Manifold vacuum is a phenomenon found in internal combustion engines burning petroleum on the Otto Cycle.
During the induction stroke , the piston descends in the cylinder and the intake valve is open. As the piston descends it effectivly increases the volume in the cylinder above it, setting up low pressure. This sucks in air via the carburettor, where it is mixed with fuel.
The carburettor needs to be mounted on, and connected to, the engine. In a simple single-cylinder engine, the carburetor can be mounted on a simple pipe. A multi-cylinder engine will require something more complex if all cylinders are to use a single carburettor so a casting where the outlet from the 'carb' branches into multiple pipes, one to each cylinder is used. This is the inlet manifold. A similar arrangement is employed for channelling exhaust gases out of the engine in the form of the exhaust manifold, where multiple pipes merge into a single one.
In a multi-cylinder engine (especially a 4-cylinder 4-stroke) there is almost constant suction through the Inlet Manifold from carburettor to engine.
To control the fuel/air mix entering the engine, a simple butterfly valve is fitted in the carburretor or at the start of the Inelt Manifold. This is simply a circular disc fitted on a spindle, fittin inside the pipework. It is connected to the throttle and via a linkage, is set to be fully open when the engine is at full throttle, and fully closed when the engine is at idle. In the latter situation the engine would be strangled of air, so a small cut-out is usually part of the butterfly valve to allow a limited amount of air into the engine.
Creating Manifold Vacuum
If the engine is operating under no load, there should be no Manifold Vacuum. As the throttle is opened, the butterfly valve opens as well and engine speed increases. The engine is pulling more air through the Inlet Manifold and the opening butterfly valve is providing more space for this air to flow through. In pratice, parasitic drag on the internal walls of the manifold, plus the restrictive nature of the venturi at the heart of the carburretor 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.
Likewise, if the engine is operating under heavy load at wide throttle openings (such as towing a heavy load up a steep hill) minimal vacuum will be created. Engine speed is low but the butterfly valve is fully open. Since the pistons are decending more slowly, the pressure differences are less marked and less air is drawn into the engine along the manifold. The manifold is actually delivering more air than the engine needs, so often there is no pressure differential at all, or a positive one.
Vacuum is created in some situations. If our engine towing up a hill is now descending the 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 it is connected to are under the influence of gravity, but the butterfly valve will be fulyl closed. The pistons will be drawing in a lot of air, but the valve will block its path, only allowing a small amount through. This sets up a vacuum on the engine side of the butterfly valve.
Uses of Manifold Vacuum
This low (or negative) pressure can be put to uses. A 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 fuel efficency- shown by either a positive pressure or no pressure difference at all. A gauge reluctant to show a vacuum shows that carburettor is out of adjustment, the butterfly valve is worn or the internal components of the engine are worn, preventing the piston sealing against the cylinder and drawing the maximum amount of air.
Vacuum is often used to drive other systems on the vehicle. Brake servos are the most common system, with a pipe running from brake servo to just down-stream of the butterfly valve. Since braking is nearly always accompanied by the closing of the throttle, this system is simple and almost fool-proof. Petrol engined 4x4 vehicles or older petrol-engined trucks used for towing sometimes have a vacuum tank fitted to allow vacuum to be available at all time 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.
The butterfly valve that creates Manifold Vacuum has its disadvantages. Whilst allowing high levels of efficency to be achieved under low-speed, high-throttle situations, when the engine is under little or no load, or at idle (such as in traffic), the restriction of the intake manifold creates huge parasitic drag on the airflow, slowing it down and creating turbulence. This reduces the engines efficency.
Manifold Vacuum in Diesel Engines
Diesel engines do not have carburretors and so do not have butterfly valves. The manifold is connected directly to the air intake and the only suction created is that caused by the decending piston with no venturi to increase it. This assists in making diesels much more efficent than petrol engines.
If vacuum is required (vehicles that can be fitted with both petrol and diesel engines often have system requring it), a butterfly valve connected to the throttle can be fitted to the Manifold. This reduces efficency 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 seperate vacuum pump fitted to provide vacuum at all times, at all engine speeds. Some modern petrol engines fitted with fuel injection (and lacking carburretors) also have pumps fitted.
