Air Intake

Internal combustion engine requires two basic things to function – oxygen and fuel. Tubes and valves guide oxygen and fuel into the cylinder, where a piston compresses the mixture to be ignited. The explosive force pushes the piston down, forcing the crankshaft to rotate, giving the user mechanical force to move the vehicle, run generators, and pump water, to name a few of the functions of an automotive engine.

The air intake system

The air intake system is critical to the function of the engine, collecting air and directing it to individual cylinders, but that’s not all. Following a typical oxygen molecule through the air intake system, we can learn what each part does to keep your engine running efficiently. (Depending on the vehicle, these parts may be in a different order.)

Air intake system

The cold-air intake tube is usually located where it can pull air from outside the engine bay, such as a fender, the grille, or hood scoop. The cold-air intake tube marks the beginning of air’s passage through the air intake system, the only opening through which air can enter. Air from outside the engine bay is typically lower in temperature and denser, therefore richer in oxygen, which is better for combustion, power output, and engine efficiency.

Engine Air Filter

Air filter

The air then passes through the engine air filter, usually located in an “air box.” Pure “air” is a mixture of gases – 78% nitrogen, 21% oxygen, and trace amounts of other gases. Depending on location and season, air can also contain numerous contaminants, such as soot, pollen, dust, dirt, leaves, and insects. Some of these contaminants can be abrasive, causing excessive wear in engine parts, while others can clog the system.

A screen usually keeps out most larger particles, such as insects and leaves, while the air filter catches finer particles, such as dust, dirt, and pollen. The typical air filter captures 80% to 90% of particles down to 5 µm (5 microns is about the size of a red blood cell). Premium air filters capture 90% to 95% of particles down to 1 µm (some bacteria can be about 1 micron in size).

Mass Air Flow Meter

Variants of mass airflow sensors

To properly gauge how much fuel to inject at any given moment, the engine control module (ECM) needs to know how much air is coming into the air intake system. Most vehicles use a mass air flow meter (MAF) for this purpose, while others use a manifold absolute pressure (MAP) sensor, usually located on the intake manifold. Some engines, such as turbocharged engines, may use both.

On MAF-equipped vehicles, air passes through a screen and vanes to “straighten” it. A small part of this air passes through the sensor portion of the MAF which contains a hot wire or hot film measuring device. Electricity heats up the wire or film, leading to a decrease in current, while air flow cools the wire or film leading an increase in current. The ECM correlates the resulting current flow with air mass, a critical calculation in fuel injection systems. Most air intake systems include an intake air temperature (IAT) sensor somewhere near the MAF, sometimes part of the same unit.

Air Intake Tube

Air intake tubes

After being measured, the air continues through the air intake tube to the throttle body. Along the way, there may be resonator chambers, “empty” bottles designed to absorb and cancel out vibrations in the air stream, smoothing airflow on its way to the throttle body. It also does one good to note that, especially after the MAF, there can be no leaks in the air intake system. Allowing unmetered air into the system would skew air-fuel ratios. At a minimum, this might cause the ECM to detect a malfunction, setting diagnostic trouble codes (DTC) and the check engine light (CEL). At worst, the engine may not start or may run poorly.

Turbocharger and Intercooler

A turbocharger

An intercooler

On vehicles equipped with a turbocharger, air then passes through the turbocharger inlet. Exhaust gases spin up the turbine in the turbine housing, spinning the compressor wheel in the compressor housing. Incoming air is compressed, increasing its density and oxygen content – more oxygen can burn more fuel for more power from smaller engines.

Because compression increases the temperature of the intake air, compressed air flows through an intercooler to reduce temperature to reduce the chance of engine ping, detonation, and pre-ignition.

Throttle Body

A throttle body

The throttle body is connected, either electronically or via cable, to the accelerator pedal and cruise control system, if equipped. When you depress the accelerator, the throttle plate, or “butterfly” valve, opens to allow more air to flow into the engine, resulting in an increase in engine power and speed. With cruise control engaged, a separate cable or electrical signal is used to operate the throttle body, maintaining the driver’s desired vehicle speed.

Idle Air Control

An idle air control unit

At idle, such as sitting at a stop light or when coasting, a small amount of air still needs to go to the engine to keep it running. Some newer vehicles, with electronic throttle control (ETC), engine idle speed is controlled by minute adjustments to the throttle valve. On most other vehicles, a separate idle air control (IAC) valve controls a small amount of air to maintain engine idle speed. The IAC may be part of the throttle body or connected to the intake via a smaller intake hose, off the main intake hose.

Intake Manifold

An intake manifold

After intake air passes through the throttle body, it passes into the intake manifold, a series of tubes that delivers air to the intake valves at each cylinder. Simple intake manifolds move intake air along the shortest route, while more complex versions may direct air along a more circuitous route or even multiple routes, depending on engine speed and load. Controlling air flow this way can make for more power or efficiency, depending on demand.

Intake Valves

The intake valve (blue)

Finally, just before getting to the cylinder, intake air is controlled by the intake valves. On the intake stroke, the intake valve opens to allow the cylinder to pull in air as the piston goes down. A few moments, the intake valve closes, allowing the piston to compress the air before ignition.

A Honda air intake system

As you can see, the air intake system is slightly more complicated than a simple tube going to the throttle body. From outside the vehicle to the intake valves, intake air takes a meandering route, designed to deliver clean and measured air to the cylinders. Knowing the function of each part of the air intake system can make diagnosis and repair easier, as well.

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