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Addition of nitroalkanes to gasoline is shown to reduce the octane quality. The reduction in the Motor Octane Number (MON) is greater than the reduction in the Research Octane Number (RON). In other words addition of nitroalkanes causes an increase in octane sensitivity. The temperature of the compressed air/fuel mixture in the MON test is higher then in the RON test. Through chemical kinetic modelling, we are able to show how the temperature dependence of the reactions responsible for break-up of the nitroalkane molecule can lead to an increase in octane sensitivity. Results are presented from an Homogenous Charge Compression Ignition (HCCI) engine with a homogeneous charge in which the air intake temperature was varied. When the engine was operated on gasoline-like fuels containing nitroalkanes, it was observed that the combustion phasing was much more sensitive to the air intake temperature. This suggests a possible means of controlling combustion phasing for HCCI.

In this work the influence of NO on combustion phasing has been studied experimentally in a single cylinder HCCI engine. A isooctane/n-heptane blend (PRF), a toluene/n-heptane mixture (TRF) and a full boiling range gasoline were tested at two different operating conditions with NO concentrations ranging from 4 up to 476 ppm in the fresh intake air. All three fuels had the same RON of 84. The first operating condition had a high intake pressure (2 bar absolute) and low intake temperature (40 °C), where low temperature chemistry is relatively prominent. The other operating condition had a high intake temperature (100 °C) and atmospheric intake pressure with significantly lower cool flame reactivity. Additionally the effect of NO at two different engine speeds, 900 and 1200 rpm were studied. The combustion phasing, represented by CA50 was advanced up to 12.5 CAD by the influence of NO.