This application is based on German Patent Application No. 10 2006 061 570.0 filed 27 Dec. 2006, upon which priority is claimed.
1. Field of the Invention
The invention relates to a fuel system, in particular of the common-rail type, for an internal combustion engine.
2. Description of the Prior Art
A fuel system of the type defined at the outset is known from German Patent Disclosure DE 102 36 314 A1. In the fuel system shown there, a prefeed pump pumps the fuel into a low-pressure line that forms a pressure region, to which a high-pressure pump is connected. The prefeed pump compresses the fuel to a pressure above the vapor pressure, so that the fuel can be delivered to the high-pressure pump in liquid form. The high-pressure pump compresses the fuel to the desire high pressure and pumps it to a distributor line, which is also known as a fuel collection line or common rail, to which in turn a plurality of injectors are connected that inject the fuel directly into combustion chambers of the engine.
The object of the present invention is to refine a fuel system of the type defined at the outset in such a way that even under unfavorable ambient conditions, an internal combustion engine employing the system can be started quickly and reliably.
In the fuel system of the invention, the pressure in the pressure region is prevented from dropping constantly below the vapor pressure after the engine and fuel system have been shut off. This avoids a delayed pressure buildup upon starting of the engine. Instead, on starting the pressure can be built up very quickly, which improves the starting quality of the engine. The proposed volume reservoir furthermore has the advantage that the pressure rise that occurs from afterheat effects in the overrunning shutoff phases, when no fuel is pumped out of the pressure region, is reduced because of the additional elasticity of an elastic volume reservoir. As a result, the components in the pressure region are subjected to a lesser load, which lengthens their service life. Moreover, expenses can be saved, since inexpensive components can be employed.
Overall, the fuel system of the invention is also simpler in construction, since provisions for pressure buildup before engine starting can be dispensed with. Such provisions are known by the term “pre-drive provisions”: For instance, upon actuation of a door contact, an advance run of the fuel pump is initiated in order to build up the pressure in the pressure region. The safety of the fuel system is improved as well, since the risk of an escape of fuel, for instance during maintenance because of an unpredictable “pre-drive event” is avoided. In a demand-responsive fuel pump, control deviations upon sudden load changes (such as a change to an overrunning shutoff or resumption after an overrunning shutoff) can furthermore be intercepted via the elastic volume reservoir provided according to the invention. Vapor formation in a downstream high-pressure pump, for instance from the pressure dropping below the fuel vapor pressure, is markedly reduced as a result.
The foundation of the invention is the fact that the fuel system and the fuel volume present in it in the vicinity of the engine expand from thermal conduction after the shutoff of the engine. As a result, the pressure in the pressure region of the fuel system, which is closed off in the shutoff situation, rises. This is particularly true for fuel systems of the kind which have a low-pressure region and a high-pressure region. In such a fuel system, the high-pressure region above all heats up first, so that the pressure rises there. As a result of attainment of an opening pressure of a pressure limiting valve that is typically present and from leakage of fuel from the high-pressure region to the low-pressure region, fuel is drained out into the low-pressure region.
From the low-pressure region, fuel is output, if a limit pressure is exceeded, via a pressure regulator or pressure limiting valve that is typically present there. If the engine and the fuel system then cool down, the fuel in the entire fuel system contracts, causing a pressure drop in the pressure region. In the prior art, the vapor pressure of the fuel or the ambient pressure is undershot in this case, causing outgassing of vapor and resulting in air dissolved in the fuel. The fuel must initially be compressed again on starting of the engine, before the pressure in the fuel system reaches a level required for engine starting. A particular problem here is the outgassed air, which can be dissolved in the fuel again only at a very high pressure.
The volume reservoir provided according to the invention prevents the vapor pressure from being constantly undershot, so that neither fuel nor air gasses out. The reason for this is that contraction volume, which is the volume by which the fuel contracts as it cools down, is stored by the elastic volume reservoir before this cooling occurs. At the same time, the characteristic curve of the volume reservoir is designed such that even after dispensing the contraction volume, it still subjects the pressure region to a pressure that is higher than the vapor pressure.
