What does a turbocharger do?
A turbocharger compresses air into the combustion space in order to generate more power. Compressed air has a larger air mass than ambient air and this increases the amount of fuel that can be burnt efficiently within the combustion space. Increasing the amount of fuel that is burnt efficiently has a corresponding effect upon the engine’s power output (power output increases).
The turbocharger consists of three main parts, the compressor, turbine and central hub rotating assembly (CHRA…i.e. the middle bit). The compressor and turbine are connected on a common shaft, so both operate at the same speed (same rpm).
How does a turbocharger work?
Exhaust gases cause the turbine to rotate which causes the compressor to rotate; this is a feedback loop because the amount of air fed to the engine is determined by the speed of the turbine (which is determined by the amount of exhaust gas discharged from the combustion space). As a larger air mass is discharged into the combustion chamber, more fuel can be burnt efficiently.
Approximately 30-35% of all the heat energy lost by an engine is lost through the exhaust gas stream, but a turbocharger reduces this loss to approximately 20%.
Pressure and thermal energy from the exhaust gas is transferred to the turbine using the turbine casing or nozzles. The shape of the turbine casing maximises the kinetic energy that the exhaust gas exerts on the turbine. The turbine begins to rotate due to the force exerted upon it and this causes the compressor to rotate (because it is on a common shaft).
Large slow speed diesel engines will usually employ a single stage axial turbine design, whilst smaller faster engines employ a radial turbine design. The radial turbine gives some advantages over the axial turbine design:
Sometimes a waste gate will be installed which allows exhaust gas to by-pass the turbine if the exhaust gas manifold pressure is too high.
Turbochargers utilise radial centrifugal style compressors. The size and dimensions of the compressor are chosen based upon the optimal compressor map. The compressor rotates and creates a pressure differential, this causes air to be drawn into the compressor eye and discharged radially (due to the centrifugal force) from the compressor into the casing. The compressor casing is colloquially called a ‘volute’ casing and consists of a diffuser and collector (the collector is the volute part). The diffuser’s unique shape turns kinetic energy to pressure energy due the compressed air’s reduction in speed and increase in pressure. Some important things are happening during the compression stage:
If the velocity decreases in a centrifugal compressor/pump, pressure increases.
If the velocity increases in a centrifugal compressor/pump, pressure decreases.
Scavenging and Charging
Compressed air from the compressor is often referred to as ‘charge air’ and the process of filling a cylinder with fresh air (a.k.a. charge air) is known as ‘charging’. The process of expelling exhaust gas from the cylinder and replacing it with charge air, is known as ‘scavenging’.
Cooling of the Compressed Air
Compressed air is cooled after exiting the turbocharger to further increase the air density. Normally the air is cooled using a heat exchanger with fluid on the non-compressed air side and the charge air on the other side, but system designs vary. The cooler used in the ‘Turbocharger System Model’ has thin radiator fins in order to increase the contact surface area with the charge air and thus increase the heat transfer rate.
Cooling of air increases the density, but condensation of moisture will form if the air is cooled too much. Moisture is undesirable because it will react with exhaust gas within the cylinder liner and become corrosive; it also reduces the lubricating properties of the lubrication oil used within the cylinder liner. Air at very low temperatures could crack engine components due to the large temperature difference between the air and the components.
People often refer to the air cooler as an ‘after cooler’ ‘charge air cooler’, or, ‘inter cooler’. The inter cooler term is often not appropriate, but is taken from compressed air terminology where there are multiple coolers and multiple compressor stages.
Applications of Turbochargers
Applications of turbochargers are numerous because they increase the overall height and weight of the engine only slightly, but give a much higher power output and efficiency. Turbochargers are significantly beneficial to the aviation industry. Planes fly at high altitudes and the mass of air reduces as their altitude increases. It is difficult for jet aeroplanes (jet aeroplanes use combustion turbines) to get enough air for combustion at higher altitudes and thus the amount of fuel fed to the engines must be reduced in order to operate the engine efficiently. This reduction in fuel reduces the total power output of the engine; this manifests itself as a reduction in speed.
If a turbocharger is installed, it is possible to compress air at higher altitudes and thus increase the density of air fed to the engine. The increased mass of air allows more fuel to be fed to the engine and a larger power output to be achieved.
Turbochargers for large two stroke engines used upon ships will often have forced draught axial fans for when the engine operates at slow speed and the turbocharger compressor is not rotating fast enough to supply the required amount of air.
If fuel is fed into the engine quickly, the exhaust gas temperatures will not increase as quickly as the fuel feed. The result is that it is possible to feed a lot of fuel to the engine quickly, but it may take several seconds before the turbine increases in speed. The turbine delay also means that the compressor is delayed and the compressed air required for efficient combustion cannot be supplied to the engine at the same rate the fuel is applied. This whole process is known as ‘turbocharger lag’.
There are often interlocks now placed upon fuel feeds to prevent any excess of fuel being delivered without the necessary amount of compressed air being supplied for efficient combustion. Interlocks are efficient because no fuel is wasted.
What is the difference between a turbocharger and a supercharger?
Turbocharger compressors are driven by exhaust gas.
Supercharger compressors are driven mechanically from the engine (using gears, belts or chains etc.).
What is the difference between a naturally aspirated engine and a forced induction engine?
Naturally aspirated engines have no turbocharger; the same mass of air is drawn into the combustion chamber irrespective of the speed of the engine.
Forced induction engines have a turbocharger or supercharger etc.; the mass of air drawn into the combustion chamber varies depending upon the engine’s speed. Forced induction engines are sometimes referred to as ‘pressure charged engines’.
1600’s Robert Boyle defined Boyle’s law.
‘The pressure of a fixed quantity of gas is inversely proportional to the volume it occupies so long as the temperature remains constant.’