An intake (also inlet) is an opening or structure through which a fluid is admitted to a space or machine as a consequence of a pressure differential between the outside and the inside. The pressure difference may be generated on the inside by a mechanism, or on the outside by ram pressure or hydrostatic pressure. Flow rate through the intake depends on pressure difference, fluid properties, and intake geometry. An intake can be an outlet at the same time – the intake for a hydro-electric generator turbine may be an outlet for a dam, where the turbine is inside relative to the dam, and the dam is inside relative to the river valley below.
An intake or (for aircraft) inlet is an opening on a car or aircraft body capturing air for operation of an internal combustion engine.
The internal combustion engine functions as an air pump during the intake and exhaust strokes, and like the exhaust system on an engine, the intake system must be carefully engineered and tuned to provide the greatest efficiency and power. An ideal intake system should increase the velocity of the air until it travels into the combustion chamber, while minimizing turbulence and restriction of flow.
However, in jet aircraft, particularly supersonic aircraft, a function of the intake is to slow and increase the pressure of the air entering the compressor.
Automobile engine intakes
The modern automobile air intake system has three main parts, an air filter, mass flow sensor, and throttle body. Some modern intake systems can be highly complex, and often include specially-designed intake manifolds to optimally distribute air and air/fuel mixture to each cylinder. Many cars today now include a silencer to minimize the noise entering the cabin. Silencers impede air flow and create turbulence which reduce total power, so performance enthusiasts often remove them.
All the above is usually accomplished by flow testing on a flow bench in the port design stage. Cars with turbochargers or superchargers which provide pressurized air to the engine usually have highly refined intake systems to improve performance dramatically.
Production cars have specific-length air intakes to cause the air to vibrate and buffet at a specific frequency to assist air flow into the combustion chamber. Aftermarket companies for cars have introduced larger throttle bodies and air filters to decrease restriction of flow at the cost of changing the harmonics of the air intake for a small net increase in power or torque.
With the development of jet engines and the subsequent ability of aircraft to travel at supersonic speeds, it was necessary to design inlets to provide the flow required by the engine over a wide operating envelope and to provide air with a high pressure recovery and low distortion. These designs became more complex as aircraft speeds increased to Mach 3.0 and Mach 3.2, design points for the XB-70 and SR-71 respectively. The inlet is part of the fuselage or part of the nacelle.
The air intakes for aircraft jet engines operating in the trans-sonic or supersonic regimes may have variable geometry to improve efficiency.