Objective 1 Turbocharger PDF

Objective One When you have completed this objective, you will be able to: State the purposes for turbocharging the engine air induction system. Purposes for Turbocharging an Engine Some diesel engines are naturally aspirated. They rely on a difference in air pressure duri ng the intake stroke in or...

Objective One When you have completed this objective, you will be able to: State the purposes for turbocharging the engine air induction system. Purposes for Turbocharging an Engine Some diesel engines are naturally aspirated. They rely on a difference in air pressure duri ng the intake stroke in order to operate. For example, when a piston moves down on th e intake stroke, a lowpressure area, called a vacuum, is created within that cylinder. The outside air is at atmos pheric pressure. This pressure is slightly higher than the pressure in the cylinder. Conseq uently, air flows from the region of higher pressure to the region of lower pressure. This natural movement, shown in Figure 1, is used to supply air to the engine for combustion purposes. Figure 1 - Naturally aspirated engine on the intake stroke. The majority of diesel engines are turbocharged. These engines do not rely solely on at mospheric air pressure to fill the cylinder for combustion purposes. Some engines such as a Detroit Diesel two stroke cycle are equipped with a blower or positive displacement air pump. As soon as the intake port is uncovered, the blower forces air into the engine cylinder at a pressure above atmospheric pressure. Turbocharging utilizes wasted exhaust gases and heat to drive a centrifugal air pump th at increases the pressure of the air entering the intake manifold (above atmospheric). Th is increased pressure or boost improves the scavenging of the cylinder during the valve overlap period and provides a positive pressure in the cylinder during the intake stroke s o there is a greater volume of air available for combustion. The basic reason for turbocharging an engine is to increase the engine’s power output. A turbocharged engine may serve several purposes such as the following. It offsets the effects of decreased performance when the engine must operate at hig her altitudes. The greater the altitude, the less dense the air. This means that less o xygen can be drawn into a cylinder through natural means. It increases power output without significantly increasing the weight of the engine. T his is critical in marine applications or with on- highway vehicles where any extra weight reduces the legal cargo carrying capacity. It increases the power of an engine without significantly increasing its size. The unde rhood space in most vehicles is often very limited. It compensates for varying load demands during operation. For example, a supercha rged engine can handle both low and high demand loads experienced when drivin g a pump on a pipeline. It reduces exhaust gas emissions by making the combustion process more efficient a nd, at the same time, increasing fuel economy. To increase the power output of an engine, more fuel must be delivered to each cylinder. However, more fuel cannot be efficiently burned in a cylinder unless more air is provide d to ensure complete combustion. A turbocharger will ensure that the cylinders receive s ufficient air by forcing air into the cylinders at an increased pressure. As the air pressure increases, the temperature of the air increases as shown in Figure 2. Although the increase in air pressure improves engine output, the gain is partially offset by the higher air temperature. (Hot air is less dense; therefore has less oxygen.) In order to obtain the most benefit from the increased airflow, the hot air must be cooled. Afterc oolers/intercoolers are used to cool the intake airflow. Figure 2 - Air temperature changes during turbocharging and aftercooling. (Court esy of Cummins Inc. ®) The increased air pressure also tends to increase the amount of turbulence in the cylind ers. This results in better mixing and combustion of the air and fuel. This generally has a positive effect on the fuel economy. In comparison to a naturally aspirated engine, a turbocharged engine has the following differences incorporated into its design. Compression ratio is lower. The increased air pressure, combined with additional fue l in the cylinder, will automatically raise the combustion pressures. If the compressi on ratio was not reduced, the stress on the engine components, such as the piston s, crankshaft, connecting rods and bearings would be excessive and result in engin e damage. The camshaft and valve timing are changed to fully gain the benefits of the turbocha rging process. In general, the valve lift, opening duration and the amount of valve overlap will be increased. This allows the pressurized air flow to completely force e xhaust gases from the cylinders while filling the engine cylinders with a fresh air ch arge. This is especially important on highspeed diesels when the intake valve is only open for a very short time. Injection timing is retarded. This limits cylinder pressures during combustion and red uces the stresses on the lower engine components. The fuel injection pump output and/or fuel injector sizes are increased. As more air i s available for combustion, more fuel can be delivered to the cylinders. The centrifugal fuel injection advance may not be required. The crankcase vent could be vented to the atmosphere. Many modern engines vent the crankcase to the intake side of the engine. When a turbocharged engine is correctly designed, the power output may be 50% or m ore than that of a comparable sized naturally aspirated engine. Higher power output is p ossible, but extensive strengthening of engine components is required.