Abstract
Nowadays concerns about methanol have increased from the
viewpoints of environmental protection and versatility of fuels at a global
scale. Energetic research on methanol-fueled automobile engines has been
forwarded from the viewpoints of low environmental pollution and the use of
alternate fuel since the oil crisis, and they are now being tested on vehicles
in various countries in the world. Desire for saving of maintenance cost and
labour prevails as well as the environmental problems in the field of marine
engines. From these motives scientists have carried out research and
development of a methanol fueled marine diesel engine which is quite different
from automobile engines in the size, main particulars, working condition and
durability. Although scientists have made a great use of invaluable knowledge
from automotive technology, some special studies were necessary due to these
differences. Ignition method is a
typical one. Dual fuel injection system was tried for trouble-free ignition of
methanol fuel. This system is thought to
be the most favourable ignition method for marine diesel engines which have to
withstand quick load change and accept no misfiring.
INTRODUCTION
Energetic research on methanol-fueled automobile engines
has been forwarded from the viewpoints of low environmental pollution and the
use of alternate fuel since the oil crisis, and they are now being tested on
vehicles in various countries in the world. Various technical issues have
already been solved or the prospect is bright for them. It can be said that
this type of engine is very close to completion at present. On the other hand,
it is an actual situation in the marine engine field that the research on this
type of engine has hardly been tested so far, since it has seldom been
evaluated from the viewpoint of environmental pollution control because it is
used at sea and the idea to use methanol on marine engines is not established
yet.
However, IMO (International Maritime Organization) is
now investigating to include exhaust gas from ships in the objects to be
controlled from the viewpoint of environmental protection on a worldwide scale
that has been loudly emphasized recently. In case clean methanol is used as
fuel, work for handling complicated machines such as centrifuges for heavy fuel
oil and for treating sludge discharged from them can be avoided, and further it
can be expected to lessen frequent engine maintenance work. It has therefore
been strongly desired to use methanol on marine diesel engines from mainly the
viewpoint of pursuing economy. Though
knowledge which has been gained with automobile engines can be used in
principle, many subjects to be solved still remain, since marine diesel engines
have large bores and mean effective pressures of more than two times as much,
their operating conditions are extremely severe and they need high reliability
and durability in comparison with automobile engines.
Methanol has a cetane number of three and, consequently, extremely low
ignitability. Marine engines with spark ignition can not exhibit mean effective
pressures as high as those of ordinary diesel engines because of the high rate
of pressure rise during ignition and they can not permit misfiring because of
the large volume of their exhaust systems. The dual fuel injection system which
has actual service results on large-sized gas engines has therefore been
selected as the ignition system for this research. Since methanol is not only
corrosive but also insufficient in lubricating ability, elemental research has
been needed to solve these issues
EXPERIMENTAL ENGINE
A single cylinder, four-stroke, direct-injection type diesel engine having
a cylinder bore of 250mm has been modified so as to be suitable for this
experiment. The rated speed of this experimental engine has been set lower than
that of the original type so that the results of this research can be utilized
as widely as possible. Table 1 and Fig.1 show the principal particulars of the
experimental engine and the schematic drawing
The combustion system of the experimental engine is of a dual fuel
injection type such that the main fuel injection valve (methanol) is located at
the centre of the combustion chamber and atomized fuel from this valve is
ignited by the pilot oil injection from the secondary injection valve (oil)
located on the cylinder head near the periphery of the combustion space. This
system has been adopted from the reasons that it has the high stability of
ignition, good low load performance and high reliability, and that it serves as
a measure to prevent corrosion, since combustion deposits made by pilot oil
injection cover the inside surface of the combustion chamber. The methanol
injection pump is of a forced lubrication type to prevent lubrication troubles.
Since methanol is highly volatile, the auxiliary equipment of the methanol
system such as the fuel tank, strainer, supply pump and valves have been
installed in an enclosed chamber (a fuel supply unit). A fan
and a gas detector have been installed to sufficiently ventilate the inside of
the unit for safety. Pipe joints are also of special structure to prevent fuel
leakage.
Though the dual fuel injection system involves such a demerit that its fuel
system becomes complicated, auxiliary machinery such as generating engines and
a boiler burn fuel oil on board in case of a ship, and large gain can not be
expected even though only the main engine adopts a system of burning only
methanol unless these auxiliary machines also burn only methanol.
The dual fuel system is therefore considered proper. Fig.3 shows the
schematic drawing of the fuel regulating linkage of the experimental engine. In
this drawing, l is the methanol injection pump, 2 is the pilot oil injection
pump, 5 is the governor and 31 is the actuator necessary for controlling the
ratio of the quantity of methanol and pilot oil to be injected. To grasp the
condition of deposits in the combustion chamber, methanol with purity of 99.9%
and JIS No.2 gas oil for pilot injection has been used.
