Week-6 : Turbocharger Modelling

AIM:

• List down different TC and locate examples from GT Power
• Explore tutorial number 6 and 7
• Plot operating points on compressor and turbine maps
• In which application Variable Geometry Turbine is beneficial?
• Explore example- Diesel VGT EGR and understand the modelling part

SOLUTION:

1. LIST DOWN DIFFERENT TC AND LOCATE EXAMPLES FROM GT POWER:

TURBOCHARGER:

A turbocharger (technically a turbosupercharger), colloquially known as turbo, is a turbine -driven, forced-induction device that increases an IC Engine power output by forcing extra compressed air into the combustion chamber. This improvement over a naturally aspirated engine's power output is because the compressor can force more air—and proportionately more fuel—into the combustion chamber than atmospheric pressure.

In naturally aspirated piston engines, intake gases are drawn or "pushed" into the engine by atmospheric pressure filling the volumetric void caused by the downward stroke of the piston (which creates a low-pressure area), similar to drawing liquid using a syringe. The amount of air actually drawn in, compared with the theoretical amount if the engine could maintain atmospheric pressure, is called volumetric efficiency. The objective of a turbocharger is to improve an engine's volumetric efficiency by increasing density of the intake gas (usually air) allowing more power per engine cycle.

The turbocharger's compressor draws in ambient air and compresses it before it enters into the intake manifold at increased pressure. This results in a greater mass of air entering the cylinders on each intake stroke. The power needed to spin the centrifugal compressor is derived from the kinetic energy of the engine's exhaust gases.

In automotive applications, 'boost' refers to the amount by which intake manifold pressure exceeds atmospheric pressure at sea level. This is representative of the extra air pressure that is achieved over what would be achieved without the forced induction. The level of boost may be shown on a pressure gauge, usually in bar, psi or possibly kPa. The control of turbocharger boost has changed dramatically over the 100-plus years of their use. Modern turbochargers can use waste gates, blow-off valves and variable geometry.

TYPES OF TURBOCHARGER:

• Single turbo
• Twin-turbo
• Twin-scroll turbo
• Variable geometry turbo
• Variable twin scroll turbo
• Electric turbo

SINGLE TURBO:

Single turbochargers alone have limitless variability. Differing the compressor wheel size and turbine will lead to completely different torque characteristics. Large turbos will bring on high top-end power, but smaller turbos will provide better low-end grunt as they spool faster. There are also ball bearing and journal bearing single turbos. Ball bearings provide less friction for the compressor and turbine to spin on, thus are faster to spool (while adding cost).

Advantages

• Cost effective way of increasing an engine’s power and efficiency.
• Simple, generally the easiest of the turbocharging options to install.
• Allows for using smaller engines to produce the same power as larger naturally-aspirated engines, which can often remove weight.

Disadvantages

• Single turbos tend to have a fairly narrow effective RPM range. This makes sizing an issue, as you’ll have to choose between good low-end torque or better high-end power.
• Turbo response may not be as quick as alternative turbo setups.

TWIN-TURBO:

Just like single turbochargers, there are plenty of options when using two turbochargers. You could have a single turbocharger for each cylinder bank (V6, V8, etc). Alternatively, a single turbocharger could be used for low RPM and bypass to a larger turbocharger for high RPM (I4, I6, etc). You could even have two similarly sized turbos where one is used at low RPM and both are used at higher RPM. On the BMW X5 M and X6 M, twin-scroll turbos are used, one on each side of the V8.

Advantages

• For parallel twin turbos on ‘V’ shaped engines, the benefits (and drawbacks) are very similar to single turbo setups.
• For sequential turbos or using one turbo at low RPM and both at high RPM, this allows for a much wider, flatter torque curve. Better low-end torque, but the power won’t taper at high RPM like with a small single turbo.

