Turbo


 A engine is said to be turbo engine, if its intake is powered using a compressor.

Requirement of power

·         An engine takes in air, due to fall in pressure when the piston moves during suction stroke.

·         To increase power of engine, we need to add more fuel, then there would not be enough air to burn all the fuel.

·         Hence air needs to supplied under force using a compressor .If there is more air, then more fuel can be injected to produce more power.

 

Working

Turbocharger is component which is used to boost the power out of a engine without changing the size of the engine.

A turbocharger harnesses the kinetic and thermal energy of the exhaust gas of a engine, To add more charge or air to the engine.
 Initially the kinetic energy of exhaust is converted into Mechanical energy by turbine. this principle is similar to a windmill toy.

 

So the turbine starts to rotate.Now the shaft of the turbine is coupled to compressor.This compressor is a Centrifugal compressor . Now the impeller of compressor rotates sucking in more air and supplying it under pressure ( 2-3 bar above atmospheric).

Since same shaft connects turbine and compressor, the outlet of compressor is Hot air. Also compression increases temperature of air . Hence it requires a inter cooler. Inter cooler resembles like a engine radiator. A inter cooler cools the Air coming out from the compressor.

As the intake air is cooled, it contains more density of oxygen molecules than hot air. Also cooling reduces specific volume of air, more air is taken inside the engine.

Generally a turbine can revolve at much higher speeds greater than engine speed (around 10000-40000RPM),Hence they require better bearings , but normal bearings may fail at these speeds ,hence oil fil is used as bearing.This oil is the engines lubricating oil,This oil also helps in cooling the turbocharger.

Turbo Lag


Many might be aware of term Lag. Mostly person who might have played online games experience "Lag". Yeah its actually the time difference between the Signal being sent and received.


Similarly Turbo lag is situation in which, A driver presses the Accelerator Pedal demanding more power, But there is time delay (1-2sec) before the engine delivers the demanded power .Its happens more when the engine is running at lower RPM (1000-1500rpm).At these speeds the Exhaust does not possess much kinetic energy. Hence they do not spool up the turbine. 

Therefore this time lag often representing the time delay for engine to produce enough exhaust to spool up the turbine.


Advanced systems to prevent turbo lag

Variable geometry turbine


Every car company might call it using different names but the principle is similar. The VGT or Variable nozzle turbine uses a movable vane which can change the size of the inlet nozzle.

This principle is shown from above image , the movable vanes are golden color lines , they change their angle ,diverting the inlet exhaust gas. The principle is similar to what we do while watering plants . By closing the tube hole with fingers we can increase velocity of water by restricting area of flow.

Twin scroll turbine


The principle is similair to above turbine ,instead of changing nozzles shape ,Twin scroll turbine uses two separate nozzles. One while running at higher RPM and other at lower RPM.

Some cars use a electric motor to Run the compressor side ,when the engine runs at lower RPM.This is used to reduce turbo lag.When the engine picks up RPM the power from Electric motor is cut off ,by using a clutch(automatic)

To know about Twin Turbos Vigneshwarraj's answer to What is the difference between a twin turbo, bi-turbo and a twin scroll turbo?

 

Fuel system


 When a turbo , forces more air to the engine,There must be increase in amount of fuel supplied to the engine.
This function is taken by fuel pressure regulator. The fuel pressure regulator senses the vacuum produced in the intake manifold to supply more fuel under pressure into the cylinder/

Some modern cars come with boost sensor which can detect the boost given by a turbo , so that more fuel is infected.

Others

·         Most cars come with a Blow off valve , which is used to prevent Surge is compressor when throttle is suddenly closed . Its because there will be more pressure built up behind the throttle body as air will start to accumulate behind the throttle valves. Blowoff valve

·         Most turbochargers have a waste gate , this is used to by pass the exhaust gas from entering the turbo charger at higher speeds , because that high pressure can increase knock tendency.