Hydrocarbon emission control

A four-stroke spark ignited internal combustion engine has a variable volume working chamber and an auxiliary chamber having a fixed volume which is smaller than the maximum volume of the working chamber, with the working chamber and the fixed chamber being communicated by a series of passages in the cylinder wall. The passages in the cylinder wall are situated such that when the piston is near the top of its exhaust stroke, the end gases stored in the auxiliary chamber are discharged into the crankcase.

FIELD OF THE INVENTION 
The invention relates to the reduction of the hydrocarbon content of the 
exhaust gas emissions of a spark ignited four-stroke internal combustion 
engine. 
BACKGROUND OF THE INVENTION 
When the cyclic variation of the hydrocarbon content of exhaust gases is 
studied in detail, it is noted that there are peaks of high hydrocarbon 
concentration immediately when an exhaust valve opens and just before the 
exhaust valve closes. In a well tuned engine, the hydrocarbon 
concentration between these two peaks is significantly lower and within 
the range expected from complete combustion. The two peaks are not 
therefore caused by an incorrect fueling map and other reasons must be the 
cause of the presence of unburnt fuel in the exhaust. 
The present invention is concerned with the cause of the second peak which 
occurs at the end of the exhaust event rather than the first peak. It is 
generally believed that a major cause of this problem is the presence in 
the combustion chamber of small crevices into which fuel can be compressed 
but into which the combustion flame cannot penetrate. One such crevice is 
that surrounding the piston top land that is to say the small space 
between the piston and the cylinder above the top piston ring. During the 
compression stroke, fuel and air are compressed into this space. During 
combustion, the expanding flame front pushes mixture ahead of it into this 
crevice tending to increase the amount of fuel stored even further. 
However, the flame cannot enter this crevice because it is bound by two 
cold walls and the flame is quenched during its attempt to penetrate into 
this gap. Consequently, a quantity of fuel remains trapped in the crevice 
throughout the power stroke until the pressure in the combustion chamber 
during the exhaust stroke drops to allow the unburnt charge to escape from 
the crevice. The unburnt charge will then reside near the top of the 
piston and will be discharged towards the end of the exhaust stroke. 
Attempts have been made in the prior art to reduce the crevice volume by 
reducing the distance between the piston crown and the top ring but this 
causes problems because the top ring then runs hotter and reduces engine 
life. 
OBJECTS OF THE INVENTION 
The invention therefore seeks to provide a four-stroke engine in which the 
hydrocarbon content of the exhaust gases is reduced, especially near the 
end of the exhaust stroke. 
SUMMARY OF THE INVENTION 
According to the present invention, there is provided a four-stroke spark 
ignited internal combustion engine which comprises a variable volume 
working chamber defined between a reciprocating piston, which has at least 
one sealing piston ring, and a cylinder, characterised by an auxiliary 
chamber having a fixed volume significantly smaller than the maximum 
volume of the working chamber and communicating with the working chamber 
by a passage disposed in the cylinder wall at a position above the top 
piston ring when the piston is near the bottom of its stroke so that end 
gases at the end of the power stroke are stored in the auxiliary chamber, 
the auxiliary chamber communicating with the crankcase when the piston is 
near the top of its exhaust stroke so that the stored end gases are 
discharged into the crankcase. 
Preferably, the auxiliary chamber is an annular chamber surrounding the 
cylinder and connected to the combustion chamber by a plurality of 
passages distributed about the circumference of the piston. 
It should be mentioned that there are described in the prior art two-stroke 
engines which have an auxiliary chamber which is at times connected to the 
working chamber and at times to the crankcase. Examples of such engines 
are to be found in GB 2 223 802 and GB 2 083 550. Because of the 
fundamental differences between two-stroke and four stroke engines, these 
auxiliary chambers serve totally different functions and their dimensions 
and geometries are different from those of the auxiliary chamber required 
in the present invention. 
In a two stroke engine, the crankcase serves to compress the charge 
supplied to the working chamber. In the case of GB 2 223 802 the auxiliary 
chamber forms part of the transfer port and does not act as a small 
reservoir storing a small portion of the compressed charge and 
transferring it to the crankcase. 
The auxiliary chamber of GB 2 083 550 follows a complex sequence of 
connections and disconnections controlled by the side of the piston but at 
the bottom dead centre piston position the auxiliary chamber is not 
compressed as in the case of the present invention and instead gases are 
discharge into the working chamber at that position.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A cylinder 10 has a piston 12 reciprocable within it to define a variable 
volume working chamber or combustion chamber 14 of a four-stroke internal 
combustion engine. The piston 12 is provided in the usual manner with 
piston rings 20 which seal off the small gap or crevice between the piston 
12 and the cylinder 10. In the drawing, the intake and exhaust valves, the 
ports and the spark plug have all been omitted in the interest of clarity, 
because these components are generally conventional and are not germane to 
the subject matter of the invention. 