A first advantageous embodiment of the fuel system of the invention is distinguished in that it includes at least one pressure limiting device, by which the maximum pressure in the pressure region is defined. This is the case in so-called “constant-pressure systems”. In such systems, the fuel pump is constantly triggered, and the desired pressure in the pressure region is regulated by way of a pressure regulator or a pressure limiting device, by which the excess pumping quantity of the fuel pump is returned to the tank. The pressure regulator also takes on the function of a pressure limiting device, because it is designed such that it establishes or regulates the pressure in the pressure region that is maximally required for operating the engine.
In a refinement of this, it is proposed that the fuel system includes at least one second pressure limiting device, having an opening pressure that differs from the first pressure limiting device, and that the maximum pressure in the pressure region is defined by the highest opening pressure. Such fuel systems are also known as “switchover systems”. They function similarly to the constant-pressure systems mentioned above, but offer the capability of establishing at least two different pressure levels in the pressure region, depending on which pressure limiting device is activated.
Finally, it may also be provided that the fuel pump can be triggered demand-responsively; and that the maximum pressure corresponds to a rated pressure, plus a pressure difference which occurs as a result of fuel trapped in the pressure region by a temperature increase caused by thermal conduction. Such a system is also called “demand-regulated”, since the pumped quantity of the fuel pump can be regulated via variable pump triggering. Such fuel systems are typically return-free; that is, no excess pumped quantity flows back into the fuel tank. Nevertheless, for safety reasons, a pressure limiting valve is typically still present whose established pressure, however, in contrast to the aforementioned systems, is not directly in communication with the system pressure. As a result of the definition according to the invention of the characteristic curve of the volume reservoir, this reservoir can be used in this kind of demand-responsive fuel system as well, and in that case assures that the vapor pressure will not constantly be undershot.
It is especially advantageous if the characteristic curve of the volume reservoir is steeper at low pressure in the pressure region than at high pressure. It can thus be attained that the pressure in the pressure region remains above the vapor pressure, not only at the two aforementioned points but during the entire cooling down process of the fuel system. Thus any type of outgassing is suppressed, which further improves the starting properties of an engine that is provided with such a fuel system. It is best if this characteristic curve is degressive, preferably even highly degressive, with a correspondingly highly parabolic or hyperbolic course.
Above all in constant-pressure systems, long-term leakage from the pressure region to the fuel tank can occur. It is therefore proposed according to the invention that the characteristic curve is designed such that the difference between the first and second volumes additionally takes leakage losses to a fuel tank into account.
In a common rail fuel system with a first fuel pump and a second fuel pump (high-pressure pump), it can happen that if the high-pressure region cools down faster than the low-pressure region, a lower pressure will occur in the high-pressure region, as a result of which fuel leakage via the second fuel pump and beyond from the low-pressure region to the high-pressure region is provoked. In such a case, the characteristic curve should therefore be designed such that the difference between the first and second volumes additionally takes such leakage losses into account.
An especially advantageous embodiment of the fuel system of the invention provides that the elastic volume reservoir is disposed in a fuel tank. The “temperature stroke” of this elastic volume reservoir that is additionally incorporated into the fuel system is thus comparatively slight after shutoff of the engine, since this reservoir is located far away from the thermally active engine. In other words, this additional volume reservoir does not additionally worsen the effect of vapor production.
It is especially preferred if the elastic volume reservoir, together with a fuel filter, is integrated into a common function module. This module is present anyway in typical fuel systems, and so the additional element of a volume reservoir can be realized in an existing system without additional sealing points. Any additional space required is also minimized.
A simple structural realization of such a volume reservoir provides that the elastic property of the volume reservoir is furnished at least also by means of the material of the housing. Furthermore, it is understood that the elastic property can be brought about by corrugated ribs or other structural elements. The spring force for maintaining the pressure in the pressure region is made available as a result of the elastic properties of the material comprising the housing. It is also possible for the elastic property to be furnished at least also by means of an additional spring action on the housing. As a result, the characteristic curve of the volume reservoir can be optimized still further. This kind of spring action can be employed for instance for prestressing the volume reservoir.
