Internal Combustion Engines. Allan T. Kirkpatrick
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Название: Internal Combustion Engines
Автор: Allan T. Kirkpatrick
Издательство: John Wiley & Sons Limited
Жанр: Физика
isbn: 9781119454557
isbn:
Figure 1.14 Poppet valve assembly. (Adapted from Taylor 1985.)
The valve timing is controlled by a camshaft that rotates at half the engine speed for a four‐stroke engine. Lobes on the camshaft along with lifters, pushrods, and rocker arms control the valve motion. The inlet valves in early (circa 1910) engines were spring loaded, and were opened during the inlet stroke by the atmosphere‐cylinder pressure differential. Most automotive engines currently use an overhead camshaft to eliminate pushrods and simplify the valve train.
A valve timing profile is shown in Figure 1.15. The valve opening and closing angles are not necessarily symmetric about top and bottom dead center, due to fluid flow considerations discussed in Chapter 05. The valve timing can be varied to increase volumetric efficiency through the use of advanced camshafts that have moveable lobes, or with electric valves. With a change in the load and speed, the valve opening duration and timing can be adjusted to optimize power and/or efficiency.
Figure 1.15 Poppet valve timing profile. (Courtesy of Competition Cams, Inc.)
Superchargers and Turbochargers
All the engines discussed so far are naturally aspirated, i.e., as the intake gas is drawn in by the downward motion of the piston. Engines can also be supercharged or turbocharged. Supercharging is mechanical compression of the inlet air to a pressure higher than standard atmosphere by a compressor powered by the crankshaft. The compressor increases the density of the intake air so that more fuel and air can be delivered to the cylinder to increase the power. The concept of turbocharging is illustrated in Figure 1.16. Exhaust gas leaving an engine is further expanded through a turbine that drives a compressor. The benefits are twofold: (1) the engine is more efficient because energy that would have otherwise been wasted is recovered from the exhaust gas; and (2) a smaller engine can be constructed to produce a given power because it is more efficient and because the density of the incoming charge is greater. The power available to drive the compressor when turbocharging is a nonlinear function of engine speed such that at low speeds there is little, if any, boost (density increase), whereas at high speeds the boost is maximum. It is also low at part throttle and high at wide‐open throttle. These are desirable characteristics for an automotive engine since throttling or pumping losses are minimized. Most large and medium‐size diesel engines are turbocharged to increase their efficiency. With the anticipated adoption of 48V electrical systems in vehicles, there will be increased use of electrically powered superchargers used in conjunction with a turbocharger to reduce ”turbo lag.”
Figure 1.16 Turbocharger schematic. (Courtesy of Schwitzer.)
Fuel Injectors and Carburetors
Revolutionary changes have taken place with engine controls and fuel delivery systems in recent years and the progress continues. The ignition and fuel injection systems of the engine are now controlled by computers. Conventional carburetors in automobiles were replaced by throttle body fuel injectors in the 1980s, which in turn were replaced by port fuel injectors in the 1990s. Port fuel injectors are located in the intake port of each cylinder just upstream of the intake valve, so there is an injector for each cylinder. The port injector does not need to maintain a continuous fuel spray, since the time lag for fuel delivery is much less than that of a throttle body injector.
Direct injection spark‐ignition engines are available on many production engines. With direct injection, the fuel is sprayed directly into the cylinder during the late stages of the compression stroke. Compared with port injection, direct injection engines can be operated at a higher compression ratio, and therefore will have a higher theoretical efficiency, since the combustion knock limitations are reduced. They can also be unthrottled, resulting in a greater volumetric efficiency at part load. The evaporation of the injected fuel in the combustion chamber will have a charge cooling effect, which will also increase volumetric efficiency.
Cooling Systems
Some type of cooling system is required to remove the approximately 30% of the fuel energy rejected as waste heat. Liquid and air cooling are the two main types of cooling systems. The liquid cooling system (see Figure 1.17) is usually a single loop where a water pump sends coolant to the engine block, and then to the head. Warm coolant flows through the intake manifold to warm it and thereby assist in vaporizing the fuel. The coolant will then flow to a radiator or heat exchanger, reject the waste heat to the atmosphere, and flow back to the pump. When the engine is cold, a thermostat prevents coolant from returning to the radiator, resulting in a more rapid warmup of the engine. Liquid‐cooled engines are quieter than air‐cooled engines, but have leaking, boiling, and freezing problems. Engines with relatively low power output, less than 20 kW, primarily use air cooling. Air‐cooling systems use fins to lower the air side surface temperature (see Figure 1.18). There are historical examples of combined water and air cooling. The Mors, an early 1920s automobile, had a finned air‐cooled cylinder and water‐cooled heads.
Figure 1.17 Liquid cooling system schematic.
Figure 1.18 Air cooling of model airplane engine. (Courtesy R. Schroeder.)
1.6 Examples of Internal Combustion Engines
Automotive Spark‐Ignition Four‐Stroke Engine
A photograph of a V‐6 3.2 L automobile engine is shown as in Figure 1.19 and in cutaway view in Figure 1.20. The engine has a 89 mm bore and a stroke of 86 mm. The maximum power is 165 kW (225 hp) at 5550 rpm. The engine has a single overhead camshaft per piston bank with four valves per cylinder. The pistons are flat with notches for valve clearance. The fuel is mixed with the inlet air by spraying the fuel into the intake port at the Y‐junction just above the intake valves.
Figure 1.19 3.2 L V‐6 automobile engine. (Courtesy of Honda Motor Co.)