TECHNICAL FIELD

[001 ] The invention concerns in general the technology of dual-fuel diesel engines that may burn both liquid and gaseous fuel. In particular the invention concerns the engine start up phase and how a successful start up of the engine is verified.

BACKGROUND OF THE INVENTION

[002] A dual-fuel diesel engine may use both liquid fuel, such as fuel oil, and gaseous fuel, such as natural gas. Ensuring reliable operation the engine re- quires that some precautions are made at engine start phase.

[003] It is known from a state of art document EP2136059A1 a micro-pilot injection type gas engine in which, at the start of the engine, operating conditions of the engine are taken into intermittent operation to make the control of the air-fuel ratio at the start of the engine accurate and in which the time period required for warm-up operation is reduced or eliminated. The gas engine has an on-off gas valve adapted so as to be able to change the area of a fuel gas path to each cylinder and the time period of opening and closing of the fuel gas path, a rotational speed detector for detecting the speed of the engine and a combustion diagnosis device for detecting combustion conditions of each cyl- inder of the engine from the pressure in the cylinder, and an intermittent valve operation control device operating according to the speed of the engine.

[004] EP2136059A1 further discloses that at the start of the gas engine, the intermittent valve operation control device intermittently opens and closes the on-off gas valve according to a detected value of the engine speed, and, based on the detected waveform of the value of the detected combustion conditions of each cylinder obtained from the combustion diagnosis device, the intermittent valve operation control device intermittently opens and closes the on-off gas valve at an opening/closing interval which causes the air-fuel ratio to be a target value. [005] In this description the following term are being used:

- A term start up refers here to a chain of events changing the status of the engine from the stand still engine where the crank shaft is not rotating to the sta- tus of engine running on main fuel or just before a steady state running mode is achieved.

- A term start refers to shorter term event than the start up, for example start of rotation of the crank shaft, start of pilot fuel injection, start of main fuel injec- tion, etc. So it is used here in general as a beginning of a certain phase or event.

[006] Today, when starting up a dual fuel engine in diesel or gas mode, a functional check of the pilot injectors is conducted. This is currently based on a study of how the exhaust gas temperature changes due to the applied fuel amount. As the temperature measurement has some dead time because the sensor element needs to heat up and this causes some delay in achieving reliable measurement. When using this measurement of exhaust gas temperature, a large engine may need to have a temperature stabilization time of approximately 60 - 90 seconds to eliminate the heat originating from other sources than the gas fuel, like from an actuator start assist. After that the actual check of the pilot injectors can be started and that takes 5 - 30 seconds. So in all this combustion check might take about 2 minutes.

SUMMARY OF THE INVENTION

[007] The requirements concerning start up time of the engine get tighter es- pecially in power plant solutions. All these preparations before the actual production run are reducing the efficiency of the power plant. Thus the object of the present invention is to reduce the time of preparations at the start up or before switching to the main fuel and thus improve the efficiency of the engine. An object is also to reduce the time the engine is running on pilot fuel before switching to the main fuel.

[008] The present invention is characterized in that an operational check of the pilot injectors is conducted such that it is based on calculated indicated mean effective pressure (IMEP). In more detail, a method for starting up a dual fuel internal combustion piston engine, the method comprises at least following steps:

- the engine is rotated by a start assist device,

- a pilot injector is activated and an amount of pilot fuel is injected to a cylinder,

- the injected pilot fuel / air mixture is compressed in the cylinder and ignites,

- a pressure sensor in the cylinder measures the cylinder pressure, - an indicated mean effective pressure (IMEP) is calculated from the combustion of the pilot injection,

- the calculated IMEP is compared to a predetermined pressure value (PPV) caused by the pilot fuel injected to the cylinder,

- and if the calculated IMEP is within the PPV limits, the pilot injector is deemed to work properly.

[009] Thus the present invention presents another possible way to detect that the pilot injection is working properly, it is done by using the calculated IMEP from each cylinder's cylinder pressure sensor. Using IMEP as control input gives the benefit that only the "work" done by the cylinder is measured, i.e. a signal that is 0 shows that no pilot fuel was injected during the time the pilot injectors were active alone, and by this a failing pilot injector is identified.