OPERATION TEST UNDER NORMAL
CONDITION
Under the full load condition of the above mentioned experimental engine
(mean effective pressure Pme: 16.13kgf/cm), influence on engine
performance, the contamination condition of engine inside and lubricating oil,
and the properties of exhaust gas have been investigated by changing the
specifications of the pilot oil injection nozzle, main fuel injection nozzle
and main fuel injection pump, fuel injection timing and the quantity of pilot
oil.
Influence of Pilot Oil Injection Nozzle
The effect of the pilot oil injection nozzle has been confirmed by changing
the number and diameter of nozzle holes and the direction of injection in the
range shown in Fig.4. As a result, the one-hole nozzle is the best in terms of
fuel consumption, the stability of cylinder pressure and the reduction in the
quantity of pilot oil.
However, the difference in performance among various types of nozzles is
not remarkable. As mentioned later, when priority is given to the issues of
startability, accelerating ability and sudden load change like the engagement
of a clutch, or to the problem when the pilot oil injection nozzle holes have
been closed, it can be said that the three-hole nozzle is the best and the largest possible nozzle hole diameter is desirable. Though the influence of
the direction of the pilot oil injection nozzle in relation to the main fuel
injection nozzle has also been confirmed, no improvement has been found. It is
conjectured that the reasons for the above are that swirls in the combustion
chamber of the experimental engine are not strong and the quantity of pilot oil
is enough.
The two-hole nozzle shows slightly better fuel consumption. However, it is
not preferable from the viewpoint of ignition stability, since the variation of
maximum cylinder pressure (P max) is large.
It has turned out that, when the total
area is made too small, it becomes difficult to start the engine, and that it
is also difficult to continue the operation of the engine on methanol/oil even
if it could be started and the engine finally stops, since the quantity of
pilot oil necessary for causing perfect ignition can not be supplied. However,
when keeping the quantity of pilot oil constant, smaller total nozzle hole area
gives the better stability of pilot injection.
Influence of Methanol Injection Nozzle
Engine performance has been confirmed using methanol injection nozzles of
which the numbers of nozzle holes are 8, 9, 10 and 12, and nozzle hole
diameters have been selected in the range from 0.39mm to 0.48mm (90% to 200% of
the nozzle area of the injection nozzle for burning only oil).As a result, it
has turned out that, in case of the experimental engine, the injection nozzle
which has ten nozzle holes of 0.46mm in diameter, i.e.150% of the nozzle area
of the injection nozzle for burning only oil, shows the best fuel consumption.
Fig.7 shows engine performance against the number of injection nozzle holes
using intake air pressure as a parameter when using injection nozzles of which
areas have been kept constant (130% of the nozzle area of the injection nozzle
for burning only oil) and the number of nozzle holes has been 8, 10 and 12.
Though the 8-hole nozzle shows the specific fuel consumption on almost the
same level as that for the 10-hole nozzle, the former shows better performance,
since both Pmax and exhaust temperature are lower. However, it is considered in
this case that thermal loads on the combustion chamber components become high
due to the longer fuel spray travel by about 7% than that for gas oil .
It shows that temperatures for the
8-hole nozzle are higher by nearly 40lC than those for other nozzles and the
above mentioned conjecture is correct. When considering the ignition
characteristic of methanol burning from the periphery of a spray, issues remain
from the viewpoint of reliability including sliding conditions, since the
quantity of atomized fuel reaching the surface of the cylinder liner is
estimated to be more. The 12-hole nozzle shows slightly worse fuel consumption
probably due to the interference of sprays. The spray angle in this case
becomes 17 or 18 degrees and sprays do not directly touch each other. However,
when taking account of the behaviour of sprays after impinging on the surface
of the liner and the entrainment of air into sprays, it is thought that the
limit of the number of nozzle holes is 12 or so.
No large
change of characteristics has been found even though the diameters of nozzle
holes have been changed except injection pressure. In order to obtain sprays
similar to those of gas oil, it is necessary to use a methanol injection nozzle
with the number of holes of l.5 to 2 times and a hole diameter of 1.1 to 1.2 times of those of a gas oi1
injection nozzle, taking account of the spray characteristic of methanol having
a shorter fuel travel and a difference in calorific value between gas oil and
methanol. However, the number of holes is limited to l2 or so in terms of the
machining of injection nozzles in practice and injection duration for methanol
becomes relatively longer than that for gas oil. It is likely that this
characteristic is cancelled out by the high combustion speed of methanol and
does not badly influence the heat release period of a running engine so much.
