Internal combustion engine of the diesel type for combustion of gas, and a method of supplying such an engine with fuel

An internal combustion engine of the Diesel type has an injection system with injectors which at high pressure injects compressed liquid gas into the combustion chambers of the cylinders. The liquid gas may be produced from volatile organic compounds evaporated from crude oil tanks. The engine can be a high-compression engine with a compression ration of at least 1:14, and the liquid gas can have a methane number of less than 10.

BACKGROUND OF THE INVENTION 
The present invention relates to an internal combustion engine of the 
Diesel type for combustion of gas which is compressed to a high pressure 
suitable for supply to the engine, which engine has an injection system 
with injectors that inject liquid fuel into the combustion chambers of the 
cylinders at a high pressure. 
Dual-fuel two-stroke crosshead engines of this type are known in which 
liquid fuel is injected in the form of fuel oil, typically acting as an 
ignition aid for the injected gas. The gas in the known engines with 
high-pressure injection is natural gas which is gaseous at its injection 
into the combustion chamber. An engine of this type is described, for 
example, in the Applicant's brochure "Large Diesel Engines Using High 
Pressure Gas Injection Technology" from 1991 and in the technical article 
"Development of the World's First Large-Bore Gas-Injection Engine" by T. 
Fukuda, P. Sunn Pedersen et al, paper D51, CIMAC 1995 at Interlaken, CH. 
In these engines, the natural gas is supplied via a pipe system supplying 
a well-defined gas quality, normally methane gas. The high-pressure 
injection of the gaseous natural gas provides the advantage that the 
engine can use different compositions of the natural gas. Thus the gas may 
be of pure methane or, for example, of methane and ethane, if they were 
fractionated together. 
Engines of the Diesel type with supply of gas-based fuel are further known 
in a number of different designs, all with the common feature that the gas 
is injected or supplied at a low pressure of, for example, about 1-5 bar 
into the intake air of the engine and thus replaces part of the fuel oil, 
which may have environmental advantages in the form of lower particulate 
emissions in the exhaust gas. For examples of such engines, reference can 
be made to EP-A 0049721, which mentions supply of LPG (propane/butane) to 
the intake air, EP-A 0102119, which mentions supply of LPG or methane, and 
EP-A 0133777, in which compressed natural gas or LPG is added to the 
intake air. In the cases where the gas is supplied to the engine as a 
liquid fuel, both evaporation and mixing of the gas with the intake air 
take place before introduction into the cylinder, while in cases of supply 
of gaseous gas only mixing takes place. 
The proportion of the gas out of the aggregate volume of fuel must not 
become too large when the gas is mixed with the intake air in an engine of 
the Diesel type because otherwise auto-ignition of the gas may occur 
during the compression stroke. It has been described as important in the 
known art that gas ignition can only take place in a controlled manner by 
means of fuel oil injection. The injection of oil can be controlled in the 
usual manner with a suitably accurate timing for achieving the desired 
operating characteristics of the engine. 
In the known engines, which, as mentioned above, may use high-pressure 
injection of gaseous gas directly into the combustion chamber or may use 
supply of gaseous or liquid gas to the intake air of the engine, it is a 
condition for gas operation that the gas is refined or in another manner 
has obtained a predetermined and stabilised composition with a predictable 
behaviour as fuel in the Diesel engine so that the actual engine can be 
adapted to the specific fuel in its structural design. If one of the known 
engines designed for the supply of gas at a specific ignitability is 
suddenly supplied with gas which is considerably more ignitable, 
auto-ignition may occur during the compression stroke with consequent 
heavy operational disturbances for the engine. 
The partial gas operation of the known engines may result in a considerable 
environmental advantage in as much as the engine combusts less oil which 
at combustion forms environmentally harmful compounds, which do not occur 
to the same extent at combustion of gas. 
BRIEF SUMMARY OF THE INVENTION 
A purpose of the present invention is to provide an engine of the Diesel 
type which at fuel combustion reduces emission of environmentally harmful 
compounds substantially more than what can be achieved by reducing the 
compounds formed at the combustion through combustion of gas instead of 
oil. 
It is another purpose of the present invention to provide an engine of the 
Diesel type which can be fuelled with any combination of gas fuel whether 
liquid, gaseous or having varying properties. 
It is yet another purpose of the present invention to provide an engine 
which can reduce the amounts of pollution released to the atmosphere. 
It is yet another purpose of the present invention to provide a reliable IC 
engine with a high degree of safety. 
In view of this, the internal combustion engine according to the invention 
has an injection system which at least includes liquid injectors for 
high-pressure injection of compressed liquid gas produced from Volatile 
Organic Compounds evaporated from crude oil tanks. 
