Adapting spark-ignited engines for gaseous fuel operation
Backup power systems typically rely on internal combustion engines as the prime mover. In the past, compression-ignited engines were considered the go-to technology for these systems. Because of diesel fuel’s high thermal efficiency, a diesel-fueled generator typically delivers lower capital cost per kW than a comparable gaseous-fueled generator in applications of 150 kW or more. Diesel fuel also allows access to backup power in remote areas that do not have a gaseous-fuel infrastructure. Finally, many applications—especially mission critical facilities such as hospitals and 911 call centers—use diesel-fueled generators because of on-site fuel code requirements.
However, gaseous fuel—particularly natural gas—continues to be increasingly popular in spark-ignited backup power applications. Below 150 kW, it is a very cost-competitive alternative to diesel-fueled systems. Refueling is not necessary to achieve long running times. Gaseous-fueled systems don’t require the kind of costly maintenance necessary in diesel-fueled backup power systems. Natural gas also has cleaner burning characteristics when compared to diesel, and as such is not as stringently regulated by the EPA. Natural gas is even slowly gaining acceptance by authorities having jurisdiction for mission critical backup power applications.
Generator manufacturers face many challenges in applying spark-ignited engine technology to backup power systems. There are no mass-produced spark-ignited engines on the market today built specifically for use with gaseous fuel. Gaseous-fueled engines are gaining some traction in the urban bus and truck markets, but are not produced in such quantities that would make them cost-effective components. Instead, gaseous-fueled generator manufacturers must avail themselves of gasoline-fueled engines—which are mass produced for use in motor vehicles and therefore very cost-competitive—but require extensive modifications to allow them to run reliably on gaseous fuel. This requires not only expertise in power system engineering, but also considerable knowledge of engine design.
Modifying the fuel system
Gaseous fuel does not behave as does liquid fuel. Because automotive-style spark-ignited engines are engineered to work specifically with gasoline (which is liquid), most fuel systems components for these engines must be completely replaced or converted to operate on natural gas or LP fuel.
For example, consider the fuel injection system typical of an automotive-style spark-ignited engine. Due to relatively low Btu content per unit of volume, gaseous fuel does not lend itself well to fuel injection. Fuel injection systems in spark-ignited engines are designed specifically to meter and atomize gasoline so that it is quickly vaporized and burns efficiently when introduced into the combustion chamber. Because gaseous fuel is already in a gaseous state, this step is not necessary. Additionally, most automotive-style engines use a multi-point fuel injection system—also called port injection—in which individual injectors administer fuel upstream from the intake valve on the cylinder head. While this type of system makes it easier to deliver the right amount of liquid fuel to the cylinder and thus provide better engine response, gaseous fuel cannot be effectively administered this way. Finally, the atomization process—combined with gasoline’s higher energy density—makes the fuel flow rates very different. In short, fuel injectors engineered for gasoline cannot be repurposed to inject gaseous fuel.
Generator manufacturers typically address this situation in two ways. One way is to apply fuel injection at the throttle body using a computer-controlled valve or flow device to administer the correct amount of fuel using feedback from various engine sensors. A second, more traditional method is to use a fuel mixer or carburetor and throttle body assembly to deliver fuel through traditional venturi/airflow velocity interaction to create fuel demand (see Figure 2). Because of the nature of gaseous fuel, either approach is excellent for introducing fuel into the system. Both methods not only accommodate the lower pressures necessary for gaseous fuel injection, but they also allow the gaseous fuel to mix properly with intake air prior to being delivered to the combustion chambers (see Figure 1).
Gasoline does not burn in liquid form and must be completely vaporized prior to ignition. Consequently, it requires precise injection to create a stoichiometric air/fuel ratio that burns effectively. Unlike gasoline, natural gas only requires mixing with air (see Figure 3). While its stoichiometric air/fuel ratio is different than that of gasoline, it is less affected by lean conditions in which there is more air in the combustion chamber than is stoichiometrically ideal for combustion.
Addressing combustion characteristics
Note that introducing fuel to the engine is not the end of the required customization (see Figure 4). Gaseous fuel and gasoline also have very different combustion characteristics that generator manufacturers must address for performance and reliability.
For example, gasoline and gaseous fuels ignite at different temperatures. Gasoline is considerably more volatile than gaseous fuels such as natural gas or LP fuel. It has simultaneously a very low ignition temperature: 536 F and very low flash point: -45 F. The flash point of a substance is the lowest temperature at which enough of the substance will evaporate into a combustible quantity of gas. By comparison, natural gas ignites at 1,076 F and LP fuel ignites at 878 F. The high ignition temperature is the reason why natural gas is ideal as a complementary fuel in a bi-fuel system. Diesel fuel ignites at about 410 F and serves as the pilot fuel for natural gas (with its higher ignition point) when it is introduced into the system. Because of gaseous fuel’s higher ignition temperature, traditional spark plugs and ignition systems are not appropriate, and must be replaced with a hotter spark and specialized plug tips, such as iridium. Conversely, high-temperature spark plugs are not appropriate for gasoline-fueled engines because the heat of the tip can cause pre-ignition and engine knocking.
