biogas/petroi dual fuelling of sl engine for rural third...

Biomass 13 (1987) 87-103

Biogas/Petroi Dual Fuelling of Sl Engine for Rural Third World Use

H. H. Jawurek, N. W. L a n e and C. J. Rallis

School of Mechanical Engineering, University of the Witwatersrand, 1 Jan Smuts Avenue, 2001 Johannesburg, South Africa

(Received 15 July 1986; revised version received 24 April 1987; accepted 28 April 1987)

ABSTRACT

Performance and combustion characteristics of a portable fixed-ign#ion- timing engine-alternator set, fuelled with simulated biogases (mixtures of CH4, C02 and minor quantities of He) are presented. Maximum power output, at optimally adjusted air-fuel ratios, falls with increasing CO2 content of the gas, the power losses (referred to pure CH4) being severe with low-quality gases (e.g. 37% at 50% C02). Fuelling with gases of C02 content greater than 30% leads to harsh running; cylinder peak pressures are variable from cycle to cycle, low in amplitude and retarded in timing. It is shown that the poor combustion, harsh running and low power output can be corrected by supplying some petrol simultaneously with the gas. Such dual fuelling is beneficial with gases of all qualities and at all gas/petrol ratios, but particularly so with small quantities of petrol and very poor gases.

Key words." Alternative fuels, biogas, biogas/petrol dual fuelling, dual fuelling, methane, spark-ignition engine.

BACKGROUND AND AIMS

The overall aims of this study were to reassess and to extend the useful- ness of biogas as a fuel for small spark-ignition (SI) engines in rural Third World applications. Such applications demand that complex or expensive machinery, modifications and operating techniques be avoided.

The composition of biogas, depending on the feed material and the method of digestion, usually lies within the following ranges: 50-70% methane (CH4), 25-50% carbon dioxide (CO2), 1-5% hydrogen (H2),

87 Biomass 0144-4565/87/S03.50- © Elsevier Applied Science Publishers Ltd, England, 1987. Printed in Great Britain

88 H. H. Jawurek, N. W. Lane, C. J. Rallis

0"3-3% nitrogen (N2) and various minor impurities, notably hydrogen sulphide (H2S); (all gas compositions in volume percent, throughout this report).

The 'raw' gas may be 'scrubbed' to rid it of CO2, but such processing is usually too expensive and complicated for developing technology implementation. Our study thus concentrates on raw biogas.

Such gas serves as a fuel predominantly for cookers and lamps 1 and also for internal combustion engines of both the spark-ignition (SI) and the compression-ignition (CI) type. 2 The engines are used to drive various stationary agricultural machines or small alternators.

In the case of CI engines dual fuelling with biogas and a pilot quantity of injected diesel oil is necessary. 2,3 The required modifications can be quite complex, particularly for small high-speed engines subjected to variable loads. CI engines were thus excluded from our study.

Biogas as SI engine fuel

The conversion of SI engines to gas fuelling is a straightforward matter, requiring only the fitting of a simple gas-fuel adaptor and, possibly, hardened valves and valve seats. 2 Such engines fuelled with pure CH4, instead of petrol, suffer a reduction in peak power output of some 10-20°/o, 4-6 but apart from this, perform very well. The power loss can in any event be offset by increasing the compression ratio. At the high CO2 content, typical of raw biogas, however, problems with regard to starting, smooth running and power output can arise.

Our previous study 5 employed a small, governed, side-valve engine, directly coupled to a revolving field alternator. The unit was fitted with a commercially available venturi-type gas-fuel adaptor upstream of the petrol carburettor, but was otherwise unmodified. Biogases were simulated by mixtures of pure CH 4 and CO2; the H2 component was omitted, but recognised (in view of its high burning velocity) to be potentially important. The mixture strength for each fuel was adjusted, as would be expected of the rural operator, by feel and ear. The resulting power outputs thus did not necessarily reflect the maximum capabilities of the unit, but rather those that could be reasonably expected in practice.

The engine ran smoothly on gases containing up to 23% CO2, slightly noisily at 31% CO 2 and harshly at 42% CO2. The impression of harshness was formed subjectively and was not based on any measure- ments. Maximum power output was 17% lower with CH4-fuelling than with petrol. Increased CO2 content of the gas led to further losses, with a 45% loss (compared with petrol) at 41% CO2.

