A T

Metering Spring Diaphragm

Vacuum Transfer Passage Gas Metering Valve Idle Air Bypass Adjustment Power Mixture Adjustment Throttle Valve

Air In

Exhaust

(Closed)

Air In

Exhaust

(Closed)

Metering Spring Diaphragm

Vacuum Transfer Passage Gas Metering Valve Idle Air Bypass Adjustment Power Mixture Adjustment Throttle Valve

Intake (Down) Stroke

Fig. 9-19 Illustration of Air-Fuel Mixture Admission in Four-Stroke-Cycle Spark-Ignited Gas Engine. Source: Waukesha Engine Div.

Intake (Down) Stroke

Fig. 9-19 Illustration of Air-Fuel Mixture Admission in Four-Stroke-Cycle Spark-Ignited Gas Engine. Source: Waukesha Engine Div.

The camshaft and drive assembly operate the opening and closing of valves at a controlled rate of speed, as well as at a precise time in relation to piston position. Camshafts, which are usually made of cast iron or forged steel, perform valve actuation with one cam for each valve on the camshaft. Valves are closed by springs and opened by cam lobes. When the camshaft is in the engine frame and the valves are overhead, a push rod is used to transmit motion of the cam and lifter to a rocker arm on the cylinder head that opens the valves. In the case of overhead camshafts, the cams may operate directly on the followers or may first operate on a rocker lever. Figures 9-20 through 9-22 illustrate camshaft and valve opening mechanisms.

Fig. 9-20 Gear-Driven Camshafts on 17,000 hp (12,700 kW) In-Line Four-Stroke-Cycle Engine. Source: MAN B&W

Valve timing refers to the adjustment of valves to open and close at the proper time for smooth and efficient engine operation. The valve actuating mechanisms are adjusted so that the valves open and close a designated number of degrees before and after the piston has reached TDC or BDC. If timing is inaccurate in a given cylinder, it will adversely effect power production and extraction.

Timing gears located at one end of the engine drive the camshaft and other components using power supplied by the crankshaft. In smaller applications, a chain- or cog belt-driven camshaft may be used. Figure 9-23 illustrates use of a chain drive applied to a large long-stroke engine to allow the camshaft to be located high up on the engine. Such engines are provided with a hydraulic chain tightener, which automatically maintains the tension of the chain throughout its service life.

During the intake and exhaust strokes of a four-stroke-cycle engine, the engine essentially acts as an air or

Fig. 9-21 Essential Moving Parts of Valve-Operating Mechanism for One Cylinder. Source: Waukesha Engine Div.

fuel-air pump. In two-stroke-cycle engines, which include only the compression and power strokes, a separate pump or compressor may be used.

The operation of filling and clearing the cylinder in two-stroke engines is called scavenging. A series of ports or openings is arranged around the cylinder in such a position that the ports are open when the piston is at the bottom of its stroke. Scavenging arrangements are classified as cross-, loop-, and uniflow-scavenging, depending on the location

Fig. 9-22 Illustration of Valve and Valve Spring. Source: Waukesha Engine Div.

and orientation of the scavenging ports. Cross-and loop-scavenging systems use exhaust and inlet ports in the cylinder walls, uncovered by the piston as it approaches. Uniflow systems may use inlet ports with exhaust valves in the cylinder head or inlet and exhaust ports with opposed pistons. Figure 9-24 illustrates a hydraulically operated exhaust valve assembly for a two-stroke-cycle engine.

With a typical uniflow design, exhaust valves open when the piston is more than half way down and a blowdown (or free exhaust) process begins. As the piston continues toward BDC, the scavenging ports open. Exhaust flow continues toward the exhaust valves, which now have a large open area. When the cylinder pressure falls below the inlet pressure, air enters the cylinder and the scavenging process begins. Flow continues while inlet pressure exceeds cylinder pressure. As cylinder pressure exceeds exhaust pressure, the fresh charge displaces the burned gases.

Volumetric efficiency is a term used to express the effectiveness of an engine intake system's induction process. Volumetric efficiency of a four-cycle, naturally aspirated engine is the ratio of the actual volume of air (stated in terms of standard temperature and pressure) taken into the engine cylinder during the intake stroke to the piston displacement. It essentially measures the weight of air actually in the cylinder compared to the weight of an equivalent volume of free air.

Engine power capacity is directly related to volumetric efficiency. As the weight of air in the cylinder increases, more fuel can be burned, producing more power. For two-stroke-cycle engines, the term scavenge efficiency is used to describe how thoroughly the burned gases are removed and the cylinder is filled with fresh air. The term volumetric efficiency as defined above is not applied to supercharged or turbocharged engines.

Regardless of the engine type, thermal fuel efficiency is impacted by how thoroughly the products of combustion are swept out from the cylinder and a fresh air charge is admitted. Lower-speed engines generally can achieve greater fuel efficiency because there is more time in each cycle for these events to occur. Engine designs that optimize mass flow through the engine by minimizing restrictions on both the air intake and exhaust sides are also able to achieve greater fuel efficiency.

