Intake Manifold Science

Any increase in velocity will also increase kinetic energy of air and fuel, making it difficult for both to navigate changes in direction from carburetor to combustion chamber. So what we have is sort of a seesaw effect. Fuel can be more easily suspended in air if both are moving rapidly, but it’s difficult to keep air and fuel mixed when it comes time to change direction. In recent years, considerable research has provided ways to compensate for many of the problems associated with high velocity air/fuel separation. And as engine sizes continue to decrease, you’ll likely see more changes in intake manifold design, to maintain high flow rates and low air/ fuel separation levels.

A part of current exhaust emissions devices is the practice of putting exhaust gas back into the intake manifold during times when the engine is under load (or at times when combustion temperatures are higher than thought desirable). Exhaust gas recirculation (EGR) passages are cast into an intake manifold in such a way that allows direct communication between the air cavity (plenum) just below the carburetor and an exhaust gas source. Usually, there is a vacuum-operated valve that keeps recycled gas from flowing into the intake manifold all the time. But when exhaust gas is allowed into the intake manifold, some amount of air/ fuel mixture dilution results. This dilution reduces the amount of combustion heat that follows, thereby decreasing the amount of oxides of nitrogen (NOx) during operation of the EGR system. But for good combustion efficiency, it’s already been mentioned in a previous Shop Series that dilution of any sort can reduce both power and net efficiency.

This isn’t to suggest that you run out and plug your engine’s EGR system, but it brings us up to the point of examining any other sources of fresh air/ fuel mixture contamination. Perhaps the most important one is the dilution that takes place during the first few thousandths of intake valve opening. As you might expect, the more time there is for this dilution to pass into the intake manifold the more of a problem it is. At low rpm, there’s

more time. So the old problem of “reversion” (back-flow into the intake manifold) is also one of contamination, since it is exhaust gas that is coming out of the cylinder and into the intake manifold. Some new-car manufacturers have even exercised this idea in the design of camshafts so that some amount of “built-in” EGR could result from slight encouragement of reversion at the lower engine speeds. Curious, huh?
And finally, before we leave the subject of contamination, there’s one more source to ponder.

E. Two basic types of intake manifolds are: (1) two-level (two-plane) and (2) single-level (single-plane). Typically, two-plane designs provide two separate air cavities, while single-plane manifolds join all cylinders of the engine into one air cavity.

1. By combining the best features of both single-plane and two-plane manifold designs, this cutaway shows how each runner can be brought directly to the plenum chamber while maintaining conventional 180-degree firing order in a two-level configuration. Note very small cross-sectional area of each runner. High mixture velocities at low engine speed result from such a design. This results in more torque output. 2. As an example of how pressure varies in each runner of an intake manifold, this oscilloscope photograph of pressure variations can be compared to Figure A, inasmuch as the basic trace is the same as that produced by theoretical data. It’s also what’s happening inside your favorite intake manifold—like it or not.

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