COMBUSTION ENGINE

FIELD OF THE INVENTION

[0001] The invention relates to an exhaust after-treatment for a dual-fuel internal combustion engine (ICE) with the features of the preamble of claim 1 and a method for operating a dual- fuel combustion engine.

BACKGROUND

[0002] Dual-fuel internal combustion engines can be operated in two operating modes. A distinction is made between an operating mode primarily using a liquid fuel supply ("liquid operation", called "diesel mode" when using diesel as liquid fuel) and an operating mode primarily using a gaseous fuel supply, in which liquid fuel serves as the pilot fuel initiating combustion. This operating mode is referred to as pilot operation or pilot-injection operation.

[0003] To keep fuel costs as low as possible and to meet emissions requirements, the goal is to operate dual-fuel internal combustion engines in pilot-injection operation at the highest substitution rates possible. The substitution rate specifies what proportion of the energy supplied to the internal combustion engine is supplied in the form of gaseous fuel. The goal is to achieve substitution rates greater than 98%. Such high substitution rates require an internal combustion engine configuration that is similar to a gas engine, e.g. regarding its compression ratio. The partly conflicting requirements of an internal combustion engine for pilot-injection operation and liquid operation force compromises in design, e.g. in terms of the compression ratio.

[0004] As a rule, strict emission requirements require the after-treatment of exhaust gases. Commonly, selective catalytic reduction (SCR) is employed to comply with NO _x (nitrogen oxide) emissions; in this process, the nitrogen oxide in combustion engine exhaust is reduced to nitrogen and water through after-treatment of exhaust gases by adding a reductant (typically a urea solution).

[0005] Diesel engines are known to be operated at high efficiency and low particle emissions. A resulting higher adjusted NO _x concentration in the diesel engine's raw emission can be brought via exhaust after-treatment via SCR to the permissible value of NO _x concentration in the treated exhaust.

[0006] A system for after-treatment of exhaust gases is known from US 8,899,018 B2, in which the relative consumption levels of diesel fuel and reductant are measured and evaluated in terms of their costs, allowing for the formulation of a cost-effect function, according to which one can adjust an internal combustion engine's operating point.

SUMMARY OF THE INVENTION

[0007] The aim of the present invention is to specify an exhaust after-treatment or dual-fuel internal combustion engine with after-treatment of exhaust gases that facilitates the cost- optimized operation of the overall system.

[0008] Said aim is achieved by an exhaust after-treatment having the features of claim 1 or by a dual-fuel internal combustion engine having the features of claim 6 or a genset having the features of claim 7 or by a method for operating a dual-fuel internal combustion engine having the features of claim 8.

[0009] In that the processing unit is designed to take a substitution rate into account when adjusting the operating point of the internal combustion engine, it becomes possible to operate the dual-fuel internal combustion engine at minimal operating costs depending on the respective costs of the gaseous fuel, liquid fuel and reductant, while still satisfying locally applicable statutory emission limits.

[0010] According to the invention, a substitution rate is specified to begin with in a first iteration as the starting value for calculating an exhaust gas composition prior to an after- treatment of exhaust gases. This composition of the exhaust gas is typically referred to as raw emission or also as cylinder-out emission.

[0011] The optimal concentration of NO _x in the raw emission of the internal combustion engine can be determined based on the respective costs of the consumable resource (gaseous fuel, liquid fuel and reductant) and the relationship between specific fuel consumption, the NO _x emissions (cylinder-out emission) and the NO _x conversion limit (the anticipated maximum conversion of NO _x ) of the after-treatment system.

[0012] The relationship between the specific fuel consumption and the NO _x emissions is usually referred to as the BSFC-NO _x trade-off. BSFC stands for "Brake Specific Fuel Consumption", the specific fuel consumption.

[0013] The substitution rate and concentration of NO _x in the raw emission are input into a model calculation, from which the correcting variables follow that are required by combustion control for the highest possible efficiency. The model calculation can be implemented as a physical model function or as a transfer function. The physical model function is a model calculation based on physical relationships, the transfer function is a statistical relationship between influencing factors and target variables.

[0014] The amount of reductant required to meet an internal combustion engine emission requirement can be determined from a calculated target concentration of NO _x in the raw emission (taking into account the expected or maximum conversion of NO _x in the SCR catalytic converter). The NO _x concentration in the treated exhaust complies with local emission targets and is indicated to the processing unit as a preset value that must be maintained.

[0015] The calculation of the target concentration of NO _x in the raw emission is iterative, which means the substitution rate can also be adjusted during the course of the calculation. Adjustments of the target concentration of NO _x and the substitution rate are carried out under conditions of highest possible efficiency and favorable thermal management for the exhaust after-treatment.

