Fuel system for crankcase scavenged two cycle spark ignition engines

A fuel system for crankcase scavenged two cycle spark ignition engines which includes a specialized valve assembly connecting a source of propane gas and a source of oil to the engine. An automatic main valve connects the source of gas to the engine port and also senses pulses from the engine crankcase. A control valve interconnected to a venturi controls the air and oil supply to the engine.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to the fuel system of small two cycle 
crankcase scavenged spark ignition engines typically used for yard blowers 
and trimmers and similar equipment, to operate on LPG (liquified petroleum 
gas) such as propane, to eliminate air pollution, fuel mixing and 
providing single step starting, making these engines more user friendly. 
The present invention further relates to the substitution of a novel fuel 
system and carburetor assembly to replace the gasoline fuel tank and gas 
line conventionally used with such two cycle crankcase scavenged spark 
ignition engines. 
2. Description of the Prior Art 
A conventional two stroke engine is provided with a scavenging fan. At the 
start of the first stroke, the piston is in its highest position. When the 
compressed petrol-and-air over the piston is ignited, the latter is thrust 
downwards and, in so doing, releases the exhaust port. The burned gases in 
the cylinder, which are still under high pressure, can thus escape through 
this port. When the piston descends further, its upper edge releases the 
inlet port, which admits fresh petrol-and-air mixtures, delivered by a 
fan, into the cylinder, so that the remaining burned gases are flushed 
out. When the piston rises again on the second stroke, all the ports are 
closed off for a time, and during this period the petrol-and-air mixture 
is compressed, so that a fresh cycle can commence. 
The crankcase-scavenged two-stroke engine has no scavenging fan. Instead, 
the crankcase is hermetically sealed, so that it can function as a pump in 
conjunction with the piston. When the piston ascends, a partial vacuum is 
produced in the crankcase, until the lower edge of the piston releases the 
inlet port and thus opens the way to the fresh petrol-and-air mixture into 
the crankcase. When the piston descends, the mixture in the crankcase is 
compressed a little so that, as soon as the top of the piston releases the 
transfer port and overflow duct which connects the crankcase to the 
cylinder, it can enter the cylinder. Meanwhile, what happens above the 
piston is the same as in the fan scavenged engine. The lubricating oil is 
mixed with the petrol or is, alternatively, supplied to the points of 
lubrication dropwise by small lubricating oil pumps. The oil which enters 
the crankcase is liable to be carried through the overflow duct and 
transfer port into the cylinder, where it passes through the exhaust port 
and into the exhaust system. 
The two cycle crankcase scavenged spark ignition engines known as two cycle 
SI engines are predominantly used to power hand tools made for the garden 
industry and to a lesser extent for the construction and building 
industry. The power rating of these engines are from a few tenth of a 
horsepower to several horsepower. The two cycle SI engines dominate this 
field because they provide mobility at a light weight and a reasonable 
initial cost. 
The fuel for these engines is a mixture of gasoline and lubricating oil in 
a predetermined ratio which must be mixed by the user. This is an 
unpleasant chore for the consumer, often resulting in improper mixture 
ratio. The fuel mixture provides fuel for the combustion process and the 
oil contained in it provides lubrication for the engine components. 
Because of the two cycle operating principle, part of the incoming mixture 
is discharged through the exhaust port as part of the scavenging process, 
leaving unburned hydrocarbons in the exhaust. Furthermore, much of the 
lubricating oil burns and contributes to smoke and unfavorable exhaust 
emission and odor. 
The fuel system consists of a fuel tank attached to the engine and 
connected to a gasoline carburetor which is typically a membrane 
carburetor as manufactured by Walbro Co., Tillotson and several domestic 
and foreign manufacturers. This is a precision device consisting at least 
of thirty components and several minute holes. The engine crankcase 
pressure pulses are used to operate a small pulse pump to feed a metering 
chamber which dispenses fuel into the carburetor venturi as a function of 
air flow and throttle position. Providing starting fuel at cranking speed 
and controlling fuel at peak power is a considerable task which explains 
the need for complexity and required precision of this type of carburetor. 
As a consequence, the cost of the carburetor is high and is a considerable 
part of the total engine cost. Furthermore, it is one of the weak points 
of engine reliability and cause of hard starting. Starting of these 
engines is accomplished with a rope starter which has a spring rewind 
pulley and a ratchet that engages the crankshaft through pawls. When the 
engine starts, the pawls disengage due to the centrifugal force acting on 
them. The starting procedure involves several steps which must be 
performed in sequence. If it is done incorrectly, the engine will not 
start which is the most frequent consumer complaint. The typical starting 
sequence requires: to turn on the ignition, to close the choke, set the 
throttle to a starting position, briskly pull on the starter until the 
first pop is heard, readjust or open the choke, briskly pull on the 
starter until the engine starts, warm up the engine before loading. 
Starting a warm engine has its own problems such as vapor lock because of 
heat soak. There are also variations on the above procedure such as hand 
priming prior to starting. 
Therefore, there is a significant need for an improved fuel system which 
operates on cleaner burning fuel such as liquid propane and which 
eliminates or significantly reduces the problems addressed above. 
