JPS63201312A - Two-cycle internal combustion engine - Google Patents

Abstract

PURPOSE:To carry out favorable combustion all over the load zones of an engine by providing two new air ports which extend in parallel on both sides of the center line of a combustion chamber and providing new air valves on both of said new air ports respectively while providing a new air control valve on one of said ports. CONSTITUTION:New air ports 20 and exhaust ports 22 are oppositely provided on a cylinder head 14 and new air valves 24 and exhaust valves 26 are provided to these ports respectively. Also, two new air manifolds 30, 32 are installed on the cylinder head 14, and connected to the new air ports 20 respectively. And, a new air control valve 48 which is opened and closed in accordance with an engine load and an engine speed is provided on the new air manifold 32 on the high load side. In this case, the new air ports 20 consists of two new air ports 20, 20 which extend in parallel on both sides of the center line of a combustion chamber 18 and the flow resistance of the two new air ports 20, 20 are set equally. Thereby, both stratification at the time of low load and crossflow scavenging at the time of high load can be attained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a two-stroke internal combustion engine in which a fresh air valve and an exhaust valve are provided in the cylinder head.

[Conventional technology]

Japanese Patent Publication No. 60-5770 discloses an open chamber type two-stroke internal combustion engine having an intake (fresh air) valve and an exhaust valve. In this two-stroke internal combustion engine, the intake valve and exhaust valve open almost simultaneously when the piston is at bottom dead center, and the fresh air flowing in from the intake valve is directed downward, reverses at the top of the piston, and enters the cylinder. It forms a vertical U-shaped flow. The interface between fresh air and exhaust air is first near the intake valve, then in the lower middle part of the cylinder, and then moves closer to the exhaust valve, such that exhaust and fresh air are replaced throughout the cylinder. It has become.

However, although such two-stroke internal combustion engines do not have any problems in a high load range, they have problems with combustion at idle or under light loads. During idling or in a light load range, the amount of fresh air supplied is small and a large amount of exhaust gas remains in the cylinder, so the fresh air is dispersed into the residual exhaust gas and becomes thin (thin). It cannot be collected near the spark plug.

That is, in a vertical U-shaped flow within the cylinder, the main stream of fresh air moves downward in the cylinder. For this reason, ignition by the spark plug provided in the cylinder head and generation of a flame kernel are inhibited, and flame propagation speed is reduced, making misfires and combustion fluctuations more likely to occur.

Furthermore, forming a swirl around the cylinder axis in the intake air is conventionally known. For example, U.S. Pat.
No. 543,928 supplies air from two oppositely arranged intake valves to form a swirl around the cylinder axis. The exhaust valve is located in a subchamber formed at the center of the top of the cylinder.

Therefore, in the internal combustion engine disclosed in this patent, combustion starts in the pre-chamber and then spreads into the swirling cylinder, causing swirls in the exhaust gas and residual exhaust gas. It does not create stratification between gas and fresh air.

The applicant of this application previously discovered that during low loads including idle, part of the exhaust gas discharged from the exhaust port when the exhaust valve is opened is re-introduced into the combustion chamber while swirling around the cylinder axis. We proposed a two-stroke internal combustion engine that can perform stratified charge combustion by collecting fresh air on the cylinder head side against residual exhaust gas on the cylinder head side. In this two-stroke internal combustion engine, stratification is performed as described above at low loads, and cross scavenging is performed at medium to high loads.

[Problem that the invention seeks to solve]

As mentioned above, in a two-stroke internal combustion engine, exhaust gas remains in the combustion chamber because exhaust gas is exhausted by scavenging, and at idle or in a light load range, the ratio of fresh air supplied to the residual exhaust gas in the combustion chamber becomes small, resulting in combustion failure. There was a problem with stability. In order to solve this problem, in the earlier application of the present application, a part of the exhaust gas discharged from the exhaust port is swirled around the cylinder axis and re-introduced into the combustion chamber, thereby stratifying the fresh air and the residual exhaust gas. This problem was solved by collecting fresh air near the cylinder head. However, in a two-stroke internal combustion engine that has a fresh air valve and an exhaust valve in the cylinder head, when a swirl is formed around the cylinder axis at high loads, both fresh air and exhaust gas circulate around the cylinder, and the cylinder A problem arose in that it was not possible to scavenge the exhaust gas at the bottom of the engine. Therefore, it is preferable to form a swirl when the load is low, but when the load is high, it is preferable to perform cross-scavenging without forming a swirl, and it is preferable to change the fresh air supply characteristics depending on the engine load. It was revealed. In this case, if the cross-scavenging efficiency under high load is not excellent, the possibility of high output, which is a characteristic of a two-stroke internal combustion engine, will be reduced.