[0016] The model calculation provides combustion control correcting variables for maximum efficiency at a given substitution rate. The model calculation can also provide an improved substitution rate for higher efficiency at the given NO _x .

[0017] The combustion control takes values for the following correcting variables from the model calculation: start of a pilot-injection (start of injection), duration of pilot-injection (duration of injection), supply pressure of the liquid fuel (rail pressure) and an air- fuel ratio (λ) of the supplied gaseous fuel and air.

[0018] Taking the substitution rate into account is, among other reasons, so relevant, because the optimal efficiency substitution rate depends on the specified NO _x raw emission. [0019] Thus, given relatively high liquid fuel costs and simultaneously low costs for the reductant, operating the combustion engine at maximum efficiency in accordance with the model calculation may be advised if this approach results in minimal overall operating costs. An optimization of the operation of the combustion engine in terms of efficiency results in high NO _x raw emissions because of the disassociation of air and nitrogen at the high pressures and temperatures selected for high efficiency or given an increased proportion of liquid fuel.

[0020] In other words, in this case, a higher NO _x raw emission— at a lower-priced exhaust gas after-treatment due to the low costs of the reductant— is accepted, since the result is the minimum in terms of total operating costs (consumption of the operating resources gas, liquid fuel, reductant).

[0021] At high costs of the reductant and simultaneously low costs of the gaseous fuel and/or liquid fuel, the consumption of the reductant may be reduced to the detriment of efficiency, in order to minimize total operating costs.

[0022] One can also specify consumption of the reductant should be minimized at low tank levels in order, in this way, to prolong operating time.

[0023] The preferred embodiment provides— preferably in the processing unit— that an optimizer be configured to determine the optimal operating point for the combustion engine in terms of cost, based on the respective costs of liquid fuel, gaseous fuel, and reductant, taking into account a definable maximum concentration of NO _x in after-treated exhaust that has been processed by the after-treatment device.

[0024] The preferred embodiment provides a control loop for iterative adjustment of the substitution rate.

[0025] The optimizer can be configured as a physical or statistical function.

[0026] The preferred embodiment provides a feedback loop from which the actual consumption of resources and actual NO _x concentration before and optionally/preferentially also after exhaust after-treatment (i.e. in the treated exhaust gas) can be supplied to the processing unit.

A virtual engine and exhaust after-treatment model could also be used as an alternative to the feedback loop. The feedback loop can, on the one hand, render the BSFC NO _x trade-off more precisely; on the other hand, further optimization of the substitution rate can be performed: for example, if the costs of the reductant are low (high cylinder-out NO _x is acceptable), the model will recommend a substitution rate that is optimized for efficiency (from approximately 99% to 98%). In other words, the highest substitution rate is not necessarily expedient for a cost- optimized operating point.

[0027] The invention is particularly relevant for the use of stationary combustion engines and for marine applications or mobile applications such as so-called non-road mobile machinery (abbreviated NRMM).

[0028] The internal combustion engine can be used as a mechanical drive, for example to run compressor units, or coupled with a generator to a genset.

[0029] Protection for an internal combustion engine and a genset is also desirable.

[0030] Protection for a procedure to run a dual- fuel combustion engine is also desirable.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The invention is explained in greater detail through figures in the following: In this regard:

[0032] Fig. 1 depicts a flow chart of the invention,

[0033] Fig. 2 depicts a graph of the specific fuel consumption and the consumption of the reductant plotted against the raw emission of the internal combustion engine and

[0034] Fig. 3 is a schematic representation of a combustion engine.

DETAILED DESCRIPTION

[0035] Figure 1 shows a simplified flow chart according to an exemplary embodiment of the invention. The specific costs of the resources, e.g. gaseous fuel G, liquid fuel D and reductant U (from the English for urea), can be entered into an operator interface 6 (English: human machine interface, HMI). Optionally, the substitution rate SR can also be entered into the HMI.

[0036] The substitution rate specifies what proportion of the energy supplied to the internal combustion engine is supplied in the form of gaseous fuel. The rest up to 100% is supplied in the form of liquid fuel D. As a rule, the aim is to achieve the highest possible substitution rate e.g. 99%. [0037] The values stored in the human machine interface 6 are collected in an optimizer (5), where— in a first step— an initial optimal substitution rate SRi and an initial NO _x emissions target value in the raw exhaust gas of the combustion engine are calculated (based on the cost of the resource and on a BSFC-NO _x trade-off stored in the optimizer (5)). The index for the substitution rate SR points to the current iteration.

[0038] The optimizer (5) passes the calculated— or in another variant— the preset substitution rate SR to a model or transfer function. The model or transfer function provides the values for the correcting variables, e.g. start of injection, duration of injection, supply pressure of the liquid fuel and the air-fuel ratio λ of the gaseous fuel G. These values are passed via motor control unit (7) to the internal combustion engine.

