Cooling System Development & Validation For The Urban Bus

Transit buses operate in very dusty and debris laden urban environments. These conditions quickly result in clogged or restricted engine coolant radiator and charge-air-cooler cores despite the best maintenance efforts. This coupled with normal degradation of the anti-recirculation baffling, etc., results in reduced cooling capacity. This underscores the need for transit buses to be equipped with adequately sized units capable of maintaining optimum cooling system performance under degraded conditions. Unless specified by the purchases, bus OEMs are not motivated to provide cooling packages with enough excess capacity to maintain this optimum engine performance under real world conditions. This paper shares NYCT's efforts and experiences in its quest to develop and test cooling system units capable of delivering desired performance under clogged and degraded conditions. In an aggressive repower program, NYCT tested cooling systems on Orion V and Nova buses with Series 50 EGR-equipped engines. These emission-enhanced engines operate with increased heat rejection to the cooling system in comparison to the original 2-cycle 6V92 ENGINES. With engine compartment real estate at a premium, design options were limited. In addition, some options can compound the clogging problem (for example increased fan size, core thickness and core fin density). Qualified cooling packages for this program were required to pass the engine manufacturer's requirements under simulated degradation conditions such as 25% blockage of the face area of the cooling unit. Testing was performed on a test track with a towed dynamometer. A description of the test plan, including simulated degradation conditions, will be presented along with the analyzed data obtained. The Nova bus was also tested in a low speed wind tunnel under controlled conditions. The data obtained from testing several cooling packages will be discussed and comparisons made with the test track results. The conclusions obtained from reviewing both sets of test results will be presented and recommendations made for upgrading or specifying cooling system requirements, which parallel the needs dictated by the service environment.