Manifold vacuum
Manifold
Manifold vacuums should not be confused with
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
do not have throttle plates).The
When the throttle is opened (in a car, the
manifold pressure above atmospheric pressure.Modern developments
Modern engines use a manifold absolute pressure (abbreviated as MAP) sensor to measure air pressure in the intake manifold. Manifold absolute pressure is one of a multitude of parameters used by the engine control unit (ECU) to optimize engine operation. It is important to differentiate between absolute and gauge pressure when dealing with certain applications, particularly those that experience changes in elevation during normal operation.
Motivated by government regulations mandating reduction of fuel consumption (in the USA) or reduction of
Manifold vacuum vs. venturi vacuum
Manifold vacuum is caused by a different phenomenon than venturi vacuum, which is present inside carburetors. Venturi vacuum is caused by the venturi effect which, for fixed ambient conditions (air density and temperature), depends on the total mass flow through the carburetor. In engines that use carburetors, the venturi vacuum is approximately proportional to the total mass flow through the engine (and hence the total power output). As ambient pressure (altitude, weather) or temperature change, the carburetor may need to be adjusted to maintain this relationship.
Manifold pressure may also be "ported". Porting is selecting a location for the pressure tap within the throttle plate's range of motion. Depending on throttle position, a ported pressure tap may be either upstream or downstream of the throttle. As the throttle position changes, a "ported" pressure tap is selectively connected to either manifold pressure or ambient pressure. Older (pre-
Manifold vacuum in cars
Most
To control the amount of fuel/air mix entering the engine, a simple butterfly valve (throttle plate) 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 pressed 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, or the carburetor has a separate air bypass with its own idle jet.
If the engine is operating under light or no load and low or closed 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
Uses of manifold vacuum
This low (or negative) pressure can be put to use. A
Vacuum used to be a common way to drive auxiliary systems on the vehicle. Vacuum systems tend to be unreliable with age as the vacuum tubing becomes brittle and susceptible to leaks.
Before 1960
- Windshield wiper motors - Prior to the introduction of Federal Motor Vehicle Safety Standards in the USA 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 and simple but resulted in the comical yet unsafe effect of wipers which operate at full speed while the engine idles, operate around half speed while cruising, and stop altogether when the driver depresses the pedal fully.
- Power lock motors
- "Autovac" fuel lifter,[1] which uses vacuum to raise fuel from the main tank to a small auxiliary tank, from which it flows by gravity to the carburetor. This eliminated the fuel pump which, in early cars, was an unreliable item.
1960–1990
Automotive vacuum systems reached their height of use between the 1960s and 1980s. During this time a huge variety of vacuum switches, delay valves and accessory devices were created. As an example, a 1967 Ford Thunderbird used vacuum for:
- foolproof. Vacuum tanks were installed on trailers to control their integrated braking systems.
- Transmissionshift control
- Doors for the hidden headlamps
- Remote trunk latch release
- Power door locks
- HVAC air routing - Vehicle HVACsystems used manifold vacuum to drive actuators controlling airflow and temperature.
- Control of the heater core valve
- Rear cabin vent control
- Tilt-away steering wheel release
Other items that can be powered by vacuum include:
- Exhaust gas recirculation solenoid
- Power steering pump
- Fuel pressure regulator
Modern usage
Modern cars have a minimal amount of accessories that use vacuum. Many accessories previously driven by vacuum have been replaced by electronic accessories. Some modern accessories that sometimes use vacuum include:
- Vacuum-assist brake servos
- Positive crankcase ventilationvalve
- Charcoal canister
- HVAC air routing
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 overrun (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.
References
See also
Part of the Automobile series | |
Engine block and rotating assembly | |
Valvetrain and Cylinder head | |
Forced induction | |
Fuel system | |
Ignition |
|
Engine management | |
Electrical system | |
Intake system | |
Exhaust system | |
Cooling system | |
Lubrication | |
Other | |
Aircraft piston engine components, systems and terminology | |||||||
---|---|---|---|---|---|---|---|
Piston engines |
| ||||||
Propellers |
| ||||||
Engine instruments |
| ||||||
Engine controls | |||||||
Fuel and induction system | |||||||
Other systems |