JP7575873B2 - Method for adjusting richness in a spark-controlled internal combustion engine - Google Patents

Description

The present invention relates to a method for adjusting the richness of an air-fuel mixture in an internal combustion engine of the ignition-controlled type (especially a gasoline-fueled engine). The present invention is advantageously suited for application in the automotive field.

In internal combustion engines of the ignition-controlled type (gasoline-fueled), particularly in motor vehicles, it is known to determine the richness of the mixture based on a set of engine operating parameters, including at least the engine speed and torque required for the operation of the vehicle. The boost or amount of air that the engine can take in is particularly regulated by the opening of the engine butterfly valve, and the richness is adjusted accordingly by adjusting the time for injecting fuel into the engine.

For example, it is known to set the richness of such engines near the stoichiometric value, i.e., near a richness of 1, for most engine operating points so that a catalytic converter of the three-way catalytic converter type mounted in the exhaust pipe of an engine can remove pollutants from the engine's combustion gases. By using closed-loop control, for example, adjusting the richness through the use of indications from at least one oxygen probe mounted at the inlet of the catalytic converter, the catalytic converter can be operated within its catalytic window, reducing the nitrogen oxides (NOx) produced in the engine's combustion gases and oxidizing unburned hydrocarbons (hydrocarbons or HC) and carbon monoxide (carbon monoxide or CO).

However, it is also known that near full load, engine exhaust circuit components can reach extremely high temperatures, which can compromise their mechanical strength and reduce vehicle reliability. When a torque (or load) value above a certain threshold is detected as being required to operate the vehicle, the mixture as a whole is enriched, such as to a richness value above 1.20, in order to limit the exhaust temperature.

The richness value set to obtain a given maximum temperature in the exhaust pipe, such as a given exhaust manifold temperature or a given turbocharger turbine temperature (in the case of a supercharged engine), can be determined by prior tests on an engine test bed in stable mode, i.e. by keeping the speed and torque constant. The richness value is in fact set so that the exhaust gas temperature itself is equal to a given predetermined value, which after a certain time related to the inertia of the materials, becomes that of the components of the engine's exhaust circuit.

Numerous methods are known from the prior art for adjusting the richness of a vehicle, increasing the richness to values higher than 1 when the load on the engine reaches a value close to full load.

For example, Japanese Patent Publication No. JP-S-6043144 discloses a method for setting fuel injection time according to engine operating conditions in order to avoid overheating of the exhaust circuit. Upon detecting a load above a threshold, a temperature sensor measures the exhaust gas temperature and increases richness after a predetermined delay that decreases with gas temperature.

This delay is intended to avoid unnecessary overconsumption of fuel. In particular, as soon as a high load is detected, there is no need to immediately increase the richness, since the components that make up the exhaust circuit have a certain thermal capacity and therefore do not immediately rise to temperatures that are limiting in terms of their reliability, even when the engine is running at high load, due to the heat caused by the exhaust gases. Therefore, in delaying the increase in richness, this characteristic can be used advantageously, without compromising the reliability of the engine, and theoretical fuel economy can be achieved.

However, this method is inaccurate because the temperature measured by the sensor after the detection of a high load does not correctly indicate the temperature of the components in the engine's exhaust circuit at that same moment. More specifically, the temperature of the components depends on the amount of heat provided to them before the high load is detected; the more heat energy provided to the component, the higher this temperature will be.

Thus, if the delay is reduced when the measured gas temperature is high, a situation may arise where the richness is increased almost immediately, even though the temperature of the exhaust circuit components has not yet risen to the measured gas temperature value and there is no need to change the richness at that particular moment. Fuel consumption is therefore unnecessarily worsened.

From US-A-5239965 a method of adjusting the richness is also known, in which the application of the increased richness value is delayed by a delay whose length depends on the thermal condition of the components of the exhaust circuit, which is measured by dedicated measuring means.

More specifically, it detects whether the engine load exceeds a high reference load (denoted by the acronym PMOTP1), which helps determine whether the components in the exhaust circuit are under high-load thermal conditions. If the test is positive, this means that the heat from the exhaust gases is increasing the temperature of the components and that they may overheat if the richness is not increased.

Under these high load conditions, the value of a delay counter (designated by the acronym COTPCY), which represents the rate at which the temperature of the exhaust circuit components has increased as a result of heat from the gases before the high load is detected, is periodically incremented and the value of this counter is compared with a reference delay threshold (designated by the acronym QAOTP) previously calibrated according to the throughput of air through the engine. This delay threshold decreases with the throughput.

