A stratified-charge cross-flow scavenged two-stroke cycle spark-ignition engine (1) includes a piston (2) having a raised crown (6) with a baffle (7) and chamfered sides (8, 9) and reciprocal in a cylinder (3) between a combustion chamber (4) and a crankcase (5). Transfer passage structure is disclosed wherein a pair of elongated scavenging air passages (17, 18) extend between the crankcase and combustion chamber on opposite sides of a shorter fuel-air transfer passage (11). A pair of scavenging air inlet ports (20, 21) in the combustion chamber are adjacent the fuel-air inlet port (12) therebetween and substantially distally opposite the exhaust port (16). The scavenging air inlet ports face the baffle at its edges along the chamfered sides of the piston crown.

DESCRIPTION 
1. Technical Field 
The invention relates to a stratified-charge cross-flow scavenged 
two-stroke cycle spark-ignition engine. One application is marine 
propulsion systems. 
2. Background 
In a cross-flow two-stroke cycle engine, the piston has a raised crown with 
a curved deflection shoulder or baffle facing the fuel-air mixture inlet 
port to redirect the incoming mixture into the cylinder. This is in 
contrast to a loop scavenged engine having a flat or spherical piston 
crown. 
In a cross-flow engine, the piston crown also has chamfered sides extending 
from the tip of the crown at the edges of the baffle. The exhaust port is 
approximately 180.degree. opposite the fuel-air inlet port. During the 
scavenging downstroke of the piston, the incoming fuel mixture expels 
combustion products out the exhaust port, with the baffle on the piston 
crown preventing short circuiting thereacross of the incoming fuel 
mixture. However, there is some short circuiting to the exhaust port of 
unburned fuel mixture across the ends of the baffle along the chamfered 
sides of the crown adjacent the peripheral cylinder wall. 
Shaping the baffle edges to prevent the mixture from flowing thereacross is 
not an acceptable solution because of interference problems with the 
combustion chamber. The baffle is thus normally chamfered at the edges. In 
some applications, a fairly large chamfer is necessary in order to prevent 
the baffle from causing pre-ignition. 
DISCLOSURE OF THE INVENTION 
A scavenging air system provides air inlet means in the combustion chamber 
proximate the chamfered side of the piston crown. A pair of elongated air 
passages have air inlet ports facing the edges of the crown baffle at the 
chamfered sides. The air inlet ports are on opposite sides of and adjacent 
the fuel-air mixture inlet port and substantially distally opposite the 
exhaust port.

BEST MODE FOR CARRYING OUT THE INVENTION 
A stratified-charge cross-flow scavenged two-stroke cycle spark-ignition 
engine 1, includes a piston 2 reciprocal in a cylinder 3 between a 
combustion chamber 4 and a crankcase 5. Piston 2 has a raised crown 6 with 
a curved deflection shoulder or baffle 7 and chamfered sides 8 and 9, at 
the edges of the baffle. A fuel supply system is provided for supplying 
fuel to crankcase 5. A fuel-air transfer passage 11 extends between 
crankcase 5 and a fuel-air inlet port 12 in combustion chamber 4. During 
the upstroke of piston 2, the fuel-air mixture is compressed in chamber 4, 
and upon ignition of spark plug 13 combustion of the mixture drives piston 
2 downwardly to rotate crankshaft 14 through connecting rod 15. The 
combustion products exit through exhaust port 16. 
A plurality of transfer passages such as 11, 17 and 18 extend between 
crankcase 5 and combustion chamber 4. These passages include the fuel-air 
transfer passage 11 extending between crankcase 5 and fuel-air inlet port 
12 in combustion chamber 4 facing baffle 7 of piston crown 6 and 
approximately 180.degree. opposite exhaust port 16. The transfer passages 
further include air passages 17 and 18 having an input 19 for receiving 
external air and extending between crankcase 6 and respective air inlet 
ports 20 and 21 in combustion chamber 4 proximate respective chamfered 
sides 8 and 9 of pistion crown 6. 
Air inlet ports 20 and 21 face baffle 7 of the piston crown. Air inlet 
ports 20 and 21 are adjacent fuel-air inlet port 12 therebetween. The 
three ports 20, 12 and 21 are substantially distally opposite exhaust port 
16. 
External air input 19 is an air throttle, including butterfly valve 22 and 
one-way reed valve 23, connected to the pair of air passages 17 and 18 by 
bridging passage 24 therebetween. In one embodiment, air is inducted 
through input 19 only to the outside transfer passages 17 and 18. In 
another embodiment, air is also inducted into central transfer passage 11 
as shown at 25. In the latter embodiment, the amount of air introduced 
into central passage 11 may be limited by an orifice or valve so that the 
first portion of a charge entering the cylinder from that passage is air, 
and as the scavenging progresses, fuel-air mixture is introduced to the 
central passage. 
Air transfer passages 17 and 18 each have a length between crankcase 5 and 
respective air inlet ports 20 and 21 substantially great enough to reduce 
fuel mixture therein and afford substantially only air at the respective 
air inlet ports 20 and 21 to in turn short circuit along respective 
chamfered sides 8 and 9 of piston crown 6 to exhaust port 16 in 
substitution for unburned fuel mixture from port 12. The greater the 
length of passages 17 and 18 between crankcase 5 and combustion chamber 4 
the lesser the amount of fuel and the greater the amount of air at ports 
20 and 21. This greater amount of air reduces the amount of unburned fuel 
mixture from ports 20 and 21 otherwise lost to exhaustion, and also 
substitutes for more of the unburned fuel mixture from port 20 otherwise 
lost to exhaustion. Fuel-air transfer passage 11 extends from crankcase 5 
at a port 26 adjacent piston 2 and has a length approximately equal to the 
height of piston 2. Air passages 17 and 18 extend from crankcase 5 at 
ports 27 and 28 substantially distally removed from piston 2. The length 
of air passages 17 and 18 between respective ports 27 and 20, and 28 and 
21, is substantially greater than the length of fuel-air transfer passage 
11. The length of passage 11 is approximately equal to the height of 
piston 2. 
Fuel supply means 10 comprises carburetor means 29, including butterfly 
valve 30 and one-way reed valve 31, mounted to crankcase 5 substantially 
coaxially with piston 2 to enable narrow engine design. By supplying the 
fuel into the crankcase, there is better mixing of fuel and air because 
the fuel goes into a hot crankcase and is stirred up by the crankshaft and 
connecting rods to provide better atomization and vaporization. In an 
alternative, the carburetor could be located on top of the short transfer 
passage 11, in which case the fuel mixture would flow down the short 
passage into the crankcase and afford the noted atomization and 
vaporization. In each case, no lubrication pump is required because the 
fuel-oil mixture is supplied to the crankcase. 
The carbureted cross-flow scavenged engine with reduced fuel comsumption 
thus provides short circuiting of substantially only air from the outside 
transfer passages to the exhaust port distally opposite thereto. Such 
engine is particularly useful in low cost low horsepower engine 
applications.