Category: Term of the day
A turbocharger is an exhaust gas-driven compressor used to increase the power output of an internal-combustion engine by compressing air that is entering the engine thus increasing the amount of available oxygen. A key advantage of turbochargers is that they offer a considerable increase in engine power with only a slight increase in weight.
A turbocharger is a dynamic compressor, in which air or gas is compressed by the mechanical action of impellers, vaned rotors which are spun using the kinetic movement of air, imparting velocity and pressure to the flowing medium.
The mechanical concept turbocharger revolves around three main parts. A turbine is driven by the exhaust gas from a pump, most often an internal combustion engine, to spin an impeller whose function is to force more air into the pump's intake, or air supply. The third basic part is a center hub rotating assembly (CHRA) which contains bearing, lubrication, cooling, and a shaft that directly connects the turbine and impeller. The shaft, bearing, impeller, and turbine can rotate at speeds in the tens or hundreds of thousands of RPM (revolutions per minute).
The lubrication system can be either a closed system or be fed from the engine's oil supply. The lubrication system may double as the cooling system, or separate coolant may be pumped through the center housing from an outside source. Oil lubrication and water cooling using engine oil and engine coolant are commonplace in automotive applications.
The turbine and impeller are each contained within their own folded conical housing on opposite sides of the center hub rotating assembly. These housings collect and direct the gas flow. The size and shape can dictate some performance characteristics of the overall turbocharger. The area of the cone to radius from center hub is expressed as a ratio (AR, A/R, or A:R). 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 as well. This allows the designer of the engine system to tailor the compromises between performance, response, and efficiency to application or preference. Both housings resemble snail shells, and thus turbochargers are sometimes referred to in slang as snails.
By spinning at a relatively high speed the compressor turbine draws in a large volume of air and forces it into the engine. As the turbocharger's output flow volume exceeds the engine's volumetric flow, air pressure in the intake system begins to build, often called boost. The speed at which the assembly spins is proportional to the pressure of the compressed air and total mass of air flow being moved. Since a turbo will spin to RPMs far beyond what is needed or of what it is mechanically capable of, the speed must be controlled, and thus is also the property used to set the desired compression pressure. A wastegate is the most common mechanical control system and is often further augmented by an electronic boost controller.
The implementation 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 automobile 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 determine 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.
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 heat 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.