Renewable Energy 31 (2006) 849–863
www.elsevier.com/locate/renene
Technical note
Performance of direct-injection off-road diesel
engine on rapeseed oil
Gvidonas Labeckas*, Stasys Slavinskas
Department of Transport and Power Machinery, Lithuanian University of Agriculture, Student Str.
15, P.O. Box LT-53067, Kaunas Academy, Lithuania
Received 12 January 2005; accepted 5 May 2005
Available online 19 July 2005
Abstract
This article presents the comparative bench testing results of a naturally aspirated, four stroke,
four cylinder, water cooled, direct injection Diesel engine operating on Diesel fuel and cold pressed
rapeseed oil. The purpose of this research is to study rapeseed oil flow through the fuelling system,
the effect of oil as renewable fuel on a high speed Diesel engine performance efficiency and injector
coking under various loading conditions.
Test results show that when fuelling a fully loaded engine with rapeseed oil, the brake specific fuel
consumption at the maximum torque and rated power is correspondingly higher by 12.2 and 12.8%
than that for Diesel fuel. However, the brake thermal efficiency of both fuels does not differ greatly
and its maximum values remain equal to 0.37–0.38 for Diesel fuel and 0.38–0.39 for rapeseed oil.
The smoke opacity at a fully opened throttle for rapeseed oil is lower by about 27–35%, however, at
the easy loads its characteristics can be affected by white coloured vapours.
Oil heating to the temperature of 60 8C diminishes its viscosity to 19.5 mm2 sK1 ensuring a
smooth oil flow through the fuel filter and reducing the brake specific energy consumption at light
loads by 11.7–7.4%. Further heating to the temperature of 90 8C offers no advantages in terms of
performance. Special tests conducted with modified fuel injection pump revealed that coking of the
injector nozzles depends on the engine performance mode. The first and second injector nozzles that
operated on pure oil were more coated by carbonaceous deposits than control injector nozzles that
operated simultaneously on Diesel fuel.
q 2005 Elsevier Ltd. All rights reserved.
Keywords: Diesel engine; Rapeseed oil; Fuel consumption; Injector coking
* Corresponding author. Tel.:C370 37 752 285; fax: C370 37 752 311
E-mail addresses: gvidonas@info.lzuu.lt (G. Labeckas), sslavins@tech.lzuu.lt (S. Slavinskas).
0960-1481/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.renene.2005.05.009
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1. Introduction
In relation with a fast depletion of crude oil resources of the earth and increased
market prices of mineral Diesel fuel (DF), investigations have been intensified over the
last few years in order to examine the technical properties of Vegetable Oils Methyl
Esters (VOME) [1,2] and vegetable oils [3–6], including Rapeseed Oil (RO) [7–10].
The main purpose of the usage of renewable fuels comes from the urgent concern about
rapidly growing ambient air pollution, especially in the urban areas. Polluted air is one
of the reasons for climate changes that provoke frequent hurricanes, heavy rains and
floods having negative impact on the environment and people health. Using vegetablederived fuels for direct-injection Diesel engine fuelling reduces the carbon monoxide
(CO) and hydrocarbons (HC) emissions by approximately 10% and unburned carbons
(C) by up to 52% [11]. Therefore, vegetable-based renewable fuels are becoming more
popular in Germany, Austria, France, Italy, Czech Republic and other countries. The
increasing popularity of RME and RO rests on well established advantages:
(1) As a result of considerably higher oxygen content (10.8%) in the fatty acids, a
more complete combustion along with reduced emissions of harmful components
can be achieved [9].
(2) Renewable fuel is almost sulphur-free (0.04–0.002%), therefore, under heavy loads
and high cylinder gas temperature no sulphates are formed and the emissions of
particulate matters (PM) can be reduced by up to 24% [11].
(3) Using the RME or pure RO and their blends with Diesel fuel, contributes to the
closed cycle CO2 circulation diminishing the air pollution and a so-called ‘greenhouse’ effect [12].
(4) A higher RO flash point (220–300 8C) improves fire security during fuel
transportation and storage because this renewable fuel is neither flammable nor
explosive.