The invention will be better understood and further objects and advantages thereof will become more apparent from the ensuing detailed description of preferred embodiments, taken in conjunction with the drawings, in which:
A fuel system according to the invention is identified overall in
The fuel system 10 includes a fuel tank 12, in which a first fuel pump 14, also called a prefeed pump, is disposed. Via a check valve 16, it pumps fuel into a low-pressure line 18, which forms an at least intermittently closed-off pressure region. It leads out of the fuel tank 12 via a function module 20, represented here only by dot-dashed lines and described in detail hereinafter, and to a high-pressure pump 22. Pump 22 compresses the fuel to a very high pressure and pumps it onward into a high-pressure line 24, which leads to a fuel distributor 26 that is also known as a “common rail”. A plurality of injectors 28 are connected to the common rail and inject the fuel directly into combustion chambers (not shown) of the engine that are associated with them.
From the low-pressure line 18, a return line 30 branches off between the prefeed pump 14 and the function module 20; a pressure regulator 32 is disposed in this return line. The aforementioned function module 20 includes a fuel filter 34 and an elastic volume reservoir 36. The fuel filter 34 and the elastic volume reservoir 36 are accordingly jointly integrated into the function module 20, specifically in such a way that the housing of the fuel filter 34 is at the same time the housing of the elastic volume reservoir 36, as
It can be seen from
The fuel system 10 shown in
When the engine is shut off, the typically electrically driven prefeed pump 14 is also switched off, and the typically mechanically driven high-pressure pump 22 also ceases its operation. The low-pressure line 18 now acts as a pressure region that is in principle closed off, in the same way as do the high-pressure line 24 and the common rail 26. Especially the region of the fuel system 10 in the vicinity of the engine, which means at least the common rail 26, the high-pressure line 24, the high-pressure pump 22, and at least a portion of the low-pressure line 18, now heat up from thermal conduction from the engine, and the fuel volume present and closed off in this region also heats up. As a result, the fuel expands, causing the pressure in the low- and high-pressure regions to rise.
From attainment of the opening pressure of a pressure limiting valve, although it is not shown in
In
The elastic volume reservoir 36 is now designed such that the difference dVK (“contraction volume”) between the first volume V1 and the second volume V2 corresponds at least approximately and at least to a value by which the volume V of the fuel in the low-pressure line 18 decreases, upon cooling from a maximum temperature to ambient temperature. The maximum temperature is the temperature that the fuel system 10, or the fuel trapped in the low-pressure line 18, reaches after the shutoff of the engine or of the fuel system 10 because of thermal conduction from the engine. In addition, the difference dVK takes leakage losses via the prefeed pump 14 and beyond to the fuel tank 12 into account, along with leakage from the low-pressure line 18 back into the high-pressure line 24. Such losses can occur whenever the high-pressure line 24 and the common rail 26 cool down faster than the low-pressure line 18 and the fuel trapped in it. In that case, it can in fact happen that a lower pressure prevails in the high-pressure line 24 than in the low-pressure line 18, so that fuel flows from the low-pressure line 18 into the high-pressure line 24 via the inlet and outlet valves of the high-pressure pump 22.
By means of the characteristic pressure/volume curve 50 shown in
In
In
In
In
In a distinction from the fuel system 10 of
Still another variant of a fuel system is shown in
After the engine and fuel system 10 have been shut off, the pressure in the low-pressure line 18 therefore first rises to a pressure that is higher than the normal operating pressure. This is shown in
An alternative embodiment of the elastic volume reservoir 36 is shown in
The spring 72b of the piston reservoir 36b has a flatter characteristic curve than the spring 72a of the piston reservoir 36a. At the same time, however, the spring 72b is more strongly prestressed than the spring 72a. The result is the characteristic pressure/volume curve 50, comprising two essentially linear portions; the first portion, associated with the piston reservoir 36a, is relatively steep and is marked 50a. The second portion, which is flatter, is marked 50b. In operation, up to the rated pressure pN, that is, the normal operating pressure, only the piston reservoir 36a is operative. If the pressure rises in response to afterheating (when the elastic volume reservoir 36 is used in a demand-responsive fuel system as in
The foregoing relates to preferred exemplary embodiments of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.
Number | Date | Country | Kind |
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10 2006 061 570 | Dec 2006 | DE | national |
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5590631 | Tuckey | Jan 1997 | A |
5701869 | Richardson et al. | Dec 1997 | A |
20020157646 | Hiraku et al. | Oct 2002 | A1 |
20040250795 | Stroia et al. | Dec 2004 | A1 |
Number | Date | Country | |
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20080283026 A1 | Nov 2008 | US |