Effect of trace impurities and of additives on diesel-fuel ignition

Summary of research on additives with special emphasis on cold-starting

(1986-1999)

supported jointly by NSERC and by Esso Petroleum Canada

Collaborators:Sarnia: Dave Shaw, Dave Steere, Lai Wong, Marc-André PoirierAbingdon: Dave RickeardYork University: (engine) Peter Clothier, Avygdor Moise (modelling) Baltazar Aguda, DeLin Shen, Wai-To Chan, Simone Heck

6-cylinder car engine rear-most cylinder equipped with a pintle instead of a spark plug and gasoline intake blocked injector pump driven off ring gear pressure transducer in diesel-fuel line and in rear cylinder

With engine block at 100ºC, and compression ratio 7.5, compression temperatures of 350–400ºC were obtained

The idea attracted wide suspicion from the diesel engine community, but was cheap and useful as an ignition

quality tester for cold-starting conditions costs:engine from wrecker's: $1,000instrumentation – (injector pump, pintle, transducers, storage oscilloscope, local fabrication): < $10,000

(expected cost of refrigerated laboratory and real diesel engine – probably around $150K -- $200K)

typical engine traces

base fuel, τ = 7.0 msec 1% EHN, τ = 4.8 msec

Wartime study of a wide range of organic additives showedthat either acceleration or retardation of ignition could occur

but:organic nitrates or peroxides always accelerated ignition

By the 1980s, 2-ethyl-hexyl nitrate (known then as DII-3),also called iso-octyl nitrate or EHN, was the cheapest and

most efficient nitrate additive commercially available

By1990/91, the mechanism by which all organic nitratesworked was accepted: a simple gas-phase kinetic

chain reaction

Mechanism for acceleration ofdiesel-fuel ignition by organic nitrates

RO-NO2 → RO + NO

2

NO2

+ RH → HONO + R

HONO → NO + OH

OH + RH →H2O + R

R + O2 → RO

2 .... etc.,

RO + O2 → RO

3 .... etc.,

2NO + O2 → 2NO

2

The reason why EHN is one of the most effectiveorganic nitrates is because when it decomposes,

it creates a lot of formaldehyde (CH2O):

                 RO =

→ 2-heptene + CH2O

followed byNO

2 + CH

2O →HONO + CHO

NO2 + CHO →HONO + CO

which are much faster reactions thanNO

2 + RH → HONO + R

This was confirmed by artificially adding extraformaldehyde to the system, which increases

the efficiency of the nitrate

Also, it can be proved that the nitrate works in the gas phase because introducing NO

2 via the

air intake causes the same acceleration as does the equivalent amount of nitrate present

in the fuel

In 1989, we studied a selection of available peroxidesand found no correlation with any kinetic or

thermochemical parameter

However, we disproved the contemporary theory thatthe efficiency of the peroxide was related to the

number of free radicals produced in the decomposition of that peroxide

The most effective peroxide wasdi-tert-butyl peroxide (DTBP)

being produced by ARCO at about the same costas EHN, around $2 per pound

We also found that the effects of DTBP and EHNwere additive, i.e. they worked independently

Before 1990, the elemental sulphur content of Canadian diesel fuel could be as high as ~1.5%,

i.e. ~5−10% of organic sulphur compounds

Although some sulphur compounds will accelerate ignition,some will retard it, and others are neutral,

on balance the sulphur content was ignition-inhibiting

strong accelerants (thiophenol, thianthrene)

strong retardants

(benzothiophene, dibenzothiophene)

no effect

(dibenzothiophene sulphone)

These inhibitors could be removed by shaking the fuel with

silica gel or alumina, with the result that EHN still accelerated the ignition of the fuel in the usual manner,

but DTBP had no effect at all

Thus, EHN and DTBP work in completely different waysas ignition improvers

Moreover, entraining DTBP in the air stream has no effect onignition delay, in contrast with the EHN case whereNO

2 entrained in the air stream was equally effective

From numerical modelling, we suspected that DTBP mightinhibit ignition, and a slight retardation of some fuels by

DTBP had been seen previously by researchers at ARCO,but they had discounted these effects as “outliers”

We made a test fuel, CN ≈ 45, a mixture of n-hexadecanewith diphenylmethane and found that DTBP significantly

retarded the ignition of this fuel in our engine

By this time, 1997/1998, the sulphur content of Canadiandiesel fuel had been reduced 100-fold, but DTBP still

accelerated its ignition

Hence, instead of working on sulphur-containing impurities,it must now work in some other way

Nowadays, the non-hydrocarbon impurities in Canadiandiesel fuel are mainly nitrogen compounds

Most of these can be removed by washing with dilutesulphuric acid, with a significant shortening of the

ignition delay in our engine

Neutralising the sulphuric acid releases about 0.4 gmof mixed amines per litre of fuel

andwhen added back restores the fuel to its original

colour, smell, and ignition delay

As in the sulphur case, added nitrogen compounds canaccelerate ignition, retard ignition or have no effect

The most potent inhibitors we found were, in orderbenzylamine ≈ indoline > dibenzylamine

benzylamine

indoline

dibenzylamine

(Ph.CH2NH

2)

Benzylamine is easier to purify than indoline

Heating benzylamine with DTBP gives high yields of awhite crystalline substance called lophine

Reactions to form lophine were known as far back as 1836,for example, heating benzaldehyde with ammonia,

(although DTBP itself wasn't synthesised until 1947)

Lophine has many uses, as a precursor in pharmaceuticalsynthesis, and because of its luminescent properties

So, how does DTBP work? There are four choices:

1) in the cold liquid in the fuel tank;2) in the hot pintle body before injection, T ~ 100°C;3) in the liquid spray droplets, T >> 100°C, probably > 250°C near the surface of the droplet;4) in the gas phase, as with the nitrate additives.