[0039] In the present exemplary embodiment, the optimizer (5) and the motor control unit (7) are combined in a processing unit (4). A configuration where individual processing units are executed separately is conceivable or also one where they are integrated in the motor control unit (V).

[0040] Calculation of the required amount of reductant U is performed in an SCR control 51 based on the initial calculated target value of NO _x concentration in the raw exhaust gas. The required amount of reduction agent U is the amount that must be supplied to the exhaust after- treatment device (3) in order to lower the NO _x concentration in the raw emissions of the combustion engine in the exhaust after-treatment device (3) to a definable proportion of treated exhaust. An example of a definable proportion of NO _x concentration in the exhaust gas in the treated exhaust is a permissible concentration based on local legislation. The exhaust after- treatment device (3) is preferably configured as an SCR catalytic converter.

[0041] A control loop for the iterative adjustment of the substitution rate SR may be provided. The iterative adjustment of substitution rate SR can be performed via an optimizer (5), which uses the model calculation to recommend an efficiency-optimized substitution rate SR (not shown). Alternatively, or in addition, it is possible to provide a feedback loop that produces an optimized substitution rate SR. This option is indicated as a dashed arrow pointing from internal combustion engine (2) to optimizer (5). The actual consumption of fuels G, D and/or an actual NO _x concentration in the raw emissions of the combustion engine (2) are passed via this feedback loop to optimizer (5), in which [0042] the optimal substitution rate SR and the target value of NO _x concentration in the raw emissions of the internal combustion engine (2) are calculated. Flow rate sensors are provided in the respective fuel lines, for example, to determine fuel consumption figures. To determine the NO _x concentration in the raw emissions of the internal combustion engine (2), a NO _x sensor can be placed in an exhaust line of the internal combustion engine (2) in front of the exhaust after- treatment device (3). These structural details are not shown in the schematic flow diagram of Figure 1.

[0043] An adjustment in substitution rate SR can, for example, consist in lower substitution rate SR. To give a numerical example, substitution rate SR could be lowered from 99.0% to 98.4% for a given load point and specified NO _x emissions. As a result, more ignition energy is available due to the increased proportion of liquid fuel D, which in this example means efficiency improves.

[0044] Likewise, an option can be provided for iteratively adjusting the target concentration of NO _x in the raw exhaust (cylinder-out emission).

[0045] The preferred embodiment provides a feedback loop with respect to thermal management of the exhaust after-treatment device:

[0046] in the context of this disclosure, thermal management of the exhaust after-treatment device (3) is understood to mean the temperature dependence of NO _x generation in the internal combustion engine as well as the temperature dependence of NO _x conversion in exhaust after- treatment device (3). For example, if the temperatures in the exhaust after-treatment device (3), which is preferably configured as an SCR catalytic converter, are too low for sufficient conversion, the exhaust gas temperature and thereby the operational temperature of the exhaust after-treatment device (3) can be increased by influencing the setting parameters of internal combustion engine (2).

An example of such a measure for increasing the operational temperature of exhaust after- treatment device (3) would be shifting the combustion position or raising the proportion of liquid fuel.

[0047] This type of thermal management is particularly advantageous if the internal combustion engine (2) is intended to operate under different load conditions (engine loads). Different engine loads result in different exhaust temperatures, which has a critical impact on NO _x conversion rates.

[0048] If the NO _x conversion rate to be expected is low, a measure for lowering engine raw emissions can also be implemented to satisfy the emission limits in this manner.

[0049] A thermal management can be implemented, for example, by measuring a temperature on the exhaust after-treatment device (3) and the corresponding calculation of the associated NO _x conversion rate.

[0050] The preferred embodiment provides for detecting NO _x conversion before and after the exhaust gas after-treatment device (3). The relationship between the NO _x conversion and temperature is in principle known and can be stored, for example, in processing unit (4) as a model or look-up table.

[0051] Instead of or in addition to manual input via the human machine interface 6, the prices of the resources can also be retrieved automatically, e.g. from a web-based service.

[0052] Optionally, an oxidation catalytic converter 13 can be provided between the internal combustion engine (2) and the exhaust after-treatment device (3) and/or after the exhaust after- treatment device (3) as catalytic converter against ammonia slip and after the oxidation catalytic converter.

