Factors affecting the decomposition of biodiesel under simulated engine sump oil conditions

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DOI: 10.1243/09544070JAUTO1395

2010 224: 927Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile EngineeringC D Bannister, C J Chuck, J G Hawley, P Price and S S Chrysafi

Factors affecting the decomposition of biodiesel under simulated engine sump oil conditions

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Factors affecting the decomposition of biodieselunder simulated engine sump oil conditionsC D Bannister1*, C J Chuck1, J G Hawley1, P Price2, and S S Chrysafi2

1Department of Mechanical Engineering, University of Bath, Bath, Avon, UK2Ford Motor Company Limited, Dunton, UK

The manuscript was received on 2 October 2009 and was accepted after revision for publication on 8 April 2010.

DOI: 10.1243/09544070JAUTO1395

Abstract: Biodiesel is a renewable fuel derived from plant, waste, or algal oils. It is synthesizedby the transesterification of triglycerides with an alcohol to yield fatty acid alkyl esters. Theseesters are prone to oxidative deterioration, yielding a variety of products which increase theviscosity of the fuel beyond acceptable levels. A proportion of the fuel used will always find itsway into the vehicle’s lubricating oil with dilution becoming increasingly significant on vehiclesequipped with a particulate filter because late injections lead to increased wall wetting.Biodiesel will also accumulate in the lubricating oil but, unlike mineral diesel, the biodiesel willnot evaporate at the normal operating temperature of the oil and will instead accumulate. Asthe biodiesel oxidizes within the oil, degradation products will eventually form, leading to asignificant increase in the oil viscosity with a potential impact on base engine durability. Thisstudy investigates the relative effects of a number of factors likely to be relevant to biodieseloxidation within a simulated engine lubrication oil environment. The findings of this studysuggest that determining the point at which oxidation occurs is inherently difficult and theimpact of various factors can vary depending on the oxidation indicator examined. The mostsignificant factors in total oxidation were found to be the temperature and air flowrate withother factors, such as the presence of iron and pro-oxidant, having a significant impact onlyafter initial oxidation reactions had occurred.

Keywords: biodiesel, oxidation, engine sump oil, fatty acid methyl ester

1 INTRODUCTION

Fossil fuels are a finite resource sourced from

potentially unstable geopolitical areas, the combus-

tion of which creates large volumes of various

greenhouse gases. To combat these issues, various

alternative fuel sources, such as biodiesel, are being

sought. The term biodiesel refers to the fatty acid

alkyl esters derived from plant, animal, or waste oil

feedstocks and is produced by the reaction of the

glyceride starting material with an alcohol over an

acid or base catalyst [1].

This alternative fuel is increasingly becoming a

significant part of the transport sector with over

176106 t sold worldwide in 2008. Biodiesel, derived

from rapeseed, soybean, waste oils, or, more re-

cently, algal oils is composed of a range of saturated

and polyunsaturated esters and, on exposure to air,

these esters are prone to oxidative degradation [2].

The eventual products of this degradation are

polymeric gums and solid material that can, if

present in the fuel prior to combustion, have adverse

affects on fuel system performance or, commonly,

fuel starvation caused by clogged filters. Various

properties of biodiesel, such as a higher surface

tension, lower volatility, and higher density, lead to

the formation of larger drop sizes when injected into

the cylinder. This leads to more impingement on the

cylinder wall and, especially in the case of on-road

diesel vehicles fitted with diesel particulate filters

(DPFs) requiring regeneration via late-injection

strategies, to an increased amount of biodiesel in

the sump oil (often termed oil dilution) [3]. As bio-

diesel is generally not as volatile as diesel, little of

*Corresponding author: Department of Mechanical Engineering,

University of Bath, Bath, Avon, BA2 7AY, UK.

email: [email protected]; [email protected]

927

JAUTO1395 Proc. IMechE Vol. 224 Part D: J. Automobile Engineering

at Universitats-Landesbibliothek on January 20, 2014pid.sagepub.comDownloaded from



the biodiesel evaporates from the crankcase and

instead will start to degrade. This causes the vis-

cosity of the sump oil to increase significantly,

resulting in a loss of performance and an increase

in engine wear, and generally necessitates a pre-

mature oil change [4].

