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How a Turbo Works


Exactly how does a turbocharger "breath life into your machine"? This page looks at different sections of the turbo and how these function. 

In simple terms, a turbocharger comprises of a turbine and a compressor connected by a common shaft supported on a bearing system. The turbocharger converts waste energy into compressed air which it pushes into the engine. This allows the engine to produce more power and torque and improves the overall efficiency of the combustion process.

PRINCIPLES OF TURBOCHARGING

To better understand the technique of turbocharging, it is useful to be familiar with the internal combustion engine's principles of operation. Today, most passenger car and commercial diesel engines are four-stroke piston engines controlled by intake and exhaust valves. One operating cycle consists of four strokes during two complete revolutions of the crankshaft.

Suction (charge exchange stroke)
When the piston moves down, air (diesel engine or direct injection petrol engine) or a fuel/air mixture (petrol engine) is drawn through the intake valve.

Compression (power stroke)
The cylinder volume is compressed.

Expansion (power stroke)
In the petrol engine, the fuel/air mixture is ignited by a spark plug, whereas in the diesel engine fuel is injected under high pressure and the mixture ignites spontaneously.

Exhaust (charge exchange stroke)
The exhaust gas is expelled when the piston moves up.
These simple operating principles provide various possibilities of increasing the engine's power output:

Swept volume enlargement
Enlargement of the swept volume allows for an increase in power output, as more air is available in a larger combustion chamber and thus more fuel can be burnt. This enlargement can be achieved by increasing either the number of cylinders or the volume of each individual cylinder. In general, this results in larger and heavier engines. As for as fuel consumption and emissions are concerned, no significant advantages can be expected.

Increase in engine rpm
Another possibility for increasing the engine's power output is to increase its speed. This is done by increasing the number of firing strokes per time unit. Because of mechanical stability limits, however, this kind of output improvement is limited. Furthermore, the increasing speed makes the frictional and pumping losses increase exponentially and the engine efficiency drops.

Turbocharging
In the above-described procedures, the engine operates as a naturally aspirated engine. The combustion air is drawn directly into the cylinder during the intake stroke. In turbocharged engines, the combustion air is already pre-compressed before being supplied to the engine. The engine aspirates the same volume of air, but due to the higher pressure, more air mass is supplied into the combustion chamber. Consequently, more fuel can be burnt, so that the engine's power output increases related to the same speed and swept volume.

Basically, one must distinguish between mechanically supercharged and exhaust gas turbocharged engines.

Mechanical supercharging
With mechanical supercharging, the combustion air is compressed by a compressor driven directly by the engine. However, the power output increase is partly lost due to the parasitic losses from driving the compressor. The power to drive a mechanical turbocharger is up to 15 % of the engine output. Therefore, fuel consumption is higher when compared with a naturally aspirated engine with the same power output.

Exhaust gas turbocharging
In exhaust gas turbocharging, some of the exhaust gas energy, which would normally be wasted, is used to drive a turbine. Mounted on the same shaft as the turbine is a compressor which draws in the combustion air, compresses it, and then supplies it to the engine. There is no mechanical coupling to the engine.


The Turbine section 
The turbine stage comprises of two components; the turbine 'wheel' and the collector, commonly referred to as a 'housing'. The turbine wheel can be of radial mixed or axial design. Generally, in turbochargers used on high speed engines the turbines are of radial design. On larger engines such as ship propulsion axial turbines are used. 

The exhaust gas is guided into the turbine wheel by the housing. The energy in the exhaust gas turns the turbine. Significant amounts of power can be generated in the region of 50kW on a typical 12 litre diesel engine. 

Once the gas has passed through the blades of the wheel it leaves the turbine housing via the exhaust outlet area. 

The speed of the engine determines how fast the turbine wheel spins. If the engine is in idle mode the wheel will be spinning but at a minimal speed. As you put your foot on the accelerator the wheel starts spinning faster. As more gas passes through the turbine housing, the faster the turbine wheel rotates.

 


Compressor section 
Compressors are the opposite of turbines. Again the compressor stage comprises of two sections, the impeller or 'wheel' and the 'housing'. The compressor wheel is connected to the turbine by a forged steel shaft. As the compressor wheel spins air enters through an area known as the inducer and is compressed through the blades leaving the exducer at a high velocity. The housing is designed to convert the high velocity, low pressure air stream into a high pressure, low velocity air stream through a process called diffusion. 
Air enters the compressor at a temperature equivalent to atmosphere, however it leaves the compressor cover at a temperature up to 200 degrees celsius. 
Because the density of the air decreases as it is heated up, even more air can be forced into the engine if the air is cooled after the compressor. This is called intercooling or aftercooling and is achieved either by cooling the charge air with water or air. 

