DE102004045154A1 - Air-fuel ratio determining method for e.g. gasoline engine, involves determining air-fuel ratio of engine based on ratio of total turnover of cylinder to fuel mass supplied to cylinder - Google Patents

Abstract

The method involves determining fuel mass supplied to a cylinder in an internal combustion engine (3) and pressure of the cylinder. A total turnover (Qmax) of the cylinder is determined as a maximum of a sum of heating gradients of the cylinder, based on the cylinder pressure. Air-fuel ratio of the engine is determined based on the ratio of the total turnover to the supplied fuel mass. An independent claim is also included for a device for determining an air-fuel ratio of an internal combustion engine.

Description

The present invention relates to a method and a device for determining an air-fuel ratio in an internal combustion engine, in particular in a gasoline engine. To operate a gasoline engine, a certain air-fuel ratio is required. This ratio between an air mass ml supplied to the gasoline engine and a fuel mass mk supplied to the gasoline engine is ideal, theoretically complete, ie stoichiometric, at ml / mk = 14.7 =: Lst. Usually, the air-fuel ratio is specified using the so-called "air ratio" λ, also referred to as "combustion air ratio" or simply air ratio, which represents an air-fuel ratio normalized to the value for stoichiometric combustion:

One Value λ = 1 therefore means stoichiometric Combustion.

from Air-fuel ratio hang significantly the specific fuel consumption of a gasoline engine as well its pollutant emissions, that is exhaust emissions and a Degree of conversion of a built-in gasoline engine catalyst from. Usually is a value for λ means a so-called lambda probe is measured and dependent on the measured value of the gasoline engine preferably exactly on λ = 1 regulated to an optimal conversion rate of usually used to achieve three-way catalysts.

such However, lambda probes have the disadvantage that they have a certain warm-up time need. she are therefore not immediately ready for operation from a starting time of the gasoline engine. Therefore, during this warm-up phase conventionally the engine operated and not operated regulated, what terms fuel consumption and pollutant emissions, however optimal.

When Alternative or supplement for the use of a lambda probe was therefore proposed that Air-fuel ratio an internal combustion engine from one in cylinders of the internal combustion engine to determine measured cylinder pressure.

From the EP 0 851 107 A2 In this regard, it is known to measure a cylinder pressure at a given crank angle before a top dead center of the cylinder and a predetermined crank angle after top dead center of the cylinder and to determine therefrom the air-fuel ratio. However, such a determination of the air-fuel ratio is relatively inaccurate, particularly with regard to the long-term stability.

Out Norbert Müller, Adaptive engine control in the gasoline engine using combustion chamber pressure sensors, VDI Progress Report Series 12 No. 545, Dusseldorf 2003, pages 108-112 a method is known in which a value for λ is derived from the difference of a modeled one calculated moments and a measured moment is calculated, where the measured torque is based on cylinder pressure measurements is determined. However, this method has the disadvantage that it only for Air ratios λ ≥ 0.9 applicable and thus, for example, not for the controlled operation of Katalysatoraufheizkonzepten with secondary air pumps can be used.

It is therefore an object of the present invention, a method or a device for determining the air-fuel ratio to provide one as possible accurate determination of air-fuel ratio also for values of λ <0, 9 is possible.

These Task is solved by a method according to claim 1 or by a device according to claim 19. The dependent claims define advantageous or preferred embodiments of the present invention Invention.

According to the invention, for determining an air-fuel ratio of an internal combustion engine, it is proposed to determine a fuel mass supplied to at least one cylinder of the internal combustion engine and a cylinder pressure in the at least one cylinder of the internal combustion engine, depending on the cylinder pressure to determine a total revenue in the at least one cylinder and dependent of the Total sales and the fuel mass supplied to determine the air-fuel ratio.

to Determination of the air-fuel ratio can in particular a relationship of total sales to the supplied Fuel mass, also referred to as the combustion characteristic, used become. It has become shown that this combustion characteristic is quadratically dependent on the air-fuel ratio and thus the air-fuel ratio can be determined from the combustion characteristic value.