Currently, biodiesel is primarily produced from

rapeseed, palm, and soybean oil, although numerous

other vegetable oils as well as waste cooking oils are

also suitable feedstocks. Biodiesel produced from

tropical oils mainly consists of saturated esters such

as palmitic acid (C16:0), biodiesel produced from

rapeseed and jatropha oils mainly consists of

monosaturated fatty acids, whereas biodiesel pro-

duced from soybean, sunflower, and corn oils is high

in polyunsaturated fatty acids such as linoleic acid

(C18:2) [5, 6].

Numerous studies have been undertaken into the

oxidation of biodiesel. Originally studies detailed the

degradation of normal biodiesel under storage con-

ditions over a period of one year [7, 8] Although

arguably the most realistic test possible, the extended

timeframe of this type of study is clearly not practical

in assessing the oxidative stability of a fuel after

production for certification purposes. For this, many

studies have used the Rancimat test or oxidative

stability index to evaluate the stability of the biodie-

sel samples [9–13]. In these tests the oxidation is

monitored by assessing the acidity of the volatile

oxidation products. It is also possible to link oxidation

to a change in the viscosity, density, acid value, and

peroxide value of the biodiesel sample [14, 15], as well

as measuring the decline in the antioxidant level [16–

18]. More in-depth studies have employed various

analytical techniques such as nuclearmass resonance

(NMR), mass spectrometry, Fourier transform infra-

red spectroscopy, and gas chromatography–mass

spectrometry to elucidate further the mechanism of

degradation and to examine the formation of the

various oxidation products [19–21]. An overview of

themajor oxidation process is presented in Fig. 1. The

fatty acid alkyl chains have variable levels of un-

saturation. The susceptibility of the different fatty

acid esters to oxidation differs according to the

number of double bonds in the sample. Polyunsatu-

rated compounds are more prone to oxidation than

are the monounsaturated esters, which in turn are

more reactive than the saturated esters [19]. Oxi-

dation happens via a radical mechanism, where a

hydrocarbon free radical is formed on the bisallylic

carbon, the double bonds which lie around this

radical are able to reorganize (isomerize) to a more

stable structure, and the resulting radical species then

reacts with oxygen to form a peroxide species [22]. As

the most likely site for this to happen is the bisallylic

carbon, it is the triunsaturated (three-double-bond)

components that are more susceptible to attack than

are the diunsaturated components. The recently

formed peroxide species can then proceed to react

further, forming acids, ketones, and aldehydes. It is

also possible for the peroxide to react with another

fatty acid chain to form oligomers (chain consisting of

a limited number (10–100) of monomer units), which

eventually are deposited in the oil or react with fatty

acid methyl ester (FAME), resulting in peresters and

alcohols [21].

In this study, a technical-grade FAME dissolved in

dodecane was oxidized over 6 h in the presence of

iron and a fuel-borne pro-oxidant usually added to

aid particulate filter regeneration. The levels of these

factors were investigated as well as the flowrate into

the system, the reaction temperature, the amount of

FAME in the dodecane, and the type of FAME. 1H

NMR was used to analyse samples from the reaction

mixture with the pH of the collected volatile com-

ponents also being measured. As biodiesel derived

from natural sources contains antioxidants (such as

vitamin E) the examination of degradation would be

difficult over a feasible NMR timescale. To address

this issue, model solutions, made up of the pure

FAME starting materials, were used and a radical

initiator (2-ethylhexyl nitrate (2-EHN)) was added.

Fig. 1 Simplified flow diagram of the major oxidation process in the degradation of biodiesel

928 C D Bannister, C J Chuck, J G Hawley, P Price, and S S Chrysafi

Proc. IMechE Vol. 224 Part D: J. Automobile Engineering JAUTO1395




2 EXPERIMENTAL METHODS

2.1 Materials

Dodecane, methanol (reagent grade; 99.8 per cent

pure minimum), iron filings, methyl oleate (oleic

acid methyl ester (OAME); technical grade; 70 per

cent pure minimum) and linoleic acid (technical

grade 74 per cent pure minimum) were purchased

from Sigma-Aldrich Chemicals. In order to promote

DPF regeneration at lower temperatures, a fuel-

borne organometallic pro-oxidant has historically

been used to dose the fuel [23, 24]. The impact of a

common cerium-based pro-oxidant (referred to as

Pro-ox in the tables and figures) is investigated.

