Difference between revisions of "Turbocharging"

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Paralleled twin-turbo refers to the turbocharger configuration in which two identical turbochargers function simultaneously, splitting the turbocharging duties equally. Each turbocharger is driven by half of the engine's spent exhaust energy. In most applications, the compressed air from both turbos is combined in a common intake manifold and sent to the individual cylinders. Usually each turbocharger is mounted to its own individual exhaust/turbo manifold, but on inline-type engines both turbochargers can be mounted to a single turbo manifold. Parallel twin turbos applied to V-shaped engines are usually mounted with one turbo assigned to each cylinder bank, providing packaging symmetry and simplifying plumbing over a single turbo setup. When used on inline engines parallel twin turbos are commonly applied with two smaller turbos, which can provide similar performance with less turbo lag than a single larger turbo
 
Paralleled twin-turbo refers to the turbocharger configuration in which two identical turbochargers function simultaneously, splitting the turbocharging duties equally. Each turbocharger is driven by half of the engine's spent exhaust energy. In most applications, the compressed air from both turbos is combined in a common intake manifold and sent to the individual cylinders. Usually each turbocharger is mounted to its own individual exhaust/turbo manifold, but on inline-type engines both turbochargers can be mounted to a single turbo manifold. Parallel twin turbos applied to V-shaped engines are usually mounted with one turbo assigned to each cylinder bank, providing packaging symmetry and simplifying plumbing over a single turbo setup. When used on inline engines parallel twin turbos are commonly applied with two smaller turbos, which can provide similar performance with less turbo lag than a single larger turbo
  
[[File:Twin-turbo.jpg|200px|thumb|left|twin turbo V engine]]
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[[File:Twin-turbo.jpg|300px]]
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
  
 
==='''Sequential Turbos'''===
 
==='''Sequential Turbos'''===

Revision as of 23:36, 15 February 2016

Introduction

A turbocharger uses an engine’s exhaust gas to drive a turbine wheel at speeds up to 280,000 rpm.

The turbine wheel is connected by a shaft to a compressor wheel and the two wheels turn together to suck in and compress large amounts of ambient air. This air is very dense and very hot, so it is passed through a charge-air cooler, where it cools and gains even higher density before entering the engine. The advantage of compressing the air is that it lets the engine squeeze more air into a cylinder, and more air means that more fuel can be added. Therefore, you get more power from each explosion in each cylinder. A turbocharged engine produces more power overall than the same engine without the charging. This can significantly improve the power-to-weight ratio for the engine

Increasingly, turbos are coupled with high pressure fuel injection systems, a combination that makes for even more thorough, efficient and cleaner combustion.


Turbo-parts.gif How-a-turbocharger-works.jpg




Types of Turbos and setups

Wastegate type (internal)

An internal wastegate is a built-in bypass valve and passage within the turbocharger housing which allows excess boost pressure to dump into the downstream exhaust. Control of the internal wastegate valve by a pressure signal from the intake manifold is identical to that of an external wastegate. Advantages include simpler and more compact installation, with no external wastegate piping. Additionally, all waste exhaust gases are automatically routed back into the catalytic converter and exhaust system. Many OEM turbochargers are of this type. Disadvantages in comparison to an external wastegate include a limited ability to bleed off boost pressure due to the relatively small diameter of the internal bypass valve, and less efficient performance under boost conditions.

Internal wastegate.jpg Internal WasteGate operation.jpg Internal waste gate open.jpg



Wastegate type (external)

An external wastegate is a separate self-contained mechanism typically used with turbochargers that do not have internal wastegates. An external wastegate requires a specially constructed turbo manifold with a dedicated runner going to the wastegate. The external wastegate may be part of the exhaust housing itself. External wastegates are commonly used for regulating boost levels more precisely than internal wastegates in high power applications, where high boost levels can be achieved. External wastegates can be much larger since there is no constraint of integrating the valve or spring into the turbocharger and turbine housing. It is possible to use an external wastegate with an internally gated turbocharger. This can be achieved through a specially designed bracket that easily bolts on and restricts the movement of the actuator arm, keeping it from opening. Another route involves welding the internal wastegate shut which permanently keeps it from opening, but failure of the weld can allow it to open again.

