An Experimental Investigation into the Effect of Fuel Addition to Intake Air on the Performance of a Compression-ignition Engine

Abstract

The investigation arose from a programme of research into the means of reducing combustion noise and engine roughness in a compression-ignition engine. Information on pre-flame reactions in spark-ignition engines from published literature indicates that these reactions, which yield active partial products of combustion that finally lead to the auto-ignition of the end gas, or ‘knock’, may reduce the ignition delay in a compression-ignition engine and contribute to the smooth running of the engine. Preliminary tests with part of the fuel introduced with the intake air in the form of spray confirmed such a view. They showed how, by slow oxidation of the aspirated fuel, the compression pressure is increased and the ignition delay of the main fuel is reduced with consequent smoothing of the pressure diagram and elimination of combustion noise. The effects on combustion and performance under various loads and speeds are presented, it being found that overall thermal efficiency is improved at high load but made worse at low load. Further investigations were then carried out on the various factors that may have effects on the result of introducing fuel with the intake air. Among these were the types of fuel used for the aspirated as well as the main charge, combustion chamber design, compression ratio, and timing of both parts of the charge. An analysis is made of the processes of combustion under such conditions and a theory advanced to account for the phenomena observed. It is concluded that the lean aspirated mixture does not proceed to complete combustion. The use of intake spray was found particularly effective when the main fuel was of low ignition quality. The technique of using the slow oxidation of the aspirated fuel was then applied to study the ignition of various hydrocarbons in a compression-ignition engine. It is shown that the differences in ignition delay of various fuels lie chiefly in the chemical part of the delay while the physical part remains essentially constant in spite of widely different physical properties of the fuel. It is shown also that with the use of intake spray, the compression-ignition engine can run on any fuel irrespective of its cetane number and with only small differences in thermal efficiency. Finally, the effect of some additives on the reactions of the aspirated fuel and the ignition of the main fuel were studied. The results showed that the reactions were suppressed by the addition of formaldehyde and tetraethyl lead and slightly accelerated by nitrogen peroxides. The indication was that the reaction is of the ‘low-temperature’ mode. This is discussed in relation to the recently published works on oxidation of hydrocarbons in general and the reactions leading to knock in the spark-ignition engine.