For some years it has been recognised that the evaporation of Volatile 
Organic Compounds (VOC) from crude oil, among others, constitutes a 
serious environmental impact, but despite various attempts to overcome 
this, and inter-governmental agreements on a reduction thereof, the VOC 
emission is steadily increasing. The volatile organic compounds 
evaporating from crude oil have no well-defined composition, but rather 
vary during a period for oil recovered from a specified oil field and also 
vary between oils recovered from different fields. 
By using the volatile organic compounds as high-pressure injected fuel in 
an internal combustion engine of the Diesel type VOC emission to the 
atmosphere is avoided, which results in a considerable environmental 
benefit, at the same time achieving the effect known per se of the exhaust 
gasses being purer when gas is combusted instead of oil. An economic 
advantage is also gained in that purchased refined fuel is at least 
partially replaced by gas compounds which were formerly thrown away, and 
which in late years have required payment to get rid of. The use of VOC as 
fuel in an engine of the Diesel type does, however, mean that the 
combustion properties of the fuel can vary much within a very short 
interval of time. 
When the crude oil at loading flows over into a tank, the oil splashes down 
into the tank and is exposed to vigorous movements and circulation, which 
results in release of relatively large amounts of VOC in the form of 
evaporated alkanes of a very mixed composition depending on the type of 
the crude oil. These alkanes typically contain relatively large amounts of 
each of the compounds methane, ethane, propane and butane in both normal 
and branched compounds as well as some amounts of higher alkanes C.sub.5 
and C.sub.6+. At the subsequent storage in the tank, VOC evaporate with 
not quite such a spread in the alkanes, as this evaporation is mainly 
controlled by the partial pressures of the components of the crude oil in 
the tank space above the oil. The liquid phase of each component will thus 
seek towards a balance with the associated vapour phase, but at the same 
time the vapours in the tank space also tend to achieve higher 
concentrations of the heavier components near the surface of the crude 
oil, which slows down the evaporation of the higher alkanes. If the crude 
oil tank is located on a ship, the motions of the ship during bad weather 
voyages may give rise to such splashing of the crude oil that the gases in 
the tank space become more evenly distributed, which results in a higher 
evaporation of the heavy components than when the ship sails in more calm 
conditions. 
Thus both over some days slow variations may occur in the alkane 
composition of the VOC and over some minutes or hours rapid variations 
that radically change the ignition properties, etc., of the fuel. These 
variations render impossible the use of the fuel as a premix addition to 
the intake air of the internal combustion engine. By using high-pressure 
injection of the fuel, premature ignition is avoided, and therefore the 
rapidly varying fuel properties can only affect the speed at which the 
fuel is combusted. 
It is also a substantial advantage that the fuel is liquid at injection 
into the combustion chamber. Firstly the liquid gas can be compressed to a 
pressure suitably high for the injection, for example ranging from 200 to 
1000 bar, at a substantially lower energy consumption than at compression 
of gaseous gas. Secondly the liquid gas renders it possible during a short 
period to inject a gas volume of a large energy content, and the entire 
injection sequence with variations, if any, in the rate of injection can 
be controlled by the means known from oil injection. Thirdly, the main 
part of the aggregate energy content of the VOC evaporated from the crude 
oil can be liquefied by advantageously simple and energy-economical means, 
such as by compression to a higher pressure than the condensation limit of 
the desired alkanes and/or by cooling. Before injection, the condensate 
then only has to be compressed to the pressure of injection. 
The methane and ethane components of the VOC cannot be liquefied in any 
suitable manner. It is possible to store the methane and ethane gasses 
temporarily by re-introducing these gases into the crude oil, but this 
provokes increased evaporation of VOC at a later time, so this is merely a 
process postponing the problem. The methane and ethane gases can also be 
vented to the atmosphere such as has been effected previously with all 
VOC. In all circumstances, combustion of the liquid C.sub.3+ alkanes will 
involve a considerable gain compared with former times. 
In one embodiment the injection system of the internal combustion engine 
includes secondary injectors for high-pressure injection of gaseous 
mixtures which at least partially contain gas evaporated from crude oil 
tanks as well as inert gas, if any, which has been filled into the crude 
oil tanks as a detonation-preventing gas. The secondary injectors can 
inject the methane and ethane gases, etc., not liquefied by processing of 
the evaporated VOC. When emptying crude oil tanks it is normal to add 
inert gas to the tank to avoid gas explosions in the tank. This inert gas 
is an oxygen-poor mixture of gases, such as nitrogen or carbon-dioxide and 
up to about 7 per cent of oxygen. When crude oil is loaded into the tank, 
the oil will displace the inert gas in step with the loading, but at the 
same time the released VOC gases will be mixed with the inert gas. The 
gaseous mixtures fed to the secondary injectors will therefore during and 
immediately following a tank loading contain large amounts of inert gas 
which cannot be combusted in the engine. When only the proportion of 
combustible gases is sufficiently high to have an energy content more than 
double the compression work required for turning the gaseous mixture into 
a form suitable for the injection, it can pay to inject the gaseous 
mixture into the combustion chamber of the engine. Environmentally it will 
be an advantage to inject the gaseous mixtures into the combustion chamber 
of the engine, even though the energy content of the combustible gases 
does not cover the compression work. 
