Fig. 6.—Otto Cycle.
The Exhaust Stroke.
(1) On the first downstroke made by the piston a suction effect or partial vacuum is produced in the cylinder; the air and petrol vapour in the induction pipe being at atmospheric pressure, which is in excess of that now existing in the cylinder, flow into the cylinder as soon as the inlet valve I is opened by the engine mechanism. At the end of this, the suction stroke, the inlet valve closes and traps the charge of explosive mixture in the engine cylinder. This is shown in Fig. [3].
(2) On the first upstroke made by the piston the charge of explosive mixture is compressed ready for firing. Both valves are shut. This is shown in Fig. [4].
(3) On the second downstroke made by the piston the sparking plug S passes a spark which explodes the charge at the very commencement of the downward movement of the piston. The force of the explosion drives the piston downwards, doing useful work. Both valves are shut. This is the power stroke, and sufficient power must be developed on this stroke not only to do the work required from the engine but also to tide it over the other three idle strokes. On this stroke the piston drives the crank by means of the connecting rod, but on the other three strokes of the cycle the crank has to drive the piston by means of the power or energy stored in the engine flywheel on the power stroke. Towards the end of the power stroke (or explosion stroke) the engine mechanism opens the exhaust valve E and allows part of the burnt gases to escape to the silencer along the exhaust pipe. This is shown in Fig. [5].
(4) On the second upstroke of the cycle the piston pushes the remaining burnt gases out of the cylinder through the exhaust valve. When the piston reaches the top of its stroke the exhaust valve closes. This is shown in Fig. 6. The cycle of operations then begins again, giving one power stroke and three idle strokes each time as already described.
[CHAPTER II]
DESCRIPTION OF A TYPICAL PETROL ENGINE
For the purpose of explaining the cycle of operations we have considered only a diagrammatic sketch of an imaginary motor-car engine, but in Fig. [7] we illustrate an up-to-date motor-car engine. In the first place we note the position and arrangement of the four water-cooled cylinders, A1, A2, A3, A4, containing their pistons and mushroom type valves. These are conveniently placed in a vertical position and mounted on top of the crankchamber C, to the bottom of which is attached the oil-base B. At the front of the engine are shown the timing wheels in their casing E, and at the rear end the flywheel F. The starting-handle connexion is at S, the fan pulley being shown at M. The high tension magneto which supplies the current to the sparking plugs is shown at H, and I is the induction pipe connected to the carburettor K. The water circulating pump is on the off side of the engine and does not appear in the illustration, but L1 is the inlet water pipe leading from the radiator (not shown) to the water pump, and L2 is the delivery pipe from the pump to the respective cylinder jackets, L3 being the outlet water pipe. The exhaust pipe is shown at D, and the oil pump at P. The valve springs, valve tappets and guides can also be clearly seen. In examining the several parts of the engine in detail we must not lose sight of their respective positions in the general arrangement view of Fig. [7].