(5) Finally, the RO is harmless to the soil, and when the oil spills, it causes less harm
to the environment.
However, besides benefits, the usage of crude RO as fuel creates some technical
problems:
(1) Because of a lower net heating value of RO, one can expect a slight power loss and an
increase in the brake specific fuel consumption [13].
(2) Due to a poor atomisation of viscous RO at light loads and low cylinder gas
temperature, auto-ignition problems of low volatile oil may arise leading to the
unstable performance of Diesel engine [8].
(3) A more frequent change of fuel and sump oil filters is necessary and checking for fuel
injectors is recommended at least twice as often because of their coking and rapid
ageing [4].
(4) Its organic nature may damage rubber parts of the fuelling system such as oil-seals and
gaskets, therefore special elastomers should be used in replacing [1].
(5) A higher iodine number and corrosion activity of RO may affect the inner parts of an
engine, especially produced from brass, copper, tin and other coloured metals.
G. Labeckas, S. Slavinskas / Renewable Energy 31 (2006) 849–863
851
Already in 1900 the inventor Rudolf Diesel used peanut oil to fuel one of his engines at
the Paris exhibition. He predicted that in the future plant oil could gain the same
importance as fossil fuel. Rapeseed oil is renewable, safe to store and easy to handle, its
production is much cheaper than the RME. In order to reduce viscosity of vegetable oils,
three methods are currently used-transesterification [1,11], heating [3,6–8] and mixing
with Diesel fuel [4,5,9,13–15].
Preheating to the temperature of about 90 8C reduces rapeseed oil viscosity to the level
of Diesel fuel ensuring its smooth flow in the fuel lines. Tests conducted on a single
cylinder, four stroke, air-cooled direct-injection Diesel engine Yanmar 60A5-DTM
proved that heating of crude palm oil (CPO) offered no advantages in terms of
performance, but to dissolve solid phase of CPO and minimize its resistance to flow,
heating temperature of 60 8C is necessary [3].
In addition to technical problems regarding cold-flow properties and engine
reliability, a major obstacle towards widespread application of the RO based products
for engine fuelling is their high price. Consequently, the main task of de-centralised
production of the RO is minimisation of raw material transport expenses in order to
increase its competitiveness in the market. Therefore, it would be reasonable to study
possibilities to use pure RO and its mixtures with Diesel fuel for the local engine
fuelling. The production of vegetable oil is less depended on the fiscal policy and more
economically attractive especially when applied along with pressing of oilcake for
farming. Although the RO is not the superior option to other alternative fuels, it can be
regarded as a choice considering that inexpensive low energy cold-pressing (!50 8C),
filtering, sedimentation and decanting facilities could be established in some rural
areas.
Tests with a single cylinder, four stroke, direct-injection Diesel engine run on nine
different vegetable oil fuels at 1300 rpm disclosed lower brake power than that of
Diesel fuel, as well as greater smoke opacity and higher carbon monoxide and PM
emissions, but lower emissions of NO2 were found [5]. Again, the biggest decrease (up
to 20%) in engine power output was measured when running on raw sunflower oil, raw
soybean oil and distilled opium poppy-seed oil, whereas rapeseed oil ensured power
loss by 3% only.
Cold pressed rapeseed oil was brought from the RME production factory ‘Rapsoila’,
Lithuania that started in 2004 with the capacity of 10.000 ton of RME. The fuel properties
of the tested RO are compared with those of mineral fuel in Table 1. The elementary
composition of RO is as follows: 77.3% carbons, 11.9% hydrogen and 10.8% oxygen.
Actual proportion in mass between the carbons and hydrogen in this sort of oil is 6.5,
which may lead to more complete combustion and, consequently, to a bit higher engine
efficiency than in the case of Diesel fuel (6.9). However, the tested RO includes a lot of
complicated long-side chains of fatty acids and 2.7 times bigger amount of water that
significantly increases its density and viscosity, reduces cetane number, stimulates acidity
and corrosion activity. Cold pressed RO exhibit a higher pour point and cold filter
plugging point related to Diesel fuel that affects fuel flow through the fuelling system. In
addition, the total contamination of crude RO is 98.6 times higher related to Diesel fuel.