 


The oil supply 
The turbocharger bearing system is lubricated by oil from the engine. The oil is fed under pressure into the bearing housing, through to the journal bearings and thrust system. The oil also acts as a coolant taking away heat generated by the turbine. 
The Journal Bearings are a free-floating rotational type. To perform correctly, the journal bearings should float between a film of oil (i.e. between bearing & shaft, and bearing & bearing housing.) The bearing clearances are very small, less than the width as a human hair. 
Dirty oil, or blockages in the oil supply holes, can cause serious damage to the turbocharger. 


The Piston Ring Seals 
Piston Ring Seals can be found at both ends of the turbocharger. They are designed to keep the exhaust pressure out of the bearing housing, and the air pressure out of the bearing housing.

The surface finish on both Turbine and Compressor end bore must be smooth and score free.

VARIABLE GEOMETRY 

A more effective, though complex, method of turbocharging uses a turbine stage where the swallowing capacity is automatically varied while the engine is running. This permits turbine power to be set to provide just sufficient energy to drive the compressor at the desired boost level wherever the engine is operating. 

This is achieved by varying the area of a nozzle, a set of guide vanes that control the flow through the turbine. Conventional designs pivot the vanes to achieve different nozzle areas. 
Variable Geometry Turbocharging yields several benefits in engine matching:

  • Good transient response
  • Good fuel economy.
  • Increased useful engine operating speed range.
  • Enhanced compression brake capability.
  • Reduced engine swept volume and package size for a given rating

WASTEGATE

The wastegate bypasses exhaust gas around the turbine using a valve in the turbine inlet controlled by compressor outlet pressure. This serves to limit turbocharger speed at high engine speeds and loads. In doing this, it reduces the boost pressure attained at full speed full load. 
Wastegate turbochargers are matched to give good performance at low engine speed with the valve closed. This improves transient response, reduces exhaust temperatures and emissions. As engine speed increases, the wastegate valve begins to open at a pre-set boost pressure. This has the effect of increasing the swallowing capacity of the turbine; reducing shaft power and avoiding excess air delivery and rotor overspeed. 

TURBOCHARGERS COMPONENTS

Turbine Housing
Turbine housings are manufactured in various grades of spheroidal graphite iron to deal with thermal fatigue and wheel burst containment. As with the impeller, profile machining to suit turbine blade shape is carefully controlled for optimum performance. 
The turbine housing inlet flange acts as the reference point for fixing turbocharger position relative to its installation. It is normally the load bearing interface.

 

Wheel
The Turbine Wheel is housed in the turbine casing and is connected to a shaft that in turn rotates the compressor wheel.


Compressor Cover
Compressor housings are also made in cast aluminium. Various grades are used to suit the application. Both gravity die and sand casting techniques are used. Profile machining to match the developed compressor blade shape is important to achieve performance consistency.


Compressor Wheel (Impellor)
Compressor impellers are produced using a variant of the aluminium investment casting process. 

A rubber former is made to replicate the impeller around which a casting mould is created. The rubber former can then be extracted from the mould into which the metal is poured. Accurate blade sections and profiles are important in achieving compressor performance. Back face profile machining optimises impeller stress conditions. Boring to tight tolerance and burnishing assist balancing and fatigue resistance. The impeller is located on the shaft assembly using a threaded nut.

Bearing Housing
A grey cast iron bearing housing provides locations for a fully floating bearing system for the shaft, turbine and compressor which can rotate at speeds up to 170,000 rev/min. Shell moulding is used to provide positional accuracy of critical features of the housing such as the shaft bearing and seal locations. CNC machinery mills, turns, drills and taps housing faces and connections. The bore is finish honed to meet stringent roundness, straightness and surface finish specifications.

Bearing Systems
The bearing system has to withstand high temperatures, hot shut down, soot loading in the oil, contaminants, oil additives, dry starts. 
Journal bearings are manufactured from specially developed bronze or brass bearing alloys. The manufacturing process is designed to create geometric tolerances and surface finishes to suit very high speed operation.

Hardened steel thrust collars and oil slingers are manufactured to strict tolerances using lapping. End thrust is absorbed in a bronze hydrodynamic thrust bearing located at the compressor end of the shaft assembly. Careful sizing provides adequate load bearing capacity without excessive losses.

