By Douglas J. Smith IEng, Senior Editor
Reciprocating engines in sizes from a few kilowatts to several MW are used to generate electricity by utilities, hospitals, manufacturing plants and commercial buildings. For many years these engine generators were used primarily for emergency power. However, with engine technology developments and design improvements, reciprocating engines are being used in additional applications:
- Peak shaving to reduce peak demand charges
- Baseload units paralleled with the grid
Engine owners are beginning to realize that the lifecycle costs of reciprocating engine generators�fuel, operations and maintenance�represent a major concern. Fortunately, in recent years, research and development by engine manufacturers has extended the life and reduced the maintenance costs of operating reciprocating engines. In addition, many of the manufacturers have short-term or long-term operations and maintenance programs available. For example, Cummins has a program, “Cummins Power Guaranteed Maintenance Program,” for their lean burn gas engine generators that guarantees the maintenance costs.
Over the last few years, engine manufacturers have made some significant design improvements to increase the running hours before maintenance is required. At Wärtsilä the pre-chamber of their 20V34SG model natural gas fired reciprocating engines is one of the major components that has been redesigned. Because the pre-chamber redesign has optimized the combustion of the fuel, it has improved the performance, efficiency and emissions of the engines.
To increase the service intervals of the engines, Wärtsilä and its suppliers have developed spark plugs with better temperature and wear resistant materials. In addition, the spark plug and pre-chamber cooling has been improved, which further helps to extend the intervals before the spark plugs require changing. According to Wärtsilä, the new designed spark plugs have a minimum life of 2,000 hours before they need replacing. However, they believe the spark plugs have the potential to operate for 4,000 hours.
Cummins gas-fired engine generator installation. Photo courtesy of Cummins Power Generation
In the past Wärtsilä had problems with the pre-chamber’s gas supply check valve. Over a period of time the valves became fouled from the build-up of carbon deposits. As a result, mechanical cleaning of the check valve was required every 1,000 hours. A new valve, designed with bigger openings, has eliminated the problem. Although the maintenance interval is still being evaluated, Wärtsilä believes that the engine will be able to run for more than 4,000 hours before any maintenance on the valves is required.
According to Cummins Power Generation, their newer reciprocating engines have been designed to make maintenance easier. To facilitate bearing maintenance, Cummins has enlarged the crankcase inspection doors. Cylinder head maintenance has also been simplified by locating the intake and exhaust manifolds, which are self-supporting, in the center of the engines.
Although heavy fuel oil is not the fuel of choice for reciprocating engines, MAN B&W Diesel AG of Germany uses this fuel to power their largest four-stroke medium speed electric generating engines. MAN B&W has been able to extend the time between overhaul of their heavy fuel oil engines by incorporating:
- Water-cooled fuel nozzles
- Better bearing materials
- Chromium-ceramic upper piston rings
- Rotating outlet valves
- Hard faced exhaust valve seats
Table 1 shows the maintenance intervals and service life of various components of MAN B&W’s largest medium speed diesel engines burning heavy fuel oil. Horst Koehler, general manager, corporate four-stroke marketing, MAN B&W, Germany, says in the 1970s the average wear of the top ring was 0.3 mm/10,000 hours and average wear of the liner was 0.2 mm/10,000 hours. Today, even though the firing pressure of the medium speed engines has been increased to 2,900 psi from 1,880 psi, the wear of the top ring has been reduced to 0.1 mm and the liner wear to 0.05 mm/10,000 hours.
Neil Blythe, manager product engineering, Fairbanks Morse Engine, Beloit, Wisconsin, says that they have extended the intervals between maintenance and doubled the wear life of the rings and liners by using anti-bore polishing rings. The anti-bore polishing rings reduce lube oil consumption and wear of the liners and the rings from friction. Blythe’s experiences with spark ignited and dual fuel engines indicate that the ring and liner wear is 50% lower than a similarly rated diesel fueled engine. Bearing wear rates are also lower for spark ignited and dual fuel engines, he says.
Caterpillar Motoren GmbH, Germany, is using a new type of nozzle that has improved the fuel spray and reduced nozzle wear. With improved atomization of the fuel, fuel consumption has decreased and combustion has improved. Similarly, extending the service life of the cylinder heads has been accomplished by improved cooling of the valve inserts. In addition, Caterpillar has chromium plated the first ring groove on engines burning heavy fuel oil. Since there is less ring groove wear, running time can be extended before maintenance is required.
Installing piston rings. Photo courtesy of MAN B&W Diesel AG
Optimizing crankshaft balancing and reducing engine vibrations has also helped Caterpillar to reduce bearing wear. Over the last few years Caterpillar has reduced the number of engine parts by 40% and minimized the number of sealing joints used. As a result of these design changes Caterpillar has been able to improve the availability and reliability of their engines and extend the periods between major and minor overhauls.