The structural differences in the RO composition, its high viscosity, surface tension and
overall contamination may affect spray properties causing poor atomisation and uneven
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Table 1
Properties of the diesel fuel and rapeseed oil
Property parameters
Diesel fuel (Grade C)
Rapeseed oil
Chemical formula
Density at 15 8C, g/cm3
Viscosity at 40 8C, mm2/s
Flash point, open cup 8C
Cold filter plugging point 8C
Pour point, 8C
Cetane number
Sulphur, mg/kg
Contamination, mg/kg
Iodine number, J2g/100 g
Acid value, mg KOH/g
Oxygen content, max%
Carbon to hydrogen ratio (C/H)
Net heating value, MJ/kg
Theoretically necessary air/fuel ratio, kg/kg
Ash content, mass-%
Water content, mg/kg
C13H24
0.842
2.94
68
K5
0
51.6
33
0.2
6
0.06
0.4
6.9
42.55
14.45
0.01
28
C57H105O6
0.916
38.0
220–300
C15
C20
44–48
2
25
111
2.0
10.8
6.5
36.87
12.63
0.01
75
distribution across the combustion chamber of small oil portions injected per cycle.
Its poor volatility and flammability along with higher flash point at temperature of about
220–300 8C may create auto-ignition problems, especially under light loading conditions.
This may lead to misfiring cycles and incomplete combustion at low gas temperature in the
cylinder, formation of lacquer and carbon deposits on the surfaces of inner parts of the
combustion chamber. On the other hand, the higher oxygen content in the RO composition
and negligible amount of sulphur suggest advantages related to the lower PM and gas
smoke along with the reduced CO and HC emissions.
The results of various studies conducted on single cylinder engines [3,5,10,15], are not
always comparable with the results obtained on the multi-cylinder powerful Diesel engine.
Furthermore, the most harmful NOx emissions depend much on the feedstock oil used for
Diesel fuelling and increase linearly by 29.3% with a change of iodine number from 7.88
to 129.5 [12].
The purpose of this research was to investigate the effect of cold pressed rapeseed oil as
a fossil fuel extender on a four cylinder, direct-injection Diesel engine performance, fuel
energy conversion efficiency and injector coking. The objectives of this study may be
stated as follows:
1. Determine the effect of the heating of rapeseed oil on its viscosity, oil pressure drop
across the fuel filter and oil flow capacity as well as study the brake specific fuel
consumption and fuel energy conversion efficiency when running on crude RO over a
wide range of loads and speeds.
2. Examine the influence of engine performance modes on injector coking when
operating under similar conditions on pure rapeseed oil and Diesel fuel.
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853
2. Experimental apparatus and methodology of the research
Experiments were conducted on completely commissioned four cylinder, four stroke,
naturally aspirated, water-cooled, 59 kW direct-injection Diesel engine D-243 with splash
volume VlZ4.75 dm3, bore of 110 mm, stroke of 125 mm and compression ratio 3Z16:1.
The fuel was delivered by an in line fuel injection pump through five holes injection units
into a toroidal type compression-ignition combustion chamber in a piston head. The fuel
injection pump was adjusted to the initial fuel delivery start at 258 before top dead centre
(BTDC). The needle valve lifting pressure for all injectors was set to 17.5G0.5 MPa.
To maintain the necessary oil flow the engine fuelling system was modified by means of
installing two joined in parallel fine porous fuel filters, non-return valve and an electrical
thermometer. The non-return valve was installed to reduce fuel pulsations resulting from
overruns of RO and improve measurements accuracy. Fuel was fed to the injection pump
with an electrical rotor transfer pump. The fuel returning from the injection pump line was
connected to the transfer pump, whereas the returning tube from the injection units was
inserted directly into the fuel meter vessel. The oil was heated in the heat exchanger
connected to the engine cooling system. Heating temperature of the oil was handled with
the water tap by changing the flow rate through the heat exchanger.
To obtain the baseline parameters, the engine was operated on Diesel fuel grade C first.
