Focusing on dual-fuel engine benefits

Converting diesel gensets to dual-fuel operation can lower operating costs while complying with emission regulations.

11/10/2011


The fundamental concept of a dual-fueled reciprocating engine is not new. In the 1890s, Rudolph Diesel experimented with this approach during his research and development of the diesel engine. He introduced what is commonly referred to as pipeline natural gas into the air intake and observed improvements in engine performance.

Since then, dual-fuel engines have been available in many markets including mobile and stationary applications. These types of engines can be found in locomotives, automobiles, and the electric power generation industry where dual-fuel engines were used as early as the 1930s. Prior to the availability of grid power in more rural areas, hundreds of these engines could be found in many countries.

Although many dual-fuel engines were taken out of service due to the economical benefits of grid power, this concept is gaining popularity again. The topic of decreasing dependence on imported oil is a recurring issue. Emission regulations and a renewed push for clean technologies are at the forefront of many government initiatives. With diesel fuel prices rising and increasing regulation on emissions, many diesel genset owners and operators are searching for an alternative to conventional fuel. Modern dual-fuel systems with electronic controls that enhance system performance represent a viable answer to growing concerns about the widespread installed base of diesel engines (see right). 

(Above right) Figure 1: Modern dual-fuel systems with electronic controls (inset) can enhance system performance, lower operating costs, and help meet emission regulations.Courtesy: Governor Control Systems Inc.

Typical applications

Since a diesel engine is a compression ignition engine and does not have a spark-plug-type ignition system, the primary diesel fuel is used as the ignition source or pilot ignition for the mixture in the combustion chamber. Therefore, dual-fuel engines retain the fundamental principles of diesel engine operation and the efficiency of the diesel engine compression ratio while enabling the engine to run on a cheaper, cleaner fuel.

Industrial dual-fuel engine applications are typically separated into two segments: low-speed and high-speed. Low-speed is defined as 1,000 rpm or lower. High-speed engines generally run between 1,200 and 1,800 rpm.

Single-point fuel admission 

Gaseous fuel admission in a high-speed industrial diesel engine is similar to the methodology used on a traditional gas engine. The gaseous fuel is admitted into the engine’s air intake through mixers installed upstream of the turbocharger in a concept referred to as fumigation or single-point admission. The incoming gas supply is filtered prior to the pressure regulator and shutoff valves (see below).

Figure 2: For single-point fuel admission on high-speed engines, the electronic control system uses fuel pressure, manifold air pressure, and temperature to calculate the optimum diesel-to-gas ratio, which determines the appropriate fuel valve position. Courtesy: Governor Control Systems Inc.

The fuel flow is regulated through the use of a butterfly-style throttle valve controlled by the main control system before being admitted through the mixer. The control system uses a series of parameters obtained from sensors and transducers, including fuel pressure, manifold air pressure, and temperature, to calculate the optimum diesel-to-gas ratio and position the fuel valve to the appropriate position to admit the proper amount of gaseous fuel. This technique commonly allows between 50% and 70% gas substitution rates—and higher in some applications.

Multipoint fuel admission

Gaseous fuel is not admitted into a low-speed engine in the same manner as in a high-speed engine. On low-speed engines, fuel is injected through individual valves on each cylinder in a concept referred to as multipoint admission or injection, as opposed to using pre-turbocharger fuel mixers (see below).

Figure 3: For multipoint admission on low-speed engines, the electronic control system governs the injection of fuel through individual valves on each cylinder. Courtesy: Governor Control Systems Inc.

The driving factor for the difference in methodology is intake and exhaust valve timing. There is a period of time during the engine cycle where the intake and exhaust valves are open at the same time, which is referred to as valve overlap (see below, right). It is during this period that the cylinder is flushed with clean, cool air, which is often called scavenging. In order for this operation to be maintained in a dual-fuel engine, the gas flow to the cylinder must be shut off for a period of time to eliminate the possibility of the presence of gas in the exhaust manifold, a situation that could be potentially dangerous and explosive.

(Right) Figure 4: The electronic control system drives the electrically operated solenoid valves that shut off the gas flow to the cylinder to allow proper scavenging during intake and exhaust valve overlap. Courtesy: Governor Control Systems Inc.

This interruption of the gas supply to the cylinder is accomplished through the use of electrically operated solenoid valves. On a high-speed engine, the valve overlap time is much shorter than that of a low-speed engine, so a continuous gaseous fuel flow is possible. In low-speed applications, an electronic control drives the individual fuel solenoid valves controlling timing and duration of fuel injection into the cylinder. This technique commonly allows between 60% and 80% gas substitution rates—and higher in some applications.

Benefits of dual-fuel operation 

Economic benefits: With the cost of diesel fuel rising, and dual-fuel engines considerably reducing diesel fuel usage, converting an engine to operate primarily on a cheaper gaseous fuel is economically attractive. In addition, spark plugs and an ignition system are not required, eliminating the costly spark plug maintenance associated with traditional natural gas engines, which helps to further reduce overall cost of operation. Depending on the expected number of running hours and the cost of diesel and gaseous fuels, the up-front installation cost of retrofitting an existing diesel engine to dual-fuel operation can be recovered quickly.

Environmental benefits: Gaseous fuels—and natural gas in particular—are much cleaner than diesel. Diesel engines that have been converted to dual-fuel operation have exhibited significant reduction in NOX and CO2 over their original diesel operation. This is even more important in areas with increasingly tough emissions regulations. In addition, on-site diesel storage capacity can be reduced.

Technical benefits: Retrofit systems can be installed in the field quickly, minimizing engine downtime. No modifications are required to the core engine or to the factory fuel management system. With the engine’s main fuel becoming gaseous fuel rather than diesel and the electronic control system maximizing fuel efficiency, installing an alternative fuel system enables the on-site diesel supply to last much longer, extending engine uptime without compromising performance.

