BAOTURBO

How a Turbo Works

How a Turbo Works
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.