Load characteristics were taken at fixed loading modes and constant crankshaft
revolutions of 1400, 1600, 1800, 2000 and 2200 minK1. After load characteristics were
taken from the engine performance on Diesel fuel, similar experiments with crude
rapeseed oil were conducted over the same range of engine loads and revolution
frequencies.
Torque of the engine was measured with a three phase asynchronous 110 kW electrical
AC dynamometer with a definition rate of G1 Nm. The engine load characteristics were
taken with a gradual increase from the point that was close to zero up to its maximum
value of 290–310 Nm. This means that the effective power of the engine at the rated
2200 minK1 speed had been changed from the minimum up to 110% of its rated value.
The revolution frequency of the crankshaft was measured with the universal ferritedynamic stand tachometer TSFU-1 and its counter ITE-1 connected to the meter sensor
DTE-2. This aeronautical device determines the engine speed with an accuracy of G0.2%.
The fuel mass consumption was measured by weighting it on the electronic scale VLK500 and volumetric air consumption was determined by means of the rotor type gas
counter RG-400-1-1.5 installed at the air tank for reducing pressure pulsation.
In order to study the influence of rapeseed oil on carbon deposits formation on the
injector nozzles, the first and the second engine cylinders were fuelled with pure RO,
whereas the third and the fourth ones operated simultaneously on Diesel fuel. For this
purpose fuel delivery channels of the injection pump were divided into two volumes by
installing special plugs between them. Such modification of the fuel injection pump
enabled us to maintain similar testing conditions that were essentially important for the
study the comparative effects of both fuels on injector coking (Fig. 1).
In order to avoid the engine performance instability that could occur due to different
calorific values of both tested fuels, the volumetric delivery of rapeseed oil was increased
by approximately 7%. During these tests the engine was run alternately for some time
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Injectors
4
3
2
1
Diesel
fuel tank
DF injection
plungers
Plunger
pump
RO injection
plungers
Rapeseed
oil tank
Electrical
Filter Heat exchanger rotor pump
Filter
Fig. 1. Schematic layout of the Diesel engine fuelling system during simultaneous operation of the first and
second engine cylinders on pure rapeseed oil and the third and fourth cylinders on Diesel fuel.
at close to the idle speed, then at moderate torque and, finally, at about rated power output.
After each testing stage was completed, the injectors were dismantled from the engine,
their exterior view was thoroughly investigated and registered by digital photo camera.
Afterwards, injector performance was checked and possible changes in the sump oil levels
were evaluated.
Smoke opacity D (%) of the exhaust gases was measured with the Bosch device RTT
100/RTT 110 in I—100% scale with G0.1% accuracy. Gas temperature in the exhaust
manifold was measured with the chrome-aluminium thermocouple TChK-400U
connected to the galvanometer MKD-50M.
3. Test results of fuel delivery system
When operating on pure rapeseed oil, one of the main problems is linked with the
increased oil viscosity that may result in aggravation of fuel flow through the system and
thus lead to a drop in engine power. Because experiments were started with ordinary
fuelling system, the increased flow pressure of viscous RO crashed the filter container and
damaged the paper micro-fiber layer of radial vee-shaped filter element (Fig. 2b). As it
became clear later, such construction of filter container was liable to be fractured because
of a large number of round holes made on its body that apparently affected its strength
(Fig. 2a). Therefore such filters could not withstand increased oil pressures. In order to
eliminate this problem, the only reinforced fuel filters, as shown in Fig. 2c, were used for
further experimentations.
To solve viscosity problem, the effect of RO heating on oil pressure drop Dp across the
filter element and fuel flow capacity were investigated. The test results show (Fig. 3) that
at the very beginning the fuel pressure p1 built up by plunger fuel transfer pump increases
G. Labeckas, S. Slavinskas / Renewable Energy 31 (2006) 849–863
855
Fig. 2. The external view of a new fine porous filter element (a) that was damaged during experiments by pure
rapeseed oil (b) in comparison with reinforced filter (c) suitable withstand oil pressure.
