1996-12-01

Impact of Accelerative Forces on Performance in Race Engines With Wet-Fuel Manifolds 962499

Accelerative forces induce cylinder imbalance in carbureted or throttle-body injected engines by steering the fuel in the direction of the force. This cylinder imbalance manifests itself as decreased performance, decreased fuel economy, and decreased reliability. Accelerative forces do not induce cylinder imbalance in port-injected engines because fuel distribution is not readily impacted.
Race engines are subjected to a variety of extreme accelerative forces, including fore-aft forces during acceleration and braking, and lateral forces during cornering. It is not unusual to see forces much greater than the force of gravity. Both airborne fuel droplets and fuel deposited on the manifold walls and floor are steered by these accelerative forces, impacting engine performance and reliability adversely.
In this paper, the impact of accelerative forces on combustion performance in a Winston Cup engine are examined. Results indicate that maximum dynamic engine performance under race conditions may require manifold designs which intentionally maldistribute fuel when accelerative forces are not present. This is especially true in applications where the accelerative forces are reasonably constant under race conditions, such as on short ovals. Power gains achievable by tailoring the manifold design to provide good dynamic air-fuel ratio cylinder balance are small, on the order of 1 percent. Lateral force direction can be simulated on an engine dynamometer by tilting the engine from vertical, but this will not produce the correct lateral force magnitude.

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