Characteristics of the Intake System

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Volumetric Efficiency

The main goal behind tuning an intake manifold is to increase the volumetric efficiency of the engine. The volumetric efficiency itself is a misleading term, in that itself refers to the mass flow of air into the engine. The volumetric efficiency takes into account the losses throughout the system from the air filter to the intake valves themselves. When the right kind of components the efficiency can actually be greater than 100% due to the addition of a super charger or turbo charger by increasing the density of the air charge going into the engine. Volumetric efficiency itself is not a constant as the values vary for various engine speeds and pipe lengths as illustrated by the jaguar D-type engine in Figure 1. In this figure it shows that at a higher rpm value the longer pipes will actually hinder the efficiency of the engine leading to a poor performance curve. This phenomenon has led to the development of a new intake technology that involves manifold folding. Here the intake is able to vary its length based on engine rpm, by doing so the power band is maintained and an increase in the overall performance of the engine is gained. In order to create more horsepower the runners become shorter in length allowing the air a more direct path, and when torque is desired the pipe length is extended though the use of a valve to control the flow through a separate set of tubing.

Physical Characteristics

On the intake side of the engine, air and fuel are mixed and burnt in the combustion chamber providing the desired output. There are several components to the intake of an internal combustion engine. The main components of the intake manifold include the air filter, throttle body, plenum, fuel injectors, and runners. Each component provides an important role, with the air filter removing impurities in the air so it will not hinder the combustion process. The throttle body provides the user with a means to manage the flow of air into the engine itself, increasing the opening in the throttle with demand for more air. The plenum serves as a reservoir for the incoming air to be pulled from when each cylinder requires a charge of air. When dealing with fuel injected into the system there are a number of methods, from carburetion to electronic fuel injection into the port. Finally, the air flows through the runners and into the engine where it is combusted. Choices for each system must be made when tuning the intake, and this is where the complications begin.

Intake manifolds use a wide variety of shapes and sizes to take advantage of every possible combination of all the various components to maximize the needs of a particular engine. In doing so, it is important to understand the qualities that are desired in an intake system. Smooth laminar flow is wanted for the area of the intake where no fuel is present. This minimizes the losses due to wall friction and bends in the manifold system. Yet, when fuel becomes introduced into the system it is necessary for the flow to become turbulent. This will increase the flow velocity at the same time increasing the atomization of the fuel air mixture to create a proper burn once inside the combustion chamber. Variations in the lengths and bends can be seen in Figure 2, where four types of plenums are illustrated.

Figure 2. Types of intakes.

Fuel Selection

When creating an intake manifold it has been its dependency on a few parameters that are preset, one being the type of fuel used by the IC engine. Most internal combustion engines run either on diesel or gasoline, with some exceptions in racing applications that may run on methanol, or ethanol. When presented with the design of the gasoline engine, more than likely the system has been set forth with the fuel being introduced in the intake manifold that is then ignited by a spark. The throttle body controls the power output of the engine. However, with a diesel engine there is not a throttle body because the intake is controlled via the fuel injection process. The diesel engine also does not use a spark to ignite the fuel instead using very high compression ratios to achieve combustion, as well as direct fuel injection into the cylinder. These are the reasons why diesel engines are more efficient than that of a gasoline engine.

Fuel Injection vs. Carburetion

Another important consideration in the design of the intake manifold comes from the manner in which the fuel is introduced into the engine. Currently there are two main methods of fuel addition, carburetion and fuel injection. Most modern systems have become increasing operated by a computer controlled fuel injection system due to a more efficient control mechanism. In a carburetor the fuel is introduced far upstream of the intake valves. This becomes inefficient because turbulent flow must be maintained in order to create a good atomization of the fuel and air. Also, in carbureted engines, the fuel and air mixture can collect on the walls of the runners and plenum causing a loss in flow rate among other losses. The benefit of a carburetor for most old time car enthusiasts is that these types of fuel systems can be manually tuned. Yet, for most people, the current standard of multi-point fuel injection is a much better option. For with this type of injection the fuel is introduced at the intake valve making for easier flow upstream of the valves thus minimizing losses in the system. The fuel injection system also makes it possible to change and monitor the fuel air ratio at a given rpm and change accordingly due to feedback from the engine itself to a computer module, hence giving a cleaner more accurate burn. The drawback to this system is the modification of the adjusting the injection by hand is no longer possible because of the computer control involved.