Linoleic acid was converted to methyl linoleate

(linoleic acid methyl ester (LAME)) according to

standard literature procedures [25]. 1H NMR analysis

of the technical-grade methyl oleate and linolenic

acid was undertaken according to the method

reported by Knothe and Kenar [26] with the fatty

acid profile of methyl oleate revealed to be 78 wt %

OAME, 13 wt % LAME, and 9 wt % saturated methyl

esters. The fatty acid composition of LAME (techni-

cal grade) was found to be 26 wt % OAME, 70 wt %

LAME, and 4 wt per cent saturated esters. None of

the FAME samples was treated with additional

antioxidants and all the FAME samples were stored

at 28 uC for the duration of the testing to minimize

oxidation during storage.

2.2 Test procedure

A sealed three-necked flask was filled with 100ml of

the relevant FAME sample in dodecane, which was

used as a simplified substitute for engine lubricating

oil. Engine oil itself is a complex mix of hydro-

carbons with various chain lengths, as well as a

number of additives such as viscosity modifiers and

detergents. Analysis of biodiesel decomposition pro-

ducts would be substantially more complicated if

engine oil was used owing to interference between

the NMR signatures of the oxidation products and

oil constituents. The use of dodecane, while not

corresponding to the main constituents of engine

oil, provided a hydrocarbon environment with a

clearly discernible NMR peak which would not mask

the presence of oxidation products. The flask was

placed in a silicone oil bath heated to the required

temperature. The solution was mechanically stirred

at a constant rate while air was bubbled into the

solution via a needle inserted through a sealed

rubber stopper. Subsequent volatile oxidation pro-

ducts, namely organic acids, were collected, via a

reflux condenser, into a vessel containing deionized

water. 1ml samples were taken from the reaction

mixture at 0min, 15min, 30min, 45min, 60min,

120min, 180min, 240min, 300min, and 360min with

a syringe. The pH of the water was also measured

at these time intervals (Fig. 2).

The samples taken were dissolved in deuterated

chloroform and an NMR spectrum was taken. The

spectrum was referenced to the residual solvent

peak. The 1H NMR spectrum was analysed according

to the method set out by Fang and McCormick [21]

in order to obtain concentrations for various oxida-

tion indicators. 1H NMR is a powerful technique

used in quantifying, as well as qualifying, chemical

compounds. A sample is held in a uniform magnetic

field; an electromagnetic wave, normally of a radio

frequency, is applied which causes the nuclei to

absorb the energy, and this energy is radiated back

out at a specific resonance as the protons relax to

Fig. 2 Schematic diagram of the apparatus set-up

Decomposition of biodiesel under simulated engine sump oil conditions 929





their original energy state. This resonance is in part

relative to the specific molecular environment that

the proton exists in, whereas the intensity of the

peak is directly proportional to the number of pro-

tons. By this method the conjugation of the double

bonds (6.0–6.6 p/min), the formation of peroxides

(3.9–4.2 p/min) and aldehydes (9.4–10 p/min), as

well as the reduction in the amount of double bonds

(5.5 p/min) and the reduction in bisallylic sites

(2.7 p/min) could be quantified.

2.3 Experimental design

The first run was undertaken under the most severe

oxidative conditions. Subsequent experiments were

undertaken, reducing the impact of one of the

variables (Table 1).

Within a running engine, the oil will experience a

number of different environmental conditions such

as cyclic temperature changes, exposure to metals,

and various levels of aeration. The oil temperature

increases as it passes through the bearings and is

sprayed onto the underside of the pistons to provide

cooling. The oil then experiences a reduction in

temperature as it is passed through a heat exchanger

and is cooled by the engine coolant. The general

operating temperature range of the oil is between

90 uC and 140 uC, although higher local temperatures

can be experienced under certain tribological re-

gimes. The temperatures chosen for this study

represent the worst-case operating condition of

140 uC and a 50 per cent reduction corresponding

to a temperature just below the normal operating

range.

