터보차저이야기-compressor wheel <명준 Turbo ATD>

압축compressor wheel 에 대한 이야기입니다.

터보관련문의: 010 6294 3481 

 

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정품신품터보와 터보엑츄에이터

중국산터보, 중고터보는 취급하지 않습니다.

차대번호와 엔진형식으로 문의하면 보다 정확한 정보를 제공해 드립니다.

 

과급(charger)시스템은 크게 터보차저와 슈퍼차저로 나누어집니다.

각각은 장단점이 있지만 추세는 터보차저가 주를 이룹니다.  

 

터보차저는

가솔린터보차저. 디젤터보차저, 최근엔 전자식터보차저로 나눌수 있습니다.

 

가솔린터보 turbocharger cutaway  2장

 

 

디젤터보의 특성을 보여주는 cutaway 사진(가솔린터보와는 달리 디젤터보는 베인이 부착(파란색네모)

전자식터보차저

 

아래 네모칸의 부속품(compressor wheel)에 대한 이야기입니다.

 

CHRA-splitter blade wheel design

 

 

 요즘은 터보고장에 대한 문의가 들어오면 고장원인이나 새로운터보 또는 튜닝터보에 대한 이유를 물어보는 분들이 많습니다.

사전지식이 매우 풍부한 분들도 많은 분들도 있고 남들이 하니깐 나도 해 보겠다는 분들도 있습니다.

재생이나 중국산터보를 찾는 경우도 많이 봅니다. 이를 권유하는 업체는 거르는 것이 좋습니다.

제가 보는 입장에서는 위험한 발상들이 많습니다.

최신의 다운사이징터보차저는 분당30만 rpm(초당 6,000회전)이상을 회전하는 것들도 있습니다.

 

 경험이나 호기심등 기타등등의 이유로 터보를 분해해 보신 분들은 단순한 구조에 인터넷에서 구입한 터보부품으로 조립을 하는 경우가 있습니다. 혹은 인터넷에서 구입한 조악한 터보를 장착하여 사고를 치는 경우도 많이 봅니다.

터보차저의 구조와 특성을 알고 있으면 선택에 도움이 되고자 합니다.

 

 

하나씩 살펴보도록 합니다.

먼저 compressor입니다.

compressor는 두개의 주요한 구성부품으로 이루어져 있습니다.

  compressor wheel과 compressor cover 입니다.

정확한 용어를 알아야 터보차저와 엔진시스템과의 매칭관계,  재생, 그리고 터보고장의 원인을 이야기하는데 도움이 됩니다.

단순히 '골뱅이' , '터보압축하우징'등의 용어로 설명하는 것이 아직도 현실입니다.

 

compressor을 이해하기 위해 기본적인 열역학원리를 간략하게 소개해야 합니다.

 

PV / T =상수 ( P = pressure of the gas,  V = the volume it occupies, T = the temperature of the gas)

부피가 일정하고 온도가 상승하면 이에 비례하여 압력이 증가, 반대로도 해석할 수 있습니다. 주목할 점은 상수(일정한 값)에 의미를 두시면 됩니다.

 

 튜닝은 저의 분야가 아닙니다. 다만 위의 원리를 이해해야 어떻게 그리고 왜 부스트압력을 이용하여 최대한의 출력을 이끌어낼수 있는지 알수가 있습니다.

 Turbocharger compressor은 intake air을 효율적으로 압축을 잘 하는데 목적이 있습니다.  각각의  compressor는 모양에 따라 optimum flow efficiency, maximum flow capacity (choke)가 정해져 있습니다. 아주 이상적인 compress에서도 대략 76%가 한계로 알려져있습니다.

 

1.  

 

compressor wheel은 터보부품중에서도 가장 많이 거론이 되는 부품입니다. 

 

터보차저 compressor wheel을 'radial compressor'이라고 합니다.  90 도 방향으로 배출하기 때문입니다.

반면에  jet engine의 compressor wheel은 같은 방향으로 배출을 하기 때문에  'axial compress'이라고 합니다.

 

The compressor wheel has a number of critical areas, many of which are changeable within turbocharger model families, resulting in the various trims available to adjust flow parameters and correctly match the compressor to the engine. These changes must not be done by the turbo owner and must be done by the manufacturer at the time the turbo is manufactured. Changing these shapes by the turbo owner will destroy the design relationship between the compressor wheel and the compressor cover contour.

Most compressor wheels have been made from various aluminum alloys. However, there are more and more applications that push the limits of what the current aluminum alloys will stand. The higher the rotational speed of the aluminum, the shorter the life cycle. This is typically not a problem in most performance automotive applications due to the relatively low hours of operation. However, in some extreme high-boost applications, such as tractor pulling, this can pose a problem with wheel burst.

The compressor wheel has several important areas of design consideration: (1) inducer diameter, (2) tip height or tip width, (3) wheel contour, (4) splitter blade, (5) full blade, (6) backwall, (7) wheel diameter, tip diameter, or exducer diameter, (8) tip, impellor, or exducer, and (9) nose. (Courtesy Diesel Injection Service Company, Inc.)

 

While most compressor wheels are a casting, this compressor wheel has been machined from billet. Careful examination of the wheel floor reveals the machining marks left after the formation of the wheel’s hub as the blades were made. This process is done by a five-axis milling machine and is used in special low-volume, high-durability, or high-performance applications where no production wheel will fit the intended need. These types of wheels are tremendously expensive but are justifiable in many instances. (Courtesy Diesel Injection Service Company, Inc.)

