small, carburetted two-stroke engines are mainly used for hand-held tools, like e.g. chain saws, weed cutters, trimmers, lawn mowers, etc. The main reasons for using two-stroke engines for such tools/machines are that they are cost effective and that they have a high power-to-weight ratio. A further advantage of the two-stroke engine compared to other engine options is that the mechanical design is very simple, principally only containing three moving parts (the piston, the connecting rod and the crankshaft).
The major problem with small, crankcase scavenged, carburetted two-stroke engines is the emission level of unburned hydrocarbons (uHC) and carbon monoxide (CO). For the past decade, legislation and authorities have demanded a decreased level of these emissions. Legislation also requires low amounts of nitric oxides (NOx), but due to the general function of two-stroke engines, the emission of NOx is inherently low. In the following, the formation processes of the above-mentioned emissions will be briefly explained.
Carbon monoxide is formed when a hydrocarbon, such as gasoline, Liquid Petroleum Gas (LPG), diesel fuel, or any compound containing coal, is combusted in presence of too small amounts of oxygen to complete the combustion to carbon dioxide (CO2). The only way of decreasing the emission of CO is to lean the combustion, i.e. to mix the coal containing fuel with more oxygen (i.e. in most cases more air). Leaning out the fuel/air mixture has however some severe drawbacks regarding engine cooling, lubrication and engine behaviour.
NOx is formed whenever a gas containing nitrogen and oxygen is heated, e.g. in a combustion chamber of an internal combustion engine. The NOx formation is dependent on the temperature, the time the gas mixture is heated, the nitrogen and oxygen concentration, and the temperature decrease rate. As mentioned earlier, NOx formation is not a severe problem in a two-stroke engine. The reasons for this are;                The temperature in the combustion chamber does not reach high levels, due to fuel rich combustion and excessive dilution of the combustible fuel/air mixture with exhaust gases.        Due to the fuel rich mixture, virtually all oxygen present in the combustion chamber prior to combustion is consumed during the combustion. This leaves no oxygen for the formation of NOx.        
The formation of unburned hydrocarbon emissions (uHC) is a little bit more complicated than the formation of the NOx and CO emissions:                One main source for uHC emissions is the clearance volume over the piston ring pack, since unburned air/fuel mixture in pressed down into this volume and hence escapes combustion.        Wall quenching is another major contributor to uHC emissions. Wall quenching means that the combustion flame is not able to travel all the way to a combustion chamber wall, leaving an unburned zone close to the combustion chamber walls.        Incomplete combustion is a third source of uHC emissions. Incomplete combustion mainly occurs when the fuel air mixture is too diluted with an excessive air or exhaust gas amount to burn.        Short-circuiting is the main source of uHC emissions from two-stroke engines, and occurs since the exhaust port is open during the scavenging of the cylinder with unburned fuel/air mixture.        
In order to decrease the emissions of uHC from two-strokes engines, many measures have been taken in the past. Mostly, those efforts have been directed towards redesigning the so-called transfer channels, i.e. the channels from which the unburned air/fuel mixture enter the cylinder; different transfer channel designs give different scavenging flow patterns in the cylinder.
For the last decades, an old scavenging method called “air-head” scavenging has gained the interest from scientists and engine researchers as a means of reducing the emissions of uHC from two-stroke engines. The basic idea behind the air-head engine is that the first air-fuel mixture that enters the cylinder through the transfer channels is the most likely to short-circuit. Hence, an air-head scavenging system starts by letting pure air flow through the transfer channels, which increases the probability that pure air is short-circuited.
As mentioned, the idea behind the air-head scavenging is not new. In fact, Dugald Clerk, the man who is generally recognised as the inventor of the two-stroke engine, described an air-head system as early as 1881 (see GB-B-1089), but he did not use he air-head scavenging as a means for reducing the short-circuiting losses, rather as a means for avoiding premature ignition of the fresh charge, due to contact with the hot exhaust gases. More recent development has shown that there is no or little risk that uncompressed fresh air/fuel mixture ignites on hot combustion gases. Further, Clerk describes use of an air-head scavenging for a dual piston engine, with a uniflow type scavenging system of the power cylinder.
