Charge Air Cooling

Hannu Jääskeläinen, Magdi K. Khair

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Abstract: Charge air cooling is an important feature of many modern boosted diesel engines that can be used to reduce emissions and fuel consumption and increase power density. Charge air can be cooled with engine coolant, ambient air or a separate low temperature liquid circuit.

Introduction

In modern engines, it is also important to ensure the temperature of the charge does not become excessive. In modern boosted engines, this is a real possibility. Excessive temperatures can lead to reduced charge density and higher combustion temperatures which can affect torque, power and emissions.

While turbochargers and superchargers increase charge air density, they also increase the temperature of the air in the intake manifold. This arrangement with intake air compression with no subsequent cooling was suitable for applications such as North American heavy-duty diesel engines until the 1990s. As emission standards became increasingly stringent, additional increases in charge air density were needed. While this could be achieved through compression to higher pressures, this would require more expensive compression equipment and would further increase cycle temperatures. On the other hand, if intake manifold temperature could be reduced, the intake density could be further increased and more air could be supplied to the engine without necessarily increasing the intake manifold pressure. While this would require a compressor capable of higher flow, the cost would be considerably less than a compressor that was also capable of higher pressures. Cooling the air with a heat exchanger as it leaves the compressor is a common way to achieve this charge air cooling. Such a heat exchanger is referred to as a charge air cooler (CAC), intercooler or aftercooler (Figure 1). These terms are commonly used interchangeably. The term intercooler refers to the fact that this heat exchanger performs its task in between two stages of compression, i.e., between compression in the compressor and compression in the cylinder of the engine. The term aftercooler refers to the charge air being cooled after being compressed in the compressor. Increasing demand for improvements in fuel economy and exhaust emissions has made the charge air cooler an important component of most modern turbocharged engines.

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Figure 1. Schematic representation of turbocharger and charge air cooler

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