Introduction to Turbochargers
Apr 08 2016
Kunal Khetawat

We all know how a car engine works. Right? They make power by burning fuel in cylinders. A small explosion in the cylinder above the piston head puts the piston in motion turning the shafts and gears that spin the car's wheels. The pistons move down in their cyclic motion making way for the exhaust gases to leave the cylinder. Now here is the thing!

The amount of power a car can produce is directly related to a lot of factors. A prominent factor among them is how fast the engine burns fuel. The more cylinders you have and the bigger they are, the more fuel the car can burn each second and (theoretically at least) the faster it can go. One way to make a car go faster is to add more cylinders. That's why super-fast sports cars typically have eight and twelve cylinders instead of the four or six cylinders in a conventional family car.

Another option is to use a turbocharger, which forces more air into the cylinders each second so they can burn fuel at a faster rate. A turbocharger is a simple, relatively cheap kit that can get more power from the same engine! Let's take an example. The new Volkswagen Polo GT TSi, which has 1.2Lt turbocharged engine under its hood, churns 104BHP , which is equal to the power produced by naturally aspirated 1.6 Lt Polo. Now that's some power, isn't it?

A turbo charger housing assembly comprises of the two impellers or air fans known as turbine and compressor, and the centre housing/hub rotating assembly, CHRA for short, which serves as the mounting point for both impellers. There are two more components, the wastegate and a blow-off valve, which act as a control for the entire housing and control the boost pressure produced. The basic idea behind a turbocharger is pretty simple. The objective here is to convert the energy contained in your exhaust stream, which would normally go to waste, into positive pressure within the intake manifold, forcing air into the engine and thus producing more power.

The turbocharger is bolted to the exhaust manifold of the engine. The turbine is connected by a shaft to the compressor, which is located between the air filter and the intake manifold.

The exhaust from the cylinders passes through the turbine blades, causing the turbine to spin. The more exhaust that goes through the blades, the faster they spin. The waste gate which is usually mounted before the turbine housing, acts as a controlled bypass for a percentage of exhaust gas to regulate turbine speed and, thus, overall boost. It bleeds off exhaust gas before it reaches the inlet of the turbine housing. A turbocharger without a wastegate would create an excessive amount of boost/airflow and destroy parts, leaving one with a couple of melted pistons at best or a giant hole in the block (much more likely).

On the other end of the shaft that the turbine is attached to, the compressor pumps air into the cylinders. The compressor is a type of centrifugal pump -- it draws air in at the centre of its blades and flings it outward as it spins. Here we have the second control for the turbocharger, the blow-off valve. A blow-off valve is essentially a pressure relief valve which blows off excess boost pressure trapped in the system when the throttle blade closes. The turbine shaft rotates at around 1,50,000 rpm.

In order to handle such high speeds the turbine shaft has to be supported very carefully. Most bearings would explode at speeds like this, so most turbochargers use a fluid bearing. This type of bearing supports the shaft on a thin layer of oil that is constantly pumped around the shaft. This serves two purposes: It cools the shaft and some of the other turbocharger parts, and it allows the shaft to spin without much friction.

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