Biogas/petrol dual fuelling of SI engine 89

Hand-starting from cold was possible with gases containing up to 31% CO2; at higher CO2 contents, starting on petrol and 'blending-over' to gas was necessary. During this blending-over period the engine ran in the dual fuel mode, that is, simultaneously on petrol and biogas. This was extended to the concept of steady-state dual fuelling as a means of alleviating the harsh running and low power output which are obtained with low-quality (high CO2) biogases. The technique seemed particularly relevant to developing technology situations, where the alternative, namely scrubbing of the gas, is impractical. The concept of simultaneous fuelling of SI engines with biogas and liquid fuels is not novel: Picken and Soliman 6 have reported successful tests on supplementation of gas with diesel oil and kerosene, and Picken 2 and Sharma 7 make mention of supplementation with petrol. On the latter technique, however, no experimental results appear to be available.

Specific aims

In view of the above, the specific aims of this study were defined as follows:

(1) To establish for benchmark purposes the performance characteristics of the engine-alternator set for various CH4/CO2 fuel mixtures, with the air-fuel ratio adjusted to maximum power for each fuel.

(2) To check whether the presence of H 2 in the fuel gas materially affects engine-alternator performance.

(3) To elucidate, by measurement of the combustion chamber pressure transients, the nature of combustion and of harsh running with biogas fuels of high CO 2 content.

(4) To assess petrol/biogas duel fuelling as a means of correcting the harsh running and low power output on fuelling with low quality gases.

TEST FACILITY AND PROCEDURE

Engine, gas-fuel adaptor and alternator

These items were the same as those employed in our previous study. 5 The engine was a Briggs and Stratton 195 400, that is, a single-cylinder, four-stroke, side-valve machine having a swept volume of 314.5 cm 3 and a rated (sea level) power output of 5-2 kW at 3000 rev min-~. The

90 14. H. Jawurek, N. W. Lane, C J. Rallis

compression rato was 6.2 and the ignition system was of the magneto type with an essentially fixed timing of 14 ° before top dead centre.

A Beam l120B venturi-type 'gas carburettor' was fitted to the engine's air intake upstream of the petrol carburettor. The unit (previously described) 5 required a supply of fuel gas at ambient pressure and could be operated with or without simultaneous petrol fuelling. Mixture strength could be set by a needle valve in the gas inlet port. A pointer and scale arrangement indicating percentage of full throttle opening was retrofitted to the butterfly valve.

Directly coupled to the engine was a Pincor revolving-field alternator rated at 13.65 A at 220 V output, at a frequency of 50 Hz (3000 rev min-i).

Gas supply system

Simulated biogases were made up of CH4, C O 2 and H2, all of 99"9% purity. The gases were supplied from high pressure cylinders, reduced to approximately 0.8 MPa and fed to a gas mixer manufactured to our requirements by Witt-Gastechnik of Witten, Germany. This unit merely required the dialling-in of the desired gas compositions and automati- cally metered, mixed and supplied the gases to a storage vessel at 120 kPa absolute pressure. (The accuracy of the gas compositions was checked for a few mixtures by gas chromatography; the mixer readings and the analysis figures were found to be indistinguishable). The gas was reduced to ambient pressure by two Beam 52B regulators arranged in parallel and fed to the gas carburettor. The regulators were rated for 12 kW engines on natural gas fuelling. Since natural gas has a higher calorific value than CH4, and biogases of over 50% CO2 content were to be investigated, two regulators in parallel were deemed necessary to ensure sufficient capacity for the 5.2 kW engine.

Measurement of engine input and output

The following mass flow rates were determined: ( 1 ) air, by measurement of the pressure drop across an orifice meter, fitted with the usual surge- damper arrangement; 8 (2) gaseous fuels, by means of rotameters; (3) petrol, by weighing and timing. The electrical output of the alternator was dissipated in a step-switchable resistance box and a rheostat; power output was measured by means of a wattmeter, and frequency by a digital counter.