a bit with a temperature reaching 0.14 MPa and remains about constant until the pressure
control valve is opened. The oil flow capacity v in the fuelling system increases smoothly
with a pressure drop Dp across the fuel filter. It should be noted, that the increase in
oil flow is not in-line with its temperature. At initial temperature of between 25–50 8C,
p1
∆p
p2
v
n = 1000 rpm
0.15
0.03
0.025
0.02
0.09
0.015
v l/s
p1, p2, ∆p MPa
0.12
0.06
0.01
0.03
0.005
0
0
0
10
20
30
40
50
60
70
80
Oil temperature ˚C
Fig. 3. The effect of RO heating on fuel pressures in the filter inlet (p1) and outlet (p2) lines, oil pressure drop (Dp)
across the filter and flow capacity (v) when operating with plunger fuel transfer pump.
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the increase of oil flow is slow enough whereas at the upper temperature range of between
50–65 8C more rapid oil pressure drop occurs and corresponding increase in flow capacity
follows. Further oil heating beyond the temperature of 65 8C does not lead to reasonable
pressure drop in the fuelling system and the expected oil flow capacity.
In order to obtain the necessary oil flow, the plunger fuel supply pump was replaced by
an electric rotor fuel transfer pump. The rotary pump driven by a DC electric motor
produces oil pressure p1 against fuel filter by approximately 0.03 MPa higher (Fig. 4) than
the plunger fuel injection pump does. The main priority of the electric transfer pump is that
it could be switched on in advance to circulate oil in the fuelling system and warm it up
before the engine is started. Another important feature of the rotary fuel supply pump is
that because of a higher capacity it ensures the necessary oil flow at about ambient
temperature without extra heating the oil.
The density of the tested rapeseed oil is by 8.8% higher than that of Diesel fuel,
therefore volumetric fuel metering used often by an in line injection pump results in a
slightly greater delivery in units of mass. As it was determined in tests [16], reasonably
higher viscosity of RO along with reduced internal leakages in the injection pump the
volumetric oil delivery per stroke increase by about 2.6%. The causes mentioned above
tend to compensate the lower net heating value of RO by 13.35%, therefore the actual
oil energy content delivered per cycle may only be a little bit lower than that of Diesel
fuel.
As a result, Diesel fuel replacement with crude oil may only lead to negligible losses in
power output. For example, in the case of 2.4 l indirect injection naturally aspirated
Toyota Dyna engine fitted with a modified ‘Biocar kit’ (Lohmann) and tested with
preheated to 30 8C camelina oil on a chassis dynamometer, the engine maximum power
was found to be 43.25 kW compared to 38.50 kW while running on mineral Diesel fuel
[6]. In spite of a lower calorific value of camelina oil a significant power increase (12.3%)
occurred, probably, because of higher oil viscosity that reduced internal leakages and
boosted up at adequate control lever positions the volumetric delivery per cycle.
p2
∆p
v
0.18
0.05
0.15
0.04
0.12
0.03
0.09
0.02
0.06
0.01
0.03
0
v l/ s
p 1, p 2, ∆p MPa
p1
0
10
20
30
40
50
60
70
80
0
Oil temperature ˚C
Fig. 4. The effect of RO heating on fuel pressures in the filter inlet (p1) and outlet (p2) lines, oil pressure drop (Dp)
across the filter and flow capacity (v) when operating with electrical rotary fuel transfer pump.
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857
4. The engine test results and analysis
Because of a considerable number of test results, it was decided to examine the data
obtained at the minimum 1400 minK1 revolutions, the maximum torque regime
1800 minK1 and rated 2200 minK1 speed mainly. Other results that reflect engine
performance characteristics at the intermediate crankshaft revolutions 2000 and
1600 minK1 can be taken into account as supplementary measures for better
interpretation.
Fig. 5 presents the Diesel engine performance map with contours of constant brake
specific fuel consumption (bsfc) as a function of brake mean effective pressure (bmep) and
crankshaft revolution frequency (n) when operating on Diesel fuel and pure rapeseed oil.
As it follows from the analysis of data, application for engine fuelling of crude RO does
not lead to significant changes in general view of the brake specific fuel consumption map.