Preferably, the injection system includes pilot injectors for injection of 
ignitable pilot fuel which initiates a combustion process on injection. 
The pilot fuel may be oil or another very easily ignitable fuel. If the 
compressed liquid gas is of a quality making ignition aid unnecessary, 
pilot injectors may be omitted in the cylinders having liquid injectors. 
Nevertheless, it may be advantageous to arrange at least one pilot 
injector at each cylinder. If the production of VOC is insufficient to 
cover the aggregate fuel requirement of the engine over a long period, the 
engine may at intervals be operated only on oil injected via the pilot 
injectors. 
In one embodiment a number of the liquid injectors and of the secondary 
injectors are combined in a corresponding number of dual-fuel injectors 
capable of injecting both the liquid gas and the gaseous gas-containing 
mixtures. The dual-fuel injector requires less space in the cylinder cover 
than a liquid injector and a secondary injector and is therefore easier to 
position, especially if the same cylinder is already provided with 
injectors for injection of oil. 
The reliability of the gas injection can be improved by the injection 
system at intervals actuating the secondary injector also when no gaseous 
fuel is to be injected into the associated cylinder. The actuation may, 
for example, take place at least once every ten minutes, and at actuation 
the nozzle holes are blown clean of any deposits. If no gas-containing 
mixture is available at the actuation, compressed air or any gas 
available, such as inert gas, may be used instead. The interval between 
each clean-blowing actuation need not be ten minutes, but may range 
between once per engine cycle and once per day. The interval is selected 
in consideration of the fuel combusted when no gas is injected. If the 
fuel gives rise to a heavy formation of particles and soot, a short 
interval is chosen. 
If the engine is provided over a long period with gaseous and liquid gas in 
certain ratios, it is possible to obtain a simplification of the injection 
system in that only some of the engine cylinders are provided with 
secondary injectors, while others of the cylinders are provided with 
liquid injectors, all the cylinders also optionally having pilot and/or 
fuel oil injectors. The simplification lies in the fact that three 
different kinds of fuels are not to be supplied to all the cylinders of 
the engine. If, for example, the VOC composition is so that only 10-15 per 
cent of the calorific value of the VOC derives from methane and ethane, 
one or two of the cylinders of the engine can burn off all the gaseous 
gas, so there is no need for distribution and injection systems for 
gaseous fuel to the other cylinders. 
Preferably, the engine is the main engine of a ship with crude oil tanks, 
such as a shuttle tanker or a crude oil carrier, and volatile organic 
compounds of ignition properties, calorific values and/or evaporated 
amounts varying over time evaporated from these tanks constitute a 
substantial proportion of the fuel consumption of the main engine. A very 
large amount of the VOC today vented to the atmosphere are released on 
loading of crude oil at offshore oil recovery sites or at coastal oil 
terminals and during the subsequent voyage to refinery or other unloading 
location. By using the VOC as fuel in the main engine of the ship, the 
volatile compounds are removed suitably rapidly after their release from 
the crude oil. 
The engine may have an electronic control unit which, on the basis of 
monitoring of the current cylinder pressures, controls at least the 
injection pressure for the gaseous fuel gas. With a continuous monitoring 
of the pressure sequence of a cylinder, the combustion in the cylinder can 
be analyzed by the electronic control unit, the energy development at 
combustion and the speed of combustion can be determined, and on this 
basis the control unit can define fuel parameters for use in subsequent 
injection sequences. When the engine is supplied with gaseous gas mixtures 
collected from crude oil tanks, the gas may contain varying amounts of 
inert gas. The incombustible inert gas affects the combustion of the 
combustible VOC so that the combustion speed is higher when the inert gas 
content is higher. To achieve a more homogeneous combustion the control 
unit preferably adjusts the injection pressure in a downward direction 
when the inert gas content of the gas is high. This also provides the 
advantage that the compression work for high-pressure compression of the 
gas is reduced. 
The invention further relates to a method of fuel supply of an internal 
combustion engine of the above type, which method is characterized in that 
volatile organic compounds evaporated from crude oil tanks after optional 
temporary storage and compression are supplied to the fuel system of the 
engine and used as fuel in the engine regardless of the fact that these 
organic compounds have ignition properties, calorific values and/or 
evaporated amounts varying over time. The method achieves the above 
advantages that the environment is spared venting of at least part of the 
VOC and at the same time the engine uses a more pure fuel than oil, and 
the shipowner achieves an economic benefit from using a waste product as 
fuel instead of purchased bunker oil. 
