Huw O. Pritchard

How do diesel-fuel ignition improvers work?

An important descriptor of diesel fuel is its Cetane Number: this is an indicator of the time delay between injection and spontaneous ignition of fuel in a standard diesel engine running under specified conditions; the shorter the ignition delay, the higher the cetane number. Thus, those groupings of atoms within a hydrocarbon molecule that are beneficial in conferring a resistance to spontaneous ignition in a gasoline, i.e. a high octane number, are undesirable when they occur in a diesel fuel, and vice versa. The cetane number scale uses two standard compounds, cetane (n-hexadecane) defined as 100, and heptamethyl-nonane defined as 15, so that, assuming linear mixing, a 1:1 mixture would have a cetane number of 57.5, a 1:2 mixture would have one of 43.3, &c. Long-chain paraffins tend to have high cetane numbers, e.g. n-dodecane = 80, n-tridecane = 83, in addition to cetane itself = 100. On the other hand, hydrocarbons containing benzene rings tend to have very low cetane numbers, e.g. diphenyl = 21, diphenylmethane = 11 and 1,2-diphenyl-ethane = 1. Extremely low cetane numbers are also found for hydrocarbons with a benzene ring carrying short-chain substituents, e.g. xylene = –10 and m-di-iso-propyl-benzene = –12, but as the side chain becomes longer, the cetane number rises, to 26 for n-hexyl-benzene and to 50 for n-nonyl-benzene. Substances containing fused rings also exhibit very low cetane numbers, e.g. α-methyl-naphthalene = 0. A corollary is that the minimum spontaneous ignition temperatures for aromatic hydrocarbons are higher than for non-aromatics. Legislated National Standards usually require that the cetane number of commercial diesel fuel shall exceed a certain value, say 40. Most diesel engines do not perform well with fuels of cetane number below this: for

Reaction of nitrogen dioxide with hydrocarbons and its influence on spontaneous ignition. A computational study

Estimates are made, by using BHandHLYP/6-311G** density functional molecular orbital theory, of the activation energies and frequency factors for the reaction of NO2 n with methane, ethane, propane, isobutane, and benzene. For the aliphatic hydrocarbons, over the temperature range 600–1100 K, the rate of formation of a new n isomer of nitrous acid, HNO2, is very similar to that for the formation of the common isomer, HONO. This complicates our description of the acceleration of spontaneous ignition of diesel fuels by organic nitrates. These rate data are used in a reduced kinetic model to examine the effect of NO2 upon the spontaneous ignition of some linear- and branched-chain aliphatic hydrocarbons. It is concluded that, under typical diesel engine operating conditions, the spontaneous ignition of linear-chain paraffins is accelerated by the presence of NO2, but may be retarded for heavily branched-chain isomers. An Appendix discusses the relative importance n of tunnelling in hydrogen-transfer reactions.

Stimulation of diesel-fuel ignition by benzyl radicals☆

Benzyl radicals are often regarded as being inert, and not capable of initiating chain reactions. However, during the preignition phase in a diesel engine, the benzylperoxy radical, formed by the addition of O{sub 2} to the benzyl radical can be present in significant proportions. Using the most recent data, the equilibrium constant for the formation of the adduct is about 6.6 atm{sup {minus}1} at 600 K, and about 0.066 atm{sup {minus}1} at 800 K; thus, if the partial pressure of O{sub 2} is 5 atm, 97% of the benzyl radicals are in the peroxy form at 600 K, 25% at 800 K. One possible fate of the benzylperoxy radical, proposed to explain the formation of benzaldehyde in the low-temperature oxidation of toluene between 723 and 788 K, is the release of an OH radical. In this Note, the authors demonstrate that at low preignition temperature, dibenzyl mercury and {omega},{omega}{prime}-azo-toluene are more effective ignition promoters than is the usual diesel-fuel ignition improver, iso-octyl nitrate. They also show, by using ab initio molecular orbital calculations, that there is a plausible mechanism by which thermal decomposition of the benzylperoxy radical could lead to the production of OH radicals.

Theoretical study of benzyl radical reactivity in combustion systems

It has recently been suggested that benzyl radicals may play an important role in stimulating spontaneous nignition, both in diesel and in petrol engines. We examine here one of the proposed mechanisms. The energies and structures of the intermediate nbenzylhydroperoxide, and of the initial reactants and final products, were determined at the nMP2/6-311G**//B3LYP/6-311G** level of theory. An estimate was made of the k(E) function for the nunimolecular dissociation of into and thence, the relative fractions of ncollisions that lead directly to the formation of OH, as a function of temperature and pressure, as opposed to being stabilized to the hydroperoxide. The computed rate constants were then nincorporated into a kinetic model in order to assess the importance of benzyl radicals in stimulating nspontaneous ignition in hydrocarbon - air mixtures.