[010] According to an embodiment of the invention the IMEP is calculated from each of the engine cylinders. If the calculated IMEP corresponds to the prede- termined pressure value (PPV) of given pilot fuel combustion, the cylinder is deemed to be running on pilot. So when the necessary number of cylinders is running on pilot, the engine may be switched to a main gas fuel mode. Using the present method the previously used temperature stabilization time for pilot injector functionality check is completely eliminated, and the check of the IMEP readings are done in a couple of seconds. Thus especially a large engine like in a power plant solution will gain in the start up time. As the present method is significantly faster than the methods used at the moment, the present invention clearly meets the objectives of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS [01 1 ] I Fig. 1 it is shown a flow chart explaining a part of the engine start procedure.

DETAILLED DESCRIPTION

[012] The present method for starting up a dual fuel internal combustion piston engine, the method comprises at least following steps:

- The engine is rotated by a start assist device, this start assist device may be for example a pneumatic or electric device or system feeding pressurizer air to the engine cylinders.

- A pilot injector is activated and an amount of pilot fuel is injected to a cylinder. The pilot fuel can be diesel oil or other corresponding, preferably liquid form fuel.

- The injected pilot fuel / air mixture is compressed in the cylinder and ignites. This step in conducted at the compression stroke of the cylinder.

- A pressure sensor in the cylinder measures the cylinder pressure. The actual position and type of the pressure sensor is a matter of selection by the engine manufacturer. There are plenty of suitable sensors for the purpose and the exact location depends on the construction of the combustion chamber and the cylinder head.

- An indicated mean effective pressure (IMEP) is calculated from the combustion of the pilot injection. This is in most cases done by the engine control system.

- The calculated IMEP is compared to a predetermined pressure value (PPV) caused by the pilot fuel injected to the cylinder. Again, this is an internal pro- cedure done in the engine control system.

- If the calculated IMEP is within the PPV limits, the pilot injector is deemed to work properly.

[013] The limits of the PPV are determined separately, they are based on engine manufacturer knowledge on the engine in question, how it reacts on cer- tain pilot injection and how much that is supposed to create pressure when converted to IMEP as the IMEP needs to be compared to some reference value, otherwise it does not reveal the failing pilot injector. Preferably the predetermined pressure value (PPV) is determined as a function of amount of the pilot fuel injected to the cylinder. The limit determination also gives understand- ing about the functionality of the pilot injector. Thus according to an embodiment the predetermined pressure value (PPV) is a range value having tolerance, wherein the lower limit is PPV _min and the upper limit is PPV _max . At the lower limit there is a value where the pilot is not injecting enough fuel due to some failure and on the upper limit the amount of pilot fuel is too large or there is some other failure like leakage in a main fuel system. Thus it is preferred that the comparison fulfills the criteria PPV _min < IMEP < PPV _max . Both the lower limit PPVmin and the upper limit PPV _max is determined by the engine manufacturer based on simulations, other theoretical calculations or actual engine experiment results. [014] According to an embodiment the pressure sensor is arranged to measure the cylinder pressure in a continuous manner. Thus the measurement data can be used for example to determine exactly the moment when the pilot injection starts as it can be seen from the pressure curve. Thus even the pilot injection timing can be determined from the pressure sensor measurement. Also it can be seen when actually the compressed fuel / air mixture ignites. [015] In FIG. 1 it is shortly presented as a flow chart the main features of the present method in start up of the engine:

1 . Start up of the engine, initiate the procedure

2. Rotate the crankshaft

3. Activate the pilot injection

4. Compress the fuel / air mixture

5. Measure cylinder pressure at ignition

6. Calculate IMEP

7. Compare PPV _min < IMEP < PPV _max

8. Cylinder running on pilot [016] So if everything goes according to the procedure and the status from the step 7 is TRUE, the engine control system is given a status information that this particular cylinder is now running on pilot, as it is supposed to do. However if that last comparison gives FALSE, the process goes back to step 3 where the pilot is tried to activate again. After the same procedure is repeated with each of the engines cylinders intended to be started up and all are showing the status "Running on Pilot", the engine can start a switch to the main fuel mode -procedure.

[017] As can be noted, this present procedure is not dependent on temperatures to be stabilized or like, just that the pressure level is at the wanted range. The status of "Running on Pilot" can be achieved in minimum of even one cycle of the engine, but in practice a couple of cycles to make sure the running on pilot is stabilized would be the normal procedure before switching to the main gas fuel mode.

[018] As the previously described embodiments present only some of the op- tions, especially the flow chart can be arranged to be performed various sub steps, the above explained is only one example. Thus as evident to those skilled in the art, the invention and its embodiments are not limited to the above-described embodiment examples. Expressions representing the existence of characteristics are non-restrictive such that the description of charac- teristics does not exclude or prerequisite the existence of such other characteristics which are not presented in the independent or dependent claims.