The Advantages of Air to Air Aftercooling
One way to substantially improve the performance of a turbocharged engine is to lower the intake air temperature that has been increased by the compression process in the turbocharger compressor. This increase in air temperature has been presented in Bulletin No. 3 on a graph that shows air temperature versus compressor pressure ratio with compressor efficiency as a parameter. This graph is being reproduced here as Figure 1.
For example referring to the graph, a pressure ratio of 3.0 (approx. 30 psig) will heat the intake air to 350°F if the compressor efficiency is 76%. Thus, engine performance will be much improved if the compressed air temperature can be reduced to a much lower value before it enters the engine cylinders.
Charge air cooling has been used for many years, initially employing water-to-air heat exchangers, using engine coolant as the cooling medium. However, except for marine engines, the water-to-air charge cooling has been replaced by air-to-air charge air cooling due to greater cooling effect provided by the use of ambient air as the cooling medium.
The air-to-air aftercooling system is shown diagrammatically in Figure 2. It is uncomplicated, has no moving parts, and is in widespread use in commercial applications. To illustrate the potential of the system, referring to Figure 1, the ambient air temperature has been taken as 80°F. A pressure ratio of 3.0 produces a compressor outlet temperature of 350°F. Thus the maximum cooling effect will be 350-80=270°F. If the air-to-air heat exchanger has an effectiveness of .80, then the temperature of the air will be reduced by 216°F (270X.80). The compressed air entering the engine intake manifold will be 134°F (350-216).
If engine coolant were used as the cooling medium, the coolant temperature can typically be 180°F. Then the maximum cooling effect becomes 350°-180°=170°F.
Using the same heat exchanger effectiveness of .80 the compressed air temperature will be reduced by 136°F (170X.80). The air temperature entering the intake manifold will be 214°F (350-136). Comparing this temperature of 214°F with the 134°F produced by the air-to-air system, the advantage of the latter system is obvious.
Lowering the engine intake manifold temperature can result in large improvements in engine performance. Due to the large increase in intake air density, more fuel can be burned, resulting in higher horsepower output. Greater air density in the cylinders can increase combustion efficiency and lower engine fuel consumption. Exhaust temperatures are lower which lowers engine emissions and smoke in the engine exhaust is improved. Air-to-air aftercooling produces lower combustion gas temperatures in the cylinders. This can result in longer engine life since the potential for thermal stress is reduced.
As an illustration of the performance improvement of a 6.7 liter engine, normally rated at 155 HP, Figure 3 shows the actual performance of a the non-cooled engine compared with the air-to-air aftercooled version. In addition to the higher HP output, exhaust temperature is lower and fuel consumption has been improved. The engine performance shown in Figure 3 is indicative of an engine with a very modest power rating. Performance improvements of highly rated engines would be more dramatic.
The application of air-to-air charge air cooling to turbocharging is no doubt the biggest step in the development of turbocharged engines since the turbocharger itself was introduced.
If additional information or assistance is needed in the application of air-to-air aftercooling to an engine, contact a technical staff member of Comp Turbo Technology Inc.