Naturally-aspirated engine
Encyclopedia
A naturally aspirated engine is one common type of reciprocating piston
internal combustion engine
that depends solely on atmospheric pressure
to counter the partial vacuum in the induction tract to draw in combustion air. This is in contrast to a forced induction
engine, in which a mechanically driven supercharger
, or an exhaust-driven turbocharger
is employed to facilitate in increasing the mass of intake air beyond what could be produced by atmospheric pressure alone.
In a naturally aspirated engine; air for combustion (diesel cycle
in a diesel engine
, or specific types of Otto cycle
in gasoline engines - namely gasoline direct injection
), or an air/fuel mixture (traditional Otto cycle petrol engines) is drawn into the engines cylinders
by atmospheric pressure acting against a partial vacuum that occurs as the piston
travels downwards toward bottom dead centre
during the induction stroke. Owing to innate restriction in the engine's inlet tract which includes the intake manifold, a small pressure drop occurs as air is drawn in, resulting in a volumetric efficiency
of less than 100 percent - and a less than complete air charge in the cylinder. The density of the air charge, and therefore the engine's maximum theoretical power output
, in addition to being influenced by induction system restriction, is also affected by engine speed and atmospheric pressure, the latter which decreases as the operating altitude
increases.
Most automobile
petrol engines, as well as many small engines used for non-automotive purposes, are naturally aspirated. Most modern diesel engines powering highway vehicles are turbocharged to produce a more favourable power-to-weight ratio
, as well as better fuel efficiency
and lower exhaust emissions
. Turbocharging is nearly universal on diesel engines that are used in railroad, marine engine
s, and commercial stationary
applications (electrical power generation, for example). Forced induction is also used with reciprocating aircraft engine
s to negate some of the power loss that occurs as the aircraft
climbs to higher altitudes.
A two-stroke diesel engine is incapable of natural aspiration as defined above. Some method of charging the cylinders with scavenging air must be integrated into the engine design. This is usually achieved with a positive displacement blower driven by the crankshaft
. The blower does not act as a supercharger in this application, as it is sized to produce a volume of air flow that is in direct proportion to engine displacement and speed. A mechanically scavenged two-stroke diesel engine is considered to be naturally aspirated.
Reciprocating engine
A reciprocating engine, also often known as a piston engine, is a heat engine that uses one or more reciprocating pistons to convert pressure into a rotating motion. This article describes the common features of all types...
internal combustion engine
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...
that depends solely on atmospheric pressure
Atmospheric pressure
Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...
to counter the partial vacuum in the induction tract to draw in combustion air. This is in contrast to a forced induction
Forced induction
Forced induction is the process of compressing air on the intake of an internal combustion engine . A forced induction engine uses a gas compressor to increase the pressure, temperature and density of the air...
engine, in which a mechanically driven supercharger
Supercharger
A supercharger is an air compressor used for forced induction of an internal combustion engine.The greater mass flow-rate provides more oxygen to support combustion than would be available in a naturally aspirated engine, which allows more fuel to be burned and more work to be done per cycle,...
, or an exhaust-driven turbocharger
Turbocharger
A turbocharger, or turbo , from the Greek "τύρβη" is a centrifugal compressor powered by a turbine that is driven by an engine's exhaust gases. Its benefit lies with the compressor increasing the mass of air entering the engine , thereby resulting in greater performance...
is employed to facilitate in increasing the mass of intake air beyond what could be produced by atmospheric pressure alone.
In a naturally aspirated engine; air for combustion (diesel cycle
Diesel cycle
The Diesel cycle is the thermodynamic cycle which approximates the pressure and volume of the combustion chamber of the Diesel engine, invented by Rudolph Diesel in 1897. It is assumed to have constant pressure during the first part of the "combustion" phase...
in a diesel engine
Diesel engine
A diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber...
, or specific types of Otto cycle
Otto cycle
An Otto cycle is an idealized thermodynamic cycle which describes the functioning of a typical reciprocating piston engine, the thermodynamic cycle most commonly found in automobile engines....
in gasoline engines - namely gasoline direct injection
Gasoline direct injection
In internal combustion engines, gasoline direct injection , also known as petrol direct injection or direct petrol injection, is a variant of fuel injection employed in modern two-stroke and four-stroke gasoline engines...