In an environmentally optimal further development of the method, the 
evaporated and compressed compounds comprise both a gaseous and a liquid 
phase being substantially separated from each other at the supply to the 
engine. By using both the liquid and the gaseous phase as fuel, largely 
all the evaporated amount of VOC can be burnt. For the engine it is 
essential that the two phases are kept mutually separated at the supply to 
the engine, because unsuitably large variations in the calorific value of 
the fuel supplied for a combustion would occur if during injection of, for 
example, a gas phase, a drop of liquid phase suddenly came through the 
same injector. 
With a view to avoiding separation of liquid phase in the gas phase the 
temperature of the gaseous phase in the fuel and injection system of the 
engine is preferably kept higher than the temperature of the gaseous phase 
after compression to the pressure at which it was supplied to the fuel 
system of the engine. In an advantageous embodiment the temperature of the 
gaseous phase in the fuel system is controlled so as to rise towards the 
engine so that any risk of condensation is eliminated. As an alternative 
to this, at the inlet for gaseous phase to the fuel system of the engine 
there may be a freeze trap for separation of condensate from the gas 
phase. 
Preferably said liquid and said gaseous phases are both supplied to all the 
cylinders of the engine, the injectors for the gas phase thus being kept 
operable on all cylinders, just as the cylinders can be uniformly 
controlled. 
When the engine is the main engine of a ship with crude oil tanks from 
which evaporation of volatile organic components occurs, the engine is 
preferably only supplied with fuel oil to the extent required as ignition 
aid or required because the current fuel requirement of the engine exceeds 
the supply of fuel gas to the engine. This provides the optimum saving on 
bunkered fuel oil. Control of supply of fuel gas to the engine need not be 
dictated by the fuel requirement of the engine or by the current VOC 
production, but may very well be subject to an overall control aiming, as 
far as possible, to combust the environmentally friendly fuel gas in the 
coastal areas where it is desired to avoid environmentally harmful 
emission products. The supply of the gaseous and the liquid fuel gas may 
also be controlled individually, for example so that the gaseous fuel gas 
is supplied to the engine in step with the production thereof to avoid 
storage, while the liquid fuel gas is stored provisionally as needed in 
the ship and is supplied at the times when the environmental benefit is 
greatest. 
The engine may be connected with a shaft generator in a shuttle tanker or 
in a vessel for collection of hydrocarbons from a drilling or production 
well, and in this case at least part of the volatile organic compounds 
evaporated at the oil loading are preferably combusted in the engine 
driving the shaft generator for power production for driving units in the 
dynamic positioning system of the tanker or vessel. As the greatest amount 
of VOC is formed at loading of the crude oil, it is especially 
advantageous to provide the engine with a shaft generator of a dimension 
so that the power requirement for bow propellers, etc., used in the 
dynamic positioning of the vessel can be covered by the shaft generator, 
and then to operate the main engine by VOC during loading. 
The invention further relates to an internal combustion engine of the 
Diesel type for combustion of gas compressed to a high pressure suitable 
for injection into the engine cylinders of 200 bar, which engine is 
supercharged to a charging pressure of at least 3 bar absolute and has a 
volumetric compression ratio of at least 1:14 and a mean effective 
pressure of at least 15 bar and has an injection system with injectors 
injecting liquid fuel into the combustion chambers of the cylinders at a 
high pressure. 
Such an engine is known from the Applicant's above brochure "Large Diesel 
Engines . . . .", where the liquid fuel is pilot oil, and the gas is 
gaseous natural gas pre-compressed to a supply pressure of about 250 bar 
before supply to the engine. The gas is injected at this pressure after 
control oil has opened the injector. The natural gas used mainly consists 
of methane. 
Prior art also discloses old four-stroke engines where LPG has been used as 
fuel, see for example the above publications where LPG is evaporated in 
the intake air of the engine. These old engines were relatively small, 
high-speed engines with compression ratios of maximum 1:13, and it is 
well-known that the requirement to a suitably high methane number heavily 
increases with the compression ratio of the engine, its cylinder diameter, 
its mean pressure and with lower speeds. 
The methane number is an expression of the ignition properties of the gas, 
approximately like the octane number for petrol, and a gas with a methane 
number of 100 auto-ignites like pure methane, while a gas with a methane 
number of 0 auto-ignites like pure hydrogen. The ignition properties are 
important to achieve good utilisation of the calorific value of the fuel. 
It is not desirable that the ignition is a detonation, as this results in 
a steep pressure increase and a very high combustion pressure, usually 
leading to damage on the combustion chamber components with a risk of 
complete breakdown of the engine. 