As long as the counter value is below the threshold, it is assumed that the exhaust gas components will not overheat for a very short period of time and no enrichment measurements are performed. On the other hand, if the counter value exceeds the threshold, the counter value is frozen at the exact moment the threshold is crossed and the richness is immediately increased.

Such a method entails a lot of work in pre-calibrating the delay thresholds. Moreover, to fix them, only the air throughput through the engine is taken into account. Inaccurately applied thresholds result in the risk of exceeding the permissible temperature (if the thresholds are set too high, this will cause the enrichment to be delayed too long) or excessive fuel consumption (if the thresholds are set too low, this will cause the enrichment to be too soon).

From US-A-4,400,944, it is also known how to adjust the richness in order to avoid thermal failure of the turbocharger of an engine, especially at high speeds and loads, when the engine has a very slow ignition advance, which leads to high turbocharger temperatures.

According to these documents, when the temperature of the exhaust gases passing through the turbocharger is below a predetermined target value (designated T1) and this temperature increases over time, open-loop control is used to adjust the richness and add a fuel injection time correction to the injection time that depends on the difference between the temperature and the temperature target, which corresponds to a stoichiometric mixture.

On the other hand, if the temperature is below the target value and the mixture is rich, and the temperature decreases over time, the richness is adjusted by subtracting a fuel injection time correction from the injection time, which corresponds to a stoichiometric mixture.

Such a method, with variable richness, allows the exhaust gas temperature to be adjusted to a target value and does not require the selection of a delay time to increase the richness to a value greater than one. However, this has the potential to exceed the target, reducing the reliability of exhaust circuit components, causing excessive fuel consumption, and increasing pollutant emissions from the engine.

The present invention shows how to overcome the shortcomings of existing methods for adjusting richness.

This is aimed, inter alia, at presenting a method of adjusting richness that is capable of protecting the engine's exhaust circuit from excessive temperatures, is easy to implement, and limits excessive fuel consumption by the engine without accentuating the undesirable effects of pollutant emissions.

To this end, the present invention provides a method for adjusting the richness of an ignition-controlled internal combustion engine. The richness value is set near the stoichiometric mixture value unless the engine is operating near full load, and is set near a value higher than the stoichiometric mixture value when the engine torque exceeds a torque threshold below the maximum engine torque.

This method comprises the steps of: when the torque exceeds a torque threshold;
- if the engine exhaust gas temperature value is less than a first temperature threshold, the richness value is set to a first richness value higher than the stoichiometric value;
as soon as the temperature value exceeds a second temperature threshold, which is higher than the first threshold, the richness value is set to a second richness value, which is higher than the first richness value;
a step in which, when the temperature value lies between the first and second threshold values, the richness value is gradually increased from the first richness value up to the second richness value.

Further features and advantages of the present invention will become apparent from a reading of the non-limiting embodiments thereof with reference to the accompanying drawings, in which:

FIG. 1 shows an example of an ignition-controlled internal combustion engine suitable for implementing the method according to the invention. FIG. 2 is a flow diagram of the steps of a method for adjusting richness, according to one embodiment of the present method.

Figure 1 shows a cross-section through an internal combustion engine of the ignition-controlled type (particularly gasoline-fueled), and more specifically through one cylinder 1 of the engine block. An intake circuit 2 and an exhaust circuit 3 communicate with the intake duct 4 and exhaust duct 5 of cylinder 1, respectively.

The intake circuit 2 includes, without limitation, in the direction in which air circulates from upstream to downstream, a compressor 6 of a turbocharger 7 used to supercharge the engine, an air flow control valve 8 or butterfly valve 8, and an intake manifold 9 or splitter 9.

In this example, the engine is in the form of an indirect injection engine, without limitation, with a fuel injector 10 opening into the intake duct 4 for injecting gasoline into the duct 4. Alternatively, although not shown, the engine may be of the direct injection type.

A cylinder head 11 of the engine is located above the cylinder 1. The cylinder head 11 houses an intake valve 12 used to open and close the intake duct 4, and an exhaust valve 13 used to open and close the exhaust duct 5. The cylinder 1 surrounds a piston 14 that can move reciprocally between a bottom dead center (BDC) position and a top dead center (TDC) position inside a bore 15 of the cylinder 1. A combustion chamber 16 is formed in the space defined between the piston 14 and the cylinder head 11. A spark plug 17 is placed on the cylinder head 11, and its electrode is connected to the inside of the combustion chamber 16.

The exhaust circuit 3 includes, without limitation, from upstream to downstream in the direction in which the combustion gases circulate, an exhaust manifold 18, a turbine 19 of a turbocharger 7 mounted on a common shaft 20 with the compressor 6, and a device 21, such as a three-way catalytic converter 21, that removes pollutants from the engine's combustion gases.

