44 . SOME UNUSUAL ENGINES the inlet or scavenging ports open in the cylinder wall, the gas expansion has already gone so far that a depression of 0035 to 0-05 atmospheres below ambient is present in the cylinder. As a result, air from the atmosphere begins to ow into the cylinder through the bellshaped inlet chest surrounding it; and the air stream is directed by the ports towards the piston crown, where violent swirl is promoted. Just before the piston completes its downwards stroke to bottom dead centre, the exhaust valve opens and exhaust gases are led into the exhaust pipe. Since however there is still a slight vacuum in the cylinder. at higher vacuum must be produced in the exhaust pipe , and this is accomplished by connecting two cylinders (with explosions displaced by 180 of crank angle) into a single branch of the exhaust pipe, During the upward travel of the piston, airow from the atmosphere to the cylinder continues, and the cylinder is scoured by fresh air until the closure of the piston-controlled ports at about 55" past bottom dead centre. In the nal exhaust stage, until the closure of the exhaust valve 115 past bottom dead centre, exhaust gases in the pipe must have a velocity sufcient to develop a depression to scavenge the second cylinder whose exhaust valve will open 50 alter the closure of the valves of the rst cylinder. Thus it may be seen that only a small proportion of the cylinder volume is used for combustion, and this is determined (as is the compression ratio, which is no different from what normal engines feature) by the timing of the exhaust valve closure. Until this occurs the engine is being cooled internally by the large quantity of air ingested and passing through it, Due to the high expansion ratio, the mean temperature of the cycle is low and the cylinder wall is exposed to high gas temperatures only for one-third of the time approp- riate to a normal cycle Delayed closure of the exhaust poppet valve is the accepted way of con- triving a more~complete-expansion cycle in a two-stroke engine. In a four- stroke the early closing of the inlet valve achieves the same effect by giving an elTective compression ratio lower than the expansion ratio. However. most engines working on this cycle have been conceived as compression ignition types, for in practice maximum cylinder pressure is limited by con- siderations of stress, whereas in spark~ignition engines it is limited by detona~ tion. The advantage of the cycle is the possibility of an efciency higher than could be obtained with an expansion ratio equal to the compression ratio but the disadvantage is considerable, amounting to a mean e'ective pressure lower than the conventional arrangement with the same maximum pressure. In the Simplic it is only about 40 or 50 lb/inz, though the lightness and com- pactness of the engine makes it feasible that an aluminium V4 weighing 220 lb might produce 40 bhp fairly reliably. It must be stressed, however.