Normally it is therefore desirable with a gas with a high methane number. 
Ordinary prevalent natural gas has a methane number of about 90 when the 
gas is pure, and when admixed with carbon-dioxide or nitrogen the methane 
number may vary between 90 and 130, i.e., the methane number can be 
higher, which is perceived by the engine as a positive variation. The high 
volumetric compression ratio of at least 1:14 in combination with the high 
mean effective pressures of at least 16 bar in the more recent Diesel 
engines leads to a presumption that gas operation at full load is only 
possible on gaseous natural gas having a methane number of at least 80. 
The high compression ratio involves the disadvantage that the natural gas 
has to be high-pressure compressed to be able to be injected at a suitable 
high pressure into the combustion chamber at the end of the compression 
stroke, which requires a noticeable energy consumption of about 5 per cent 
of the shaft power of the engine for the gas compressors. 
With a view to reducing the energy consumption for high-pressure 
compression of the gas, the engine according to the invention is 
characterized in that the injection system includes at least liquid 
injectors for high-pressure injection of compressed liquid gas. 
It is a condition for injecting liquid gas that the gas includes propane, 
butane and/or C.sub.5+ hydrocarbons. Pure propane has a methane number of 
35, butane a methane number of only 10, and the higher hydrocarbons have 
substantially lower methane numbers. When, despite the expectation to the 
contrary, it is nevertheless possible to combust liquid gas with such low 
methane numbers in a controlled manner in a high-compression supercharged 
engine, this is probably due to the fact that combustion of the gas 
requires a certain oxygen content. The injected gas evaporates immediately 
upon injection, but even though the temperature in the combustion chamber 
is very high, the gas cannot combust until it has been suitably mixed with 
the air in the combustion chamber. The crucial step for the combustion is 
thus the mixing, and not the methane number itself as has been presumed so 
far. 
The liquid gas can be compressed to very high pressures at a very small 
energy consumption. The compression may either take place independently of 
the injection itself in a form of a common rail system where the injectors 
are controlled by control oil, or the compression may be carried out by 
piston pumps in the same manner as is done conventionally with fuel oil 
for Diesel engines, viz., the piston of the pump is actuated and 
pressurizes the liquid gas when injection is to take place. In the latter 
case the gas injector is opened by the pressure increase in the liquid 
gas, for which reason control oil can be omitted.

DETAILED DESCRIPTION OF THE INVENTION 
High-pressure gas injection engines of the Diesel type and with 
supercharging may be four-stroke engines of the medium-speed type or be 
large two-stroke crosshead engines, which with the engines of today of the 
Applicant's type MC-GI may have an output per cylinder ranging from 250 to 
5800 kW at speeds ranging from 75 to 250 rpm with a stroke/bore ratio 
ranging from 2.45 to 4.20, and with volumetric compression ratios of, for 
example, 1:14, 1:15, 1:16, 1:17, 1:18 or higher. Volumetric compression 
ratio means the classical compression ratio related to the volumes above 
the piston when the latter is in its top or bottom dead centre positions. 
FIG. 1 shows a crude oil tank 1 in a ship during loading. The ship may, for 
example, be a crude oil carrier or a shuttle tanker. Through a tank 
connection 2, crude oil is supplied from an on-shore terminal plant or 
from an offshore plant, such as a loading buoy at a production platform or 
at a Floating Production Storage Offloading (FPSO) vessel. The ship may 
also be such an FPSO vessel receiving crude oil from a production well on 
the sea bed. 
As the tank is being loaded with crude oil 3, any inert gas in the tank and 
volatile organic compounds (VOC) 4 evaporated from the crude oil are 
pressed out through an outlet pipe 5 leading to a compressor 6 which 
delivers VOC to a condenser 9 via an intermediate pipe 7 with a cooler 8. 
From the condenser, condensed gas is drawn and passed through a pipe 10 to 
an insulated tank 11 in which the liquid gas, typically containing propane 
and higher alkanes, can be provisionally stored at atmospheric pressure 
and at a temperature of approximately -42.degree. C. When the liquid gas 
is to be used as fuel, it is passed via a suction pipe 12 to a compressor 
13 shown in FIG. 3, which compresses the gas to a supply pressure 
typically of 400 bar in a common rail system and of 20 bar if the final 
compression to the injection pressure takes place by means of piston pumps 
at each cylinder. 
From the top of the condenser 9, a pipe 14 passes the non-condensed 
components, methane and ethane, to a multistage compressor 15 shown in 
FIG. 4, which compresses the gaseous gas phase to an injection pressure 
typically of about 250 bar, and from this compressor a common rail system 
distributes the gas to the individual cylinders of the engine. 
