Patent Description:
A possibility to operate a piston engine using a gaseous fuel, such as natural gas, provides many advantages, for instance in the form of lower emissions compared to many liquid fuels. Gaseous fuels usually need an ignition source, which is in most cases either a spark plug or pilot fuel. In dual-fuel engines, the gaseous main fuel is ignited by utilizing liquid pilot fuel, which is injected into the cylinders of the engine either directly or via a prechamber. The gaseous fuel is introduced into the intake duct and flows through the intake ports into the cylinders during the intake stroke. The liquid pilot fuel is injected by means of pilot fuel injectors at the end of the compression stroke. The pilot fuel is self-ignited, and the ignition of the pilot fuel triggers combustion of the gaseous main fuel. The air/fuel mixture is often a lean mixture comprising more air than is needed for the complete combustion of both the gaseous fuel and the liquid pilot fuel. In transient situations, where engine load is rapidly increasing, typically the fuel amount increases faster than the air amount, which leads to a rich air/fuel mixture. The rich mixture can lead to auto-ignition of the fuel or to too fast combustion speed thus causing knocking.

<CIT> and <CIT> disclose dual fuel engines operating on a gaseous fuel as main fuel being ignited by liquid pilot fuel injection. In order to reduce the risk of knocking during a transient engine operation, namely during acceleration, by unduly increasing the amount of gaseous fuel, the amount of liquid fuel is increased instead, thereby substituting for the gaseous fuel, while at the same time the required load demand is reached.

An object of the present invention is to provide an improved method of operating a piston engine in a transient state using a gaseous main fuel that is introduced into the main combustion chamber of a cylinder of the engine during an intake stroke and a liquid pilot fuel that is introduced into the main combustion chamber or into a prechamber as one or more injection events taking place during a compression stroke for igniting the gaseous main fuel. The characterizing features of the method according to the invention are given in of claim <NUM>. Another object of the invention is to provide an improved control system for a piston engine. The characterizing features of the control system are given in the other independent claim.

The method according to the invention comprises the steps of.

The control system according to the invention is configured to operate the engine in a transient state in accordance with the method defined above.

A piston engine according to the invention comprises a control system defined above.

In the method according to the invention, the output power is thus primarily controlled by adjusting the amount of the gaseous fuel introduced into combustion chambers. However, if the fuel demand of the engine is greater than an allowable amount of gaseous fuel, the amount of the gaseous fuel introduced into the cylinders is limited to a limit value. The difference between the fuel demand and the amount of gaseous fuel that can be introduced into the cylinders is determined and used as an input for controlling additional liquid fuel injection.

With the method and control system according to the invention, an engine can respond to rapidly increasing load while auto ignition or too fast combustion speed of the gaseous fuel can be avoided. By injecting the additional liquid fuel during the combustion stroke, a limitation in the gas supply can be compensated without affecting the start of combustion.

According to an embodiment of the invention, a first variable or a set of first variables indicative of the operation of the engine is measured or calculated, and the first limit value is determined based on the value of at least one first variable. The first variable used for determining the first limit value is chosen so that the first limit value reflects a limit above which knocking risk increases.

According to an embodiment of the invention, the first variables include at least one variable from the group of charge air pressure, engine load, engine speed and air/fuel ratio. Also a combination of two or more variables could be used to determine the first limit value.

According to an embodiment of the invention, a value from a look-up table is used as an additional input for generating the third control signal. The additional liquid fuel injection can thus have a basic level, which can be retrieved from a look-up table, and a variable portion, which is based on the difference between the first control signal and the first limit value.

According to an embodiment of the invention, the third control signal is capped to a second limit value, in case the duration of the additional liquid fuel injection reaches an upper limit determined for the duration of the additional liquid fuel injection. The second limit value can be used, for instance, to prevent smoke.

According to an embodiment of the invention, the upper limit for the duration of the additional liquid fuel injection is determined based on an allowable air/fuel ratio, maximum speed of the engine or maximum load of the engine.

According to an embodiment of the invention, a decrease rate of the third control signal is determined and compared to a third limit value, and in case the decrease rate exceeds the limit value, the decrease rate of the third control signal is capped to the third limit value. By preventing too rapid decrease of the additional liquid fuel injection, it is ensured that the liquid fuel injection is not terminated too early and the supply of the gaseous fuel can meet the fuel demand.