A Such determination of the air-fuel ratio depends on the total turnover and the supplied Fuel mass allows a relatively accurate determination of the air-fuel ratio also for values the air ratio λ <0.9.

Of the Total sales can be determined in particular by a cumulative heating process for the at least one cylinder is determined and the total sales of the Maximum value of the Sumheizheizverlaufs is determined. The cumulative heating process can in particular by means of indexing to Hohenberg from the course of the at least one cylinder pressure over a Crank angle of the at least one cylinder can be determined. These Evaluation is possible with relatively little computational effort in real time.

The Air-fuel ratio In particular, a deviation of the total sales and the supplied Fuel mass determined combustion characteristic of a modeled Combustion characteristic for λ = 1 determined become. The modeled combustion parameter can be used during operation of the internal combustion engine by means of a lambda probe measured Value for the air-fuel ratio be adapted. Instead of a sine function, the modulation also with other functional approaches respectively.

to Determination of the air-fuel ratio may include other parameters such as For example, considered an engine or cooling water temperature become.

alternative can the air-fuel ratio also dependent of a slope of the reciprocal combustion characteristic, which determined as described above. For this purpose can the internal combustion engine, in particular a sinusoidally modulated fuel mass supplied and a corresponding amplitude of the reciprocal combustion characteristic over a recursive least-squares estimator become. Instead of a sine function, the modulation can also with other functional approaches respectively.

The for the execution the method according to the invention necessary Electronics can be easily integrated into a motor control be as needed Signals like the cylinder pressure usually be measured anyway.

The Invention will now be described with reference to the accompanying drawings using preferred embodiments explained in more detail. It demonstrate:

1 Measuring curves which show a dependence of a combustion characteristic of λ,

2 a block diagram of an embodiment of the present invention,

3 a block diagram of another embodiment of the present invention,

4 Graphs illustrating the recovery of an air-fuel ratio from a reciprocal combustion characteristic,

5 a block diagram of yet another embodiment of the present invention, and

6 a block diagram of a motor controller for implementing the method according to the invention.

The following is intended with reference to 1 the basic principle of the present invention will be explained.

For this purpose, the so-called combustion characteristic is considered. For a cylinder of an internal combustion engine, the combustion characteristic value VKW denotes the ratio between the total conversion Qmax, that is to say the total energy converted in the cylinder, to the fuel mass mk supplied to the cylinder, ie

One Such value can also be for all cylinders of an internal combustion engine, for example a gasoline engine, calculated by the combustion characteristics of individual cylinders be added.

in principle however, considering cylinders separately is preferable Finally, for each cylinder separately to determine an air-fuel ratio.

The Air-fuel ratio is given below by means of the air ratio λ, which in the introduction to the description in equation (1) has been defined.

wobei a0, a1 und a2 Konstanten sind, welche vom jeweiligen Arbeitspunkt (Motordrehzahl, Ansaugdruck) abhängen. Werden also beispielsweise diese Konstanten durch Bestimmung des Verbrennungskennwerts mit dem nachfolgend beschriebenen Verfahren und des Luftverhältnisses λ mit einer herkömmlichen Lambda-Sonde bestimmt und gespeichert, kann nachfolgend durch Auflösen von Gleichung (3) nach λ das Luftverhältnis λ aus dem Verbrennungskennwert bestimmt werden. 1 shows measured curves of the combustion characteristic in J / g over the air ratio λ at two different operating points of a gasoline engine, where measured curve 1 at 1100 revolutions per minute and a suction pressure of 650 millibars and trace 2 was recorded for an engine speed of 1500 revolutions per minute and a suction pressure of 700 millibars. In both cases, a quadratic relationship between the combustion characteristic and λ, that is, shows

where a0, a1 and a2 are constants which depend on the respective operating point (engine speed, intake pressure). If, for example, these constants are determined and stored by determining the combustion characteristic value with the method described below and the air ratio λ with a conventional lambda probe, the air ratio λ can subsequently be determined from the combustion characteristic value by solving equation (3) for λ.