Typical area of the minimum bsfc values when fuelling the engine with rapeseed oil
locates itself between crankshaft’s revolution frequencies 1600–2000 minK1 and loads of
bmepZ0.53–0.70 MPa, i.e. approximately remains at the same region as it has usually
been monitored during engine operation on conventional fuel. The only feature that clearly
distinguishes the engine performance maps for both fuels tested is the difference in the
brake specific fuel consumption. As it would be expected, the minimum bsfc value for the
engine fuelled with Diesel fuel amounts 225 g kWK1 hK1 whereas when operating on less
calorific oil, the bsfc increases up to 250 g kWK1 hK1 or by 11.1%. During engine
performance on RO at the maximum torque and rated power, the bsfc values in mass were
obtained higher by 12.2 and 12.8%, respectively.
0.9
Diesel fuel
0.8
Rapeseed oil
0.7
225
bmep MPa
0.6
bsfc = 250g/kWh
0.5
230
255
0.4
0.3
235
260
265
240
250
260
300
280
0.2
0.1
1400
340
300
1600
270
280
380
1800
2000
2200
n rpm
Fig. 5. The brake specific fuel consumption (bsfc in g kWK1 hK1) map as a function of engine load (bmep) and
crankshaft revolution frequency (n).
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The higher rapeseed oil consumption in grams per unit energy developed can be
attributed, primarily, to its net heating value lower by 13.35% (Table 1). As it follows from
the map in Fig. 5, the increase of oil specific consumption occurs about proportionally to
its lower calorific value. This statement is true over the entire engine performance range
including the maximum and moderate loads mainly. However, as the engine load goes
down below a certain limit of bmep %0.20–0.25 MPa, oil consumption starts to increase
with the declining load more rapidly reaching at critically light loads fuel consumption
values of 380 g kWK1 hK1 and higher.
Increased oil consumption at reduced speeds and light loads has been experienced,
probably, because of low gas temperature in the cylinder to burn completely small portions
of oil injected. In spite of negligible amounts of fuel delivered per cycle and overall fuellean mixture at close to the idle operation modes, poor atomisation and distribution across
combustion chamber of viscous RO may also contribute to aggravation of oil mixing with
ambient air, provoking misfiring cycles and incomplete combustion too. To take a close
look, when operating at critically diminished loads one can observe small unburned oil
droplets rushing out through the exhaust pipe.
Experiments conducted on Petter model ACI single cylinder, high speed, indirect
injection Diesel engine proved that at standard timing the engine operation on RO exhibits
longer auto-ignition delay and slower burning rate leading to late combustion during
expansion stroke, especially under low load operating conditions [15]. However, under
normal operating conditions and high gas temperature in the cylinder, the bsfc
characteristics with increasing load and crankshaft revolutions for both fuels remain
similar enough suggesting good opportunities for rapeseed oil as potential Diesel fuel
extender.
Fig. 6 further indicates that in spite of a higher bsfc in units of mass, the energy
conversion efficiency for rapeseed oil appears as being even slightly higher than that of
0.45
Brake thermal efficiency
0.4
0.35
1400 rpm DF
0.3
1800 rpm DF
2200 rpm DF
1400 rpm RO
0.25
1800 rpm RO
2200 rpm RO
0.2
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
bmep MPa
Fig. 6. The brake thermal efficiency as a function of engine load (bmep) at different revolution frequencies (n).
G. Labeckas, S. Slavinskas / Renewable Energy 31 (2006) 849–863
859
Diesel fuel. The brake thermal efficiency increases smoothly with the load reaching the
maximum efficiency value of 0.39 for RO and 0.38 for Diesel fuel. It means that fuel
energy conversion efficiency for both Diesel fuel and rapeseed oil is very similar. An
exception belongs to light loads of bmep %0.20 MPa only, where the brake thermal
efficiency at the engine speed lower than 1600 minK1 for oil is lower than that for Diesel
fuel.