[Means for solving problems]

The two-stroke internal combustion engine according to the present invention has a fresh air supply system having a supercharging means, and fresh air driven in synchronization with the crank angle to open and close the fresh air port and exhaust port provided in the cylinder head. In a two-stroke internal combustion engine having a valve and an exhaust valve, a means is provided for causing part of the exhaust gas discharged from the exhaust port when the exhaust valve is opened to re-enter the combustion chamber while swirling it around the cylinder axis,
The fresh air port includes two ports extending substantially parallel to each side of the centerline of the combustion chamber, a fresh air valve is disposed in each of the two ports, and one of the two ports or a passageway upstream thereof is connected to the engine. A fresh air control valve that opens and closes depending on the load is disposed, and the two ports are formed so that their respective flow resistances are approximately equal.

[For production]

According to the present invention, stratification is achieved in the following manner at least in a low load range including idle. That is, when the exhaust valve opens during the downward stroke of the piston, a weak exhaust blowdown occurs, and the inside of the exhaust port becomes positive pressure. Exhaust blowdown ends in a short time, and as the piston continues to descend, the inside of the cylinder becomes negative pressure with respect to the exhaust port, and some of the exhaust gas that was once exhausted flows back into the combustion chamber.

In the present invention, a means for swirling the re-inflowing exhaust gas around the cylinder axis is provided, and the exhaust gas is swirled around the cylinder axis within the combustion chamber. After that, the fresh air valve opens.

During idling and in a light load range, the amount of fresh air supplied is small, and the fresh air flows in relatively slowly. Therefore, the fresh air that slowly flows in slowly rides on the exhaust gas that is swirling around the cylinder axis, and the fresh air collects near the cylinder head while riding on the exhaust swirl.

Therefore, there is fresh air on the cylinder head side and residual exhaust gas on the piston side, and due to this stratification, a relatively rich mixture gathers near the spark plug, making it easy to ignite and burn. It is. In this case, the fresh air ports include two ports extending approximately parallel to each side of the centerline of the combustion chamber, with only one port being used at low loads so that the fresh air supply is aligned with the exhaust swirl. It will be done.

When the load is high, both fresh air ports are used and the fresh air flow increases, so the exhaust swirl effect is no longer effective (and cross scavenging becomes possible. If the flow resistance of the fresh air ports is different, a swirl is formed around the cylinder axis again by the flow of fresh air from the fresh air port with the stronger flow.
Scavenging efficiency decreases. In the present invention, since the two fresh air ports are formed so that their flow resistances are approximately equal, swirl cannot occur due to the mutual flow relationship, and preferable scavenging can be performed. It can be done.

〔Example〕

FIG. 1 is a diagram showing a six-cylinder two-stroke internal combustion engine 10 to which the present invention is applied, FIG. 2 is a diagram showing details of the vicinity of the combustion chamber of one cylinder in FIG. 1, and FIG. The engine body 1O, which is a vertical cross-sectional view passing through the fresh air valve and the exhaust valve, is composed of a cylinder block 12 and a cylinder head 14, and a piston 1
A combustion chamber 18 is formed above 6. cylinder head 1
4 is formed with a fresh air port 20 and an exhaust port 22 arranged opposite to each other, each having a fresh air valve 24 and an exhaust valve 26, each of which is a bope 7) valve.

As shown in FIG. 2, two fresh air valves 24 and two exhaust valves 26 are provided, and a spark plug 28 is inserted into the combustion chamber.
It is provided approximately in the center of 8. The exhaust valves 26 are designated Es and E, while the fresh air valves 24 are designated FA and FB. This shows that the functions of the fresh air valves 24 are different from each other.Hereafter, the fresh air valve represented by FA will be referred to as a low-load fresh air valve, and the fresh air valve represented by FB will be referred to as a high-load fresh air valve. I'll call it kiben.

