- Its combustion characteristics are reasonably compatible with typical diesel engine designs
- Extensive distribution infrastructure has been developed to make it an economical, commonly available, utility supplied fuel
Throughout the Twentieth Century, various methods were developed to properly deliver natural gas into a diesel engine. Extreme care is required during this process, as an excess concentration of natural gas can cause engine damage due to pre-ignition, or "knock". Early dual-fuel designs used conventional mechanical control systems of the day to control the process. Due to limited capabilities of these controls, performance was compromised and commercial success was restricted to niche applications. More recently, microprocessor controls along with advanced sensor and actuator technologies have provided new opportunities to meet the challenges.
The desire for dual-fuel engines is driven by several environmental and economic factors. Combining diesel fuel with natural gas in dual-fuel operation provides several benefits compared to engines fueled only by diesel or natural gas. Major benefits include:
- Extended run time capabilities
- Reduced diesel fuel storage requirements
- Lower capital cost per kilowatt (kW) compared to spark-ignited engines
- Improved reliability with redundant fuel supply
- Reduced maintenance costs
- Potential for less fuel costs
- Lower exhaust emissions than diesel engines
Dual-fuel methods and operation
Several characteristics distinguish the diesel engine from four-cycle spark-ignited (also referred to as Otto cycle) engines commonly used today to burn gasoline or vaporized fuels such as natural gas:
- A diesel engine uses compression ignition rather than spark ignition. The heat generated by compressing air to high pressures provides the source of ignition for the diesel fuel.
- A diesel engine compresses only air and then injects fuel directly into the cylinder for combustion. Most Otto cycle engines mix the fuel with air before it enters the cylinder(s), using either a carburettor or "indirect" fuel injector(s), often referred to as throttle body or port fuel injection. After the mixture is compressed in the cylinder, an electrical spark (delivered through a spark plug) provides the energy to ignite the fuel.
- Since diesel engines compress only air, they can safely operate at higher compression ratios (typically 13:1 ~ 23:1 compared to 8:1 ~ 12:1 for spark-ignited engines) without concern for pre-ignition. A major benefit of the higher compression ratio is that diesel engines are inherently more energy efficient than lower compression spark-ignited engines. In other words, more of the fuel energy gets converted to mechanical energy rather than being rejected as heat (this was the primary motivation for Diesel's invention in the first place).
Natural gas ignites at a much higher temperature (620° ~ 650° C) compared to diesel fuel (260° ~ 400° C). A diesel engine cannot operate on 100% natural gas. Because the heat generated during compression is not sufficient to ignite this fuel. To create ignition in dual-fuel engines, a small amount of diesel fuel must be injected. Cylinder temperatures are high enough to ignite the diesel fuel, and the flame created reaches a temperature sufficient to ignite the natural gas.
A dual-fuel engine uses a conventional diesel engine as its basis. With most designs, the diesel fuel is delivered using the injectors that already exist on the engine. Additional components are installed to deliver natural gas into the combustion chamber. These are three proven methods that have been employed to do this:
- Low pressure injected natural gas introduces the natural gas using port injection, so it mixes with combustion air just before it enters the cylinder. This is done under moderate pressure, usually less than 3,5 bar. As many diesel engines use turbochargers to feed air into the cylinders, injection pressures must be greater than the boost pressure developed. This approach has been used in large stationary installations.
- High pressure injected natural gas delivers natural gas directly into the combustion chamber under extremely high pressures of approximately 200 bar. This is necessary since the natural gas is injected when the cylinder pressure is very high - at the end of the compression stroke and after diesel fuel has been injected to initiate combustion. This approach has found application in very large dual-fuel engines that typically operate for extended periods producing prime or continuous power. This is due to the economics involved, as separate high pressure natural gas injectors (or sophisticated combination diesel/natural gas injectors), pumps and fuel delivery lines system drive a large price premium for these engine systems.
- Combustion air gas integration introduces the natural gas with intake combustion air just prior to the turbocharger. Since a single, low pressure delivery system is used, additional engine component costs are minimized. Advanced microprocessor, sensor and actuator technologies can provide the precision and response necessary to control the system.
During initial startup, the engine operates on 100% diesel fuel. After certain permissive criteria are satisfied (for instance, the engine coolant temperature reaching 70° C, or acceptance of the electrical load), the microprocessor commences dual-fuel operation and more fuel energy is provided by the natural gas.
Throughout the process, the controller continuously monitors a variety of engine and generator parameters, including intake air temperature, engine coolant temperature, intake manifold temperature and pressure, kW load, engine speed. Through extensive mapping of these variables and their effect upon engine performance, the microprocessor automatically adjusts the dual-fuel ratio and fine tunes the mixture for optimum engine operation.
With utilization of publicly available materials.
