Page:Motors and motor-driving (1902).djvu/168

136 require frequent attention, and paraffin should be squirted down the compression tap at regular intervals. Occasionally, too, the cylinder head should be taken off, and the rings cleaned with a tooth-brush and paraffin. In double-cylinder engines, this constant attention is not required, for in addition to the splash system of lubrication, there are pipes running to the sides of the cylinders, through which oil drops constantly, and so keeps them lubricated. The speed of the engine, too. being so much less, there is not such danger of the oil being used up rapidly. Therefore it is as a rule sufficient to squirt paraffin every few days on to the top of the piston. Failing paraffin petrol is almost as effective.

It was the general practice until a short time ago, a practice which is still in some cases continued, to place the exhaust valve directly under the induction valve. By this arrangement a simpler casting for the cylinder head is possible, as it only necessitates one opening for both valves. This system has now been improved upon by leaving the induction valves in their former position and making a second opening in the head for the exhaust valves. In the Daimler type of engine this alteration had a distinct advantage, for it enabled the exhaust gases to be got rid of at once (instead of, as previously, having to travel from one side to the other of the cylinder head), thus keeping the head much cooler and the induction valves free from the sudden and wide variations of temperature through the exhaust gases passing under them. This second opening in the head is also utilised to take the sparking plugs when electric ignition is used, as in this position they are constantly subjected to the scouring action of the exhaust gases which help to keep them clean and prevent any accumulation of carbonised oil to act as a short circuit.