Internal combustion engine

Details Internal combustion engine Internal combustion engine

2002-03-01

2003-09-09

Argenbright, Tony M. (Department: 3747)

Internal-combustion engines

Means to whirl fluid before, upon, or after entry into...

Having multiple oxidant inlet means

C123S315000, C123S432000

Reexamination Certificate

active

06615795

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the field of internal combustion engines, and in particular to methods of increasing the efficiency of the cylinder head charging, fuel burn and subsequent exhaust during a combustion cycle.
BACKGROUND OF THE INVENTION
Classic internal combustion engines have two valves per cylinder. One valve, the intake valve, admits a fuel/air mixture into the cylinder head. The other, exhaust valve, permits expulsion of combustion products from the cylinder head and thence to an exhaust. During combustion events the cylinder head valves are generally both closed. Once combustion has occurred and the piston head has been forced downwards in the cylinder, the exhaust valve is opened and burnt mixture is expelled from the cylinder head by return motion of the piston head. The exhaust valve is closed as the piston reaches the top of the cylinder. The intake valve is opened so that movement of the piston back down in the cylinder sucks air/fuel mixture into the cylinder head, ready for the next spark ignition to create combustion of the air/fuel mixture.
The common internal combustion engine can be likened to a complex thermodynamic air pump. The greater the rate that an engine can pass air, the greater the potential for generating more power. However, air consumption alone does not guarantee power. Unless the air/fuel charge entering the cylinder head is burned effectively, the torque and consequently the power output of the engine as a whole will not reach its full potential.
One way of increasing the efficiency of air flow through an engine is increasing the number of valves. Four valve per cylinder head engines are common and permit a greater through flow for a given valve dimension. There are two intake valves and two exhaust valves, which provide increased air flow by providing a greater “curtain area” for a given valve diameter. Curtain area is the cylindrical area swept by the valve head between its closed an open configurations, in other words the multiple of valve lift and valve circumference. For a constant valve head area, a cylinder head formed with one intake valve head will have a reduced curtain area compared to a cylinder head formed with two smaller intake valve heads having the same total valve head diameter. This is because the total circumference of the two smaller heads will be greater than that of one large valve head. The advantage of the greater circumference of the two smaller valves persists through out the valve lift range from closed up to a lift equal to 0.25 of the valve diameter. From this point on, the advantage over a single valve diminishes until the single valve reaches a lift of 0.25 of the valve diameter.
The four-valve cylinder head has a further advantage. The valve area which can be accommodated in a circular section cylinder head is greater with four valves than with two valves. The previously discussed circumferential curtain advantage plus this area advantage gives the four-valve head its superior air breathing qualities and consequently it's potential to produce greater power.
However, One of the principle functional disadvantages of a 4-valve cylinder head as compared with a 2-valve head is that the 4-valve head lacks much of the air/fuel mixture motion generated during induction of the fresh charge into the 4-valve head.
FIG. 1
illustrates the typical mixture motion generated by a well-designed 2-valve cylinder head
1
provided in a roof thereof with an inlet valve
3
and an exhaust valve
4
. As the charge (indicated by arrows) enters the combustion chamber
2
, so a swirling motion is set up. This motion, if aggressive enough, persists through out the compression stroke and on into the combustion process. The swirl effectively speeds the combustion process thus producing a higher-pressure rise and consequently more torque. This mixture motion typically allows a 2 valve per cylinder engine to produce more low rpm torque for a given size of engine than its 4-valve counterpart. At low engine speed the fact that a 2-valve engine may have less breathing area presented to the cylinder is of little consequence because the time available to fill the cylinder is more than adequate. As rpm rises so the need for greater breathing capability increases. A 4 valve per cylinder automotive engine typically exhibits its advantage in terms of breathing (or volumetric efficiency defined as the amount of air drawn into the cylinder divided by the cylinder displacement) at above about 4000 to 4500 rpm.
FIG. 1
demonstrates that the swirl of a two-valve engine's cylinder head is principally an inherent characteristic of the basic design. A 4-valve head is not without its own characteristic motion. This motion is most commonly known as tumble. Tumble is generated because the intake valve is offset to one side of the cylinder head. Hence air/fuel mixture entering the head is presented with a large free space at the other side of the cylinder head. The mixture admitted is predominantly drawn over the top of the open valve head towards the other side of the cylinder head. The mixture then ‘tumbles’ down towards the piston head, back across the piston head and back up towards the intake valves. Although tumble helps produce an accelerated combustion process at low rpm, it generally fails to match the effectiveness of the two-valve design. Four-valve cylinder heads usually have inclined valves, in which the plane occupied by the valve heads at any point in time is angled with respect to a top face of the cylinder head (or the engine block face). This configuration leads to another disadvantage of typical 4 valve cylinder heads. This is the phenomenon of ‘cross flow’. Cross flow arises where fuel/air mixture entering the cylinder head travels directly from the intake valve to the exhaust valve and out of the cylinder head. This can, of course, only occur if both intake and exhaust valves are open at the same time. This happens between a period starting just before top dead centre at the end of the exhaust stroke to just after top dead centre on the intake stroke. During this period, when both valves are open, there is a tendency for some of the fresh charge to exit the cylinder via the still open exhaust valve.
Cross flow causes an increase in fuel consumption and unwanted exhaust emissions, mostly in the form of unburned hydrocarbons. The effects of cross flow are normally countered by shortening the duration of the valve opening events to cut the overlap. However, if duration is shortened as far as is often needed, much of the power advantage of a 4-valve design may be lost.
One method of generating swirl in 4-valve engines is to disable one valve while the engine is at lower RPM.
FIG. 2
is a schematic view from above of a 4-valve cylinder head
5
, provided with two inlet valves
6
,
7
and two exhaust valves
8
,
9
. The effect of blocking one inlet valve
7
is to cause the 4-valve engine to function as a 2-valve engine at low rpm. The second intake
7
port is typically deactivated by means of a closed butterfly valve
10
. As engine rpm rises, the need for aggressive mixture motion decreases but the requirement for strong airflow increases. Hence at higher revolutions, usually in the region of 3500 rpm, the butterfly opens and allows the second valve port to supply the engine's air demand.
The foregoing method is not the only technique for introducing swirl into a 4-valve engine. Other techniques are used in the design and manufacture of Diesel engines that need high swirl values for effective combustion. A significant problem with the known methods of inducing swirl in 4-valve engines is the requirement for a complex mechanical arrangement for opening and closing the butterfly valve according to the threshold rpm for efficient operation.
A recent development, which is a simplification of 4 valve per cylinder engines, is the 3 valve per cylinder engine. In such engines each cylinder has two intake valves and a single exhaust valve. This type of cylinder head valve arrangement does not, howev

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