Replacing diesel fuel with natural gas typically extends engine maintenance intervals and overall engine life. For example, life expectancy of cylinder-head valve seats is improved due to the cleaner combustion that gaseous fuel exhibits over diesel. Benefits of the factory diesel engine, including hardware ruggedness and operational efficiency, are maintained. Returning to operation on 100% diesel fuel is possible at any time.

Safety: Gasoline or petrol is an easily ignited volatile fuel. While diesel fuel is less volatile, it presents the same storage and handling problems. Comparatively, natural gas exhibits many different characteristics. It is buoyant at temperatures above -160 F, does not pool on the ground, and dissipates rapidly in the atmosphere (See Table 1). It is nontoxic, noncorrosive, and environmentally safe.

Table 1: Fuel Comparison

Fuel

 

Chemical formula

 

Description

 

Energy density (compared to diesel)

 

Diesel

 

C10H20 to C15H28

 

Combustible hydrocarbon mixture that ignites when compressed in an internal combustion engine.

 

 

 

Gasoline

 

C8H18

 

Combustible mixture of hydrocarbons and additives. Hydrocarbon composition is modified to obtain different octane ratings.

 

90%

 

Liquefied petroleum gas (LPG

 

C3H8

 

Combustible hydrocarbon mixture obtained by refining fossil fuels.

 

65%

 

Natural gas

 

CH4

 

Mixture of combustible gases, primarily consisting of methane found beneath the Earth’s surface

 

18%

 

Compressed natural gas (CNG)

 

Obtained by compressing natural gas, typically between 2,900 and 3,600 psi.

 

25%

 

Liquefied natural gas (LNG)

 

Obtained by supercooling natural gas to approximately -260 F.

 

60%

 

This table compares fuels and their energy densities. Courtesy: Governor Control Systems Inc.

Natural gas also has a high auto-ignition temperature. The minimum temperature required to ignite methane without a spark or flame present is approximately 1,076 F. This is more than 500 degrees higher than gasoline at 536 F, making it difficult to auto-ignite. Natural gas’s narrow range of flammability is also an important safety aspect. Natural gas burns in concentrations between 5% and 15% only, making accidental ignition highly unlikely. Most importantly, natural gas does not detonate in an open environment.

Some dual-fuel engine applications and installations are subject to safety directives such as ATEX in the European Union or Canadian Standards Association in North America. These directives are enforced wherever a potentially explosive environment is present. The main requirement is to prevent the formation of this environment. This is normally accomplished through the use of either double-walled gaseous fuel piping or single-walled piping installed in a separate compartment. In the case of double-walled piping, the space between the walls can be continuously ventilated.

In addition, gas detection sensors that continuously monitor the environment for the presence of gas can be installed in the engine room. These sensors are connected to an alarm system that has the ability to switch off the gaseous fuel supply, and either return the engine to operation on 100% diesel fuel or shut down the engine completely.

OEM alternatives: A number of OEMs produce conventional gas engines. Generally, spark-ignited engines are designed by the OEM to operate on specific gaseous fuels, and are therefore optimized with a certain compression ratio, timing, and air/fuel ratio to produce the highest efficiency and power output with the lowest emissions possible. However, there are a number of disadvantages to them. First, the power output of a spark-ignited gas engine is lower than that of a similar size diesel engine. This translates to a higher capital investment during initial installation.

The spark ignition system has a high cost of maintenance as well. Although many manufacturers continue to invest in development of longer-life spark plugs, their operational life continues to be a concern. Spark-ignited engines also run hotter than their diesel counterparts, which significantly increases valve seat wear rates.

A small number of OEMs produce a factory dual-fuel engine. An engine designed specifically for dual-fuel operation can attain a higher diesel-to-gas ratio than a converted conventional diesel. While manufacturers of these engines claim operation on as low as 1% diesel fuel, the engines may not be economical for the general customer base with installed diesel engines due to their high initial cost. Converting a conventional diesel engine to an OEM factory style dual-fuel system requires change of major engine hardware such as pistons and heads, as opposed to a standard diesel conversion that requires no change to basic engine hardware.

Auxiliary systems: In addition to the advantages of a standard dual-fuel conversion, additional features can be added to the system for enhanced benefit. One example is a fuel flow metering system. When considering an alternative fuel system, it is necessary to know how much diesel fuel is actually being saved and how much gaseous fuel is being used. A flow metering system can be integrated to measure the supply of both fuels and the diesel return line to calculate fuel usage and associated cost savings.

An operator interface enables the controls system to display its information in one central location for operator control and system parameter monitoring (see Figure 5, below). All basic engine parameters can be monitored along with diesel and gaseous fuel ratio, alarm status, real-time performance trending, and available remote access via the Internet.

Figure 5: Operators can monitor basic engine parameters, fuel ratio, alarm status, and real-time system performance on an operator interface screen. Courtesy: Governor Control Systems Inc.

Conclusion

Owners or operators of existing diesel engines interested in cost savings should evaluate the benefits of a dual-fuel conversion. While the dual-fuel engine concept is not new, rising diesel fuel costs, more emphasis on emission regulations, desire to increase engine maintenance intervals, and controlling overall costs of operation are increasing interest in this technology.

Offering ease of installation and relatively low capital investment, dual-fuel conversions provide the ability to realize this cost savings and adhere to regulations through the use of gaseous fuel in both low- and high-speed industrial engine applications.


Martz is an application engineer with Governor Control Systems Inc., based in Fort Lauderdale, Fla., where he designs custom engine and turbine control systems with an emphasis on power generation and marine propulsion applications.



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