The invention relates to a method of controlling a large two-stroke turbocharged internal combustion engine having a reactor for reduction of the NO _x content of the exhaust gas connected upstream of the turbo¬ charger.

Large two-stroke internal combustion engines are typically used as propulsion engines in ships. In the latest years, a few ships have been equipped on a trial basis with a reactor which removes by far the major part of the NO _x content of the exhaust gasses of the engine. This known engine has an exhaust gas receiver which is connected on the discharge side with a reactor for reduction of the N0 _X content of the exhaust gas, a turbocharger with a turbine, the gas inlet of which is connected with the gas outlet of the reactor, a reactor by-pass conduit which may connect the exhaust gas receiver with the turbine of the turbocharger, a cutting-off means in the by-pass conduit, and an auxiliary blower for delivery of scavenging and charging air.

Threshold values for emission of N0 _X from ships apply so far only in a few near-shore waters. By means of the by-pass conduit with the cutting-off means, the known engine plant is designed so that the reactor is completely disconnected during the main part of the voyage of the ship, and complete connected up when the ship is sailing in near-shore waters where upper threshold values for emission of NO _x have been adopted. The cutting-off means in the by-pass conduit is thus either completely open or completely closed.

In consideration of environmental protection it must be expected that an increasing number of countries will adopt rules fixing still lower threshold values for emission of N0 _X . It may therefore be anticipated that large ship's engines have to run continuously with exhaust gas purification plants. The catalysts known from the automobile industry cannot be used for ship's engines as the latter are operated with a large air excess and burn heavy fuel oil containing a deal of heavy metals and sulphur. Upstream of the turbine, turbocharged cars may be provided with a catalyst which makes CO and HC in the exhaust gas react with 0 ₂ with simultaneous formation of heat, so that the exhaust gas is heated during the passage of the reactor. Thus, the operating conditions of these engines are essentially different from the conditions of an engine with a reactor for reduction of the N0 _X content of the exhaust gas.

It has proved possible to reduce the N0 _X content by means of the so-called "selective catalytic reduction process" (SCR) where the exhaust gas is mixed with ammonia and passed through a special catalyst at a temperature of between 300 and 400°C. The catalyst reduces N0 _X into N ₂ and water. The catalyst volume has to be rather large to ensure that the ammonia is spent, and that the N0 _X content of the exhaust gas is reduced to a sufficient extent.

The large catalyst volume leads to the reactor having a large heat capacity, which has rendered it necessary to completely disconnect the reactor if the engine load suddenly has to be increased, for example at emergency braking of the ship, as the reactor cools the exhaust gas when the load is increased.

The object of the present invention is to provide a method of controlling a large two-stroke turbocharged

internal combustion engine in such a manner that the engine is ensured a normal operating state as far as possible, including also purification of the largest possible amount of exhaust gas. With a view to achieving this object, the method according to the invention is characterized in that at least one engine parameter is measured to determine whether the reactor cools the exhaust gas, and that supplementary air or gas is supplied upstream of the turbine of the turbocharger when the engine parameter measured exceeds a predetermined threshold value.

As the exhaust gas has to have a temperature of at least 300° when it is supplied to the reactor, the latter necessarily has to be connected to the exhaust system before the turbocharger. When the engine load is increased, the temperature of the exhaust gas rises, which leads to a heating up of the reactor. As the reactor has a large heat capacity, the heating of the reactor to the higher temperature may take a long time, and during this period the temperature of the exhaust gas after the reactor will be somewhat lower than the temperature before the reactor, which means that the energy content of the exhaust gas after the reactor is not in itself sufficiently large for the compressor of the turbocharger to receive power corresponding to the instantaneous engine load. In other words, the large heat capacity of the reactor and its connection upstream of the turbocharger have the effect that there is a certain time delay in the influence on the turbocharger from increasing engine loads, and the delay increases with the change in the load.

