Turbocharging v/s Supercharging

Supercharging is the process in which fresh air is compressed by a compressor which is driven by the crankshaft of an engine and then fed to the cylinder. Contrary to it, turbocharging is the compression of fresh air by a compressor which is driven by the exhaust gas. This is the crude explanation of the two processes.

History Of Turbochargers

In 1905, ALFRED BUCHI patented a turbocharger while working with SULZER. The idea of forced induction is very old as it allows more air to be available in a cylinder, to attain this supercharging is used but the latter comes with less adiabatic efficiency and an increased load on the engine. Turbochargers were firstly used in marine diesel engines. 

N/A: Naturally Aspirated Engines (only uses ambient air as fresh charge)

T/C: Turbocharged Engine (Forced Induction)

Working Of Turbocharger

Source:- Internet, Courtesy:- aermec.com

The simple construction of the turbocharger comprises a turbine section, compressor section & a common shaft. The exhaust gas at high velocity and higher enthalpy enters the turbine section, the energy of exhaust gas gets transformed into mechanical energy by the turbine. The shaft connected to the compressor and turbine starts rotating as the turbine starts to rotate this drives the compressor, fresh air is being sucked by the action of the impeller and gets compressed to a certain pressure. This compressed air is then fed to the cylinder. Turbo intercoolers are used to cool down the compressed air to increase air density, to reduce NOx emissions, reduction in black smoke, for gasoline engines it raises the anti-knock limit, etc.

*Turbochargers were initially adopted by only diesel and heavy fuel type engines but after the availability of higher octane gasoline fuels, the turbocharger can be conveniently used as knock limit is increased due to higher octane number of fuel*

Advantages of T/C over N/A.

  • Lower BSFC ( Brake specific fuel consumption)
  • Higher power to weight ratio.
  • Higher thermal efficiency.
  • Lesser HC, CO, NOx output.
  • Reduction of noise from the engine.
Engine Speed Vs Brake Power graph
Source: Internet, Courtesy: ResearchGate
Brake Specific Fuel Consumption
Source: Internet, Courtesy: researchgate.net

Turbo lag: It is the time required to change output in response to a throttle change. It can be understood as the time at which the engine reaches the required RPM to kick the turbocharger in.

Some Important Types of Turbochargers (there are many types of turbochargers but only VGT and hybrid turbo are discussed)

Variable Geometry Turbocharger(VGT):-

Consider a situation when the engine is working under variable load conditions, RPM is not constant throughout the operation. The conventional turbocharger will not yield the expected output and end up increasing fuel consumption. To overcome turbo lag and ensure the continuous operation of the turbocharger, VGT is introduced. VGT turbochargers having movable vanes in the turbine section. Whenever RPM decreases the area to radius ratio decreases to create more pressure and velocity to spool up the turbine. At higher RPM, area to radius ratio decreases and let more air to enter. The control vanes are controlled by an electronically controlled motor.

Advantages of VGT:-

  1. No throttling loss
  2. Less soot formation.
  3. Higher peak torque at low speeds.
  4. Wider working load range.
  5. Fewer emissions and low BSFC.
Hybrid Turbocharger:-

It is a combination of supercharging and turbocharging. This type of turbocharger is equipped with a high-speed turbine and high-speed compressor. The main objective of this turbocharger is to reduce turbo lag and the production of electrical energy. During initial conditions, the electrically powered compressor fed compress air to the turbocharger and executes supercharging after reaching a certain RPM turbocharger starts to working and  the turbine shaft is coupled with an electrical generator, the electricity generated is then stored.

Authored By:- Cdt. Hardik Vats, TMI

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