Manifold vacuum

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Not to be confused with Vacuum manifold. ‹See Tfd›



Manifold vacuum should not be confused with venturi vacuum, which is an effect exploited in carburetors to establish a pressure difference roughly proportional to mass airflow, and a roughly constant air/fuel ratio. It is also used in light airplanes to provide airflow for air-driven gyroscopic instruments.



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 (engine speed -RPM- in a vehicle 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 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. Superchargers and turbochargers boost manifold pressure above atmospheric pressure.

Modern developments[edit]

Modern engines use a MaP sensor to measure air pressure in the intake manifold. Manifold Pressure is one of a multitude of parameters used by the ECU to optimize engine operation.

In the past, this sensor was sometimes referred to as a Manifold Absolute Pressure (MAP) sensor. This terminology is considered obsolete because only absolute pressure is meaningful in an engine-control context and it is unnecessary (and potentially confusing) to specify "absolute". This is especially true in boosted (turbocharged or supercharged) engines, whose manifold pressures are quite independent of ambient pressure and may be either greater or less than ambient; and aircraft engines, which operate at a wide variety of ambient pressures.

Motivated by government regulations mandating reduction of fuel consumption (in the USA) or reduction of carbon dioxide emissions, (in Europe) passenger cars and light trucks have been fitted with a variety of technologies (downsized engines; lockup, multi-ratio and overdrive transmissions; variable valve timing, boost, Diesel engines, et al.) which render manifold vacuum inadequate or unavailable. Electric vacuum pumps are now commonly used for powering pneumatic accessories.

Manifold vacuum vs. venturi vacuum[edit]

Manifold vacuum is caused by a different phenomenon 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).


Manifold vacuum in cars[edit]





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).

Uses of manifold vacuum[edit]


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 (valves or piston rings) 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.

Prior to the introduction of Federal Motor Vehicle Safety Standards by the National Traffic and Motor Vehicle Safety Act of 1966, it was common to use manifold vacuum to drive windscreen wipers with a pneumatic motor. This system was cheap & simple but resulted in the comical (and unsafe) effect of wipers which operate at full speed while the engine idles, operate at half speed while cruising, and stop altogether when the driver puts the pedal to the metal. Vehicle HVAC systems also used manifold vacuum to drive actuators controlling airflow and temperature.


Manifold vacuum in diesel engines[edit]






See also[edit]