WO2016018184A1

WO2016018184A1 - Method at a 2-stroke engine, and a 2-stroke engine operating according to said method

Info

Publication number: WO2016018184A1
Application number: PCT/SE2015/000049
Authority: WO
Inventors: Mats Hedman
Original assignee: Ase Alternative Solar Energy Engine Ab; Perfecter Ab
Priority date: 2014-07-26
Filing date: 2015-07-26
Publication date: 2016-02-04
Also published as: EP3212910A1

Abstract

The present invention relates to a procedure for a 2-stroke engine comprising at least one cylinder (1) with a reciprocating piston (2), a defined combustion or expansion chamber (5), at least an outlet port (7) which is exposed at the piston lower dead centre, at least one freely controllable valve actuator (8) which activates an inlet valve (17) for opening/closing and supplying combustion air through an inlet pipe (6), a control system (15) controlling the valve actuator to open or close the inlet valve for the supply of combustion air. The invention is characterized in that the inlet valve (17) closes after the outlet port (7) is closed.

Description

Method at a 2 -stroke engine, and a 2 -stroke engine operating according to said method

The present invention relates to a method at a 2-stroke engine and a 2 -stroke engine operating according to said method.

The present invention relates to a new 2- stroke engine, which basically operates as an ordinary 2- stroke engine. In addition to working with internal combustion, however, this 2- stroke engine occasionally even work via an externally created pressure media introduced in and allowed to act as an expansion stroke, and then causes an internal cooling effect on the piston and cylinder.

Background

A plunger moves between two turning positions, an upper and a lower turning position. At the upper turning position, a compression chamber will pass into the combustion or expansion chamber, which room is created by the moving piston and the cylinder head. The piston movement from the lower to the upper turning position normally means a compression stroke and its subsequent movement from top to down position means a work or expansion stroke. After the work stroke is carried out gas exchange is carried out. Exhaust gas is removed and new combustion air is supplied and a new compression stroke follows. This 2- stroke process is the main working principle of the engine.

In a conventional, modern2-stroke engines the gas exchange is carried out at the end of the power stroke by first exposing an outlet port, then the exhaust gas escape, often pulse-like because of the excess pressure, and then an inlet port is exposed and new combustion air is supplied and the compression stroke starts. A major drawback is the uncleaned exhaust gases caused by the combustion of air and fuel, and/or lubricating oil to some extent, which is exhausted via the outlet port to the surrounding atmosphere. This is because the ports for the inlet and the outlet are overlapping opened simultaneously and that the outlet port closes after the inlet port.

With freely operable valves in the cylinder head, plate valves as in today's four stroke engines, the problems with uncleaned exhaust gases can be reduced to a large extend. The engines presented in Patents SE529569 and SE529570 have free operated, electromagnetically, hydraulically or pneumatically activated valves in the cylinder head in order to achieve a 2-stroke engine with higher efficiency and lower emissions. SE529569 indicates that the inlet and/or the outlet is preferably provided with freely operated valve, but could alternatively comprise only one port, which is opened and closed by the passing piston. The gate is arranged in the cylinder wall, preferably in the region of piston bottom dead centre. SE529570 has freely operable outlet valve in the cylinder head and may have an intake port that opens at the piston lower dead centre.

Free controllable pneumatically activated valves, which are preferred, are opened by means of pressurized gas, preferably air, but also water vapour can be used. The air is compressed by a compressor to a pressure, which is adapted to stably open the valves. However, the energy needed to compress the air, the higher the pressure the more energy is consumed, and thus the aim is a driving pressure, which, with some margin always and certainly exceeds the pressure just required to open the valves. However, problems can arise when large motor load and accompanying high pressure when gas exchange is to take place adjacent to the piston bottom dead centre. At throttle activation cylinder pressure can extremely quickly increase from around atmospheric pressure or a few bar to perhaps ten, twenty bar, or more. An outlet valve must be able to safely open against the current cylinder pressure, which automatically occurs compulsively in the camshaft operated valve systems, as in today's four-stroke engines and some larger two-stroke engines. Should the opening not be performed a great risk of engine failure may occur. The pressure in the air/gas used for said freely controllable valves must be provided when the need arises, which is a problem regarding pneumatically activated outlet valves at rapidly emerging high cylinder pressures. However, other methods for freely

controllable valves, such as hydraulically or electromagnetically actuated valves, the corresponding need for adapting the drive energy to meet the current cylinder pressure.

