Overview | Technology | Applications | Strengths/Limitations | Related Links

Overview

Reciprocating or internal combustion (IC) engines are part of our everyday life. There are over a million IC engines installed for electricity backup applications worldwide, and over 100 million engines in operation counting cars, trucks, planes, and boats. IC engines are best suited for backup, intermediate, peaking, and combined heat and power (CHP) applications where unit sizes with electrical output requirements range from a few kW up to roughly 10,000 kW. IC engines are installed in manufacturing facilities, office buildings, universities, hospitals, retail stores, distribution centers, and small utilities. IC engines are generally characterized as having:

• Low initial capital cost
• Proven reliability
• Strong maintenance support networks
• Rated output that is not impacted by higher ambient temperatures or elevations
• High partial load efficiency
• Heat recovery capabilities for combined heat and power
• No requirements for external inlet fuel compression

IC engines are divided into two basic types: spark?ignition and compression?ignition engines. The spark ignition engine is common in the form of gasoline powered car engines. Below 75 kW they are produced in large volumes, but are also seeing rapid acceptance above 300kW for natural gas fired power generation with heat recovery (CHP). Excluding the lowest output models, these engines typically have four-stroke combustion cycles, operate at medium to high speed, and are powered by liquid fuels or natural gas. Higher engine speeds allow for greater engine efficiency. Natural gas is often required for longer run hours to meet environmental regulations for applications with longer run hours.

Figure 1: Internal Combustion engine installation

Courtesy Caterpillar

The compression ignition engine, often called a diesel engine, is used in heavy trucks or buses. Diesel engines can have two- or four-stroke combustion cycles and can operate at any speed. In addition to numerous emergency diesel installations, diesels can be retrofitted to operate on various fuels ranging from waste gases such as land fill gas through heavy fuels such as # 6 oil (bunker C) heavy fuels. Heavy fuels are used extensively for power production in Africa, Asia, and many islands.

When comparing DE technologies, IC engines provide the best combination of efficiency and cost effectiveness in smaller scale applications. IC engines are found in the following cycles:

1) Simple Cycle - This is the standard operational method of IC engines. Simple cycle indicates that cogeneration or combined heat and power is not being employed. They have high simple cycle efficiencies, low capital cost and startup times typically of less than ten seconds. These attributes make IC engines well suited for back-up power.

2) Cogeneration or Combined Heat and Power - Combined Heat Power (CHP) is a leading configuration for supplying electricity while capturing thermal energy in the form of process steam or hot water for industrial and commercial applications. IC engines are not as efficient as combustion turbines in converting waste heat to steam (less than 50% thermal energy can be converted to steam), but are very well suited for applications requiring small amounts of steam at low pressure or small to large volumes of hot water. Reciprocating engine CHP installations have been steadily increasing.

Technology

IC engines and combustion turbine technologies both use the energy of combustion and convert it into rotating mechanical energy. The basic operation of an IC engine is similar to a combustion turbine in that both convert combustion gases into a rotating shaft (crank). However, combustion turbines use a continuous combustion process, whereas IC engines follow discrete steps in the energy conversion process. A typical four-stroke IC engine cycle consists of the following four steps:

• Intake
• Compression
• Power (Combustion)
• Exhaust

Figure 2: Cross section IC Engine

Courtesy Caterpillar

A four-stroke process requires two complete revolutions of the crank shaft to complete its cycle versus a two-stroke machine which completes the four cycles in one revolution of the crankshaft. During the Intake step, air and fuel are inducted into a cylinder when the piston is near or at its downward stroke (assumes a vertically oriented engine) and the intake valves or ports (located at the top of the engine block) open to draw in air. Intake air is always filtered to remove particles and extend the life of the engine. Once the air and fuel mixture is in the cylinder, the compression step occurs by an upward stroke of the piston that reduces the combustion volume and compresses the mixture. The piston is connected to the crankshaft by a connecting rod that pushes the piston upward as the crankshaft rotates. The piston travels upward until it reaches the end of its stroke.