As from equation (2) must be used to calculate the combustion characteristic the total conversion Qmax and the fuel mass mk supplied to the cylinder is determined become. In this case, mk is usually in an internal combustion engine anyway determined or regulated. The total sales Qmax can be from a measurement the cylinder pressure over the current cylinder volume can be determined. It is used as a parameter for the Cylinder volume usually the so-called crank angle measured, from which the cylinder volume can be calculated directly.

bestimmt, wobei c_v die spezifische Wärmekapazität bei konstantem Volumen, R die allgemeine Gaskonstante und κ der Adiabatenexponent sind. κ beträgt bei typischen Gasgemischen etwa 1,33. Die spezifische Wärmekapazität c_v kann näherungsweise zu

bestimmt werden, wobei T eine Brennraumtemperatur in Kelvin und A eine motorabhängige Konstante ist, welche für Ottomotoren typischerweise 0,1 beträgt.To calculate the total sales Qmax from the measurement of the cylinder pressure over the cylinder volume, a so-called sum heating curve, which characterizes the amount of heat generated by combustion minus losses due to wall heat of the cylinders or so-called blow-by losses, is calculated. This can be done for example by the well-known evaluation to Hohenberg. In this case, from a multiplicity of measuring points (p _i , V _i ), i = 1, 2,..., N, where p _i is a cylinder pressure present at a cylinder volume V _i and N is a number of the measuring points

where c _{v is} the specific heat capacity at constant volume, R is the general gas constant and κ is the adiabatic exponent. κ is about 1.33 for typical gas mixtures. The specific heat capacity c _v can be approximated to

where T is a combustion chamber temperature in Kelvin and A is an engine dependent constant which is typically 0.1 for gasoline engines.

The sum heating curve Q is then determined by simple summation over the heating curve ΔQ _i :

Of the Total revenue Qmax is then the maximum of the cumulative heating Q. The method described above requires a relatively small Computational effort and is therefore particularly suitable for real-time calculations. It can but of course Other methods for calculating the total sales Qmax or be used to calculate the sum of heating Q.

In 2 is a block diagram of an embodiment of an inventive apparatus for determining the air-fuel ratio by means of the method described above. This is an internal combustion engine 3 a fuel mass mk and an air mass ml supplied. Influences E affect the combustion in the internal combustion engine, for example, an ignition angle, an engine temperature or an intake pressure.

In the internal combustion engine 3 For each cylinder, a cylinder pressure p is measured as a function of a crank angle φ and to a cylinder pressure evaluation device 4 to hand over. The cylinder pressure evaluation device 4 calculated from the equations (4) - (6) the total Qmax, where, as already described, the respective cylinder volume directly depends on the crank angle.

The total conversion Qmax, as well as the supplied fuel mass mk, an engine speed nm and the influences E descriptive parameters of a lambda modeling 5 which, based thereon, based on equation (3), outputs a value λm for the air ratio which characterizes the air-fuel ratio. The engine speed nm and influences E can be used to define a respective operating point and thus, for example, the correct parameters a0, a1 and a2 from equation (3).

In 3 Figure 4 is a more detailed block diagram of one embodiment of the lambda modeler 5 out 2 shown. This is done in a first calculation unit 6 from the total conversion Qmax and the supplied fuel mass mk according to equation (2) a measured combustion characteristic value VKWm calculated.

Furthermore, in a second calculation unit 7 a theoretical or modeled combustion parameter VKWt for a defined by engine speed nm, intake pressure ps and ignition angle ZW operating point for an air ratio λ = 1, that is calculated for stoichiometric combustion. This can be done, for example, map-based by means of previously recorded on an engine test bench maps. However, it is also a modeling over a polynomial sentence, for example according to VKWt = b0 + b1 * nm + b2 * ps + b3 * ZW + b4 * nm 2 + VKWk (7) possible, where b0-b4 are constants and VKWk is a correction offset.

