Method for managing the lightoff of a pollution-control catalytic converter

Abstract

A method is provided for managing the light-off of a 3-way catalytic converter that is located in an exhaust line of a petrol engine having a plurality of cylinders with each cylinder having at least one exhaust valve. The method includes calculating a value of enthalpy of exhaust gases to make it possible to determine a quantity of heat supplied to the three-way catalyst, determining a threshold enthalpy value signaling the light-off of the catalyst, and stopping of activation of the three-way catalyst upon determining the value of the enthalpy that was calculated has reached the threshold enthalpy value.

Claims

1. A method for managing light-off of a three-way catalyst placed in an exhaust line of a petrol engine, the engine having a plurality of cylinders with each of the cylinders being provided with at least one exhaust valve, the method comprising: calculating a value of enthalpy of exhaust gases to make it possible to determine a quantity of heat supplied to the three-way catalyst, determining a threshold enthalpy value signaling the light-off of the catalyst based on at least one of an engine water temperature on startup and a state of aging of the three-way catalyst, and stopping of activation of the three-way catalyst upon determining the value of the enthalpy that was calculated has reached the threshold enthalpy value.

2. The management method as claimed in claim 1, wherein the calculating of the enthalpy is performed by integration beginning when the engine is started up based on a time integral as follows:
ΔH=∫Qech×Cp×Tavt×dt where: Qech=mass flow rate of gas at the exhaust (kg/h), Cp=heat capacity of the exhaust gases (J/kg/K), Tavt=temperature of the gases at the exhaust valves (K).

3. The management method as claimed in claim 2, wherein the mass flow rate of gas at the exhaust is determined by using a flow meter.

4. The management method as claimed in claim 2, wherein the temperature of the gases at the exhaust valves is modelled beforehand by an estimator derived from cartographic model which is a function of torque and of engine speed, and which is corrected by the engine water temperature, by a spark advance and by a fuel-air ratio in the cylinder.

5. The management method as claimed in claim 1, wherein the threshold enthalpy value is equal to a product of a first factor which is a decreasing function of the engine water temperature on the startup of the engine, and of a second factor lying between a positive value close to 0 and a value close to 1 and which depends on the state of aging of the three-way catalyst.

6. The management method as claimed in claim 5, wherein the second factor tends toward a value close to 0 when the three-way catalyst is new and tends towards a value close to 1 when the three-way catalyst is very old.

7. The management method as claimed in claim 5, wherein the state of aging of the catalyst is determined from a damping of an amplitude of a signal of fuel-air ratio downstream of the three-way catalyst relative to an amplitude of a fuel-air ratio signal upstream of the three-way catalyst.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Referring now to the attached drawings which form a part of this original disclosure:

(2) FIG. 1 is a diagram of the temperature of the catalyst as a function of time, illustrating stoppages of activation of the catalyst according to several configurations, incorporating, in four different cases, a particular driving style and a state of ageing of the catalyst.

DETAILED DESCRIPTION OF EMBODIMENTS

(3) A detailed description of a preferred embodiment of a management method according to the invention is given hereinbelow.

(4) FIG. 1 is a diagram of the temperature of the catalyst as a function of time, illustrating stoppages of activation of the catalyst according to several configurations, incorporating, in four different cases, a particular driving style and a state of ageing of the catalyst.

(5) The principle of a management method according to the invention consists in estimating the quantity of heat or the number of calories to be sent to the catalyst to stop its activation at exactly the right moment. It is assumed that this management method is implemented by a computer embedded in a vehicle having a petrol engine, said engine comprising cylinders each equipped with at least one intake valve and at least one exhaust valve.

(6) Thus, such a method comprises the following steps:

(7) A step of calculation of the enthalpy H of the exhaust gases, making it possible to determine the quantity of the heat supplied to the catalyst. In fact, the detection of the light-off of the catalyst is performed by monitoring the enthalpy H of the exhaust gases, which is calculated from the following time integral, the integration beginning when the engine starts up:
ΔH=∫Qech×Cp×Tavt×dt

(8) where: Qech the mass flow rate of gas at the exhaust (kg/h). This flow rate can for example be measured by means of a flow meter. Cp the heat capacity of the exhaust gases (J/kg/K) which is a constant. Tavt the temperature of the gases at the exhaust valves (K).

