Reciprocating Engine Modifications

The differences between Diesel-cycle and Otto-cycle engines (described in Chapter 9) affect the production and control of NOx. While the Diesel cycle is characterized by compression of air only with heat addition at constant pressure, the Otto cycle includes compression of an air/fuel mixture with heat addition at constant volume. Compression ignited (Diesel-cycle) engines produce high localized flame temperatures and, therefore, higher thermal

NOx emissions than lean-burn spark-ignited (Otto-cycle) engines. However, dual-fuel compression-ignited engines using a mixture of liquid and gaseous fuels can achieve significantly lower NOx emissions than liquid fuel engines. Rich-burn (below the stoichiometric level) spark-ignited engines generally produce moderate levels of NOx emissions, as well as high levels of CO and HC emissions.

Very lean mixtures generally cause slow burning combustion and can reduce bmep and, therefore, thermal fuel efficiency and capacity. To minimize thermal fuel efficiency losses, it is necessary to achieve thorough fuel-air mixing to limit peak flame temperatures and increase flame speed. Combustion chambers are designed for a high degree of turbulence. Swirl is used to promote more rapid mixing between the inducted air charge and injected fuel in compression ignition engines and is also used to speed up the combustion process in spark-ignited engines.

Figure 17-17 compares exhaust NOx output to air-to-fuel ratio for a natural gas-fired spark-ignition engine. The graph shows NOx emissions in ppmd (@ 15% O2) versus air-to-fuel ratio and Lambda (^). With rich combustion (left side of graph), NOx decreases due to lower combustion temperatures and lack of oxygen. With lean combustion (right side of graph), NOx initially reaches a peak because combustion temperature remains high and there is an abundance of oxygen. In establishing optimal air-to-fuel ratios, a balance must be achieved between numerous variables, including NOx, CO, and HC emissions, as well as bmep and thermal fuel efficiency. In spark-ignition engines, care must be taken to

Fig. 17-17 NOx Emissions of Natural Gas-Fired Spark-Ignition Engine as a Function of Air-to-Fuel Ratio. Source: Waukesha Engine Div.

avoid both knocking limits and incomplete combustion. Figure 17-18 illustrates the relationship of these variables. As can be seen, there is a narrow band between knocking limits and incomplete combustion, where NOX formation is lowest, fuel efficiency and specific load are greatest, and CO and HC emissions are relatively low.

Several methods are used for precise air-to-fuel adjustment. In one cogeneration system, the microprocessor controller considers engine output, air-to-fuel mixture pressure (boost pressure), and mixture temperature after the intercooler. The signal output by the controller moves the adjusting cone of the air-to-gas mixer into the desired position to set the required ratio. This system is reportedly capable of achieving NOx emissions levels of 45 ppmd (@ 15% O2), or about 0.5 g/bhp-h (0.7 g/kWh). In injection-type reciprocating engines, including all Diesel and many dual-fuel and natural gas engines, the air-to-fuel ratio can be adjusted for each cylinder. A precise, homogenous combustion mixture is necessary in all cylinders in order to minimize NOX emissions while avoiding misfire and detonation operating regions.

Ignition in a normally adjusted reciprocating engine is set to occur shortly before the piston reaches its uppermost position (top dead center, or TDC). At TDC, the air or air-to-fuel mixture is at maximum compression and power output and fuel consumption are optimum. Ignition timing retard is a NOx control technique that causes more of the combustion to occur during the expansion stroke, thus lowering peak temperature, pressure, and residence time. Typical retard values range from 2 to 6

degrees, depending on the engine. Some increase in CO emissions and a reduction in fuel efficiency generally does occur.

Pre-stratified charge (PSC) combustion is a retrofit system that has been applied to small and mid-sized 4-stroke-cycle, carbureted, rich-burn reciprocating engines. Controlled amounts of air are introduced into the intake manifold before the fresh air-to-fuel mixture, causing stratification that decreases the combustion temperature. The sequence must be precisely controlled to prevent any mixing of the dilution layer and the fresh mix. If the two layers have time to mix, misfires can occur. Engines require an increase in intake air capacity via charging or derating of maximum power output when this system is used.

Pre-combustion chamber systems, which are described in detail in Chapter 9, are commonly used with newer lean-burn spark-ignition and certain dual-fuel compression-ignition designs. They allow the engine's main combustion chamber to operate at higher air-to-fuel ratios than open chamber lean-burn engines. To ensure proper combustion, a small volume of fuel-rich mixture (below the stoichiometric level) is burned in a pre-combustion chamber or ignition cell, which comprises about 1 to 5% of the clearance volume located in the cylinder head. This fuel mixture may be spark or compression ignited. The flame from the cell reaches into the main combustion chamber and ignites the remaining lean mixture. This technology, which is also sometimes referred to as torch ignition or stratified charge, is more expensive than open chamber design, but produces far lower NOx levels than traditional lean-burn technology.

Figure 17-19 illustrates a pre-chamber design for a spark-ignited engine and Figure 17-20 illustrates the injection of pilot fuel into the pre-chamber of a dual-fuel compression-ignition engine. Reportedly, NOx emissions levels of under 0.5 g/hp-h (0.75 g/kWh) can be achieved along with CO and non-methane HC (NMHC) emissions of under 1.5 g/hp-h and 0.75 g/hp-h (2.0 g/kWh and 1.0 g/kWh), respectively, while maintaining high simple-cycle thermal fuel efficiency.

A key factor in dual-fuel engine designs is the ability to minimize the amount of pilot oil used. Figure 17-21 shows the influence of the pilot fuel quantity on pollutant emissions and thermal fuel efficiency, with direct pilot fuel injection. While thermal fuel efficiency and CO and HC emissions remain fairly constant from 1 to 3.5% pilot fuel proportion, NOx emissions vary greatly. In the figure, the optimal pilot fuel proportion is indicated at about 1%.

Load bmep Emissions Thermal efficiency

Load bmep Emissions Thermal efficiency

o.a 1.0 1.2 1.4 1.« 1.8 2.0 2.2 Excess air ratio <U

Fig. 17-18 Relationship Curves for Various Key Factors in Optimizing Air-to-Fuel Ratios for Spark-Ignited Engines. Source: Wartsila Diesel o.a 1.0 1.2 1.4 1.« 1.8 2.0 2.2 Excess air ratio <U

Fig. 17-18 Relationship Curves for Various Key Factors in Optimizing Air-to-Fuel Ratios for Spark-Ignited Engines. Source: Wartsila Diesel

Fig. 17-19 Illustration of Pre-Chamber Design for Spark-Ignited Reciprocating Engine. Source: MAN B&W
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