Biogas/petrol dual fuelling of $1 engine 91

Measurement of combustion chamber pressure

The cylinder head of the engine was drilled and fitted with a Kistler 9121A piezo-electric pressure transducer enclosed in a water cooling jacket.

The output signal of the transducer was fed, via a charge amplifier, to a transient recorder having a 10 bit resolution. Pressure versus time traces for either single or multiple engine cycles could be obtained by varying the sampling rate of the recorder. The records were displayed on an oscilloscope and hard-copied, using an XYrecorder.

The transient recorder was triggered with the piston at bottom dead centre (BDC). The triggering signal derived from the interruption of the fight beam of an optical emitter-sensor unit by a tag mounted on the flywheel. A voltage comparator circuit eliminated noise originating from the engine's ignition system, and ensured consistent operation. BDC signals were recorded in parallel with transient pressure.

Since records of pressure versus time were obtained, crankangle positions were estimated by linear interpolation between the BDC marks. The accuracy of this procedure is dependent on the steadiness of engine speed during the cycle.

Test procedure

In our previous study 5 the engine was started on the desired fuel, mixture strength was adjusted by ear, and the throttle governor was set to give an engine speed of 3100 rev min -1 (51.7 Hz) as specified by the manu- facturers for no load. With progressive loading the throttle butterfly valve opened under the action of the governor and the engine speed dropped slightly, initially linearly with power, and more rapidly as maximum power and full throttle were approached (see Fig. 2 in Ref. 5). In the present study maximum power only was sought, but with the mixture strength optimally adjusted for each fuel. Thus the engine was started, loaded to give approximately full throttle opening, and fine- tuned by alternately varying the load rheostat and the mixture screw (petrol or gas), to the point of maximum wattmeter deflection. All readings listed above were now taken, fo l lowed- when d e s i r e d - by the recording of the cylinder pressure versus time traces. Since the settings giving maximum power were generally rather difficult to detect unambiguously, most determinations were repeated, in some cases several times. The results reported below reflect the arithmetic means of these data points.


RESULTS AND DISCUSSION

Engine-alternator performance versus carbon dioxide content of gas

Figure 1 summarises the performance of the engine-alternator set on fuelling with various mixtures of C H 4 andCO2.

Figure l(a) shows the maximum electrical output at optimal mixture strengths (solid line), and maximum power with subjectively adjusted

I I I I I 141 . . . . . . . l - , . ~ p r e s e n t s t u d y

2.5 I-- petrol . ~ , ~ . . . . . . . . . . . Jawurek & Rallis (1984)

[ - ~o 2. o ~ l t a , , . , , ~ . ~....:~1,,~ (a)

"°*oo.o. ~ t l ~ O E:m ......... ~ o

3 . . \ (b) -- "\

1.0 ~ O ~ o ~ o ~ o m O ~ ' - - ' , , , O k '

.

03 o ~5

g 6 2(

15

0 5 o

__.~. (d) .

(e) ~o

I I I I I 20 40 60

45

4o o~ C

"-s 35 O"

U_

30

o 1.o

8 0.8 E

0.6 UJ

C o n c e n t r a t i o n o f C O 2 in b i o g a s , vo l %

Fig. 1. Engine-al ternator performance v s C O 2 content of biogas.


mixture strengths 5 (dotted line), both v e r s u s CO 2 content of the gas. Fine-tuning of the mixture strengths clearly led to a considerably more gradual power fall-off with increasing CO2 content than did casual tuning. At 40% CO2, for example, the power losses (referred to pure CH4) were 13 and 32% respectively.

Further, it appeared that the condition of the engine had deteriorated since the completion of our previous study: maximum powers on fuelling with pure CH4 and with petrol were both lower by some 6%. Subsequent stripping confirmed the need for a piston and cylinder overhaul.

The large disparity between the rated maximum power of the engine (5-2 kW at 3000 r ev mil l -1 ) and the measured maximum electrical output (2.45 kW on fuelling with petrol) may be attributed to the following: First, tests were conducted at an altitude of 1800 m above sea level; this causes a reduction in engine output of some 20%. Secondly, the energy conversion efficiency of the alternator may be taken to be 70-75%. Thirdly, the engine speed at maximum power with petrol fuelling was 2570 rev rain- 1, that is 86% of the rated speed. Finally, the engine was of necessity fitted with special air intake and exhaust systems having pressure drop losses larger than those of the standard items.