Synergy between additives in stimulating diesel-fuel ignition

Synergy between several pairs of free-radical producing additives in stimulating the ignition of fuel in a diesel engine was reported recently by Al-Rubaie et al. At a compression ratio of 17:1, it required only about two thirds as much of a 50/50 mixture of di-tert-butyl peroxide and iso-propyl nitrate to cause the same acceleration in ignition as of either additive on its own; however, with the same fuel and a compression ratio of 13:1, the mixture was only about 10% more effective than the individual additives. They also reported that in one engine, an equal mixture of di-tert-butyl peroxide and iso-octyl nitrate (2-ethyl-hexyl nitrate) was about 25% more effective than would have been expected if they had worked independently. The authors, on the other hand (but at a lower compression ratio of 7.5:1), have found that there was no direct co-operative effect between di-tert-butyl peroxide and iso-octyl nitrate, except for a second-order one in reducing the cycle-to-cycle variability. It is clear that these synergistic effects must depend on the nature of the fuel, and upon the test conditions in the engine. Furthermore, they described several instances in which pairs of additives did not reinforce each other, but actually interfered strongly;morexa0» it becomes apparent that every interaction between pairs of additives has to be considered individually. Here, they report several new examples of synergy between pairs of additives, as well as some null results. they conclude that there is no unique mechanism by which synergy arises, but they demonstrate fairly conclusively that one way is through the interaction between NO[sub 2] and CH[sub 2]O during the preignition phase.«xa0less

Retardation of spontaneous hydrocarbon ignition in diesel engines by di-tert-butyl peroxide

Abstract Di- tert -butyl peroxide is a well-known accelerant for spontaneous ignition of practical fuels in a diesel engine. We have found, however, that it retards the spontaneous ignition of pure hydrocarbon fuels in an engine under simulated cold-starting conditions. We report on modeling calculations which suggest why di- tert -butyl peroxide could inhibit the spontaneous ignition of n -hexadecane, and on experiments designed to elucidate the mechanisms by which this peroxide works as an ignition improver in commercial fuels. It appears that di- tert -butyl peroxide neutralizes the inhibiting effects of some naturally present amines in the hot fuel spray droplets.

Atmospheric oxidation and self-heating in rubber-containing materials

Among the materials listed by Bowes [1] that are susceptible to self-heating and spontaneous ignition, rubber-containing products are conspicuously absent. Tire-dump fires are a special environmental problem due to the difficulty in fighting them, the large amounts of water needed, and the noxious quality of the aqueous run-off and of the smoke. Many of these fires are maliciously set, but legislation in various jurisdictions concerning the height of perimeter fences and the clear space between the fence and the edge of the dump itself [2] help to reduce their frequency. Nevertheless, tire-dump fires sometimes do not fit the profile of having been set near the perimeter with the help of an accelerant: these are now classified by the Scraptire Council of the (U.S.) Rubber Manufacturers’ Association as accidents, although an earlier, now defunct, version of this web page blamed either lightning strikes or “Acts of God.” It is the latter class that we attempt to illuminate in this study. In fact, we will show that selfheating caused by atmospheric oxidation can occur in tire rubber, and therefore the possibility of spontaneous ignition cannot be discounted. Moreover, the observation [2] that some such fires collapse in the middle (similarly, in some of our experiments below) only tends to strengthen this conclusion. 2. Experimental

Anaerobic operation of an internal combustion engine

A process for running an internal combustion engine is disclosed. The process comprises running the engine in communication with ambient air and with a fuel that is, at the compression ratio of the engine, capable of undergoing combustion in air containing a sufficient amount of oxygen for combustion and of undergoing spontaneous explosion in the absence of sufficient oxygen in said air for combustion.

On the atmospheric oxidation of liquid toluene

This communication presents preliminary computational results on the interaction between triplet (3Sigma) and singlet (1Sigma, 1Delta) oxygen molecules with toluene. All three oxygen species form very weak complexes with toluene and all also appear capable of abstracting a benzylic hydrogen atom to form the HO2 radical. Reaction with singlet molecular oxygen does not convincingly explain the formation of benzylhydroperoxide from toluene residues stored over a long time in brown glass bottles, and it is speculated that this may be a surface-catalysed photochemical reaction. The possible involvement of singlet oxygen molecules in the spontaneous ignition of tyre rubber and of soft coal is discussed briefly and the need for new experimental studies is stressed.

Distribution of vibrational potential energy in molecular systems

It is shown that for a collection of n classical harmonic oscillators, the long-time distribution of potential energies P is approximated by sinm(πP) for n⩾4, where m=(8n/π2−1/√2) and P is scaled to lie between 0 and 1. As n→∞, the distribution tends to a δ-function centered about P=0.5. When coupling is present between the oscillators, the effective value of m is reduced, so that the breadth of the potential energy distribution reflects the degree of randomization in the system.