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.

10

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.

Choosing the right turbo part1

This article is a bit more involved and will describe parts of the compressor map, how to estimate pressure ratio and mass flow rate for your engine, and how to plot the points on the maps to help choose the right turbocharger. Have your calculator handy!!

The compressor is the part of the turbocharger that compresses air and pumps it into the intake manifold. Air molecules get sucked into the rapidly spinning compressor blades and get flung out to the outside edge. When this happens, the air molecules get stacked up and forced together. This increases their pressure.
insideTurbo_0001

The compressor map is a graph that describes a particular compressor’s performance characteristics, including efficiency, mass flow range, boost pressure capability, and turbo speed. Shown below is a figure that identifies aspects of a typical compressor map:

Compressor_Maps_Explained_0001
This entry was posted in Turbo Tech and tagged compressor map on May 7, 2013.

technology info regarding compression ratios

Now that we have introduced knock/detonation, contributing factors and ways to decrease the likelihood of detonation, let’s talk about compression ratio.

Compression ratio is defined as:1
Or2
where CR = compression ratio Vd = displacement volume Vcv = clearance volume
3

The compression ratio from the factory will be different for naturally aspirated engines and boosted engines. For example, a stock Honda S2000 has a compression ratio of 11.1:1, whereas a turbocharged Subaru Impreza WRX has a compression ratio of 8.0:1. There are numerous factors that affect the maximum allowable compression ratio. There is no single correct answer for every application. Generally, compression ratio should be set as high as feasible without encountering detonation at the maximum load condition. Compression ratio that is too low will result in an engine that is a bit sluggish in off-boost operation. However, if it is too high this can lead to serious knock-related engine problems. Factors that influence the compression ratio include: fuel anti-knock properties (octane rating), boost pressure, intake air temperature, combustion chamber design, ignition timing, valve events, and exhaust backpressure. Many modern normally-aspirated engines have well-designed combustion chambers that, with appropriate tuning, will allow modest boost levels with no change to compression ratio. For higher power targets with more boost , compression ratio should be adjusted to compensate. There are a handful of ways to reduce compression ratio, some better than others. Least desirable is adding a spacer between the block and the head. These spacers reduce the amount a “quench” designed into an engine’s combustion chambers, and can alter cam timing as well. Spacers are, however, relatively simple and inexpensive. A better option, if more expensive and time-consuming to install, is to use lower-compression pistons. These will have no adverse effects on cam timing or the head’s ability to seal, and allow proper quench regions in the combustion chambers.