The temperature sensor 22 is mounted in the exhaust circuit 3 at the inlet of the turbine 19. This sensor can measure the temperature of the exhaust gases upstream of the turbine.

The richness sensor 23 or oxygen probe 23 is mounted in the engine exhaust circuit 3 upstream of the three-way catalytic converter 21. This sensor is capable of measuring the oxygen concentration value of the engine exhaust gas.

In a known manner, for example, an engine control unit (not shown) includes means for determining at least one of the following: a value of the load (or supercharged air flow rate) Qair, a value of the flow rate of injected fuel Qfuel and the timing of fuel delivery relative to top dead centre, and a value of the spark advance AA as a function of a set of parameters describing the operation of the engine, including at least the engine torque C and the engine speed N.

In a conventional manner, the control unit adjusts the boost air Qair by adjusting the opening of the butterfly valve 8 and/or the turbine power via the opening of the turbine wastegate (or exhaust) valve (not shown). The control unit adjusts the fuel flow Qfuel and the timing of its injection by adjusting the injection time Ti of the injector 10, more specifically its opening and closing moment relative to top dead center. The control unit adjusts the spark advance AA by firing a spark between the electrode terminals of the spark plug 17 at a given angle in the engine cycle relative to top dead center of the cylinder 1.

With reference to FIG. 2, the method for adjusting richness according to the invention may comprise the following steps, implemented by the engine control unit and performed at regular time intervals dt repeatedly at each instant t _n+1 after the previous instant t _n :

The method begins at step 100, where the engine control unit determines an engine speed value N and a torque reference value C required for operation of the vehicle. The speed value may be from a sensor (not shown in FIG. 1) mounted on the end of the engine crankshaft, and the torque value may be subtracted from the driver's throttle pedal depression.

In a first test step 200, it is checked whether the engine torque is close to full load, in other words whether the torque C is between a torque threshold Cs, which is less than or equal to the maximum torque Cmax that the engine can produce, and the maximum torque Cmax. The torque threshold Cs and the maximum torque Cmax depend on the speed N. They define a range of operating points at which a mixture richness r equal to 1 may cause the temperature _θexh (measured by the sensor 22) of the engine's combustion gases upstream of the turbine to exceed a threshold value corresponding to a reliability limit (typically a temperature of the order of 950°C to 980°C).

If the test result is negative, i.e. the torque is not near full load (in other words, is less than the torque threshold Cs), the method proceeds to step 300, where the mixture richness r is set near the stoichiometric mixture richness (richness of 1). The method then resumes from step 100.

If the opposite is true, the method proceeds to step 400, where the richness r is set to a first richness value _r1 corresponding to a slightly rich mixture, such as a richness between 1.00 and 1.05.

Preferably, the first richness value is substantially equal to 1.01, and the richness is immediately set to this first richness value equal to 1.01, so that the engine exhaust gases can be immediately pre-cooled without significantly affecting the vehicle's nitrogen oxide emissions. Alternatively, although not shown, the first richness value may be substantially equal to 1.05. Setting the richness to this first richness value is achieved progressively, e.g., linearly as a function of time, from the stoichiometric mixture value to the first richness value substantially equal to 1.05.

The method continues with step 500, where the exhaust gas temperature θ _exh is measured, such as using temperature sensor 22. Although not shown, alternatively, the order of steps 400 and 500 may be reversed.

In test step 600, the temperature θ _exh is compared with a first temperature threshold θ ₁ , such as a temperature of the order of 900° C. If this temperature is below the first threshold, the method resumes from step 100. In other words, the richness remains set to the stoichiometric value. If the opposite is true, i.e. if this temperature exceeds the first threshold, the method continues further with test step 700. Here, the temperature θ _exh is compared with a second temperature threshold θ _{2, which corresponds to the thermomechanical limit of the exhaust circuit, such as a temperature of the order of 950° to 980° C. The method suggests that this second temperature threshold θ 2 is} not exceeded.

If the temperature _θexh of the exhaust gases exceeds the second threshold value _θ2 , the method proceeds to step ₈₀₀ , where the richness is immediately set to a second richness value _r2 , which is higher than the first richness value _r1 . The determination is made in such a way as to obtain a given maximum temperature of the components of the exhaust circuit, such as a given exhaust manifold temperature or a given turbocharger turbine temperature (in the case of a supercharged engine). The second richness value _r2 may be determined by carrying out preliminary tests on an engine test bed in stable mode, i.e. keeping the speed and torque constant. The richness value is in fact set so that the exhaust gas temperature itself is equal to a given predetermined value, which after a certain time related to the inertia of the materials, becomes that of the components of the exhaust circuit of the engine.