As shown in FIGS. 1 and 2, the cylinder head 1
Two fresh air manifolds 30 and 32 are attached to 4. Each branch pipe of one fresh air manifold 30 is connected to the fresh air port 20 on the side where the low load fresh air valve FA is arranged, and each branch pipe of the other fresh air manifold 32 is connected to the fresh air port 20 on the side where the high load fresh air valve FB is arranged. It is connected to the fresh air port 20 on the side where the Both fresh air ports 20 extend substantially parallel to each other in a direction substantially perpendicular to the longitudinal axis of the engine, and are located on either side of the centerline of the cylinder in a direction substantially perpendicular to the longitudinal axis of the engine, and are arranged such that at least the low-load fresh air valve FA is arranged. The fresh air port 20 on the side opened tangentially into the combustion chamber 18 . A fuel injection valve 34 is arranged in each branch pipe of the fresh air port 20 or the fresh air manifolds 30 and 32, respectively. Further, a check valve 36 made of a reed valve is arranged upstream of the fuel injection valve 34.

As shown in FIG. 1, air is taken in from an air cleaner 38, its flow rate controlled by a throttle valve 40, and supercharged by a supercharger 42. Supercharger 4
An intercooler 44 is disposed downstream of the engine 2, and the two fresh air manifolds 30 and 32 are both connected to this intercooler 44.The supercharger 42 is a mechanical type driven by the output of an engine such as a Roots pump. A supercharger can be used. Further, an air flow meter 46 is arranged upstream of the throttle valve 40.

A butterfly-type fresh air control valve 48 is disposed at a collecting portion of the fresh air manifold 32 on the high load side. While the fresh air valve 24 and exhaust valve 26 are driven by a camshaft in synchronization with the engine crankshaft, this fresh air control valve 48 is opened and closed according to the engine load and rotation speed. be. The fresh air control valve 4B is closed during low load, including at least when the engine is idling, and therefore, at this time, air is supplied only from the fresh air manifold 30 on the low load side.

The fresh air control valve 48 is opened during high engine loads, so that at this time air is supplied through both the high load side fresh air manifold 32 and the low load side fresh air manifold 30.

As shown in FIG. 1, two exhaust manifolds 50 are provided for six cylinders, one of which is the exhaust manifold 50.
0 is connected to the 1st, 2nd, and 3rd cylinders, and the other exhaust manifold 50 is connected to the 4th, 5th, and 6th cylinders. A catalyst 52 is disposed at each gathering portion of each exhaust manifold 50, and each exhaust manifold 50 further passes through a muffler 54 and terminates independently of each other. In this case, the firing order is
The order is the 1st, 6th, 2nd, 4th, 3rd, and 5th cylinders. Each exhaust manifold 50 has three branch pipes, so one branch pipe is connected to the exhaust port 22 for one cylinder.

FIG. 2 shows one such branch 50a, which is mounted approximately at right angles to the longitudinal axis of the engine. By the way, each cylinder has two exhaust ports 22, and these two exhaust ports 22 are connected to the cylinder head 1.
4 are merged into one port. The exhaust port 22 on which the exhaust valve 26, which is indicated with Es, is disposed is formed approximately at right angles to the longitudinal axis of the engine so as to be in line with the branch pipe 50a, and opens tangentially into the combustion chamber 18. do. The other exhaust port 22 merges at an angle to form a side exhaust port 22 formed approximately perpendicular to the longitudinal axis of the engine. The configuration of the exhaust port 22 is such that the exhaust gas exhaust speed can be increased by having two exhaust valves 26, and the exhaust gas is exhausted to the exhaust port 22 and the exhaust manifold 50, and a part of it is When re-entering the combustion chamber 18, due to the effect of inertia flowing in a straight line, the exhaust port 2 on the angled side
There is almost no re-inflow from 2, and it mainly passes through the exhaust port 22 where the exhaust valve 26 shown with Es is located.
This re-inflow exhaust gas swirls around the cylinder axis.
It is designed so that it can be formed.