Mentioned problems with the adaptation of the outlet valve driving energy to the cylinder pressure is dissolved in this proposed 2-stroke engine by the exhaust gases are evacuated via at least one exhaust port, which in today's two-stroke engine and that the combustion air and fuel in a preferred embodiment is so late introduced in the compression stroke that nothing will escape by the outlet port, and/or introduced only when the exhaust port is closed early in the compression stroke.

At least one outlet port which is uncovered in connection to the piston bottom dead centre is arranged, so that high cylinder pressure is no longer a problem at the same time as a combination with at least one freely operable, particularly pneumatically activated valve in the cylinder head, results in advantages.

The present invention also admits that a steam expansion stroke can be carried out so there is no compression during the compression stroke, but instead cylinder gas remaining is evacuated before a power stroke based on expanding steam for example is implemented.

Additionally, the engine according to the invention may operate in 4-stroke as today's 4-stroke engines without camshaft and with additional differences and control benefits to be explained below.

The main object of the present invention is to provide an engine technology that is substantially more fuel efficient and more versatile compared to engine technology of to day.

Another object of the invention is to provide an engine technology that can be regulated to give a good fuel economy and low emissions also at part loads.

Another object of the invention is to provide an engine technology where the amount of re-supplied residual gases in the compression stroke can be detail regulated towards a minimum of NOx in the exhaust gas discharged after the combustion stroke.

Another object of the invention to provide an engine technology that also allows expansions cycles with external pressure gas for optimal overall fuel efficiency.

The above objects of the present invention are achieved by the scope of protection as is defined by the claims followed by the description. By having at least one outlet port being uncovered by the piston in its bottom dead centre and at least one freely operable inlet valve in the cylinder head, several facilities and advantages are obtained for drive with low emissions, maximum fuel efficiency, and recirculation of residual gas in the intake air or boost air.

By opening the inlet valve sharp and short before the exhaust port closes at the pistons way to the top dead centre, and closing the same after the outlet port has been closed, it is possible to have an optimum of supercharged air with or without fuel. The fuel can be fed from an injection system directly into the cylinder or into the intake manifold. In a preferred embodiment, there are several small discharge ports, with the result of lower port height compared to a single large outlet port and with a necessary aggregate exit area. This also means that the effective displacement volume for a given cylinder volume is increased and the work rate is significantly longer than existing 2-stroke engine and that more work is performed with increased efficiency as a result. The work stroke can be made longer than the compression stroke, because of the freely operable inlet valve, which is an advantage in so-called quantitative combustion.

Multiple small ports with a smaller height with retained or increased total outlet area means that the outlet is uncovered in a significantly shorter time than is the case in a conventional 2-stroke engine, which also leads to a significantly increased potential for exploitation of the under-pressure created in the cylinder when the exhaust gas pulse-like flows out and thus remains and lasts when the outlet is closed and can be utilized as the inlet opening for the supply of combustion air.

Other advantages are that the exhaust pulses from the engine can drive a turbocharger and thus the engine can be supercharged, which is difficult to implement in 2-stroke engines with ports for both inlet and outlet that to an extend are open simultaneously, and where the outlet is closed after the inlet, so that part of the pressurized air from the inlet do not stay in the cylinder but at least partly flows out through the outlet. According to the invention it is possible to provide the cylinder head, as mentioned above, with an extra free pneumatically activated valve for admitting high pressure steam produced in a heat exchanger in the exhaust system by the boiling of the water. When the vapour pressure is suitably high it may replace a regular work stroke by a steam expansion stroke after decision of the control system. When the control system has decided to carried out a steam expansion stroke the at least one exhaust valve opens and is kept open during the piston movement to the upper dead centre where it is closed, and where the steam valve is opened for supply of high pressure steam and a steam expansion stroke begins. The control system decides when the steam valve will close and the steam expansion stroke is controlled by the control system to perform as much work as at the previous work stroke performed. Introduction of steam expansion strokes, which can be performed more and more often at lower engine loads and at reduce efficiency, the exhaust heat share of the added energy increases, which increases the efficiency significantly. After a steam expansion stroke combustion air and fuel are introduced as usual, as specified below.

When the engine is warm, the pneumatic valve can as mentioned, be activated by pressurized steam instead of pressurized air, which increases the overall engine efficiency.