Influence of Plunger Diameter of Methanol Injection Pump
The test has been carried out with injection timing
being set at 23 degrees before TDC (statically) for pumps having plunger
diameters from 22mm to 27mm and at 20 degrees before TDC (statically)for the
pump having plunger diameter of 28mm, since maximum cylinder pressure has been
predicted to exceed an allowable limit in this case. As seen in this figure,
the injection duration and the specific fuel consumption are almost constant in
the range of plunger diameter from 26mm to 28mm. Since Pmax has an allowable
limit and injection timing must be changed when the rate of injection is
increased, the improvement in fuel consumption is small even
though the plunger diameter of the methanol injection pump is made too
large. It can therefore be said that the limit to the plunger diameter is about
l.3 times of that for only oil burning.
Influence of Injection
Timing
As seen in this figure, the engine performance
becomes better in case where pilot oil is injected earlier by two degrees than
methanol. Though the test where pilot oil is injected later than methanol has
also been carried out, combustion has not stabilized and continuous running has
been difficult. Another test has also been carried out, where the relative
difference in injection timing between methanol and pilot oil has been fixed
and the timing for both fuels has been advanced in parallel. However it has
turned out that the improvement in fuel consumption is small.
Influence of the Quantity
of pilot Oil
It can be seen from this figure that the lowest points of specific fuel
consumption differ with the specifications of pilot oil injection valves. That
is, the percentage of pilot oil in total consumed fuel for the lowest point of
specific fuel consumption is between 11 and 12% for the one-hole nozzle and
that is near 15% for the three-hole nozzle. Thus, the lowest point shifts
toward the larger percentage of pilot oil. Though the quantity of pilot oil can
be decreased down to about 4% by making the pilot oil injection nozzle area
smaller, proper quantity is considered to be 12-15% in practice, since the
startability of an engine must be considered as mentioned later. Smoke density
and NOx have also been measured during these tests. Though detailed results
will be explained later, the results can be summarized as follows. Compared
with a diesel engine being operated on gas oil, the smoke density is lower by
one order by Bosch scale and NOx is about half under the same load condition.
Thus, exhaust gas characteristics have been confirmed to be superior.
Furthermore, overhaul inspection and the results of lubricating oil analysis
after tests have shown less contamination of the engine inside. As mentioned
above, it has been confirmed that the possibility of lowering environmental
pollution and decreasing maintenance work for diesel engines is large.
STARTING TEST
Test Method
The stable combustion of dual fuel engines under normal operation can be
ensured by pilot oil of several percent of total fuel which is injected under
full load condition. However, a considerably large quantity of fuel is needed
when starting engines, since accelerating torque is necessary in addition to
normal running torque. For this reason, starting tests have been carried out
under the following conditions.
a) Constant quantity of
methanol (full load) and varying quantity of pilot oil
b) Constant quantity of pilot
oil and varying quantity of methanol
c) Operation on only pilot oil
d) Starting on pilot oil and
injection of methanol after that
e) Constant quantity of
methanol (50%) and varying quantity of pilot oil
For all conditions except e), cold
conditions of intake air temperature ts=191C , cooling water
temperature twt=191C , lubricating oil temperature to=01C
and liner temperature tL=201C have been adopted. For a part of e)
condition, warm conditions of tst=301C
, tw=58 C , to=50 t and tL=391C have been
adopted.
Test Results
Mark shows that no ignition has been detected. Figure show that, though
ignition has been detected, it has not been continued and torque has not been
generated. Figure show that ignition has been detected and continued stably and
engine speed has risen up to its set speed. Suffixes show test numbers.
As can be seen from this figure,
under the cold condition , ignition has not been detected at all like Test Nos.
1-5 in case where a large quantity of methanol has been injected together with
oil. On the other hand, under the warm condition like engines just after operation, starting
has been possible like Test Nos. 21, 22 and 13. However, there has been an
example such as Test No.12 where operation could not be continued due to pilot
oil less by few percent than that of Test No.13. When pilot oil is plenty,
starting even under the cold condition is possible like Test No.25 even though
a considerably large quantity of methanol is injected. Test Nos.8, 9 and l0
have been carried out in such a way that the engine has been started on only
pilot oil and methanol has been injected after detecting ignition. These are
examples where the engine has misfired and not generated effective torque and
operation could not be continued because of much methanol and less pilot oil.
It has turned out that, since pilot flames are blown out by the injection of
methanol, energy necessary for starting can not be made up by methanol and a
necessary quantity of pilot oil must be injected under the cold condition.
Test No.3 shows the case where methanol of the quantity corresponding to
the limit of the injection pump rack has been injected under the cold
condition. Engine speed rises up to only that by starting air. Test No.7 shows
that accelerating torque is not generated though slight ignition is detected,
because the quantity of methanol has been decreased to 30%.