WHY TURBOCHARGER

A turbocharger acts in a similar way as a supercharger and pressurises the air at the inlet manifold. As the inlet valve in the cylinder opens, a greater mass of air is drawn into the cylinder to be burnt with the fuel. More power is generated at each engine speed. 

Unlike the supercharger it does not feed off the power output of the engine. The turbocharger uses the waste energy from the exhaust gas to drive a turbine wheel that is linked to the compressor through a shaft. At high altitudes, there is insufficient oxygen to burn the fuel, resulting in low power and black smoke. 

At high altitudes the turbocharger rotates faster to increase delivery of air to the engine to compensate. So a turbocharger maintains power from the engine and produces clean emissions. 

Fitting a turbocharger and an air cooler can increase engine power even more. An Intercooler removes the heat of compression between the stages of a compressor whereas an aftercooler reduces the temperature of the air leaving the compressor. Delivering cold air means that there is more oxygen per cylinder (cold air has a higher density than warm air) thus more engine power.

To conclude, the benefits of turbocharging are: 

• increased engine power output (in the region of 50% increase)
• improved fuel consumption (improved pressure balance across the engine)
• improved emissions
• altitude compensation

HISTORY OF TURBOCHARGING

1900's 

Patent obtained by Dr Alfred J Büchi in Switzerland in 1905, for an arrangement of a reciprocating internal combustion engine, rotary compressor and turbine driven by exhaust gas energy. 

1910's 

Experimental turbocharging plant opened in Sulzer Bros Ltd, Winterthur, Switzerland in 1911

1920's 

1925 marked the first successful application on 2 German ships fitted with 2,000 hp turbocharged diesel engines. This success led to Buchi licensing many manufacturers in Europe, USA, Japan

1930's 

Turbochargers with axial turbines used in marine, railcar and large stationary applications.

1940's

 The advent of the aircraft gas turbine led to major advances in materials technology and design. This had the following implications for turbocharging : • development of improved heat resisting materials
• development of precision casting techniques for high temp materials
• this allowed the development of radial turbines and led to the use of radial flow turbos on small automotive diesel engines

1950's 

Major engine producers such as Cummins, Volvo and Scania start experimenting with turbocharged engines for trucks using turbochargers supplied by Elliot and Eberspächer. These early designs were unsuccessful due to the large size of the turbocharger. German engineer, Kurt Beirer produces an innovative compact design that is taken up by Schwitzer Corporation, Indianapolis. In 1954 Cummins offer a range of turbocharged engines, the VT12, six cylinder NT, NRT's and JT's. 

Also in 1954, Volvo offered their first turbocharged truck diesel, the TD96AS, rated at 185 bhp compared with the 150 bhp naturally aspirated D96AS. 

Pole position at Indianapolis in 1952 won by a car powered by a turbocharged Cummins engine.

The Chevrolet Corvair Monza and the Oldsmobile Jetfire were the first turbo-powered passenger cars, and made their debut on the US market in 1962/63. Despite maximum technical outlay, however, their poor reliability caused them to disappear quickly from the market. 

After the first oil crisis in 1973, turbocharging became more acceptable in commercial diesel applications. Until then, the high investment costs of turbocharging were offset only by fuel cost savings, which were minimal. Increasingly stringent emission regulations in the late 80's resulted in an increase in the number of turbocharged truck engines, so that today, virtually every truck engine is turbocharged. 

In the 70's, with the turbocharger's entry into motor sports, especially into Formula I racing, the turbocharged passenger car engine became very popular. The word "turbo" became quite fashionable. At that time, almost every automobile manufacturer offered at least one top model equipped with a turbocharged petrol engine. However, this phenomenon disappeared after a few years because although the turbocharged petrol engine was more powerful, it was not economical. Furthermore, the "turbo-lag", the delayed response of the turbochargers, was at that time still relatively large and not accepted by most customers. 

The real breakthrough in passenger car turbocharging was achieved in 1978 with the introduction of the first turbocharged diesel engine passenger car in the Mercedes-Benz 300 SD, followed by the VW Golf Turbodiesel in 1981. By means of the turbocharger, the diesel engine passenger car's efficiency could be increased, with almost petrol engine "driveability", and the emissions significantly reduced. Today, the turbocharging of petrol engines is no longer primarily seen from the performance perspective, but is rather viewed as a means of reducing fuel consumption and, consequently, environmental pollution on account of lower carbon dioxide (CO2) emissions. Currently, the primary reason for turbocharging is the use of the exhaust gas energy to reduce fuel consumption and emissions. 