This swirl S is in a clockwise direction as viewed in FIG. The exhaust port 22 (on the Es side) that forms this swirl S faces the fresh air port 20 having the fresh air valve FB on the high load side in a straight line, and the fresh air port 20 having the fresh air valve FA on the low load side faces the fresh air port 20 having the fresh air valve FA on the low load side. It faces the port 20 with an offset across the center line. Therefore, when the fresh air supplied from the fresh air port 20 having the fresh air valve FA on the low load side generates a swirl by itself, the swirl has the same clock as the swirl S of the re-inflowing exhaust gas. It becomes a rotation direction. When the load is low, the fresh air control valve 48 is closed, so there is no flow of fresh air from the fresh air port 20 on the high load side facing the exhaust port 22 that forms the swirl S, which prevents the swirl S of the re-inflowing exhaust gas. Thus, the swirl S can be maintained without disappearing.

As shown in FIG. 2, the inner wall of the cylinder head 14,
That is, a mask 56 is formed on the earthen wall of the combustion chamber 18.

This mask 56 extends approximately parallel to the longitudinal axis of the engine into the combustion chamber 1.
8, most of the center is formed by a sharply rising terrace-like ridge, and the slope becomes gentler at the side edges. The spark plug 28 is placed on the fresh air valve 24 side.

This mask 56 also helps form the swirl S. That is, not only is there almost no exhaust gas re-inflowing from the exhaust valve (H) 26 as described above, but even if there is inflow, it is blocked by the mask 56. The re-inflowing exhaust gas from the exhaust valve Es tends to swirl by itself as described above, and furthermore, if it deviates from the swirl and flows toward the center of the combustion chamber 18, this is also blocked by the mask 56. Therefore, the re-inflowing exhaust gas from the exhaust valve Es has no choice but to pass through the gently sloped region of the side edge of the mask 56, and increasingly flows along the combustion chamber 18 and the cylindrical surface of the cylinder. Also, during high loads, two fresh air ports 20
Fresh air supplied in parallel from the air hits the mask 56, directs the flow downward, and prevents it from blowing through to the exhaust port 22.The spark plug 28 is on the fresh air valve 24 side, so the mixture is richer even at low loads. You can relate to them.

To explain the pressure of exhaust gas here, when the engine is under low load, there is a weak exhaust blowdown immediately after the exhaust valve 26 is opened, and the inside of the exhaust port becomes positive pressure, and the exhaust pressure in the combustion chamber 18 rapidly decreases. do. When the pressure in the combustion chamber 18 falls below the pressure at the exhaust port due to the downward movement of the piston 16, a portion of the exhaust gas discharged from the combustion chamber due to exhaust blowdown is combusted due to the pressure difference between the exhaust port 22 and the combustion chamber 18. It will now flow back into chamber 18 (backflow).

Thus, when the engine is under low load, there is a backflow of exhaust gas into the combustion chamber immediately after the weak exhaust blowdown.

In the present invention, the exhaust gas flowing back into the combustion chamber is
As shown in the figures and FIG. 3, a swirl S around the cylinder axis is formed within the combustion chamber 18. Conventionally, it has been proposed to generate a swirl in the intake air taken in from the intake port, but a major feature of the present invention is to generate a swirl in the exhaust gas flowing back.

Figure 4 shows the opening period (FO) of the fresh air valve 24 and the opening period (EO) of the exhaust valve 26, which are driven in synchronization with the crankshaft.
). In a two-stroke internal combustion engine, there are only two strokes: an expansion stroke in which the piston 16 descends from top dead center TDC to bottom dead center BDC, and a compression stroke in which the piston 16 ascends from bottom dead center BDC to top dead center TDC. The intake and suction are performed near the bottom dead center BDC between these two strokes, and basically include scavenging to perform gas exchange while pushing out the exhaust gas with fresh air pushed in by the supercharger 42. When the load is high, the amount of fresh air and fuel is large, and therefore, if scavenging is performed reliably, there is little exhaust gas remaining in the cylinder, so there are few combustion problems.However, scavenging must be done reliably and efficiently. When idling or under low load, the amount of fresh air and fuel is small, and combustion must be carried out in a large amount of residual exhaust gas. When fresh air and exhaust mix, the air-fuel ratio becomes dilute, making ignition combustion extremely difficult. It becomes.

In the present invention, the exhaust valve 26 opens at 8 foreshock 0 degrees before bottom dead center BDC, and at this time, the descending speed of the piston 16 is fast, so the pressure in the combustion chamber 18 after a weak exhaust blowdown at idle and low load. The back pressure in the exhaust port 22 and the negative pressure in the combustion chamber 18 ensure that a backflow of exhaust gas occurs. The exhaust valve 26 closes at a point 4 degrees after the bottom dead center BDC, where the compression stroke has not progressed much.

