The Intake System
The system that allows air and fuel into the engine is known as the intake system. This system is comprised of the air filter, the intake manifold, and either a carburator or a throttle body along with pressurized fuel injectors depending on the engine. Most modern cars use a fuel injection system as opposed to the carburator to more precisely measure the amount of fuel entering the engine.

In carburated engines, fresh air enters the engine through the air filter, whose purpose is to filter out particles that could damage the engine. From the air filter, the fresh air is ducted into the carburator. The carburator works on a venturi principle whereby the air being drawn through accelerates through a narrowed passage called the venturi. As the passage begins to widen again, a vacuum occurs. It is this vacuum that draws fuel into the moving column of air through the carburator's jets. The jets are sized to permit a fairly exact amount of fuel to be drawn into the air stream, thus creating the proper mixture of air and fuel.

A modern fuel injected engine has no carburator. Instead, it relies on pressurized fuel injectors that are triggered electronically to squirt just the right amount of fuel into the air stream. Fuel injection has several advantages over a carburator. First, the electronic controls (see electronic controls) can better meter the exact amount of fuel needed to make the engine work efficiently. Second, by squirting under pressure, the fuel can be better vaporized (broken up into smaller particles) which will promote more even and complete combustion in the engine. Finally, the fuel injection system usually squirts fuel into the intake manifold runners (more on the intake manifold below) near each cylinder rather than ahead of the plenum, so the fuel injection can be better timed with the intake stroke.

In either case, a throttle is used to control the amount of air into the engine. The throttle is connected directly to the gas pedal. It consists of a plate hinged in its center that when closed blocks all but a minute amount of air from entering the engine. Thus when a cylinder is on the intake stroke, it can only draw in a minimum amount of the air-fuel mixture. Less air and fuel weakens the cumbustion significantly, and the engine will create a great deal less power. In fact, power at this point is only sufficient to allow the engine to idle. Opening the throttle wider, as happens when the driver pushes the gas pedal, allows a greater amount of the air-fuel mixture into the combustion chamber, creating greater power output from the engine.

The Intake Manifold

The intake manifold consists of a plenum and runners. The plenum is simply a main pipe that distributes the intake air to the various cylinders. Picture a main water line with smaller pipes running to each house on a street. The main line would be the plenum while the smaller pipes that go to each house would be the runners. So, each runner will go from the plenum to the intake valve of each cylinder. If the engine is fuel injected, the fuel injector will be located in the runner, near the intake valve, with its operation timed to squirt fuel into the air stream while the valve is opened.

It would be an oversimplification to say that the intake manifold is merely plumbing, as there is a great deal more science involved in tuning a manifold than one would think. One might think of a pipe organ, with columns of air flowing through a tube having some resonant frequency. Intake manifolds are designed with a resonant frequency as well, along with a specific quantity of volumetric air flow. Ideally, the intake manifold will be tuned so that its resonant frequency promotes the greatest amount of air-fuel mixure to enter each combustion chamber on each intake stroke. The wave of forward pressure then must be timed to match the timing of the valve opening, so that the highest intake pressure in the runner occurs when the valve opens and the piston is simultaneously drawing the air-fuel mixture in. Of course, when the valve closes, a reverse wave occurs in the runner, which if properly timed, will increase pressure back to the plenum and can help force intake air down another runner to an intake valve that might be opening at the specific monemt that its intake valve is opening. You can see that tuning an intake manifold can be a bit more scientific than what might be expected. 

Intake tuning is part of a larger equation that engine designers must incorporate when deciding at what RPM band the engine will be most efficient. The exhaust system can also be tuned, as it affects how efficiently the engine forces all the CO2 and other non-combustible gasses out of the combustion chamber. There's even a certain degree of effect that exhaust and intake tuning have on one another. Keep in mind that racing engines generally are tuned to operate at a much higher RPM band than engines used in normal cars. While this makes for a more powerful engine, it has a negative effect on both fuel economy and engine longevity.

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