According to Leo LeBlanc, managing director, Nixon Energy Solutions, North Carolina, electronic controls and monitoring systems have played a big part in optimizing the operation and maintenance of reciprocating engines. Fuel delivery, gas mixing and direct injection have all been improved through the use of electronics, says LeBlanc.
Reciprocating engines use a variety of fuels other than diesel and natural gas, including low Btu landfill and coal-bed methane gas, flare gas and heavy fuel oil. Flare gas is not corrosive and only differs from natural gas in that it contains more of the higher hydrocarbons. According to Cummins, this reduces the methane number, or the resistance to knocking. To combat this problem, engine manufacturers can supply engines designed with lower compression ratio pistons. In some instances where flare gas is used the engines may have to be de-rated. Even so, flare gas generally has no effect on the maintenance schedules. Low Btu coal bed methane and digester gas, which are non-corrosive, also have little or no effect on engine maintenance.
In engines burning low Btu landfill gas, the main concern is with lube oil oxidation. Due to sulfur compounds and other elements in the gas, acids are formed in the lube oil. As a result the maintenance cycles are shortened considerably. Engines at some sites may operate for only 250 hours before the lube oil needs changing. Another concern with using landfill gases is solid particles in the form of silicates. These particles cause abrasive wear in the engines and particularly on the cylinder heads and valves.
According to Valerie Bartkus, business development analyst, Stewart & Stevenson Distributed Energy, siloxanes in landfill gas cause major engine wear and lead to more frequent oil changes, valve adjustments, inspections and frequent cylinder head replacement. Siloxanes are silicates formed from the decomposition of plastics.
According to LeBlanc, natural gas is the lowest cost fuel for reciprocating engines, followed by diesel. Because dual fuel engines require more costly fuel systems, the fuel costs are higher than for engines burning natural gas or diesel separately. LeBlanc estimates the maintenance costs for natural gas engines at 1-1.5 mils/kWhr; diesel, 2 mils/kWhr and dual fuel, 2.5-3 mils/kWhr.
Operations and Maintenance
While most maintenance schedules are generally based on hours of operation, wear and tear of an engine is a function of not just operating hours but also the number of starts, loading regime, fuel type, and oil and water quality, says Blythe. Since peaking and emergency standby units typically have less operating hours per year than baseload units, the annual maintenance costs are lower.
However, Blythe says that due to the higher frequency of starts, rapid loading and relatively high loads when in operation, the maintenance cost per hour for peaking and standby units is higher than for baseload units. On average MAN B&W overhauls standby emergency units once every five years and baseload units once every three years. Low load operation of engines generally allows maintenance intervals to be extended.
Inspecting crankcase of medium speed diesel engine. Photo courtesy of MAN B&W Diesel AG
Although the intervals for changing lube oil depends upon the fuel consumption and the engine’s load and running hours, it does not mean that the lube oil in standby emergency engines can be used for several years before being replaced. Oil changes on standby emergency and peaking units are generally changed on time elapsed, or number of starts, rather than running hours.
Reducing maintenance cost does not have to be complicated or costly. Cummins believes the most common maintenance problem that leads to higher costs of operating diesel engines is not doing maintenance in the first place, preventive and/or predictive. Plugged air filters, or the engine coolant not having the correct concentration of corrosion inhibitor, can contribute to higher maintenance costs. On smaller engines used for standby and emergency, the failure of batteries for starting the engines is a big issue.
Cummins recommends that key components, such as turbochargers, vibration dampers, water pumps, fan clutch and idler assembly, engine/generator mounts, be checked every 10,000 hours of operation or every two years.
According to Cummins, the most important routine maintenance activities include:
- Changing the lube oil
- Topping the batteries
- Cycling the diesel fuel in storage tanks
- Changing air filters
- Replacing belts
- Checking coolant additives
LeBlanc believes that giving priority to the cleanliness of the fuel, lube oil and air is the most cost effective way of extending the time between major engine overhauls. Keeping the fuel clean will extend the running hours before the cylinder head and its components must be removed for inspection and/or repair. Similarly, keeping the lube oil clean will decrease bearing problems. According to LeBlanc, the bottom line for reducing maintenance costs and extending running hours is: Keep the fuel clean, analyze the lube oil regularly, have a clean working environment during any engine maintenance, and recycle stored fuel at least once/year.
Technical and design factors also have a big role to play, however. In Blythe’s opinion, higher injection pressures, material improvements and the application of electronic controls have been instrumental in improving availability, reducing maintenance costs and increasing efficiency. The proof is in the class of engines being put into service today. Modern diesel engines have a thermal efficiency between 45% and 50% while natural gas-fired engines have efficiencies of 40% to 45%.