2 stage turbo systems

r2sSchemeThe basic development goals for future combustion engines for automobile and commercial vehicle applications make more refined charging systems necessary. The design of such a charging system leads to conflicting goals in terms of the rated output of the engine on one hand and the transient response and the range of maximum torque on the other hand. You need a relatively large exhaust turbocharger to attain the nominal output point. The desire for a very high boost pressure even at low engine speeds means, however, that the turbine and the compressor need to be made much smaller. A combination of the two would be ideal.

To resolve this conflict, BorgWarner Turbo Systems has developed regulated 2-stage turbocharging. It meets the demands of an optimal design and allows for the continuously variable adaptation of the turbine and compressor sides of the system for each engine operating point.

With this newly developed charging system, BorgWarner Turbo Systems offers the engine manufacturer an additional extre

rough the system.

At low engine speeds, i.e. when the exhaust mass flow rate is low, the bypass remains completely closed and the entire exhaust mass flow is expanded by the HP turbine. This results in a very quick and high boost pressure rise. As the engine speed increases, the job of expansion is continuously shifted to the LP turbine by increasing the cross-sectional area of the bypass accordingly.

Regulated two-stage turbocharging therefore allows for continuous adaptation on the turbine and compressor sides to the actual requirements of the operating engine.

The system can be regulated via pneumatic actuators that control the bypass valve in the same manner as when used in mass-produced turbochargers with swing valves. This makes it possible to model a compact charging system (when detailed knowledge of the complex system response is available) that fulfills the highest torque, response and power requirements while utilizing proven components.

mely high-performing charging system for future engine generations that fulfills the highest requirements in terms of power, fuel consumption and emissions.

The regulated 2-stage turbocharger consists of two turbochargers of different sizes connected in series that utilize bypass regulation. The exhaust mass flow coming from the cylinder flows into the exhaust manifold first. Here it is possible to expand the entire exhaust mass flow using the high pressure turbine (HP) or to redirect some of the mass flow through a bypass to the low pressure turbine (LP). The entire exhaust mass flow is then utilized again by the low pressure turbine (LP).
The entire fresh air flow is first compressed by the low pressure stage. In the high pressure stage, it is compressed further and then the charging air is cooled. Due to the precompression process, the relatively small HP compressor can reach a high pressure level so that it can force the required amount of air to flow through the system.

At low engine speeds, i.e. when the exhaust mass flow rate is low, the bypass remains completely closed and the entire exhaust mass flow is expanded by the HP turbine. This results in a very quick and high boost pressure rise. As the engine speed increases, the job of expansion is continuously shifted to the LP turbine by increasing the cross-sectional area of the bypass accordingly.

Regulated two-stage turbocharging therefore allows for continuous adaptation on the turbine and compressor sides to the actual requirements of the operating engine.

The system can be regulated via pneumatic actuators that control the bypass valve in the same manner as when used in mass-produced turbochargers with swing valves. This makes it possible to model a compact charging system (when detailed knowledge of the complex system response is available) that fulfills the highest torque, response and power requirements while utilizing proven components.
Courtesy BorgWarner Turbo systems.

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VNT what it does

Nozzle Ring

VNT 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, VNT 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.

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oil related failure

In the event of an lubrication failure to the turbocharger bearing system and shaft, I often get asked why the main bearing of the engine survive a lubrication failure.
There are numerous reasons for this.

  • The turbo is generally the furthest away from the oil pump and is the first to suffer when oil pressure drops or fails completely
  • The crank turns at an average of 5000 rpm in general operation conditions and the turbo at 160,000 rpm.
  • The main and big end bearings are made from a much more complex material and operating design and have completely different load functions to contend with. Where main bearing are designed to absorbs impurities, the turbo bearings are not.
  • Turbo bearings are more susceptible to two types of wear. Abrasive wear. Abrasive wear is the characteristic for a third material to get in between the bearing and shaft. Carbon deposits and impurities from machining and cam grinding.
  • The heat and speed generated inside the turbo shaft system makes it vulnerable to adhesive wear. This is when the two surfaces begin to fuse together or bond and pieces are torn off of one of the surfaces and bonded to the other
  • The oil supply pipe from the block to the turbo is generally pretty close to the exhaust and heat source.In engine shut down periods this oil can boil in the pipe and form deposits which are then forced through the bearings. These deposits cannot be removed by general cleaning.