External wastegate.jpg External Wastegate Setup.jpeg External Wastegate.jpg

Variable geometry Type

List of common manufacturer acronyms

  • VGT — Variable Geometry Turbocharger (Cummins/Holset),
  • VNT — Variable Nozzle Turbine (Honeywell/Garrett),
  • VTG — Variable Turbine Geometry (BorgWarner and ABB)
  • VG — Variable Geometry turbocharger (MHI)
  • VGS — Variable Geometry System turbocharger (IHI)
  • VTA — Variable Turbine Area (MAN Diesel & Turbo)


Variable-geometry turbochargers, are a family of turbochargers, usually designed to allow the effective aspect ratio (A:R) of the turbo to be altered as conditions change. This is done because optimum aspect ratio at low engine speeds is very different from that at high engine speeds. If the aspect ratio is too large, the turbo will fail to create boost at low speeds; if the aspect ratio is too small, the turbo will choke the engine at high speeds, leading to high exhaust manifold pressures, high pumping losses, and ultimately lower power output. By altering the geometry of the turbine housing as the engine accelerates, the turbo's aspect ratio can be maintained at its optimum. Because of this, VGTs have a minimal amount of lag, have a low boost threshold, and are very efficient at higher engine speeds. VGTs do not require a wastegate.

VGTs tend to be much more common on diesel engines as the lower exhaust temperatures mean they are less prone to failure. The few early gasoline-engine VGTs required significant pre-charge cooling to extend the turbocharger life to reasonable levels, but advances in material technology has improved their resistance to the high temperatures of gasoline engine exhaust and they have started to appear increasingly in, e.g., gasoline-engined sports cars.

VNT-Turbo.jpg

Twin Turbos

Paralleled twin-turbo refers to the turbocharger configuration in which two identical turbochargers function simultaneously, splitting the turbocharging duties equally. Each turbocharger is driven by half of the engine's spent exhaust energy. In most applications, the compressed air from both turbos is combined in a common intake manifold and sent to the individual cylinders. Usually each turbocharger is mounted to its own individual exhaust/turbo manifold, but on inline-type engines both turbochargers can be mounted to a single turbo manifold. Parallel twin turbos applied to V-shaped engines are usually mounted with one turbo assigned to each cylinder bank, providing packaging symmetry and simplifying plumbing over a single turbo setup. When used on inline engines parallel twin turbos are commonly applied with two smaller turbos, which can provide similar performance with less turbo lag than a single larger turbo

Twin-turbo.jpg

Sequential Turbos

Sequential turbos refer to a set-up in which the motor utilizes one turbocharger for lower engine speeds, and a second or both turbochargers at higher engine speeds. Typically, larger high-flow turbochargers are not as efficient at low RPM, resulting in lower intake manifold pressures under these conditions. On the other hand, smaller turbos spool up quickly at low RPM but cannot supply enough air at higher engine speed. During low to mid engine speeds, when available spent exhaust energy is minimal, only one relatively small turbocharger.(called the primary turbocharger) is active. During this period, all of the engine's exhaust energy is directed to the primary turbocharger only, providing the small turbo's benefits of a lower boost threshold, minimal turbo lag, and increased power output at low engine speeds. As RPM increases, the secondary turbocharger is partially activated in order to pre-spool prior to its full utilization. Once a preset engine speed or boost pressure is attained, valves controlling compressor and turbine flow through the secondary turbocharger are opened completely. (The primary turbocharger is deactivated at this point in some applications.) In this way a full twin-turbocharger setup provides the benefits associated with a large turbo, including maximum power output, without the disadvantage of increased turbo lag.

BMW 335d sequential turbos











Twin Scroll

Twin-scroll or divided turbochargers have two exhaust gas inlets and two nozzles, a smaller sharper angled one for quick response and a larger less angled one for peak performance.

With high-performance camshaft timing, exhaust valves in different cylinders can open at the same time, overlapping at the end of the power stroke in one cylinder and the end of exhaust stroke in another. In twin-scroll designs, the exhaust manifold physically separates the channels for cylinders that can interfere with each other, so that the pulsating exhaust gasses flow through separate spirals (scrolls). With common firing order 1-3-4-2, two scrolls of unequal length pair cylinders 1-4 and 3-2. This lets the engine efficiently use exhaust scavenging techniques, which decreases exhaust gas temperatures and NOx emissions, improves turbine efficiency, and reduces turbo lag evident at low engine speeds

twin scroll setup










Hybrid Turbo

A Hybrid Turbo simply means a non-standard or potentially upgraded turbocharger. This consists of a turbo with combined parts from different frame sizes and potentially different manufactures to make the ultimate performance turbo.


There are many variants of the hybrid theme ranging from a turbo with only one component changed or modified through to one with all the components being changed or modified in some form or another, therefore there are no hard and fast rules on hybrid turbo designs or the performance you can achieve… it's a bit like a modified engine, it looks the same from the outside but is crammed full of more exotic material and higher flowing components on the inside. (Quick tip, to gain a much better response and an increase in power, convert your turbo to a ball bearing turbocharger especially on a road car. The non ball bearing units often compromises with response versus power. More power and torque for the same given displacement equals more lag.


