Optimization of the Engine Intake Air Temperature through the Air Conditioning Unit

In modern turbocharged internal combustion engines the cooling of the air after the compression stage is the standard technique to reduce temperature of the engine intake air aimed at improving cylinder filling (volumetric efficiency) and, therefore, overall global efficiency. At present, standard values for the intake air temperature are in the range 30-70°C, dependently on engine load, external air conditions and vehicle speed and the adoption of a dedicated cooling fluid operating at low temperatures (-10-0°C) is addressed as the most viable option to achieve an effective temperature reduction. This paper investigates a pilot engine set-up, featuring an evaporator on the intake line of a turbocharged diesel engine, tested on a high speed dynamometer bench: the evaporator was a part of an air refrigeration unit - the same used for cabin cooling - composed also by a compressor, a condenser and a thermostatic expansion valve. The effects of the undercooling of the charge air have been experimentally assessed in terms of fuel consumption and regulated emission reduction, evaluated on the most common engine operating points. Mechanical power needed by the compressor was obviously taken into account in order to assess the overall benefits. A fuel consumption reduction has been demonstrated in the order of 2.5% when the intake air subcooling is turned on. A benefit on the regulated emissions has been observed (NOx, PM). HC and CO behavior, on the contrary, deserves some more attention and involves engine control parameters (for instance, EGR rate) and combustion performances.