Manifold vacuum
Encyclopedia
Manifold 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
.
Manifold vacuum is an effect of a piston's movement on the induction stroke and the choked flow
through a throttle
in the intake manifold of an engine. It is a measure of the amount of restriction of airflow through the engine, and hence of the unused power capacity in the engine. In some engines, the manifold vacuum is also used as an auxiliary power source
to drive engine accessories. Manifold vacuum should not be confused with venturi vacuum
, which is an effect that is exploited in carburetor
s to achieve a fixed mix ratio between fuel and air.
that restricts intake airflow, while a diesel engine
is controlled by the amount of fuel supplied to the cylinder, and so has no "throttle" as such. Manifold vacuum is present in all naturally aspirated engines that use throttles (including carbureted
and fuel injected
gasoline engines using the otto cycle
or the two-stroke cycle; diesel engine
s do not have throttle plates).
The mass flow through the engine is determined by the rotation rate of the engine, multiplied by the displacement
of the engine, and the density of the intake stream in the intake manifold. In most applications the rotation rate is set by the application (road speed in a car
or machinery speed in other applications). The displacement is dependent on the engine geometry, which is generally not adjustable while the engine is in use (although a handful of models do have this feature, see variable displacement
). Restricting the input flow reduces the density (and hence pressure) in the intake manifold, reducing the amount of power that is produced. It is also a major source of engine drag (see engine braking
), as the engine must pump material from the low-pressure intake manifold into the exhaust manifold (at ambient atmospheric pressure).
When the throttle is opened (in a car, the accelerator pedal is depressed), ambient air is free to fill the intake manifold, increasing the pressure (filling the vacuum). A carburetor
or fuel injection
system adds fuel to the airflow in the correct proportion, providing energy to the engine. When the throttle is opened all the way, the engine's air induction system is exposed to full atmospheric pressure, and maximum airflow through the engine is achieved. In a "naturally aspirated" engine, total engine output is thus determined by the ambient barometric pressure. Supercharger
s and turbocharger
s can "boost" manifold pressure to above atmospheric pressure.
which measures the manifold absolute pressure to control fuel flow.
s. Venturi vacuum is caused by the venturi effect
and depends on the total mass flow through the carburetor. In engines that use carburetors, venturi vacuum is proportional to the total mass flow through the engine (and hence the total power output).
Manifold vacuum may also be "ported" where the opening is placed so it is normally above the throttle plate at idle, but as the butterfly valve opens, the opening is effectively below it, and the opening sees nearly the full manifold vacuum. Ported vacuum is often used for distributors and emissions items.
s use four-stroke Otto cycle
engines with multiple cylinder
s attached to a single intake manifold. During the induction stroke, the piston
descends in the cylinder and the intake valve is open. As the piston descends it effectively increases the volume in the cylinder above it, setting up low pressure. Atmospheric pressure pushes air through the intake manifold and carburetor
or fuel injection system
, where it is mixed with fuel. Because multiple cylinders operate at different times in the engine cycle, there is almost constant pressure difference through the inlet manifold from carburetor to engine.
To control the amount of fuel/air mix entering the engine, a simple butterfly valve
(the throttle
) is generally fitted at the start of the intake manifold (just below the carburetor
in carbureted engines). The butterfly valve is simply a circular disc fitted on a spindle, fitting inside the pipe work. It is connected to the accelerator pedal of the car, and is set to be fully open when the pedal is fully depressed and fully closed when the pedal is released. The butterfly valve often contains a small "idle cutout", a hole that allows small amounts of fuel/air mixture into the engine even when the valve is fully closed.
If the engine is operating under light or no load and low throttle, there is high manifold vacuum. As the throttle is opened, the engine speed increases rapidly. The engine speed is limited only by the amount of fuel/air mixture that is available in the manifold. Under full throttle and light load, other effects (such as valve float
, turbulence
in the cylinders, or ignition timing
) limit engine speed so that the manifold pressure can increase -- but in practice, parasitic drag
on the internal walls of the manifold, plus the restrictive nature of the venturi at the heart of the carburetor, means that a low pressure will always be set up as the engine's internal volume exceeds the amount of the air the manifold is capable of delivering.
If the engine is operating under heavy load at wide throttle openings (such as accelerating from a stop or pulling the car up a hill) then engine speed is limited by the load and minimal vacuum will be created. Engine speed is low but the butterfly valve is fully open. Since the pistons are descending more slowly than under no load, the pressure differences are less marked and parasitic drag in the induction system is negligible. The engine pulls air into the cylinders at the full ambient pressure.