Fig. 9-23 Chain-Driven Camshaft on Large Long-Stroke Engine. Source: MAN B&W

Fig. 9-24 Exhaust Valve Assembly for Two-Stroke-Cycle Engine. Source: MAN B&W

Superchargers and Turbochargers

The higher the air pressure (or charge density), the greater the amount of air and oxygen that can be provided to the cylinder and the greater the amount of fuel that is combusted. When the air-charging device is driven mechanically by an accessory shaft from the engine, it is called a supercharger. When the device is driven by the exhaust gases, it is called a turbocharger.

Turbocharging and supercharging have become extremely common in today's high-performance engines. Capacity is commonly increased by 35 to 100% without increasing engine size, resulting in increased power density (increased power ratio per unit of space). However, the increase in power density does produce additional stress on engines, causing greater wear and shorter engine life.

Turbocharging/supercharging is an important part of the movement toward leaner air-fuel mixtures, designed to achieve low NOx emissions. As engines decrease the total fuel in the air-fuel mixture, a lower-flame temperature is achieved, which reduces NOx emission levels. It also reduces power output. Increasing the air-charge density compensates for this, allowing engines to maintain power density while operating under very lean combustion conditions.

Common positive displacement charging devices used as superchargers include sliding-vane or rotary compressors and roots blowers. Centrifugal compressors are continuous flow devices, which are well suited for the high-speed operation achieved with an exhaust-driven radial- or axial-flow turbine. Figures 9-25 and 9-26 illustrate two turbocharger turbine designs. Figure 9-27 is a charge-air schematic for a four-stroke-cycle natural gas-fired, spark-ignited engine.

The energy to drive the turbocharger comes from the blow-down energy in the engine exhaust. The potential exhaust gas energy available to a turbocharger turbine placed in the exhaust stream is called the blow-down energy because it represents the combustion products being blown-down from cylinder pressure at the point where the exhaust valve opens to atmospheric pressure.

Charge cooling with a heat exchanger, commonly referred to as an intercooler or aftercooler, prior to entry

Fig. 9-25 Radial Turbocharger Design. Source: MAN B&W

Fig. 9-26 Axial Turbocharger Design. Source: MAN B&W

used for charging air. Some designs feature two blowers in series or parallel. Figure 9-29 shows four configurations used for increasing charge density and optimizing volumetric efficiency with turbochargers and superchargers on a two-stroke-cycle opposed piston engine, which uses two pistons in each cylinder.

Figure 9-29(a) features a mechanical blower or supercharger only. Figure 9-29(d) features a turbocharger only. Figure 9-29(b) features both a turbocharger and supercharger in parallel and Figure 9-29(c) features a tur-bocharger and supercharger in series. In the series configuration, air is drawn into the turbocharger, where it is compressed and discharged through a cooler to the engine-driven blower. This second-stage blower, operating

Fig. 9-25 Radial Turbocharger Design. Source: MAN B&W

to the cylinder, can be used to further increase the air or air-fuel mixture density. By reducing charge pressure and temperature in spark-ignited engines, higher charge pressure and/or compression ratios can be used.

Figure 9-28 is a schematic drawing of a turbocharger on a four-stroke-cycle engine. The flow path of charge air can be seen from the compressor section through the intercooler and intake manifold, through the intake valve to the cylinder. In the exhaust path, the combustion gases exit the cylinder through the exhaust valve, pass through the exhaust manifold, then to the turbine section before being exhausted to atmosphere. Also shown is the intercooler water circuit.

There are numerous other designs and configurations

Fig. 9-27 Charge-Air Schematic for Four-Stroke-Cycle Spark-Ignited Engine. Source: Waukesha Engine Div.

Fig. 9-28 Schematic Illustration of Turbocharger with Intercooler, on Four-Stroke-Cycle Engine. Source: Waukesha Engine Division

Upper Crank-

Upper Crank-

Biewer Scavenging System

Fig. 9-29a-d Various Configurations for Increasing Charge Density. Source: Fairbanks Morse Engine Div.

at a low pressure ratio, discharges the air directly into the engine intake manifold and then to the individual cylinder ports. The inlet air both scavenges the cylinder and supplies a sufficient air charge for proper combustion. The blower provides particularly good response during engine starting and to sudden load changes.

Fig. 9-28 Schematic Illustration of Turbocharger with Intercooler, on Four-Stroke-Cycle Engine. Source: Waukesha Engine Division

Turbo-Blower or Tu rbo ■ I ntercoo 1er Scavenging System

Upper Crank-

Upper Crank-

Compressor Air Assist

Turbocharged Scavenging System

Fig. 9-29a-d Various Configurations for Increasing Charge Density. Source: Fairbanks Morse Engine Div.

Fuel Delivery and Combustion Systems

Specific fuel consumption (sfc) and thermal fuel efficiency are based on the power delivered by an engine and the fuel energy consumed. Sfc is the ratio of the mass flow of fuel (irif), typically expressed as an amount of fuel, in number of lbm, gallons or cf used by the engine, per hour, to the power (P) produced or delivered by the engine (typically expressed as hp or kW). It is expressed as follows:

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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