Figure l(b) gives output frequency (engine speed) at maximum power as a function of fuel gas composition. In the CO2 concentration range of 0-40% the frequency remained essentially constant at 42.5 Hz and then fell off sharply. Figure l(c) shows the corresponding gas consumption (CH 4 plus CO2, normalised to 20°C, 101 kPa) to have risen roughly linearly with CO2 content, to have peaked at 40% CO2, and then to have dropped. However, the overall efficiency of the unit (electrical output divided by calorific input), shown in Fig. 4(e), continued to rise slightly with CO 2 content, up to 50% CO2. Thus the sharp fall-off in speed (and power) at 40% CO 2 appears to have been due not to incomplete combustion, but rather to a gas supply problem.

At this point a number of symbols require to be defined. The most important of these is the equivalence ratio 4. This quantity is the ratio of stoichiometric to actual air-fuel mass ratio, where fuel refers to combustibles only (here CH 4, CO2 excluded). Equivalence ratio is thus a measure of mixture strength, with ~ = 1 signifying a mixture that is stoichiometric, ~ < 1 one that is lean and ¢ > 1 one that is rich. For any engine/fuel combination a particular value of ¢ has associated with it a maximum attainable power output, Pmax" Different mixture strengths, 4, lead to different values of Pmax, and generally there exists an optimum value of 4, namely ~ *, for which Pm,x itself is a maximum, namely P*m~x. Figure l(a) shows these values of mixture-optimised (or attempted mixture-optimised) maximum power output.

94 H. H. Jawurek, N. W. Lane, C. Z Rallis

Figure l(d) shows the values of ¢* corresponding to P'max for gases of various CO2 contents. All values are seen to lie in the stoichiometric to lean region and to fall with increasing CO 2 content of the gas. This is contrary to normal (liquid fuel) experience, according to which maximum power is obtained with rich mixtures (typically 4 = 1.15).

A theoretical check on the combustion behaviour of CH 4 and CH4/ CO2 mixtures was thus carried out. Values of the adiabatic flame tempera- ture at constant pressure (an approximate indication of power output) were calculated as a function of 4 for a number of gases, using a procedure which took account of the presence of 12 combustion product species? The theoretical flame temperatures exhibited maxima at 4 = 1-02 for pure CH4, and at progressively lower values of 4, asymptot- ing to 4--1.00, with gases of progressively increasing CO 2 content. These findings indicate that maximum power P'max, should have occurred with mixtures ranging from slightly rich for pure CH4, to stoichiometric for highly CO2-diluted gases.

In view of this discrepancy additional and more detailed tests towards determining the optimum mixture strength, 4", were carried out, initially on two gases, namely pure CH 4 and a 70% CH4/30% CO 2 mixture. With each gas 4 was varied systematically, Pmax was determined at each setting, and the Pmax versus 4 data were plotted and smoothed. The resulting curves, though rather flat, exhibited clear maxima, and the coordinates of the latter were taken as P'max and 4*. (This procedure, while elaborate, is more reliable on statistical grounds than the previously employed once-off, trial-and-error search for P'max') The resulting values of 4" are shown as square data points in Fig. l(d). Our previous experimental findings, namely that mixture-optimised maximum power, P'max, occurs with lean mixtures, were thus confirmed. We are at a loss to explain this behaviour. However, since operation at mixture strengths greater (richer) than 4" was regularly achieved in these additional tests, we can state that the fuel supply was more than adequate for gases of 0-30% CO2 content.

With fuels having CO 2 contents of 40% and greater, however, similar detailed tests did reveal a fuel starvation problem. In this region of CO 2 concentration no genuinely optimal values of 4 were achievable. Pmax increased with increasing 4 until the gas supply needle valve was fully open (see triangular data point at 40% CO2). This fact had escaped us in our previous tests since the Pmax versus 4 curves are flat near their maxima (where achievable) and the needle valve could be advanced (apparently opened) several turns beyond its fully open condition.

