Prime Mover Technologies

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Chapter Nine /

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Reciprocating engines are the dominant type of internal combustion engines (engines in which the fuel is burned within the working cylinder) and are the most commonly used prime movers. Reciprocating engines serve innumerable vehicular and equipment applications, as well as stationary applications that can range in capacity from a few hp (or kW) to about 90,000 hp (67,000 kW).

Heavy-duty reciprocating engines are extremely efficient and reliable power producers with the ability to generate usable thermal energy. Reciprocating engines generally provide the highest simple-cycle full-load and part-load thermal efficiency among comparably sized prime movers.

Reciprocating Engine Types

Spark- and Self-ignited Reciprocating Engines

There are two general types of reciprocating internal combustion engines:

• Spark-ignited engines operate on the Otto cycle and use gaseous or readily vaporized liquid fuels such as natural gas, gasoline, propane and various biomass-type and manufactured gases.

• Self-ignited, or Diesel, engines use liquid fuels and achieve ignition through the heat of compression. Diesel engines operate on the full range of liquid petroleum fuels, both distillate and residual. Some Diesel engines are modified to operate as dual-fuel engines using a mixture of liquid and gaseous fuels that are ignited by a compression-ignited pilot oil charge.

Two- and Four-Stroke Cycle Reciprocating Engines

The majority of reciprocating engines operate on what is known as the four-stroke cycle. In this cycle, power is generated through a series of four combustion process stages: air intake, compression, power, and exhaust. Two revolutions of the crankshaft occur in each four-stroke cycle (one power stroke every two crankshaft revolutions).

Figure 9-1 illustrates the piston location within the cylinder at the completion of each of the four combustion process strokes. From right to left, the four-stroke process operates as follows:

1. The intake stroke starts with the piston at top dead center (TDC), which is the upper most part of its stroke, and ends with the piston at bottom dead center (BDC), which is the end of the downward stroke. As the piston moves downward, it creates a partial vacuum in the cylinder. The intake valve is open and air, or a mixture of air and vaporized fuel, is drawn into or injected into the cylinder from the intake manifold past the open intake valve.

2. The compression stroke starts with the piston at BDC. Both the intake and exhaust valves are closed and remain so as the piston moves upward. The air (compression-ignition Diesel cycle) or air-fuel mixture (spark-ignition Otto cycle) is confined within the cylinder and is compressed to a small fraction of its initial volume. Toward the end of the compression stroke, combustion is initiated and the cylinder pressure begins to rise more rapidly.

3. The power (or expansion) stroke starts with the piston at or near TDC again and ends at BDC. The high-temperature, high-pressure gases resulting from fuel combustion push the piston down and force the crank to rotate. As the piston approaches BDC, the exhaust valve opens to initiate the exhaust process and drop the cylinder pressure to close to the exhaust pressure level.

4. The exhaust stroke starts with the piston at BDC. The exhaust valve remains open and the remaining burned gases exit the cylinder. The process continues first because the cylinder pressure may be significantly higher than the exhaust pressure, and then because the upward motion of the piston sweeps the now fully expanded gases through the exhaust valve and into the exhaust manifold. As the piston again reaches TDC, the exhaust valve closes and the cycle repeats.

In a two-stroke cycle engine, there are no separate intake and exhaust process strokes. Intake and exhaust begin near BDC during the last part of the power stroke and end shortly after the beginning of the compression stroke. Most modern designs feature some type of external scavenge blower. In simpler types, ports in the cylinder liner, opened and closed by the piston movement, control the exhaust and inlet flows. The two-stroke cycle requires

Fig. 9-1 Illustration of Four-Stroke-Cycle Reciprocating Engine Combustion Process Cycle.

only one revolution of the crankshaft, yielding one power stroke per revolution. The basic two-stroke process operates as follows:

1. The compression stroke starts by closing the inlet and exhaust ports and then compresses the cylinder contents. As the piston approaches TDC, combustion is initiated and pressure increases.

2. The power (or expansion) stroke is similar to that in the four-stroke cycle until the piston approaches BDC. At that point, the exhaust ports open and the intake ports are uncovered.

Engine Aspiration

The intake manifold distributes the air (self-ignited Diesel cycle) or the air-fuel mixture (spark-ignited Otto cycle) to the various cylinders of a multi-cylinder reciprocating engine. There are two types of engine aspiration:

• Natural aspiration draws combustion air into the piston cylinder at atmospheric pressure. Gas/fuel need only be supplied at low pressure (less than 1 psig).

• In charged aspiration, an air blower or mechanical compressor is used to pressurize the air or air-fuel mixture before it is inducted into the cylinder. When driven by an engine auxiliary output shaft or a separate driver, the device is known as a supercharger. When driven by an exhaust-powered turbine, the device is known as a turbocharger. In some cases, an engine may feature both a supercharger and a tur-bocharger. Because compressing the air or air-fuel mixture increases its temperature, an aftercooler (or intercooler) is used to cool the heated charge to further increase its density.

Charging increases the power output from a given cylinder size by increasing the amount of oxygen available for combustion. Thus, for a given engine size, weight, displacement, and piston speed, significantly more power is produced when supercharging or turbocharging is used. This increase in power density greatly reduces engine capital cost per hp (kW). It also, generally, increases engine volumetric efficiency.

Operating Cycles

The working fluid (air or air-fuel mixture) in an actual reciprocating engine does not go through a complete cycle in the engine, meaning it is not returned to its original condition. Instead, the engine actually operates on the so-called open cycle. To analyze the engine, however, it is convenient to devise a closed cycle that approximates the open cycle. This approximation is known as the air-standard cycle.

In practice, engines operating on Otto and Diesel cycles do not duplicate the cycle or approach the theoretical efficiencies of these ideal cycles. In fact, in many modern engine designs, the characteristics of Otto and Diesel engines have become more similar to each other. It is still, however, useful to consider the air-standard cycles as a starting point for understanding the basic principals of operation. Following are descriptions of the Otto and Diesel cycles.

Otto Cycle

Figure 9-2 provides representations of the air-standard, or ideal, Otto cycle in both pressure/volume (P-V) and temperature/entropy (T-s) ordinates. The ideal Otto cycle consists of an isentropic compression (A-B), followed by constant-volume combustion (B-C), an isentropic expansion from which work is extracted (C-D), and, finally, a reversible constant-volume rejection of heat (D-A).

Specific heat (c) is the energy, as heat, transferred during a process per unit mass flow (m) of fluid (working fuel), divided by the corresponding change of temperature (T) of the fluid (working fuel) in the process (subscripts v and p

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