The basic idea is to get more flow from the turbocharger at a given rotor speed… this can be achieved in various ways from larger or more efficient compressor wheels, higher flow compressor covers with a larger A/R, cut back turbine blades and larger A/R turbine housings etc…

Simply “winding the boost up” is not the answer. Yes, in most cases it will give you more power but what are you doing? If you take a standard turbocharger and increase it’s boost pressure output you are doing two things that are or can be detrimental to the turbo and/or engine. Firstly, you are making the turbo work harder/spin faster to make that extra pressure. The turbine is rated to rotate at a maximum reliable speed. Spinning it faster could take you outside that zone and then you can run the risk of turbine blade failure. Running the compressor wheel faster will certainly give you more flow and pressure but at the expense of a lower efficiency and therefore a higher charge temperature.

This can cause a higher likelihood of pre-detonation and limit power output. A hybrid turbo, if designed properly, will allow the extra flow and pressure to be achieved at a safe and reliable turbine speed and at higher compressor efficiencies.

Example Hybrid Turbo










Turbo Talk

Ever read about turbos and upgrades and have been baffled by the tech talk? this should help


A/R

Describes a geometric property of all compressor and turbine housings. Increasing compressor A/R optimizes the performance for low boost applications. Changing turbine A/R has many effects. By going to a larger turbine A/R, the turbo comes up on boost at a higher engine speed, the flow capacity of the turbine is increased and less flow is wastegated, there is less engine back pressure, and engine volumetric efficiency is increased resulting in more overall power

A/R diagram










A/R (Area/Radius) describes a geometric characteristic of all compressor and turbine housings. Technically, it is defined as:

The inlet (or, for compressor housings, the discharge) cross-sectional area divided by the radius from the turbo centerline to the centroid of that area (see A/R Diagram.).

The A/R parameter has different effects on the compressor and turbine performance, as outlined below.

Compressor A/R - Compressor performance is comparatively insensitive to changes in A/R. Larger A/R housings are sometimes used to optimize performance of low boost applications, and smaller A/R are used for high boost applications. However, as this influence of A/R on compressor performance is minor, there are not A/R options available for compressor housings.

Turbine A/R - Turbine performance is greatly affected by changing the A/R of the housing, as it is used to adjust the flow capacity of the turbine. Using a smaller A/R will increase the exhaust gas velocity into the turbine wheel. This provides increased turbine power at lower engine speeds, resulting in a quicker boost rise. However, a small A/R also causes the flow to enter the wheel more tangentially, which reduces the ultimate flow capacity of the turbine wheel. This will tend to increase exhaust backpressure and hence reduce the engine's ability to "breathe" effectively at high RPM, adversely affecting peak engine power.

Conversely, using a larger A/R will lower exhaust gas velocity, and delay boost rise. The flow in a larger A/R housing enters the wheel in a more radial fashion, increasing the wheel's effective flow capacity, resulting in lower backpressure and better power at higher engine speeds.

When deciding between A/R options, be realistic with the intended vehicle use and use the A/R to bias the performance toward the desired powerband characteristic.

Here's a simplistic look at comparing turbine housing geometry with different applications. By comparing different turbine housing A/R, it is often possible to determine the intended use of the system.

Imagine two 3.5L engines both using GT30R turbochargers.

The only difference between the two engines is a different turbine housing A/R; otherwise the two engines are identical:

  • Engine #1 has turbine housing with an A/R of 0.63
  • Engine #2 has a turbine housing with an A/R of 1.06.


What can we infer about the intended use and the turbocharger matching for each engine?

  • Engine#1: This engine is using a smaller A/R turbine housing (0.63) thus biased more towards low-end torque and optimal boost response. Many would describe this as being more "fun" to drive on the street, as normal daily driving habits tend to favor transient response. However, at higher engine speeds, this smaller A/R housing will result in high back pressure, which can result in a loss of top end power. This type of engine performance is desirable for street applications where the low speed boost response and transient conditions are more important than top end power.


  • Engine #2: This engine is using a larger A/R turbine housing (1.06) and is biased towards peak horsepower, while sacrificing transient response and torque at very low engine speeds. The larger A/R turbine housing will continue to minimize backpressure at high rpm, to the benefit of engine peak power. On the other hand, this will also raise the engine speed at which the turbo can provide boost, increasing time to boost. The performance of Engine #2 is more desirable for racing applications than Engine #1 since Engine #2 will be operating at high engine speeds most of the time.


Choke Line

The choke line is on the right hand side of a compressor map and represents the flow limit. When a turbocharger is run deep into choke, turbo speeds will increase dramatically while compressor efficiency will plunge (very high compressor outlet temps), and turbo durability will be compromised.


CHRA

Center housing rotating assembly - The CHRA includes a complete turbocharger minus the compressor, turbine housing, and actuator.