In addition, the fresh air valve 24 opens at a time when exhaust gas backflow occurs after the exhaust valve 26 opens, for example, at 0 degrees before bottom dead center BDC, and at 7 after dead center BDC
Closes at 0 degrees.

FIG. 5 is a diagram illustrating the backflow and swirl generation of exhaust gas during idle and low load, and the resulting stratification of fresh air and residual exhaust gas. At this time, the intake control valve 48 is closed and fresh air is supplied to the low-load fresh air valve (
FA) Only from the 20th side. This supply of fresh air is slow because the amount of air is small and the supercharging pressure is low. As shown in Fig. 5(a), when the 8 foreshocks occur 0 degrees before bottom dead center BDC, the exhaust valve 26 opens and a weak exhaust blowdown of pressure P occurs.This exhaust blowdown occurs during idle and low load. It will finish in a short time.

For example, the pressure at the exhaust port 22 during mild exhaust blowdown at idle and low load is instantaneous.

Generally it will be about 2-3kg/aJ, but soon it will be 1.05k
g/cd, and stabilizes while maintaining positive back pressure.

As the piston 16 descends, the inside of the combustion chamber 18 becomes negative pressure,
As shown in FIG. 5), exhaust gas re-inflows in the direction of arrow Q, and a swirl S of exhaust gas is formed within the combustion chamber 18 by the structure of the exhaust port 22 and swirl forming means such as the mask 56. Ru. When the temperature reaches 60 degrees before bottom dead center BDC, the low load fresh air valve (F^20) opens. Fresh air is throttle valve 4
The amount of iPi is 0, and the supercharging pressure of the supercharger 42 is also relatively low. In addition, since it takes time for the fresh air valve 24 to substantially fully open, fresh air does not flow in immediately, and the low-load fresh air valve (F^
20) Even at the beginning of the valve opening, the exhaust gas backflow and swirl formation continue. In this way, the formation of the exhaust swirl continues for a considerable period of time, and since this swirl is formed around the cylinder axis, it is maintained without disappearing until after the end of the compression stroke.

After that, as shown in FIG. 5(C) and (dl), fresh air enters when the fresh air valve 24 is substantially fully opened.At this time, the descending speed of the piston 16 is also small, so combustion Since almost no negative pressure is formed in the chamber and the supercharging pressure is low as described above, fresh air slowly enters the combustion chamber 18.Therefore, the fresh air that has flowed in slowly rides on the exhaust swirl, causing the above-mentioned Since the air is supplied so as to rotate in the same direction, it swirls together with the exhaust swirl on the exhaust swirl.In this way, the fresh air gathers at the part near the spark plug 28 on the cylinder head 14 side, i.e. , stratification of the fresh air on the cylinder head 14 side and the exhaust gas on the piston 16 side is achieved.This stratification of fresh air and exhaust gas occurs when the piston 16 is at the bottom of the cylinder head, as shown in FIG. It descends to a point, then rises a little, closes the exhaust valve 26, and is maintained even when the fresh air valve 24 closes.In addition, even if the piston 16 passes the bottom dead center and begins to rise, the movement speed remains unchanged for a while. Since the catalyst 52 provided in each exhaust manifold 50 prevents the pressure from decreasing in the exhaust port 22 and the exhaust manifold 50, the outflow of fresh air from the combustion chamber 18 to the exhaust port 22, so-called fresh air blow-through, is almost impossible. Does not happen.Also, each exhaust manifold 5
It has been confirmed that the catalyst 52 installed at 0 has the effect of reflecting the pressure at the time of exhaust blowdown, and this reflected pressure is applied to the combustion chamber 18 after the piston 16 passes the bottom dead center and begins to rise. By applying back pressure to the air, venting fresh air is effective in preventing air pollution. In order to take advantage of this effect, it is necessary to prevent the valve opening timings of the exhaust valves 26 from overlapping, and thus, as shown in FIG. It is preferable that Note that the fuel is injected while the fresh air valve 24 is open.

At idle and under light load, the air-fuel mixture gathers in the cylinder head 14 side near the ignition plug 28 side as described above, and as a result, the air-fuel ratio does not become lean near the ignition plug 28, and the air-fuel ratio is easily reduced by the ignition plug 28. This allows for reliable combustion to be achieved by ignition. This air-fuel mixture is placed on top of the exhaust gas and is activated by the high-temperature exhaust gas, forming an activated thermal atmosphere containing radical fuel components, and combustion occurs in a state where ignitability is enhanced. It can be done.

