Object of the invention

The object of the invention is a dual liquid fuel injection system for an internal combustion direct injection engine, a fuel injection process with said system and an engine comprising said system, wherein the fuels are low-pressure liquid fuel and a high-pressure liquid fuel.

The technical problem

The technical problem is how to design a dual liquid fuel injection system for internal combustion engines, wherein the fuels are a low-pressure liquid fuel, such as gasoline (petrol), and a high-pressure liquid fuel, such as for example liquid gas, which will upon change of the fuel from high-pressure liquid fuel to the low-pressure liquid fuel and during operation in various external operating conditions and engine loads prevent gasification of the high-pressure liquid fuel, while simultaneously ensuring sufficiently high pressure in the system and thus allow continuous engine operation with full power. At the same time, the system has to ensure simple installation into vehicles with a direct fuel injection system, such as gasoline, and has to be economically interesting for manufacturers as well as users.

Prior art

Today, direct fuel injection is used in internal combustion engines for greater combustion efficiency. Since such a method of engine fuelling requires high pressures, even up to 300 bar, a high-pressure pump is installed in the supply line to provide sufficient pressure to the liquid fuel entering the fuel manifold and forward via the injector elements into the combustion chamber. A conventional fuel feed and delivery system for an internal combustion engine comprises a tank for low-pressure liquid fuel, such as petrol, with a pump pushing the fuel under a certain pressure through the lines towards the high-pressure pump. From there, the fuel goes into the fuel manifold and into the combustion chamber via the injector elements. For ecological as well as economic reasons, many motor vehicle users choose to use high-pressure fuels, such as liquefied gas, which is usually a mixture of butane and propane. The liquefied gas system shall normally be installed on the vehicle as retrofitted equipment or as a first time installation and shall be designed to be supplemented by a low-pressure liquid fuel system.

In direct injection engines, newer systems, including in the case of using two fuel types, have one high-pressure pump to feed liquid fuel, either low-pressure petrol or high- pressure liquefied gas pushing the fuel forward into the fuel manifold. The switching between one fuel and another must be performed in such a way that the vehicle engine operates without interruption or limitation. When switching from one fuel to another, the problem is to switch from the high-pressure liquefied gas to a low-pressure liquid fuel due to a significantly lower required fuel pressure. Lowering the pressure of the high-pressure liquefied gas causes gasification and bubbling. Gas bubbles prevent the high-pressure pump from operating effectively, causing the pressure at the outlet of the high-pressure pump to decrease and fuel to enter the fuel manifold at a too low pressure. This results in malfunctioning or inadequate operation or even engine failure. Another problem is the high temperature of the engine, also causing gasification of the liquefied gas. Various systems have been developed to prevent the gasification of liquefied gas in a dual-fuel injection system, which prevent or reduce the possibility of gasification of high-pressure liquefied gas by incorporating different elements into the system.

Thus, EP 2143916 A1 (van Veen) describes a dual-fuel injection system comprising a low-pressure liquid fuel tank, such as petrol, with a pump, and a high-pressure liquid fuel tank, such as liquefied gas, with a pump. Both tanks are connected via lines to the same high-pressure pump, pushing the fuel under high pressure forward into the fuel manifold. An auxiliary pump is installed in the line of the high-pressure liquid fuel, which generates sufficient pressure of the high-pressure fuel in order not to gasify it before entering the high-pressure pump. Excess liquefied gas is fed back to the high-pressure liquid fuel tank through the high-pressure pump. The low-pressure liquid fuel line is connected to the high-pressure liquid fuel line at the node located in front of the auxiliary pump. In the use phase of the low-pressure liquid fuel, it flows over the bypass past the auxiliary pump. In the fuel switching phase from high-pressure liquefied gas to low-pressure liquefied fuel, the high-pressure liquefied fuel inlet shall be closed and the auxiliary pump shall continue to operate and pushes the remainder of the high- pressure liquefied fuel towards the high-pressure pump. This reduces the pressure of the high-pressure liquid fuel in the installation and at a certain point in time the pressure of the high-pressure liquid fuel in the line is lower than the pressure of the low-pressure liquid fuel. The shut-off valve opens and allows the low-pressure fuel to drain into the line and forward to the high-pressure pump. Since the pressure difference of both fuels is significant, the valve closing the supply of low-pressure liquid fuel to the installation shall be opened only when there is virtually no high-pressure liquid fuel left in the installation. The high-pressure liquid fuel has more and more space in the installation in front of the auxiliary pump, which reduces the pressure and is highly likely to gasify the liquefied gas before there is a sufficient pressure drop to allow the low-pressure fuel to pass through. At the same time, the auxiliary pump has less and less high- pressure liquid fuel available to feed it to the high-pressure pump, which is why it cannot generate sufficient pressure in the fuel manifold.