The higher ignition temperatures of gaseous fuel also lead to higher fuel
combustion and exhaust temperatures. That means many of the components in and around the combustion chamber must be made of higher-grade materials, such as high-silicon molybdenum and stainless steel, than those found in a traditional gasoline engine so that they will not break down when exposed to
the higher temperatures.
Dealing with dry fuel
Unlike gasoline or diesel fuel, gaseous fuel is considered a “dry” fuel because it brings no lubricating value to the engine. Because they are liquids—and petroleum products at that—gasoline and diesel fuel lubricate many moving parts within the upper combustion chamber. This is not the case with gaseous fuel.
The valves and valve seats are the components most affected by this lack of additional lubrication. In a traditional gasoline engine, the valves are cooled and lubricated by the gasoline when it is injected, especially in the case of multi-point fuel injection. The gasoline residue lubricates the valves as they close against the seats, thereby minimizing wear. However, in a gaseous-fueled engine, there is no lubrication between the valve and the seat. If this is not addressed, not only will it result in significant wear, but it also could produce metal filings that when heated by combustion could serve as a secondary ignition source in the chamber, leading to engine knocking and power loss. Generator manufacturers mitigate this not only by using more durable valve and seat materials, but also by reducing the valve seat angle so that friction between the two surfaces is minimized. Additional consideration is also given to the type and ash content of the lubricating oil to compensate for the lack of lubrication from the fuel itself. Without such adjustments, the valves and seats would wear quickly.
Adjusting engine timing
As a vaporized liquid, the flame speed generated by gasoline combustion is very different than that of a gaseous fuel. Generator manufacturers must make adjustments to engine timing to address these different combustion characteristics.
In all spark-ignited engines, ignition begins slightly before the piston reaches top dead center (TDC) of the cylinder during the compression stroke. This offset—formally known as the crankshaft angle and described in terms of degrees—is due to the fact that the fuel does not combust all at once. With vaporized gasoline fuel, for example, the fuel ignites at the spark plug, and then the flame front begins to propagate very predictably across the combustion chamber. Because ignition occurs before the piston reaches TDC, the piston still has time to compress the fuel vapor slightly to maximize propagation of the flame front so that the fuel burns as much as possible. This also maximizes the power of the fuel’s combustion to deliver maximum power to the crankshaft during the downward power stroke.
Because gaseous fuels generally burn at a slower rate, combustion must start earlier in terms of crankshaft angle to provide complete heat release and combustion prior to the exhaust valve opening. Generator manufacturers must address this timing difference for their gaseous-fueled engines in order to maximize engine power.
Fuel compression issues
Because of its natural volatility, gasoline does not react well to high compression. When compressed too much, gasoline will spontaneously ignite (pre-ignition). While this phenomenon is precisely why diesel-fueled (compression-ignited) engines work, it is not desirable in a spark-ignited engine. Gasoline’s predilection for pre-ignition under pressure is why gasoline fuel grades have additives to increase octane ratings. The higher the octane rating, the easier it is to compress the gasoline without risking pre-ignition. In cases of pre-ignition, there is the propagation of a second flame front, inefficient burning of fuel, loss of power, and engine knocking. As such, traditional gasoline-fueled engines tend to have relatively low cylinder compression ratios.
By comparison, natural gas has an inherently high octane rating, and therefore is extremely tolerant of compression. An increase in compression ratio will increase thermal efficiency by several points. Generator manufacturers will routinely increase the compression ratio of the gaseous-fueled engines they use in their generators. Not only does higher compression make it easier to consume the gaseous fuel more efficiently during combustion, but it can also maximize the thermal efficiency of the fuel, which tends to be lower than that of either gasoline or diesel fuel, and thus maximize engine power.
While gaseous-fueled spark-ignited engines continue to gain acceptance in a variety of industrial backup power systems, most are sourced ready to run on gasoline fuel, not dry gaseous fuel. The differences between these two fuel types are significant enough that generator manufacturers must extensively modify or purpose-design engines for use in backup power systems. As such, these manufacturers provide not only backup power expertise, but also considerable engine design expertise. Additionally, in spite of these modifications, gaseous-fueled backup power systems continue to be extremely cost-effective alternatives to diesel-fueled generators in applications below 150 kW.
Winnie is director of engineering and corporate research and development for Generac Power Systems where he has worked for nearly 18 years. He has been involved with design and development of spark-ignited, diesel, and bi-fuel generators. In his current role, he is responsible for the design and development of liquid-cooled engines, emissions compliance engineering, and Generac’s four corporate engineering labs.