By the opening of the intake valve is time controlled it is possible to decide that no more combustion air is supplied so that an appropriate amount of residual gases remain in the cylinder to minimize NOx. This is synchronized with the piston closing the outlet port. The above-mentioned precise control of the combustion air thus provides the opportunity for controlling residual gases, so called EGR. The pressure existing in the cylinder after closing the outlet can at a particular engine load and engine speed is e.g. 0.5 bar. If then combustion air is supplied to the cylinder so the pressure is 2 bar when the intake valve closes the residual gases from the previous work stroke will be around 10-12%, depending on the mix temperature that occur between the residual gas and supplied combustion air. Said percentage residual gases has proven to be an appropriate level to minimize NOx. At other engine loads, the cylinder pressure is not as in the example above and the aforementioned conditions are different. Exhaust gases may have time to bounce back into the cylinder via the exhaust port before the piston closes said port as well as new combustion air can be supplied before said closing and be able to push exhaust gases out through the outlet port before the piston closes said port. Through control of the inlet valve the amount of residual gas in the cylinder at different engine loads and engine speeds can thus be controlled to minimize NOx.

An oxygen detector may advantageously be provided in the exhaust passage wherein the opening and closing of the inlet valve can be time controlled so that minimal combustion air flows out of the cylinder. At least one freely operable valve is disposed in the cylinder head and when it opens it will take one or a few ms, depending on engine size, before the combustion air has flowed up to the at least one outlet port. With a freely controllable inlet valve, combustion air can therefore be applied starting before the port of the outlet is closed, but it should not be so early that a substantial part of the combustion air flows out. A very precise control of the process for optimal supply of combustion air is made possible by the oxygen sensor outside the exhaust port in the exhaust pipe. Should an inappropriate level of oxygen be indicated the inlet valve is controlled to open later next time the combustion air is supplied. This may prevent new combustion air containing lubricating oil residues and any fuel from coming out through the exhaust port. The inlet valve closes after the outlet valve is closed while the opposite prevails in more conventional 2-stroke engines, which allows the advantageous use of exhaust turbocharger.

In case you wish to run the engine in 4-stroke the intake valve is kept open when the piston is going up and the residual gases not evacuated via the lower exhaust ports is discharged via the open inlet valve which then closes when the piston is adjacent to its upper dead centre and again opens when a certain low pressure is created in the cylinder and fuel and air are introduced. In this context it may be mentioned that it is possible to organize a special outlet valve connected to the cylinder head for 4-stroke operation or to run a steam expansion cycle. A controllable valve actuator, one of this controlled inlet valve for the inlet of pressurized steam in the expansion chamber, one to the control system connected pressure sensor for recording the pressure in the steam from a heat exchanger arranged in exhaust system of the engine is advantageously provided for the steam expansion cycle in such a way that when a steam expansion stroke is to be performed the valve actuator is activated for admission of steam into the expansion chamber.

A valve actuator with associated outlet valve for evacuation of cylinder gas, steam or combustion gas, is useful when starting the engine in 4-stroke, whereupon the operation, depending on the power output can be transferred into 2-stroke or continue in 4-stroke.

At high power output in 4-stroke operation, the exhaust gas will be largely evacuated through the lower outlet port, and the remaining exhaust gas through the upper outlet valve.

By the piston forming part of a slide valve arrangement opening the outlet port in close connection to the bottom dead centre of the piston while the inlet port is closed, the first step in an effective dispensing of the exhaust gases at 2-stroke operation is ensured. When the piston turns and moves upwards, it will come to a position where the discharge port begin to be closed and in this position, at least a valve actuator begin to open at least one inlet valve for air and maybe for fuel mixture (at 2-stroke operation] and the possible discharge of residual gases (in 4-stroke operation).

In a developed embodiment of the two-stroke engine according to the invention steam expansion strokes are allowed where further at least one controllable inlet valve is disposed in the cylinder head and at least one free operable outlet valve is provided in the cylinder head for evacuating steam, and, at a new 4-stroke process, also for the evacuation of exhaust gases. Said free controllable valves can be electromagnetically, hydraulically or pneumatically activated.

Summary of figures Fig. 1 shows schematically a section of the engine cylinder with its piston at top dead centre after a compression stroke.

Fig. 2 show the cylinder of Fig. 1 with the piston at the bottom dead centre.