During the loading of the ship, the condensing system is in continuous 
operation, but during the subsequent voyage it will be sufficient with 
intermittent operation of the system controlled on the basis of 
measurement of the pressure in the tank 1, so that the condensing system 
is started, for example, when the gases in tank 1 have a high pressure of 
0.14 bar, and is stopped when the gases have a low pressure of 0.05 bar. 
Further details is incorporated by reference to the description in 
Norwegian patent appl. No. 970393 (corresponding to WO98/33026) of 
Statoil, Den norske stats oljeselskab a.s., filed on the same day as the 
above said Danish priority application. 
FIG. 2 shows an embodiment of an injection system for a single engine 
cylinder having a secondary injector 16 for injection of gaseous gas and a 
liquid injector 17 for injection of liquid gas and a pilot injector 18 for 
injection of oil. The three injectors may be separately mounted in 
respective housings in the associated cylinder cover. It is also possible 
to integrate two of the injectors in a common housing into a so-called 
dual-fuel injector. Although the pilot injector can naturally form part of 
such a dual-fuel injector, it is preferred in the cases where all three 
types of injectors are found in a single cylinder that the secondary 
injector 16 and the liquid injector 17 are integrated into the dual-fuel 
injector 105 (FIG. 5), the gases thus being passed to the same injector 
housing, which facilitates encapsulation of the gas-carrying systems. 
Dual-fuel injectors are described in detail in, for example, the 
Applicant's Danish patents 153240 and 155757 and the Applicant's 
publication WO95/24551 has a detailed description of a gaseous gas 
injector. Several injectors of each type may be mounted on the same 
cylinder in order to achieve a better distribution of the fuel in the 
cylinder, among other things. 
The below description discusses injection of the gas whether it is liquid 
or gaseous. Injection is to mean that the gas is either injected and 
atomized or blown in, and both actions take place at a suitable high 
pressure in relation to the current pressure in the combustion chamber. 
If the cylinder in question needs fuel oil, either as an ignition aid or 
because the gases cannot alone cover the fuel requirement, the oil can be 
supplied at intervals at the desired time in the engine cycle to the pilot 
injector 18 from a fuel oil source 19, which be of different designs. The 
fuel oil source may be a usual fuel pump, supplied with oil from a 
low-pressure feed pipe common to, the pumps and having a pump piston 
driven by a cam on a camshaft. A regulator, not shown, can turn the pump 
piston in the usual manner for adjustment of the oil volume supplied from 
the pump at a high pressure of up to, for example, 800 bar. Alternatively, 
the fuel oil source may be an electronically actuated fuel pump supplied 
with oil from a common low-pressure feed pipe and adjusted as to volume 
and controlled as to time by means of setting signals from an electronic 
control unit. A third possibility is the so-called common rail system 
where the fuel oil source comprises a high-pressure reservoir for oil 
connected to an inlet port of an electronically actuated control valve 
having further at least two ports, viz., an outlet port to a pipe 20, 20' 
leading to the oil inlet of the pilot injector 18, and a port connected 
with a drain. Based on control signals received from an electronic control 
unit the control valve can switch the pipe 20, 20' to either the oil inlet 
port or the drain port. 
When the fuel oil source 19 starts supply of high-pressure oil to the pipe 
20 at the point in time of the engine cycle desired in respect of the 
timing of the combustion, the pressure will rapidly rise above the opening 
pressure of the pilot oil injector 18, whereupon the oil is injected. 
The liquid fuel gas is supplied from a fuel gas source 25, which may be 
formed in the same ways as the fuel oil source 19. For the sake of 
simplicity only the embodiment of the common rail type will be described, 
according to which the source 25 comprises the low-pressure supply from 
the tank 11 and the high-pressure compression in the compressor 13, from 
where a pipe system 26 connects the liquid injectors 17 in the engine in 
parallel. In response to control signals received from an electronic 
control unit, a control valve 27 can connect the fuel inlet of the 
injector 17 with the high-pressure gas pipe 26 or with a drain. When the 
control valve 27 opens for supply of liquid gas at the point of time of 
the engine cycle desired in respect of the timing of the combustion, the 
liquid injection 17 opens for injection and atomization of the gas in the 
combustion chamber. 
The injection of gaseous gas by the injector 16 can only take place when 
liquid fuel has been injected for the same combustion sequence for 
initiation of the combustion. This liquid fuel may either be fuel oil from 
the injector 18 or fuel gas from the injector 17. Below is described an 
embodiment in which the combustion is initiated with pilot oil, but it 
should be understood that the liquid injector 17 can replace the pilot 
injector 18 in the safety system described. 