By a process of elimination, the answer is that it happensin the liquid spray droplets!

This makes perfect sense as:

1) heating DTBP with fuel at 150°C is 10-times more effective in improving the ignition delay than simply adding it to the engine's fuel;

2) the thermal decomposition of DTBP produces acetone (which is inert) and methyl radicals which are very feeble chain-reaction initiators:

Comparison of the thermal decomposition rates forDTBP and EHN:

T°C 100 200 300 350 400 500 EHN 750 hr 65 s 65 ms 5 ms 0.5 ms 14 μsDTBP 605 hr 45 s 41 ms 3 ms 0.3 ms 8 μs

(liquid phase or gas phase – makes no difference)

Intermediate summary1) Organic nitrates accelerate ignition via a chain reaction RO-NO

2 → NO

2 → HONO → OH + NO → NO

2 → ....

2) DTBP does not accelerate hydrocarbon ignition per se, but destroys some of the ignition inhibitors naturally present in diesel fuel, in the hot droplets in the spray.3) These naturally-present inhibitors used to include sulphur compounds, mainly dibenzothiophenes, but are nowadays mainly organic amines, amounting to about 0.4 gm per litre of (Canadian) street-bought diesel fuel.4) If these (polar) organic inhibitors are removed completely, the “lubricity” of the fuel is destroyed, and the injector pumps fail within ~1,000 miles; so they would have to be replaced by a new lubricant. It would not be cost-effective unless a use could be found for about 105 litres/day (in Canada alone) of mixed amines; it would, however, reduce NO

x emissions, country-wide.

Postscript

There are several ways of removing these amines:1) silica gel;2) alumina;3) dilute sulphuric acid.

However, it is not necessary to remove them to achieve theimprovements in ignition delay that we find in our engine. They can be sequestered in reverse micelles within the fuel:addition of one of several types of detergent will do this

micelle in water reverse micelle in fuel

All of these treatments gave fuels that had significantly reducedignition delays in our engine, i.e.

base diesel fuel 7.0 msecsilica gel 6.0 msecdilute sulphuric acid 5.9 msecDTBP reflux 5.9 msec0.75 g/L AOT 5.8 msec

The last observation implies that these reverse micellessurvive the injection process and into the spray, and one

can surmise that the true ignition delay of the hydrocarboncontent of this fuel is around 5.9 ± 0.1 msec

These fuels all had about the sameASTM D-613 Cetane Number, CN ≈ 43

Note also that diesel fuel typically contains ~100 ppm ofwater, which also gets trapped in the reverse micelles andprevents the crystallisation of ice from the fuel down to

below –40°C

Cloud points for 0.6g/L AOT in toluene

20

0

-20

-40

Deg

rees

Cen

tigr

ade

0 0.01 0.02 0.03water-toluene molar ratio

S. Fielder & H. O. Pritchard, unpublished

Cold-starting testsEsso Research Centre Abingdon

Caterpillar 3406B engine at –10°C

base fuel treated fuelcranking time to ignition 18 sec 8 sectime to 10% smoke opacity 7.0 min 5.6 minknock reduction time 60 sec zero

the AOT-containing and the silica-gel-treated fuelswere indistinguishable

AOT =

Cold-starting tests Esso Research Centre Abingdon

1.6 litre VW IDI engine at –10°C

Canadian fuel, CN ≈ 43: a 4% reduction in ignition delay, from 2.5 to 2.4 ms; and smoother cold-idle behaviour

SMOOTHER IDLE WITH TREATED FUEL

Cold-starting testsEsso Research Centre Abingdon

1.6 litre VW IDI engine at –10°C

UK fuel, CN ≈ 48: no such improvements

Comments

1) original suspicions of others were correct – what our engine measures does not have much to do with Cetane Number;2) both ignition delay and cold-starting difficulties are strongly influenced by the presence of ignition inhibitors in the fuel – as well as by the base CN value of the hydrocarbon content itself;3) one cannot remove these natural inhibitors for two reasons: a) need to find a use for a huge quantity of mixed (aromatic) amines b) need to restore lubricity of the fuel by some new additive4) if the amines are removed and replaced by a lubricity improver, DTBP cannot be used as an ignition improver;5) The addition of a surfactant to the fuel to improve its cold- starting properties was not a commercial proposition because it did not improve the Cetane Number – hence there would be no ASTM-type quantity that could be used to measure the quality of the fuel with respect to cold-starting. However, a single additive addressing both cloud point and cold-starting simultaneously could be attractive.

Future Ignition Quality Testing Consider 3 fuels with the same ASTM D-613 Cetane Number:a) pure hydrocarbon mixture;b) Canadian refinery fuel;c) UK refinery fuel [blend with pure hydrocarbon to match CN value].

The IQT ignition bomb should give the same behaviour for each towithin the normal scatter of the measurements.

Now wash each fuel with silica gel and repeat: if the ASTM D-613Cetane Number is unchanged, the results should be the same.

Then alter the IQT conditions to mimic cold-starting: lower thecompression temperature and the liquid-fuel inlet temperature,(and perhaps adjust the air mass slightly).

Does the treated Canadian fuel now ignite more quickly than theother two, as our cold-starting tests would suggest?

Future Ignition Quality Testing(continued)

If so, then the presence of contaminants and/or additivesalters the temperature dependence of the ignition delay

from that of the plain hydrocarbon fuel

[likewise, bio-diesel may exhibit a different temperature dependence in ignition delay from that of regular fuel]

This allows the definition of a new ignition qualityindex for fuels used under non-standard conditions

e.g.CSECN: Cold Starting Equivalent Cetane Number LTECN: Low Temperature Equivalent Cetane Number

Ottawa, 18th April, 2005