Automotive four-cycle engine

Internal-combustion engines – Charge forming device – Including exhaust gas condition responsive means

Reexamination Certificate

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C123S568140, C123S090150

Reexamination Certificate

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06626164

ABSTRACT:

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT
The present invention relates to a four-cycle engine for a motor vehicle designed to use so-called “internal” exhaust gas recirculation (EGR) in a positive way by causing part of already combusted exhaust gases to remain in a combustion chamber.
EGR is a widely known conventional technique in which an exhaust air line and an intake air line of an engine are connected by an external EGR passage and part of exhaust gases is returned to the intake air line through the external EGR passage to reduce nitrogen oxides (NOx), as described in Japanese Unexamined Patent Publication No. 5-86988, for example. This Patent Publication discloses an external EGR system in which a specific amount of cooled exhaust gas (EGR gas) is re-introduced into a combustion chamber through an external EGR passage which is provided with an EGR cooler and the recirculation rate (EGR rate) is increased in higher-load ranges of the engine, so that NOx emissions are decreased and increases in combustion temperature and exhaust gas temperature are avoided in the higher-load ranges. Avoiding the increases in combustion temperature and exhaust gas temperature is advantageous for improving combustion efficiency, preventing knocking combustion, and for improving the reliability of exhaust gas converting catalysts.
Internal EGR is another form of exhaust gas recirculation technique of the prior art, in which already combusted exhaust gases are intentionally caused to remain in the combustion chamber instead of recirculating the exhaust gases through the external EGR passage. A common approach used for internal EGR is to increase the amount of overlap between open periods of intake and exhaust valves. For this purpose, an internal EGR system is usually provided with a valve timing adjuster for varying opening and closing timing of the intake and exhaust valves. The valve timing adjuster increases the amount of residual exhaust gas left in the combustion chamber by causing “blowing-back” of the exhaust gas during overlap periods, for instance, by increasing the amount of overlap in a range requiring the internal EGR.
An arrangement for adjusting the opening and closing timing of the intake and exhaust valves in accordance with engine operating conditions is disclosed in Japanese Unexamined Patent Publication No. 10-266878. According to the arrangement of this Patent Publication, the exhaust valve is closed at a point before the top dead center on an intake stroke and the intake valve is opened at a point after the top dead center on the intake stroke to cause multi-point self-ignition in the combustion chamber and to reduce pumping loss in a specific low-load operating range of the engine and, as required torque decreases, the closing point of the exhaust valve is advanced and the opening point of the intake valve is retarded.
Among the aforementioned conventional arrangements, the external EGR system shown in Japanese Unexamined Patent Publication No. 5-86988 requires that intake and exhaust systems of the engine be provided with such additional facilities as the external EGR passage, an EGR valve and the EGR cooler. While a specified portion of exhaust emissions is returned to an intake passage on a downstream side of downstream of a throttle valve and re-introduced into the combustion chamber in the engine provided with the external EGR system, it is difficult to recirculate a sufficient amount of exhaust gas under higher-load conditions in a high-speed range, because negative pressure in the intake passage decreases on the downstream side of the throttle valve and an intake period of each cylinder shortens under high-speed high-load conditions. In addition, the external EGR system is associated with a problem that exhaust gas deposits are likely to accumulate in the intake air line.
On the other hand, although the aforementioned internal EGR approach enables exhaust gas recirculation by increasing the amount of overlap between the open periods of the intake and exhaust valves, eliminating the need for the external EGR passage, it is difficult to avoid increases in combustion temperature and exhaust gas temperature in higher-load ranges because the temperature of the residual exhaust gas left in the combustion chamber is high. Furthermore, if the overlap period is fixed, it is substantially shortened and it becomes difficult to achieve sufficient internal EGR due to a decrease in effective valve opening period when the engine speed increases. Accordingly, it is necessary to significantly increase the overlap period in high-speed ranges to ensure sufficient effects of internal EGR up to the high-speed ranges. The valve opening and closing timing should be made adjustable within a wide variable range to meet this requirement, and this makes the construction of the valve timing adjuster complex. If the overlap period is increased, however, both the intake and exhaust valves would be relatively widely opened at the intake top dead center, and this makes it necessary to form a deep recess in a top surface of a piston to avoid interference between the intake and exhaust valves. This recess in the piston could exert an adverse effect on combustion in the combustion chamber.
The arrangement disclosed in Japanese Unexamined Patent Publication No. 10-266878 is intended to cause self-ignition by decomposing and radicalizing fuel in a mixture supplied into the combustion chamber with the aid of high-temperature exhaust gas, and not to avoid increases in combustion temperature and exhaust gas temperature in higher-load ranges.
SUMMARY OF THE INVENTION
In view of the foregoing, it is an object of the invention to provide an automotive four-cycle engine capable of producing sufficient effects of EGR and avoid increases in combustion temperature and exhaust gas temperature by internal EGR up to medium- to high-speed high-load ranges.
In one principal feature of the invention, an automotive four-cycle engine whose power cycle has four strokes including intake, compression, expansion and exhaust strokes, each stroke being defined as a movement of a piston in a single direction between a top dead center and a bottom dead center, is constructed such that a closing point of an exhaust valve defined as a point of transfer from an acceleration portion to a constant speed portion on its valve lift characteristics curve is set to a point a specific period before the intake top dead center between the exhaust stroke and the intake stroke and an opening point of an intake valve defined as a point of transfer from a constant speed portion to an acceleration portion on its valve lift characteristics curve is set to a point after the intake top dead center at least in medium- to high-speed ranges at least in medium- to high-load regions of the engine excluding its maximum-load region.
In this construction of the invention, the internal EGR is accomplished with already combusted gases left in a combustion chamber as the exhaust valve is closed at the point the specific period before the intake top dead center in the medium- to high-speed ranges in the medium- to high-load regions of the engine. Since the intake valve is opened after the intake top dead center, the pressure in the combustion chamber during the period from the closing point of the exhaust valve to the intake top dead center and, as a consequence, heat is sufficiently dissipated during this period when the temperature in the combustion chamber before the temperature decreases due to subsequent pressure drop after the intake top dead center. As the already combusted gases in a combustion chamber are cooled in this fashion, the effect of avoiding increases in combustion temperature and exhaust gas temperature is obtained as is the case with an external EGR system which re-introduces low-temperature exhaust gases.
According to the invention, it is preferable to set the opening point of the intake valve to a point a specific period after the intake top dead center within an earlier part of the intake stroke and provide a period dur

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