Chapter 4 Principles of Engine Operation 71 Copyright Goodheart-Willcox Co., Inc. the engine’s power. Many two-cycle diesel engines are fitted with a turbocharger and a gear-driven supercharger. Charge Air Coolers A charge air cooler is simply a heat exchanger that is used to cool intake air. See Figure 4-25. A diesel engine will perform better if the intake air is cooled before it enters the combustion chamber. This is because cool air is denser than warm air and contains more oxygen per unit volume. Remember that a turbocharger will pressurize or com- press air. When air is compressed, it heats up. Cooling the air after it passes through the turbocharger increases its density and oxygen concentration. Charge air coolers use either air or coolant as a medium and are often located near the radiator. Note: Depending on the manufacturer and the application, charge air coolers are often referred to as aftercoolers or intercoolers. Diesel Engine Classifications Diesel engines can be classified in a number of ways. The broadest classification is by power. Diesel engine power can range from three to four horsepower in the smallest diesels to 40,000 horsepower in giant marine and stationary diesel engines. Speed ratings can also be used to classify engines. Diesel engine speed ratings (rpm) can also vary widely—from 125–250 rpm in large, low-speed die- sels to as high as 3,500–4,000 rpm in light-duty automotive diesel applications. Most heavy-duty truck engines operate in the middle of the range, at 1800–2200 rpm. Diesel engines can also be classified in the following ways: ❑ Engine cycle design. ❑ Cylinder arrangement. ❑ Combustion chamber design. ❑ Fuel injection system design. Engine Cycle Design Diesel engines can be built to operate using a four- stroke or two-stroke cycle. Four-stroke cycle diesel engines are the most popular. Four-Stroke Cycle Engines A four-stroke cycle engine (four-cycle engine) requires two complete revolutions of the crankshaft (720° of crank- shaft rotation) to complete the four piston strokes that make up one complete combustion cycle. The four piston strokes are the intake, compression, power (combustion), and exhaust strokes. See Figure 4-26. Exhaust turbine Air compressor Bypass Wastegate Piston Air intake Exhaust Figure 4-24. Turbochargers are driven by exhaust gases. It is actually more efficient than a supercharger because it is driven by energy that is otherwise wasted. Figure 4-25. The charge air cooler is usually located in front of the radiator. Charge air cooler Figure 4-26. The ending position of each stroke in a four-cycle engine. (Detroit Diesel Corp.) 4 Cycle Intake compression Power Exhaust