This correction offset VKWk can be adapted by means of a later explained adaptation loop and in an adder 10 to that of the second calculation unit 7 outputted value to form the modeled combustion characteristic VKWt. This is in a subtraction unit 11 subtracted from the measured combustion characteristic value VKWm to form a differential combustion characteristic value ΔVKW. For adaptation of the correction offset VKWk this difference of an adaptation device 9 fed. This adaptation device 9 is from an activation device 8th activated if an air ratio λs measured by a lambda probe is equal to 1 within a given accuracy. For an air ratio of 1, the measured combustion parameter VKWm should match the modeled combustion parameter VKWt modeled for λ = 1, and thus ΔVKW = 0. Accordingly, VKWk from the adaptation device 9 changed until ΔVKW = 0.

In a lambda calculation unit 12 finally, the air ratio λm is calculated from the difference ΔVKW, wherein additionally an engine temperature, represented by a cooling water temperature Tw, can be taken into account. The use of the difference ΔVKW instead of the measured value VKWm for the calculation of λm has the particular advantage that the operating point of the motor can be taken into account via the value VKWt.

ergibt. Für eine Modulation kleiner Amplitude kann Kurve 13 für den durch die Modulation überstrichenen Bereich näherungsweise als linear angesehen werden, wobei die Steigung der Kurve 13 dann das Verhältnis der Amplitude A_RVKW zu einer entsprechenden Amplitude A_Φ des reziproken Luftverhältnisses Φ, welche durch Kurve 15 in 4 angedeutet ist, entspricht. Diese Steigung ist andererseits gleich dem Gradienten des Polynoms aus Gleichung (8), das heißt

wobei AP den jeweils betrachteten Arbeitspunkt bezeichnet.Another possibility for calculating λ from a measured combustion characteristic value VKW is now to be made with reference to FIG 4 and 5 be explained. measured curve 13 in 4 shows the re ziproken combustion characteristic RVKW = 1 / VKW above the reciprocal air ratio Φ = 1 / λ. According to equation (3), a quadratic relationship applies here, too RVKW = c2 · Φ 2 + c1 · Φ + c0 (8) where c0, c1 and c2 are again coefficients which in principle can also depend on the operating point. According to the invention, the supplied fuel mass mk is now sinusoidally modulated during the measurement of the reciprocal combustion characteristic value. mk = mk 0 (1 + mk A sin (ωt)) = mk 0 + mk M (t) (9) where mk _{0 is} an average or DC component of the air mass supplied, mk _{A is} a modulation amplitude, mk _{M is} a momentary modulation amplitude, ω is the angular frequency of the modulation and t is the time. Such a sinusoidal excitation leads to a corresponding behavior of the reciprocal combustion characteristic, as shown in curve 14 is indicated, in which case a phase shift may occur, ie RVKW = A0 + A1sin (ωt) + A2cos (ωt) (10) where A0 represents a DC component, A1 a sine component and A2 a cosine component of the reciprocal combustion characteristic value. The factors A0, A1 and A2 are determined, for example, in an engine control unit in real time via a least square estimator, the reciprocal combustion parameter being calculated in principle according to equation (2). The phase shift of the reciprocal combustion characteristic is given as arctan (A2 / A1), while the amplitude A _{RVKW increases with respect to} the oscillation of the reciprocal combustion _parameter

results. For a small amplitude modulation curve can 13 for the area swept by the modulation approximately as linear, with the slope of the curve 13 then the ratio of the amplitude A _RVKW to a corresponding amplitude A _{Φ of} the reciprocal air ratio Φ, which by curve 15 in 4 is indicated corresponds. On the other hand, this slope is equal to the gradient of the polynomial of equation (8), that is

where AP denotes the respective considered operating point.

und entsprechend für die Amplitude A_Φ

According to equation (1) applies

and correspondingly for the amplitude A _Φ

berechnen, wobei die Funktion f gemäß

gegeben ist. Die benötigten Koeffizienten c1, c2 werden dabei beispielsweise über eine Messung mit einer Lambda-Sonde adaptiert bzw. bestimmt und in der Motorsteuerung abgespeichert.From equations (9), (12), (13) and (14) can then be the reciprocal air-fuel ratio Φ and thus a value for the air ratio λ at the operating point according to

calculate, where the function f according to

given is. The required coefficients c1, c2 are adapted or determined, for example, via a measurement with a lambda probe and stored in the engine control.

