Process for control of boost pressure limitation of a turbocharger in an internal combustion engine as a function of the density of ambient air

[0002] The invention relates to a process for control of boost pressure of a turbocharger in an internal combustion engine as a function of the density of ambient air, an internal combustion engine with exhaust gas turbocharger and with control means for control of the boost pressure in which the actual boost pressure is determined at specific time intervals and compared with assigned values for the desired boost pressure taken from stored compressor characteristics and readjusted, and with an engine control device for an internal combustion engine with exhaust gas turbocharger having a control unit for control of the boost pressure and a data storage unit.

[0003] Control of the boost pressure of an internal combustion engine with a turbocharger is of the state of the art. For example, DE 197 57 661 C1 discloses boost pressure control in an internal combustion engine with turbocharger in which a desired boost pressure is determined from characteristics determined by engine speed and load and is taken into account for control of unstable influences based on dynamic road behavior.

[0004] As soon as the supercharger speed exceeds a maximum permissible boundary value for the supercharger speed, a turbocharger can be damaged. Such excessive speed may come about if the vehicle is in operation at higher altitudes in which the ambient pressure falls below a specific minimum pressure or the absorption line of the engine changes radically as a result of leaks from the system. In order to prevent overspeed of the turbocharger in operation at a high altitude, the speed of the turbocharger is usually restricted by limiting the boost pressure as a function of the engine speed and atmospheric pressure. This is accomplished by means of previously recorded characteristics in which the boost pressure is filed on the basis of ambient pressure and engine speed. For this purpose costly tests for determining the characteristics must be carried out in advance for each altitude for every engine having a turbocharger. In addition, fluctuations in air pressure and in ambient temperature must be predicted in the design process.

[0005] The object of the invention is to limit overspeed of the turbocharger in an internal combustion engine having a turbocharger by a simple and reliable process in which allowance is made for atmospheric conditions despite high performance.

[0006] It is claimed for the invention that this object is attained both by the process for control of boost pressure limitation of a turbocharger in an internal combustion engine as a function of the density of ambient air in accordance with the characteristics specified in claim 1 and by designing an internal combustion engine having a exhaust gas turbocharger and by means for control of the boost pressure in which the actual boost pressure is determined at specific time intervals and compared with assigned values for the desired boost pressure in a particular operational condition determined from stored compressor characteristics and reset, in accordance with the characteristics specified in claim 9, and by means of an engine control device for an internal combustion engine with exhaust gas turbocharger having a control unit for adjustment of the boost pressure and a data storage unit in accordance with the characteristics specified in claim 10.

[0007] As a result, the boost pressure as a function of atmospheric conditions is determined simply at each determination point, with the atmospheric conditions taken into account, and this boost pressure is compared with the maximum boost pressure permissible for this operating condition. Without costly pretesting for determination of a set of characteristics for each individual ambient situation the measured boost pressure may be compared directly with the maximum permissible boost pressure taken from a set of characteristics to determine if the compressor speed is still below the boundary with overspeed and accordingly to prevent further increase when the boundary state is reached.

[0008] By preference the process as specified in claim 2, in which the adjusted air mass throughput at the compressor intake may be determined with very good approximation, is to be carried out.

[0009] The process as specified in claim 3 permits control and boost pressure limitation in a simple manner by means of the engine control means usually already present as part of the structure, means in which all predetermined data are present and to which all measured data are first delivered through corresponding inputs and from which corresponding control signals for boost pressure control may be forwarded by way of an output.

[0010] The process as specified in claim 4 permits especially precise determination of the pressure relationships P_LP_VEby taking into account the pressure losses between measuring point and compressor intake. This makes it possible further to minimize the confidence interval between maximum permissible compressor speed and compressor speed as measured. The performance spectrum of the engine may in this way be extended directly to higher levels.