More vacuum is created in some situations. On deceleration or when descending a hill, the throttle will be closed and a low gear selected to control speed. The engine will be rotating fast because the road wheels and transmission are moving quickly, but the butterfly valve will be fully closed. The flow of air through the engine is strongly restricted by the throttle, producing a strong vacuum on the engine side of the butterfly valve which will tend to limit the speed of the engine. This phenomenon, known as compression braking, is often used in engine braking
to prevent acceleration or even to slow down with minimal or no brake usage (as when descending a long or steep hill). Note that although "compression braking" and "engine braking" are sometimes used to describe the same thing, "compression braking" here refers to the phenomenon itself while "engine braking" refers to the driver's usage of the phenomenon. Compression braking can be greatly increased by opening the cylinders to the exhaust with a valve on downstroke while in overrun
, which is often done on large trucks (see Jake brake
).
or piston ring
s) are worn, preventing good pumping action by the engine and reducing overall efficiency.
Vacuum is often used to drive auxiliary systems
on the vehicle. Vacuum-assist brake servos, for example, use atmospheric pressure pressing against the engine manifold vacuum to increase pressure on the brakes. Since braking is nearly always accompanied by the closing of the throttle and associated high manifold vacuum, this system is simple and almost foolproof. In fact, if the throttle is gradually opened while the brakes are depressed, the driver would feel the back pressure pushing against the brake pedal. Vacuum tanks were installed on trailers to control their integrated braking systems.
Some AMC cars up into the 1970s and cars built before the 1960s used manifold vacuum to drive windscreen wiper
s via a small piston and valve arrangement connected to the wiper mechanism. This system was simple and reliable and crucially did not place any extra load on the vehicle's electrical system- cars of the period were fitted with dynamo
s that would often fail to provide sufficient power for lights, heaters, wipers etc. as well as supplying charge to the battery and power to the ignition system if all were required at once. Powering the wipers by vacuum means that the speed of the wipers inversely corresponds to engine load. Hence under certain conditions (such as when the vehicle was climbing a hill at full throttle), little or no vacuum was generated and the wiper system would either stop all together or slow to the point of being unusable. Air conditioning systems also use(d) manifold vacuum to drive the valves controlling air flow and heating.
Another obsolete accessory is the "Autovac" fuel lifter which uses vacuum to raise fuel from the main tank to a small auxiliary tank, from which it flows by gravity to the carburettor. This eliminated the fuel pump which, in early cars, was an unreliable item.
s do not have butterfly valve throttles. The manifold is connected directly to the air intake and the only suction created is that caused by the descending piston with no venturi to increase it, and the engine power is controlled by varying the amount of fuel that is injected into the cylinder by a fuel injection
system. This assists in making diesels much more efficient than petrol engines.
If vacuum is required (vehicles that can be fitted with both petrol and diesel engines often have systems requiring it), a butterfly valve connected to the throttle can be fitted to the manifold. This reduces efficiency and is still not as effective as it is not connected to a venturi. Since low-pressure is only created on the over-run (such as when descending hills with a closed throttle), not over a wide range of situations as in a petrol engine, a vacuum tank is fitted.
Most diesel engines now have a separate vacuum pump ("exhauster") fitted to provide vacuum at all times, at all engine speeds.
Many new BMW
petrol engines do not use a throttle in normal running, but instead use "Valvetronic
" variable-lift intake valves to control the amount of air entering the engine. Like a diesel engine, manifold vacuum is practically non-existent in these engines and a different source must be utilised to power the brake servo.
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
in an internal combustion engine is the difference in air pressure between the engine's intake manifold
Inlet manifold
In automotive engineering, an inlet manifold or intake manifold is the part of an engine that supplies the fuel/air mixture to the cylinders...
and Earth's atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
.
Manifold vacuum is an effect of a piston's movement on the induction stroke and the choked flow
Choked flow
Choked flow is a compressible flow effect. The parameter that becomes "choked" or "limited" is the velocity or the mass flow rate.Choked flow is a fluid dynamic condition associated with the Venturi effect...
through a throttle
Throttle
A throttle is the mechanism by which the flow of a fluid is managed by constriction or obstruction. An engine's power can be increased or decreased by the restriction of inlet gases , but usually decreased. The term throttle has come to refer, informally and incorrectly, to any mechanism by which...
in the intake manifold of an engine. It is a measure of the amount of restriction of airflow through the engine, and hence of the unused power capacity in the engine. In some engines, the manifold vacuum is also used as an auxiliary power source
Automobile ancillary power
]Automobile accessory power can be transferred by several different means. However, it is always ultimately derived from the automobile's internal combustion engine, battery, or other "prime mover" source of energy...
to drive engine accessories. Manifold vacuum should not be confused with venturi vacuum
Venturi effect
The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section of pipe. The Venturi effect is named after Giovanni Battista Venturi , an Italian physicist.-Background:...
