Case Study: Thermodynamic Performance Analysis Of a Twin-scroll Turbocharger vs Single-Scroll Turbocharger Using GT-Power.

 Turbocharger:

Literature Review:

  • A turbocharger is mechanical rotating device which forcebly induces air at high pressure into the internal combustion engine, in turn increases the power and effeciency (thermal and volumetric effeciency) of the engine.
  • A turbocharger is the assembly of turbine and a compressor connected with each other via a mechanical rotating shaft (rotor) which has a significant moment of inertia when at rest.
  • The above fig. shows the actual Twin Scroll Turbocharger model.
  • The compressor is initially turned by the turbine which rotates due to exhaust flow gases. As the engine speed increases increases, more amount of mass flow rate of exhaust gases enters the turbocasing and rotates the turbine at very high speed. The high speed turbine coupled with the compressor via an inertia shaft, rotates the compressor blade at very high speed and thus compresses inlet mass flow rate at higher pressure ratio.
  • Main drawback of turbocharger is "Turbo spool up time /turbo lag".
  • Turbo spool up time /turbo lag  is mainly causes when the engine starts and there is not enough exhaust mass flow rate to rotate the turbine at high speed which causes surge  in the compressor.
  • Apart from that if the exhaust mass flow rate is way too high than it will make the turbine rotate at very high speed which might cause mechanical failure  of the shaft  (rotor) and in compressor section choking  will be created.
  • The above figure shows a general case of a  compressor map of a turbocharger.
  • The map represents a pressure ratio vs Corrected air flow rate.
  • Surge line  indicates the line to the left of which pressure ratio ratio drops in the compressor due lack of desired amount of air flow rate in the inlet. If engine operating in that region than it might suffer  mild flutter to wildly fluctuation of boost  and even  barking from the compressor.
  • Choke line represents the line to the right of which if the engine operates then compressor effeciency will come down due to chocking. This is caused due to high mass flow rate of the exhaust gases rotating the turbine.
  • Speed lines represents  the speed  of the compressor. Higher the curve then higher the speed of the compressor. The highest speed line is the mechanical limit below which the turbo has to operate.

Types Of Turbocharger:

  • Single Turbo:  The simplest, and still most common, type of turbo is the single-turbocharger set-up. Because of its relative simplicity it is the cheapest, but arguably not the best, turbo option available. A single turbocharger can’t be effective across the whole rev range. Generally speaking,we have to choose either a smaller turbo that kicks in lower down on the rev counter and offers good low-down torque, or a larger one that will provide more top-end power. In either case, the boost range will be fairly narrow, and lag will be an issue.

 

  • Sequential Turbo: Having to choose between a small low-end turbo or a big high-end one presents a rather intuitive solution: Fit two, one small and one large. That way we have a small turbo that kicks in early and provides good torque, and a bigger one that offers top-end grunt, resulting in a wide and flat torque curve. On the downside, quite obviously, we’re left with an engine set-up that is expensive, heavy and complex.It is also known as "Twin turbo" or "Bi-turbo" .
  • The above figure shows the model of a "Twin-scroll sequential twin turbocharger"

 

  • Twin-scroll turbo
    A turbo is powered by exhaust gases that are redirected to spin turbine blades and force air into the engine. Now, an engine’s cylinders fire in sequence, meaning that exhaust gases enter the turbo in pulses. As we can probably imagine, these pulses can easily overlap and interfere with one another when powering the turbo, and a twin-scroll turbocharger solves this issue by using a divided-inlet turbine housing and a specific exhaust manifold that pairs the right cylinders to each scroll. In a four-cylinder vehicle, we can then have the first and fourth cylinders powering one scroll, and two and three powering another. This means that there’s less pulse overlap and less lag.

 

  • VGT turbo
    A variable geometry turbo (VGT) is an expensive and complex power solution that’s especially prevalent in diesel engines. A VGT has a ring of aerodynamically-shaped vanes in the turbine housing that can alter their area-to-radius ratio to match the revolutions of the engine. At low revs, area-to-radius ratio creates more pressure and velocity to spool up the turbo more effectively. At higher revolutions, the ratio increases to let in more air. The result is a wider boost range and less lag.