As the oil is pumped around the engine circuit,

there are a number of opportunities for aeration to

occur. The agitation of the oil via chain-driven front-

end ancillaries, as well as the presence of balancer

shafts, lash adjusters, and piston cooling jets can

all contribute to the total volume of air held in

suspension, or dissolved within the oil. In compar-

ison with the Rancimat test, a relatively low air

flowrate of 25 l/h was chosen for this study to

represent an engine speed condition with moderate

oil agitation.

As well as the environmental factors to which the

oil is exposed, it comes into contact with various

materials within the engine. Metals are known to

catalyse oxidation reactions with copper, iron, alu-

minium, and zinc present in abundance within

the oil circuit. Although copper is often considered

the most effective catalyst in the oxidation process,

ferrous material is present in far larger quantities

and, as such, was included as a factor in this study.

Although 25 per cent dilution rates, while very

high, are not impossible to observe under certain

conditions [27], 50 per cent is significantly more

dilution than is likely to occur during normal vehicle

use. Unfortunately the techniques used to determine

the products of degradation were not sensitive

enough to detect, without significant error, the

decomposition products formed from the biodiesel

unless it was present at these elevated levels.

3 RESULTS AND DISCUSSION

3.1 Primary oxidation reaction

3.1.1 Total number of double bonds

Other studies have shown that, in the general

oxidation of FAME, the number of double bonds

decreases throughout the reaction, eventually form-

ing saturated oligomers and volatile oxygenated

species [21]. For this reason the decrease in the

number of double bonds was measured over the

reaction time with the aim of quantifying the general

rate of oxidation (Fig. 3).

Reducing the amount of pro-oxidant had a similar

effect on the oxidation as removing the oxidant

altogether and in both reactions the total amount of

double bonds having reacted is less than in the full

baseline testing run. Removing the iron filings from

the reaction produced a similar level of double

bonds to removing the pro-oxidant; however, a 50

Table 1 List of experiments

Run Dodecane (ml) FAME (ml) Iron (g) Pro-oxidant (ml) 2-EHN (ml) Temperature (uC) Air flowrate (l/h) Sample labelling

1 50 50 (LAME) 0.445 40 2.33 140 25 FULL2 50 50 (LAME) 0.222 40 2.33 140 25 1/2 Iron3 75 25 (LAME) 0.222 20 1.17 140 25 B254 50 50 (LAME) 0.445 20 2.33 140 25 1/2 Pro-ox5 50 50 (LAME) 0.445 40 2.33 140 12.5 1/2 Air6 50 50 (LAME) 0.445 40 2.33 70 25 1/2 Temp7 50 50 (LAME) 0.445 0 2.33 140 25 No Pro-ox8 50 50 (LAME) 0 40 2.33 140 25 No Iron9 50 50 (OAME) 0.445 40 2.33 140 25 OAME






per cent reduction in the iron filings had little effect

when compared with the full test. The largest effects

on the level of double bonds are the air flowrate and

the temperature, which, coincidently, lead to similar

levels of double bonds throughout the reaction time.

3.1.2 Bisallylic site

The auto-oxidation reaction follows a radical me-

chanism and the most favoured site for radical

formation is the carbon group situated between two

double bonds, namely the bisallylic site. On forma-

tion of the radical, the double bonds immediately

reorder into a more stable conjugated structure. The

rate of radical formation can be measured by observ-

ing the decrease in the NMR signal relating to the

bisallylic protons (Fig. 4).

Removing the iron or the pro-oxidant from the

sample slows the radical formation over the course

of the reaction when compared with the full test. A

similar level of decrease is observed when the pro-

oxidant is reduced by half. There are considerably

fewer bisallylic sites available in technical grade

OAME and the B25 blend; however, beyond this

there is no discernible change in the rate of decay

compared with the full condition. The largest

observable effects on the level of bisallylic sites are

the air flowrate and the temperature of reaction,

with the rate of oxidation being slower and the final

level of bisallylic sites higher than when any other

factor is removed. The rate of bisallylic decay is fairly

smooth under all examined conditions, suggesting

that the highly variable rate of conjugation forma-

tion is caused by the latter oxidation reactions.