When the pilot injector 18 opens, the oil pressure at the same time 
actuates a safety device 21 to enable application of control oil pressure 
on the secondary injector 16. The safety device 21 may, for example, be of 
a well-known mechanical type with a piston keeping a drain port in a 
control oil pipe 22, 22' open until the fuel oil pressure displaces the 
piston for closure of the drain port by exceeding said opening pressure. 
Through a pipe 23, the drain port is connected with a reservoir 24 for 
control oil. Alternatively, the safety device may be of an electronic type 
which determines in an electronic control unit whether the fuel oil or the 
liquid fuel gas is injected and uses this information as a condition for 
actuation of the secondary injector 16. In this case the control unit can 
detect the injection on the basis of a pressure sensor in the pipe 20 or 
by a position sensor in the valve 18 for detecting the actual valve 
opening. 
In the embodiment shown the secondary injector 16 can be actuated to an 
open position by application of control oil pressure at the connection of 
the gas valve to the pipe 22, 22'. The injector may further have a 
connection 28 for sealing oil and a connection 29 leading to a 
high-pressure gas source. The sealing oil pressure may, for example, be 40 
bar higher than the gas pressure in the connection 29. Alternatively the 
injector 16 may be kept closed by the control oil pressure and be opened 
by its elimination, thus obviating the need for sealing oil. This is 
described in detail in WO95/24551. 
An electronically actuated control valve 30 has an inlet port connected 
with a pipe 31 with high-pressure oil supplied from a pump 32 which is fed 
from the reservoir 24 via a pipe 33. The control valve 30 has further at 
least two ports, viz., an outlet port to the pipe 22, 22' and a drain port 
connected with the reservoir 24. Based on control signals received from an 
electronic control unit 34, the control valve can connect the pipe 22, 22' 
to either the oil pressure pipe 31 or to the drain port. The control valve 
may, for example, be a magnetic valve, an electronically controlled 
hydraulic valve or a magnetic valve with so-called magnetic locking, which 
may result in extremely short switch times. 
In a well-known manner the electronic control unit 34 is supplied with 
information on the current angular position of the engine crankshaft and 
controls the three injectors 16-18 for injection of the most desirable 
combination of fuels for the combustion in question. 
An example of the gas systems in a crude oil carrier with a propulsion 
engine according to the invention will now be described in further detail 
with reference to FIG. 3 showing the system for the liquid gas, and FIG. 4 
with the system for the gaseous gas. The drawing only shows two cylinders 
35, but of course the engine has more. Blow-off valves 36, 36' can empty 
the gas systems of the cylinder of gas through a common drain pipe 37, if 
necessary. A venting valve 38 can empty the branch pipe 26 if the engine 
is not to be driven on liquid gas for a period. The entire feed pipe 26 
can be emptied of liquid gas by closure of a main valve 39 and opening of 
the drain valves, and of a feed valve 40 connected with an inert gas 
source 41. In a completely corresponding manner, a venting valve 38' (FIG. 
4) can empty a high-pressure gas reservoir 42, the volume of which may 
contain gaseous gas for, for example, 20 injection sequences, and a 
cut-off valve 43 can close the gas supply if the pressure lowering at an 
injection is so large that the secondary injector must be presumed not to 
be correctly closed after conclusion of the injection. The entire pipe 29 
can be scavenged with inert gas by closure of a main valve 44 and opening 
of a cut-off valve 45 in a drain pipe 46 simultaneously with opening of 
the valve 401 to the inert gas source 41. 
The gas-carrying members in the engine room are encapsulated by a mantle 47 
supplied with venting air at the outlet of the drain pipe 37, as shown by 
the arrow 48, at least one blower 49 exhausting air from the mantle 47 at 
the passage of the feed pipe into the engine room. Gas detectors 50 are 
arranged at suitable locations in the system for monitoring any gas 
leakages. 
Before being passed into the engine room, the liquid gas may be heated to, 
for example, 45.degree. C. in a unit 51 to avoid any ice formations inside 
the mantles 47. 
The gaseous gas supplied with the pipe 14 is compressed in the compressor 
generally designated 15, at least in a low-pressure compressor 15a which 
compresses the gas to about 25 bar, and in a high-pressure compressor 15b 
which delivers the gas at a pressure of, for example, 250 bar to the pipe 
29 which has been passed into the engine room. The drive motors for the 
compressor are placed in a gastight enclosure, and both the latter and the 
compressor room are vented by respective blowers. 