According to an embodiment of the invention, the pressure in at least one cylinder of the engine is monitored, and in case a measured peak pressure exceeds a predetermined value, the amount of the gaseous fuel introduced into the cylinders is decreased from the level determined by the first control signal or the second control signal.

According to an embodiment of the invention, one or more variables indicative of knocking is monitored, and in case knocking is detected, the amount of the gaseous fuel introduced into the cylinders is decreased from the level determined by the first control signal or the second control signal.

Embodiments of the invention are described below in more detail with reference to the accompanying drawings, in which.

<FIG> shows schematically pilot fuel and gaseous fuel injection systems of a piston engine <NUM>. <FIG> shows one of the cylinders <NUM> of the engine <NUM> of <FIG>. The engine <NUM> is a large internal combustion engine, such as a main or an auxiliary engine of a ship or an engine that is used at a power plant for producing electricity. The rated power of the engine <NUM> is at least <NUM> kW per cylinder and the cylinder bore is at least <NUM>. In <FIG>, four cylinders <NUM> that are arranged in line are shown, but the engine <NUM> can comprise any reasonable number of cylinders <NUM> that are arranged, for instance, in line or in a V-configuration. The engine <NUM> of <FIG> is a dual-fuel engine that can be operated at least in a gaseous fuel mode using a gaseous main fuel and a liquid pilot fuel. The gaseous fuel can be, for instance, natural gas, biogas, liquefied petroleum gas or ethane. The liquid pilot fuel can be, for instance, light fuel oil (LFO), marine diesel oil (MDO) or biodiesel. The engine <NUM> can possibly also be operated in a liquid fuel mode using only liquid fuel, such as LFO or MDO. The engine <NUM> could also be a multi-fuel engine, which can be operated using two or more different types of liquid fuels and/or gaseous fuels. For instance, the engine <NUM> could be a tri-fuel engine that can be operated in a gaseous fuel mode using gaseous main fuel and liquid pilot fuel, and in two different liquid fuel modes, for instance in a first liquid fuel mode using heavy fuel oil or other fuel containing mainly residual fuel oil and in a second liquid fuel mode using light fuel oil or other fuel containing mainly distillate fuel oil.

The engine <NUM> is connected to a turbocharger <NUM> comprising a compressor 16a and a turbine 16b for pressurizing the intake air of the engine <NUM>. The engine <NUM> could also be provided with two or more turbochargers connected in series and/or in parallel.

The engine <NUM> is provided with separate fuel injection systems for the gaseous fuel and the pilot fuel. The gaseous fuel is stored in a gas tank <NUM>, from which it is supplied via a gas line <NUM> into the cylinders <NUM> of the engine <NUM>. The gas line <NUM> is provided with a main gas valve <NUM> for controlling the supply of the gaseous main fuel into the gas line <NUM>. The main gas valve <NUM> can be closed for example when the engine <NUM> is operated in a liquid fuel mode. Each cylinder <NUM> of the engine <NUM> is provided with an own gas admission valve <NUM>. The gas admission valves <NUM> are individually controllable. A first control line <NUM> connects the gas admission valves <NUM> to a control unit <NUM>, which controls the opening and closing of the gas admission valves <NUM>. Each gas admission valves <NUM> is opened and closed by means of an actuator, such as a solenoid or a step motor. The gas admission valves <NUM> are arranged to introduce the gaseous fuel into an intake duct <NUM> of the engine <NUM>. Each gas admission valve <NUM> is located close to the intake valves of the respective cylinder <NUM>. The engine <NUM> could also comprise additional gas admission valves for introducing part of the gaseous fuel into prechambers. The gaseous fuel is introduced into the intake duct <NUM> during the intake stroke. When the intake valves of a cylinder <NUM> are open, the gaseous fuel flows into the cylinder <NUM> forming an air/fuel mixture. The air/fuel mixture is preferably a lean mixture comprising more air than is needed for the complete combustion of both the gaseous fuel and the liquid pilot fuel. It should be noted that <FIG> shows only a simplified example of a fuel injection system for gaseous main fuel. The fuel injection system can comprise many additional components not shown in the figures, for instance different valves, filters etc..