Its a known fact that any small impurities like carbon get embedded into the main bearing shell and do not cause wear. Bearing material in the crankshaft bearing is designed to do this function to prolong bearing life in an engine. adesiveWearTurbocharger bearing are generally made from a brass alloy as a bush and have very small clearances and impurities will cause wear to both shaft and bearing in a very short amount of time. The limited friction properties of brass materials cause the galling pattern of wear, caused by insufficient lubrication.

Adhesion wear is a result of micro-junctions caused by welding between the opposing rough surfaces rubbing on the counter-bodies. The load applied to the contacting surfaces is so high that they deform and adhere to each other forming micro-joints. The motion of the rubbing counter bodies result in rupture of the micro-joints where some of the material is transferred by its counter-body. This effect is called scuffing or galling. Eventually this will cause the seizure of one of the bodies by the counter-body. This is a common sight found when stripping a turbocharger and inspecting the bearing and shaft journals. The blue to purple discoloring is caused by excessive heat and the brown colouring on the shaft is the brass adhesion onto the shaft.

With this in mind it is imperative that the engine builder and, or turbo fitter has made sure that the inside of the motor and oil galleries have been cleaned properly and using the correct cleaning material. Unfortunately paraffin is one of the worst cleaning materials when used on its own. After cleaning it is essential that it gets washed again with HOT WATER and soap at high pressure. Oil supply lines need to be replaced as deposits CANNOT be removed by cleaning.

Most “warranty” failures are cased by this.

 

What is engine knock/ detonation.

Before discussing compression ratio and boost, it is important to understand engine knock, also known as detonation. Knock is a dangerous condition caused by uncontrolled combustion of the air/fuel mixture. This abnormal combustion causes rapid spikes in cylinder pressure which can result in engine damage.

engine-knockin

Three primary factors that influence engine knock are:

1. Knock resistance characteristics (knock limit) of the engine: Since every engine is vastly different when it comes to knock resistance, there is no single answer to “how much.” Design features such as combustion chamber geometry, spark plug location, bore size and compression ratio all affect the knock characteristics of an engine.

2. Ambient air conditions: For the turbocharger application, both ambient air conditions and engine inlet conditions affect maximum boost. Hot air and high cylinder pressure increases the tendency of an engine to knock. When an engine is boosted, the intake air temperature increases, thus increasing the tendency to knock. Charge air cooling (e.g. an intercooler) addresses this concern by cooling the compressed air produced by the turbocharger.

3. Octane rating of the fuel being used: octane is a measure of a fuel’s ability to resist knock. The octane rating for pump gas ranges from 85 to 94, while racing fuel would be well above 100. The higher the octane rating of the fuel, the more resistant to knock. Since knock can be damaging to an engine, it is important to use fuel of sufficient octane for the application. Generally speaking, the more boost run, the higher the octane requirement.

This cannot be overstated: engine calibration of fuel and spark plays an enormous role in dictating knock behavior of an engine.

Air/Fuel Ratio tuning: Rich v. Lean

When discussing engine tuning the ‘Air/Fuel Ratio’ (AFR) is one of the main topics. Proper AFR calibration is critical to performance and durability of the engine and it’s components. The AFR defines the ratio of the amount of air consumed by the engine compared to the amount of fuel.

A ‘Stoichiometric’ AFR has the correct amount of air and fuel to produce a chemically complete combustion event. For gasoline engines, the stoichiometric , A/F ratio is 14.7:1, which means 14.7 parts of air to one part of fuel. The stoichiometric AFR depends on fuel type– for alcohol it is 6.4:1 and 14.5:1 for diesel.

