Two stroke engine

A two-stroke engine is provided comprising at least one cylinder (1) in which is slidingly mounted a piston (2) which defines in this cylinder a combustion chamber (C) and a precompression chamber (P), the lateral cylindrical wall of the cylinder being provided with at least two ports closable by the piston (2), namely at least one transfer port (3) through which opens a transfer channel (4) connecting the two chambers together and at least one exhaust port (5) communicating with an exhaust pipe (6). These two ports are arranged and disposed so that the opening of the transfer port (3) begins before that of the exhaust port (5) and special means are provided for making the pressure of the fresh gases admitted into the combustion chamber (C) through the transfer port (3), during opening of this transfer port, sufficiently high with respect to that of the burnt gases then present in the combustion chamber.

The invention relates to two-stroke engines comprising at least one 
cylinder in which is slidingly mounted a piston which defines in this 
cylinder a combustion chamber and a precompression chamber, the lateral 
cylindrical wall of the cylinder being provided with at least two ports 
closable by the piston, namely at least one transfer port through which 
opens a transfer channel connecting together the two chambers and at least 
one exhaust port communicating with an exhuast pipe. 
It applies in particular to high efficiency engines of this type, such as 
light aeroplane engines and outboard engines for boats. 
An interesting application -- but not exclusive -- of the invention is that 
of engines in which each piston is mounted at the head of a cylindrical 
rod itself slidingly mounted in a boss connected to the corresponding 
cylinder, the shape of the boss being substantially complementary to the 
inner surface of the piston and each rod being connected to the 
crank-shaft of the engine through an excentric, as has been described in 
U.S. Pat. No. 3,946,706 of the Applicant. 
In known embodiments of the engines in question, the exhaust ports begin to 
open before the transfer ports so that the combustion chamber is emptied 
of its burnt gases before being filled with fresh gases. 
Special measures ("exhaust tuning") must then be taken so as to allow the 
burnt gases to flow back to the combustion chamber before the exhaust 
ports are totally closed, so as to prevent the escape of fresh gases 
through these ports. 
The measures in question are delicate to achieve, they lead to cumbersome 
constructions and they are only valid for certain operating conditions. 
The invention avoids recourse to such special measures. 
More generally, it allows the air and fuel supply of the engine to be 
improved and thus the efficiency of this engine and the power developed 
thereby to be increased. 
To this end, the engines of the kind in question are essentially 
characterized in that their ports are arranged and disposed so that the 
opening of the transfer port begins before that of the exhaust port and in 
that special means are provided for making the pressure of the fresh gases 
admitted into the combustion chamber through the transfer port, during 
opening of this transfer port, sufficiently high with respect to the burnt 
gases then present in this combustion chamber. 
In preferred embodiments, recourse is further had to one and/or the other 
of the following arrangements: 
the pressure of the fresh gases admitted into the combustion chamber 
through the transfer port is made at least equal to that of the burnt 
gases occupying this chamber, 
the pressure of the fresh gases is made high, during opening of the 
transfer port, by giving a low value to the volume of the precompression 
chamber, 
in an engine in accordance with the preceding paragraph, the precompression 
chamber is defined respectively at both its axial ends by the inner 
surface of the piston and by a boss of substantially complementary shape 
connected to the cylinder, inside which boss is slidingly mounted a rod 
coupled to the piston, 
the pressure of the burnt gasses is reduced, during opening of the transfer 
port, by forming in the lateral wall of the cylinder a port of small 
section communicating with the exhaust pipe of the engine and adapted to 
be opened a little before the exhaust ports properly speaking, 
in an engine according to the preceding paragraph, the small section port 
is connected to the exhaust pipe through successively a decompression 
chamber and a calibrated passage, the forms and dimensions of this chamber 
and of this passage being determined so that the pressure of the burnt 
gases inside the decompression chamber is comparable to that of the fresh 
gases which have just been introduced into the combustion chamber once the 
transfer port is closed, 
means are provided for ensuring an indirect injection of petrol or gasoline 
through the transfer ports at certain times when these latter are open, 
the intake of fresh gases into the precompression chamber comprises a 
non-return flap means and the intake pipe containing this non-return 
device has a length greater than four times the inner diameter of the 
cylinder. 
The invention comprises, apart from these main arrangements, certain other 
arrangements which are used preferably at the same time and which will be 
more explicitly discussed hereafter.