, which is an effect that is exploited in carburetor
Carburetor
A 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....
s to achieve a fixed mix ratio between fuel and air.
Overview
The rate of airflow through an internal combustion engine is an important factor determining the amount of power the engine generates. Most gasoline engines are controlled by limiting that flow with a throttleThrottle
A throttle is the mechanism by which the flow of a fluid is managed by constriction or obstruction. An engine's power can be increased or decreased by the restriction of inlet gases , but usually decreased. The term throttle has come to refer, informally and incorrectly, to any mechanism by which...
that restricts intake airflow, while 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...
is controlled by the amount of fuel supplied to the cylinder, and so has no "throttle" as such. Manifold vacuum is present in all naturally aspirated engines that use throttles (including carbureted
Carburetor
A 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....
and fuel injected
Fuel injection
Fuel 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....
gasoline engines using the 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....
or the two-stroke cycle; 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...
s do not have throttle plates).
The mass flow through the engine is determined by the rotation rate of the engine, multiplied by the displacement
Engine displacement
Engine displacement is the volume swept by all the pistons inside the cylinders of an internal combustion engine in a single movement from top dead centre to bottom dead centre . It is commonly specified in cubic centimeters , litres , or cubic inches...
of the engine, and the density of the intake stream in the intake manifold. In most applications the rotation rate is set by the application (road speed in a car
Čar
Čar is a village in the municipality of Bujanovac, Serbia. According to the 2002 census, the town has a population of 296 people.-References:...
or machinery speed in other applications). The displacement is dependent on the engine geometry, which is generally not adjustable while the engine is in use (although a handful of models do have this feature, see variable displacement
Variable displacement
Variable displacement is an automobile engine technology that allows the engine displacement to change, usually by deactivating cylinders, for improved fuel economy. The technology is primarily used in large, multi-cylinder engines...
). Restricting the input flow reduces the density (and hence pressure) in the intake manifold, reducing the amount of power that is produced. It is also a major source of engine drag (see engine braking
Engine braking
Engine braking is where the retarding forces within an engine are used to slow a vehicle down, as opposed to using an external braking mechanism, for example friction brakes or magnetic brakes....
), as the engine must pump material from the low-pressure intake manifold into the exhaust manifold (at ambient atmospheric pressure).
When the throttle is opened (in a car, the accelerator pedal is depressed), ambient air is free to fill the intake manifold, increasing the pressure (filling the vacuum). A carburetor
Carburetor
A 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....
or fuel injection
Fuel injection
Fuel 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....
system adds fuel to the airflow in the correct proportion, providing energy to the engine. When the throttle is opened all the way, the engine's air induction system is exposed to full atmospheric pressure, and maximum airflow through the engine is achieved. In a "naturally aspirated" engine, total engine output is thus determined by the ambient barometric pressure. 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,...
s and 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...
s can "boost" manifold pressure to above atmospheric pressure.
Modern developments
Some modern engines with fuel injection have a MAP sensorMAP sensor
The manifold absolute pressure sensor is one of the sensors used in an internal combustion engine's electronic control system. Engines that use a MAP sensor are typically fuel injected. The manifold absolute pressure sensor provides instantaneous manifold pressure information to the engine's...
which measures the manifold absolute pressure to control fuel flow.
Manifold vacuum vs. venturi vacuum
Manifold vacuum is caused by a different effect than venturi vacuum, which is present inside carburetorCarburetor
A 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....
s. Venturi vacuum is caused by the venturi effect
Venturi effect
The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section of pipe. The Venturi effect is named after Giovanni Battista Venturi , an Italian physicist.-Background:...
and depends on the total mass flow through the carburetor. In engines that use carburetors, venturi vacuum is proportional to the total mass flow through the engine (and hence the total power output).
Manifold vacuum may also be "ported" where the opening is placed so it is normally above the throttle plate at idle, but as the butterfly valve opens, the opening is effectively below it, and the opening sees nearly the full manifold vacuum. Ported vacuum is often used for distributors and emissions items.
Manifold vacuum in cars
Most automobileAutomobile
An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...
s use four-stroke 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....
engines with multiple cylinder
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...
s attached to a single intake manifold. During the induction stroke, 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...
descends in the cylinder and the intake valve is open. As the piston descends it effectively increases the volume in the cylinder above it, setting up low pressure. Atmospheric pressure pushes air through the intake manifold and carburetor
Carburetor
A 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....
or fuel injection system
Fuel injection
Fuel 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....
