Abstract
During the early phase of the exhaust process of an internal combustion engine, gas exits under high pressure as critical flow. Conversely, exhaust ports are often physically tested under steady, low pressure airflow and inconsistences frequently occur between test results and engine performance. Modifications are often made using traditional intake port improvement techniques that may have a negative impact on high pressure, compressible flow within the port. This research focused on the question of whether downstream exhaust port geometry can negatively affect port efficiency during low valve lift, critical flow. Typical engine analysis assumes critical flow through the entire valve curtain area with no downstream effects on flow rates or coefficients. A chamber was constructed to use compressed gases to test the critical flow performance of two Chevy SB2.2 cylinder head ports. A Weld Tech ported cylinder head was chosen based on discussions with port designers indicating the final design was modified to increase port volume and diameter which increased engine performance, but resulted in reduced flow under low pressure, steady airflow. One port was modified to decrease port volume and increase steady flow coefficients to mimic these conditions as described by designers. Then, compressed carbon dioxide and nitrogen were used as testing fluids to investigate the low lift critical flow performance of the original and modified ports. These results were compared to those obtained from steady flow, low pressure testing using air. The use of the products of lean hydrogen combustion was also investigated, but ultimately deemed infeasible with the envisioned testing apparatus. Through modifications the port volume was reduced 4.7% and steady, low pressure flow coefficients were improved by 7.2% at 0.050” valve lift and 6.1% at 0.100” valve lift compared to the original port. These improvements were mostly gained through pressure recovery. Using the compressed gas chamber, the measured effective critical flow coefficients averaged over 20 to 90 lb/in2 were decreased 2.8% and 3.2% at 0.050” lift using nitrogen and carbon dioxide, respectively. At 0.100” lift, the effective critical flow coefficients averaged over 20 to 70 lb/in2 were decreased 0.1% and 0.14% using nitrogen and carbon dioxide, respectively. Critical flow coefficient results indicated a dependence on upstream pressure and downstream port geometry, which could contribute to the inconsistencies in steady flow testing and engine performance. Engine Analyzer software was also used to demonstrate the benefits of increased exhaust flow path diameter on predicted power at high engine speeds and the beneficial translation to racing engines. These improvements were the result of reduced back pressure, increased scavenging, and decreased pumping losses.