Rajan Wilcox
The vacuum fuel system, also known as the manifold vacuum, is an essential component of antique cars with petrol engines. It is caused by the difference in air pressure between the engine's intake manifold and Earth's atmosphere, resulting from the piston's movement and airflow through the throttle. This system has been used in various accessories, including power door lock motors and fuel lifters, and was most popular between the 1960s and 1980s. In this context, the vacuum is utilised to draw fuel from the main tank to an auxiliary tank, from which the fuel flows by gravity to the carburettor, eliminating the need for an early car's unreliable fuel pump.
| Characteristics | Values |
|---|---|
| Working principle | The difference in air pressure between the engine's intake manifold and Earth's atmosphere |
| Components | Carburetor or fuel injection system, throttle, accelerator pedal, intake manifold, manifold absolute pressure (MAP) sensor, engine control unit (ECU) |
| Function | Provides energy to the engine by adding fuel to the airflow in the correct proportion |
| Advantages | Cheap and simple; eliminates the need for a fuel pump |
| Peak usage | 1960s to 1980s |
| Modern usage | Minimal; mostly replaced by electronic accessories |
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What You'll Learn
The vacuum fuel system's peak
The manifold vacuum, or engine vacuum in a petrol engine, is the pressure difference between the air inside the engine's intake manifold and the Earth's atmosphere. This pressure difference is caused by the movement of the piston and the airflow through the throttle, which is restricted by the carburetor or throttle body. When the throttle is opened, ambient air fills the intake manifold, increasing the pressure and reducing the manifold vacuum.
The manifold vacuum is used to power engine accessories and the crankcase ventilation system. It is also used in light airplanes to provide airflow for pneumatic gyroscopic instruments. In cars, the manifold vacuum is utilized for the brake servos, creating a simple and reliable system where braking increases the manifold vacuum, which in turn increases pressure on the brakes.
The "Autovac" fuel lifter is another example of a vacuum fuel system. It uses vacuum to raise fuel from the main tank to an auxiliary tank, from which the fuel flows by gravity to the carburetor. This design eliminates the need for a fuel pump, which was an unreliable component in early automobiles.
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How it differs from a fuel pump
A vacuum fuel system in antique cars uses engine suction to pull fuel into the engine, whereas a fuel pump system uses a mechanical or electric pump to draw fuel out of the tank and push it through a pipe to the carburettor.
In a fuel pump system, the pump may be mechanical and worked by the engine, or it may be electric, in which case it is usually placed next to or inside the fuel tank. The fuel tank is always placed at the opposite end of the car from the engine for safety reasons. Inside the tank, a float operates an electrical sender unit that transmits current to the fuel gauge, indicating the amount of fuel in the tank.
The tank also has an air vent, usually a pipe or a small hole in the filler cap, to allow air in as the tank empties. Some modern systems have a carbon filter to prevent fuel fumes from escaping. In a mechanical pump, the actuating lever moves up and down constantly but pulls the diaphragm down only when necessary to refill the pump chamber. The return spring then pushes the diaphragm up to deliver fuel to the carburettor.
An electric pump has a similar diaphragm mechanism, worked by a rod drawn into a solenoid switch, which opens a set of contacts to turn off the current. The solenoid attracts an iron rod that pulls the diaphragm down, drawing fuel into the chamber. When the iron rod reaches the end of its travel, it forces a set of contacts apart, breaking the current to the electromagnet and relaxing the pull on the diaphragm. When the diaphragm return spring raises the diaphragm, it also pulls the rod away from the contacts, and the cycle repeats.
Both vacuum fuel systems and fuel pump systems ultimately serve the same purpose of delivering fuel to the engine, but they differ in their methods of achieving this goal.
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The role of gravity
The Autovac fuel lifter's reliance on gravity is a simple yet innovative solution to the challenge of fuel delivery in vintage vehicles. By utilising gravity, this system ensures a consistent and controlled flow of fuel from the auxiliary tank to the carburetor. This process is essential for the overall performance and efficiency of the engine.
The carburetor plays a vital role in mixing the correct proportion of fuel with the airflow, providing the necessary energy to the engine. As the fuel descends by gravity to the carburetor, it enters this critical stage of the fuel system, where precise fuel-air mixture ratios are maintained.
Additionally, the role of gravity in antique car vacuum fuel systems is complemented by the function of the manifold vacuum. This vacuum is created by the difference in air pressure between the engine's intake manifold and the Earth's atmosphere, influenced by the movement of pistons and airflow restriction. The manifold vacuum assists in creating suction, further aiding in drawing fuel through the system.