SUMMARY OF THE INVENTION 
The present invention relates to an improved fuel system and assembly which 
replaces the conventional gasoline fuel system in crankcase scavenged two 
cycle engines. 
The present invention relates to a novel approach to resolve air pollution 
problems, to eliminate fuel mixture preparation, to simplify the starting 
procedure, to make these engines user-friendly and more acceptable for the 
consumer market with particular attention paid to the environment. 
The fuel system is retrofit with minor external modifications to the engine 
structure, such as replacing the gasoline fuel tank with an adapter to 
support an LPG propane bottle. 
To resolve the exhaust emission problem, LPG propane was selected which is 
commercially available from any hardware store in 14.1 or 16.4 oz. 
capacities. 
Propane is used as a fuel on large engines such as fork lifts so that they 
can operate indoors without poisoning the atmosphere. Large engines are 
typically for cycle SI engines which have a separate fuel system and a 
separate lubrication system. 
It has been discovered, according to the present invention, that if liquid 
propane is used instead of gasoline to burn in crankcase-scavenged two 
cycle engines, then the engine will burn much cleaner. 
It has further been discovered, according to the present invention, that if 
the carburetor is modified to comprise specialized valves and an oil 
reservoir to efficiently combine the propane fuel and oil, then the user 
will not need to separately mix the fuel and oil. 
It is therefore an object of the present invention to provide an efficient 
and clean burning crankcase scavenged two cycle engine. 
It is an additional object of the present invention to provide such a 
crankcase scavenged two cycle engine which is run by liquid propane gas 
and not gasoline. 
It is a further object of the present invention to provide a crankcase 
scavenged two cycle engine which has automatic valves and an oil supply 
reservoir which can be combined with the propane gas to provide an 
efficient system for feeding the oil and fuel to the crankcase and 
cylinder of the two cycle engine without the requirement for the user to 
separately mix the fuel and oil. 
Further novel features and other objects of the present invention will 
become apparent from the following detailed description, discussion and 
the appended claim, taken in conjunction with the drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Although specific embodiments of the invention will now be described with 
reference to the drawings, it should be understood that such embodiments 
are by way of example only and merely illustrative of but a small number 
of the many possible specific embodiments which can represent applications 
of the principles of the invention. Various changes and modifications 
obvious to one skilled in the art to which the invention pertains are 
deemed to be within the spirit, scope and contemplation of the invention 
as further defined in the appended claims. 
The FIGURE is a schematic diagram of the present invention fuel system for 
crankcase scavenged two cycle spark ignition engines. The source of fuel 
is liquid propane gas 10 which is housed inside a liquid propane gas 
bottle 12. By way of example, the liquid propane gas bottle 12 may be a 
Standard 14.1 or 16.4 oz. LPG propane bottle with a standard threaded 
adapter, built-in check valve and pressure relief valve as per DOT-39NRC 
228/286. The built in check valve is illustrated at 14 and the pressure 
relieve valve is illustrated at 16. 
A standard pressure reducing and regulating valve 20 such as sold by the 
Coleman Co., is mounted to a metal part of the engine such as the fan 
housing to support one end 18 of the LPG propane bottle 12 and to warm the 
propane gas with engine heat as it tends to cool when its pressure is 
reduced. When the LPG bottle 12 is attached to the engine, its internal 
check valve 14 is opened by the regulator valve prong 22 and permits the 
propane gas 10 to enter the regulator valve 20 which reduces the bottle 
pressure to system pressure and maintains this pressure regardless of 
flow. 
A conduit or gas line 24 with proper fittings to connect the pressure 
reducing and regulating valve with the carburetor body 30. The carburetor 
30 may be connected to the engine cylinder 60 with a thin gasket to assure 
maximum heat transfer to the carburetor 30. 
An automatic main valve 40 is down-stream from the pressure regulating 
valve 20 and it is housed in the carburetor body 30 which is attached 
directly to the intake port 62 of the cylinder 60 and in connection with 
the pulse port 64 of the crankcase 56. The automatic main valve 40 is a 
double valve with a single actuator 42, normally closed by spring load for 
initial seating and by the gas system pressure and oil head. The main 
valve 40 has a gas valve 44 and an oil valve 46. The main valve 40 
prevents gas from entering the carburetor when the engine is not running 
and when the LPG bottle is attached to the engine. It also prevents oil 
from draining when the engine is stored. The automatic main valve 40 is 
normally closed by the system pressure and spring load. The automatic main 
valve 40 also has means to sense the engine rotation to thereby open due 
to crankcase pulse or venturi vacuum or the combination of the two. When 
the rope starter is activated and the engine is turned over, vacuum 50 is 
generated in the carburetor venture 52 and pressure pulses 54 in the 
crankcase 56. A diaphragm actuator 48 senses the pressure differential 
between the carburetor venture 52 and the crankcase pulses 54 which, in 
turn, acts on the gas valve 44 and cracks it open, permitting gas 10 to 
enter carburetor 30. When the engine starts, the carburetor venture vacuum 
50 becomes predominant and keeps the main valve 40 open while the engine 
runs. When the engine is stopped by cutting out the ignition, the main 
valve 40 automatically closes and prevents the gas 10 or oil 80 from 
entering the carburetor 30. 