WO201 3/115645 A1 (Jaasma) discloses an injection system for feeding two fuel types to the fuel manifold of an internal combustion engine. The system comprises a tank for low-pressure liquid fuel, such as petrol, with a pump, and a tank for high-pressure liquid fuel, such as liquefied gas, with a pump. Both tanks are connected via lines to the same high-pressure pump, but each forms their own fuel feed circuit to the high-pressure pump. At the high-pressure pump inlet, an excess amount of high-pressure liquid fuel is provided, which is why there is an excess of liquefied gas discharged through the high-pressure pump and returned directly to the tank of high-pressure liquid fuel, i.e. liquefied gas, via the return line. When fuel is switched from high-pressure liquid fuel to low-pressure liquid fuel, the drain and the supply line of the high-pressure liquid fuel tank shall be closed. Therefore, part of the amount of reflux high-pressure liquid fuel that otherwise returns from the high-pressure pump back to the tank remains in the line. This high-pressure liquid fuel is led into the low-pressure liquid fuel circuit via an open auxiliary valve, which is otherwise closed when feeding only the high-pressure liquid fuel. Due to the opening of the auxiliary valve and thus the increase in volume, the pressure of the high-pressure liquid fuel is reduced. However, the pressure reduction is not such that the low-pressure liquid fuel could be fed from the tank through the non-return valve into the low-pressure liquid fuel circuit. Hence, there is a specific node in the circuit with a specific volume designed to accommodate additional low- pressure liquid fuel. This provides an additional amount of low-pressure liquid fuel for the pump installed downstream in the circuit. In order to ensure adequate line pressure, a significant amount of low-pressure liquid fuel must always be available in the node. The fuel must be available in the node at all times, otherwise the engine will not perform well or will not operate at all when switching from liquefied gas to liquid fuel. Since there must be a sufficient quantity of low-pressure liquid fuel in the node, the node is essentially formed as a smaller tank for which a certain space must be provided, but which is always lacking in the vehicle. Due to the high amount of fuel in the low- pressure system circuit, there may also be slower switching responsiveness, untimely filling of the resulting gas bubble in front of the high-pressure pump and thus engine malfunctioning. It may also be that, at high outdoor temperatures and a warm engine, it is not possible to provide a sufficient amount of low-pressure fuel in the node to allow for a sufficient pressure drop.

Solution to the technical problem

The technical problem is solved by the dual-fuel injection system, the main characteristics of which are given in the first independent patent claim.

The dual-fuel injection system for a direct injection internal combustion engine comprises the following: an assembly for feeding low-pressure liquid fuel with a low- pressure liquid fuel tank, a pump installed in the tank for pumping low-pressure liquid fuel, and a line receiving low-pressure liquid fuel and leading it towards the high- pressure pump; an assembly for feeding high-pressure liquid fuel with a high-pressure liquid fuel tank for storing high-pressure liquid fuel, and a high-pressure liquid fuel line receiving the high-pressure liquid fuel from the high-pressure liquid fuel tank and leading it towards the high-pressure pump; a high-pressure pump installed downstream of the low- and high-pressure liquid fuel tank and receiving the low- and high-pressure liquid fuel; a return line connecting the high-pressure pump to the high- pressure fuel tank, wherein the system comprises a switching node installed downstream of the low- and high-pressure liquid fuel tank and upstream of the high- pressure pump and connected to each of them by at least one line, wherein the switching node comprises an internal circulation line connected to the high-pressure liquid fuel line which is guided through the switching node and comprises a junction in which a low-pressure liquid line is connected; an auxiliary pump is installed on the internal circulation line so that it is in sequential connection with the pump of the low- pressure liquid fuel tank to increase the pressure of the low-pressure liquid fuel, and at the same time, it is in fluid connection with the return line which is led through the switching node, wherein the dual-fuel injection system further comprises a flexible container installed in the high-pressure liquid fuel tank and two pumps connected with each other sequentially, wherein the flexible container is in fluid connection with at least one pump. The flexible container is designed like a crucible with a non-return valve at the bottom and made of flexible memory material.