An annular auxiliary chamber 16 of small volume surrounds the working 
chamber 14 and is connected to it by passages 18 in the cylinder wall. 
Several such passages 18 are uniformly distributed about the circumference 
of the cylinder and are arranged immediately above the top of the piston 
12 when the latter is at bottom dead centre, as shown in the drawing. The 
auxiliary chamber 16 is thus communicating with the working chamber 14 
when the piston 12 is near bottom dead centre and communicates with the 
crankcase when the piston is near top dead centre. 
The auxiliary chamber 16 is intended to store the end gases which in a 
conventional engine would have left the crevice volume at the start of 
exhaust stroke and remained near the top of the piston 12. At the end of 
the power stroke and start of the exhaust stroke, the auxiliary chamber 16 
is at crankcase pressure, which is significantly lower than the pressure 
in the working chamber 14 and the gases near the top of the piston 12 will 
be forced into the auxiliary chamber 16 until the pressure in the 
auxiliary chamber 16 matches that in the working chamber 14. Unlike the 
space above the piston 12, however, the auxiliary chamber 16 is isolated 
from the working chamber 14 as soon as the piston 12 moves up from the 
bottom dead centre position and instead of being discharged to atmosphere, 
this trapped volume is released to the crankcase when the piston 12 clears 
the passages 18 leading to the auxiliary chamber 16. From the crankcase, 
the trapped gases are recirculated to the intake system by the usual 
crankcase breather to ensure complete burning of its remaining combustible 
hydrocarbon content in a subsequent engine cycle. 
The mass of end gases transferred to the crankcase in this manner is 
controlled and very small. It is known that a small leakage past the 
piston, or blow-by as it is termed, is beneficial for reducing hydrocarbon 
emissions in the exhaust gases even though it is undesirable for other 
reasons. In effect, the auxiliary chamber of the present invention 
provides a controlled and selective leakage past the piston which succeeds 
in reducing hydrocarbon emissions but without reducing combustion pressure 
and engine efficiency. 
Because of the small volume of the auxiliary chamber, crankcase pressure is 
not seriously affected by the controlled leakage. However, though the 
total mass of the gases is small, it does contain the high hydrocarbon 
fraction and thereby permits the hydrocarbon emissions discharged to 
atmosphere to be reduced significantly. 
The action of the auxiliary chamber at the bottom of the induction stroke 
is also beneficial in reducing the hydrocarbon content of the exhaust 
gases. At the end of an engine cycle, the auxiliary chamber 16 contains 
mainly burnt gases at the same pressure as the crankcase, that is slightly 
above atmospheric pressure. The pressure in the working chamber 14 at the 
end of the induction stroke will be slightly below atmospheric and at the 
end of the induction stroke the gases will be drawn in from the auxiliary 
chamber into the working chamber. 
Unlike the remainder of the intake charge, which is a fuel and air mixture, 
the gases now resting near the top of the piston have a relatively low 
hydrocarbon concentration. As these are the gases which will be forced 
into the crevice volume surrounding the piston top land in the compression 
and power strokes, they will tend to reduce the hydrocarbon content of the 
gases stored in the crevice. 
In this respect, it is desirable to ensure that the gases entering from the 
auxiliary chamber 16 into the working chamber 14 remain near the top of 
the piston during the compression stroke and this can be assisted by 
inducing swirl about a vertical axis, this being achieved by directing the 
passages 18 tangentially. 
It is essential that the chamber 16 be sealed in the direction of 
preventing escape of gases from the auxiliary chamber 16 directly to the 
atmosphere but it would assist the process of placing a hydrocarbon free 
layer of gases over the top of the piston if the volume of the 
hydrocarbon-free gases could exceed the volume of the auxiliary chamber 
16. To that end, in the illustrated embodiment, a one-way valve 22 is 
provided which prevents gases from leaving the auxiliary chamber 16 but 
allows additional ambient air or slightly compressed air to be drawn into 
the auxiliary chamber 16 and from the auxiliary chamber 16 to the working 
chamber 14 at the bottom dead centre at the end of the induction stroke. 
If a significant amount of air is drawn into the working chamber by this 
technique and the hydrocarbon free air is successfully confined to the 
bottom half of the working chamber then a stratified charge engine is 
achieved which permits the engine to operate at low load without external 
throttling, thereby reducing pumping losses.