Fig. 3 show the cylinder of Fig. 1 with the outlet port just closed and the inlet valve just opened at the beginning of the compression stroke.

Fig. 4 show the cylinder of Fig. 1 where the piston during the compression stroke is keeping the outlet port closed while the inlet valve is closed.

Fig. 5 show the cylinder of Figure 1 and where a valve actuator with associated outlet valve is added.

Fig. 6 show the exhaust system with heat exchanger for boiling water and with a heat exchanger where exhaust gases are cooled.

Fig. 1 shows a 2-stroke engine according to the invention comprising at least one cylinder 1 with a cylinder head 11, a reciprocating piston 2 in the cylinder covering the exhaust port 7 and the inlet valve 17 is closed. A combustion and expansion stroke is to be initiated. A flywheel 3 is mounted on a crankshaft 4 on which a piston rod for said piston is disposed in usual manner. Between the piston and the cylinder head there is a combustion or expansion chamber 5 to which combustion air is supplied, via an inlet valve 17 controlled to open and close by a valve actuator 8, from an intake pipe 6. On the crankshaft there is a sensor or a crankshaft indicator 19 the signals of which are read by an engine control system 15 thus registering the position of the piston in the cylinder and, depending on the need of torque/power, orders the valve actuator to open and close the valve at the correct time. The piston is shown in its upper turning position where it covers an outlet port 7. The exhaust pipe has an oxygen detector 22 and the signals of which are read by the engine control system 15 which is used to control the valve actuator 8 to keep the inlet valve 17 closed so that the combustion air has time to flow out of the cylinder prior to the piston closes the outlet port. It should be mentioned that in some cases, the oxygen sensor may not be necessary as the smaller engines for e.g. chain saws, which either go idle or full throttle. This is valid on engines already factory-set so that combustion air does not have time to escape through the exhaust port before the piston closes this. Possibly spark plugs 16 are arranged for engines where fuel will be ignited by a spark ordered by the engine control system at the optimum time with regard to motor efficiency. At the diesel engine application 16 represents an injectors for diesel fuel. This illustrated embodiment is suitable for smaller engines as garden machinery, power saws, motorcycles, etc. But it should be noted that the figure shows in principle what is required for application of the invention.

Fig. 2 shows the lower dead centre of the piston in the cylinder where an outlet port 7 are uncovered, wherein the pressurized exhaust gases, typically pulselike, flow out through said port to an exhaust system. If there exists atmospheric pressure in the exhaust system and cylinder pressure is more than 2 bar when the port is uncovered exhaust gases initially flow at sonic speed, so-called critical flow and there arises momentarily a vacuum in the cylinder. Exhaust gases can drive an exhaust gas turbocharger with which the motor can be recharged, which normally do not work well in a conventional 2-stroke engine.

Fig. 3 show how the piston during the compression stroke start just closes an outlet port 7, wherein said pressure in the cylinder usually will persist. The valve actuator 8 is commanded by the engine control system to open the inlet valve 17, wherein the combustion air with or without fuel is supplied.

Fig. 4 illustrate a later stage during the compression stroke where the intake valve is closed and the piston just reaches its top dead centre and in connection to which combustion begins.

Fig. 5 show a valve actuator 9 with associated outlet valve 18 via which cylinder gas, steam or combustion gas is evacuated. The outlet valve will cause the engine to advantageously start in 4-stroke after which the operation may be turned to 2- stroke. 4-stroke operation can be compared with the possibility of an extra gear when the engine speed can be doubled in comparison with 2-stroke operation with about the same power output. At high power output in relation to the 4- stroke operation the exhaust gases will to a great extend be evacuated via the outlet port 7 and the remaining exhaust gases through the exhaust valve 18. At engine load where the cylinder pressure is less than 1 bar at the bottom dead centre of the piston, 4-stroke operation could be an attractive option.

Fig. 6 shows the exhaust system 12 with heat exchangers 13 for boiling water in which the exhaust gas is as hottest and with a heat exchanger 14 where the exhaust gases are cooled for condensing out water. A valve actuator 10 with an inlet valve 20 for supplying vapour is introduced. The valve actuator 9 of figure 5 is not visible because it is behind the actuator 10.