The return line is designed as a first return line connecting the high-pressure pump to the switching node and as a second return line connecting the switching node to the high-pressure liquid fuel tank.

The pumps in the high-pressure liquid fuel tank are installed horizontally and fixed using magnets arranged in the flexible container.

The dual-fuel injection system can be retrofitted into any engine of an internal combustion vehicle with a direct fuel injection system. The dual-fuel injection system allows, especially in the case of switching from high- to low-pressure liquid fuel, controlled mixing of low- with high-pressure liquid fuel, ensuring smooth engine operation at all loads and even at higher outdoor temperatures in summer. In the case of the reduction of high-pressure liquid fuel in the tank, when the fuel temperature in the tank increases further due to the high temperature of the fuel returning to the tank from the high-pressure pump, it is usually not possible to switch the liquid fuel without disrupting the engine operation. The dual-fuel injection system uses a switching node and an auxiliary pump to add low-pressure liquid fuel to the return line, which guides the high-pressure liquid fuel from the high-pressure pump back to the high-pressure liquid fuel tank. Consequently, this causes a temperature and pressure drop in the high-pressure fuel tank. With a multiple supply of the low- pressure liquid fuel, or by feeding low-pressure liquid fuel for a sufficiently long time into the high-pressure liquid fuel tank, the temperature and pressure decrease and it is possible to switch to low-pressure liquid fuel.

Due to the flexible container, which is significantly smaller than the high-pressure liquid fuel tank in which it is installed, even a small added amount of low-pressure liquid fuel is sufficient to achieve a reduction of the gasified pressure in the fuel mixture captured by the pump.

Adding a low-pressure fuel to the flexible container increases the pump’s performance in the high-pressure fuel tank by 10 to 20% as well as the fuel mixture in the circuit, thus increasing the gasification conditions above those present in the system under extreme engine operating conditions. When adding a low-pressure fuel to the flexible container, due to its higher density and the full tank, it mostly remains in the flexible container and falls to its bottom to be captured by the pump.

The dual-fuel injection system will be described in more detail in the following, with an embodiment and a figure showing a schematic representation of the dual-fuel injection system. The dual-fuel injection system (100) of a direct injection internal combustion engine comprises a basic assembly for feeding low-pressure liquid fuel originally installed in the vehicle, i.e. petrol fuel, and an assembly for feeding high-pressure liquid fuel in the form of a liquefied gas, which is usually a mixture of propane and butane, with the latter assembly usually retrofitted to the vehicle. The assembly for feeding low-pressure liquid fuel comprises a low-pressure liquid fuel tank (1) with a pump (2) that feeds low- pressure liquid fuel with a pressure of 3-8 bar into line (3) and to the high-pressure pump (4). The high-pressure pump (4) pushes the low-pressure liquid fuel with the required pressure through outlet (4.1) into the fuel manifold (5), from where the fuel is injected into the combustion chamber of the engine via the injector elements (6).