Higher thermal efficiency at moderate and heavy loads has been obtained, probably,
because of oxygenated nature of rapeseed oil that leads to more complete combustion of
fuel-rich mixtures. On the other hand, a slightly lower proportion in mass between carbon
and hydrogen may also contribute to burn oil more efficiently at adequate loads. Similar
experiments on single cylinder, energy cell Diesel engine proved an increase in the brake
thermal efficiency when running on neat RO with standard timing compared with mineral
Diesel fuel [15]. A higher thermal efficiency has been obtained in spite of a lower net
heating value of RO, its higher density and viscosity that worsened fuel spray
characteristics aggravating evaporation and combustion. As an outcome, the brake
specific RO consumption based upon fuel energy content at moderate and heavy loads is
slightly lower comparing with Diesel fuel.
To examine the effect of oil heating temperature on engine performance at light modes
special experiments for easy loads of bmepZ0.15 and 0.20 MPa were performed. First the
engine operated on rapeseed oil with normal temperature of 35 8C. After then it was run on
the oil warmed to the temperature of 60 8C and, finally, on the oil heated up to the
maximum temperature of 90 8C. During these tests the crankshaft revolution frequency
within the speed range of 1400–2200 minK1 was increased by every 200 minK1.
Data analysis presented in Fig. 7 shows, that for both loads oil heating to the
temperature of 60 8C at low revolutions of 1400 minK1 results into reduction of the brake
specific energy consumption (bsec) by 11.7 and 7.4%, respectively, indicating that the fuel
Fig. 7. Dependencies of the brake specific energy consumption (bsec) in MJ kWK1 hK1 on rapeseed oil heating
temperature at light loads (bmep) and different speeds (n).
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energy savings occur in specific regions of the engine performance map, i.e. at the light
loads mainly.
As the engine speed increases beyond a certain level of 1600 minK1, oil heating does
not indicate reasonable advantages in terms of performance. Heating to the maximum
temperature of 90 8C suggests less benefits in fuel savings at light loads, resulting even in
contrary effects and tending to increase the brake specific energy consumption at moderate
and heavy loads. Poor engine performance on overheated RO results in smoke opacity
higher by 10–20% comparing with that emerging from an unheated oil. Therefore, oil
heating to the optimal temperature of about 60 8C could be considered as an effective
measure reducing its viscosity and boosting flow capacity, improving atomisation and
combustion of small RO portions injected per cycle.
Tests with rapeseed oil and the Bosch CR system disclosed that mean drop size and
spray penetration profiles are less affected by the oil temperature than by the injection
pressure. Rapeseed oil temperature of 60 8C actually does not improve spray penetration to
be sufficient to reach the penetration equivalent at 25 8C for standard oil [7]. Having tested
the atomisation of 15 various types of biodiesel fuels, researchers determined that most of
the oils had the Sauter Mean Diameter from 20 to 30% higher than Diesel and came to
conclusion that atomisation quality may not be the primary cause for the different results
reported in the literature [2].
Bearing in mind injector coking problems, it was decided to conduct special
experiments with an in line fuel injection pump that was modified to ensure simultaneous
performance of the first two engine cylinders on rapeseed oil and the other two cylinders
on Diesel fuel. Photos of the injector nozzles taken 1 h after the operation close to the idle
950 minK1 speed and an easy 18 Nm torque show that the control injector nozzles 3 and 4
were slightly covered by soot particles whereas the injector nozzles 1 and 2 that operated
on RO heated up to the temperature of 60 8C look a bit glazy and wet (Fig. 8a).
Pictures indicate that due to an easy load and low gas temperature in the cylinder,
combustion process of rapeseed oil does not proceed properly. This point of view has been
supported by critically low temperature of the exhaust gases that was as low as 175 8C.
Uneven distribution of small oil portions across a combustion chamber, slow evaporation
and poor auto-ignition may also contribute to incomplete combustion. Consequently, after
short-term operation on rapeseed oil close to the idle mode on surfaces of the first and
second nozzles, no carbon deposits were found at all. The sump oil level during this
experiment was monitored as increasing more rapidly because of poor oil combustion
quality.