The fuel injection system for the liquid pilot fuel comprises an own pilot fuel injector <NUM> for each cylinder <NUM> of the engine <NUM>. The pilot fuel injector <NUM> can be arranged to inject the fuel either directly into a main combustion chamber <NUM>, as shown in <FIG>, or into a prechamber. The pilot fuel injectors <NUM> are individually controllable. The pilot fuel injectors <NUM> can be, for instance, electrically actuated. A second control line <NUM> connects the pilot fuel injectors <NUM> to the control unit <NUM>, which controls the opening and closing of the pilot fuel injectors <NUM>. Each pilot fuel injector <NUM> comprises a valve needle, which can be lifted for injecting fuel through the pilot fuel injector <NUM>.

In the embodiment of <FIG>, the pilot fuel injection system is a common rail fuel injection system comprising a fuel rail <NUM> for storing pressurized fuel. All the pilot fuel injectors <NUM> are connected to the same fuel rail <NUM>. A low-pressure pump <NUM> supplies fuel from a liquid fuel tank <NUM> to a high-pressure pump <NUM>, which raises the pressure of the pilot fuel to a level that is suitable for direct fuel injection and supplies it into the fuel rail <NUM>, from which the fuel is supplied to the pilot fuel injectors <NUM>. The fuel injection pressure can be in the range of <NUM> to <NUM> bar. Instead of a single fuel rail <NUM>, each pilot fuel injector <NUM> could be provided with an own fuel accumulator, or the pilot fuel injection system could comprise two or more fuel rails <NUM>, each serving two or more pilot fuel injectors <NUM>. In addition to the use as a pilot fuel injection system, the fuel injection system of <FIG> can also be used for injecting liquid fuel into the cylinders <NUM> of the engine <NUM> in other situations.

The pilot fuel injection system could also be used when the engine <NUM> is operated in a liquid fuel mode using only liquid fuel. In that case, the pilot fuel injectors <NUM> would function also as main fuel injectors. It is also possible that only parts of the fuel injection system of <FIG> are used as part of a fuel injection system for liquid main fuel. For instance, additional fuel injectors could be connected to the fuel rail <NUM> of <FIG> for injecting liquid main fuel. In the gaseous fuel mode, the liquid pilot fuel would be injected by the pilot fuel injectors <NUM>, and in a liquid fuel mode the liquid fuel would be injected by main fuel injectors. In both operating modes, the same low-pressure pump <NUM> and high-pressure pump <NUM> would be used. The pilot fuel injectors <NUM> and the main fuel injectors could be integrated in fuel injector units. The pilot fuel is injected into the cylinders <NUM> of the engine <NUM> via the pilot fuel injectors <NUM> during the compression stroke when the piston is close to top dead center.

In addition to the fuel injection systems shown in <FIG>, the engine <NUM> can be provided with at least one additional fuel injection system, for instance for injecting liquid main fuel into the cylinders <NUM> of the engine <NUM>. The additional fuel injection system could be a common rail system or a fuel injection system having an own fuel injection pump for each cylinder <NUM> of the engine <NUM>.

<FIG> shows schematically one cylinder <NUM> of the engine <NUM> of <FIG>. Each cylinder <NUM> of the engine <NUM> is provided with a piston <NUM>, which is configured to move in a reciprocating manner within the cylinder <NUM>. The piston <NUM> is connected via a connecting rod <NUM> to a crankshaft <NUM>. A flywheel <NUM> is attached to one end of the crankshaft <NUM>. Together with the walls of the cylinder <NUM> and a cylinder head <NUM>, the piston <NUM> delimits a main combustion chamber <NUM>. Each cylinder <NUM> of the engine <NUM> is provided with at least one intake valve <NUM>. Each cylinder <NUM> can be provided for example with two intake valves <NUM>. The intake valves <NUM> are used for opening and closing fluid communication between the intake duct (intake receiver) <NUM> and the main combustion chamber <NUM>. Each cylinder <NUM> of the engine <NUM> is provided with at least one exhaust valve <NUM>. Each cylinder <NUM> of the engine can be provided for example with two exhaust valves <NUM>. The exhaust valves <NUM> are used for opening and closing fluid communication between the combustion chamber <NUM> and an exhaust duct <NUM>.

