Turbo charging basics

**Kirk** · 11-14-2007, 01:58 PM

Turbocharger Operation

The power generated by a engine is directly related to the amount of air compressed in the cylinders. In other words, the greater the compression (within reason), the greater the output of the engine. One process of "artificially" increasing the amount of airflow into a engine is a turbocharger.

Turbochargers do not require a mechanical connection between the engine and the pressurizing pump to compress the intake gases. Intead, they rely on the rapid expansion of hot exhaust gases exiting the clyinders. The gases spin the turbine blades (hence the name turbocharger) of the pump. Because exhaust gas is a waste product, the energy developed by the turbo is said to be free since it theoretically does not use any of the engine's power that it helps to produce.

A typical turbo usually consists of the following components.

Turbine aka "hot wheel"
Shaft
Compressor aka "cold wheel"
Wastegate valve
Actuator
Center housing and rotating assembly. This component contains the bearings,shaft,turbine seal assembly, and compressor seal assembly.

The turbo is normally (even though I have seen some mounted at the end of the exhaust) located close to the exhaust manifold. An exhaust pipe runs between t he exhaust manifold and the turbine housing to carry the exhaust flow to the turbine wheel. Another pipe connects the compressor housing intake to the throttle body.
Inside the turbo, the turbine wheel (hot side) is attached via a shaft to the intake compressor wheel (cold wheel). Each wheel is encased in its own spiralshaped (looks kinda like a snail shell) housing that controls and directs flow of the exhaust and intake gases. The shaft that joins the two wheels rides on bearings.
The compressing process typically starts when the engine speed is above 2k rpm. The force of the exgaust flow is directed through a nozzle against the side of the turbine wheel. As the hot gases hit the turbine wheel causing it to spin, the specially curved turbine fins direct air toward the center of the housing here it exits. Once the turbine starts to spin, the compressor wheel (shaped like the turbine wheel in reverse) also starts to spin, causing air to be drawn into the center, where it is caught by the whirling blades of the compressor and thrown outward by centrifugal force. From there, the air exits under pressure through the remainder of the induction system on its way to the cylinder.
A turbo is capable of pressurizing the intake charge above normal atmoshperic pressure. Turbo boost or boost are the terms used to describe the positive pressure increase created by a turbo. Here is an example, 10psi (68.9kPa) of boost means the air is being fed into the engine at 24.7psi (170kPa) (14.7psi [101kPa] atomospheric + 10 pounds [44.48 Newtons] of boost)

Various Turbo Designs

In an effort to increase the efficiency of turbo engines, manufactures have developed various designs of turbos and their control systems. One common design is the variable nozzle turbine turbo (VNT). In this design, the cross sectional area through which the exhaust flows is variable. This area is adjusted, via guide vanes, accoring to engine speed. At lower engine speeds, the vanes restrict exhaust flow, thereby increasing boost pressure. At higher engine speeds, the vanes open wider and exhaust back pressure decreases. VNT turbos do not have a wastegate, provide higher boost at lower engine speeds, and are more responsive o changes in engine load. They also help reduce the effects of turbo lag.
Some engines have two turbos, each of a defferent size. The smaller of the two spins up to speed very quickly, this reduces turbo lag, The larger one is slower to get up to speed but adds boost at higher engine speeds. This design is two stage: one for lower engine speeds and immediate increase of speed and one for sustainer power. Other engines with two turbos have one for each bank of a V-type engine.

Wastegate

If the turbocharged air pressure becomes too high, knocking or detonation occur and engine output acctually decreases. To prevent this, a turbo uses a wastegate valve. This valve allows a certain amount of exhaust gas to bypass the turbine once the ideal boost is exceeded. The wastegate is usually operated by an actuator that senses the air pressure in the induction system. When the ressure gets too high, the actuator opens the wastegate valve.
Some wastegates are controlled by the PCM that directly controls a solenoid that, in turn, controls vacuum to the waste gas. The PCM also coordinates ignition timing and air/fuel mixtures with the output of the turbo.

Intercooler

The intercooler cools the preasurized air before it reaches the combustion chamber. When intake air is compressed, its tempature increases greatly. As you should know, air density is reduced and the cylinders recieve less fresh air than what could be gotten from the boosted intake air. Also, the increase tempature increases the chances of engine knock/detonation. To offset these consequences, many if not all turbo systems are fitted with an intercooler. The air leaving the turbo passes through the cooler; cooling the air makes it denser and lowers the tempature produced in the combustion chamber. These factors help reduce engine knock/detonation and increase engine output. Intercoolers are like radiators in that heat from the air passing though them is removed and dissipated into the atmosphere. There are many types of intercoolers, they can be air or water cooled or cooled by the A/C system.

Turbo lubrcating system

Most turbos are lubricated by engine oil that is line-fed into the turbo's oil inlet. The oil drains back to the engine through a separate line. A turbo should never be operated if it has less than 30psi of oil pressure.

Some other stuff

Lack of lubrication and oil lag are major causes of turbo failure. Never immediatly stop a turbocharged engine after pulling a trailer, driving at high speed, or uphill. Idle the engine for 20seconds to 2 mins. The turbo wll continue to rotate after the engine oil pressure has dropped to 0, which can cause bearing damage.