[0011] Preference is to be given to the process as specified in claim 5, a process by which it is possible additionally to minimize the danger of short-term occurrence of boost pressure peaks situated above the maximum permissible boost pressure and accordingly the occurrence of short-term compressor speed peaks situated above the maximum permissible value for the compressor speed on the basis of dynamic processes. Overspeed of the compressor as a result of dynamic processes and the speed peaks caused by them can thus be prevented by simple means.

[0012] Claims 6 to 8 represent especially advantageous processes for control of boost pressure limitation.

[0013] The invention is described in detail in what follows with reference to the exemplary embodiments presented in FIGS. 1 to 3, of which

[0014] FIG. 1 presents a schematic diagram of an internal combustion engine with turbocharger;

[0015] FIG. 2 a control graph illustrating control of boost pressure limitation as claimed for the invention;

[0016] FIG. 3 a qualitative representation of a set of compressor characteristics.

[0017] FIG. 1 shows an internal combustion engine 1 to which exhaust gases are transferred by a conventional methods by way of a suction pipe 2 which delivers charged air by way of a turbocharger 4 and from which exhaust gases are delivered by way of a exhaust gas line 15 to the turbine 6 of the turbocharger 4, if supercharging is desired. When the turbocharger 4 is operated, the turbine 6 of the turbocharger 4 is driven by the exhaust gases, the process being accompanied by stress relief, and the exhaust gases are transferred from the turbine 6 by a conventional method by way of a exhaust gas line 16. The turbine 6 of the turbocharger 4 drives the compressor 5 of the turbocharger by a conventional method, as a result of which the air drawn in by way of an air duct 13 and moving through an air filter 12, an air duct 11, an air mass gauge 10, and an air duct 9, is supercharged and while in this supercharged state is delivered by conventional means by way of an air duct 8, a boost air cooler 3, an air duct 7, to the intake pipe 2 and accordingly the engine 1. Supercharging of the boost air by the turbocharger 4 is controlled by state-of-the-art conventional means (not shown) in accordance with the desired performance requirements.

[0018] In order to limit the supercharge speed to prevent overspeed of the turbocharger under charged atmospheric conditions, such as at higher altitudes, the supercharger pressure or boost pressure P_List, the ambient temperature T_∞, and the ambient pressure P_∞, and, by way of the air mass gauge 10, the air mass m_Ldrawn in and by way of a boost pressure and supercharge air temperature sensor 17 the boost pressure P_Listare measured in this operational situation. The measurement data are transmitted to the calculation unit 20 of the engine control device 18. The set of compressor characteristics specific to the turbocharger 4 is stored in a databank 19 of conventional design of the engine control device 18.

[0019] As is to be seen from FIG. 3, this specific set of compressor characteristics shows the characteristic curve of the maximum permissible compressor speed n_Vmaxplotted against the air mass m_Lkor, adjusted to the standard condition of the compressor, and the pressure ratio of the boost pressure downstream from the compressor p_Lto the pressure upstream from the compressor P_VE.

[0020] As is to be seen from FIG. 2, the temperature at the compressor intake T_VEis determined in the calculation unit 20 of the engine control device 18 from the ambient temperature T_∞, allowance being made for a stored, predetermined, air mass dependent correction curve value K_T, as also the pressure at the compressor intake is determined from the ambient pressure P_∞by application of a stored, predetermined, air mass dependent correction curve value K_Pat the compressor intake P_VE.

[0021] The correction curves K_Tand K_Pmay, for example, be stored in the form of air mass dependent correction factors. The calculation unit 20 then calculates the air mass throughput at the compressor intake m_Lkoradjusted to standard conditions, from the value for the air mass drawn in, m_L, the value for the temperature at the compressor intake, T_VE, and the value for the pressure at the compressor intake, P_VE, in accordance with the following formula:

[0022] m_Lkor=m_L·((T_VE+273.15 K)/T_OV)^0.5/(p_VE/P_OV)),

[0023] T_OVbeing the standard temperature and P_OVthe standard pressure of the compressor characteristic. The standard conditions are the conditions under which the compressor characteristic was plotted.