Disadvantages

• Cost and complexity, as you’ve nearly double the turbo components.
• There are lighter, more efficient ways of achieving similar results

TWIN SCROLL TURBO:

Twin-scroll turbochargers are better in nearly every way than single-scroll turbos. By using two scrolls, the exhaust pulses are divided. For example, on four cylinder engines (firing order 1-3-4-2), cylinders 1 and 4 might feed to one scroll of the turbo, while cylinders 2 and 3 feed to a separate scroll. Why is this beneficial? Let’s say cylinder 1 is ending its power stroke as the piston approaches bottom dead centre, and the exhaust valve starts to open. While this is happening, cylinder 2 is ending the exhaust stroke, closing the exhaust valve and opening the intake valve, but there is some overlap. In a traditional single-scroll turbo manifold, the exhaust pressure from cylinder 1 will interfere with cylinder 2 pulling in fresh air since both exhaust valves are temporarily open, reducing how much pressure reaches the turbo and interfering with how much air cylinder 2 pulls in. By dividing the scrolls, this problem is eliminated.

Advantages

• More energy is sent to the exhaust turbine, meaning more power.
• A wider RPM range of effective boost is possible based on the different scroll designs.
• More valve overlap is possible without hampering exhaust scavenging, meaning more tuning flexibility.

Disadvantages

• Requires a specific engine layout and exhaust design (eg: I4 and V8 where 2 cylinders can be fed to each scroll of the turbo, at even intervals).
• Cost and complexity versus traditional single turbos.

VARIABLE GEOMETRY TURBOCHARGER (VGT):

Perhaps one of the most exceptional forms of turbocharging, VGTs are limited in production (though fairly common in diesel engines) as a result of cost and exotic material requirements. Internal vanes within the turbocharger alter the area-to-radius (A/R) ratio to match the RPM. At low RPM, a low A/R ratio is used to increase exhaust gas velocity and quickly spool up the turbocharger. As the revs climb, the A/R ratio increases to allow for increased airflow. The result is low turbo lag, a low boost threshold, and a wide and smooth torque band.

Advantages

• Wide, flat torque curve. Effective turbocharging at a very wide RPM range.
• Requires just a single turbo, simplifying a sequential turbo setup into something more compact.

Disadvantages

• Typically only used in diesel applications where exhaust gases are lower so the vanes will not be damaged by heat.
• For gasoline applications, cost typically keeps them out as exotic metals have to be used in order to maintain reliability. The tech has been used on the Porsche 997, though very few VGT gasoline engines exist as a result of the cost associated.

VARIABLE TWIN SCROLL TURBOCHARGER:

It combines the advantages of a twin scroll turbo and a variable geometry turbo. It does this by the use of a valve which can redirect the exhaust airflow to just a single scroll, or by varying the amount the valve opens can allows for the exhaust gases to split to both scrolls. The VTS turbocharger design provides a cheaper and more robust alternative to VGT turbos, it means it is a viable option for petrol engine applications. It allows for a wide flat torque curve. It is more robust in design versus a VGT, depending on the material selection.

Advantages

• Significantly cheaper (in theory) than VGTs, thus making an acceptable case for gasoline turbocharging.
• Allows for a wide, flat torque curve.
• More robust in design versus a VGT, depending on the material selection.

Disadvantages

• Cost and complexity versus using a single turbo or traditional twin-scroll.
• The technology has been played with before (eg: quick spool valve) but doesn’t seem to catch on in the production world. There are likely additional challenges with the technology.

ELECTRIC TURBOCHARGER:

Aeristech’s patented Full Electric Turbocharger Technology is a new enabling technology that will help vehicle manufacturers meet stringent future emissions legislation whilst providing excellent response throughout the engine operating range, even at low engine rpm and vehicle speed.FETT is the ultimate solution for extreme engine downsizing and improved engine efficiency using a single stage turbocharger. 