The engine described in GB 1089 has very little in common with the engine according to the present invention. The GB 1089 engine has e.g. two different piston/cylinder arrangements. One of the cylinders has as its only task to provide the other cylinder with the scavenging action for the new charge, whereas the other cylinder is the power cylinder, in which the combustion takes place.
A slightly more recent publication (U.S. Pat. No. 968,200, from 1910) describes an air-head scavenging for a crankcase scavenged two-stroke engine with a fairly complicated design. The piston is namely divided into two portions, wherein the power cylinder portion has a considerably smaller diameter than the scavenging portion of the piston. This means that the scavenging volume will be much larger than the cylinder volume, making short-circuiting of unburned fuel/air mixture unavoidable. Hence, the main reason for the air-head scavenging of U.S. Pat. No. 968,200 was probably to scavenge the cylinder from exhaust gases prior to letting in unburned fuel/air mixture. According to U.S. Pat. No. 968,200, a piston controlled ducting system is used to fill the crankcase with fuel/air mixture and the single transfer channel with pure air. In this way, air only will enter the cylinder during the initial phase of the scavenging. In order to separate the pure air from the fuel/air mixture, the transfer channel of U.S. Pat. No. 968,200 is very long, and contains a spiral path, in order to increase the flow-path length.
Further, the design according to U.S. Pat. No. 968,200 uses cross-scavenging, i.e. the transfer channel is connected to the cylinder at a position opposite the exhaust port. Excessive short-circuiting is avoided by means of a deflector on the piston top.
F. W Lanchester and R. H. Pearsall (The institution of automobile engineers, “An investigation of certain aspects of the two-stroke engine for automobile vehicles”, pp 55-62 February, 1922) describe a further arrangement for an air-head scavenged two-stroke engine. The concept described in that publication also uses very large transfer channels, in order to avoid mixing of the pure air with the fuel/air mixture in the crankcase. Lanchester and Pearsall even describe the use of a honeycomb structure in the transfer channel in order to reduce the mixing of the pure air with the fuel/air mixture in the crankcase. Further, the engine described in the above publication uses a cross scavenging similar to the type described above with reference to U.S. Pat. No. 968,200.
SAE paper 980761 (Society of automotive engineers, Inc, 1998) describes an air-head engine with reed valve (e.g. one-way valves) control, both for the incoming air-head air and for the air-fuel mixture. The scavenging pattern of the cylinder according to SAE 980761 is a so-called loop-scavenging, i.e. the scavenging flow from the transfer channels is directed towards a point in the cylinder on the side opposite the exhaust port.
WO-A-00/40843 describes a modified air-head scavenging, wherein two transfer channels close to the exhaust port scavenge the cylinder with pure air during the entire scavenging phase, and two transfer channels remote from the exhaust port scavenge the cylinder with a fuel-rich fuel/air mixture. Reed valves are used to control the airflow from the air scavenging transfer channels, which have a very large internal volume.
WO-A-99/18338 describes an air head engine with reed valve control of the air-head air flow and the fuel/air mixture flow. The transfer channels of this engine are also very large, actually it is stated on page 2, lines 34-37 that “the total volume of the scavenging hole and scavenging channel is set so as to be greater than 20% of the stroke volume”.
There are severe problems with the prior art designs:                In all prior art designs, the length of the transfer channels is very large. This leads to a lower high-speed power than is the case for shorter transfer channels. Until now, long channels have been regarded as necessary in order to get acceptable function of air-head engines. The long transfer channels also lead to a larger volume being connected to the crankcase, leading to a lower crankcase compression ratio, which in turn leads to a lower scavenging efficiency. Further, long and bulky channels add to the total size and volume of the engine.        The above-described designs comprising loop scavenging all utilise reed valves as the control means for the airflow to the crankcase and to the transfer channels. This is an expensive and complicated way of controlling the airflow.        
A further problem with the prior art designs is related to the characteristics of the carburetor. In order to get an acceptable idling running of the engine, the carburetor is usually set to provide a very fuel-rich mixture. As mentioned above, fuel-rich mixtures lead to excessive amounts of CO emissions. CO emissions are very harmful for all animals, and are of course a major problem for handheld tools that usually are used in the vicinity of the respiratory organs of a user. For present air-head engines, which mainly short-circuit air, the fuel-air ratio in the cylinder stays very fuel rich, even at high load. Obviously, this contributes to the CO emission levels.