COMPUTATIONAL FLUID DYNAMICS: HOW TURBOCHARGERS CAN BOOST YOUR ENGINE EFFICIENCY

A turbocharger increases the output of the engines while surviving extreme operations. It is a type of forced induction system that acts as an air pump driven by exhaust gas.

It pressurizes the intake air to allow more air into the cylinder. The compression increases the density of air, which leads to more fuel intake. 

This compression results in higher air to fuel ratio and hence more torque with a smaller capacity engine. This way, a turbocharged engine produces more power than the same engine without charging.

Significant Components of a Turbocharger

Turbine

The turbocharger uses the engine's exhaust flow to spin the turbine, which in turn rotates the air pump. It spins at speeds of up to 150,000 rotations per minute (rpm). The exhaust gases enter the turbine radially and exist axially. Due to the exhaust air, the temperature of the turbine is very high as well.

Compressor

A shaft connects the turbine to the compressor, located between the air filter and the intake valve. The compressor pressurizes the air travelling to the pistons and pumps the air into the cylinder. The fresh air enters the compressor axially and exits radially.

Central Hub-Shaft

The central hub-shaft is a shaft connecting the turbine and the compressor to boost the performance of the engine. The size of the turbine, compressor wheels, and the size of their housing affect the turbo's dynamic range. Hence, the choice is made according to the requirement.

Design Aspects of a Turbocharger

The turbocharger has a few design conditions that help in selecting the turbo for a particular engine.

  • The turbine size affects the amount of power the engine can produce.
  • A large turbine will pose little resistance to outgoing exhaust gas so the engine will be able to make more horsepower, but a large turbine will spin up to speed ("spool up") much more slowly.
  • A small turbine spools up quickly but chokes down the exhaust.
  • Generally, a large compressor will produce cooler compressed air and will be able to generate more turbo boost.
  • CFD engineers must strike a balance between a small and big turbocharger such that it can handle a wide range of exhaust flow rates with ease.

Working of a Turbocharger

The turbocharger turbine consists of a turbine wheel and turbine housing. The turbine uses energy from the exhaust gases to convert heat energy into rotational motion. 

The gas, restricted by the turbine's flow cross-sectional area, results in a pressure and temperature drop between the inlet and outlet. This pressure drop is converted by the turbine into kinetic energy to drive the turbine wheel.

The turbine, mechanically coupled with compressor through the shaft, is rotated at high speeds (up to 280,000 rpm). It converts the engine exhaust gas into mechanical energy to drive the compressor. 

The compressor then draws in ambient air and pumps it at high pressure and temperature (PV = RT, pressure increases, temperature increases).

As the temperature increases, the air density decreases in the system. Hence, a charged air cooler is essential to cool down the compressed air and make the inlet air denser.

Turbocharger Boost Control

The turbocharger boost control is of two types: 

  • Waste-gate valve
  • VGT or VNT

Waste-Gate Valve
The waste-gate valve bypasses exhaust gases away from the turbine wheel in a turbocharged engine system. Its primary function is to regulate the maximum boost pressure in turbocharger systems and protect the engine and the turbocharger.

The valve remains closed for average driving purposes. When the lid is open, it prevents overpressure and suppresses fuel consumption.

VGT or VNT (Variable Geometric Turbine or Variable Nozzle Turbine)

VGT or VNT will control the area through which the exhaust gases flow. At low engine speeds (low rpm), the nozzle closes up, which will increase the flow momentum. 

At high engine speeds, the mass flow rate is higher, and therefore the vent opens up to handle a higher exhaust flow rate. This process will aid in better boost control over a wide range of engine speeds.

Turbocharger Specifications

Inducer and Exducer
The inducer diameter refers to the diameter where the air enters the wheel, whereas the exducer diameter is where the air exits from the turbine wheel.

The inducer has a small inlet side for the compressor and turbine wheel, whereas the exducer has a big inlet side for both.

Trim 
Trim is a term to express the relation between the inducer and exducer of both turbine and compressor wheels. More accurately, it is an area ratio.

  Trim   

    Lag     

   Peak Volume   

       Application    

  Medium  

 Less  

Low

Passenger car

Large

More

High

Specific

A/R Calculation

Image Source

A/R ratio is well known as the Aspect Ratio. It is the cross-sectional area divided by the radius from the turbo centerline to the centroid of that area. It is to compare the turbine/compressor of the same wheel size. Extensive A/R refers to less back pressure and later spool-up. 

Turbocharger Flow Process

Image Source

The change in area and pressure ratio affects the mass flow rate, which consequently affects the power produced by the turbine or compressor. Ultimately, this flow will affect the shaft speed.

Compressor Map

The compressor map depicts the compressor's efficiency through various terms such as Surge Margin, Choke Margin, and speed margin.

  • Surge Margin: It is the ratio of the highest pressure and lowest mass flow rate for the compressor at a given constant speed. It mainly occurs when the wheel is too big for expected boost and pressure or when the throttle is suddenly closed.
  • Choke Margin: It is the right-hand boundary for the compressor map. The point where efficiency drops below 58% refers to the choke line. Plus, the turbo speeds will approach or exceed the allowable limit.
  • Speed Margin: It gives the maximum speed of a turbo. An increase in turbo speed results in increased pressure ratio and mass flow. In the choke line description, the turbo speed lines are very close together. Once a compressor operates past the choke limit, turbo speed increases quickly, and over-speed conditions are very likely.

Turbine Map

This map gives the information of the engine operating point on the compressor map. If the point shifts to the right, the pressure ratio increases without a further increase in the mass flow rate. In such a case, the flow gets choked.

Comparing the Features of a Normal Engine and Turbo-Enabled Engine

  • Temperature: Mass flow is directly proportional to the volumetric efficiency. When the turbo increases the air pressure, the temperature also increases. Intercooler works to keep the heat and increase density.
  • Peak Pressure: When the cylinder fills with more air, more fuel burns in each cycle, so there is an increase in the peak pressure. Therefore, the turbocharger's peak pressure is almost 200 times the NA (Naturally Aspired) engine.
  • Engine Weight: The engine weight is less for turbochargers as compared to the NA engines. As the engine weight reduces for the turbocharger, the fuel consumption also reduces.
  • Throttle Loss: Turbocharger reduces lots of throttle loss, and therefore, the thermal efficiency is high for the turbocharged engine.
  • Altitude Behavior: As the altitude increases, the atmospheric pressure decreases. For an NA engine, it is difficult to suck more air, but in the turbocharger, the efficiency is not affected.
  • Noise Pollution: Unlike the NA engine that produces considerable noise and high exhaust gas emissions, the exhaust gas emission is extremely low for the turbocharger, and there is no noise produced.

Conclusion

A turbo can boost an engine's horsepower without significantly incrementing its weight. A turbocharger uses engine exhaust energy to intake more air into the combustion chamber for more efficient engine operation.

CFD engineering students need a strong theoretical base if they want to make and design efficient turbochargers. To know more about turbochargers and its functionality in CFD, register at Skill-Lync, and enhance your knowledge today. 


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