[0053] Figure 2 shows a graph of the specific fuel consumption (BSFC from "Brake Specific Fuel Consumption") plotted in g/kWh, as well as the consumption of reductant (urea mass flow) plotted in g/kWh via the NO _x concentration of the raw emissions of the combustion engine (Engine Out NO _x ) plotted in g/kWh, which are required to comply with the specified NO _x emission after treatment of the exhaust. The scales were generalized in that a reference value of the specific fuel consumption and the consumption of reductant was designated as "X" and then the respective changes in relation to this reference value were plotted in g/kWh. The scale of NO _x concentration was generalized in a similar manner, in that a reference value of the NO _x of the raw emission was designated with "Y" and the respective changes in relation to this reference value were plotted in g/kWh.

[0054] The curve labeled "Diesel" displays the specific fuel consumption in liquid operation of the combustion engine. The curves of "90% SR" and "99% SR" give examples of the specific fuel consumption for a substitution rate of 90% or 99%. The straight line labeled "Urea" displays consumption of the reductant U depending on the NO _x concentration in the raw exhaust of the internal combustion engine (2) for a specified, permissible NO _x concentration in treated exhaust gas. The scale associated with the consumption of reductant U is the right vertical axis of the graph.

[0055] What should be noted is that at a given NO _x concentration in the untreated exhaust gas of the internal combustion engine (2), a high substitution rate of 99% is favorable in terms of the specific fuel costs within the range shown.

[0056] It is further evident, that in order to determine an operating point that is optimal in terms of operating costs, one must take note of the substantial influence of specific fuel consumption on NO _x raw emissions.

[0057] Figure 3 shows a schematic of an internal combustion engine (2) with an exhaust after-treatment device (3) for reducing the nitrogen oxides (NO _x ) present in the exhaust of the internal combustion engine (2).

[0058] The combustion engine (2) has a gas feed device (8) for metering gaseous fuel G. The gas supply device 8 can, for example, can be configured as a port-injection valve for air- supercharged combustion engines or as air-gas mixer for mixture super-charged combustion engines. In addition, the combustion engine (2) has a fuel injector (9) for metering liquid fuel D. The substitution rate can be set by regulating the respective quantities of gaseous fuel G and liquid fuel D.

[0059] The exhaust after-treatment device (3) is preferably configured as SCR catalytic converter in which nitrogen oxides present in the exhaust are reduced to nitrogen by the supply of reductant U. The exhaust gas escaping from the internal combustion engine (2) enters into the exhaust gas after-treatment device (3). Optionally, one can place an oxidation catalytic converter (13) upstream. In a variant there can also be a temperature sensor 11 provided before the oxidation catalytic converter 13.

[0060] An injector (10) for reductant U is located upstream from the after-treatment device (3). The reductant U can, for example, be an aqueous urea solution. Optionally, a temperature sensor (11) and a NO _x sensor (12) may be provided. The preferred embodiment provides for a temperature sensor (11) and NO _x sensors (12) before and after the exhaust after-treatment device (3), to be able to determine the difference between the respective values (in the figure, only a single temperature sensor (11) and NO _x sensor (12) are depicted before the exhaust after- treatment device, for the sake of simplicity).

[0061] A processing unit (4) comprises an optimizer (5) and a motor control unit (7). Via human machine interface (6), the values for the costs of resources, e.g. gaseous fuel G, liquid fuel D and reductant U, and optionally, a substitution rate, can be transferred to the processing unit (4). A model calculation is stored in optimizer (5), which can be configured as calculation unit within processing unit (4), which produces a substitution rate and NO _x raw emission based on the respective costs of the resource for a specified operating point and preset NO _x emission (after treating the exhaust).

[0062] The model calculation provides combustion control correcting variables for maximum efficiency at a given substitution rate. The combustion control takes values for the following correcting variables from the model calculation: start of pilot-injection, duration of a pilot injection, supply pressure of the liquid fuel D and air- fuel ratio of the gaseous fuel G.

[0063] The amount of reductant U required to ensure compliance of the emitted NO _x concentration can be calculated in an SCR controller (51). Usually, the amount of reductant that is required is determined based on the following values:

• a temperature of the SCR catalytic converter, an exhaust mass flow,

• an engine NO _x raw emission and a target NO _x emission to be achieved in the treated exhaust,

• ammonia storage state on the catalytic converter (volume of ammonia currently stored in the catalytic converter), and

• aging factors of the catalytic converter. [0064] The preferred embodiment provides that information on the operating status of the exhaust after-treatment device (3) is reported back to the processing unit (4). This information may concern the temperature in the exhaust gas after-treatment device (3), which can be detected by a least one temperature sensor (11). The temperature can indicate the conversion rate in the exhaust after-treatment device (3). For example, if the temperature is too low, the required conversion rate will not be achieved. To address this, the exhaust temperature of the internal combustion engine (2) can be raised by changing the parameter settings in the motor control unit (7), which raises the temperature level in the exhaust after-treatment device (3).