Extensive attempts were made to identify and to cure this fuel starvation problem. The capacity of the gas mixer was checked and found to be


more than adequate. The inlet orifices ('jets') of the gas carburettor, as well as that of the mixture needle valve, were successively enlarged; larger fuel lines and fittings were installed; and the liquid carburettor was bypassed, the gas being injected into the inlet manifold. These all had no effect. Bypassing of the demand regulators (the gas pressure being maintained at its previous value by a regulator internal to the gas mixer) led to insignificant increases in ¢ and Pmax" Feeding the fuel gas under positive pressure initially appeared more effective, but the supply pressure could not be extended to more than some 10 kPa gauge (see open data point at 40% CO2, Fig. ld). Further increases in pressure led to sudden cutting-out of the engine, apparently owing to choking of the air supply. At no stage and by no techniques could stoichiometric, let alone rich, mixtures be achieved. It thus appears that the fuel starvation problem resided not in the gas supply system, but (for reasons at present unidentified) in the engine itself.

In short, the stoichiometric to lean mixtures associated with maximum power output were: (a) truly optimal, for fuel gases of low CO 2 content (< 30%), and (b) inevitable for gases of high CO2 content. Similar behaviour must be expected to be encountered in general practice, particularly with engines of similar design to that tested.

Figure l(e) further shows a fall in overall efficiency on fuelling with gases of greater than 50% CO2 content. (The point at 60% CO2 derives from an earlier study conducted with mixture tuning techniques differing slightly from these described here, but is included to lend added credi- bility to the reading at 55% CO2.) Thus it would seem that incomplete combustion commenced at a CO2 concentration somewhat beyond 50%. This is roughly in tune with the results of Picken's study (unpublished, quoted by Hobson et al. 1°) in which increased exhaust emission of unburnt fuel set in the region of 45-50% CO2. However, harsh and apparently irregular running was, as in our previous study, 5 already well established at 40% CO2.

Effect of hydrogen content of biogas

Biogases are known to contain 1-5% H2, with occasional reports of a maximum of 10%. 11,12

Since the laminar burning velocity of H2 is at least six times that of CH413'14 its presence in biogas fuels should to some extent correct the drastic reduction in burning velocity caused by the dilution of C H 4 with CO2 .15 The molar calorific value of H 2, however, is about a third of CH4; 13 therefore the replacement of some CH4 with H 2 in a particular


C H 4 / C O 2 mixture reduces the energy input per combustion cycle, thus tending to counteract the above benefit.

Performance tests were carried out, first, on gases of 30, 40, 50 and 60% CO2 content, each of 0 and 10% H 2 (balance CH4), and secondly, on gases of 40% CO2, with the H 2 content varied incrementally between 0 and 10%. The presence of H2 was in all cases beneficial; thus the effects of improved combustion consistently outweighed those of reduced energy input. However, for gases of up to 50% CO 2 content, the increases in maximum power were less than 1% for each 1% H 2. (At 60% CO2 the corresponding power increase was somewhat larger, but such CO2 levels are not likely to occur in natural biogases).

Thus for all practical biogas fuels (containing at most 50% CO2 and 5% H2) the effect of H 2 content on engine performance is small and the main trends shown in Fig. 1 may be expected to hold.

Combustion and harsh running with gas fuels of high CO 2 content

Figure 2 shows typical cylinder pressure diagrams for the combustion of various fuels (solid lines) and for estimated compression without combustion (dotted lines), both versus time (crankangle).

On fuelling with petrol of octane number 87 (Fig. 2(a)) the pressure rise was steep, peak pressure occurred at approximately 4 ° after top dead centre (TDC) and knock -- although inaudible -- appears to have been present. With pure C H 4 (Fig. 2(b)) the behaviour was similar, the more gradual pressure rise and the retarded peak pressure position (some 17 ° after TDC) reflecting the lower burning velocity. Again, mild knock was apparently present, but inaudible. The engine ran smoothly on both fuels.