The two-stroke engine considered comprises at least one cylinder 1 in which 
is slidingly mounted a piston 2 which defines in this cylinder a 
combustion chamber C and a precompression chamber P, the cylindrical 
lateral wall of the cylinder being formed with at least two ports closable 
by piston 2, namely at least one transfer port 3 through which opens a 
transfer channel 4 connecting the two chamber P and C together and at 
least one exhaust port 5 communicating with an exhaust pipe 6. 
Ports 3 and 5 are here arranged and disposed so that opening of the 
transfer port 3 begins before that of the exhaust port 5. 
Moreover, special means are provided for making the pressure of the fresh 
gases admitted into combustion chamber C through transfer port 3, during 
opening of this port, sufficiently high with respect to that of the burnt 
gases then present in said chamber C. 
Experience shows that, because of this relatively high pressure of the 
fresh gases admitted -- which pressure is preferably at least equal to 
that of the burnt gases then present in the combustion chamber -- , these 
burnt gases do not prevent intake of these fresh gases into the chamber in 
question and, despite that, the exhaust concerns practically only the 
burnt gases, which arrive first at the level of the exhaust ports 5. 
The required increase in relative pressure may be obtained by increasing 
the pressure of the fresh gases admitted and/or by reducing the pressure 
of the burnt gases removed. 
To increase the pressure of the fresh gases admitted, they are 
precompressed by giving more especially to the precompression chamber P a 
very small minimum volume, this chamber being defined by a boss 7 
connected to cylinder 1 and whose shape is substantially complementary to 
the inner surface of piston 2. 
In this case, this piston is mounted on a rod 8 itself slidingly mounted in 
boss 7 and connected to the crankshaft of the engine through an excentric, 
for example in the way described in the above-mentioned U.S. patent. 
The operation of such an engine is the following. 
When the transfer ports 3 open, the fresh gases begin to penetrate into 
combustion chamber C, whose volume increases because of the advance of 
piston 2 to its bottom dead center. When the exhaust ports open in their 
turn, a violent current is established, the burnt gases then being sucked 
up on the one hand by the depression of the exhaust and pushed back on the 
other hand by the high pressure fresh gases which are then projected 
towards the inside of chamber C, and preferably towards the top of the 
cylinder. 
When the exhaust ports close again, the fresh gases continue to fill 
chamber C, because of their kinetic energy, until the transfer ports 3 
close in their turn. 
The dynamic filling of chamber C thus obtained is much better than in a low 
compression two-stroke engine, it is no longer necessary to have recourse 
to special measures for causing the exhaust gases to flow back towards the 
cylinder and the exhaust pipe is simplified and less cumbersome. 
If the pressure of the burnt gases is still too high at the beginning of 
opening of the transfer ports 3, it may be advantageous to provide a 
channel or "leak" 9 of very small section with respect to that of the 
exhaust ports 5 properly speaking, this small section being accurately 
calibrated: the channel in question opens into combustion chamber C at a 
level in this chamber a little closer to the bottom of the cylinder than 
the exhaust ports 5, this level possibly almost reaching that of the edge, 
of the transfer ports 3, the closest to said bottom. 
Channel 9 in question allows the pressure of the burnt gases contained in 
combustion chamber C to be reduced to a suitable value during opening of 
the transfer ports 3, but its very small section prevents undesirable 
losses of the fresh gases at the end of admission. 
In a more perfected version illustrated in FIG. 3, channel 9 does not open 
directly into the exhaust pipe 6: it ends at a decompression chamber 10 
itself connected to the exhaust pipe through a second calibrated passage 
11. The shape and volume of this chamber 10 and the section of passage 11 
are calculated so that the pressure of the burnt gases inside this chamber 
10 is of the same order as that of the fresh gases in combustion chamber C 
during closing of the transfer ports 3, so as to avoid any undesirable 
loss of the fresh gases to the exhaust pipe. 
Since the opening of the transfer ports 3 is maintained for a large angular 
movement of the drive shaft, which movement is greater than that 
corresponding to the opening of the exhaust ports 5, advantage may be 
taken thereof to ensure an indirect injection of petrol through these 
openings without loss of fuel to the exhaust, under conditions close to 
those observable for the indirect injection into four stroke engines. 
Of course, in this case, the injection pump will be adjusted so as to allow 
injection at opportune moments, corresponding generally to the period when 
the exhaust ports have just closed and when the transfer ports are still 
open, with retard for slow running and advance for fast running. 
In the pipe 12 for the intake of fresh gases into precompression chamber P, 
there is advantageously provided a non return disk or flap device 13. 
Under very high running conditions, if a flap device is provided, the flaps 
may be perturbed in their operation: in such a case, it is advantageous to 
provide the non return by conferring directly on the column of fresh gases 
admitted a sufficient kinetic energy; to this end, the intake pipe 12 is 
advantageously given a relatively great length L, which length is 
preferably at least four times greater than the inner diameter D of the 
cylinder. 
The long column of fresh gases then contained in this intake pipe opposes 
by its very inertia the precompressed gases from being driven outwardly 
through said pipe. 
As is evident and as it follows moreover already from what has gone before, 
the invention is in no wise limited to those of its modes of application 
and embodiments which have been more especially considered; it embraces, 
on the contrary, all variations thereof.