3.1.3 Conjugated double bonds

The conjugated radical reacts with oxygen to form

peroxides; these peroxides are still conjugated,

however, and the conjugated bonds decompose in

subsequent chemical reactions to give smaller-chain

oxygenated compounds. The observed conjugated

bonds in the reaction are a balance between the rate

of formation (which as seen above is reasonably

smooth) and the rate of the numerous chemical

reactions involved in their decay. All these rates will

Fig. 3 Decrease in the number of double bonds throughout the reaction time. All trend lines aresecond-order polynomials and have R2. 0.99






vary greatly over time depending on the reactions of

the various oxidation products formed and the loss

of volatile products.

Figure 5 shows the level of conjugated double

bonds observed throughout the reaction time.

Reducing the amount of iron or pro-oxidant has

little effect on the number of conjugated bonds in

the reaction. However, after 240min, there is a

substantially larger amount of conjugated bonds

when all the pro-oxidant has been removed. There

are approximately half as many conjugated bonds

when the pro-oxidant is reduced by 50 per cent,

suggesting that the pro-oxidant has a large effect on

the breakdown of the oxygenated FAME later in the

oxidation process.

Removing iron from the reaction vessel has a

similar effect to removing the pro-oxidant, and there

are a significantly larger proportion of conjugated

bonds after 6 h. Reducing the metal by 50 per cent

has very little effect compared with the full run; this

follows the consensus that iron behaves catalytically

in the oxidation of biodiesel and was in abundance.

The temperature of the reaction and the air flowrate

in the reaction vessel also have significant impacts

on the conjugation of the methyl linoleate and,

although the initial levels of conjugation are similar,

the numbers of conjugated bonds in the two runs

are much higher at the end of the reaction than in

the full test. The rates of change in the levels of

conjugated bonds in the B25 and full test are highly

similar, suggesting that the blend level has little

effect on the primary reaction mechanism other

than the fact that there is less oxidizable material.

3.2 Subsequent oxidation products

3.2.1 Soluble oxygenated product formation

The newly conjugated radical species will then react

with oxygen to form peroxides and hydroperoxides.

Subsequently these peroxides will break down

through numerous reactions to form aldehydes,

ketones, acids, and water. Because of the complexity

of the 1H NMR spectrum and the relatively small

amounts of soluble organic oxidation products

formed, it was impossible to measure the level of

ketone or acid formation; however, it was possible to

quantify the levels of peroxide and aldehydes pres-

ent (Fig. 6).

Fig. 4 Decrease in the bisallylic protons throughout the reaction. All trend lines are second-order polynomials and have R2. 0.99






There was very little peroxide observed under the

full testing procedure, suggesting that, under the

conditions imposed, the peroxide is degrading into

the various decomposition products quickly. No

clear trends were observed for the levels of alde-

hydes or peroxides in the reaction mixture under any

of the testing conditions. A large number of reac-

tions are taking place in the formation and decay

of both of these species and, as the rates of all these

reactions vary, the total quantity of peroxides and

aldehydes present is cyclic, allowing no clear inter-

pretation of the results.

3.2.2 Volatile oxygenated products

Peroxides and aldehydes eventually break down into

volatile acids. The volatiles given off throughout the

experiments were passed through distilled water and

the pH of this solution was measured (Fig. 7).

On removal of half the iron, using B25, or

substitution of the LAME with OAME the final pH

values are all similar to those of the full oxidation

run. This suggests that, in the production of volatile

acids, 50 per cent of the iron is having the same

effect as when the full amount is present. There is

little observable change in the pH when the blend

level is reduced or when OAME is used in place of

LAME. The temperature and the air flowrate have

the largest effect in the production of volatiles,

closely followed by removing all the pro-oxidant or

iron. Reducing the pro-oxidant, rather than remov-

ing it entirely, results in a pH value that falls between

the full and the no pro-oxidant reactions.

3.3 General oxidative trends

3.3.1 Oxidation rate

As with Fig. 3, Fig. 8 shows the decrease in the NMR

signal representing double bonds for the full testing

procedure. As discussed previously, the oxidation of

biodiesel is complex with primary, secondary, and

tertiary reactions occurring simultaneously. For this

reason it was decided to split the experimental data

into three smaller time periods with the intention of

gleaning additional information pertaining to the

Fig. 5 The number of conjugated double bonds observed throughout the reaction. All trend linesare second-order polynomials and all R2 values are greater than 0.90 unless statedotherwise






impacts of the factors under investigation on

primary and secondary oxidation processes. The

data were split into three time periods, namely 15–

60min (initial), 120–240min (midtest), and 300–

360min (final), as shown in Fig. 8, with a linear

least-squares fit applied to each. The gradient of the

fit line represents the rate of change in the number

of double bonds (termed the oxidation rate) during

the initial, midtest, and final phases.

