Turbocharging Basics 101: Session 1:Pumped Up
An article from Import Racer! By Dino G. Tadokawa


All diagrams courtesy of Garrett Engine Boosting Systems

In the most basic form a turbocharger can be viewed as an exhaust driven air compressoróor simply, an air pump. A highly sophisticated air pump of course! In general, the more air you can cram into an engine, the more power your engine can create; this is accomplished with the right proportion of fuel and correct spark timing. Engine design integrity as well as engine swept volume, maximum rpm and volumetric efficiency plays a big part in this assumption. In any case, with a turbocharger onboard it makes your small displacement engine think it's really a large cubic inch displacement engine. Your little turbocharged 1.8 VTEC can hang with the domestic big boys and in many cases they can blow the Detroit iron into oblivion. Letís revisit the turbo gurus (all with the help of our lovely teaching aide) to figure out what an engine can do with a bunch of compressed air.

A turbocharger is a centrifugal air compressor powered by an exhaust driven radial inflow turbine stage which utilizes hot expanding exhaust gas energy that is normally wasted on a naturally aspirated engine.

Center Housing Rotational Assembly

The Guts
A turbocharger system is comprised of three integral stages known as the Compressor Stage, Turbine Stage and the CHRA. The heart, or main "guts" of the unit, is housed in what is called the CHRA (Center Housing Rotational Assembly). The CHRA consists of the compressor wheel and turbine shaft wheel assembly. The turbine wheel is the driving or propelling end of the turbocharger. It is often known as the "hot side," or "iron side," since it's always exposed to the heat of the spent exhaust gases. Integrated on a common shaft, the steel turbine wheel assembly is designed to drive the aluminum "cold side" compressor stage. The rotational unit "shaft wheel assembly" is supported both radially on free floating journal bearings, and axially the movement is controlled via a thrust bearing/collar assembly. The center housing itself is manufactured in both air cooled and water cooled configurations. As far as sealing is concerned, there are piston rings seals located at both ends to prevent oil/gas leakage within the CHRA assembly.

Turbine stage cutaway view

The turbine stage is the combination between the turbine wheel and turbine housing. Proper matching of turbine wheel size (trim and diffusion ratio) and turbine housing A/R (aspect ratios) determines a turbine stage match based on an engineís given flow requirements and ultimate application needs. This same concept applies to the compressor stage. Again, proper matching of compressor wheel size and aspect ratios determines the required flow and pressure capabilities for a given application. When determining a match for the proper power, balance between turbine and compressor stage is required. Leave the matching up to qualified turbo guru's though since they can best tailor the intended flow, pressure and efficiency range for your intended application requirements.

Compressor stage cutaway view

The Spin
Looking at the total system the turbocharger is mounted to the engine's exhaust system via a turbocharger manifold. As the engine expends its exhaust, this pressurized exhaust gas enters the turbine housing and drives the impeller (device which resembles a propeller) turbine shaft wheel assembly. The hot expanding exhaust gases enter the turbine housing at higher and higher speeds as engine loads increase. As the common shaft wheel assembly spins under load, the compressor stage begins to spool (spin) up and develop compressed air pressure. This form of compressed air is known as "boost" pressure. This compressed air flows into your engine through your intake manifold (again, with the correct amount of fuel and spark) and the engine will develop more horsepower. Pretty simple, huh! It's funny that the mother earth gives us typically 14.696 psi (or 1 atmosphere, which equates to 101.3 kPa or 29.9 in. Hg., the same as 760mm Hg @ 0 degrees C) absolute pressure at sea level. In a naturally aspirated engine this is all we get to help the flow of the air and fuel mixture into our engines. But under forced induction (such as supercharging and turbocharging) the air is compressed and force-fed into the engine. This allows the engine to burn more fuel and air at the same time, thereby increasing the engine's power output.

Turbocharging is an example of advanced automotive technology at its best. It's a proven fact that boosted engine performance under the right conditions can outclass many of the so-called big displacement naturally aspirated performance engines.

Systems installation diagram

What's the purpose of turbocharging?
The primary purpose of a turbocharger is to increase the power output of an internal combustion engine. The power output is a direct result of the turbocharger compressorís ability to provide more air. The more air you can provide, with the correct amount of fuel, the more power your engine can produce.

Future Boost
In the next installment of BT101, the Turbo Guru extraordinaires will school you on more subjects related to turbocharging matching, purchasing a turbo and the questions you must ask when making a purchase, turbocharger installation and a final article on true factory "werks" racing turbochargers. Stay tuned for ongoing coverage in Basic Turbocharging 101.