Figures 2(c) and (d) show two combustion cycles on fuelling with a 50% CH4/50% CO2 gas. Peak pressures were extremely low in amplitude, absurdly late in position (at 42 and 34 ° after TDC, respectively) and inconsistent from cycle to cycle. The first two effects can be explained in terms of lean mixture, low burning velocity and low energy input per cycle; the third was probably due to fluctuations in mixture strength. While one might reasonably predict such fluctuations (well known with liquid fuelling) to be greatly reduced on gas fuelling, Yu 16 has shown this not to be the case, unless extensive precautions are taken. Thus variations in mixture strength must be expected to have occurred in our tests and it seems possible that their effects on pressure rise were magnified by the lean mixtures and by the severe dilution of the fuel by C O 2.

Biogas/petrol dual fuelling of $1 engine 97

(a) petrol

2.0

~ 1.o a~

o

~ ' i _ . . . . . . '

o l i ~ , l r ~ ' , ~ ~ r ! / ,

B D C TDC BDC

(b) 100% CH4

BDC

~ ~.i !i~ ~ i ¸ .i

t-,! ~! i~ i~ i ~.ii! i

i i ! ~ _ I i TDC B D C

, i i i i .-

(c) B 1.o 50% 0H4/50% 002

o."

i ' :

.i

ii

BDC TDC BDC

(d) 500 /0 0H4/50O/o CO 2

' I

~il i ! i l i ! i , ~ ~!~ ~.1.0 -i I1:-I i I I ] : ilJ ' I I ! ] I !

• : l - r l " t ~ k ~ t [ = - ] L : l i f [ r l ] T [ ] [ I ] ] ] i I ! I

B D C T D C BDC

Fig. 2. Combustion chamber pressure, P, vs crankangle for various fuels. (BDC and TDC signify bottom dead centre and top dead centre, respectively; the vertical arrows

indicate ignition.)


Fig. 3.

2.o~i i

~-" 1.0 ~"

TDC

i l ~1, I~! ~ I : I I

Ii Gas: 50%CHJ50%CO2t, i ~ i i ' ~ ~ i ' i ~ '1

, i i i ~ l i t , I ' ,

I ! i : : K I i 1' ! ! I I i ,;i

TDC TDC Inconsistent combustion on fuelling with biogas of high CO2 content.

A more pronounced example of inconsistent combustion, for three consecutive cycles, is shown in Fig. 3. The first maximum in each case was due to compression at TDC and the second to combustion. In the first two cycles the combustion peak pressure was approximately equal to that of Fig. 2(d); in the third it was lower than that corresponding to compression only. The impression of harsh or irregular running of this engine on fuelling with gases of high CO2 content appears to have been largely due to such variations in peak pressure. No evidence of outright missing, however, was found.

It was noted subjectively that with the admixture of H 2 to the gas (increased burning velocity) the engine ran somewhat more smoothly. This was, however, not investigated quantitatively.

D u a l f u e l l i n g

The enforced operation at lean mixtures, the low power output and the rough running that are obtained with fuel gases of high CO2 content could all be corrected by simultaneous fuelling with an appropriate, generally small, quantity of petrol.

In practical execution this entailed starting the engine on petrol (normally necessary with poor quality gases), gradually blending over to pure gas fuelling, readmitting some petrol as required, and readjusting the gas mixture strength for maximum power.

Results obtained in this manner for a gas of 45% C O 2 and 2% H2 content are shown in Fig. 4, with maximum power plotted against petrol content, the latter expressed as mass percentage of the total combustibles (CH4, H2 and petrol). Fuel mixtures ranging from pure gas to pure petrol were covered. Dual fuelling resulted in increases in maximum power throughout the range, but most markedly so at low percentages of petrol.

Biogas/petrol dual fuelling of Sl engine 99

Fig. 4.

Jz¢

O.

O

O

E -'l

E x ~E

2.5

2.0

1.5 0

! I I I

/ Gas: 53% CH4/2% H2/45% CO~

20 40 60 80 100

P e t r o l c o n t r i b u t i o n t o t o t a l combustibles, mass %

Biogas/petrol dual fuelling at optimal air-fuel ratio for each fuel mixture.

I t i : ~ :] P, MPa ~.i.. Gas: 50% OH J50% CO2 ~ ......... i t -

' i . I • i / I

(a) ~ i" l~ :4-'~4 i ]!" " ,347%petrol ' lJ : : t ' t :Pl : : f f ! i t

1.0-t : : : : : : : : : : : : : : : : : : : : : ~' 4LL i -P - i-

j : l / r ; i 1~ BDC TDC BDC

i Gas: 50% CH~/50% CO2 i,;.. ,:!..

