Natural gas engines are consolidating themselves as an efficient and cleaner alternative in heavy transport. Its use in trucks and buses responds to strict environmental regulations and requires an understanding of its technical differences compared to diesel engines.
By Francisco Aristizábal, technical specialist at AERA
Natural gas (NG) engines are increasingly used in non-industrial applications, particularly in automotive sectors such as trucks and buses. These engines comply with Euro V/VI emission standards and offer advantages such as low noise and vibration. They feature upgraded components to withstand high temperatures and typically have displacements ranging from 4 to 17 liters, delivering up to 500 kW of power. Although they are structurally similar to diesel engines, the main differences are in the fuel and ignition systems.
Diesel engines, like natural gas engines, work in a slightly different way, although the four-strokes are the same. At intake time, only air is sucked or forced into the combustion chamber of the cylinder. During the compression time, the air is compressed and therefore heated; Just before the piston reaches top dead center, the fuel is injected at high pressure. The air-fuel mixture is automatically switched on at the start of the power time.
Diesel engines are often limited by their ability to withstand structural loads, with maximum pressures of approximately 1,500 psi. Gas engines, on the other hand, are limited by their ability to handle thermal loads, specifically high exhaust temperatures. Gas engines operate at higher exhaust temperatures because they maintain a constant air-to-fuel ratio at any load. Diesel engines, on the other hand, run on excess air on all loads; only the amount of fuel burned increases with charging. This extra air also helps to cool the load in diesel engines.
As for the fuel system, in addition to the mixer or carburetor, a fuel pressure regulator is a major component. A governor (mechanical or electronic), similar to those used in diesel engines, is also required in larger natural gas engines, along with magnets. However, gas quality has a significant impact on engine performance. Its composition affects parameters such as methane number and calorific values (both lower and higher). This becomes especially important when using natural gas from exploration fields.
Detonation, which can cause the engine to stop, is an undesirable phenomenon, and its causes are not limited to the timing of ignition. Liquid hydrocarbons and other components present in the gaseous fuel mixture have an important influence (see graph).
Detonation produces severe pressures and temperatures that can affect not only the temperature of the spark plugs, but also impose significant stress on insulators, electrodes, pistons, valves, bearings, and other engine components.
In the ignition system, electronic ignition systems were designed to replace traditional magneto systems. Electronic ignition eliminates magneto and other components subject to mechanical wear, as well as providing greater diagnostic and fault detection capabilities. However, magnets are still widely used. A magneto is an alternating current generator that produces electrical energy, precisely timed, for spark-ignition motors. Ignition transformers are also necessary for each cylinder in order to increase the voltage coming from either the magneto or the electronic ignition system, allowing the spark to jump between the spark plug electrodes. Although spark plugs are the smallest components in the system, they are among the most critical to engine performance due to their direct role in ignition.
The ignition system must be able to supply the voltage necessary to create a spark between the spark plug electrodes. In practice, this requires a considerable "ignition reserve" to compensate for the normal wear and tear of the spark plugs and other components of the ignition system. The ignition reserve is defined as the difference between the available voltage (Va) of the ignition system and the required voltage (Vr) by the spark plug. If at any point Vr equals Va, a misfire is likely to occur (see image).
The temperature and condition of the spark plug electrodes are key factors to consider in spark plug design and selection. Voltage requirements decrease as the temperature of the electrodes increases and increase when the temperature drops. New, sharp electrodes concentrate the arc of the spark by providing an easier path for current flow. As new spark plugs, with well-defined electrodes, wear out, the voltage required for ignition increases. Changes in electrode materials, ranging from traditional copper/nickel to advanced materials such as platinum, iridium, and gold-palladium, must also be considered, in descending order of voltage requirements.
As always, this information is meant to serve as a general guide. Specific details may vary depending on the equipment manufacturer and application. Always refer to the manufacturer's documentation and other OEM service materials. AERA's technical line is also available to help with questions about these topics.
Francisco Aristizábal, technical specialist at AERA
*This article was originally published in AERA's Engine Professional magazine, is published in Aftermarket International, with the author's permission.
