The present invention relates to a method of operating a free-piston engine according to the diesel principle, wherein a combustion room in a cylinder is reduced by displacing a piston thereby compressing air supplied to the cylinder, fuel being injected into the combustion room allowing the mixture of air and fuel to ignite by spontaneous combustion and to expand for returning the piston.

It is known that the moment of injection of the fuel has influence on the ignition and combustion process in the combustion room of a diesel engine. There have been many investigations to optimize combustion conditions and to optimize the combustion progress. The most favourable moment to start the injection of fuel may for instance be judged from the efficiency as it has been found that when the injection is successively latened the efficiency first increases and subsequently decreases.

The object of the invention is to provide a method of operating a free-piston engine according to the diesel principle realizing an improved efficiency. For this purpose, the method according to the invention is characterized in that the main fuel injection is started at such a moment that more than half of the mixture of fuel and air is combusted only after the piston has started to return. During experiments it has surprisingly be found that the optimum moment of injection of the fuel for obtaining a high efficiency is late, even so late that at least half and preferably the greater part of the mixture of fuel and air is only combusted after the top dead centre of the piston, that is after the piston has already started to return. As a result thereof, the combustion pressure in the cylinder will remain low and may have a value, for example, which is similar to the compression end pressure. The fuel should of course still be injected so early that the mixture gets the change to ignite by spontaneous combustion. The method according to the invention has not only a favourable effect on efficiency, but

also the content of contaminants in the exhaust fumes is lowe than in the conventional prior art methods. The low maximum combustion pressure within the cylinder further has a favourable influence on the noise level of the engine. The most optimum combustion conditions are obtained at a certain ignition moment, wherein there is supplied an amount of energ to the compressed mixture which is set within narrow limits. The invention also provides for means enabling the regulation of the amount of supplied energy with an accuracy better than 5 %. For this purpose there are used a.o. pressure sensors within the compression pressure accumulator, and the starting point of the compression stroke is accurately fixed.

The invention will hereafter be elucidated with reference to the drawing showing an embodiment of the invention by way of example.

Fig. 1 is a very schematic longitudinal sectional view of a free-piston engine including a scheme of the hydraulic parts thereof.

Fig. 2 and 3 are a p-V-diagram and p-t-diagram, respectively, belonging to a method of the prior art.

Fig. 4 and 5 are a p-V-diagram and p-t-diagram, respectively, belonging to an exemplary embodiment of the method according to the present invention.

Fig. 1 schematically shows an embodiment of a free- piston engine in which the method according to the invention may be used. This free-piston engine includes a cylinder 1 in which a piston 2 may reciprocate in order to enlarge and reduce a combustion room 3 on one side of the piston which will be referred to later on. As shown in Fig. 1 a rod 4 having plunger elements 5 and 6 is formed on the piston 3, the plunger element 5 mainly serves for displacing the piston 2 to the top dead centre, that is in a direction to reduce the combustion room 3 for compressing combustion air introduced therein, by means of an hydraulic compression device 7. The plunger element 6 serves for converting mechanical energy delivered to the piston 2 and rod 4 af er combustion in the combustion room 3, into hydraulic energy, which takes place by means of a hydraulic working device 8. Since the compression device 7 and the working device 8 do not form part of the present invention,

they will not be further elucidated. It may be noted, however, that there are arranged pressure sensors 16 within the compression pressure accumulator 15 of the compression device 7 in order to enable an accurate regulation of the compression energy supplied to the piston 2.

As further shown in Fig. 1, an air inlet channel 9 including a back pressure valve 10 communicates to the part of the cylinder 1 below the piston 2, while a connecting or scavenging channel 11 can ensure that air from the space below the piston 2 can arrive in the combustion room 3 when the piston 2 moves to the bottom dead centre. Through an outlet channel 12 communicating with the cylinder 1 combustion gasses may be removed from the combustion room 3. A sensor 17 is provided for sensing the bottom dead centre of the piston 2. In a cylinder head 13, limiting the combustion room 3 on the side facing away from the piston 2, there is arranged an injector 14 adapted to inject liquid fuel, such as diesel oil, into the combustion room 3 by atomizing it. Control means 18 control the start and the duration of the fuel injection. The present invention is concerned with the injection of the fuel by this injector 14, and in particular with the time it happens.