, where it is mixed with fuel. Because multiple cylinders operate at different times in the engine cycle, there is almost constant pressure difference through the inlet manifold from carburetor to engine.
To control the amount of fuel/air mix entering the engine, a simple butterfly valve
Butterfly valve
A butterfly valve is a valve which can be used for isolating or regulating flow. The closing mechanism takes the form of a disk. Operation is similar to that of a ball valve, which allows for quick shut off. Butterfly valves are generally favored because they are lower in cost to other valve...
(the throttle
Throttle
A throttle is the mechanism by which the flow of a fluid is managed by constriction or obstruction. An engine's power can be increased or decreased by the restriction of inlet gases , but usually decreased. The term throttle has come to refer, informally and incorrectly, to any mechanism by which...
) is generally fitted at the start of the intake manifold (just below the carburetor
Carburetor
A 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....
in carbureted engines). The butterfly valve is simply a circular disc fitted on a spindle, fitting inside the pipe work. It is connected to the accelerator pedal of the car, and is set to be fully open when the pedal is fully depressed and fully closed when the pedal is released. The butterfly valve often contains a small "idle cutout", a hole that allows small amounts of fuel/air mixture into the engine even when the valve is fully closed.
If the engine is operating under light or no load and low throttle, there is high manifold vacuum. As the throttle is opened, the engine speed increases rapidly. The engine speed is limited only by the amount of fuel/air mixture that is available in the manifold. Under full throttle and light load, other effects (such as valve float
Valve float
Valve float is an adverse condition which occurs when the poppet valves on an internal combustion engine valvetrain do not remain in contact with the camshaft lobe during the valve closure phase of the cam lobe profile...
, turbulence
Turbulence
In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic and stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time...
in the cylinders, or ignition timing
Ignition timing
Ignition timing, in a spark ignition internal combustion engine , is the process of setting the angle relative to piston position and crankshaft angular velocity that a spark will occur in the combustion chamber near the end of the compression stroke...
) limit engine speed so that the manifold pressure can increase -- but in practice, parasitic drag
Parasitic drag
Parasitic drag is drag caused by moving a solid object through a fluid medium . Parasitic drag is made up of many components, the most prominent being form drag...
on the internal walls of the manifold, plus the restrictive nature of the venturi at the heart of the carburetor, means that a low pressure will always be set up as the engine's internal volume exceeds the amount of the air the manifold is capable of delivering.
If the engine is operating under heavy load at wide throttle openings (such as accelerating from a stop or pulling the car up a hill) then engine speed is limited by the load and minimal vacuum will be created. Engine speed is low but the butterfly valve is fully open. Since the pistons are descending more slowly than under no load, the pressure differences are less marked and parasitic drag in the induction system is negligible. The engine pulls air into the cylinders at the full ambient pressure.
More vacuum is created in some situations. On deceleration or when descending a hill, the throttle will be closed and a low gear selected to control speed. The engine will be rotating fast because the road wheels and transmission are moving quickly, but the butterfly valve will be fully closed. The flow of air through the engine is strongly restricted by the throttle, producing a strong vacuum on the engine side of the butterfly valve which will tend to limit the speed of the engine. This phenomenon, known as compression braking, is often used in engine braking
Engine braking
Engine braking is where the retarding forces within an engine are used to slow a vehicle down, as opposed to using an external braking mechanism, for example friction brakes or magnetic brakes....
to prevent acceleration or even to slow down with minimal or no brake usage (as when descending a long or steep hill). Note that although "compression braking" and "engine braking" are sometimes used to describe the same thing, "compression braking" here refers to the phenomenon itself while "engine braking" refers to the driver's usage of the phenomenon. Compression braking can be greatly increased by opening the cylinders to the exhaust with a valve on downstroke while in overrun
Overrun
Overrun may refer to:* Overrun brake* Overrun, the condition of a vehicle travelling without throttle, see freewheel** Overrunning clutch, see freewheel* Buffer overrun, see buffer overflow...
, which is often done on large trucks (see Jake brake
Jake brake
A compression release engine brake, frequently called a Jake brake or Jacobs brake, is an engine braking mechanism installed on some diesel engines...
).