The combustion or power step in the four-stroke cycle occurs when either compression is high enough (16:1) to cause the mixture to self ignite, or an external spark is introduced. The pressure ratio is the ratio of the pressure at full compression, or minimum volume, divided by the pressure of the cylinder at its maximum volume. The expanding exhaust gases push the piston downward, creating mechanical energy that causes the connecting rod to rotate the crank shaft. In the exhaust step, the valves or ports in the exhaust manifold open to allow hot exhaust gases to escape, completing the cycle. Many IC engines use turbochargers (essentially a small CT) to compress fuel and air for injection into the combustion cylinder. The turbocharger is powered by the post combustion exhaust gases. Non-turbocharged engines are called naturally aspirated.

In contrast to the four-stroke cycle, two-stroke machines complete their cycle in one revolution. For this reason, two-stroke air aspirated engines generate more mechanical power than their four-stroke counterpart with the same cylinder volume. Both types of engines go through the four steps listed above. When the piston moves downward, the two-stroke engine exposes an exhaust port that allows exhaust to escape and then introduces a fresh air/fuel mixture into the cylinder. The mixture is compressed with a subsequent upward stroke of the piston, followed by the combustion process that drives the piston back downward and creates mechanical power through the crankshaft. The exhaust valves or ports in the exhaust manifold open to allow hot exhaust gases to escape. Although two-strokes can generate more power than a four-stroke with equivalent displacement (cylinder volume), they are also less efficient and have higher emissions.

Figure 3: IC Engine Package

Courtesy Caterpillar

Some of the design considerations that impact the overall IC engine performance of the IC engine include the use of turbocharging, engine speed, brake mean effective pressure (BMP), and emissions control technologies. More specifically:

• Turbocharging - This is a concept almost as old as IC engines. In 1885 and 1896, Gottlieb Daimler and Rudolf Diesel researched the possibility of increasing engine output and efficiency by pre-compressing the intake air. The first commercial turbochargers were seen in truck engines in 1938. By compressing the fuel and air mixture that is introduced into the cylinder, turbochargers create a higher energy environment for combustion. Turbocharger design has advanced substantially with higher efficiencies being achieved through improvements in air flow design, materials metallurgy and air flow studies. Now most IC engines in the 100 kW range and above use turbocharging to boost engine output. The two main components of a turbocharger are the turbine and the compressor. Both share a single shaft. The turbine is driven by the hot exhaust gases leaving the exhaust manifold, which in turn drives a compressor on the same shaft. The turbocharger must be cooled to keep the temperature to the engine under its specified limit.
• Engine Speed - Increasing engine speed proportionally improves output power. A 50% increase in engine speed, for example, produces roughly 50% more in rated output. This greatly reduces the cost to produce a kWh of electricity. Unfortunately, increasing engine speed also leads to shorter maintenance intervals on engines. In back-up power applications, where the equipment is typically run for shorter time spans, IC generator packages tend to utilize higher engine speeds.
• Brake Mean Effective Pressure (BMP) - BMP is a unit that describes the air flow through an IC engine. BMP is equal to the product of rated horsepower divided by the sum of the piston area x the length of a piston stroke x the engine speed (horsepower / (piston area x stroke x speed). Increasing BMP directly increases horsepower, and in the case of a generator unit, also electric output. Air flow through an IC can be improved with well designed filters, intakes and exhaust, advanced turbochargers, and intercooling. Improved air filter, intake, and exhaust designs allow air to flow with less resistance, allowing the engine to operate with less resistance as it directs less energy to pushing and pulling air through the cylinders. Intercoolers function like a radiator used to cool the compressed air of a turbocharger before it enters the engine cylinder. When air is cooled, its density increases. Intercoolers provide a higher energy environment in the cylinder as more molecules of air and fuel can be introduced into the cylinder than is possible with a turbocharger alone. This higher energy environment provided by intercoolers and other advanced air flow designs translate directly into increased engine horsepower equal to Rated Horsepower / (piston area x stroke x speed). This means that increasing BMP directly increases HP (or kW).
• Emissions Controls Technologies - IC engines can produce a variety of pollutants including sulfur dioxides, nitrogen oxides, volatile organic compounds (VOCs), carbon monoxide, and carbon dioxide. The Environmental Issues section provides a description of these pollutants and some of the regulatory and siting issues that need to be considered before choosing a particular technology. Emission control technologies provide important environmental benefits. They can also affect engine performance. Lean burn natural gas fueled engines reduce emissions by lowering firing temperatures and optimizing the air/fuel mixture. Both of these techniques for reducing emissions can improve overall engine performance and can be employed on compression ignition engines as well. Aftercoolers, for example, are an effective method for lower firing temperature to reduce NOx emissions. Lowering peak pressures can also help reduce emissions, but have a detrimental effect on engine performance. Emission control technologies involving post combustion exhaust treatment, such as catalytic converters, can result in increased air flow resistance that causes a decrease in engine performance.

