Internal-combustion engines – Combustion chamber means having fuel injection only – Combustible mixture stratification means
Reexamination Certificate
2001-04-20
2003-07-01
Argenbright, Tony M. (Department: 3747)
Internal-combustion engines
Combustion chamber means having fuel injection only
Combustible mixture stratification means
C123S305000, C701S104000, C701S105000, C060S274000, C060S308000
Reexamination Certificate
active
06584952
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method and device for controlling the mode of combustion of an internal combustion engine, and in particular a four-stroke petrol engine.
BACKGROUND
In a controlled-ignition four-stroke petrol engine:
the fuel is injected directly into the combustion chamber via high-pressure fuel supply system;
the control parameters are calculated and applied by a central control unit;
the exhaust gases are treated by one or more catalytic converters placed in the exhaust line.
The engine control unit processes the various demands on the engine (the wishes of the driver, the on-board electronic systems of the course-control or gearbox type, etc.), assimilates them and produces a reference torque value to be achieved by action on the control parameters which are:
the air flow rate
the amount of fuel injected
the ignition advance applied.
For a direct-injection petrol engine, the engine control unit has various modes of combustion available in order to achieve this reference torque value.
At each instant it has to evaluate the mode of combustion that will produce the best fuel consumption/driveability/exhaust gases clean-up compromise.
SUMMARY OF THE INVENTION
One of the fundamental characteristics of the modes of combustion is the richness of the air/fuel mixture that they allow.
The richness of the mixture is a dimensionless quantity defined as the ratio between the air/fuel proportions of a stoichiometric mixture and the same air/fuel proportion of the mixture in the mode of combustion considered.
Richness for the mode of combustion
i
=(airflow rate/fuelflow rate) stoichiometric/(airflow rate/fuelflow rate)mode i.
By definition:
the richness is equal to 1 when the mixture is stoichiometric,
the richness is greater than 1 when the proportion of petrol in the mixture is greater than that of the stoichiometric mixture. The mixture is said to be “rich”,
the richness is less than 1 when the proportion of petrol in the mixture is lower than that of the stoichiometric mixture. The mixture is said to be “lean”.
The mode of combustion which provides the best efficiency is the so-called “stratified charge” mode.
In this mode, the fuel is injected into the combustion chamber at the end of the compression phase so that the richness of the mixture near the spark plug at the time of ignition is high enough to ensure combustion.
The overall mixture has a very great excess of air (mean richness of the order of 0.4) which allows:
an increase in the combustion efficiency of the engine,
an increase in the mean pressure in the intake manifold and thus a reduction in “pumping losses”.
The range of use of this mode of combustion is physically restricted by the maximum air-filling of the cylinders, which filling is associated with the maximum pressure in the plenum chamber (full air charge).
This “stratified charge” mode of combustion is therefore favoured for low torque demands but cannot meet all the demands placed on the engine by the driver.
For higher torque demands, there are two modes of combustion that may be employed, both characterized by the injection of fuel into the chamber during the inlet phase.
This injection allows homogenous mixing of air and fuel.
The distinction between these two “homogenous” modes of combustion is in the associated mean richness level:
Lean Homogenous Mode
The mean richness of the mixture is of the order of 0.75.
This mode has the same advantages of the aforementioned stratified charge mode, limited by the overall richness level which has to be high enough to allow combustion of the mixture.
Stoichiometric Homogeneous Mode
The richness of the mixture is equal to 1.
This mode is needed for high demands for engine torque, which demands require high fuel flow rates.
A rich homogeneous mode is also defined for when the engine is running at full load. This mode will not be mentioned here because it is not specific and can be likened to the stoichiometric homogenous mode for the aspects dealt with.
The favoured mode of combustion for optimizing consumption can be represented schematically by the graph of engine torque against engine rotational speed as shown in FIG.
1
.
The transition from one mode of combustion to another is to be made with no appreciable affect as far as the driver is concerned.
This constraint entails complex management of the actuators by the engine control unit.
For example, without any specific action by the control unit, the change from one mode of operation with a lean mixture to the homogenous stoichiometric mode of operation would give rise to a sudden and significant increase in torque, which needs to be avoided.
To do this, the engine control unit calculates the air, fuel and ignition advance controls in order at every instant to meet the reference torque value.
This control “by torque” makes it possible to ensure a torque which is equal to the demand from the driver, including in changes of mode.
However, the quality with which the reference torque value is adhered to is dependent on spread which may be brought about by the ageing of engine parts, manufacturing spread or even the varying characteristics of the commercially available fuels.
These variations carry the risk of disrupting the control by torque and thus of making the changes in mode perceptible to the user.
It is therefore important that changes in mode of combustion should be brought about only for lasting changes in the engine operating point.
The influence of the mode of combustion on emissions of pollutants will now be discussed.
Emissions or pollutants from the engine are treated by a catalytic system that forms cart of the exhaust system.
This system may be made up of one or more elements intended to oxidize or to reduce the toxic components of the exhaust gases.
The most dangerous components are unburnt hydrocarbons (HC), carbon monoxide (CO) and the oxides of nitrogen (NOx).
The CO and the HCs need to be oxidized to convert them into CO
2
+H
2
O.
The NOx have to be reduced to be converted into H
2
+O
2
.
When the air-petrol mixture is stoichiometric, the dual function or oxidation and reduction is carried out by a three-way catalytic converter.
When associated with fine adjustment of the richness of the air-fuel mixture allowing the richness to be made to fluctuate by small amounts around 1, this catalytic converter allows excellent overall conversion of the two pollutants.
When the air-petrol mixture is lean, only the oxidation function can be performed by the three-way catalytic converter.
The reduction function can then be provided in various ways:
NOx can be stored while the mixture is lean and then reduced during phases in which the engine is operating at a richness greater than or equal to 1,
chemical formulation can be used to allow a reduction in lean mixtures.
Whatever the definition of the catalytic system, its treatment efficiency is very low for as long as its temperature has not reached a threshold light-off temperature at which the chemical reactions begin.
As long as this light-off temperature (of the order of 250°) has not been reached, specific engine management is required in order to:
minimize basic engine emissions as far as possible,
increase the temperature of the catalytic system as quickly as possible.
This management favours the clean-up constraint at the expense of fuel consumption. It has therefore to be suppressed as soon as the control unit detects sufficient efficiency for the pollutants to be converted in the mode of combustion that favours consumption.
SUMMARY OF THE INVENTION
The invention aims to create a method and a device for the overall management of the constraints associated with the choice of combustion mode.
The constraints taken into consideration are the fuel consumption, the driveability of the vehicle and the efficiency with which the pollutants are treated as the temperature rises after the engine has been started.
A subject of the invention is therefore a method for controlling the mode of combustion of a controlled-ignition four-stroke petrol engine equipped w
Argenbright Tony M.
Hoang Johnny H.
Peugeot Citroen Automobiles SA
LandOfFree
Method and device for controlling the combustion mode of an... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and device for controlling the combustion mode of an..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and device for controlling the combustion mode of an... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3085840