J-::l ~: :[ :::1: -~.-I:L:l i~i i ..... (b) .... i t i ; t i - q . . . ! ' = t : i

i 16 9%pet ro l l = I ' ]'t;'/ ! I ~ !

.............. i i } ! ~ : I !

~ _ L I : : :i I I,i:14:17

BDC TDC BDC

Fig. 5. Combustion chamber pressure, P, vs crankangle for biogas-petrol dual fuelling.

The equivalence ratio varied from 0.74 with pure gas fuelling, to unity at 40% petrol, and to 1"28 on pure petrol fuelling.

Rough running was alleviated even by small admixtures of petrol. Figure 5 shows combustion chamber pressure versus time (crankangle) for fuelling with a 50% CH4/50% CO2 gas together with (a) 3-47% of petrol and (b) 16.9% petrol (mass percent of total combustibles). Comparison with Figs 2(c) and (d) shows the greatly improved pressure rise and peak pressure.

It has been suggested to us that since the engine ran in a fuel-starved, over-lean mode on gases of high CO2 content, supplementation with

1 O0 H.H. Jawurek, N. W. Lane, C. J. Rallis

petrol was effective merely by supplying additional combustibles; supplying additional gas would have served equally well. The point, however, is that significant increases of gas flow into the engine were impossible to achieve, not even by injection under pressure. It was the addition of a dense fuel, for example petrol, that was required. Supple- mentation with diesel oil and kerosene 6 holds similar benefits. These fuels, however, cannot in general serve as sole fuels in SI engines, thus rendering them unsuitable as back-ups in the event of failure of the gas supply.

The benefits of dual fuelling, however, go beyond the increase of maximum power and the prevention of rough running. Since operation at any gas/petrol ratio is possible, even minor quantities of gas -- too small for normal gas fuelling -- can be used to advantage in a blend. For example, fuelling with the gas of Fig. 4 simultaneously with 80% petrol yields a maximum power practically indistinguishable from that for pure petrol, but nevertheless leads to a substantial 20% saving in petrol. Thus, biogas should be viewed not only as an alternative fuel to petrol, but also as an additional fuel.

In rural situations dual fuelled engines would probably not be tuned quantitatively for optimum mixture strength. More likely, smooth running and sufficiency of power output will be casually checked on gas fuelling, and if found unsatisfactory, will be corrected simply by admit- ting some petrol. Results using this technique are shown in Fig. 6 for gases of 40 and 55% CO2 content and 2% H2. In both cases mixture strengths were adjusted for maximum power on pure gas fuelling only. (The tests on the 40% CO2 gas were, apart from the additional Pmax versus ~ tests, the last to be conducted in this study and were preceded by an engine overhaul; the resulting incompletely run-in state of the engine led to maximum power on gas fuelling, lower than that given in Fig. l(a).) Thereafter the petrol mixture valve was progressively opened and tests were continued until the rich limit of smooth running was reached. Maximum power increased significantly and approximately linearly with petrol content of fuel. In the case of the gas of 55% CO2 content (admittedly rare in natural biogases) the power increase was dramatic: the admixture of 10% petrol to the total combustibles led to a doubling of maximum power output.

CONCLUSIONS

These refer to the engine of the present study and, more generally, to fixed-timing SI engines of similar size and design.

Biogas[petrol dual fuelling of SI engine 101

Fig. 6.

2.0

1.5 O

0 Q.

E

.E 1.C x ¢u ~ ,

0.5

I I I

Gas: 5 8 % C H J 2 % H 2 / 4 0 % C O 2

. J "

./•Gas:" 43% CHJ2% H2/55% CO 2 "S-

* I i i i I 5 10 15

Petrol contribution to total combustibles, mass %

Dual fuelling with mixtures rendered increasingly rich by addition of petrol.

On fuelling with various C H 4 / C O 2 mixtures and with the air-fuel ratio adjusted for maximum power in each case, the engine suffers progressive leanness of mixture and losses in peak power output with increasing CO2 content of the gas. The losses (referred to pure CH4) are small with gases of high quality (2 and 9% at 20 and 35% CO2 respectively) and severe with poor gases (60% at 55% CO2).

