LHV, Air Required for Combustion & Engine Power
An engine is a conversion device. It takes in the chemical energy stored in the fuel, releases that energy as heat during combustion, and then converts the heat energy into mechanical work. Part of the engine’s job, then, is to make certain enough fuel gets into the cylinder to enable it to reachits full rated power output. But, as we learned earlier, combustion requires fuel and air, so getting enough air into the cylinder is just as important as getting enough fuel.
The path the air takes, from first entering the air cleaner until it reaches the cylinder, poses a restriction to flow of the air. This is characteristic of the design of the engine, so it is a property that varies fromengine model to engine model.
Also characteristic of the engine design is whether or not it has a device that helps to push air into the cylinder to increase the power-producing potential of the engine. Such devices are known as superchargers, with the type most commonly seen on large engines being called turbochargers since the power used to pump the air into the engine comes from a turbineplaced in the engine exhaust stream.
The most basic engine air inlet design uses no supercharger; it relies only on the vacuum drawn by the piston moving down in the cylinder to pull air in. This design is known as “naturally aspirated” (abbreviated “NA”). An NA engine will be sensitive to any factors that restrict airflow, and one such factor is the altitude and ambient temperature of theinstallation where the engine is in use. Increased altitude or ambient air temperature causes the air to become less dense, making it more difficult for the engine to draw in enough air to support combustion.
The fuel gas makeup comes into play in that the amount of air required to support combustion is determined by the air requirements of the fuel as shown by the stoichiometric air/fuel ratio.
Addinga turbocharger to an engine is an effective way to offset this breathing problem. The turbocharger forces air into the engine, allowing it to overcome the effects of decreased ambient air density. But, pumping up the pressure of the inlet air to increase the density also pumps up the temperature of that air, and higher temperatures mean decreased air density. For this reason, many turbochargerinstallations also include an intercooler or aftercooler, a heat exchanger designed to cool the inlet air, allowing it to become more dense. The turbocharged-aftercooled (“TA”) engine design gives greatly expanded breathing ability to the engine, supporting the production of power.
The fuel system has a similar role in enabling the engine to produce rated power. It also poses a restriction to theflow of fuel into the engine and so is critical in the design of the engine. It is also affected by the inlet pressure of the fuel supply much like the air system is impacted by the site altitude and ambient temperature. But, unlike the air system, the fuel system may also be impacted by the LHV of the fuel gas. Too high an energy content may cause the fuel system to be unable to mix air and fuelin the proper ratios, resulting in poor combustion and reduced power. To low an LHV results in starving the engine for the fuel energy necessary to produce power.
Methane Number, Detonation & Engine Power
Earlier we discussed the creation of a rating scale based on the detonation-resistant characteristics of methane and hydrogen. This Methane Number scale serves as a basis for rating fuelmixtures on their relative ability to resist detonation.
To understand how this information is used, we start with an understanding of detonation itself.
Detonation and Pre-ignition
Detonation and pre-ignition are two forms of abnormal combustion that involve uncontrolled burning of the fuel-air mixture in the cylinder. Pre-ignition is the term used to describe premature ignition of the...