Manifold vacuum
From Wikipedia, the free 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 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 carburetors to achieve a fixed mix ratio between fuel and air.
The rate of airflow through an internal combustion engine determines the amount of power the engine generates, and most engines are controlled by limiting that flow with a throttle that restricts intake airflow. 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 engines 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 compression 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. Superchargers and turbochargers can "boost" manifold pressure to above atmospheric pressure, usually by a maximum of 0.7 bar.
Some modern engines using the Atkinson cycle rely on variable valve timing to regulate mass flow through the engine, and hence have no throttle and no manifold vacuum at all. In those engines, mass flow is regulated by phasing the intake valve to change the effective displacement of the engine.
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[edit] Manifold vacuum vs. venturi vacuum
Manifold vacuum is caused by a different effect than venturi vacuum, which is present inside carburetors. 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).
[edit] Manifold vacuum in cars
Most automobiles use four-stroke otto cycle engines with multiple cylinders 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. This sucks in air through the intake manifold and carburetor or fuel injection system, where it is mixed with fuel. Because multiple cylinders suck on the manifold at different times in the engine cycle, there is almost constant suction through the inlet manifold from carburetor to engine.
To control the amount 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 intermediate throttle, the throttle is closed and the engine pumps the air out of the intake manifold as fast as it can leak in through the throttle. 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.
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 due to the fact that 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 closing the exhaust with a valve on the over-run, which is often done on large trucks (see jake brake).
[edit] 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 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. Petrol engine 4x4 vehicles or older petrol-engine trucks used for towing sometimes have a vacuum tank fitted to allow vacuum to be available at all times to provide power to run trailer-mounted braking systems.
Some cars built before the 1960s used manifold vacuum to drive windscreen wipers via a small piston and valve arrangement connected to the wiper mechanism. This system was simple and reliable, but meant that the speed of the wipers changed often- speeding up as the car went downhill and slowing down or stopping as the car accelerated.
[edit] Manifold vacuum in diesel engines
Many diesel engines 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 valve 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 an exhauster must be added to power the brake servo. Valvetronic requires very high modulus valve springs and results in a heavy valvetrain, so despite its advantages in fuel economy, it is currently unsuitable for high-revving engines.