So what is meant by a rich or lean AFR? A lower AFR number contains less air than the 14.7:1 stoichiometric AFR, therefore it is a richer mixture. Conversely, a higher AFR number contains more air and therefore it is a leaner mixture.
For Example:
15.0:1 = Lean
14.7:1 = Stoichiometric
13.0:1 = Rich
Leaner AFR results in higher temperatures as the mixture is combusted. Generally, normally-aspirated sparkignition (SI) gasoline engines produce maximum power just slightly rich of stoichiometric. However, in practice it is kept between 12:1 and 13:1 in order to keep exhaust gas temperatures in check and to account for variances in fuel quality. This is a realistic full-load AFR on a normally-aspirated engine but can be dangerously lean with a highly-boosted engine.

Let’s take a closer look. As the air-fuel mixture is ignited by the spark plug, a flame front propagates from the spark plug. The now-burning mixture raises the cylinder pressure and temperature, peaking at some point in the combustion process. The turbocharger increases the density of the air resulting in a denser mixture. The denser mixture raises the peak cylinder pressure, therefore increasing the probability of knock. As the AFR is leaned out, the temperature of the burning gases increases, which also increases the probability of knock. This is why it is imperative to run richer AFR on a boosted engine at full load. Doing so will reduce the likelihood of knock, and will also keep temperatures under control.

There are actually three ways to reduce the probability of knock at full load on a turbocharged engine: reduce boost, adjust the AFR to richer mixture, and retard ignition timing. These three parameters need to be optimized together to yield the highest reliable power.afr

Choosing the right turbo part3

GTX4294R_Compressor_Map

6) Choke Line: Overspin Choke Any points plotted to the right of the graph mean the wheel in question is too small and will have to spin too quickly to make the expected boost/power. At these extreme wheel speeds, efficiency goes out the door because the wheel chops the air so badly that the pore density of the charge air will likely cause a dramatic loss of power.

7) Speed Lines: Compressor Wheel Speed This measurement, illustrated as lines across the graph, represents the shaft speed of the wheel. Remember, the faster it spins, the hotter the charge air. This is why they invented intercoolers

Compressor_Maps_Explained_0002

The missing pieces of the puzzle here are the engine’s redline speed, the A/R of the turbine side and the volumetric efficiency of the engine. How fast an engine runs will impact the shaft speed of the turbo and when optimal efficiency is realized. The turbine side of the turbo will determine the spool-up characteristics and responsiveness of the unit. Turbine performance and engine redline speed are closely related, and together both of these factors will have a dramatic effect on the efficiency of the compressor wheel.

This entry was posted in Turbo Tech and tagged compressor map on May 17, 2013.

Choosing the right turbo part2

Turbocharger systems are a complex combination of many different parts. From the turbo itself and intercooler to the fuel management system and the quality of the engine’s internal components, a vehicle must have many different things just in the right order to run properly.

Here are some things to consider:

? Think in horsepower not boost.

? Boost is just a number that you will have to run on your engine to make a certain horsepower.

? How much power do you want to make? Be realistic, the more accurate that you are, the better tuned your forced induction system will be.

? Can your vehicle (not just the engine, but the entire setup) handle such power?

? Remember the turbocharger is generally not the weakest link.

? Forged pistons, connecting rods, head studs, etc.

? As much as possible” is not a goal.

Intended usage:

? What are you using the vehicle for?

? Race or street use?

? The way that you will be using the vehicle dramatically changes the sizing of the turbocharger and intercooler needs.

? Your choice of transmission type and gearing will greatly affect the performance and characteristics of the turbocharger, keep this in mind.

Packaging:

? Will the turbocharger(s) fit in your vehicles space constraints? Consider using differently sized compressor housings to more easily fit a given location.

compressor_map_01

This entry was posted in Turbo Tech and tagged turbocharger on May 14, 2013.