A downdraft balanced induction system provides optimum  airflow to each of the individual cylinders throughout a wide  operational range.  [Figure 3-4]  Better matched fuel-to-air  ratios provide a much smoother and more efficient engine  operation. Air from the induction manifold flows into the  intake ports where it is mixed with fuel from the fuel nozzles  and then enters the cylinders as a combustible mixture as the  intake valve opens. Figure 3-2  is a diagram of an induction system used in an  engine equipped with a carburetor. In this induction system,  carburetor normal flow air is admitted at the lower front nose  cowling below the propeller spinner and is passed through an  air filter into air ducts leading to the carburetor. A carburetor  heat air valve is located below the carburetor for selecting  an alternate warm air source (carburetor heat) to prevent  carburetor icing.  [Figure 3-5]  Carburetor icing occurs when  the temperature is lowered in the throat of the carburetor and  enough moisture is present to freeze and block the flow of air  to the engine. The carburetor heat valve admits air from the  outside air scoop for normal operation, and it admits warm  air from the engine compartment for operation during icing  conditions. The carburetor heat is operated by a push-pull  control in the cockpit. When the carburetor heat air door is  closed, warm ducted air from around the exhaust is directed  into the carburetor. This raises the intake air temperature.  An alternate air door can be opened by engine suction if the  normal route of airflow should be blocked by something.  The valve is spring loaded closed and is sucked open by the  engine if needed.  The carburetor air filter, shown in  Figure 3-6,  is installed in  the air scoop in front of the carburetor air duct. Its purpose is  Induction systems can consist of several different  arrangements. Two that are used are the updraft and downdraft  induction systems. An updraft induction system consists of  two runners and a balance tube with intake pipes for each  cylinder to deliver induction air to each cylinder’s intake  port.  [Figure 3-3]  The balance tube is used to reduce pressure  imbalances between the two side induction runners. With  carbureted engines, it is important to maintain a constant and  even pressure in the induction system so that each cylinder  receives equal amounts of fuel. On fuel-injected engines,  the fuel is injected at the intake port just before the intake  valve. It is important with this system to keep the pressure  consistent at each intake port.  3-2 Figure 3-2.  Nonsupercharged induction system using a carburetor. Figure 3-1.  Inlet scoop in engine cowling.  Basic Carburetor Induction System Drain line Carburetor air valve Air intake duct Cold air actuator lever Carburetor Temperature bulb Air filter Warm air Cold air