Cooling Systems - Circuit Configurations
(Adapted from Cat Marine Engine - Application & Installation Guide)
The connections to the external cooling systems can be connected together to form circuits, defined sequences of the components involved. The cooling circuit configurations available for a given engine model will vary, with the most commonly seen described below.
A single circuit cooling system may be found on a naturally aspirated (NA)A single circuit cooling system may be found on a naturally aspirated (NA)engine, with no turbocharger and thus no aftercooler to be placed on a separate circuit. A turbocharged engine that cools the aftercooler with jacket water (a jacket water aftercooled, or JWAC engine) also does not require a separate circuit since the heat loads use a common coolant flow.
Combined Circuit Cooling
(An engine cooling system schematic for combined-circuit cooling)
Many engines use mutliple cooling circuits. A combined circuit arrangement can be created by operating the jacket water system as a closed loop that is cooled by the aftercooler flow. In isolating the jacket coolant, there is only one external cooling flow.
Separate Circuit Cooling
(Engine cooling system schematic for separate-circuit cooling)
The most commonly seen configuration for turbocharged engines is the separate circuit arrangement, sometimes abbreviated as SCAC for “separate-circuit aftercooled.” The aftercooler is moved to a circuit distinct from that of the jacket water, which allows the two circuits to be regulated to different temperatures.
Running the aftercooler at a much lower temperature than the jacket water allows the intake charge temperatures to be lowered. The greater air density and reduced charge temperature that result both contribute to enabling higher engine power levels. Separate-circuit cooling is a necessity on gas engines. The lower temperature of the air-fuel mixture delivered to the cylinders is essential to avoiding detonation, one of the key limiting parameters for gas engine performance. The separate circuit configuration does require one circulating pump, one thermostat, and one external coolercore for each circuit.
Air-to-Air Aftercooling (ATAAC)
A special case of separate circuit cooling seeks to use the best properties of liquid- and air-cooled systems on the same engine. The engine’s jacket water system is retained as it is found on other separate-circuit configurations but the engine-mounted, liquid-cooled aftercooler is replaced with a direct air-to-air heat exchanger, typically installed off-engine. This is done in instances where high ambient air temperatures are expected to limit the ability to achieve the required temperature of the coolant provided to the engine’s aftercooler. By eliminating the liquid coolant circuit, one intermediate step of the heat exchange is removed. This allows only one step of temperature difference (approach temperature) instead of two, allowing the combustion intake air to be cooled to a temperature much closer to the ambient air temperature than would be possible with the liquid-cooled circuit. The layout of an ATAAC arrangement is shown below.
Air enters at the air cleaner (2), is boosted by the turbocharger (4), and then is routed to the air-to-air core of the cooler (5). Temperature measurement at the engine air inlet (3) allows control of the resulting air temperature via a regulated bypass valve (1) on the external air lines.
For an ATAAC configuration, instead of pumping liquid coolant from the engine to an external cooler, the combustion intake air itself is sent to an external cooler section. Such a configuration does come with some limitations. Consider that:
- It is more difficult to regulate the actual combustion intake air temperature on an ATAAC engine than it is on a conventional liquid-cooled arrangement. This makes it even more important that the engine management system monitor variations in the achieved intake manifold air temperature. These measurements are used to make any necessary adjustments to the injection/ignition timing or air/fuel ratio to maintain the engine’s performance and emissions.
- Having the combustion air cooled directly by the ambient air can create challenges for operating the engine in an arctic environment – very cold intake air temperatures can have a significant impact on combustion. Automated controls become necessary to compensate for this. A thermostatically-controlled valve that allows some air to bypass the cooler provides temperature control of the intake air similar to the liquid-cooled arrangement. Other options, such as variable speed fans or adjustable shutters on the cooler, can also be used to offset the effects of the extreme cold environment.
- Routing the intake air from the turbocharger to an external cooler adds restriction to the air flow path. Because the nominal engine performance is based on a design air mass flow rate, restriction can affect the performance envelope of the turbo, essentially decreasing the altitude capability of the engine if the added restriction is excessive. The maximum allowable pressure drop in the air flow path should be available in the engine’s technical data.
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