The intake valves <NUM> are connected to intake valve actuating means <NUM>. The intake valve actuating means <NUM> are used for opening and closing the intake valves <NUM>. The intake valve actuating means <NUM> can be configured to allow variable intake valve timing. The crank angle at which the intake valves <NUM> are opened and/or closed can thus be varied. The intake valve actuating means <NUM> can be implemented in many alternative ways. The intake valve actuating means <NUM> can comprise an electrical, hydraulic or mechanical actuator. The intake valve actuating means <NUM> could also be any combination of electrical, hydraulic and/or mechanical means. For instance, the intake valves <NUM> can be opened by means of a camshaft. The closing force for closing the intake valves <NUM> can be created by means of one or more springs, such as helical springs and/or air springs. The closing of the intake valves <NUM> can be delayed by means of a hydraulic system. Alternatively, both the opening and closing timing could be determined by means of an electrical actuator, such as a solenoid. Alternatively, the intake valves <NUM> could be both opened and closed hydraulically. The intake valve actuating means <NUM> are connected to the control unit <NUM>, which is configured to transmit a control signal to the intake valve actuating means <NUM> for determining the opening and/or closing timing of the intake valves <NUM>.

In the embodiment of <FIG>, the exhaust valves <NUM> are connected to similar actuating means <NUM> as the intake valves <NUM>. However, the exhaust valves <NUM> could also be provided with different actuating means. For instance, it is not necessary that the actuating means <NUM> of the exhaust valves <NUM> allow variable valve timing. In the embodiment of <FIG>, also the exhaust valve actuating means <NUM> are connected to the control <NUM> unit which is configured to transmit a control signal to the exhaust valve actuating means <NUM> for determining the opening and/or closing timing of the exhaust valves <NUM>. Instead of the arrangements described above, both the intake valves <NUM> and the exhaust valves <NUM> could be cam-controlled. The valve timings could be either variable or fixed.

The gas admission valves <NUM> and the pilot fuel injectors <NUM> are connected to the control unit <NUM>. The injection timings and durations of both the pilot fuel injection and the main fuel injection can thus be individually controlled in each cylinder <NUM> of the engine <NUM>.

Each cylinder <NUM> of the engine <NUM> is provided with a cylinder pressure sensor <NUM>. The cylinder pressure sensor <NUM> is arranged to measure pressure in the cylinder <NUM>. The engine <NUM> is provided with data processing means, such as the control unit <NUM>, which receives measurement data from the cylinder pressure sensors <NUM>. The engine <NUM> further comprises a crank angle sensor <NUM> or other means for determining the angular position of the crankshaft <NUM>. In the embodiment of <FIG>, the crank angle sensor <NUM> monitors the angular position of the flywheel <NUM>. On the basis of the angular position of the flywheel <NUM>, the position of the piston <NUM> in each cylinder <NUM> can be determined. In addition to the pressure measurement data, the control unit <NUM> also receives measurement data from the crank angle sensor <NUM>. The cylinder pressure can thus be determined in respect of crank angle.

During normal steady-state operation of the engine <NUM> in the gaseous fuel mode, intake valves <NUM> of each cylinder <NUM> of the engine <NUM> open at the end of the exhaust stroke. At the beginning of the intake stroke, the control unit <NUM> transmits a control signal via the first control line <NUM> to the gas admission valve <NUM> for opening the gas admission valve <NUM>. The gas admission valve <NUM> opens and the valve <NUM> is kept open for a certain period of time for allowing the gaseous fuel to flow into the intake duct <NUM> and further into the cylinder <NUM>. Before the end of the intake stroke, the gas admission valve <NUM> is closed. Also the intake valves <NUM> are closed, and during the compression stroke following the intake stroke, the air/fuel mixture in the cylinder <NUM> is compressed. As the piston <NUM> in the cylinder <NUM> approaches top dead center, the control unit <NUM> transmits a control signal via the second control line <NUM> to the pilot fuel injector <NUM> for opening the pilot fuel injector <NUM> and for injecting liquid pilot fuel into the cylinder <NUM>. The valve needle of the pilot fuel injector <NUM> is lifted for a short period of time and pilot fuel is injected into the cylinder <NUM>. After a certain ignition lag, the pilot fuel is self-ignited. The amount of the pilot fuel is relatively small. The combustion of the pilot fuel can release, for instance, less than five percent of the heat released by the combustion in the cylinder <NUM>. The combustion of the pilot fuel ignites also the mixture of air and the gaseous main fuel. When the piston <NUM> is close to bottom dead center, the exhaust valves <NUM> open. During the exhaust stroke, the exhaust gases flow out of the cylinder <NUM>, and a new engine cycle begins.