), or an air/fuel mixture (traditional Otto cycle petrol engines) is drawn into the engines cylinders
Cylinder (engine)
A cylinder is the central working part of a reciprocating engine or pump, the space in which a piston travels. Multiple cylinders are commonly arranged side by side in a bank, or engine block, which is typically cast from aluminum or cast iron before receiving precision machine work...
by atmospheric pressure acting against a partial vacuum that occurs as the piston
Piston
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from...
travels downwards toward bottom dead centre
Dead centre
In a reciprocating engine, the dead centre is the position of a piston in which it is farthest from, or nearest to, the crankshaft. The former is known as top dead centre while the latter is known as bottom dead centre ....
during the induction stroke. Owing to innate restriction in the engine's inlet tract which includes the intake manifold, a small pressure drop occurs as air is drawn in, resulting in a volumetric efficiency
Volumetric efficiency
Volumetric efficiency in internal combustion engine design refers to the efficiency with which the engine can move the charge into and out of the cylinders. More specifically, volumetric efficiency is a ratio of what quantity of fuel and air actually enters the cylinder during induction to the...
of less than 100 percent - and a less than complete air charge in the cylinder. The density of the air charge, and therefore the engine's maximum theoretical power output
Power (physics)
In physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit...
, in addition to being influenced by induction system restriction, is also affected by engine speed and atmospheric pressure, the latter which decreases as the operating altitude
Altitude
Altitude or height is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often varies according to the context...
increases.
Most automobile
Automobile
An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...
petrol engines, as well as many small engines used for non-automotive purposes, are naturally aspirated. Most modern diesel engines powering highway vehicles are turbocharged to produce a more favourable power-to-weight ratio
Power-to-weight ratio
Power-to-weight ratio is a calculation commonly applied to engines and mobile power sources to enable the comparison of one unit or design to another. Power-to-weight ratio is a measurement of actual performance of any engine or power sources...
, as well as better fuel efficiency
Fuel efficiency
Fuel efficiency is a form of thermal efficiency, meaning the efficiency of a process that converts chemical potential energy contained in a carrier fuel into kinetic energy or work. Overall fuel efficiency may vary per device, which in turn may vary per application, and this spectrum of variance is...
and lower exhaust emissions
Automobile emissions control
Vehicle emissions control is the study and practice of reducing the motor vehicle emissions -- emissions produced by motor vehicles, especially internal combustion engines....
. Turbocharging is nearly universal on diesel engines that are used in railroad, marine engine
Marine engine
A marine engine is an engine that propels a ship or boat. Types of marine engine include:*Marine steam engine*Petrol engine or gasoline engine*Diesel engine*Steam turbine*Gas turbine-See also:*Marine propulsion*Engine room*Marine automobile engine...
s, and commercial stationary
Stationary
Stationary can mean:* In statistics and probability: a stationary process.* In mathematics: a stationary point.* In mathematics: a stationary set.* In physics: a time-invariant quantity, such as a constant position or temperature....
applications (electrical power generation, for example). Forced induction is also used with reciprocating aircraft engine
Aircraft engine
An aircraft engine is the component of the propulsion system for an aircraft that generates mechanical power. Aircraft engines are almost always either lightweight piston engines or gas turbines...
s to negate some of the power loss that occurs as the aircraft
Aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...
climbs to higher altitudes.
A two-stroke diesel engine is incapable of natural aspiration as defined above. Some method of charging the cylinders with scavenging air must be integrated into the engine design. This is usually achieved with a positive displacement blower driven by the crankshaft
Crankshaft
The crankshaft, sometimes casually abbreviated to crank, is the part of an engine which translates reciprocating linear piston motion into rotation...
. The blower does not act as a supercharger in this application, as it is sized to produce a volume of air flow that is in direct proportion to engine displacement and speed. A mechanically scavenged two-stroke diesel engine is considered to be naturally aspirated.
See also
- CarburetorCarburetorA carburetor , carburettor, or carburetter is a device that blends air and fuel for an internal combustion engine. It is sometimes shortened to carb in North America and the United Kingdom....
- Fuel injectionFuel injectionFuel injection is a system for admitting fuel into an internal combustion engine. It has become the primary fuel delivery system used in automotive petrol engines, having almost completely replaced carburetors in the late 1980s....
- Manifold vacuumManifold vacuumManifold vacuum, or engine vacuum in an internal combustion engine is the difference in air pressure between the engine's intake manifold and Earth's atmosphere....
- Twin-turboTwin-turboTwin-turbo refers to a turbocharged engine, in which two turbochargers compress the intake charge. There are two commonly used twin turbo configurations: parallel twin-turbo and sequential twin-turbo...
- TurbodieselTurbodieselTurbodiesel refers to any diesel engine with a turbocharger. Turbocharging is the norm rather than the exception in modern car and truck diesel engines...