 

  • Electric turbo
    A very recent development is the introduction of turbos with electric compressors. An example is BorgWarner’s eBooster, which is an electrically powered compressor. The compressor provides instant boost to the engine, until the turbocharger has spooled up enough. A similar version of this can be found in Audi’s SQ7. With instant boost, lag becomes a thing of the past, but again, the system is expensive and complex. A compressor needs a motor, which in turn needs to be powered, so this is not a simple system to implement.

PROBLEM DEFINITION:

  • Perform a Thermodynamic analysis of Twin-scroll and single -scroll Turbocharger And compare the results.

 

Engine Map (GT-Suite)

Model Description:

  • Engine:-
  • The engine is a 4-stroke 6-cyl-turbocharged 5.65L CI engine. It has 2 valves (Single overhead cam system) each at intake and at exhaust port.
  • Engine size: 100 x 90 mm, CR = 21, head clearance = 1.
  • Firing Order: 1-5-3-6-2-4
  • The engine runs in "SPEED MODE" condition, where the user enters the engine RPM value and in return power, torque and other performance parameters are calculated.
  • The combustion is modeled using "DI Wiebe" combustion model.
  • The heat transfer model used is " Woschni GT".
  • The simulation will run for 3-cases at 2 different engine rpm (4500 rpm/5000 rpm).

 

  • Intake System And Intercooler:-
  • The inlet condition is set to absolute pressure and temperature: 1bar / 297k.
  • The intercooler wall temperature is "user imposed" and it is controlled via a controll system which actuates the intercooler oulet gas temperature based on intercooler effectiveness data.The intercooler(Charge-Air-Cooler) is thermally seperated from its inlet and outlet using a "no-cond" orifice as because the intercooler inlet and outlet manifold is defined with a temperature solver instead of userdefined condition as in the main intercooler. So a "no-cond" orifice is used instead of a "Belmouth" orifice.
  •  
  • The Fig is the graphical representation of "mass flow rate vs Effectiveness"  of the intercooler.

 

  • Exhaust System:- 
  • In the exhaust manifolds, the exhaust ports are set to a "user defined wall temperature"  and from the exhaust runners to outlet the thermal conditions are soved using  solver. Hence, the "exhaust port""  is thermally isolated with the rest of the exhaust system using a "no-cond" orifice.
  • There is only 1 exhaust entry is modeled to enter the "turbine" which makes it a "Single -Scroll Turbocharger".

 

  • Turbocharger:-
  • The compressor is a "Radial" type compressor connected with the Turbine with an "Inertia Shaft".
  • For the compressor, the absolute pressure and temperature are set to "Ambient Conditions".
  • The following is the  compressor map for " pressure" ratio vs "mass flow rate"
  • The turbocharger is a "Free-float" turbocharger as it doesn't has "waste gate" to actuate its flow automatically.

Case Setup:-

Results:-

Speed Map Monitors Of Compressor

Case:1 [Speed Monitor Plot]

Case:2[Speed Monitor]

Case 3: [speed monitor]

 

PERFORMANCE RESULTS:

SIMULATION DASHBOARD RESULTS:-

VOLEF IMEP Pmax Ieff REScs FAeff
cylinder-1: 0.948 9.21 173.28 37.9 3.4 0.0247
cylinder-2: 0.934 9.31 170.21 38.2 3.5 0.0250
cylinder-3: 0.934 9.40 169.91 38.6 3.4 0.0250
cylinder-4: 0.945 9.50 171.98 39.1 3.3 0.0248
cylinder-5: 0.936 9.44 170.33 38.8 3.3 0.0250
cylinder-6: 0.950 9.25 173.97 38.0 3.3 0.0246

 

ENGINE: IMEP = 9.35 bar, VOLEF = 0.955 VOLEFm = 0.941

FLOW STEADY STATE:

YES Mass Flow (%) = 0.0325 at con : 42
YES Pressure (%) = 0.0247 at cmp : exhrunner-3
ODE STEADY STATE - ODE Solution Cluster: ODE Cluster 1 (Column 1)