By determining according to the invention when the reactor is heated with simultaneous cooling of the exhaust gas, and during these periods supplying supple- mentary air or gas to the turbocharger, it is possible

to ensure that the engine receives an amount of scaveng¬ ing and charging air which correctly matches the engine load, also in the period immediately following a change of the engine load. To ensure the highest possible purification of the exhaust gas, the supplementary air may suitably be supplied as charging air by means of an auxiliary blower which is able to deliver air at engine loads of above 55 per cent. The previously known auxiliary blowers turn off at engine loads of about 50 per cent. By dimension¬ ing the blower for larger loads it is possible to obtain the result, partly that the auxiliary blower has sufficient capacity at the starting of the engine to compensate for the heating up of the reactor, partly that the auxiliary blower may be active during manoeu¬ vring, when the engine load often exceeds 50 per cent. In an environmentally optimum design,—the engine is optimized and controlled so that part of the exhaust gas downstream of the reactor is by-passed the turbocharger when the engine parameter measured is below the threshold value, and is passed fully or partially through the turbocharger when the engine parameter measured exceeds the threshold value. Such a control, however, requires high efficiency of the turbocharger. If this is not the case, or if the method is to be used for an existing engine without replacing the turbo¬ charger, it is also possible to control the engine so that part of the exhaust gas downstream of the exhaust valves is by-passed the reactor and passed directly to the turbocharger when the engine parameter measured exceeds the threshold value. In this case, N0 _X is not removed in the reactor from the diverted part of the exhaust gas.

The determination of the point of operation at which the supplementary air/gas is to be supplied, may

take place based on registration of the instantaneous load of the engine and comparison with values known from experience for how fast the load may be changed without supply of supplementary air/gas. Alternatively, said point of operation may be determined by the temperature difference of the exhaust gas over the reactor exceeding a predetermined value.

The invention further relates to a turbocharged internal combustion engine for use in the above method, which engine comprises an exhaust gas receiver which is connected on the output side with a reactor for reduc¬ tion of the N0 _X content of the exhaust gas, a turbocharger with a turbine the gas inlet of which is connected to the gas outlet of the reactor, and an auxiliary blower for supply of scavenging and charging air. According to the invention, this engine is charac¬ terized by a by-pass conduit which may connect the exhaust passage between the reactor and the turbine of the turbocharger with the exhaust passage downstream of said turbine, a control means which may fully or partially cut off the by-pass conduit, at least one sensor for measurement of an engine parameter, and a control unit for control of the control means in dependency of signals received from the sensor. If it is desired to modify one of the known engines mentioned in the introduction, where the reactor may be either completely disconnected or connected up, so that the engine may be run in accordance with the method of the invention, the engine has to be supplemented with at least one sensor for measurement of an engine parameter and a control unit which is able to control the cutting-off means in dependency of signals received from the sensor, to an at least partially open position, where a normally small part of the exhaust gas is by- passed the reactor and passed directly to the turbocharger.

Examples of the invention will now be described in further detail with reference to the very schematic drawings, in which

Fig. 1 shows a diagram of a first embodiment of an internal combustion engine according to the invention, and

Fig. 2 is a correspondingly schematic diagram of another embodiment according to the invention.

Fig. 1 shows a large two-stroke internal combustion engine which may, for example, yield a power of 40 MW. The engine is supplied with air via a compressor C of a turbocharger.and possibly also via an auxiliary blower AB.which delivers the air to a charging air cooler, not shown, from where the air flows on up to the combustion chambers and further out through the exhaust valves to the exhaust gas receiver ER, which is connected via an exhaust passage 1 with a reactor R, which may remove more than 90 per cent of the N0 _X content of the exhaust gas. From the outlet of the reactor, an exhaust passage 2 leads to the inlet in the turbine part T of the turbocharger, the outlet of which communicates with the open air via an exhaust passage 3.

A by-pass conduit 4 extends from the exhaust passage 2 upstream of the turbine T to the exhaust passage 3 downstream of said turbine. The by-pass conduit contains a control means V which is adjustable continuously from a fully open to a fully closed position.

The turbocharger T/C has such high efficiency that in case of stationary operation of the engine it may deliver a surplus of power. As the turbocharger thus does not require all the exhaust gas in order to deliver the required amount of scavenging and charging air to the engine E, the control means V is adjusted to such a partially open position that the compressor C of the

turbocharger delivers exactly the desired amount of air. The exhaust gas flowing through the by-pass conduit 4 may, for example, be exploited in a power turbine, not shown, although it is also possible, of course, to pass the exhaust gas directly out into the exhaust passage 3. With regard to the efficiency of the turbocharger it may be mentioned that for operation of the engine without a reactor, an efficiency of about η = 64 per cent is required. Today, it is possible to get turbo- chargers which have an efficiency of η = 73 per cent. The reactor R is a so-called SCR reactor manufac¬ tured by the Danish company Haldor Topsøe A/S. In connection with this in itself known reactor, it should merely be mentioned that an engine with a power of 40 MW requires a reactor which contains about 13.5 tonnes of catalyst material. It is possible to use a proportionally smaller amount of catalyst material, but this would entail a larger emission of ammonia together with the purified exhaust gas. As the reactor R has a large heat capacity, an increasing engine load will lead to a cooling of the gas during the passage of the reactor, as mentioned above, and only after some time will the increasing load lead to an increasing energy supply to the turbine T of the turbocharger.