The control valve 70 is down-stream from the automatic main valve 40. This 
is a needle valve rotatably seated in the carburetor body 30. Its function 
is to control the gas flow to the engine cylinder 60 by closing or 
opening. This is accomplished by a spring biased arm 72 attached to the 
control valve stem 74. The spring 76 tends to close the valve 70 and is 
acting against a manual control force typically generated by a finger 
trigger mechanism to open the valve 70 to increase speed. The preferred 
orientation of the needle valve is on center with the venture 50 and 
perpendicular to its axis. It can thereby control both air and gas flow. 
To the extension of the small end of the needle valve, a butterfly valve 
78 may be attached to control air/fuel ratio and also to facilitate the 
mixing of air, gas and oil in the venturi prior to entering the engine. 
Test engines operated satisfactorily without the butterfly valve 78 where 
air/fuel ratio was controlled by venturi throat size and control valve 
opening. 
Oil 80 is housed in an oil reservoir 82. Oil flow to the engine is 
controlled by a properly sized orifice 84 which is down-stream from an 
automatic main valve 40 and its oil valve 46. The differential pressure 
between the oil reservoir 82 and the venturi vacuum 50 forces the oil 80 
into the venturi throat 51. The form of oil discharge can be controlled by 
the oil exit hole to the venturi. A protruding tube 86 into the air stream 
88 provides atomization of the oil 80. Elimination of the discharge tube 
86 permits the oil 80 to ooze out of the hole and forced to the engine 
port 62 on the venturi wall by the air motion. A proper compromise between 
these two limits can minimize suspended oil in the air to leave the engine 
during the scavenging period. For lubrication, approximately 7cc. oil is 
needed per 14.1 oz. propane. The oil reservoir 82 must be transparent so 
that the oil level can be observed. It may be attached to the carburetor 
or it may be remotely mounted depending from the installation. The oil 
reservoir 82 is equipped with a rubber check valve 90 permitting air to 
enter and preventing oil to spill. The gravity feed arrangement is the 
most cost-effective and sufficient for string trimmer and blower 
installation. This does not exclude a more sophisticated system such as an 
all position metering pump. The gravity fed oil supply 82 also may include 
means to automatically shut off when the engine control is deactivated. 
This prevents oil from draining into the engine when stored. Gravity 
discharge is increased by the throat vacuum when the engine is running 
because of the pressure differential between the atmosphere and the 
carburetor throat. 
To start the engine, the ignition must be ON. This is normally the case 
when a momentary kill switch is used. The LPG propane bottle 12 is 
attached to the engine which automatically activates the pressure 
regulator 20. The throttle trigger is placed into the start position which 
opens the control valve 70 for the starting gas requirement to the engine. 
When the starter rope is activated, the automatic main valve 40 opens as 
described earlier and provides gas 10 and oil 80 to the venture 52. When 
the engine starts and sustain operation, the throttle trigger can be 
released which will take up the idle position. After warm up, the engine 
is controlled by the throttle trigger to match the power demand. The 
engine is stopped by the customary ignition kill switch. 
In accordance with propane fueled consumer products such as stoves and 
torches, it is recommended to remove the LPG propane bottle 12 from the 
engine to prevent leakage in case of a faulty valve. Assuming that the 
bottle will not be removed despite the operating instruction, it may be 
prudent to incorporate a shut-off feature into the pressure regulator 20 
and eliminate the ignition kill switch so that the operator is forced to 
shut off the engine by cutting off the fuel supply. Naturally, the 
ignition kill switch may be retained at a very small cost penalty. 
The operating time on a standard 14.1 oz. propane bottle is a function of 
load and fuel consumption which varies from engine to engine. A calculated 
spread of 45 minutes to 60 minutes was found which corresponds to a 
typical one-month consumer use of blowers or trimmers. A back pack LPG 
propane bottle in combination with a hand-held blower would provide more 
than adequate fuel supply for a professional gardener for several days. 
The cost of the LPG propane bottle is relatively high compared to gasoline. 
This is because of the cost of the disposable bottle which costs more than 
the propane in it. The economic consideration greatly improves when a 
bottle exchange program is considered either by the equipment manufacturer 
or the gas supplier. 
The LPG fuel system as described above provides a user-friendly engine and 
environmentally acceptable tool at a reasonable cost penalty to the user 
and considerable opportunity to the fuel supplier. 
Of course the present invention is not intended to be restricted to any 
particular form or arrangement, or any specific embodiment disclosed 
herein, or any specific use, since the same may be modified in various 
particulars or relations without departing from the spirit or scope of the 
claimed invention hereinabove shown and described of which the apparatus 
is intended only for illustration and for disclosure of an operative 
embodiment and not to show all of the various forms of modification in 
which the invention might be embodied or operated. 
The invention has been described in considerable detail in order to comply 
with the patent laws by providing full public disclosure of at least one 
of its forms. However, such detailed description is not intended in any 
way to limit the broad features or principles of the invention, or the 
scope of patent monopoly to be granted.