LO 11-2 Identify and explain the purposes and functions of the cooling system

Diesel Engine Heat Loads

While compression ignition (CI) combustion is one of the most thermally efficient processes for converting fuel energy into mechanical energy, it still involves the release of a substantial amount of heat to the cooling system. The release of heat to the cooling system from the combustion chamber is called heat rejection. Diesel engines have several exclusive, or diesel-only, systems and components that are not found in automotive engines but that require additional cooling. These systems use engine coolant to transfer combustion heat away from the engine and into the atmosphere.

Exhaust Gas Recirculation
Cooled EGR, which has been used by HD diesels in North American since 2002, places an even larger heat load on the cooling system Figure 6. Up to 30% of intake air mass in most current diesels includes exhaust gases, which can reach temperatures over 1200° F (649° C). These temperatures are reduced to 400° F (204° C) or less by a coolant-type heat exchanger before being mixed with intake air originating from the outlet of the air charge cooler. This means the diesel-powered vehicle’s cooling system needs to dissipate 15–40% more heat absorbed from the exhaust gas. Until model year 2002, when cooled EGR was introduced, most diesels transferred 100% of exhaust heat directly into the atmosphere. Now larger heat loads are absorbed by the cooling system to accommodate EGR cooling. Engine modifications are needed to remove the additional heat from the cooling system and include larger water pumps, cooling fans, and radiator cross-sectional areas combined with increased underhood airflow Figure 7.

FIGURE 11-6: Diesel cooling systems absorb additional heat loads from components. High-capacity EGR coolers, air compressors, oil coolers, and turbochargers represent just a few additional heat loads.

FIGURE 11-7: Larger frontal air intakes, such as this truck, are required on EGR-equipped engines to help remove additional underhood heat loads generated by hot surfaces.

Lubrication Oil Heat Loads
In diesels, heat generated by combustion is rejected and transferred to engine parts and lubrication oil. Because every turbocharged engine cools the underside of the piston with engine oil, more heat is transferred to the lubrication system. If excess heat is not removed from the oil, it can lose its viscosity and thin too much to support bearing loads and properly lubricate parts. At temperatures above 250° F (121° C), conventional lubricating oil can quickly oxidize and even burn. To remove this heat from the oil, virtually all diesel engines use oil coolers that circulate engine coolant to remove heat from the engine oil Figure 8.

FIGURE 11-8: Oil cooler nozzles spray the underside of all engine piston crowns with oil to remove heat. The heat absorbed by the oil is removed by the cooling system through the oil cooler.

FIGURE 11-9: The cooling of compressed intake air is sometimes performed by liquid heat exchangers. This MaxxForce DT engine uses jacket water aftercooling engine coolant between the low- and high-pressure turbochargers. ATAAC is used for the final stage of cooling.

Air Intake System Heat Loads
Intake air compressed by the turbocharger easily heats to over 400° F (204° C). Temperatures are even higher than this in engines using series turbocharging. The cooling of heated intake air is necessary to reduce emissions and prevent the thermal overloading of cylinder components such as piston crowns and valves. While most engines use air-to-air aftercooling (ATAAC), engine coolant is often used to help reduce air charge temperatures. Liquid heat exchangers, air charge coolers, or aftercoolers may be located in the intake manifold to circulate coolant, which lowers air inlet temperatures Figure 9. More commonly, liquid interstage coolers are used on series turbocharged engines to reduce air temperatures between the low- and high-pressure turbochargers. Heat removed from intake air is simply transferred to the atmosphere through the radiator.