In 5 Fig. 12 is a block diagram for implementing the method for determining λm described above. internal combustion engine 3 and cylinder pressure evaluation device 4 corresponding to the corresponding blocks 2 and will therefore not be explained again. A calculation unit 21 which is essentially the first calculation unit 6 out 4 calculated from the total conversion Qmax and the supplied fuel mass mk (t) corresponds to the reciprocal combustion characteristic value RVWK (t) from equation (10). The bracketed t indicates that it is now in contrast to the case of 4 are sinusoidally temporally modulated quantities. The reciprocal combustion index RVWK (t) becomes a filter 22 fed. The purpose of this filter is to filter out implausible values of RVWK (t), which can arise, for example, as a result of burnout. For this purpose, the filter 22 be configured for example as a gradient filter.

The filtered reciprocal of combustion value then becomes a least squares estimator 23 for determining the amplitude A _RVKW supplied. A weighting factor V can be used to weight the values of RVKW (t) such that "older" measured values are no longer taken into account in the calculation of A _{RVKW and} V can therefore also be referred to as the "forgetting factor". This provides a dynamics of the least squares estimator 23 affected.

Another calculation unit 24 then calculates the desired value for the air ratio λm according to equations (15) and (16).

It should be noted that the described methods for determining λ are preferably used separately for cylinders. Of course, it is also possible to determine an averaged value of the air ratio λ for all cylinders. The functions necessary for carrying out the method according to the invention, ie the different means 1 . 2 and 5 , are preferably integrated within a motor control, as in 6 is shown.

It is powered by an internal combustion engine 3 the cylinder pressure p (φ) of a motor control 16 fed. The engine control 16 includes funds 17 for combustion analysis, means 18 to form λm according to the method of the invention, means 19 to the lambda control and means 20 for lambda diagnosis. The means 17 for combustion analysis use cylinder pressure measurement for purposes other than those described above, for example, to determine induced engine torque or to determine a loss. The means 20 for lambda diagnosis receive a value λs of the air ratio of a lambda probe and compare it - for example, to check the operability of the lambda probe or for adapting the method according to the invention 4 - with the means 18 λm supplied to lambda modeling. The means 19 to lambda control give as a function of λm - especially in the starting phase of the internal combustion engine 3 Or from λs a control signal e to the internal combustion engine 3 , which regulates the injection of the internal combustion engine.

The inventive method is particularly suitable for determining the air-fuel ratio in gasoline engines, is in principle also on other internal combustion engines such as diesel engines applicable.

1

measured curve

2

measured curve

3

internal combustion engine

4

calculation unit

5

calculation unit

6

calculation unit

7

calculation unit

8th

release device

9

adaptation device

10

summing

11

summing

12

modeling means

13

measured curve

14

stimulation

15

vibration

16

motor control

17

medium for combustion analysis

18

medium for lambda modeling

19

medium to lambda control

20

medium for lambda diagnosis

21

calculation unit

22

filter

23

Least-squares estimator

24

modeling unit

mk

Fuel mass

ml

air mass

λ

air ratio

FMV

Combustion parameter

p (φ)

cylinder pressure

nm

Engine speed

Qmax

total sales

.lambda..sub.m

modeled air ratio

.lambda..sub.s

measured air ratio

e

Injection signal

A _RVKW

amplitude

V

weighting factor

RVKW

reciprocal Combustion parameter

ps

suction

ZW

firing angle

VKWk

correction value

VKWt

calculated Combustion parameter

VKWm

measured Combustion parameter

ΔVKW

Combustion characteristic value difference

tw

Cooling water temperature

Claims (21)

