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OVERVIEW The Bourke engine has two opposed cylinders with the Piston s in a Scotch Yoke mechanism. Because the motion of the pistons is a perfect sine wave with regards to time vs displacement the fuel burns in a smaller volume. The use of the Scotch Yoke reduces vibration from the motions of the connection rod — for example calculations show, the peak acceleration in a Scotch yoke is 25% less than the acceleration in a conventional crank and slider arrangement, with a conrod to stroke ratio of 1.57, which is about the minimum seen in practice. The intake valves are replaced by ports and so reducing the total number of parts. DESIGN FEATURES The following design features have been identified
CLAIMED AND MEASURED PERFORMANCE
It should be noted that the high power to weight and efficiency claims have not been demonstrated on a running engine in an independent test. Such figures are regularly claimed for experimental engines, by the time they reach the outside world the performance falls back into line with engineering expectations, as demonstrated in the witnessed test. Any emissions generated cannot be cleaned up with a conventional Catalytic Converter , due to the low exhaust temperature, of less than 100°C (212° F). A conventional engine runs its catalytic converter at approximately 700-900°C. VIDEOS (large files, will take time to download on low speed connections)
OPERATION (0)Starting from BDC the intake port is covered. As the piston travels toward TDC energy is used to create a partial vacuum in the compression (lower) chamber. As the piston approaches TDC the intake port is opened and air is drawn into the compression chamber from the intake duct. (1) TDC, with a full charge of air in the compression chamber, the cool air is warmed by the cylinder walls and piston. (2) The piston moves down, so the skirt closes the intake in the beginning of the down stroke. The air is then compressed by the piston, its temperature and pressure rising roughly in an adiabatic compression. In the early stages of compression it absorbs heat from the cylinder walls. In the later stages of compression it warms the cylinder, resulting in a loss of internal energy (This is inevitable according to second law of thermal dynamics regarding energy transfers). Some of this heat is also lost to the cooling system.. (3) Approaching BDC the piston uncovers the transfer port and opens the exhaust port of the combustion chamber. Energy stored in the compressed air in the compression chamber is used to help blow the exhaust out of the exhaust port. As it does so the compressed air expands and cools some and fuel is injected and mixed with the incoming charge. If the scavenge ratio exceeds 40% some fresh mixture is discharged unburnt out of the exhaust port. (4) At BDC the residual exhaust in the chamber and the walls of the chamber heat the incoming mixture. (5) As the piston moves up the piston ring closes the transfer port in the combustion chamber and the exhaust port. As the piston moves up the bore it re-compresses the mixture causing it to heat up and transfer heat back into the walls. As in (2) the heat transfer to and from the mixture increases the internal energy loss from the mixture. Also as in (2) some of this heat lost to the walls is lost to the cooling system. Since the Bourke engine has extended dwell time near TDC the air charge is held in a compressed heated state, exacerbating the heat loss to the walls. (6) TDC - the mixture is fully compressed, and is now ignited, either by self ignition or by the spark plug. (7) The rising pressure due to the combustion forces the piston back down the bore. Since the burning/burnt mixture is hot it heats the cylinder walls. The extended dwell time around TDC ensures almost complete combustion of all the fuel. However as in (5), the extended dwell time also increases the amount of heat transferred to the walls which is later lost to the cooling system. (8) The piston ring uncovers the transfer port and the exhaust port (as in 3), and the exhaust flows out of the exhaust port, pushed out partly by the incoming charge. ENGINEERING CRITIQUE OF THE BOURKE ENGINE The Bourke Engine has some interesting features, but the extravagant claims for its performance are unlikely to be borne out by real tests. Many of the claims are contradictory. 1) Seal friction from the seal between the air compressor chamber and the crankcase, against the conrod, will reduce the efficiency. 2) Pumping losses, the air charge is compressed and expanded twice but energy is only extracted for power in one of the expansions. 3) Engine weight is likely to be high as it will have to be very strongly built to cope with the high peak pressures seen as a result of the rapid high temperature combustion, and the scotch yoke/triple slipper bearing are heavier than a conventional crankshaft. 4) Each piston pair is highly imbalanced. This will limit the speed range and hence the power of the engine, and increase its weight due to the strong construction necessary to react the high forces in the components. (videos of running engines do not appear to show imbalance) 5) High speed two-stroke engines tend to be inefficient compared with four-strokes because some of the intake charge escapes unburnt with the exhaust. 6) When the charge is transferred from the compressor chamber to the combustion chamber it will cool down, reducing the efficiency of the engine. 7) Use of excess air will reduce the torque available for a given engine size. 8) Forcing the exhaust out through small ports will incur a further efficiency loss. 9) Operating an internal combustion engine in detonation reduces efficiency due to heat lost from the combustion gases being scrubbed against the combustion chamber walls by the shock waves. 10) Emissions - although some tests have shown low emissions in some circumstances, these were not necessarily at full power. As the scavenge ratio (ie engine torque) is increased more HC and CO will be emitted. See HCCI engine SEE ALSO EXTERNAL LINKS
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