The information presented here will help clarify some of the often misunderstood effects of turbocharger construction, repair and test details surrounding the requirements of accurate shaft balancing.
All turbochargers require that the rotating shaft utilising a pair of turbine wheels is dynamically-balanced at rotation speeds of 150,000rpm and sometimes higher.
This specialised procedure is carried out on all new, remanufactured and repaired turbos.
The test equipment consists of highly sophisticated devices such as accelerometers, high-speed stroboscopic light detectors, fine-tuned tachometers and state-of-the-art computer software.
To achieve such high levels of accuracy, the manufacturers use special fasteners designed to retain compressor-wheels to meet these exacting standards.
The balancing process uses a machine that identifies where small amounts of metal need removing from the compressor-wheel.
A small reflective line of paint helps the machine to determine the exact position of where there is an excess of material. The procedure is highly technical and requires a great deal of skill to perform the turbos balancing needs, correctly.
There is no known method for detecting the levels when a turbocharger is ‘out of balance’. The out of balance effect can cause vibrations when attached to a running engine. The amounts of natural vibration caused by a running engine are far too high.
But since a turbo can spin far beyond what is safe, the speeds restricted by the wastegate.
The wastegate is a common way to control the speed of something within a system. This can be further expanded by an electronic boost controller. The introduction of a wastegate allows the exhaust gases to bypass the turbine.
In this third instalment of ‘How a Turbo Works’ series – Part 3, we discuss specifically the boosting potential a turbo can give a car and tease the wastegate -more on that in part 4.
How a turbo works -Boost
Boost/thrust refers to the increase in pressure within the manifold. This is the level of thrust can be read by looking at the pressure gauge within the instrument panel. This is usually translated into bar, psi or kPa. The pressure reading is representative of the extra air pressure that is achieved over what would be achieved without the forced induction.
Boost is limited by controlling the wastegate. This wastegate stops the exhaust gases from crossing over. In some cars, the maximum boost depends on the fuel’s octane rating and is electronically regulated using a knock sensor or Automatic Performance Control (A.P.C.).
Many diesel engines do not have any wastegate because the amount of exhaust energy is controlled directly by the amount of fuel injected into the engine.
In this second instalment of ‘How a Turbo Works’ series – Part 2, we discuss the various parts of a turbo and how they all play an important role in producing more power.
For a turbo to work it has to have four main components listed below and illustrated.
Center Hub Rotating Assembly (C.H.R.A.)
The turbine and impeller wheels are housed at opposite sides of the Centre Hub Rotating Assembly. This is also known as the C.H.R.A.
The housings fitted around the compressor and turbine wheels suck in and direct the gas through the turbo. The size and shape of the wheels dictate the performance characteristics of the overall turbo.
Often the same basic turbocharger assembly will be available from the manufacturer with multiple AR* choices for the turbine housing and sometimes the compressor cover.
We mentioned earlier the turbine and impeller wheels ‘size and shape of the wheels dictate the performance’. Generally, the larger the turbine wheel and compressor wheel, the larger the flow capacity. Measurements and shapes can vary, as well as curvature and the number of blades on the wheels all introduce a varying factor as to how well the turbo will perform.
The Centre Hub Rotating Assembly (C.H.R.A.) houses a single shaft that connects and suspends the compressor and impeller wheels. At the rotating speeds the turbo wheels spin, bearings are necessary to have it rotate with minimal friction. This is usually performed by a thrust or ball bearing lubricated by a constant supply of pressurized oil. The C.H.R.A. can also be “water-cooled” by allowing the engine coolant to be circulated in place of oil.
How a turbo works are down to the principles of design. This series of articles will attempt to explain the principles of those designs.
A turbocharger consists of a turbine and a compressor linked by one shared and central axis. The turbine inlet receives exhaust gases from the engine exhaust manifold causing the turbine wheel to rotate. This rotation drives the compressor, compressing ambient air and delivering it to the air intake of the engine.
The Objectiveof how a turbo works
The objective of a turbocharger is to improve upon the size-to-output efficiency of an engine by solving for one of its cardinal limitations. A naturally aspirated car engine uses only the downward stroke of a piston to create an area of low pressure in order to draw air into the cylinder. Since the number of air and fuel molecules determines the potential energy available to force the piston down on the combustion stroke, and because of the relatively constant pressure of the atmosphere, there ultimately will be a limit to the amount of air and consequently fuel filling the combustion chamber. This ability to fill the cylinder with air is its volumetric efficiency. Since the turbocharger increases the pressure at the point where air is entering the cylinder, and the amount of air brought into the cylinder is largely a function of time and pressure, more air will be drawn in as the pressure increases. The intake pressure, in the absence of the turbocharger determined by the atmosphere, can be controllably increased with the turbocharger.
Application of how a turbo works
The application of a compressor to increase pressure at the point of cylinder air intake is often referred to as forced induction. Centrifugal superchargers operate in the same fashion as a turbo; however, the energy to spin the compressor is taken from the rotating output energy of the engine’s crankshaft as opposed to exhaust gas. For this reason turbochargers are ideally more efficient, since their turbines are actually heat engines, converting some of the kinetic energy from the exhaust gas that would otherwise be wasted, into useful work. Superchargers use output energy to achieve a net gain, which is at the expense of some of the engine’s total output.
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