Internal-combustion engines – Charge forming device – Exhaust gas used with the combustible mixture
Reexamination Certificate
2003-02-28
2004-10-26
Wolfe, Jr., Willis R. (Department: 3747)
Internal-combustion engines
Charge forming device
Exhaust gas used with the combustible mixture
C123S090160, C123S090110
Reexamination Certificate
active
06807956
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for controlling the cylinder charge in an internal combustion engine having variable timing of the gas-exchange valve of its cylinders. An internal recirculation of the residual gas in the cylinder is implemented by controlling the closing times of the at least one exhaust valve of the respective cylinder and by opening the at least one intake valve near top dead center, including the intermittent discharge of residual exhaust gas in front of the at least one intake valve.
BACKGROUND INFORMATION
Appropriate methods for exhaust control may include the early closing of all exhaust valves of a cylinder before the piston in the cylinder has reached its top dead center or the delayed closing of at least one exhaust valve of a cylinder only after at least one intake valve of the cylinder has opened. In the second case, a so-called valve overlap, a pressure drop from the exhaust side to the intake side is used for the internal recirculation of residual gas. This drop may be caused or intensified by the use of a throttle valve in the intake manifold.
In MTZ Motortechnische Zeitschrift 60 (1999) 7/80, pages 476-485, a choke-free load control including fully-variable valve gears for a spark-ignition engine is described. In engines having variable valve timing, the charge, or power output, is controlled, fully or in part, by controlling the opening curve of the gas-exchange valves (intake and exhaust valves of the cylinders). A throttle valve in the intake manifold may be present in addition, but may also be dispensed with entirely.
To be considered as variables of the opening curve of a valve, i.e., as parameters that may be adjusted by valve timing, are the following:
1. Beginning and end of the valve opening. This is also referred to as operating points for the valve opening. Generally, the operating points may be characterized by the crankshaft's angular position relative to a, e.g., cylinder-specific, reference position. An important factor in this context may be the phase relation to the working cycle of the respective cylinder, such as the position during the compression cycle, expansion stroke, discharge stroke or intake stroke.
2. Valve lift.
3. The average or maximum velocity of the valve at opening or closing. This is also referred to as curve steepness of the valve lift curve.
In an engine having fully variable valve gears, as is described in the MTZ 60 (1999) 7/8, pages 479-485, the gas-exchange valves may be controlled directly and actuated by electromagnetic or electro-hydraulic valve actuators, for example. In particular, the cylinder charge may be metered solely through the appropriate controlling of the valves, the use of a throttle valve not being required. By reducing throttling losses, i.e., the pumping action of the engine, the engine efficiency may be increased and the specific consumption may thereby be reduced. Engines dethrottled in this manner may present a problem in that the conditions for the external mixture formation in the intake manifold may be more difficult. In engines controlled by throttle valves, a vacuum pressure may exist in the intake manifold, except in the case of a full load, which may allow a desirable vaporization of the fuel normally injected in the vicinity of the intake valves. In the case of a dethrottled engine, however, ambient pressure practically prevails in the intake manifold, causing a marked reduction in the vaporization rate and a corresponding increase in the fluid fuel portion (wall film). This may have a serious effect when the engine is cold, i.e., at cold start and during the subsequent warm-up phase. For instance, the control strategy of “early intake—closing” which may be desirable in partial load operation when the engine is warm (cf. MTZ 60 (1999) 7/8) may turn out to be undesirable in a cold engine since fuel, partly in liquid form, may be drawn into the combustion chamber and not sufficiently processed, i.e., vaporized and homogenized together with the air, by the time ignition occurs. Moreover, by the early closing of the intake valves and due to the cooling of the cylinder charge during the subsequent expansion, fuel that has already evaporated may condense in the vicinity of bottom dead center of the piston. Under such conditions, the combustion quality, running smoothness, fuel consumption and the pollutant concentration in the exhaust gas may be correspondingly poor. In such timing of the intake valves, which is intended to reduce throttle losses to the greatest possible extent, these are opened near the piston's top dead center and, given a partial load, are closed early, i.e., before bottom dead center.
With respect to the control strategy used for the gas-exchange valves, the following two conventional measures may improve the mixture formation and carburetion in a choke-free spark-ignition engine:
1. An intake-guided exhaust-gas recirculation, where residual gas is discharged via the intake valves and reaspirated. This internal recirculation of residual gas is described, for instance, in M. Pischinger, J. Hagen, W. Salber, T. Esch: Möglichkeiten der ottomotorischen Prozessführung bei Verwendung des elektromechanischen Ventiltriebs, 7. Aachener Kolloquium Fahrzeug—und Motorentechnik, 1998, p. 987-1015.
2. Delayed opening of the intake valves, as it is described from the previously cited publication.
The two mentioned conventional methods may have the effect of lowering the pollutant concentration of the raw exhaust gases during warm-up of the engine. Furthermore, the method of internal recirculation of residual gas also may have this effect in engines at operating temperature. It may be important to reduce the pollutant concentration of the raw exhaust gas during the warm-up phase, such as, for example, in the early phase of warm-up during which the light-off temperature of the catalytic converter has not yet been reached and the catalytic converter may thus be only able to filter or convert the pollutants to a very limited degree.
By the strategy of a delayed opening of the intake valves, a vacuum pressure may be generated in the cylinder after the charge-change process, due to the expansion of the cylinder volume in top dead center. Intake valves are opened only once the pressure in the cylinder is sufficiently low, or once a supercritical pressure relationship between cylinder and intake manifold (<approx. 0.5) has been reached. This may result in a high inflow velocity (maximally sonic velocity) of the fresh gas into the respective cylinder. This highly dynamic, turbulent inflow-process may improve the mixture carburetion, namely due to the feature that liquid fuel portions are better atomized and are subsequently nearly completely evaporated. Moreover, the homogenization of the gas components air, fuel vapor and residual gas may be improved. In this manner, an improvement in the pollutant concentrations in the raw exhaust gas may be achieved, such as, for example, of unburned hydrocarbons (HC). Moreover, the improved combustion—apart from the achieved carburetion, the increased turbulence of the cylinder charge may also exert a positive influence here—may be able to mostly compensate for the increased charge-change losses of this method, such as in the case of a cold engine.
The other mentioned strategy may cause an increase in the portion of hot residual gas contained in the cylinder charge, for instance by closing the exhaust valves ahead of top dead center and the early opening of at least one intake valve, also before or near top dead center, optionally also by using a cylinder overlap. In addition, the pressure drop existing between cylinder and intake channel (intake manifold) in the vicinity of top dead center, given an already open inlet, may cause part of this hot exhaust gas to stream into the intake manifold. This residual gas, discharged in the vicinity of the fuel injector and fuel wall film, may cause a quick warming of the intake elbow after a cold start and may improve the mixture formation, i.e., the vaporizati
Beuche Volker
Diehl Udo
Gaessler Hermann
Grosse Christian
Mischker Karsten
Kenyon & Kenyon
Robert & Bosch GmbH
Wolfe, Jr. Willis R.
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