[0024] The maximum permissible boost pressure ratio (p_L/P_VE) of boost pressure P_Lto pressure upstream from compressor P_VEto be associated with the value determined for the adjusted air mass drawn in at the compressor intake m_Lkorat the maximum permissible compressor speed n_Vmaxis determined from the compressor characteristics stored in the databank 19. By multiplication by the pressure determined upstream from the compressor P_VEthis value is determined for the maximum permissible boost pressure P_Lmaxfor the air mass throughput at this turbocharger 4.

[0025] The computed maximum permissible boost pressure P_Lmaxis adjusted to an adjusted maximum permissible boost pressure p_Lmaxkorin accordance with the equation

[0026] p_Lmaxkor=p_Lmax·K_PLLK, with K_PLLK=f(m_L),

[0027] by means of a predetermined correction factor K_PLLKstored in the databank 20 [sic] for the boost pressure loss between boost pressure value measured at the position of the boost pressure sensor 17 and the compressor outlet immediately downstream from the compressor 5, in particular because of the high pressure loss in the boost cooler 3.

[0028] This corrected, maximum permissible, atmosphere dependent on the boost pressure P_Lmaxkoras determined is compared with the measured boost pressure P_L.

[0029] So long as the boost pressure P_Lat downstream of the compressor is smaller than the adjusted maximum permissible adjusted boost pressure P_Lmaxkor, according to the compressor performance characteristics in FIG. 3, the compressor speed n_vcorresponding to this state is smaller than the maximum permissible compressor speed n_Vmax. Resort to boost pressure adjustment is not yet necessary. As soon as the boost pressure P_Lcorresponds to the maximum permissible boost pressure P_Lmaxkor, action is taken so that adjustment for the time being no longer allows boost pressure increase in this condition.

[0030] In another embodiment short-term overspeed due to peak boost pressures generated by dynamic processes can be counteracted, after calculation of the maximum permissible pressure ratio (P_L/P_VE)_maxor after calculation of the adjusted maximum permissible boost pressure p_Lmaxkorallowing for pressure losses, by making an adjustment of the maximum permissible pressure ratio allowing for the gradient of the turbocharger speed over time, through application of a predetermined correction parameter K_nstored in the databank 20 [sic.], so that a corrected, maximum permissible pressure ratio (P_L/P_VE)_maxntaking the speed changes of the supercharger into account is determined, one which corresponds, for example, to the equation

[0031] (P_L/P_VE)_maxn=(P_L/P_VE)_max−K_n,

[0032] K_n=f((P_Lkor/P_VE); m_Lkor) being additionally filtered.

[0033] The gradient is calculated in the calculation unit 20.

[0034] In FIG. 1 the air pressure and air temperature sensor 14 is mounted upstream from the air filter 12 in the direction of flow. The air pressure and air temperature sensor 14 may also be provided in any other suitable place for measurement of ambient air pressure and/or ambient temperature. For example, the air pressure and the air temperature may be measured in a position situated downstream from the air filter 12.

[0035] In individual instances it may be advisable to equalize the boost pressure sensor with the ambient pressure sensor when the vehicle is not in movement, in order to reduce the tolerance.

[0036] 1 Internal combustion engine

[0037] 2 Intake pipe

[0038] 3 Boost air cooler

[0039] 4 Turbocharger

[0040] 5 Compressor

[0041] 6 Turbine

[0042] 7 Air duct

[0043] 8 Air duct

[0044] 9 Air duct

[0045] 10 Air mass gauge

[0046] 11 Air duct

[0047] 12 Air filter

[0048] 13 Air duct

[0049] 14 Sensor for air pressure and air temperature

[0050] 15 Exhaust gas line

[0051] 16 Exhaust gas line

[0052] 17 Sensor for boost pressure and boost air temperature

[0053] 18 Engine control device

[0054] 19 Databank

[0055] 20 Calculation unit