A turbine designed to charge the diesel engine cylinders with air at a higher-pressure utilizing energy from exhaust gas and hence increase the density of charge air above atmospheric pressure.
Turbochargers increase power for an engine of the same size or reduction in size for an engine with the same power output. They also reduce specific fuel oil consumption - mechanical, thermal and scavenge efficiencies are improved due to less cylinders, greater air supply and use of exhaust gasses. Thermal loading is also reduced due to shorter more efficient burning period for the fuel leading to less exacting cylinder conditions. A turbocharger is made up of two main sections: the turbine and the compressor.
The turbine consists of the turbine wheel and the turbine housing. The turbine housing guides the exhaust gas into the turbine wheel. The energy from the exhaust gas turns the turbine wheel, and the gas then exits the turbine housing through an exhaust outlet area.
The compressor consists of the compressor wheel and the compressor housing. The compressor wheel is attached to the turbine by a forged steel shaft, and as the turbine turns the compressor wheel, the high-velocity spinning draws in air and compresses it. The compressor housing then converts the high-velocity, low-pressure air stream into a high-pressure, low-velocity air stream through a process called diffusion. The compressed air is pushed into the engine, allowing the engine to burn more fuel to produce more power.
The power for a turbocharger comes from the exhaust gas stream of the engine but this can be done in one of two ways – pulse or constant pressure. In a multi-cylinder engine each exhaust stroke will result in a new injection of exhaust gas into the system. This can either be fed directly to the turbocharger or to a collecting chamber. When the exhaust valve is opened and the cylinder contents expelled, it is under high pressure and if fed directly to the turbocharger through a pipe of sufficient bore to maintain that pressure and deliver high energy to the turbocharger.
In pulse turbocharging each cylinder is exhausted in turn and the turbocharger will receive regular pulses of energy. Each cylinder will either have a direct feed to the turbocharger or the cylinders will be grouped into sets each having its own exhaust pipe or lead and sharing a final pipe. If grouped into sets, the appropriate cylinders will be selected so that there will be no interference with the scavenging of cylinders caused by blowback of gases from one cylinder to another when one is exhausted. Successful pulse turbocharging requires that the leads should be as short and as straight as possible and of a small bore to prevent energy dissipation. This method of operation is very responsive to changes in the engine speed and in theory provides for better scavenging. It also permits for multiple turbochargers to be used.
In a constant pressure system, there is only one lead to the turbocharger and all of the cylinders are connected to a single exhaust manifold. Some of the energy is lost through dissipation due to larger diameter but it does mean that there is a constant pressure from the manifold to the turbocharger. This type of system is best suited to high power output engines as the energy dissipation is of less consequence. It is a simpler system and therefore should require less maintenance and there also other advantages. It provides a higher turbine efficiency overall and the number of turbochargers required can be reduced. Since the pipes need not be so short or straight, there is more flexibility in turbocharger placement.
Constant pressure turbocharging allows for higher efficiency at normal engine speeds. On the downside, constant pressure is less efficient at part loads as there can be insufficient energy to run the turbocharger. The turbocharger is also less responsive to changing engine conditions because the volume of gas in the manifold will increase or reduce slower when changing engine speeds.
Aside from there being two methods of operation, there are two basic variations of turbochargers which relate to the direction of gas flow to the turbine rotor. In a radial flow turbocharger, the exhaust gas enters from the side and flows out along the axis of the shaft. In an axial flow turbocharger, the exhaust gas enters and leaves along the axial direction. Radial flow turbochargers are more suited to smaller high-speed engines while the axial flow turbochargers are designed for more powerful low-speed engines. There is however a considerable overlap area in between where either type may be employed.
The demands of environmental regulations on modern engines with regard to both emissions and efficiency is spurring development of turbochargers. Variable turbine geometry and two-stage turbocharging have emerged in recent years.