Figure 9 shows the results of a partial least-

squares multi-variate analysis examining the impact

of each factor on the rate of change in the number of

double bonds. The horizontal axis in Fig. 9 repre-

sents the magnitude of the coefficients attributed to

Fig. 7 pH of the deionized water, after the full 6 h reaction time. Shaded areas show significantlyhigher values than does the FULL test run

Fig. 6 The level of aldehydes and peroxides observed for the reactions based on the iron leveland environmental factors






each factor in the equation for the fit obtained

during multi-variate analysis. The greater the mag-

nitude of the coefficient, the greater the impact that

factor will have on the output, in this case the rate

of decrease in the number of double bonds. In

addition, interaction terms are also presented to

demonstrate whether combinations of factors cause

a greater or smaller impact than would be expected

by the summation of their individual constituents.

It can be seen in Fig. 9 that, of the primary (first-

order) factors, the temperature has the most domi-

nant effect on the initial rate of oxidation. An

increase in temperature causes the gradient of the

fit lines, shown in Fig. 8, to become more negative;

this implies a faster rate of decrease in the number of

double bonds or an increase in the oxidation rate.

Similarly, the air flowrate through the sample and the

saturation of the FAME also had significant impacts

on the oxidation rate, causing an increase in oxida-

tion at higher air flowrates and higher levels of

unsaturation. Interestingly, an increase in the quan-

tity of iron added, normally considered to catalyse

the oxidation process, actually led to a decrease in

the rate of oxidation during this initial period of the

test.

More significant than the primary factors are the

interactions between them, with combinations such

as an unsaturated FAME with a high temperature or

air flowrate vastly increasing the rate of oxidation. As

with the primary factors, the interaction between a

high temperature and air flowrate also resulted in an

increased rate of oxidation.

The second subplot of Fig. 9 shows the impact of

each factor and interaction on the rate of oxidation

during the middle of the test (120–240min). As with

the initial data, the air flowrate remains significant

during this portion of the test but, unlike initially, the

pro-oxidant concentration becomes dominant. This

suggests that while the pro-oxidant may have

relatively little impact on primary oxidation, it does

increase the rate of secondary oxidation processes.

The third subplot of Fig. 9 shows the impact of

each factor and interaction on the rate of oxidation

during the final phase of the test (300–360min). In a

reversal of trends seen during the previous time

periods, many of the factors which had previously

increased the rate of oxidation actually result in a

reduction in rate. For example, an increase in the air

flowrate during the initial and middle sections of the

reaction caused a large increase in the rate of

oxidation; however, during the final phase of the

test a higher air flowrate caused a reduction in the

rate. This can be explained by considering the test as

a whole, with a finite number of double bonds being

oxidized at a given rate. If the initial rate is high, as

the number of bonds remaining decreases, so too

does the rate of oxidation of those remaining bonds.

This effect is highlighted by the increasing impor-

tance of the blend ratio as the test time increases,

with higher blends sustaining higher oxidation rates

near the end of the test. Similarly, if the initial rate is

low, such as in the case of iron content, then the

oxidation rate may increase as there are still suf-

ficient double bonds available for oxidation.

Fig. 8 Reduction in double bonds with test time for the FULL sample






Fig. 9 Impacts of factors on the initial, midtest, and final rates of change in the number ofdouble bonds. FAME(Mono) represents when OAME was substituted for LAME within theFAME






When Fig. 9 is examined as a whole, it is possible

to observe how the balance of factor importance

shifts as the reaction progresses. It should be noted

that the axis scales are not the same but instead

decrease with time, suggesting that the impacts of all

factors diminish gradually. Generally speaking, the

rate of a reaction, under constant environmental

conditions, is dependent on the relative concentra-

tions of the reactants and the products. As the

reaction progresses, the concentrations of products

increase and this results in a reduction in the rate of

reaction. This scenario can be clearly observed in

Fig. 9 when examining the effects of factors such as

the temperature and air flowrate. Higher tempera-

tures and air flowrates result in significantly in-

creased rates of reaction in the early stages of the

test but a complete reversal by the end as the

concentration of reactants has decreased and the

products increased. In contrast, it is suggested that

the presence of iron and the cerium-based pro-

oxidant only becomes significant for secondary

oxidation reactions once the concentrations of the

products of primary oxidation reactions have in-

creased sufficiently.