The effect of the presence of H 2 in the fuel gas (at concentrations typical of real biogases and with realistic levels of CO2) is small: peak power increased by less than 1% for each 1% H 2 content.

Elevated concentrations of CO2 in the gas lead to unavoidably lean and harsh running of the engine. This becomes noticeable at 30% CO 2 and increasingly marked at higher CO2 contents.

The combustion of gases of high CO 2 content is poor and irregular. Cylinder peak pressures are severely retarded, low in amplitude and inconsistent from cycle to cycle. This is due, first, to the greatly reduced burning velocity of the over-lean and CO2-diluted gas, and secondly, to fluctuations in air-fuel ratio. The combined effects of these phenomena manifest themselves as harsh or rough running.

Low power output, poor combustion and harsh running can be corrected by the simple technique of dual fuelling, that is, supplying


some petrol simultaneously with the gas. Such admixture of petrol is beneficial for all biogases, but particularly so, in small quantities and with very poor gases. For example, for a gas of 55% CO2 content the admission of 10% petrol combustibles yielded a doubling in peak power output.

Dual fuelling is possible at any gas/petrol ratio. Thus even small quantities of biogas, insufficient for normal gas fuelling, can be utilised with resultant savings in petrol.

ACKNOWLEDGEMENTS

The authors offer their warm thanks to the following: Union Liquid Air Company Ltd, for making available gas mixing equipment, analytical services and gaseous fuels; Autolec (Transvaal) Ltd, for their donation of the engine-alternator set and numerous spares; the Anglo-American Corporation Ltd, for financial support; Messrs R. W. Bluff, D. A. Course, W. S. Gallie, P. R. Roberts, C. M. Sealey, I. Tingle and D. G. Torricelli for their enthusiastic contributions at various stages of the project.

REFERENCES

1. Van Buren, A. (1979). A Chinese biogas manual, Intermediate Technology Publications, London.

2. Picken, D. J. (1984). In: Biomethane, production and uses, eds R. Buvet, M. F. Fox and D. J. Picken, Turret-Wheatland, Rickmansworth, Herts, pp. 192-200.

3. Karim, G. A. (1982). In: Methane fuel of the future, eds P. M. McGeer and E. Durbin, Plenum Press, New York, pp. 113-29.

4. Durbin, E. (1982). In: Methane fuel of the future, eds P. M. McGeer and E. Durbin, Plenum Press, New York, pp. 83-99.

5. Jawurek, H. H. & Rallis, C. J. (1984). Proceedings, 19th Intersociety Energy Conversion Engineering Conference, San Francisco, 2,670-4.

6. Picken, D. J. & Soliman, H. A. (198l). Journal of Agricultural Engineering Research, 26, 1-7.

7. Sharma, Y. N. (1980). Paper presented at Workshop on Low Cost Energy for Water Pumping in Botswana.

8. Judge, A. W. (1955). The testing of high speed internal combustion engines, 4th ed., Chapman and Hall, London, p. 126.

9. Lane, N. W. (1986). Research Report No 87, School of Mechanical Engineering, University of the Witwatersrand, Johannesburg.

10. Hobson, P. N., Bousfield, S. & Summers, R. (1981). Methane production from agricultural and domestic wastes, Applied Science Publishers, London, p. 152.


11. Wase, D. A. J. & Forster, C. E (1984). Biomass, 4, 127-42. 12. Maramba, E D. (1978). Biogas and waste recycling, Maya Farms -- Liberty

Flour Mills, Metro Manila, Philippines, p. 146. 13. Obert, E. E (1973). Internal combustion engines and air pollution, Harper

and Row, New York, p. 96. 14. Rallis, C. J. & Garforth, A. M. (1980). Progress in energy and combustion

science, 6, 303-29. 15. Lewis, B. & yon Elbe, G. (1951). Combustion, flames and explosions of

gases, Academic Press, New York, p. 466. 16. Yu, H. T. C. (1963). Society of Automotive Engineers Transactions, 71,

596-613.

biogas/petroi dual fuelling of sl engine for rural third...

Documents