Traces of nitric oxides (NO or NO₂) are now well known to promote the ignition of simple fuels. This promoting effect is important in several practical systems among which:

- the NO-NO₂ conversion by hydrocarbons in recent NOx-reduction strategies

- the modeling of combustion with exhaust-gas recirculation (EGR) in mild combustion

The complex interaction of nitric oxides and fuel mixtures is still poorly understood, especially at high level of NOₓ.

New experimental results were obtained in Jet-Stirred Reactor (JSR) for the oxidation of a mixture of ca. 85% of iso-octane and 15% of n-heptane (PRF85) in presence (or not) of NO or NO₂ under fuel-lean conditions (φ = 0.2, 0.5, and 0.7), high pressure (10 atm) and over a wide temperature range (550-1000K). Concentration profiles of reactants, intermediates and final products were measured by GC and FTIR.

## EXPERIMENTAL DEVICE

<figure><img src="image_1.png" /></figure>

**❖ Conditions:**

$$p = 10 \text{ atm} / \varphi = 0.2 / \tau = 0.5 \text{ s}$$

$$X_{\text{iC8H18}} = 890 \text{ ppm} / X_{\text{nC7H16}} = 170 \text{ ppm} / X_{\text{NO}} \text{ or } X_{\text{NO2}} = 60 \text{ ppm} / X_{\text{O2}} = 6.1\%$$

<figure><img src="image_2.png" /></figure>

**❖ Conditions:**

$$p = 10 \text{ atm} / \text{various } \varphi / \tau = 0.5 \text{ s}$$

$$X_{\text{iC8H18}} = 890 \text{ ppm} / X_{\text{nC7H16}} = 170 \text{ ppm} / X_{\text{NO}} = 200 \text{ ppm}$$

<figure><img src="image_3.png" /></figure>

**❖ Conditions:**

$$p = 10 \text{ atm} / \varphi = 0.2 / \tau = 0.5 \text{ s}$$

$$X_{\text{iC8H18}} = 890 \text{ ppm} / X_{\text{nC7H16}} = 170 \text{ ppm} / X_{\text{O2}} = 6.1\% / \text{various } X_{\text{NO}}$$

<figure><img src="image_4.png" /></figure>

New data were obtained for the oxidation of a mixture of primary reference fuels (PRF85) in absence and presence of NO or NO₂. NO and NO₂ have very similar effects. When NOₓ addition is increased, the Negative Temperature Coefficient (NTC) is erased. When NOₓ addition is too high (> 200 ppm), the cool flame is erased. The oxidation of NO by HO₂ promotes the conversion of the fuel but, when the concentration of NO increases the cool flame tends to disappear due to the formation of alkoxy radicals via ROO + NO → RO + NO₂ and possible stabilization of HONO (OH + NO → HONO).