05

2'3 Speed

50 Hz

60 Hz

Fig. 9-51 Jacket Water Duty as a Function of Speed and Capacity. Source: Cogen Designs, Inc.

Waukesha engine. The engine is a turbocharged, intercooled four-stroke-cycle lean-burn unit, featuring pre-chamber combustion design and four valves per cylinder. Compression ratio is 9:1 and displacement is 8,699 in3 (142.5 liter). The performance curves are based on continuous duty rating, which is defined as the highest load and speed that can be applied 24 hours per day, 365 days per year, except for normal maintenance. It is also based on 10% overload rating, which is allowed 2 hours per 24 hours.

The figure graphs bsfc, or fuel consumption per bhp-h output, versus load for three operating speeds: 800, 900, and 1,000 rpm, for operation with a 32:1 air-fuel ratio. Notice the decrease in capacity and increase in thermal fuel efficiency as speed is reduced: capacity is reduced from 2,090 bhp (1,558 kW) at 1,000 rpm to 1,670 bhp (1,245 kW) at 800 rpm, while the full load heat rate is reduced from 6,556 Btu/bhp-h (9,851 kJ/kWh) at 1,000rpm to 6,369 Btu/bhp-h (8,793 kJ/kWh) at 800 rpm.

In the United States, standard conditions referenced for engines are 14.696 psia (101.325 kPa) and 60°F

Fig. 9-52 Performance Curve for Spark-Ignited Gas Engine. Source: Waukesha Engine Div.

(16°C). Fuel flow, in standard cubic feet (scf), is referenced to a gas at standard conditions. New ISO standard conditions applicable to the example in Figure 9-52 are: 29.54 in. Hg (100 kPa) barometric pressure, 77°F (25°C) ambient and induction air temperature, and 30% relative humidity (1 kPa/0.3 in. Hg. water vapor pressure). Fuel is specified as dry natural gas with 900 Btu/ft3 (33.5 J/cm3) on an LHV basis and 118 octane rating. The performance curves are based on operation at 180°F (82°C) jacket water outlet with 130°F (54°C) intercooler water at a bmep of 190 psi (13.1 Bar).

Based on the standard conditions used in establishing performance ratings, adjustment factors may be applied to calibrate the ratings for operation at conditions other than those specified. In the equipment specification package, heat balance data is provided for a range of characteristic factors, including: Speed (rpm) Power (bhp or kW) Bsfc (Btu/bhp-hr or kJ/kWh) Fuel consumption (Btu/hr or kW) Heat to jacket water (Btu/hr or kW) Heat to lube oil (Btu/hr or kW) Heat to intercooler (Btu/hr or kW) Heat to radiation (Btu/hr or kW) Total energy in exhaust (Btu/hr or kW) Exhaust temperature after turbine (+/-50°F or 30°C) Induction airflow (scfm or nm3/h) Exhaust gas flow (lbm/hr or kg/h)

Following are examples of the impact of changes in the heat balance values resulting from changes in one variable: • At 1,000 rpm, 180°F (82°C) jacket water temperature, and 130°F (54°C) intercooler water temperature, the continuous power is 2,090 bhp (1,558 kW) at a bmep of 190 psi (13.1 bar). By decreasing the intercooler water temperature to 90°F (32°C), the continuous power rating increases to 2,200 bhp (1,640 kW) at a bmep of 200 psi (13.8 bar). Bsfc increases only slightly.

• With the intercooler water maintained at 90°F (32°C) and changing jacket water temperature to 250°F (121°C), capacity at 1,000 rpm remains constant. Heat rejection to jacket water is reduced 18%, heat rejection to lube oil increases 29%, and heat loss to radiation increases 55%.

• At 209 bmep (14.4 bar), corresponding to the 10% overload rating, capacity increases to 2,299 bhp (1,714 kW), while bsfc decreases by about 1.5%.

