70 Diesel Technology Copyright Goodheart-Willcox Co., Inc. is usually cast in a single piece. The intake manifold uses a soft gasket to maintain a tight seal with the cylinder head. Exhaust Manifold The exhaust manifold collects burned gases from each exhaust port and channels them to the exhaust pipe on naturally aspirated (air intake) engines. On turbo- charged engines, the exhaust gases are routed from the exhaust manifold, through the turbocharger, and into the exhaust pipe, Figure 4-22B. Because they are subjected to rapid and extreme varia- tions in temperature, exhaust manifolds are usually made of cast iron. Marine and large stationary diesel engines may have a water-jacketed exhaust manifold to lower the tem- perature in the engine compartment. Truck engine exhaust manifolds usually are not liquid-cooled. Therefore, heat- sensitive components must be shielded from the exhaust manifold or located away from it. Superchargers A supercharger or blower is an air pump that is mechan- ically driven by the engine. See Figure 4-23. Its job is to force more air into the combustion chamber. More air permits a larger fuel charge to be burned in the cylinder, creating addi- tional power from the same basic engine. Because the blower is driven by the engine, it robs the engine of power. However, this loss is more than offset by the power gained. Currently, almost all four-cycle engines use turbo- chargers, which are more efficient than superchargers. Nevertheless, superchargers are still used on most two- cycle engines, where they scavenge or blow exhaust gases out of the combustion chamber during the period in the exhaust/intake cycle when both the air inlet ports and the exhaust valves are open. Turbochargers A turbocharger is also an air pump designed to put more air in the combustion chamber. However, it differs from a supercharger in that it is driven by energy that nor- mally would be wasted—the exhaust gases rushing out of the engine, Figure 4-24. One advantage of a turbocharger is that it operates hardest when it is needed the most. For example, when the engine is at idle or coasting downhill, it produces very little exhaust gas. This is fine because extra air and fuel are not needed under these conditions. When the engine is under heavy load, it produces more exhaust, the turbocharger spins faster, and more air is forced into the cylinder. Turbochargers allow a small diesel engine to produce up to 40% more power than the same size engine could generate if it were naturally aspirated. Turbochargers also help an engine run better at high altitudes, where air is thin. At high altitudes, normally aspirated diesels smoke unless they are equipped with smaller injectors that limit Turbocharger air to air charge air cooler Intake air inlet to turbocharger Charge air cooler or aftercooler Intake valve A B Intake manifold Exhaust manifold Turbocharger inlet Exhaust valve Turbocharger exhaust outlet Figure 4-22. Intake and exhaust systems of a mobile engine. A—The air intake manifold brings fresh air to the engine’s combustion chambers. B—The exhaust manifold routes the burned gases to the exhaust system. (Cummins Engine Co., Inc.) Inlet air (low pressure) Exhaust gas (low pressure) Pulley Belt Cell rotor High pressure air High pressure exhaust Piston Air Exhaust Figure 4-23. Superchargers increase the amount of combustion air entering the cylinder. Superchargers are either belt-or chain- driven by the engine.