The second part of the dual-fuel injection system (100) constitutes the assembly for feeding high-pressure liquid fuel, comprising the following: a high-pressure liquid fuel tank (7), in which high-pressure liquid fuel in the form of liquefied gas, such as a mixture of propane and butane, is stored; a switching node (10) with a low-pressure liquid fuel auxiliary pump (9), which is activated when switching from high- to low-pressure liquid fuel to increase the pressure of the low-pressure liquid fuel; a high-pressure liquid fuel line (8) for guiding the high-pressure liquid fuel from the high-pressure liquid fuel tank (7) to the high-pressure pump (4), wherein line (8) comprises the first line (8.1) from the high-pressure liquid fuel tank (7) to the switching node (10) and the second line (8.2) from the switching node (10) to the high-pressure pump (4); a return line (27), which comprises the first return line (11) from the high-pressure pump (4) to the switching node (10) and the second return line (12) from switching node (10) to the high-pressure liquid fuel tank (7).

High-pressure liquid fuel is located in the high-pressure liquid fuel tank (7), the pressure of which is between 1.5 and 5 bar in the non-operative state at a normal ambient temperature of 10°C-30°C, and in the operating state, an average of up to 10 bar. As fuel is heated on its way to the high-pressure pump (4), there is a high probability of the liquefied gas passing into the gas phase, resulting in the high-pressure pump (4) being ineffective. Therefore, two pumps (13.1 and 13.2) are installed in the high- pressure liquid fuel tank 7, which are connected sequentially and generate a pressure in the high-pressure liquid fuel of 6 to 24 bar if necessary. In the example shown, pumps (13.1 and 13.2) are installed sequentially in the high-pressure liquid fuel tank (7) and fixed using magnets to the bottom of the high-pressure liquid fuel tank (7). Pumps (13.1 and 13.2) may also be arranged differently concerning each other. Pump (13.1) capturing fuel from the high-pressure liquid fuel tank (7), is connected with its inlet to a flexible container (15) designed as a crucible with a non-return valve at the bottom in which the liquid fuel is contained. In more detail, the flexible container (15) will be described below. The operation of pumps (13.1 and 13.2) is controlled by the engine control unit (ECU) in the manner of the electric current measurement.

High-pressure liquid fuel with a pressure of up to 24 bar comes from the high-pressure liquid fuel tank (7) through the solenoid valve (14) into line (8) passing through the switching node (10) located between the high-pressure liquid fuel tank (7) and the high- pressure pump (4). Line (8) enters the switching node (10) at the second entry (10.2) and exits it at the exit (10.4). The high-pressure pump (4) is connected to the high- pressure liquid fuel tank (7) by a return line (27) via which excess high-pressure liquid fuel is discharged back into the high-pressure liquid fuel tank (7) so that the excess high-pressure liquid fuel is guided via a first return line (11 ) entering the switching node (10) at the third inlet (10.3) and via a second return line (12) back into the high-pressure liquid fuel tank (7). The outflow of the high-pressure liquid fuel from the high-pressure liquid fuel tank (7) back into the second return line (12) prevents the non-return valve (16). The high-pressure liquid fuel entering the high-pressure liquid fuel tank (7) from the second return line (12) is guided forward via the internal line (17) into the interior of the high-pressure liquid fuel tank (7) to the flexible container (15) so that the end of the internal line (17) extends to a height of one-third to two-thirds of the height of the flexible container (15). Thereby, pump (13.1) is always provided with at least a certain minimum amount of liquid fuel even when there is already a relatively small amount of fuel in the high-pressure liquid fuel tank. Since the end of the internal line (17) terminates at a certain spacing from the bottom of the flexible container (15), air bubbles, possibly present in the returning high-pressure fuel, may be withdrawn from the high-pressure liquid fuel and floated from the flexible container (15) into the high- pressure liquid fuel tank (7) before the high-pressure liquid fuel reaches the pump suction area (13.1).

The flexible container (15) is made of a flexible memory material which, under the influence of external force, deforms and returns back to its basic shape after the external force has ceased to be applied. In size, the flexible container (15) is substantially smaller than the high-pressure liquid fuel tank (7) in which it is installed. In a deformed state, when it is exerted by an external force, e.g. squeezing by hand, its diameter is smaller than the diameter of the solenoid valve (14) port of the standard high-pressure liquid fuel tank (7). Consequently, the installation of the flexible container (15) and the two pumps (13.1 and 13.2) does not require a special opening on the high-pressure liquid fuel tank (7); it is even possible to install them outside the high- pressure liquid fuel tank (7).