The output pressure from the high-pressure compressor 15b can vary with the 
composition of the gaseous mixture being compressed. The pressure may be 
lower, such as 175 bar, when the inert gas content is high, and higher 
when the gas mixture consists mainly of combustible gas. It has been 
mentioned above that the pressure can be controlled by means of the 
electronic control unit of the engine based on pressure measurements from 
combustion sequences and pertaining calculation of the energy development 
at combustion, which, by comparison with the duration of the injection, 
provides a measure of the inert gas content of the gas mixture. Thus, in a 
feed-back control, data for a combustion can be used regularly for 
adjustment of the output pressure of the high-pressure compressor for the 
subsequent engine operation. It will often be so that a ship with crude 
oil tanks sails between fixed destinations and loads crude oil of a 
uniform quality into the crude oil tanks. This renders it possible to 
record at the first loading how the inert gas content of the gas mixture 
varies during and after the tank loading. These data can then be used at 
subsequent loadings as an empirical basis for adjustment of the output 
pressure of the compressor in a feed-forward control. 
When the engine is a high-compression Diesel engine, liquid gas can also be 
supplied in the form of LPG or an LPG mixture which can be refined, 
fractionated or pre-processed in any other manner into a more stable 
composition than liquid gas produced by condensation of VOC. The 
high-compression Diesel engine according to the invention can be a 
four-stroke engine having, for example, a maximum speed of 700 rpm. 
Preferably the engine is a two-stroke crosshead engine having a cylinder 
bore of at least 200 mm, suitably at least 250 mm, and a speed of maximum 
250 rpm. The engine may have a mean pressure of at least 16 bar and may be 
super-charged to at least 3 bar absolute at full load. The mean pressure 
may also be higher, such as 17 or 18 bar, and the supercharging may also 
be higher. As fuel the engine may also be supplied with compressed liquid 
gas with methane numbers as low as maximum 15, but may, of course, also 
use fuel of a higher methane number. 
It may be appropriate to operate the high-compression engine by means of 
dual-fuel systems using liquid gas and fuel oil either separately or 
together. The oil may optionally be used as an ignition aiding pilot fuel. 
The liquid gas is injected at a pressure higher than the pressure in the 
combustion chamber. The injection pressure usually ranges from 200 to 1200 
bar, typically from 350 to 900 bar. At the injection the fuel is atomized 
into clouds of liquid droplets which evaporate immediately, whereupon a 
mixing takes place with the other gases in the combustion chamber. The 
injection of liquid gas occurs during one or more periods which may 
possibly start immediately before the piston reaches its top dead centre 
at the end of the compression stroke, for example from 6.degree. CA before 
TDC, but otherwise occurs during the expansion stroke. The gas may combust 
when it has become suitably mixed with oxygen-containing air. 
Each liquid injector 17 for injection of liquid gas is connected with the 
associated gas supply pipe 26 supplied from a suitable external gas source 
which may be of the above type on a crude oil carrier or may be connected 
to a permanent main line, if the engine is a stationary power generator. 
The pipe 26 passes the liquid gas to the liquid injector and is 
encapsulated by the mantle 47. The blower 49 maintains an airflow in the 
space between the inner surface of the mantle and the outer surface of the 
pipe. Preferably, the venting air has been preheated before being passed 
between the mantle and the pipe by means of freely available waste heat. 
This may be effected, for example, by heating the air in a heat exchanger 
with the coolant of the engine. The hot air can be supplied to the mantle 
in a counterflow with the inflowing gas. This may, for example, be 
effected by locating the air inlet near the engine, but this in turn may 
make it necessary with two blowers. The result of the heating is that no 
ice formations occur in or on the pipes in case of leaks and complete or 
partial venting of the gas to the atmosphere. At such a venting, the 
pressure will first be relieved to atmospheric pressure. Then the liquid 
gas will evaporate by bumpily boiling inside the pipes which are thus 
being cooled. The hot blower air counteracts a complete cooling down of 
the gas-carrying system. The gas-carrying pipes may also be emptied in a 
controlled manner without any substantial amounts of gas boiling in the 
pipes by the liquid gas being displaced by another fluid applied to the 
gas-carrying pipes at a suitable high pressure, for example fuel oil or a 
gas, such as inert gas. 
If the engine is a ship's engine, the liquid gas may be stored in a 
pressurized and/or cooled tank. 
With reference to FIG. 5, the liquid injectors 16 and the secondary 
injectors 17 are combined in a corresponding number of dual-fuel injectors 
105. Also, according to a preferred embodiment, the engine is a main 
engine 101 of a ship 102 with crude oil tanks, and the volatile organic 
compounds of ignition properties, calorific values and evaporated amounts 
varying over time evaporated from these tanks constitute a substantial 
proportion of the fuel consumption of the main engine. The engine is 
connected to a shaft generator 103 in a shuttle tanker or ship 102 for 
collection of hydrocarbons from a drilling or production well. At least 
part of the Volatile Organic Compounds evaporated at the oil loading are 
combusted in the engine driving the shaft generator for power production 
for driving units 104 in a dynamic positioning system of the tanker or 
vessel.