(9) This temperature can be modelled beforehand by an estimator derived from a cartographic model which is a function of the engine speed and torque, corrected by the T° of the water of the engine, by the spark advance and by the fuel-air ratio in the cylinder. To recap, the fuel-air ratio is the ratio of the quantity of fuel divided by the quantity of air. This temperature of the gases at the valves will therefore depend on the driving style of the driver, which can, for example be sporty driving or flexible driving.

(10) A step of determination of a threshold enthalpy value S signaling the light-off of the catalyst. This threshold enthalpy is a function of two parameters, which are the water temperature on startup and the state of ageing of the catalyst. Thus, as a nonlimiting example, the threshold enthalpy S is equal to the product: of a first factor which is a decreasing function of the water temperature when the engine starts up. Thus, the higher the water temperature is on startup the fewer the calories that have to be added to heat the catalyst, and a second factor lying between a value close to 0 and a value equal to 1, depending on the state of ageing of the catalyst. This way, this second factor tends toward 0 when the catalyst is new and is 1 when the catalyst is very old. This reflects the fact that the newer the catalyst is, the less time it will take to heat said catalyst to bring it to its light-off temperature. The ageing of the catalyst, which corresponds to its loss of effectiveness, can for example be determined from the damping of the amplitude of a signal of fuel-air ratio downstream of the catalyst, measured by an oxygen probe downstream of the catalyst, relative to the amplitude of a signal of fuel-air ratio upstream of the catalyst, measured by an oxygen probe upstream of the catalyst. Any other diagnostic method known to the person skilled in the art, such as, for example, a calculation of the maximum oxygen storage capacity, can also be used to determine the state of ageing of the catalyst.

(11) A step of stopping of the activation of the catalyst when the calculated enthalpy value H reaches the threshold enthalpy value S.

(12) Referring to FIG. 1, which illustrates the change in temperature of the catalyst as a function of the period of activation of said catalyst, for a given type of driving of the vehicle, and for a given state of ageing of the catalyst, the curve 1 relates to sporty driving and the curve 2 relates to slower driving. Temperature corresponding to sporty driving increases more rapidly than that which corresponds to slower driving because of the discharging of a greater number of calories at the exhaust of the engine. For one and the same catalyst light-off temperature, the necessary activation period is therefore shorter in the case of sporty driving than in the case of slower driving.

(13) Also, the light-off temperature of a new catalyst T.sub.1-one is lower than the light-off temperature of an old catalyst T.sub.1-o,odd, as can be seen on the ordinate axis of the diagram of FIG. 1. For an identical driving profile, the necessary activation period is therefore shorter in the case of a new catalyst than in the case of an old catalyst.

(14) The result of this dual comparison is a ranking of the activation periods corresponding to the following four cases A, B, C, D represented in FIG. 1: Case A: activation period t.sub.A; case of a new catalyst for sporty driving; Case B: activation period t.sub.B; case of an old catalyst for the same sporty driving as in the case A; Case C: activation period t.sub.C; case of the same new catalyst as in the case A for slow driving; Case D: activation period t.sub.D; case of the same old catalyst as in the case A, for the same slow driving as in the case C.

(15) It is found that the following inequations are borne out: t.sub.A<t.sub.B and t.sub.C<t.sub.D (effect of just the ageing of the catalyst), t.sub.A<t.sub.C and t.sub.B<t.sub.D (effect of just the driving profile).

(16) Also, it is found that the ageing of a catalyst has a greater influence on the activation period than the type of driving profile, which is reflected by the verification of the inequation: t.sub.B<t.sub.C, such that the following hierarchy is verified:
t.sub.A<t.sub.B<t.sub.C<t.sub.D.

(17) In a nonlimiting manner, it is possible to implement different types of modification of the setting of the engine to form this heating of the catalyst.

(18) For example, for a very short activation (case A for example), it is possible to simply increase the idle overspeed. For a slightly longer activation (cases B and C for example), it is possible to add, to the increase of the overspeed which takes place only when idling, a specific setting mode of the engine over all of its operating points which consists in degrading the spark advance. Finally, if the activation is destined to be even longer (case D for example), it is possible, for example, after a certain time, to degrade the combustion efficiency of the engine even further, for example by retarding the injection of the fuel into the cylinders, at the cost of increased overconsumption. Obviously, other setting choices can be made, either as alternatives depending on the activation period, or in combination, without departing from the scope of the invention.