To gain more information about the effect of performance modes on injector coking,
the Diesel engine was run for 2.5 h at the intermediate 1600 minK1 speed and moderate
69 Nm torque. At this performance regime the engine produces a brake power of 11.5 kW
that could be regarded as sufficient for a standard tractor to cope with many agricultural
tasks. The exhaust temperature during this test was measured as high as 210 8C. After
these tests had been completed, injectors were dismounted from the engine and their
appearance was investigated again. The control injector nozzles 3 and 4 were found as
normally covered with soot particles, whereas the surfaces of the injector nozzles 1 and 2
were heavily coated with carbonaceous deposits (Fig. 8b). Accumulation of carbon
deposits is linked, probably, with the oxidised fraction of the rapeseed oil that due to heavy
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861
Fig. 8. Carbon deposits formation on the injector nozzles after: a—1.0 h of operation at close to the idle; b—2.5 h
of operation at moderate load; c—2.0 h of operation at about rated power output. Numbering of injectors
corresponds to engine cylinders: 1 and 2 cylinders operated on pure rapeseed oil, 3 and 4 cylinders operated on
Diesel fuel.
molecular weight and high viscosity of crude rapeseed oil did not burn completely. In
contrast to Diesel fuel, rapeseed oil does not proceed through distillation process, therefore
its composition differ as having some complicated fractions that at high cylinder gas
temperature are fragmented into smaller hydrocarbons before fuel evaporation occurs.
Finally, the engine operated for 2.0 h close to the rated 2050 minK1 speed under the
torque of 245 Nm. At this performance regime the engine produces the brake power of 52.
6 kW that corresponds to 89.2% of its rated value. At high loads and increased portions of
the injected fuel, cylinder gas pressure and temperature become high enough to speed up
crude oil evaporation and combustion processes. Temperature of the exhaust gases
reached up to 550 8C, which can be considered as normal. The comparison of pictures
presented in Fig. 8b and c shows that after the operation close to the maximum power, the
injector nozzles 1 and 2 were found as considerably less coated with carbonates comparing
with their coatings at moderate loads, nevertheless they still remained a little bit more
affected than the control nozzles.
After every test stage, the visual inspections of injector surfaces as well as their checks
for possible functioning troubles were provided. Control studies of valve needle opening
pressure and visual evaluation of fuel sprays confirmed that all injectors were still within
the norm suitable for further performance. It would be worth noticing, that the nozzle
surface of the first injector became a bit more covered with carbons than the other one.
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G. Labeckas, S. Slavinskas / Renewable Energy 31 (2006) 849–863
This may be linked with cylinder pressure sensor that was installed into the first cylinder,
which increased slightly the volume of the combustion chamber and reduced an actual
compression ratio. This may affect the air-fuel mixture composition and result in delay of
auto-ignition, causing a slightly higher soot particle formation on this particular nozzle.
5. Conclusions
1. The minimum brake specific fuel consumption when fuelling a Diesel engine with
rapeseed oil increases up to 250 g kWK1 hK1 or by 11.1% in comparison with Diesel
fuel (225 g kWK1 hK1). The bsfc for RO at the maximum torque and rated power is
higher by 12.2 and 12.8%, respectively. The higher rapeseed oil consumption (248–
255 g kWK1 hK1) in comparison with Diesel fuel (221–226 g kWK1 hK1) can be
related to its lower net heating value because the maximum brake thermal efficiency
(0.37–0.38) for RO is a bit higher (0.38–0.39).
2. Heating of rapeseed oil to the temperature of 60 8C ensured a smooth flow through the
fuel filter and reduced the brake specific energy consumption at light loads by 11.7–7.
4%, again depending on the load. Oil heating to the temperature of 90 8C offers no
advantages in terms of performance even increasing the fuel energy consumption at
moderate and heavy loads and boosting visible smoke.
3. Tests with oil heated up to temperature of 60 8C revealed that after 1.0 h of operation
close to the idle, 2.5 h at moderate torque and, afterwards, 2.0 h at approximately rated
power, the injector nozzles were found wet and glazy, heavily coated with
carbonaceous deposits and a little bit less affected by unburned carbons, respectively.
The first and second injector nozzles were, however, more coated by carbon deposits
than the control injector nozzles that operated simultaneously on Diesel fuel.
Before practical usage of crude rapeseed oil for unmodified Diesel engines fuelling,
long-term endurance tests are necessary to learn more about potential problems mentioned
in the literature.
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