The switching node (10), which is installed downstream of the low-pressure liquid fuel tank (1) and the high-pressure liquid fuel tank (7) and upstream of the high-pressure pump (4), ensures sufficient quantity and adequate pressure of the liquid fuel supplied to the high-pressure pump (4) in combination with loads of the pumps (13.1 and 13.2) at the time of switching from one to another type of liquid fuel, preferably when switching from high- to low-pressure liquid fuel, as well as in the case of high external temperatures heating the high-pressure liquid fuel and high engine loads requiring a higher quantity of fuel. The switching node (10) comprises an internal circulation line (24) which is connected to a high-pressure liquid fuel line (8) which is guided through the switching node (10). A line (3) of low-pressure liquid fuel is connected to the internal circulation line (24) at the junction (20) and enters the switching node (10) at the first inlet (10.1) and is thereby coupled to a high-pressure liquid fuel line (8). In the case of low-or high-pressure liquid fuel being supplied to the engine when only low-pressure liquid fuel, e.g. petrol, or only high-pressure liquid fuel, e.g. liquefied gas, is supplied as a liquid fuel to the engine, the pressure of the liquid fuel in the path through the switching node (10) in the direction towards the high-pressure pump (4) remains substantially constant. Thus, in the case of feeding low-pressure liquid fuel, it enters the switching node (10) at a pressure of 3-8 bar through the first inlet (10.1) and exits through the outlet (10.4) at an unchanged pressure of 4-7 bar and is guided to the high-pressure pump (4), which compresses the low-pressure liquid fuel to the pressure required in the fuel manifold (5).

The switching node (10) further comprises an auxiliary pump (9) which is installed on the internal circulation line (24) in such a way that the pump (2) located in the low- pressure liquid fuel tank (1) is arranged sequentially. At the same time, the auxiliary pump (9) is also connected via a magnetic valve to a return line (27) passing through the switching node (10) and thus also to a high-pressure liquid fuel tank (7). The auxiliary pump (9) is activated when switching from high- to low-pressure liquid fuel to increase the pressure in the low-pressure liquid fuel.

In the case of feeding low-pressure liquid fuel, pump (2) feeds low-pressure liquid fuel with a pressure of 4-7 bar into line (3), which merges into the high-pressure liquid fuel line (8) at the switching node (10). Liquid fuel with unchanged pressure exits at the outlet (10.4) from the switching node (10) and is guided to the high-pressure pump (4). The high-pressure pump (4) pushes the low-pressure liquid fuel with the required pressure through outlet (4.1) into the fuel manifold (5), from where the fuel is injected into the combustion chamber of the engine via the injector elements (6).

In the case of feeding the high-pressure liquid fuel, it enters the switching hub (10) at a pressure of 6-24 bar through the second inlet (10.2) and exits through the outlet (10.4) at a substantially unchanged pressure of 6-24 bar and is guided forward to the high-pressure pump (4), which compresses the high-pressure liquid fuel to the pressure required in the fuel manifold (5). The excess high-pressure liquid fuel is discharged from the high-pressure pump (4) via the return line (27), namely via the first return line (11 ) to the switching node (10) through the third inlet (10.3) to the valve (23), which reduces the pressure by 2-12 bar, preferably by 5-8 bar to maintain the pressure at the high-pressure pump (4) and to provide the minimum required flow of high-pressure liquid fuel exiting via the outlet (10.5) into the second return line (12) and then into the high-pressure liquid fuel tank (7). The pressure of the high-pressure liquid fuel fed by the pumps (13.1 and 13.2) and entering the high-pressure pump (4) is measured by a pressure sensor (18), which is installed at the third inlet (10.3) of the first return line (11 ) into the switching node (10).