This is explained with reference to measurements on a free-piston engine having the following relevant data: piston weight: 11.2 kg minimum oscillation time: ca. 70 ms piston diameter: 104 mm piston stroke: 165 mm compression volume: 1.02 dm (volume of combustion room when the outlet channel closes) compression energy input: 650-850 J

Figs. 2 and 3 illustrate by means of a p-V and p-t- diagram the progress of the pressure within the combustion space 3 when fuel is injected in the conventional manner. Through the lower curve in the p-V-diagram it can be recognized that, during the compression stroke of the piston 2 when consequently the combustion room 3 is reduced by displacement of the piston 2 in the direction to the cylinder head 3 and hence the air supplied to the combustion room 3 is

compressed, the volume decreases and the pressure increases, firstly linear and then after ignition of the mixture in a very rapid progressive manner to a high pressure, whereafter the pressure decreases again according to the upper curve in the diagram by increase of the volume as a result of the expansion stroke of the piston 2. It is shown that the pressure within the combustion room 3 rises almost up to 100 bar. This is also shown in the p-t-diagram, in which the pressure is depicted as a function of the time. It can be see that substantially 36.3 ms after the bottom dead centre, that is 36.5 ms after start of the compression stroke of the pisto 2, fuel is injected into the combustion room 3 through the injector 14, whereafter an ignition of the fuel-air mixture takes place while the piston is still making its compression stroke. As a result the pressure rises up to said very high value before the piston starts again to return in its expansion stroke. This high combustion pressure puts of cours a high load on the parts, this high pressure also leading to high NOx-content of the exhaust fumes and a high noise level. Figs. 4 and 5 show the measurements on a free-piston engine when the fuel is injected in accordance with an exemplary embodiment according to the invention. When Figs. 3 and 5 are compared, it can be seen that the fuel injection according to the invention does not start ca. 36.5 ms but ca. 39 ms after the beginning of the compression stroke of the piston 2. The result thereof is clearly shown by comparison with Fig. 2 and 3, namely that the ignition of the fuel-air mixture only takes place after the piston has already started its expansion stroke. In Fig. 4 this can be recognized by the curve of the expansion stroke first returning via the curve of the compression stroke before the pressure rises again due to the ignition of the fuel-air mixture. The combustion pressure caused thereby is much lower than in Fig. 2 and 3. This can also be recognized clearly in Fig. 5, where the pressure, as a function of the time, first falls back after 40 ms because the volume of the combustion room is increasing as a result of the starting expansion stroke of the piston 2 and only after this drop of the pressure a rise of the pressure occurs as a consequence of the ignition and combustion of the fuel-air mixture. The maximum combustion pressure is of the order of

magnitude of the compression end pressure. As mentioned, the late ignition of the fuel-air mixture is a result of the fact that the injection of fuel into the combustion room 3 by the injector 14 starts at a later moment, which moment is approximately 39 ms after the bottom dead centre. The duration of the injection is, however, equal to that of the embodiment according to Fig. 2 and 3.

The method according to the invention does not only lead to an improvement in the efficiency of the combustion (judged from the area between the curves going back and forth in the p-V-diagrams and the area below the curve in the p-t- diagrams) , but also to a lower NOx-emission and a lower noise level of the internal combustion engine. It is noted that if one drafts a graph of the induced efficiency as a function of the injection time and the supplied compression energy, the position of the optimum curve is influenced by the variable factors as the ambient temperature, the engine temperature, the amount of fuel injected, the quality of the fuel (cetane rating) , the stroke frequency, the pre-injection, and engine characteristics as piston weight, stroke/diameter ratio, piston displacement and charging pressure. Tests have shown that a significant optimum in the efficiency diagram occurs if the fuel injection is started at such a moment and so much compression energy is supplied to the piston that the ignition just starts if the piston is in its top dead centre. The regulation of the compression energy within narrow limits appears to be an important theme for optimizing the engine. The known measures should be taken such that this regulation can take place with an accuracy of + 5 % under operational conditions.

The invention is not restricted to the embodiment shown in the drawing and described above by way of example, which may be varied in different manners within the scope of the invention. For example, the invention may also be used in free-piston engines having two opposed pistons and the combustion room hence being limited on two sides by a piston. Furthermore, it is noted that when, for example, pre-injection of fuel is used, the ignition of the fuel-air mixture may start before the piston is in its top dead centre, but at least half and preferably the greater part (for example 80 to

90 %) of the combustion will still occur after the top dead centre of the piston. Also in that case the favourable effect on efficiency and emission described above will occur.