At medium to high loads, the fresh air control valve 48 is opened, so fresh air is supplied through both fresh air valves 24, and in particular,
A large amount of air can now pass through the fresh air port (FB24) on the high load side. When the amount of fresh air increases in this way, the swirl effect of the re-inflowing exhaust gas disappears, and a large amount of fresh air is used to perform cross-scavenging. At this time, since there is a mask 56 that crosses the center of the cylinder head 14, fresh air supplied toward the exhaust port 22 hits the mask 56 and flows downward, so that the fresh air first flows downward and then into the piston. It hit 16 and turned upwards,
Scavenging is performed with a U-shaped flow.

At this time, if there is a difference in the flow strength of the two fresh air ports 20, the dominant one will be pushed and a swirl will be formed around the cylinder axis again.

At high loads, such a swirl around the cylinder axis cannot increase the scavenging efficiency, and in particular, the effect of U-shaped cross-scavenging cannot be achieved.Therefore, in the present invention, two fresh air ports are used. 20 are formed so that their respective flow resistances are approximately equal.

The first means to make the flow resistances approximately equal is to make the cross-sectional areas of both fresh air ports 20 approximately equal. Sometimes it is convenient to slowly supply fresh air from the low-load side fresh air valve (FA). In other words, when the load is low, the amount of air required is much smaller than when the load is high.
It is considered preferable to form the fresh air valve (FA), which can be said to be exclusively for low loads, and its fresh air port to be small, but it is preferable to form it larger than what is actually considered necessary (to the same size as the high load side FB). By doing so, it is possible to increase the cross-sectional area and thus assist in slow feeding. A second means for making the flow resistance approximately equal is to form the mask 56 and the wall structure near the valve seat of the fresh air valve 24 approximately equivalently for both fresh air ports 20. However, in order to form an exhaust swirl, it is preferable that the slope of the side edge of the mask 56 is reversed between the upper side (Es side) and the lower side (E side) like the blades of a propeller. The sloped part is a gentle slope, but the slope becomes steeper as you go over the ridgeline, whereas the lower part slopes gently from the FA to the ridgeline and then descends with a steep slope. In such a case, the fresh air port 20 on the high load side has greater flow resistance, so the fresh air port on the low load side may be made slightly smaller, or the bubble lift of the fresh air valve 24 may be slightly reduced. It is better to keep it small.

〔Effect of the invention〕

As explained above, according to the present invention, reliable combustion can be achieved by achieving stratification between fresh air and residual exhaust gas especially at idle and low load, and cross scavenging is performed at high load. It is possible to obtain a two-stroke internal combustion engine that can obtain high output and can perform good combustion from low loads to high loads.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing a six-cylinder two-stroke internal combustion engine to which the present invention is applied, Fig. 2 is a diagram showing the vicinity of the combustion chamber of one cylinder in detail in Fig. 1, and Fig. 3 is a diagram showing a new version of Fig. 2. A vertical cross-sectional view passing through the air valve and the exhaust valve, FIG. 4 is a diagram showing pulp timing, and FIG. 5 is an explanatory diagram explaining stratification at low load. 14... Cylinder head, 16... Piston,
18... Combustion chamber, 20... Fresh air port,
22... Exhaust port, 24... Fresh air valve, 26
...Exhaust valve, 42...Supercharger, 48...
- Fresh air control valve, 56...mask.

Claims (1)

[Claims] A two-stroke internal combustion engine with a fresh air supply system that includes supercharging means, and a fresh air valve and exhaust valve that are driven in synchronization with the crank angle to open and close the fresh air port and exhaust port provided in the cylinder head. In an engine, a means is provided for causing part of the exhaust gas discharged from the exhaust port when the exhaust valve is opened to re-inflow into the combustion chamber while swirling it around the cylinder axis. a fresh air control valve that includes two ports extending substantially parallel to each other, a fresh air valve disposed in each of the two ports, and a fresh air control valve that is opened and closed in one of the two ports or in a passageway upstream thereof; A two-stroke internal combustion engine, characterized in that the two ports are formed such that their respective flow resistances are approximately equal.