Management & Operations

Cooling a Repowered Bus Presents Significant Challenges

Posted on April 1, 2001 by Harry Oberg, New York City Transit

Repowering older buses in your fleet has obvious advantages: reduced emissions, technology advancements and additional years of service. However, new engine systems, especially those with advanced emission controls, can increase the burden on the cooling system. This article describes how New York City Transit (NYCT) is handling this problem as it swaps out engines in its older buses. As part of an aggressive emission-reduction program, NYCT is replacing the two-cycle Detroit Diesel (DDC) engines in older buses with new, four-cycle Series 50 DDC engines. The Series 50 features exhaust gas recirculation (EGR) and variable nozzle turbocharging. To further reduce particulate emissions, continuously regenerating technology soot filters (CRT) are installed at the time of repower. The Series 50 and other diesel engines with advanced emission-control systems increase the stress on cooling systems because of design elements like cooled EGR and charge-air-cooling (CAC), which were not present on the two-cycle engine. The CRT reduces the amount of particulates released into the air, albeit at the cost of increased exhaust restriction. The dilemma is that although the overall heat load on the cooling system increases and must be managed, the CRT depends on high exhaust temperatures in order to function properly and reduce soot. Then and now The older transit bus’ engine had far less horsepower (the 6V71N version of this engine was rated at 190 hp) than the technologically advanced Series 50 EGR (rated at 275 hp). Despite the technological simplicity of the 6V71N and its successor, the 6V92T (rated at 253 hp with turbocharging and after-cooling), cooling the engine in the older bus was a barely manageable task. Unlike trucks and cars, most transit buses have side- and/or rear-mount cooling systems. As a result, a bus does not benefit from ram air flow to help cool the engine. A bus is also burdened by its low-mount radiator which, like a vacuum cleaner, sucks up all manner of dirt and debris, clogging the core very quickly. As the core clogs, air flow through it becomes increasingly restricted, which reduces the transfer of heat to the atmosphere. Therefore, buses require more frequent cleaning and repair of the cooling system than any other on-highway vehicle. Experience and testing revealed that even when new, the cooling system left little room for expansion in terms of heat load and very little allowance for degradation of the system. Experience in the form of overheat road calls revealed that the heat rejection characteristics of the 253-hp 6V92T and 275-hp Series 50 (non-EGR) engine stressed the cooling system to its limit. Testing confirmed the need for additional design work on the cooling system. Installation of the turbocharged 6V92T introduced additional heat to the cooling system by the addition of air-to-water aftercooling. Although the Series 50 uses air-to-air charge-air-cooling, the liquid cooling was affected because the CAC now occupies space that previously contained part of the radiator. NYCT specifies a side-by-side radiator/CAC arrangement because it is not desirable to add heat load from the CAC to the liquid coolant. In addition, NYCT assumed that it would be difficult to clean debris that lodged in the middle of the radiator/CAC stack. The surface area of the radiator is now less than one-half the area of the original radiator in a bus equipped with a 6V71N. Certainly, enlarging the surface area or volume of the cooling system would be an easy way to increase the capacity of a bus’ cooling system. Unfortunately, limited space in the engine compartment prevents significant enlargement of the cooling system, so NYCT tried to improve the efficiency of the system through improved heat transfer. This will likely involve striking a balance between the radiator and charge air cooler. Generally, transit buses are kept in their original fleet much longer than over-the-road trucks and are therefore more likely to be affected by advancing technology. Few trucking fleet managers would consider replacing the 8V92 with a Series 60 in an over-the-road tractor, yet several bus fleets have these programs underway or are considering them. Thus it is necessary to validate the cooling system design in order to avoid problems cooling the engine, which can degrade performance, invalidate warrantees and increase emissions. Reduced emissions are the very reason for performing most of these conversions. Basic design considerations A fleet manager or contractor assumes the role and responsibility of OEM and/or system integrator when performing a repower. Although the fleet manager does not usually have the engineering resources of a traditional OEM, the task is no less necessary to ensure success and to limit ongoing maintenance. With the proper approach and assistance from manufacturers and suppliers, the task of cooling the repowered bus will be less daunting. Although one may find that many of the parts necessary for the repower are available from OEM, the finished job will likely include some specially fabricated parts. When replacing the 71 or the 92 Series DDC engines with a modern, low-emissions diesel, consideration is given to the size and placement of the charge-air-cooling required by most engines. To do this, the existing radiator is scrapped and replaced. In addition, radiator shutters/winter fronts are not compatible with CAC, which demands a constant flow of air across the core to ensure adherence to intake air temperature requirements. In terms of bodywork, the radiator/ CAC aperture should be opened to the extent possible to gain access to extra cooling air. Over the years, it is likely that some of the original baffling and shrouds have been damaged or lost, especially in the case of the baffling in front of the radiator. Testing revealed that recirculation of heated air back through the radiator/CAC core caused a significant temperature rise. This equipment must be repaired and/or reinstalled. It is possible that new baffling/shrouds will have to be designed in order to limit recirculating air. It may mean the difference between a bus that passes a cooling index test and one that does not, so its importance cannot be ignored or downplayed. On buses with hydraulic or other than direct drive fans, the fan speed needs to be checked with a stroboscope and set to the OEM recommendations. Simply stated, the cooling demands of a modern low-emission diesel engine requires that the entire system be in a state of proper design and good repair if you are to have any chance of cooling the bus properly. Certifying the design Because of the size of the program (800-plus buses), NYCT did not take a trial-and-error approach to cooling system qualification. The cooling package for repower buses needs to be certified like a new bus. This will assure proper cooling is available and maintain the engine warranty, as well as ensure reliability. Two types of testing were performed:

  • Testing with a towed dynamometer
  • Dynamometer testing in a climatic, environmental wind tunnel Certification with a towed dynamometer is the most common method for testing the cooling system of a heavy vehicle. While it is perfectly valid, it is somewhat cumbersome and time consuming if there is a need to modify or adjust the design. Extensive modifications, like adjusting the fan penetration into the shroud, will likely be performed at an off-site location. It is also dependent on the weather conditions at the test track. For example, testing cannot be performed in wet weather or in temperatures below 70º F. During the winter, even test tracks in the southwestern United States have weather-related problems. Meanwhile, the wind tunnel method is less readily available but enables faster testing of various configurations of a cooling system. With this test methodology, the fleet manager is able to tailor the test to the conditions the bus is expected to operate in. Because little time is lost transporting the bus and the weather is not a factor, this method lends itself to experimentation with a variety of configurations, e.g., adjusting fan-to-shroud depth, using different fan configurations and different core configurations. The cost of testing is not insignificant and may burden a smaller property with fewer buses to repower. For the smaller repower project, it is probably best to procure certified packages rather than try to spread the cost of testing over a few buses. Degradation factors Because of the difficulty and expense encountered maintaining the cooling system, NYCT factored in a 25% radiator blockage to simulate dirt loading of the radiator core. For the test, that was accomplished by blocking 25% of the core’s surface area with cardboard. Admittedly it is not a high-tech method, but test results indicated that it was effective. There are some issues with the placement of the cardboard. NYCT blocked the lower radiator and CAC because it felt that dirt would tend to accumulate in that area first. Some people advise blocking the corners or other areas. There are arguments for each. Unfortunately, NYCT is unable to report successful results at this point with a variety of radiator, CAC, fan and shroud configurations, but it is getting closer. ****************************************** Investigating poor system performance Cooling system tests are very rigorous and tend to reveal any weaknesses in the bus’ mechanical systems. Therefore, some effort should be made to ensure that other systems on the bus are in good repair. A careful look at the test results will usually point to the reason for poor cooling system performance. For instance, high intake manifold temperature differential may be the result of a charge-air-cooling (CAC) that is simply not as efficient as it needs to be. An indication of that might be a low internal pressure drop across the core. In that case your options may be limited. It may be necessary to look for a more efficient core. That may also be caused by restricted air flow across the CAC core, which can sometimes be corrected by increasing the restriction across the radiator core, effectively diverting air back to the CAC. High air-to-water temperatures can be the result of poor airflow, recirculation of heated air or poor internal design. A careful look at the data will usually indicate the cause. For example, high inlet air temperatures on the front (roadside) of the radiator are indicative of hot air recirculation. If the data indicates that the problem is caused by lack of adequate air circulation through the core, a solution might be to balance the airflow through the cores as was done with the CAC. Perhaps a more efficient fan is required or some redesign of the fan shroud by adjusting the penetration of the fan into the shroud. There are many solutions to cooling problems. A thorough analysis of the test data will point to an efficient and effective solution.
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