A control unit CU is connected with three sensors SI, S2 and S3 via respective wires 5, 6, and 7. The sensors measure engine parameters (in-service or performance parameters), and on the basis of these engine parameters the control unit determines whether the reactor R is heated to such a degree by the exhaust gas that the turbocharger has to be supplied with supplementary energy in order to be able to deliver the desired amount of air.

As examples of suitable engine parameters may be mentioned that the sensor SI may measure the instan¬ taneous engine load based on the amount of fuel supplied to the working cylinders. The sensor S2 may measure the temperature of the exhaust gas in the exhaust passage upstream of the reactor R, and the sensor S3 may measure the temperature of the exhaust gas immediately upstream of the inlet to the turbine T. Alternatively the engine parameter measured by a sensor may be the pressure in the air/gas system of the engine at a measuring point positioned downstream of the compressor C and upstream of the turbine T.

The control unit CU may determine the need for supplementary air based on only one measured engine parameter which is measured continuously in dependency of time, and the change in time of the engine parameter may then be compared to predetermined threshold values for the time variation of the parameter to determine whether the reactor R takes up so much energy from the exhaust gas that supplementary air/gas has to be supplied. The change in time of the engine load may, for example, be compared in the control unit with predeter¬ mined threshold values for the change in load. A corresponding comparison may also be made on the background of the temperature Tl of the exhaust gas upstream of the reactor R measured by the sensor S2. Alternatively, the control unit may receive temperature signals from both the sensor S2 and the sensor S3, and on the basis of this calculate the difference between the temperature upstream Tl and downstream T2 of the reactor.

If the engine parameter(s) measured exceed(s) the predetermined threshold value, the control unit CU transmits a signal for supply of supplementary air/gas to the engine E. Via the wire 9, this signal may be

transmitted to the motor control of the auxiliary blower AB which either starts or increases the output of the auxiliary blower. Alternatively, via the wire 8 the control unit may transmit the signal to the control means V which is thus adjusted towards a closed position so that a smaller part of the exhaust gas flows through the by-pass conduit 4, and a larger part through the turbine T.

The control unit CU may also be designed to start the auxiliary blower when a first threshold is exceeded, and then to reduce the gas flow through the by-pass conduit 4 when a second threshold value is exceeded. In the embodiment of Fig. 1 the second threshold value will often preferably be lower than the first one, so that the control closes the by-pass conduit 4, before the auxiliary blower is started. If a power turbine is connected to the conduit 4, it may, however, be appro¬ priate to start the auxiliary blower first. The first threshold value may, for example, be set at T2/T1 = 0.5, while the second threshold value may be set at T2/T1 = 0.25.

When the heating of the reactor R is close to being finished, its energy absorption will decrease, and the control unit CU may therefore reduce the supply of supplementary air/gas as the engine regains a stable operational condition.

Fig. 2 shows another embodiment in which elements having the same function as elements in Fig. 1 have been designated with numerals which are 20 higher than the reference numerals of Fig. 1.

The by-pass conduit 24 extends here from the upstream to the downstream side of the reactor. This embodiment works by the control unit CU opening for the control means V when the threshold value is exceeded so that part of the exhaust gas is by-passed the reactor

R and passed directly to the turbine T of the turbocharger. This embodiment is particularly useful for engine plants where the turbocharger has a low effi¬ ciency. Apart from this, the embodiment of Fig. 2 functions in the same manner as the embodiment of Fig. 1. If the embodiment of Fig. 2 works with two threshold values, the lower threshold value will normally prefer¬ ably make the control unit CU start the auxiliary blower, as the emission of unpurified exhaust gas is thus restricted.

In the embodiment where the turbocharger has high efficiency, it is also possible to position the by-pass conduit elsewhere, for example to connect it downstream of the compressor of the turbocharger and connect it with the open air for blowing off excess air or connect it with the turbine inlet of the turbocharger. In both cases, the excess air of the turbocharger will not be passed through the engine, and it is therefore possible to supply supplementary air to the engine by reducing the air flow through the by-pass conduit.