That the condensed water is fed to the heat exchanger for boiling, or that the exhaust system is insulated to keep the exhaust heat prior to boiling or non- insulated and cooled for said condensation is not displayed but are considered to be self-evident for a man skilled in the art. A freely controllable inlet valve 20 activated by steam pressure is provided and connected to the control system 15 via a valve actuator 10, as well as a pressure sensor 21 also connected to the control system. The control system may decide by the signals from the pressure sensor when a routine work rate is replaced by a steam expansion stroke.

A preferred embodiment of the invention, regarding supply of fuel that is ignited but a spark (methanol, ethanol, hydrogen, gas, gasoline, etc) will be described. If the inlet valve 17 is opened just before or when the outlet port has closed, and when a vacuum exists above the piston in the cylinder, maximum of air is supplied to the combustion for maximum torque if the inlet valve immediately closes after the exhaust port is closed. At spark ignited engines is the mass of combustion air is either regulated by air damper/throttle or by closing the inlet valve 6 is closed during the compression stroke when the combustion air mass corresponds to the mass to be combusted with fuel, so called quantitative combustion. If combustion air is supplied via the throttle, the intake valve opens momentarily when the piston is closing the outlet port. At a diesel engine the maximum of air is supplied when the outlet port is closed. It must be performed prior to said closing as air in the exhaust pipe is not considered harmful to a diesel process, and further, adjacent to the upper dead centre as much diesel as the engine control system decided in dependence on the need of torque, so called qualitative combustion. It should be noted that fuels when combusted deliver a lot of water, such as methanol and hydrogen, is to be preferred for high pressure steam to be boiled on exhaust heat, because the rich content of steam in the exhaust gas simplifies a water condensing process and also provides cleaner emissions by collecting particles by the so-called scrubbing effect in the condensed water to be filtered out before the water is boiled. When fuels contain some hydrogen a tank of distilled water can be used and to a large extend can be regained by said mentioned method.

Examples of driving cycles - Starting a cold engine:

An engine with outlet valve is started using e.g. a starter engine in the 4-stroke mode and crank house compressed air can drive the valve actuator. The crankshaft can also be rotated by a starter, or manually via a so-called kick- starter, wherein the piston moves from the top dead centre to the bottom dead centre while the inlet valve is closed. When displacing the piston a low pressure is established in the cylinder above the piston while the crankcase air is compressed. The actuator is by the control system trigged to enable the valve opening using crank house compressed air prior to the outlet port is to be exposed by the piston and this port opening is facilitated by said low pressure. Combustion air possibly including fuel is supplied through the inlet after which the compression and ignition takes place. If the piston is in an inappropriate position, for example near the bottom of the cylinder to perform said process, preferably a so-called manual decompression valve is used so that the piston can be brought to, or near top dead centre, whereupon said process is implemented. If fuel is direct injected an alternative embodiment may be that the inlet valve is kept closed and when the piston uncovers the outlet port, air flows, optionally containing residual exhaust gases into the cylinder from the cylinder exhaust port. Presumably then also sufficient oxidation air exists for ignition and consequent engine start.

Idling:

At an engine with outlet valve idling takes place in 4-stroke and e.g. crank house compressed air drives the actuator. Otherwise the flywheel mass will have enough energy during the power stroke to drive the piston between its end positions. While at the end of the work stroke out flowing exhaust gases create a certain low pressure in the cylinder by the outlet and opens briefly the inlet valve for supply of combustion air and fuel, which are ignited after compression. By remaining residual gases mixed with the combustion air an inhibition of the formation of NOx is achieved. The time for the brief opening should not be so long that the combustion air has time to escape through the exhaust port before being closed by the piston at the beginning of the compression stroke.

Moderate engine load: The engine runs in 2-stroke at about half of its maximum speed, e.g. 1500 rpm, and the turbo is working so that a certain pressure in the combustion air prevails. The turbo gives a surplus of air supplied to the crank house and increases the crank house pressure for the benefit of the activation of the opening of the inlet valve. Of course, some and/or all of the pressure air are to be used both to the opening of the inlet valve and to supercharge the combustion air. After the stroke the opening of the intake valve occurs just before the piston closing the outlet port and the combustion air and fuel is supplied during the compression stroke. The control system determines, dependent on the demanded torque, in which piston position the inlet is to be closed and thus when the supplied combustion air starts to be compressed. The working strokes will be longer than the compression strokes, which can be said to correspond to a so-called late Miller cycle in a 4-stroke engine.