In steady-state operation the engine <NUM> is operated using a lean air/fuel mixture. In transient situations, where the engine load rapidly increases, the supply of fuel into the cylinders needs to be increased. The turbocharger <NUM> cannot immediately respond to the increased load, and therefore the air/fuel mixture becomes richer. This can lead to auto ignition or too fast combustion of the gaseous fuel thus causing knocking. The present invention addresses this problem.

In the method according to the invention, the engine is operated in a transient state using a gaseous main fuel that is introduced into the main combustion chamber <NUM> of a cylinder <NUM> of the engine <NUM> during an intake stroke and a liquid pilot fuel that is introduced into the main combustion chamber <NUM> or into a prechamber as one or more injection events taking place during a compression stroke for igniting the gaseous main fuel.

One example of a gas supply system suitable for use in the method according to the invention has been described above. However, the system does not need to be identical to the system of <FIG>. In addition to the gas supply into the main combustion chambers <NUM>, part of the gaseous fuel can be introduced into prechambers, in case the engine <NUM> is provided with prechambers.

For igniting the gaseous main fuel, liquid pilot fuel is introduced into the main combustion chambers <NUM> and/or into prechambers. A liquid fuel injection system described above can be used for injecting the liquid fuel. The liquid pilot fuel is injected during the compression stroke. The pilot fuel injection can be divided into one or more injection events. The pilot fuel injection can take place for example in the range <NUM>-<NUM> degrees before top dead center.

The steps of the method according to the present invention are shown in <FIG>. In a first step of the method, a first measurement signal indicative of the engine output power or engine speed is monitored <NUM>. The first measurement signal is compared to a set point <NUM>. For instance, if the first measurement signal is a speed signal, it is compared to the desired engine speed. The use of a speed signal as the first measurement signal is suitable for many different applications, for instance for engine generator sets where the aim is to keep the rotation speed of the engine and the generator constant. Instead of a speed signal, for example a signal indicative of the electrical power produced by a generator that is driven by the engine could be monitored and compared to a set point.

Based on the comparison between the first measurement signal and the set point, a first control signal for controlling the amount of the gaseous main fuel is generated <NUM>. The engine <NUM> can comprise a PID controller 10a, which can be part of the control unit <NUM>. The PID controller 10a can receive the first measurement signal and the set point. The first control signal can be generated by the PID controller 10a based on the difference between the first measurement signal and the set point.

The first control signal is compared to a first limit value <NUM>. The first limit value is indicative of the maximum amount of gaseous fuel that can be introduced into the cylinders of the engine without a significant knocking risk. If the first control signal is below the first limit value, the knocking risk is low. The first limit value does not have to be constant, but it can be a changing limit value. The first limit value can be determined based on the value of one or more first variables, which are measured or calculated. The first variables can include charge air pressure, engine load, engine speed and air/fuel ratio. For instance, the charge air pressure of the engine can be monitored, and based on the charge air pressure, the first limit value can be determined. A higher charge air pressure allows more gaseous main fuel to be introduced into the cylinders of the engine without a knocking risk, and the first limit value could thus increase as the charge air pressure increases. If the first limit value is determined based on the air/fuel ratio, the mass of the injected fuel and the mass of the air introduced into the cylinders are calculated, and the first limit value is determined so that a minimum allowed air/fuel ratio is achieved. The first limit value could also be determined based on a combination of two or more first variables. For instance, both the charge air pressure and the engine load or engine speed could be monitored, and the first limit value could be determined based on the charge air pressure and the engine load or engine speed.

If the first control signal is below the first limit value, the amount of the gaseous main fuel is controlled based on the first control signal <NUM>. When the first control signal is below the first limit value, the knocking risk is low. The output power of the engine can thus be controlled based on the first control signal, such as the output signal of the PID controller 10a.

If the first control signal is above the first limit value, a second control signal capped to the first limit value is generated and the amount of the gaseous main fuel is controlled based on the second control signal <NUM>. When the first control signal is above the first limit value, it is an indication that the knocking risk increases if the amount of the gaseous fuel introduced into the cylinders is increased on the basis of the first control signal. The first control signal, such as the output signal of the PID controller 10a, is therefore not used to directly control the amount of the gaseous main fuel. The amount of the gaseous main fuel is thus not increased as much as the output signal of the PID controller 10a indicates.