It should be noted, however, that the data

presented in Fig. 9 are intended to highlight the

impact of the different factors on the rate of

oxidation as the reaction progressed and does not

necessarily indicate the degree of oxidation or the

quantities of oxidation products which have been

produced.

3.3.2 Relative impact on oxidation

As has been discussed in sections 3.1 and 3.2 the

oxidation of FAME is a complex process with varying

interpretations of what constitutes ‘oxidized’. In

practice, limits are often set for acceptable levels of

viscosity, acid value, peroxide value, and induction

time; however, owing to the cyclic nature of the

formation of some products (such as peroxide

shown in Fig. 6), these measures may not be suffi-

cient to indicate the degree of oxidation. In section

3.3.1 the impacts of different factors on the rate of

oxidation were examined but, in order to quantify

how far the oxidation reactions have progressed, it is

necessary to define oxidation as either the total

decrease in initial reactants (i.e. FAME) or the

absolute increase in oxidation products.

Table 2 summarizes the relative impact on FAME

oxidation as determined by examining five different

indicators of initial reactant reduction or product

formation.

1. Final pH value, i.e. the pH of the distilled water

containing volatile compounds at the end of the

test period. This is a useful indicator as it will

quantify the amounts of volatile compounds

released during the course of the reaction in a

similar way to the Rancimat test.

2. Percentage decrease in the number of double

bonds, i.e. the decrease in the double-bond

NMR equivalent at the end of the test period as

a proportion of the value at the start of the

experiment. The oxidation process involves radi-

cals interacting with double bonds within the

FAME molecule, causing them to reorganize into

a more stable compound. When this happens, the

number of carbon–carbon double bonds is de-

creased. The percentage change in the number of

double bonds should give an indication of the

degree of oxidation as well as accounting for any

differences in initial levels.

3. Absolute decrease in the number of double bonds,

i.e. the absolute reduction in NMR equivalence

between the beginning and end of the experi-

ment. As with the percentage decrease in the

number of double bonds, the absolute decrease

should also give an indication of the total

progression and rate of the oxidation process;

however, this measure may be more susceptible

Table 2 Factor oxidation rank orders based on different indicators (using the sample labelling in Table 1). Theoverall rank order was obtained by averaging the rank values for all indicators

Final pH value

Percentagedecrease in thenumber of doublebonds

Absolute decreasein the number ofdouble bonds

Final conjugatedvalues

Averagealdehydevalue

Overall rank order

Averageranking

More oxidation 1/2 Iron B25 FULL B25 1/2 Iron FULL 2.2

Q

FULL FULL 1/2 Iron OAME FULL 1/2 Iron 2.2OAME OAME 1/2 Pro-ox 1/2 Iron 1/2 Pro-ox B25 3.8B25 1/2 Iron No Pro-ox FULL No Iron 1/2 Pro-ox 4.21/2 Pro-ox 1/2 Pro-ox No Iron 1/2 Pro-ox 1/2 Air OAME 5No Iron No Pro-ox B25 No Iron No Pro-ox No Iron 5.6No Pro-ox No Iron 1/2 Air 70 uC B25 No Pro-ox 6.41/2 Air 1/2 Air 70 uC 1/2 Air OAME 1/2 Air 7.2

Less oxidation 70 uC 70 uC OAME No Pro-ox 70 uC 70 uC 8.4






to differences in the initial number of double

bonds present in the sample.

4. Final conjugated values, i.e. a measure of the

number of conjugated double bonds at the end of

the test. During the first stage of the oxidation

process the double bonds within the FAME

molecule will conjugate to form a more stable

compound containing alternating single and

double bonds with delocalized electrons. As the

reaction progresses, conjugation will increase and

then decrease as the conjugated double bonds are

broken down. The final conjugated value should

give an indication of how far this process has

progressed.

