to full power output is maintained well below rated engine speed. Therefore, climbing a hill requires fewer gear changes and speed loss is lower.
The high-altitude performance of a turbocharged engine is significantly better. Because of the lower air pressure at high altitudes, the power loss of a naturally aspirated engine is considerable. In contrast the performance of the turbine improves at altitude as a result of the greater pressure difference between the virtually constant pressure upstream of the turbine and the lower ambient pressure at outlet. The lower air density at the compressor inlet is largely equalized. Hence, the engine has barely any power loss. Because of reduced overall size, the sound-radiating outer surface of a turbo engine is smaller; it is therefore less noisy than a naturally aspirated engine with identical output. The turbocharger itself acts as an addition silencer.

Development
As turbochargers have to meet different requirements with regard to map height map width, efficiency characteristics, moment of inertia of the rotor and conditions of use, new compressor and turbine types are continually being developed for various engine applications. Furthermore, different regional legal emission regulations lead to different technical solutions.
The compressor and turbine wheels have the greatest influence on the turbocharger’s operational characteristics. These wheels are designed by means of computer programs which allow a three-dimensional calculation of the air and exhaust gas flows. The wheel strength is simultaneously optimized by means of the finite-element method (FEM), and durability calculated on the basis of realistic driving cycles.

CAD-assembled model of a turbocharger
Despite today’s advanced computer technology and detailed calculation programs, it is testing which finally decides on the quality of the new aerodynamic components. The fine adjustment and checking of results is therefore carried out on turbocharger test stands.

Matching
The vital components of a turbocharger are the turbine and the compressor. Both are turbo-machines which, with the help of modeling laws, can be manufactured in various sizes with similar characteristics. Thus, by enlarging and reducing, the turbocharger range is established, allowing the optimal turbocharger frame size to be made available for various engine sizes. However, the transferability to other frame sizes is restricted, as not all characteristics can be scaled dimensionally. Furthermore, requirements vary in accordance with each engine size, so that is not always possible to use the same wheel of housing geometries.
The Model similarity and modular design principle, however, permit the development of turbochargers which are individually tailored to every engine. This starts with the selection of the appropriate compressor on the basis of the required boost pressure characteristic curve. Ideally, the full-load curve should be such that the compressor efficiency is at its maximum in the main operating range of the engine. The distance to the surge line should be sufficiently large.
The thermodynamic matching of the turbocharger is implemented by means of mass flow and energy balances. The air delivered by the compressor and the fuel fed to the engine constitute the turbine mass flow rate. In steady-state operation, the turbine and compressor power outputs are identical (free wheel condition). The matching calculation is iterative, based on compressor and equations to describe interrelationships which are difficult to express in an analytical way.

Testing
The turbocharger has to operate as reliably and for as long as the engine. Before a turbocharger is released for series production, it has to undergo a number of tests. This test program includes tests of individual turbocharger components, tests on the turbocharger test stand and a test on the engine. Some tests from this complex testing program are described below in detail.
Containment test
If a compressor or turbine wheel bursts, the remaining parts at the wheel must not penetrate the compressor or turbine housing. To achieve this, the shaft and turbine wheel assembly is accelerated to such a high speed that the respective wheel bursts. After bursting, the housing’s containment safety is assessed. The burst speed is typically 50% above the maximum permissible speed.

Low-cycle fatigue test (LCF test)
The LFC test is a load test of the compressor or turbine wheel resulting in the component’s destruction. It is used to determine the wheel material load limits. The compressor or turbine wheel is installed on an over speed test stand. The wheel is accelerated by means of an electric motor until the specified tip speed is reached and then slowed down. On the basis of the results and the component’s S/N curve, the expected lifetime can be calculated for every load cycle.

Rotor dynamic measurement
The rotational movement of the rotor is affected by the pulsating gas forces on the turbine. Though it’s own residual imbalance and through the mechanical vibrations of the engine, it is stimulated to vibrate. Large amplitudes may therefore occur within the bearing clearance and lead to instabilities, especially when the lubricating oil pressures are too low and the oil temperatures too high. At worst, this will result in metallic contact and abnormal mechanical wear.
The motion of the rotor is measured and recorded by contactless transducers located in the suction area of the compressor by means of the eddy current method. In all conditions and at all operating points, the rotor amplitudes should not exceed 80% of maximum possible values. The motion of the rotor must not show any instability.

Start-stop test
The temperature drop in the turbocharger between the gasses at the hot turbine side and the cold compressor inlet can amount to as much as 1000 degrees C in a distance only a few centimeters. During the engine’s operation, the lubricating oil passing through the bearing cools the center housing so that no critical component temperatures occur. After the engine has been shut down, especially from high loads, it can accumulate in the center housing, resulting in cooking of maximum component temperatures at the critical points, to avoid the formation of lacquer and carbonized oil in the turbine-side bearing areas and on the piston ring. After the engine has been shut down at the full-load operating point the turbocharger’s heat build-up is measured. After a specified number of cycles, the temperatures are not exceeded and the carbonized oil quantities around the bearing are found to be low, is this test considered passed.

Cyclic endurance test
During engine operation, the waste gate is exposed to high thermal and mechanical loads. During the waste gate test, these loads are simulated on the test stand. The checking of all components and the determination of the rates of wear are included in the cycle test. In this test, the turbocharger is run on the engine for several hundred hours at varying load points. The rates of wear are determined by detailed measurements of the individual components, before and after the test.
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