===Clipped Turbine Wheels=== (Cut Back Turbine Blades)

When an angle is machined on the turbine wheel exducer (outlet side), the wheel is referred to as being ‘clipped’. Clipping causes an increase in the turbine wheel’s flow capability as it reduces backpressure in the turbine housing. However, it dramatically lowers the turbo efficiency at low speeds. This reduction causes the turbo to come up on boost at a later engine speed (increased turbo lag). High performance applications should only use a clipped turbine wheel where outright power is the prerequisite. All Garrett GT turbos use modern unclipped wheels.


Corrected Airflow

Represents the corrected mass flow rate of air, taking into account air density (ambient temperature and pressure)

Example: Air Temperature (Air Temp) - 60°F Barometric Pressure (Baro) – 14.7 psi Engine air consumption (Actual Flow) = 50 lb/min

Corrected Flow= Actual Flow Ö([Air Temp+460]/545) Baro/13.95

Corrected Flow= 50*Ö([60+460]/545) = 46.3 lb/min 14.7/13.95


Efficiency Contours

The efficiency contours depict the regional efficiency of the compressor set. This efficiency is simply the percentage of turbo shaft power that converts to actual air compression. When sizing a turbo, it is important to maintain the proposed lugline with a high efficiency range on the map.


Free-Float

A free floating turbocharger has no wastegate device. This turbocharger can't control its own boost levels. For performance applications, the user must install an external wastegate.


GT

The GT designation refers to Garrett's state-of-the-art turbocharger line. All GT turbos use modern compressor and turbine aerodynamics which represent huge efficiency improvements over the old T2, T3, T3/T4, T04 products. The net result is increased durability, higher boost, and more engine power over the old product line.


On-Center Turbine Housings

On-center turbine housings refer to an outdated style of turbine housing with a centered turbine inlet pad. The inlet pad is centered on the turbo's axis of rotation instead of being tangentially located. Using an on-center housing will significantly lower the turbine's efficiency. This results in increased turbo lag, more back pressure, lower engine volumetric efficiency, and less overall engine power. No Garrett OEM's use on-center housings.


Pressure Ratio

Ratio of absolute outlet pressure divided by absolute inlet pressure

Example: Intake manifold pressure (Boost) = 12 psi Pressure drop, intercooler (DPIntercooler) = 2 psi Pressure drop, air filter (DPAir Filter) = 0.5 psi Atmosphere (Atmos) = 14.7 psi at sea level

PR= Boost +DPIntercooler+ Atmos Atmos-DPAir Filter

PR= 12+2+14.7 = 2.02 14.7-.5


Surge Line

The surge region, located on the left hand side of the compressor map, is an area of flow instability typically caused by compressor inducer stall. The turbo should be sized so that the engine does not operate in the surge range. When turbochargers operate in surge for long periods of time, bearing failures may occur.


Trim

Trim is a common term used when talking about or describing turbochargers. For example, you may hear someone say "I have a GT2871R 56 Trim turbocharger". What is 'Trim?' Trim is a term to express the relationship between the inducer* and exducer* of both turbine and compressor wheels. More accurately, it is an area ratio.

  • The inducer diameter is defined as the diameter where the air enters the wheel, whereas the exducer diameter is defined as the diameter where the air exits the wheel.

Based on aerodynamics and air entry paths, the inducer for a compressor wheel is the smaller diameter. For turbine wheels, the inducer it is the larger diameter

Illustration of the inducer and exducer diameter of compressor and turbine wheels










Intercooling

Turbochargers and superchargers are engineered to force more air mass into an engine's intake manifold and combustion chamber. Intercooling is a method used to compensate for heating caused by supercharging, a natural byproduct of the semi-adiabatic compression process. Increased air pressure can result in an excessively hot intake charge, significantly reducing the performance gains of supercharging due to decreased density. Increased intake charge temperature can also increase the cylinder combustion temperature, causing detonation, excessive wear, or heat damage to an engine block.

Passing a compressed and heated intake charge through an intercooler reduces its temperature and pressure. If the device is properly engineered, the relative decrease in temperature is greater than the relative loss in pressure, resulting a net increase in density. This increases system performance by recovering some losses of the inefficient compression process by rejecting heat to the atmosphere. Additional cooling can be provided by externally spraying a fine mist onto the intercooler surface, or even into the intake air itself, to further reduce intake charge temperature through evaporative cooling.

Intercoolers increase the efficiency of the induction system by reducing induction air heat created by the supercharger or turbocharger and promoting more thorough combustion. This removes the heat of compression (i.e., the temperature rise) that occurs in any gas when its pressure is raised or its unit mass per unit volume is increased.

A decrease in intake air charge temperature sustains use of a more dense intake charge into the engine, as a result of forced induction. The lowering of the intake charge air temperature also eliminates the danger of pre-detonation of the fuel/air charge prior to timed spark ignition. This preserves the benefits of more fuel/air burn per engine cycle, increasing the output of the engine.

Stock vs Upgrade Intercooler
Front Mount Intercooler Upgrade VW Golf 5