For each given speed value N, there is a range of torque C, near full load, where the richness must actually be increased to limit the exhaust gas temperature. Therefore, a second richness value _r2, which depends on speed and torque, needs to be determined.

If this is not the case, i.e. if the exhaust gas temperature _θexh is less than the second threshold value _θ2 , the method proceeds to step 900. Here, the richness is gradually increased, i.e. each time the exhaust gas temperature is between the first threshold value _θ1 and the second threshold value _θ2 , iterating successively every time interval dt, from a first richness value _r1 up to a second richness value _r2 .

Preferably, the richness is increased linearly as a function of time and at the same time limited by a maximum value equal to the second richness value _r2 , i.e. according to a formula of the following type:

(Formula 1) rn ₊₁ = max ( _r2 ; _rn + k × ( _r2 - _r1 ) × dt),

In this formula,
- r _n+1 denotes the richness value calculated at instant t _n+1 of the method,
r _n denotes the richness value calculated at instant t _n of the method,
- k denotes a positive constant coefficient indicating the rate at which richness increases.

At the end of step 800 or step 900, the method is restarted from step 100. From the above, and in particular from the succession of steps 600 to 900, it can be seen that when the engine is under conditions close to full load, the temperature of the exhaust gases starts to rise successively to values that increasingly approach the second temperature threshold θ _2. As soon as the temperature reaches the first temperature threshold θ ₁ , which may be considered as an alarm threshold, the richness starts to increase. No delay is applied and all the richnesses implemented are below the second richness value r ₂ , which corresponds to the thermomechanical limit of the exhaust circuit. The temperature of the exhaust gases therefore rises slowly. This gives time for the temperature of the components of the exhaust circuit to catch up with the temperature of the gases, insofar as the rate at which the richness increases (represented by the positive constant coefficient k) is rather low overall. This value can be determined by previous tests, or empirically depending on the value of the heat capacity of the materials, in order to take into account the inertia in the temperature rise of the components of the exhaust circuit.

In particular, in the instance where, in a particular severe driving cycle, the temperature of the exhaust gases nevertheless reaches the second temperature threshold θ ₂ before the richness reaches the second richness value r ₂ , the method foresees, as a conservative measure, immediately increasing the richness r to this second richness value r _2. This immediately prevents the exhaust gases from rising any further in temperature, ensuring that the temperature of the components making up the exhaust circuit does not exceed the second temperature threshold θ ₂ and thus guarantees their reliability.

Claims (5)

A method for adjusting a richness value (r) of an ignition-controlled internal combustion engine, the richness value (r) being set to a value close to a stoichiometric mixture value when an engine torque (C) of the engine is equal to or less than a torque threshold (Cs) lower than a maximum engine torque (Cmax), and being set to a value close to a value higher than the stoichiometric mixture value when the engine torque (C) exceeds the torque threshold (Cs) lower than the maximum engine torque (Cmax);
When the engine torque (C) exceeds the torque threshold value (Cs),
- if the engine exhaust gas temperature value (θ _exh ) is less than a first temperature threshold value (θ ₁ ), said richness value (r) is set (400) to a first richness value (r ₁ ) higher than said stoichiometric value;
- if said temperature value (θ _exh ) exceeds a second temperature threshold (θ ₂ ) that is higher than said first temperature threshold (θ ₁ ), said richness value (r) is immediately set (800) to a second richness value (r ₂ ) that is higher than said first richness value (r ₁ );
a step (900) of gradually increasing said richness value (r) from said first richness value (r ₁ ) up to said second richness value (r ₂ ) if said temperature value (θ _exh ) is between said first temperature threshold (θ ₁ ) and said second temperature threshold (θ ₂ ) ;
method. The method of claim 1 , wherein the first richness value (r ₁ ) is substantially equal to 1.01. 3. The method according to claim 1 or 2, wherein the second richness value ( _r2 ) is set by a preliminary engine test performed under a stable mode, in which a temperature value ( _θexh ) of the exhaust gas of the engine during operation of the engine at a stable operating point for a given engine speed (N) and a given engine torque (C) is set to be equal to the second temperature threshold value ( _θ2 ). The method according to any one of the preceding claims, wherein the second temperature threshold (θ ₂ ) is between 950 and 980°C. The method according to any one of claims 1 to 4, wherein the gradual increase in the richness value (r) from the first richness value (r ₁ ) up to the second richness value (r ₂ ) is a linear increase as a function of time.

Images