Chapter 6 Engine Types and Classifi cations 139 Copyright by Goodheart-Willcox Co., Inc. Open Closed Small diameter Large diameter Open Closed Small diameter Large diameter A B Figure 6-29. A variable intake engine alters the path that air takes when entering combustion chambers. A—At low engine speeds, the airstream is directed through a long, small-diameter runner to provide good air velocity and mixing of fuel. B—At high engine speeds, the airstream is directed through a short, large-diameter runner to allow for higher power output. Figure 6-30. This is a variable intake system that has a dual- resonance, variable-length intake manifold. At low speeds, intake air takes the long route through the curved intake run- ners. At high speeds, a butterfly valve opens to shorten the path for the incoming air. (Mercedes-Benz) Variable Intake Engine A variable intake engine (VIE) can adjust its intake manifold runner length to match engine speeds. At low engine speeds, long, small-diameter intake manifold run- ners increase the speed of the air-fuel charge, creating a “ram effect” that allows a denser charge into the combus- tion chamber. However, at high engine speeds, short, large- diameter intake manifold runners are needed to produce the same effect. Conventional, fixed-runner intake mani- folds are designed as a compromise between low-speed and high-speed efficiency. Various methods are used by engine designers to provide variable length intake manifolds. Some of these include dual intake runners (one long and one short), different routing of intake manifold runners using butter- fly valves, or by a rotating runner device that lengthens or shortens the airflow path. Figure 6-29 shows one type of VIE design. This engine simply has two intake manifold runner lengths. A butterfly valve in the intake manifold plenum can open and close to select either route for incoming air. Figure 6-30 shows a variable runner, also called variable resonance, intake manifold design. A flap or butterfly valve in the intake manifold can be opened or closed to alter the manifold runner length. The long intake runner length helps low-speed operation. The short intake runner is for high-speed operation. Another variation uses a stack of hollow plastic runner wheels mounted inside the manifold plenum to vary the intake. By rotating the runner wheels, each manifold run- ner can be shortened or lengthened as needed. During low-engine-speed operation, the runner wheels are turned so that incoming air has a very long path to flow through before entering the combustion chambers. Then, as engine speed increases, the runner wheels are turned to shorten the effective runner length. Variable Backpressure Exhaust Engine A variable backpressure exhaust engine (VBEE) uses different paths through the engine exhaust manifolds to improve engine efficiency. Similar to a variable intake manifold, the system can increase exhaust runner length for low-engine-speed operation and decrease exhaust runner length for high-engine-speed performance. A VBEE is rare and presently only used on exotic, high-performance engines from manufacturers such as Aston Martin, Ferrari, Honda, Lamborghini, Nissan, and Mitsubishi.