Method for determining an air-fuel ratio (λm) of an internal combustion engine ( 3 ), wherein at least one cylinder of the internal combustion engine ( 3 ) and a cylinder pressure (p (φ)) is determined in the at least one cylinder, wherein depending on the cylinder pressure (p (φ)) a total conversion (Qmax) of the at least one cylinder is determined, and depending on the total conversion (Qmax) and the supplied fuel mass (mk), the air-fuel ratio (λm) is determined. Method according to claim 1, characterized, that the cylinder pressure (p (φ)) dependent on determined by a respective volume of the at least one cylinder will, and that the total sales (Qmax) depending from the cylinder pressure (p (φ)) and the respective volume is determined. Method according to claim 2, characterized in that that the total sales (Qmax) as the maximum of a cumulative heat flow (Q) of the at least one cylinder is determined. Method according to one of the preceding claims, characterized in that the air-fuel ratio (λm) is dependent on a ratio of the Total sales (Qmax) to the fuel mass supplied (mk) is determined. Method according to claim 4, characterized in that that the air-fuel ratio (λm) in dependence from a difference of the ratio of total sales (Qmax) to the fuel mass (mk) supplied and a modeled relationship is determined. Method according to claim 5, characterized in that that the modeled ratio in dependence of an engine speed (nm), a suction pressure (ps) and / or a firing angle (ZW) is calculated. Method according to claim 5 or 6, characterized that the modeled ratio is calculated with a polynomial function. Method according to one of claims 5 to 7, characterized that the modeled relationship like that is calculated that the difference for a stoichiometric value of the air-fuel ratio is substantially equal to 0. Method according to one of claims 5 to 8, characterized in that the modeled ratio during normal operation of the internal combustion engine ( 3 ) is adapted by means of a measured air-fuel ratio (λs). Method according to claim 9, characterized in that in that the measured air-fuel ratio (λs) is measured with a lambda probe becomes. Method according to one of claims 1 to 4, characterized, that the supplied Fuel mass (mk) according to a predetermined function is modulated, that caused thereby Modulation of the ratio of total sales (Qmax) to the fuel mass (mk) supplied is measured and that depends from the measured modulation of the ratio of total sales (Qmax) to the supplied Amount of fuel (mk) the air-fuel ratio is determined. Method according to claim 11, characterized in that that the given function is a sine function. Method according to claim 11 or 12, characterized that the air-fuel ratio dependent on from an amplitude of the modulation of the ratio of the total revenue (Qmax) to the supplied Fuel mass (mk) is determined. A method according to claim 13, characterized in that the amplitude of the modulation of the ratio of the total revenue (Qmax) to the supplied fuel mass (mk) by a least square estimator ( 23 ) is determined. A method according to claim 13 or 14, characterized in that the air-fuel ratio according to

where λm is an air ratio corresponding to the air-fuel ratio, mk _{0 is} an average of the modulation of the amount of fuel supplied, mk _{A is} an amplitude of the modulation of the supplied fuel _mass (mk), A _{RVKW is} an amplitude of the modulation of the ratio of the total revenue to the supplied fuel mass and c1 and c2 are predetermined constants. Process according to one of the claims 11 to 15, characterized in that that the modulated ratio of total sales (Qmax) to the fuel mass (mk) supplied is filtered to influences of disorders to damp a combustion in the internal combustion engine. Method according to claim 16, characterized in that the filtering takes place with a gradient filter. Method according to one of claims 1 to 17, characterized in that the air-fuel ratio in dependence on a temperature of the internal combustion engine ( 3 ) is determined. Device for determining an air-fuel ratio (λm) of an internal combustion engine ( 3 ), with means for determining at least one cylinder of the internal combustion engine ( 3 ) supplied fuel mass (mk), means for determining a cylinder pressure (p (φ)) of the at least one cylinder of the internal combustion engine ( 3 ), with first calculation means ( 4 ) for the calculation of a total turnover (Qmax) depending on the Cylinder pressure (p (φ)), and with second calculation means ( 5 . 6 - 12 . 21 - 24 ) for determining the air-fuel ratio (λm) as a function of the total conversion (Qmax) and the supplied fuel mass (mk). Device according to claim 19, characterized in that that the device is carrying the method according to one of claims 1 to 18 is configured. Apparatus according to claim 19 or 20, characterized in that the device is at least partially in a Moto3rsteuerung ( 16 ) of the internal combustion engine ( 3 ) is integrated.