Test No.11 shows the case where only pilot oil is injected. Though the rate
of speed increase is small, engine speed rises up to the set speed. Test No.13
shows the case where methanol of the quantity of 50% has been injected under
the warm condition. It can be seen that engine speed quickly rises by the
combustion of methanol.
Results of starting tests using accelerating time and
mean effective pressure (Pmi) obtained from indicator diagrams as coordinates.
O and X marks show cases where starting has succeeded and failed respectively.
The solid line shows the relationship between minimum mean effective pressure
necessary for accelerating engine speed which has been calculated from mean
accelerating torque and accelerating time.It can be seen from this figure that, apart from the length of accelerating
time related to inertial mass, Pmi of at least 4 or 5 kgf/cm2 must be generated
for starting engines.
QUICK LOAD THROW-IN TEST
Test Method
Tests simulating the condition of engaging clutches which are often
installed on medium to high speed engines have been carried out by quickly
throwing-in 1oads on the dynamometer (eddy current type) according to the
procedures shown in Fig.18.
Rotating mass is added between the engine shown in Fig.l and the
dynamometer to be able to simulate a shafting of a marine engine. The engine
has been imposed with a load during four or five seconds after changing over
from oil operation to methanol/oil operation under no load condition, and
engine speed and pressure in the cylinder have been recorded. Supposing the
loaded condition of an engine after engaging a clutch, 1oads (40-150 kgf )
corresponding to 20-70% of the load at full engine output and also the quantity
of fuel to be injected corresponding to these loads have been selected.
For intake air pressure, two cases of naturally aspirated and supercharged
(0.35 kgf/cm) conditions have been selected. Since intake air of this
experimental engine is supplied by an independent motor driven blower, the
transient characteristics of a turbocharged engine can not be simulated
exactly. However, it is considered that engine characteristics can
qualitatively be grasped by this test. The governor of this engine is Woodward
UG8 type with a torque limiter.
Test Results
Test results of the quick load throw-in test. Measured
points are p1otted by selecting Pmi, which has been converted from a
dynamometer load, for the abscissa and the percentage of methanol injected,
which has been calculated from the rack position of the injection pump, for the
ordinate. This figure show cases where engine speed has returned to its set
values after quickly imposing loads. Also show cases where the engine has
stalled and could not carry loads. Figure show cases where the engine had not
sta1led but the engine speed has not returned to its set values. Suffixes show
test numbers.
The magnitude of load which can be thrown-in is effective only in the
hatched range under naturally aspirated condition and the engine output is
limited to Pmi
= 9-1Okgf/cm2. Since this limit can not be raised even under the warm
condition, it is not influenced by the phenomenon of blowing out pilot flames
by methanol as detailed under the section of starting test, and it is thought
that output can not be increased due to the shortage of intake air even though
the quantity of methanol is increased. It can be seen from examples that the
limit of output can considerably be increased by d small degree of
supercharging and Pmi of 12.5kgf/cm2 can be developed. It means that
the magnitude of load which can be thrown-in, i.e. the speed of engaging the
c1utch (the rising speed of oil pressure for operating the clutch), depends on
the accelerating ability of the turbocharger and it can be said that the clutch
must be operated linking with intake air pressure.
Though Pmi = 1Okgf/cm2 can be obtained just after load
acceptance in every case under naturally aspirated condition, the balance
between generated engine torque and load can not be maintained due to the
shortage of air (small air/fuel ratio) and the cooling effect by the latent
heat of vaporization of methanol when the quantity of injected methanol is
much. Both Test Nos.19 and 25 have this tendency, and engine speed lowers
halfway and can not recover.
Under supercharged condition, the
engine generates Pmi = 15kgf/cm2 and engine speed quickly returns to
the set value, since a considerably large quantity of air, i.e. specific air
consumption 4kgf/PSh, is supplied to the engine. Test Nos.41 and 42 shows the
cases where the engine can not develop enough output because of too little
quantity of injected methanol against the thrown-in engine load. As mentioned
before, the experimental engine does not represent the dynamic characteristics
of actual turbocharged engines, since the experimental engine is not equipped
with an exhaust turbocharger. However, it is expected that the abovementioned
load throw-in test can offer matters to be considered when methanol is applied
to diesel engines.
CONCLUSION
Tests have been carried out under static and dynamic conditions in order to
grasp engine performance when methanol is applied to marine diesel engines. As
a result, it has turned out that the performance of a methano1/oil burning
engine can be improved near to the performance level of an oil burning engine
by
1. Optimizing the fuel injection
system and the combustion chamber geometry
2. Adapting the fuel regulating
system and the intake air system of the former.
its very good work go head
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