Understanding Air-Fuel Ratio in Compression Ignition Engines

A compression ignition engine, often called a diesel engine, works differently from a spark ignition (petrol) engine. In a compression ignition engine, only air is drawn into the cylinder during the intake stroke. Fuel (diesel) is injected towards the end of the compression stroke. The high temperature of the compressed air ignites the fuel automatically.

The air-fuel ratio is the mass of air divided by the mass of fuel used in the combustion process. It is a critical parameter affecting engine performance, emissions, and efficiency.

In a spark ignition engine, combustion typically occurs close to the stoichiometric air-fuel ratio (around 14.7:1 for petrol), which is the ideal ratio for complete combustion. SI engines can run slightly rich (more fuel) or slightly lean (more air) near this ratio.

However, compression ignition engines always operate with a lean mixture. This means there is significantly more air than needed for complete combustion of the fuel injected. The reasons for operating lean include:

• Diesel engines use throttling of fuel injection, not air intake (except sometimes for emissions control). The amount of air drawn in is relatively constant, while the fuel injected varies depending on the load.
• Operating lean helps control combustion temperature and pressure rise, preventing knocking (diesel knock is different from petrol knock but operating too rich can still lead to harsh combustion).
• Lean operation helps reduce the formation of particulate matter (soot), although NOx emissions can be higher.

The typical range of air-fuel ratios for compression ignition engines is quite wide, from about 18:1 at full load to 70:1 or even higher at no load (idling). The given options represent typical operating ranges.

• The range 6 to 14 is a rich to stoichiometric range, typical for spark ignition engines, not compression ignition engines.
• The range 40 to 48 and 28 to 36 represent very lean mixtures, which might occur at part load or idling in a CI engine, but the range 18 to 25 covers a significant part of the operating spectrum, particularly closer to the full load condition where the engine uses more fuel relative to air.

Considering the options provided which represent practical operating conditions, the range 18 to 25 is a common air-fuel ratio range encountered in compression ignition engines, particularly towards higher loads where the ratio is lower (closer to stoichiometric, but still lean) compared to idling.

Therefore, the air-fuel ratios used in a compression ignition engine lie in the range of approximately 18 to 25, covering conditions from full load towards part load.