If the amount of the gaseous main fuel introduced into the cylinders does not equal the fuel demand, the speed of the engine drops if the effect of the limited supply of the gaseous main fuel is not compensated. Therefore, the difference between the first control signal and the first limit value is determined <NUM>, the difference between the first control signal and the first limit value is used as an input for generating a third control signal <NUM>, and additional liquid fuel injection into the cylinders <NUM> during a combustion stroke is controlled based on the third control signal <NUM>. Because the second control signal has been capped to the first limit value, the amount of the gaseous main fuel introduced into the cylinders of the engine is not sufficient for meeting the power demand. The effect of inadequate supply of the gaseous main fuel is compensated by injecting liquid fuel into the cylinders. Because the additional liquid fuel injection is carried out during the combustion stroke, the pilot fuel injection does not have to be changed, and the start of the combustion is thus not affected. The additional liquid fuel injection can take place, for example, in the range of <NUM>-<NUM> degrees after top dead center.

The third control signal could be based on the difference between the first control signal and the first limit value. However, this is not necessary, but a value from a look-up table can be used as an additional input for generating the third control signal. The additional liquid fuel injection can thus have a basic level that is determined using a look-up table, and the difference between the first limit value and the first measurement signal can determine a variable portion of the additional liquid fuel injection. The look-up table can comprise different basic levels of additional liquid fuel injection for different speed/load combinations.

An upper limit can be determined for the duration of the additional liquid fuel injection. The upper limit can be used for example to prevent smoke. In case the duration of the additional liquid fuel injection reaches the upper limit, the third control signal can be capped to a second limit value. The upper limit for the duration of the additional liquid fuel injection can be determined, for example, based on an allowable air/fuel ratio, maximum speed of the engine or maximum load of the engine.

Also the decrease rate of the third control signal can have a limit value. The decrease rate of the third control signal can be determined and compared to a third limit value, and in case the decrease rate exceeds the limit value, the decrease rate of the third value can be capped to the third limit value. By preventing too rapid decreasing of the additional liquid fuel injection, it is ensured that the additional liquid fuel injection is not terminated too early and the supply of the gaseous main fuel can meet the fuel demand.

The amount of the gaseous fuel can be further controlled based on the cylinder pressures. The cylinder pressures can be monitored and in case a measured peak pressure in one or more cylinders exceeds a predetermined value, the amount of the gaseous fuel introduced into the cylinders can be decreased from the level determined by the first control signal or the second control signal.

The amount of the gaseous fuel can be further controlled based on detected knocking. One or more variables indicative of knocking can be monitored, and in case knocking is detected, the amount of the gaseous fuel introduced into the cylinders can be decreased from the level determined by the first control signal or the second control signal. Knocking can be detected for example by monitoring cylinder pressures.

Claim 1:
A method of operating a piston engine (<NUM>) in a transient state using a gaseous main fuel that is introduced into the main combustion chamber (<NUM>) of a cylinder (<NUM>) of the engine (<NUM>) during an intake stroke and a liquid pilot fuel that is introduced into the main combustion chamber (<NUM>) or into a prechamber as one or more injection events taking place during a compression stroke for igniting the gaseous main fuel, the method comprising the steps of
- monitoring a first measurement signal indicative of the engine output power or engine speed (<NUM>),
- comparing the first measurement signal to a set point (<NUM>),
- based on the comparison between the first measurement signal and the set point, generating a first control signal for controlling the amount of the gaseous main fuel (<NUM>), and
- comparing the first control signal to a first limit value (<NUM>),
wherein
in case the first control signal is below the first limit value,
- the amount of the gaseous main fuel introduced into the cylinders (<NUM>) is controlled based on the first control signal (<NUM>), and
in case the first control signal is above the first limit value,
- a second control signal capped to the first limit value is generated and the amount of the gaseous main fuel introduced into the cylinders (<NUM>) is controlled based on the second control signal (<NUM>),
- the difference between the first control signal and the first limit value is determined (<NUM>),
- the difference between the first control signal and the first limit value is used as an input for generating a third control signal (<NUM>), and
- additional liquid fuel injection into the cylinders (<NUM>) during a combustion stroke is controlled based on the third control signal (<NUM>).