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A study has been conducted to quantify the typical internal surface roughness of a cast iron exhaust manifold. In addition, the range of surface roughness values that can be obtained with various manufacturing methods was measured. Initial investigations were conducted to measure the effect of a range of surface roughness values on the performance of the engine system, specifically torque and the thermal losses through the exhaust manifold walls. Several manifold geometries were used to represent a variety of actual manifold applications, including designs that were subjected to tight packaging constraints. Physical tests were used to show that large variations in surface roughness resulted in modest changes in manifold component pressure losses. A simulation tool was used to predict that modest improvements in manifold pressure losses have little impact on engine output.

The heat rejection rates and skin temperatures of a liquid cooled exhaust manifold on a 3.5 L Gasoline Turbocharged Direct Injection (GTDI) engine are determined experimentally using an external cooling circuit, which is capable of controlling the manifold coolant inlet temperature, outlet pressure, and flow rate. The manifold is equipped with a jacket that surrounds the collector region and is cooled with an aqueous solution of ethylene-glycol-based antifreeze to reduce skin temperatures. Results were obtained by sweeping the manifold coolant flow rate from 2.0 to 0.2 gpm at 12 different engine operating points of increasing brake power up to 220 hp. The nominal coolant inlet temperature and outlet pressure were 85 °C and 13 psig, respectively. Data were collected under steady conditions and time averaged. For the majority of operating conditions, the manifold heat rejection rate is shown to be relatively insensitive to changes in manifold coolant flow rate.