While gravity plays a key role in the fuel delivery process, it is just one aspect of the overall antique car vacuum fuel system. The interplay between gravity, vacuum pressure, and airflow restriction collectively contributes to the functionality of these vintage fuel systems, showcasing the ingenuity and simplicity of engineering in the automotive industry's early days.
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The throttle's function
The throttle is a mechanism that regulates the power or speed of an engine. In the context of a gasoline engine, the throttle is typically a butterfly valve that controls the amount of air and fuel allowed to enter the engine. The throttle is usually located at the entrance of the intake manifold or housed in the throttle body. When the throttle is fully open, the intake manifold is close to ambient atmospheric pressure, and when it is partially closed, the manifold vacuum drops below ambient pressure.
The throttle position is controlled by the driver's input on the accelerator pedal. In modern engines, the accelerator pedal is connected to a sensor that outputs a signal to the Engine Control Unit (ECU) based on the current pedal position. The ECU then determines the throttle opening based on the accelerator pedal's position and inputs from other engine sensors. In older carbureted engines, the throttle is mechanically linked to the accelerator pedal through a cable and linkage.
The function of the throttle is to regulate the amount of air and fuel entering the engine. In a fuel-injected engine, the throttle body controls the airflow in response to the driver's input on the accelerator pedal. The throttle body is usually located between the air filter and the intake manifold. As the driver presses on the accelerator pedal, the throttle plate rotates within the throttle body, opening the throttle passage and allowing more air into the intake manifold.
The throttle position sensor (TPS) and airflow sensor communicate with the ECU to determine the amount of fuel needed at the injectors. The ECU increases the fuel injected by the injectors to maintain the required air-fuel ratio. In a carbureted engine, the throttle is found in the carburetor, and it is opened and closed through a cable and linkage from the accelerator pedal. As the throttle opens, the carburetor responds by creating more fuel flow.
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Manifold vacuum vs. venturi vacuum
Manifold vacuum and venturi vacuum are two different types of vacuums found in an engine. Manifold vacuum is the most commonly recognised vacuum source in an engine. It is created in the intake manifold of an engine due to the pistons' intake downstroke on one end and the restriction created by the partially open carburettor throttle plates on the other end. The vacuum continues to increase when the engine is revved in the driveway as the throttle plates restrict airflow into the engine. When the throttle plates are closed tightly and the pistons are moving quickly, a very high vacuum is created. If the throttle plates are opened more, creating a larger "leak path", less vacuum is created. If the pistons are moving slowly and the throttle plates are wide open, the pressure in the manifold will be very close to atmospheric pressure, resulting in no intake vacuum. Thus, intake vacuum can be used as a signal source to determine how hard an engine is working.
Venturi vacuum, on the other hand, is produced in an entirely different manner and behaves independently of manifold vacuum. It is produced by the high airflow through the primary venturi, which creates low pressure and is used to open the secondary throttle shaft. The more air that passes through the primary side, the more the secondary diaphragm will open. Distributors using venturi vac are likely to use lighter springs or larger diaphragms. The spring rates are calibrated according to the size of the venturi on the carburettor. A carburettor with a larger venturi will generate less restriction to flow and, therefore, a smaller vacuum signal will be supplied to the distributor.
At both cruise and Wide Open Throttle (WOT), manifold vacuum and ported vacuum are the same, with high vacuum at cruise and virtually no vacuum at WOT. The difference in vacuum occurs only at idle. When the throttle plates are opened, the hole becomes fully exposed to manifold vacuum, and normal manifold vacuum is realised. Thus, you have a manifold vacuum "on-off" switch, turning manifold vacuum "off" at idle and restoring it to normal operation once the throttle plate is cracked open.
While manifold and venturi vacuums operate in different ways, they are both crucial components of an engine's vacuum system, contributing to its overall performance and efficiency.
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Frequently asked questions
Manifold vacuum, or engine vacuum in a petrol engine, is the difference in air pressure between the engine's intake manifold and Earth's atmosphere.
A manifold vacuum is caused by a piston's movement on the induction stroke and the airflow through a throttle in the intervening carburetor or throttle body leading to the intake manifold.
In some engines, the manifold vacuum is used as an auxiliary power source to drive engine accessories and for the crankcase ventilation system. It can also be used to power pneumatic accessories.
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.
Venturi vacuum is caused by the venturi effect, which depends on the total mass flow through the carburetor for fixed ambient conditions (air density and temperature). It is used to establish a pressure difference roughly proportional to mass airflow and to maintain a somewhat constant air/fuel ratio.