YES (1) Torq. dTqmx (%) = 0.0639 at cmp : SHAFT-1
Flow timesteps/cycle=721, average time step = 0.999 deg
Time step restricted by
99.2%: Flow Control Settings
0.8%: cylinder-1

INFO Number of flow time steps in this case = 11536

CASE COMPUTATIONS: Elapsed Time: 000:00:10.67
FINAL COMPUTATIONS: Elapsed Time: 000:05:31.54

END OF RUN
Sun Sep 22 19:06:11 2019 Creating results (.gdx) file...
Sun Sep 22 19:06:13 2019 GDX file GTModel1_Compress_6cyl.gdx generated successfully.

OBSERVATION(1):-

  • From the speed monitors of the compressor shows that all the case are within the optimum region.
  • For case-1, the pressure ratio is "2.5216 @ 0.3750 Kg/s.(73.6% effeciency)
  • For case-2, the pressure ratio is "2.5217 @ 0.3676 Kg/s.(74.5% effeciency)
  • For case-3, the pressure ratio is "2.5216 @ 0.3333 Kg/s.(74.5% effeciency)
  • Comparing cases 1,2&3  we clearly see that at higher altitude (5000 m. from sea lavel) engine seems to struggle with the air flow rate as at higher altitude air supply is low. Thus, for case-2, power effeciency as well as BSFC also decreases down.
  • The "average exhaust temperature" for each of the simulation run is nearly 850K  which is nearly at the permissible limit to avoid any durability issues.
  • The BSFC  for case-1 is the lowest due to low-altitude and higher engine rpm at higher A/F ratio.
  • The BSFC for case-2 is the lowest due to lack of proper air-supply at higher altitude.

 

  TWIN-SCROLL TURBOCHARGER vs SINGLE-SCROLL TURBOCHARGER

ENGINE MAP (TWIN-SCROLL TURBO):

The only thing that has been manipulated from the previous setup is the 2-exhaust manifold (instead of 1) running directly to the turbine inlet.

The advantages of Twin-scroll turbo compred to Single-scroll turbo are:-

• Quicker boost response
• Increases power at all engine speeds
• Reduces mixing losses
• Maximizes pulse energy to the turbine wheel
• Increases turbine efficiency
• Improves “low-end” performance similar to a twin-turbo system
• Reduces pumping losses
• Reduces fuel consumption 

RESULTS:-

Compressor plots (pressure ratio vs mass flow rate)

compressor plot (effeciency map)

The plot shows the compressor effeciency maps and speed maps stating that the operating condition of the compressor at very feasible zone for the given engine speed and other parameters..

 

 

Performance Comparision at low revs @ 3000 rpm

SINGLE-SCROLL TURBO                          VS                TWIN-SCROLL TURBO

Performance Comparision at HIGH revs @ 5000 rpm

SINGLE-SCROLL TURBO                          VS                TWIN-SCROLL TURBO

OBSERVATION:-

  • From the above thermodynamic analysis and engine calibration, we can say that:-
  • Power developed at low revs for "twin-scroll turbo" is "16.9% higher"  than the "single-scroll turbo".
  • Torque developed at low revs for "twin-scroll turbo" is "17% higher"  than the "single-scroll turbo".
  • BSFC  developed at low revs for "twin-scroll turbo" is "4.8% lesser"  than the "single-scroll turbo".
  • And IMEP developed at low revs for "twin-scroll turbo" is "2.5% higher"  than the "single-scroll turbo".

similarly, for higher revs..

  • Power developed at  "twin-scroll turbo" is "5.3% higher"  than the "single-scroll turbo".
  • Torque developed at "twin-scroll turbo" is "5.36% higher"  than the "single-scroll turbo".
  • BSFC  developed at "twin-scroll turbo" is "5.1% lesser"  than the "single-scroll turbo".
  • And IMEP developed at "twin-scroll turbo" is "2.45% higher"  than the "single-scroll turbo".

Hence from the above analysis we can clearly say that twin-scroll turbine has upper hand on both the high end and lower end of the performance. But however design  is very complex and the product is expensive.


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