5. Average aldehyde value, i.e. the average aldehyde

value during the course of the experiment. The

average value was used because of the parabolic

response. As oxidation progresses, aldehydes are

formed but later react further, leading to a

reduction in the total aldehyde concentration. It

is expected that an average value for the reaction

would give an indication of the progression of the

oxidation process.

The B25 and OAME factors have been highlighted

as these, by definition, contained smaller numbers of

double bonds from the outset of the experiment and,

as such, this difference could have an impact on the

rank order determined using certain indicators. It

can be seen that, depending on which oxidation

indicator is used, varying rank orders are obtained,

thus highlighting the difficulty in determining when

oxidation has occurred and the relative impact of

each factor.

The implications of this are that using only one

indicator as a guide to biodiesel oxidation may not

yield the necessary information, especially if a

history of the changes in that indicator over time

was not available. For example, the aldehyde con-

centration increases over time to a maximum

followed by a gradual decrease; if a single discrete

test was performed, there would not be sufficient

information to establish whether the reading was

early or late in the oxidation process during the rise

or fall in the concentration. If, however, a pH test

was conducted at the same time as determining the

aldehyde concentration, this may yield sufficient

information to infer the advancement of the oxida-

tion process.

Although each indicator does suggest a slightly

different rank order, there are trends which can be

observed. Generally the samples labelled FULL and

1/2 Iron lay at the top of the rankings, demonstrating

that the most oxidation is observed when halving the

air flowrate through the sample and that reducing

the temperature results in reduced oxidation. An

overall rank order was determined by averaging the

rank positions obtained from all the indicators for

each factor. The overall ranking suggests that the

temperature is the most dominant factor, followed

by the air flowrate, fuel-borne pro-oxidant, and

finally iron concentration.

4 CONCLUSIONS

Defining when oxidation has occurred is challenging

as there is no total measurement for the oxidation of

biodiesel. Many indicators fluctuate over time as

they are formed and subsequently break down;

however, the number of double bonds remaining

does seem to be a good measure of the progress of

oxidation. It is difficult to separate the role of each

component in the reaction, owing to the interactions

between factors; however, on the basis of the data

collected, and shown in Table 2, it appears that the

air flowrate into the system and the reaction

temperature have the largest effects on the total

oxidation of FAME. The presence of air and elevated

temperatures leads to an increase in the rate of

oxidation in the early stages of the reaction, but a

decrease in the oxidation rate later in the test as the

concentration of reactants (and absolute number of

double bonds available) decreases.

The presence of both pro-oxidant and iron filings

has an effect on the oxidation, where the removal of

these components from the reaction mixture re-

duces the total level of oxidation. Unlike with the

cerium-based pro-oxidant, merely reducing the

amount of iron in the reaction does not always yield

a significant reduction in oxidation, possibly because

the iron acts catalytically and is present in abun-

dance. The presence of iron and pro-oxidant does,

however, lead to an increased rate of oxidation in the

later stages of the reaction, implying that both

factors promote secondary oxidation processes.

Blend level has little effect on oxidation when

the data have been normalized for the amount of

LAME in the specific samples. Lower blend levels

do, however, mean that less oxidizable material is

available which, in turn, leads to lower absolute

quantities of oxidation products. The results of this

study suggest that a reduction in biodiesel oxidation

within the sump oil could be achieved by reducing

the temperature and aeration of the sump oil;

however, in reality, this is not a feasible solution.

The addition of antioxidants to the sump oil, tailored






to negate the formation of the initial radical species,

would still seem to be the most effective method.

ACKNOWLEDGEMENT

The authors would like to thank the Ford MotorCompany for their financial support through theAcademic Fellowship of the first author. The thirdauthor holds the Medlock Chair of Engineering. Thiswork was undertaken using facilities within thePowertrain and Vehicle Research Centre at theUniversity of Bath, UK.

F Authors 2010

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APPENDIX

Notation

Bx biodiesel blend ratio (where x is the

weight percentage of biodiesel)

DPF diesel particulate filter

2-EHN 2-ethylhexyl nitrate

FAME fatty acid methyl ester

LAME linoleic acid methyl ester

NMR nuclear magnetic resonance

OAME oleic acid methyl ester






Documents

Factors affecting the decomposition of biodiesel under simulated engine sump oil conditions