The switching of fuel from high- to low-pressure liquid fuel may occur at the user’s request or automatically as a result of an insufficient quantity of high-pressure liquid fuel in the high-pressure liquid fuel tank (7). In the case of insufficient amount of high- pressure liquid fuel in the high-pressure liquid fuel tank (7) and at the same time increased engine temperature, the temperature of high-pressure liquid fuel increases, so the pressure of high-pressure liquid fuel providing high-pressure liquid fuel in front of high-pressure pump (4) must be higher. Due to the insufficient amount of high- pressure liquid fuel in the high-pressure liquid fuel tank (7), pumps (13.1 and 13.2) cannot provide sufficient fuel to the high-pressure pump (4), which would lead to gasification of high-pressure liquid fuel. Instead of reducing the line pressure, which would allow switching to low-pressure liquid fuel, there would be an increase in pressure in front of the high-pressure pump (4), which is undesirable. Insufficient operation of pumps (13.1 and 13.2), as a result of the insufficient quantity of high- pressure liquid fuel in the flexible container (15) in the high-pressure liquid fuel tank (7), is detected by the engine control unit (ECU) which switches on the pump (2) to feed the low-pressure liquid fuel via line (3) to the switching node (10). At the same time as the pump (2), an auxiliary pump (9) is also switched on. The auxiliary pump (9) increases the pressure in the low-pressure liquid fuel and directs it via the open magnetic valve (21) and the outlet (10.5) of the switching node via the second return line (12) to the flexible container (15) in the high-pressure liquid fuel tank (7). In this way, pumps (13.1 and 13.2) regain a sufficient amount of liquid fuel for normal operation, which is reflected in increased current consumption. This is detected by the engine control unit (ECU) which stops the pump (2) in the low-pressure liquid fuel tank (1) and the auxiliary pump (9) in the switching node (10) and re-establishes the supply of liquid fuel from the high-pressure liquid fuel tank (7) to the high-pressure pump (4). The described actuation of pumps (2 and 9) may be repeated several times, depending on the engine type, driving mode and other external factors. The liquid fuel supplied to the high-pressure pump (4) is essentially a mixture of highl and low-pressure liquid fuel, whereby the ratio of high- to low-pressure liquid fuel changes with each new start-up in favour of low-pressure liquid fuel. The feeding of low-pressure liquid fuel to the flexible container (15) and thus to the high-pressure liquid fuel tank (7) increases the proportion of low-pressure liquid fuel in the fuel mixture, thereby lowering the gasification limit pressure of the fuel mixture. When the proportion of low-pressure liquid fuel in the high-pressure liquid fuel tank (7) and thus in the high-pressure liquid fuel line (8) is sufficiently high, the gasification limit pressure of the mixture of high- and low-pressure liquid fuel is reduced so much that it is lower than the pressure provided by the successively connected pumps (2 and 9) together. When pumps (13.1 and 13.2) are switched off, pump (2) and auxiliary pump (9) together provide a pressure higher than the gasification pressure at the outlet (10.4) into the line (8), so that the low-pressure fuel passes via the non-return valve (22) in the internal line (24) into the line (8) to the high-pressure pump (4) directly. At the same time, the solenoid valves (21 and 23) are closed. After a certain consumption of liquid fuel depending on the engine type, the entire remaining portion of the high-pressure liquid fuel in the liquid fuel mixture is consumed in the high-pressure line (8) from the switching node (10) to the high-pressure pump (4) and the auxiliary pump (9) ceases to operate. Liquid fuel, which is now a low-pressure liquid fuel, is supplied directly from pump (2) via line (3) and line (8) to high-pressure pump (4). Switching from high- to low-pressure fuel is complete.

If, after switching from one to another type of liquid fuel, the engine control unit (ECU) detects any malfunction of the engine, it shall re-start the switching to the previous phase of the fuel change process, where the number of switching and thus the switching time depends on the engine type. The liquid fuel change process shall be carried out for as long as the change has not been successfully carried out.

In the case of a reverse change of liquid fuel from low- to high-pressure liquid fuel, the switching node (10) is not active to the same extent because due to the substantially higher pressure of the high-pressure liquid fuel, which immediately fills the lines, the change is carried out in such a short time that it does not affect the engine operation. The switching node (10) as well as the other active elements of the dual-fuel injection system are controlled via an engine control unit (ECU) which is not the subject of the present theme.