TURBOCHARGER KILLERS

90 % of all turbocharger failures are due to the following causes:
Penetration of foreign bodies into the turbine or the compressor
Dirt in the oil
Inadequate oil supply (oil pressure/filter system)
High exhaust gas temperatures (ignition system/injection system)
These failures can be avoided by regular maintenance. When maintaining the air filter system, for example, care should be taken that no tramp material gets into the turbocharger.

TURBOCHARGER SAVERS

The turbocharger is designed such that it will usually last as long as the engine. It does not require any special maintenance; and inspection is limited to a few periodic checks. To ensure that the turbocharger's lifetime corresponds to that of the engine, the following engine manufacturer's service instructions must be strictly observed: 

Oil change intervals
Oil filter system maintenance
Oil pressure control
Air filter system maintenance

TROUBLESHOOTING

Black smoke possible cause:
Dirty air filter system
Suction and pressure line distorted or leaking
Excessive flow resistance in exhaust system/ leakage upstream of turbine
Fuel system/injection feed system defective or incorrectly adjusted
Valve guide, piston rings, engine or cylinder liners worn/increased blow by
Dirty compressor or charge air cooler
Boost pressure control swing valve/poppet valve does not close
Turbocharger bearing damage
Foreign body damage on compressor or turbine
Engine air collector cracked/missing or loose gaskets
Turbine housing/flap damaged
Insufficient oil supply of turbocharger
     
Blue smoke possible cause:
Dirty air filter system
Excessive flow resistance in exhaust system/ leakage upstream of turbine
Oil feed and drain lines clogged, leaking or distorted
Crankcase ventilation clogged and distorted
Coke and sludge in turbocharger center housing
Valve guide, piston rings, engine or cylinder liners worn/increased blow by
Dirty compressor or charge air cooler
Piston ring sealing defective
Turbocharger bearing damage
      
Boost pressure too high possible cause:
Fuel system/injection feed system defective or incorrectly adjusted
Boost pressure control swing valve/poppet valve does not open
Pipe assy. to swing valve/poppet valve defective
      
Compressor/turbine wheel defective possible cause:
Turbocharger bearing damage
Foreign body damage on compressor or turbine
Turbine housing/flap damaged
Insufficient oil supply of turbocharger
     
High oil consumption possible cause:
Dirty air filter system
Excessive flow resistance in exhaust system/ leakage upstream of turbine
Oil feed and drain lines clogged, leaking or distorted
Crankcase ventilation clogged and distorted
Coke and sludge in turbocharger center housing
Valve guide, piston rings, engine or cylinder liners worn/increased blow by
Dirty compressor or charge air cooler
Piston ring sealing defective
Turbocharger bearing damage
     
Insufficient power/boost pressure too low possible cause:
Dirty air filter system
Suction and pressure line distorted or leaking
Excessive flow resistance in exhaust system/ leakage upstream of turbine
Fuel system/injection feed system defective or incorrectly adjusted
Valve guide, piston rings, engine or cylinder liners worn/increased blow by
Dirty compressor or charge air cooler
Boost pressure control swing valve/poppet valve does not close
Pipe assy. to swing valve/poppet valve defective
Turbocharger bearing damage
Foreign body damage on compressor or turbine
Engine air collector cracked/missing or loose gaskets
Turbine housing/flap damaged
Insufficient oil supply of turbocharger
     
Oil leakage at compressor possible cause:
Dirty air filter system
Excessive flow resistance in exhaust system/ leakage upstream of turbine
Oil feed and drain lines clogged, leaking or distorted
Crankcase ventilation clogged and distorted
Coke and sludge in turbocharger center housing
Valve guide, piston rings, engine or cylinder liners worn/increased blow by
Dirty compressor or charge air cooler
Piston ring sealing defective
Turbocharger bearing damage
     
Oil leakage at turbine possible cause:
Oil feed and drain lines clogged, leaking or distorted
Crankcase ventilation clogged and distorted
Coke and sludge in turbocharger center housing
Valve guide, piston rings, engine or cylinder liners worn/increased blow by
Piston ring sealing defective
Turbocharger bearing damage
     
Turbocharger generates acoustic noise possible cause:
Suction and pressure line distorted or leaking
Excessive flow resistance in exhaust system/ leakage upstream of turbine
Dirty compressor or charge air cooler
Turbocharger bearing damage
Foreign body damage on compressor or turbine
Exhaust gas leakage between turbine outlet and exhaust pipe
Engine air collector cracked/missing or loose gaskets
Turbine housing/flap damaged
Insufficient oil supply of turbocharger

 

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How a turbo Works @ HowStuffWorks.com