Throwing a powerful electric motor in the mix eliminates nearly all of the drawbacks of a turbocharger. Turbo lag? Gone. Not enough exhaust gases? No problem. Turbo can’t produce low-end torque? Now it can! Perhaps the next phase of modern turbocharging, there are undoubtably drawbacks of the electric path as well.

Advantages

• By directly connecting an electric motor to the compressor wheel, turbo lag and insufficient exhaust gases can be virtually eliminated by spinning the compressor with electric power when needed.
• By connecting an electric motor to the exhaust turbine, wasted energy can be recovered (as is done in Formula 1).
• A very wide effective RPM range with even torque throughout.

Disadvantages

• Cost and complexity, as you now must account for the electric motor and ensure it remains cool to prevent reliability issues. That goes for the added controllers as well.
• Packaging and weight become an issue, especially with the addition of a battery on board, which will be necessary to supply sufficient power to the turbo when needed.
• VGTs or twin-scrolls can offer very similar benefits (though not at quite the same level) for a significantly lower cost.

GT-POWER EXAMPLES:

Normal turbocharger model:

VGT model:

Twin-turbo model:

Waste gate model:

2. EXPLORE TUTORIAL NUMBER 6 AND 7:

Tutorial 6:

In this tutorial, the EndEnvironment template is used to prescribe the upstream and downstream boundary conditions. The model is built from compressor outlet to turbine inlet.

Brake power – 217.7 kw

Air flow rate – 1093.3 kg/h

Fuel flow rate – 51.8 kg/h

BSFC – 238.1 g/kw-h

Volumetric efficiency – 185.4%

Tutorial 7:

In this tutorial, compressor and turbine templates are added. the compressor and turbine is coupled with each other using shaft.

Brake power – 220.9 kw

Air flow rate – 1565.7 kg/h

Fuel flow rate – 51.8 kg/h

BSFC – 234.7 g/kw-h

Volumetric efficiency – 265.5%

conslusion of these two models:

Model 7 has more volumetric efficiency and airflow rate for the same amount of fuel than model 6. The pressure difference across the compressor is higher in case of model 7 than model 6 that is the reason in variation of results.

3. PLOT OPERATING POINTS ON COMPRESSOR AND TURBINE MAPS:

COMPRESSOR MAP:

Model 7 is used to plot the operating points on compressor and turbine map. The points on the map represents the operating point.

TURBINE MAP:

4. IN WHICH APPLICATION VGT IS APPLICABLE?

• High A/F ratio and high peak torque at low engine speed
• Prevent over-boosting at high engine speed
• Reduce exhaust gas emissions and improve the fuel economy
• Improve response time of the turbocharger during transient engine operations

5. EXPLORE DIESEL VGT EGR:

This model contains EGR, VGT, intercooler, injection limiting, boost control and exhaust after treatment device modelling. 

The layout represents a 4 cylinder, 16 valve direct injection diesel engine VGT with EGR, air enter at ambient conditions passes through air box and goes into compressor which run through shaft on which turbine is mounted. Turbine runs through the kinetic energy of exhaust gases. Compressed air passes through the intercooler which decreases temperature of air to increase density and goes into inlet manifold. Exhaust gas is cooled into EGR cooler and mixing of exhaust into inlet air is controlled by EGR valve.

There is VGT rack controller which controls blade positions and vary the speed of turbine and hence boost pressure and target BMEP into the cylinder.

VGT requires varying rack positions and also the different turbine maps for respective rack position.

The below map shows the different maps for different rack position

VGT RACK CONTROL:

It represents RLT graph in which x axis is Engine speed and y axis is fuel injection quantity. Rest table represent boost pressure and blade opening is done according to it.

RESULTS:

BRAKE POWER: 104.9 KW

AIR FLOW RATE: 543.6 kg/h

BSFC: 253.1 g/kw-h

COMPRESSOR MAP:

TURBINE MAP:

CONCLUSION:

Thus the various turbocharger are modelled using GT power