Induction System Components

The following components are part of the engine induction system:

• Air inlet tract

• Air filter element

• Throttle butterfly valve

• Carburetor

• Manifold

Air Inlet Tract

The air inlet tract comprises all ducting and any components that are fitted into the duct­ing between the extreme point at which air for the engine induction is collected, and the engine inlet manifold, where the air/fuel mixture is accumulated prior to entering the engine cylinders. This tract commonly is constructed from high-pressure molded plastic, which means that it is lightweight and that more complex shapes can be made. This allows for better under-bonnet placement. It is essential that the designer takes into account the volume of air that the engine requires while running at high-speed/ high-load conditions. Equally, the test technician and engineer also should consider this important factor when installing engines into test cells. The air filter container is an important part of the inlet tract and either is mounted directly onto the carburetor or is connected to the carburetor by a rigid tube. Engine performance is critically dependent on the air inlet passage configuration; therefore, the air filter mounting must conform to the required standards.

Air Filter Element

Usually, this is designed as a disposable item constructed from convoluted paper. The convolutes give the element sufficient strength to maintain its shape. The element is held between the lower container body and the removable cover. It has two rubber gaskets molded to it, which form an airtight seal within the filter container. Other types include the oil-bath type and the metal gauze outer, sponge inner (K&N) washable type of filter element.

If unfiltered air reaches the engine, the following may result:

• Carburetor jet blockage

• Valve seat damage, causing a loss of cylinder compression

• Engine cylinder or piston scoring, causing a loss of cylinder compression or leading to piston-to-cylinder seizure

• Piston ring wear, causing high oil consumption and blow-by Throttle Butterfly Valve

Normally an integral part of the carburetor, the throttle butterfly valve is a variable opening disc fitted to a cross-spindle positioned to the lower portion of the carburetor body. This valve is connected to the accelerator pedal of the vehicle.

Where the engine is utilizing fuel injectors, the throttle butterfly valve is fitted within the upper portion of the manifold. On systems that utilize engine management electronics (e. g., ECU), the throttle butterfly valve is positioned to the rear of the airflow-metering device. The purpose of the butterfly valve is to control the volume of air flowing through the inlet tract at given engine speed conditions. The power output of the engine depends directly on the mass of the air and fuel charge that the cylinders of the engine actually take in and combust. Therefore, this relatively simple device can have a significant role in the running of the engine.


The carburetor can be a complex component, and with so many variants available in the workplace, it will not be discussed in any great detail. Suffice it to say that the purpose of the carburetor is to mix fuel and air to the correct ratio for any given engine condi­tion. In essence, it is a tapered tube. As the air passes through the narrowed throat, it is forced to speed up. The increased speed of this air over the main carburetor jet causes additional fuel to be drawn into the airstream and atomized, with the fuel/air ratio remaining constant over the range.

Note that with fuel injection for spark ignition engines, more and more engines are fit­ted with injectors, which negates the requirement for a carburetor. In the case of direct injection (DI) engines, the injectors are fitted into the cylinder head and inject atomized fuel directly into the individual cylinder combustion chambers.

In the case of indirect injection (IDI) engines, the injection of fuel into the airflow takes place farther upstream from the combustion chamber. Where this occurs upstream will determine the type of injector that is used. Single point injectors (SPI) are used upstream of the throttle butterfly valve, whereas multi-point injectors (MPI) are sited in the individual branches of the inlet manifold. Direct injection has the greater operating pressures.


Ten factors must be taken into consideration when designing a manifold, as follows:

1. Flow to each cylinder should be as direct as possible.

2. Charge quantities to each cylinder should be equal.

3. An equal mixture strength of uniformly distributed charge is required for each cylinder.

4. Equal aspiration intervals between manifold branch pipes will prevent charge flow interference between cylinders.

5. Designed for minimal wall wetting and collection points for unbumed fuel.

6. An amount of ram pressure charging is needed.

7. Use the smallest tract diameter to maintain adequate air velocity at low engine speeds but without hindering volumetric efficiency at high engine speeds.

8. Use the smallest amount of friction in each branch pipe.

9. Allow enough pre-heating for cold starting and warm-up periods.

10. Provide drainage of the heavier liquid fraction of the fuel.

As will be realized from this list of considerations, manifold design is a precise and complex subject. Therefore, it is sufficient to state that the manifold is a means of con­necting the engine to the rest of the induction system. At the same time, it is used as a form of heater element to prevent carburetor icing. With the new generations of plastics now available, induction manifolds can be, and indeed are, made from these plastics. The main benefits are that the manifolds can cost less to manufacture, and the finish of the end product is of a higher quality than those made from aluminum.

Should the manifold-to-cylinder-head sealing gasket have an air leak, it will have det­rimental effects on the performance of the engine. Weakening of the air/fuel mixture (i. e., overheating of the piston crown) will cause the engine to misfire on one or more cylinders, leading to loss of power.

The air/fuel charge is the source of energy within an engine when it has undergone the combustion process; however, it also has a secondary role, in that it is used to cool the piston crown area, too. If this were not true, then after a period of running, the piston crown would overheat and melt rapidly. A sign of this will be detonation whereby the air/fuel mixture is ignited prior to the designated ignition point by the spark plug. Another cause of detonation is maladjusted ignition timing; therefore, both the timing and the air/fuel ratio must be correct for the engine to run efficiently.

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