Alternative Fuel Combustion

The preceding description of the injection and combustion process is what occurs when conventional diesel or heavy fuel oil is injected at the correct conditions (temperature, pressure etc.). It must not be assumed that this will automatically occur with any proposed alternative fuel. In fact, it is the exception rather than the rule for alternative fuels to behave in a similar manner to conventional fuels. While HVO is considered a 'drop in' fuel and diesel engines with modified fuel systems can run on Di-methyl Ether, other alternative fuels such as LNG, Methanol, Ammonia and Hydrogen present significant technical challenges in the injection, ignition and combustion phases.

Fuel injection systems employ components such as fuel pumps and injectors with extremely fine clearances. Diesel oil or heavy fuel oil can provide some lubricity to allow satisfactory operation of these components. While biofuels provide this lubricity it is largely absent in other alternative fuels.

Ignition occurs when the auto ignition temperature of the fuel is reached. The ignition quality of a fuel is denoted by Cetane Number for distillate fuels and Calculated Carbon Aromacity Index (CCAI) for heavy fuel oil. Many alternative fuels, although highly flammable, have high auto-ignition temperatures and will not ignite in a conventional diesel engine. Thus pilot fuel injection is required in many instances. The pilot fuel (usually diesel or heavy fuel oil) is injected just before the alternative fuel. The pilot fuel ignites and this initiates combustion of the alternative fuel.

Ignition Energy, the actual energy required to ignite the fuel, is an important property. The pilot fuel must provide this energy. Ammonia, for instance, has a relatively high ignition energy.

The rate at which the flame propagates in the combustion space is dependent on the fuel and this differs across alternative fuels. So even when a pilot fuel is employed to initiate combustion the resulting flame speed may be very different to that of conventional fuels. For instance, the flame propagates rapidly in hydrogen whereas it propagates relatively slowly in ammonia. This clearly has a direct effect on the rate of pressure rise in the engine cylinder, a key parameter in engine performance.

Some dual fuel engines, particularly slow speed diesels, employ high pressure gas injection directly in to the combustion chamber. This may be described as Dual Fuel High Pressure Injection or Dual Fuel Gas Injection . Other engine designs, particularly medium-speed dual-fuel engines, do not inject the alternative fuel into the cylinder directly. Instead, the fuel gas or vapour is admitted to the cylinder via the air inlet ports or valve(s). This may be described as Dual Fuel Low Pressure Injection or Dual Fuel Gas Induction as opposed to Dual Fuel Gas Injection . This means that, unlike a conventional diesel engine, the fuel/air mix is compressed in the cylinder. This avoids the difficulty of high pressure injection of the fuel but can lead to other difficulties, particularly misfiring and knock in the case of LNG. Precise control of the gas/air mix is required. Lack of precision in fuel/air mix control can lead on the one hand to misfiring or on the other hand to knock (detonation).

The lower heating values (LHV) of alternative fuels differ from conventional fuels. The injection system must be capable of injecting the appropriate quantity of fuel in the correct time frame to ensure correct power output. It is not sufficient to only increase the storage capacity when using a fuel such as methanol, the injection system must be capable of delivering the significantly increased volume to each injector over the full load range.

In summary the satisfactory combustion of many alternative fuels is dependent on a range of parameters, any one of which can impair performance.