 

 

This compressor wheel has been cut in half. The shaft bore (1) typically runs all the way through the wheel, the compressor wheel hub (2) supports the blades and its shape forms the wheel floor (3) that turns the inlet airflow 90 degrees to make it a radial-type compressor. Some wheels will have an extended backwall (4), which strengthens the wheel at its highest point of stress for improved durability. The root of the blades (5) will have a small fillet to support the stresses of compression.

 

  

Compressor wheels are balanced on two planes: the nose and backwall. Note the balance stock removal off the nose and off the backwall in a process called scallop balancing. (Courtesy Diesel Injection Service Company, Inc.)

 

Special note: Any compressor or turbine wheel burst is an extremely dangerous situation and can be lethal. It is most advisable that a burst shield be employed in these extreme applications.

Some commercial diesel and performance racing applications are beginning to use titanium wheels machined on five-axis mills. While these wheels are extremely expensive, they do tend to solve some of the premature failures seen on highboost applications. In most cases however the cast aluminum alloys are more than adequate for most street and strip applications.

The compressor wheel is balanced on two planes: the nose and the back face. Because the wheels in the turbocharger rotate at such high speeds balance is critical for correct operation and life. The compressor wheel is set up on specially designed balance machines and the two planes of balance are defined. A balance spec is set by the manufacturer based upon turbo size and intended operational speed. Typically the balance spec is held to within hundredths of an ounce-inch. On extremely high-speed applications the rotor group can be dynamically balanced by stacking up all of the rotating components and indexing them, then balancing them all as a group. This is explained further later in this chapter.

 

Types of Compressor Wheels

There are many types of compressor wheel designs that have been used over the years. Each one has its own advantage and type of use. Care should be taken to understand the basics of each design to make sure that the turbo you intend to use for your application is of the type and variety that best suits your application.

Straight radial designs are not used much today. They develop high pressure but aren’t very efficient. When used in conjunction with a vaned-type diffuser the efficiency can be very high, but the tradeoff is a narrow flow range, which disqualifies it for use on an automotive application where the engine operates over a wide range of RPM. Historically they were used on diesel engines and generator applications that operate over a narrow RPM band.

A full-blade wheel is rarely seen and is typically on slower speed turbo applications. The full bladed wheel is not recommended for highperformance applications with high boost from high turbo speed. While the full bladed wheel is believed to produce a slightly higher pressure and has slightly higher efficiency, it tends to have trouble biting off enough air at higher speeds.

The splitter blade is the alternating shorter blade between each full blade. At higher speeds, the larger gap between the full blades is more capable of biting off more air. Once taken inside the compressor wheel, the splitter blade helps to efficiently manage and compress the air as it is accelerated and turned radially to the axis of rotation. The splitter blade is most commonly seen in automotive applications today.

The straight radial wheel is easily identified by the blades that emanate in a straight line perpendicular to the axis of rotation. They develop high pressures but are not as efficient or have as broad of a flow range as today’s wheel designs. (Courtesy Diesel Injection Service Company, Inc.)

 

 

 

The full-bladed compressor wheel means that all compressor wheel blades are full length rising from the max wheel diameter all the way to the inducer or inlet diameter of the wheel. (Courtesy Diesel Injection Service Company, Inc.)

 

 

The splitter blade wheel design is where every other blade is shorter than the full blade next to it. This allows for higher airflow at higher rotational speeds.

 

 

 Backward curved impellor refers to that part of the blade element as it approaches the maximum diameter. Note how it curves backward relative to the direction of rotation, which is clockwise. This feature helps to begin air diffusion by slowing the air speed before it exits the wheel thereby making the airflow range broader in the higher efficiency regions of operation. (Courtesy Turbonetics)

 

These two wheels have nearly the same exducer diameter, but the wheel on the left uses a partial backwall while the wheel on the right uses a full backwall. You will almost never see a partial backwall turbo applied to a new application. (Courtesy Diesel Injection Service Company, Inc.)

The extended tip is a special machining process that cuts the compressor wheel impellor back at an angle greater than 90 degrees relative to the backwall. This allows for the rotational diameter to extend beyond the backwall diameter and makes a little higher pressure on a large diameter wheel while minimizing the wheel mass in the outer diameter. The moment of inertia is very sensitive the farther away from the center of rotation. (See Chapter 6 for moment of inertia specifics relative to mass at a given radius.) (Courtesy Diesel Injection Service Company, Inc.)

 

 

 Eliminating the thru bore high-stress core of the compressor wheel reduces the risk of hub fatigue in a highly cyclical application and increases the compressor wheel life. A boreless design can last five times as long as a thrubore wheel. (Courtesy Honeywell Turbo Technologies)

 

The backward curved impellor or BCI wheel design, is the most common type of highly efficient compressor used today. This design creates a wider compressor map that is compatible with automotive applications and raises compressor efficiency by beginning air diffusion within the wheel before it exits into the diffuser of the compressor. The backwall of the compressor can be either a full backwall or a partial backwall. A full backwall is where the backwall is the same diameter as the overall diameter of the compressor wheel.

The idea of the partial backwall is to reduce the overall wheel mass allowing the wheel to be more responsive. However, most backward curved impellors cannot be properly supported without the aid of a full backwall, thus there are design tradeoffs.

A recent advancement patented by Honeywell Turbocharging Systems (Garrett brand) is the use of a boreless compressor wheel. The traditional compressor wheel has a shaft bore through its center that allows it to be fastened onto the turbine wheel and shaft assembly’s stub shaft.