Uses of manifold vacuum
This low (or negative) pressure can be put to uses. A pressure gauge measuring the manifold pressure can be fitted to give the driver an indication of how hard the engine is working and can be used to achieve maximum momentary fuel efficiency by adjusting driving habits: minimizing manifold vacuum increases momentary efficiency. A weak manifold vacuum under closed-throttle conditions shows that the butterfly valve or internal components of the engine (valvesPoppet valve
A poppet valve is a valve consisting of a hole, usually round or oval, and a tapered plug, usually a disk shape on the end of a shaft also called a valve stem. The shaft guides the plug portion by sliding through a valve guide...
or piston ring
Piston ring
A piston ring is a split ring that fits into a groove on the outer diameter of a piston in a reciprocating engine such as an internal combustion engine or steam engine.The three main functions of piston rings in reciprocating engines are:...
s) are worn, preventing good pumping action by the engine and reducing overall efficiency.
Vacuum is often used to drive auxiliary systems
Automobile ancillary power
]Automobile accessory power can be transferred by several different means. However, it is always ultimately derived from the automobile's internal combustion engine, battery, or other "prime mover" source of energy...
on the vehicle. Vacuum-assist brake servos, for example, use atmospheric pressure pressing against the engine manifold vacuum to increase pressure on the brakes. Since braking is nearly always accompanied by the closing of the throttle and associated high manifold vacuum, this system is simple and almost foolproof. In fact, if the throttle is gradually opened while the brakes are depressed, the driver would feel the back pressure pushing against the brake pedal. Vacuum tanks were installed on trailers to control their integrated braking systems.
Some AMC cars up into the 1970s and cars built before the 1960s used manifold vacuum to drive windscreen wiper
Windscreen wiper
A windscreen wiper or windshield wiper is a device used to remove rain and debris from a windscreen or windshield. Almost all motor vehicles, including trains, aircraft and watercraft, are equipped with such wipers, which are usually a legal requirement.A wiper generally consists of an arm,...
s via a small piston and valve arrangement connected to the wiper mechanism. This system was simple and reliable and crucially did not place any extra load on the vehicle's electrical system- cars of the period were fitted with dynamo
Dynamo
- Engineering :* Dynamo, a magnetic device originally used as an electric generator* Dynamo theory, a theory relating to magnetic fields of celestial bodies* Solar dynamo, the physical process that generates the Sun's magnetic field- Software :...
s that would often fail to provide sufficient power for lights, heaters, wipers etc. as well as supplying charge to the battery and power to the ignition system if all were required at once. Powering the wipers by vacuum means that the speed of the wipers inversely corresponds to engine load. Hence under certain conditions (such as when the vehicle was climbing a hill at full throttle), little or no vacuum was generated and the wiper system would either stop all together or slow to the point of being unusable. Air conditioning systems also use(d) manifold vacuum to drive the valves controlling air flow and heating.
Another obsolete accessory is the "Autovac" fuel lifter which uses vacuum to raise fuel from the main tank to a small auxiliary tank, from which it flows by gravity to the carburettor. This eliminated the fuel pump which, in early cars, was an unreliable item.
Manifold vacuum in diesel engines
Many diesel engineDiesel 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...
s do not have butterfly valve throttles. The manifold is connected directly to the air intake and the only suction created is that caused by the descending piston with no venturi to increase it, and the engine power is controlled by varying the amount of fuel that is injected into the cylinder by a fuel injection
Fuel injection
Fuel 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....
system. This assists in making diesels much more efficient than petrol engines.
If vacuum is required (vehicles that can be fitted with both petrol and diesel engines often have systems requiring it), a butterfly valve connected to the throttle can be fitted to the manifold. This reduces efficiency and is still not as effective as it is not connected to a venturi. Since low-pressure is only created on the over-run (such as when descending hills with a closed throttle), not over a wide range of situations as in a petrol engine, a vacuum tank is fitted.
Most diesel engines now have a separate vacuum pump ("exhauster") fitted to provide vacuum at all times, at all engine speeds.
Many new BMW
BMW
Bayerische Motoren Werke AG is a German automobile, motorcycle and engine manufacturing company founded in 1916. It also owns and produces the Mini marque, and is the parent company of Rolls-Royce Motor Cars. BMW produces motorcycles under BMW Motorrad and Husqvarna brands...
petrol engines do not use a throttle in normal running, but instead use "Valvetronic
Valvetronic
The Valvetronic system is a variable valve timing system to offer continuous and precisely variable intake valve lift, from ~0 to 10 mm, and duration. It typically works in conjunction with the independent Double VANOS system that continuously varies the timing...
" variable-lift intake valves to control the amount of air entering the engine. Like a diesel engine, manifold vacuum is practically non-existent in these engines and a different source must be utilised to power the brake servo.
See also
- Automobile accessory power
- Vacuum delay valveVacuum delay valveA vacuum delay valve is a valve with a small orifice, which delays a vacuum signal. These are commonly used in automobiles to alter the behavior of vacuum switches, vacuum motors, and other vacuum devices....