Table 9-1 lists adjustment factors for selected Waukesha engines based on altitude and temperature. In this case, all natural gas engine ratings are based on a fuel of 900 Btu/ft3 (35.3 MJ/m3) SLHV, 119 octane (per ASTM D-2700 test method). All Diesel engine ratings are based on a #2-D fuel of 18,400 Btu/lbm (42.8 kJ/g) LHV. Ratings are based on ISO 3046/1-1986 with mechanical efficiency of 90% and Tcra (clause 10.1) as specified limited to +/-10°F (5°C). Ratings are valid for SAE J1349, BS 5514, DIN 6271, and AP 17B-11C standard atmospheric conditions.

The table lists adjustment values in two categories: Intermittent/Standby and ISO Standard/Prime Power. The Intermittent Service rating is defined as the highest load and speed that can be applied in variable speed mechanical system applications only. Operation at this rating is limited to a maximum of 3,500 hours per year. The Standby Service rating applies to those systems used as a secondary source of electric power. This rating is the output the engine will produce continuously (no overload), 24 hours per day for the duration of the prime power source outage. The ISO Standard Power/Continuous Power rating is defined as the highest load and speed, which can be on a continuous basis, year-round. It is permissible to operate the engine at up to 10% overload, or maximum load indicated by the intermittent rating, whichever is lower for two hours in every 24 hour period.

Tables 9-2 and 9-3 show the manufacturer's heat balance tables for a Fairbanks Morse Diesel engine designed for dual-fuel and Diesel operating mode, respectively. The heat balances are based on maintaining the jacket water out temperature at 175°F (+/-5 degrees), the lubricating oil inlet temperature at 135°F, and the intake airflow is 90°F.

Performance curves are particularly useful for predicting performance for operation under variable load and speed. The bsfc of a reciprocating engine operating at constant speed increases with decreasing load. If power requirements vary, variable speed operation can be a big advantage. Thermal efficiency levels will remain close to full-load ratings in the middle and upper ranges of a load curve. Efficiency drops off at the lower end of the load curve, but not nearly as quickly as with constant speed operation.

When a gear box is used, efficiency is reduced by 2% to 5%. In engine generator applications, efficiency is also reduced by an additional 1.5% to 4% as a result of generator losses. Usually, when output is specified in terms of the driven load, i.e., generator output in kW, generator and gearing losses are included in the performance data.

Another factor to consider when evaluating system performance is the auxiliary component requirement, or parasitic loads. For example, high-compression engines must have fuel delivered at a pressure higher than the combustion chamber pressure at the time of injection. For example, given a delivered gas pressure of 50 psig (4.5 bar) (at the gas meter) for a 7,000 hp (5,200 kW) engine requiring a delivery pressure of 1,600 psig (111 bar), the parasitic load for gas compression is greater than 4% of the engine output.

Engine Maintenance, Reliability and Life

While first-cost and thermal efficiency are often the driving force behind investment decisions, engine life, reliability, and maintenance requirements are critical factors that greatly affect life-cycle economics.

Although there are exceptions to every rule, lower rotational speed, mep, and compression ratios for similar type engines generally result in longer engine life and a lower maintenance requirement. Higher speed engines accumulate more operating strokes, which means faster wear. Higher bmep and compression ratio mean higher firing pressures and temperatures, also promoting wear. Output and efficiency enhancement features, such as tur-bocharging or supercharging, tend to produce greater stresses and result in greater wear and shorter engine life.

The benefits of lower first cost and somewhat greater thermal efficiency of oil-fired engines come at the expense of higher maintenance requirements. This is due to deposit build-ups, oil contamination, clogging of injectors and nozzles, and higher fouling factors in exhaust and heat recovery components.

The type of operating duty is also a critical factor. In particular, frequent starting promotes for engine wear and engines running close to maximum power rating are subject to added wear. Engines used for standby or emergency use will experience a greater degree of wear, per hour of run time, than engines used for regular duty because they operate at a higher power rating, are subject to more starts and stops, and typically operate at higher speeds.

Table 9-1 Manufacturer's Performance Adjustment Factors

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