In IC engine design once an engine block design has been created, it can be modified relatively easily. First, multiple versions of an engine block design can be built by extending the size of the block and adding cylinders in line. Second, technological advances including turbocharging, engine speed, BMP, and emission control technologies can be rapidly deployed across multiple engine sizes that depend on the same core engine. As a result, manufacturers are able to offer a wide range of options that meet specific application requirements.

Applications

Reciprocating systems, much like combustion turbine systems, can be categorized as simple cycle or cogeneration. Brief descriptions of the basic system configurations are discussed below, along with some of the representative applications. The IC engine designs that are used best fit the following specific applications including:

1) Four-stroke Dual Fuel - Dual fuel systems are compression ignition engines that are either designed or later retrofitted to be powered by natural gas, but still require about 10% diesel as a pilot for ignition. This unit provides high efficiency output with low emissions for simple cycle peaking projects, as well as intermittent and continuous applications.

2) Four-stroke Diesel Fuel - This unit is suited for simple cycle emergency backup.

3) Two-stroke Diesel Fuel - These units tend to be low speed diesel units with turbocharging. These units are often used in countries where heavy fuel oils are commonly used. Emissions are high compared to other types of equipment.

4) Four-stroke Natural Gas or Propane - Similar to the dual fuel units, these units provide high efficiency with low emissions for peaking, intermittent and continuous simple cycle projects, as well as CHP applications.

Simple cycle installations are inexpensive and have low to medium efficiencies when compared to other power generation technologies. The simple cycle approach is ideal for backup or peaking power. These IC engines can ramp up to full power within 10 seconds of a loss of power. Back-up power systems are often coupled with uninterruptible power supply (UPS) systems, which typically consist of energy storage devices like large banks of batteries or flywheels. Both the IC and the UPS can be installed permanently, or placed on trailers for mobility.

When used for peaking applications engines will typically operate during the summer weekdays, when loads are highest because of air conditioning. These high electrical loads tend to occur in the mid-afternoon until early evening. In the U.S., peak summer electrical usage usually occurs between noon to 7 pm. Peaking engines are used by utilities to meet electric load requirements. Some business, government, and institutional facilities use peak reduction engines to assist with their energy management. In an attempt to reduce peak load demand, some utilities impose demand charges to dissuade businesses from using power during peak periods and to recoup revenue for the higher cost of peaking power. Other utilities approach demand reduction differently by offering to pay facilities to reduce their peak loads through demand response programs. Some of these programs allow business to use DE resources to reduce demand for grid power.

Figure 4: Small IC Engine Package

Courtesy DTE Energy

Cogeneration plants driven by IC engines provide facilities globally with low cost electricity and thermal energy (steam or hot water). This can result in thermal efficiencies exceeding 60%. IC engines offer four sources of heat:

• Hot exhaust gas
• IC engine water jacket cooling
• Turbocharger after cooling
• Lube oil cooling

IC engines are well suited for producing hot water, but not as efficient as CTs for steam production. The exhaust gas can create hot water or low to intermediate pressure steam. The steam pressure is dependent on the exhaust gas temperature and exhaust flow rate. The engine jacket cooling water, lube oil cooler and turbocharger cooler can be used to heat water through the extraction of heat from their processes. The most common uses for this hot water or steam from IC engines are for HVAC (heating or cooling with an absorption unit) or industrial processes.

Strengths/Limitations

ICs represent the largest installed base of DE in the world, with millions of units in operation. ICs dominate the marketplace below 1,000 kW and have a substantial market share for part of the market up to 10,000 kW. Units are installed principally as backup units, but peaking and cogeneration units are being installed at a rapid rate due to the low capital cost and high efficiency when compared to wind, solar and other DE technologies. In cogeneration applications requiring hot water, ICs are competitive against combustion turbines, but are not equal to CTs when steam is required.

Related Links

These links provide supplemental information on reciprocating engines. The vendor links will provide direct access to the manufacturer's Websites for manufacturers of reciprocating engines to obtain product specifications for reciprocating engines and generator sets.