High engine load: The engine operates at or near its maximum speed and exhaust gas turbocharger produce air with higher air pressure than before to combustion and crank house compression. After the work stroke the inlet valve is opened when, or just before, the piston is closing the outlet, wherein the pulse of out flowing exhaust gases have created a substantial low pressure in the cylinder and combustion air, optionally under pressure and optionally with fuel is supplied early in the compression stroke.

An extra gear with the doubling of the engine speed is made possible if the engine as mentioned before is equipped with actuator activated outlet valve. Large motor load is possible without a big drive pressure to the actuator is needed in that exhaust gas of high pressure is evacuated through the exhaust port and the residual gas via the outlet valve in the cylinder head. Possibly unclean exhaust gas can be scrubbed clean through utilization of steam expansion strokes.

Steam expansion strokes: When the control system have decided that a steam expansion stroke will be implemented the at least one inlet valve, or a separate exhaust valve opens normally after the power stroke and is open during the piston coming to an upper dead end where it is closed. Then the steam valve is opened for supply of high-pressure steam and a steam expansion stroke is initiated. Occasionally, more than one steam expansion stroke can be sequentially performed. When a combustion stroke again shall be conducted it takes place according to any of the methods described above.

The invention is not limited to the embodiments described above but the invention can be applied in the context of what is stated in the following appended claims.

Claims

1. Procedure for a 2-stroke engine comprising at least one cylinder (1] with a reciprocating piston [2), a defined combustion or expansion chamber (5), at least an outlet port (7) which is exposed at the piston lower dead centre, at least one freely controllable valve actuator (8] which activates an inlet valve (17] for opening/closing and supplying combustion air through an inlet pipe (6], a control system (15) controlling the valve actuator to open or close the inlet valve for the supply of combustion air characterized in that the inlet valve (17) closes after the outlet port (7) is closed.

2. The method according to claim 1, characterized in that the opening of the intake valve is controlled in time so that a suitable mass of residual gases for minimizing nitrogen oxides (NOx) are retained in the cylinder (1) before the piston (2) closes the outlet port (7).

3. The method according to claim 1 comprising an oxygen sensor (22),

characterized in that the opening of the inlet valve (17) is controlled in time so that the combustion air is prevented from flowing out of the cylinder (1).

4. The method according to claim 3, also comprising a controllable valve actuator (10), one of this actuator controlled inlet valve (20) for admitting pressurized steam in the expansion chamber (5), a pressure sensor (21) connected to the control system (15) for recording the pressure in the steam from a heat exchanger (13) arranged in an exhaust system (12) connected to the motor, characterized in that when a steam expansion stroke will be performed the valve actuator (10) is activated for admission of steam into the expansion chamber (5) to perform a work stroke.

5. The method according to any preceding claims, which also comprising a valve actuator (9) with associated outlet valve (18) for evacuating of cylinder gas, steam or combustion gas, characterized in that the engine is started in 4-stroke, then the operation passes to 2-stroke, or continues in 4-stroke.

6. The method according to claim 5, characterized in that at high power output in connection to 4-stroke operation, the exhaust gas that will largely be evacuated by the outlet port (7) and remaining exhaust gas through the outlet valve (18).

7. A 2-stroke engine for performing the method of claim 1 comprising;

at least one cylinder (1) and therein a reciprocating piston (2) defining a combustion or expansion chamber (5), the lower part of which is provided with at least one outlet port (7), and at least one freely operable valve actuator (8) in the cylinder head for the control of an inlet valve (17) through which combustion air is supplied via an inlet channel (6) in the cylinder (1) top (11), and with a control system (15) comprising sensors and software controlling the valve actuator (8) to open and close the inlet channel (6), characterized in that the piston (2) forms part of a slide valve arrangement which opens only the outlet port (-ports) (7) prior to the lower dead centre of the piston (2).

8. An engine according to claim 7, characterized in that the inlet valve (17), or an additional outlet valve (18) is provided and can be used for the outlet of residual gases at 4-stroke operation.

9. An engine according to claim 7, characterized by a controllable valve actuator (10), one of this actuator controlled inlet valve (20), one pressure sensor (21) connected to a control system (15) for registration of the pressure of steam from a heat exchanger (13) arranged in an exhaust gas system (12) connected to the engine, wherein the valve actuator (10) actuates the valve (20) for admission of steam into the expansion chamber (5).