Bourke engine operation

Bourke Engine Animation

(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.

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