This would seem to explain the reason a "dry" engine operates at a higher temperature than does a water injection engine. The reactions absorb considerable heat, which is of course given up by reaction No. 5. However, this last reaction occurs when the piston is fairly well advanced on the power stroke and the cylinder temperature at this point is low enough to receive an increment without overheating the engine.
To offset this claim many "dry" engines are operating at practically full load with no carbon deposits. Even water injection engines usually operate below half load without the use of any water, and no carbon deposits appear. There is no doubt that water injection does prevent preignitions at full load and assists in clearing the cylinder of exhaust gases. This, however, is better obtained by proper design of the air ports.
Cylinder Wear Due to Water has frequently been observed that engines using water injection are subject to serious cylinder wear. This is more prevalent in the Southwest where the oil carries considerable sulphur. This sulphur un doubtedly does unite, to some extent, with the water introduced into the cylinder, forming- sulphurous acid. This acid cuts the cylinder and pistmi walls. From reports of a number of engines using Eastern oils that contain little or no sulphur it appears that cylinder wear occurs without the presence of sulphur. This could be explained on the grounds that the oxygen of the water unites with the iron of the walls, forming ferric oxide. This oxide is very abrasive and cuts the walls, presenting a fresh iron surface to the action of the water.
Faulty Lubrication Caused by Water opponents of the use of water injection claim, with some degree of truth, that it tends to wash the lubrication off the piston and that faulty lubrication is the chief cause of the rapid cylinder wear. From reports obtained it would appear that rapid cylinder wear takes place in those cases where the operator has followed shop instruc tions as to the amount of lubricating oil to use. Since the engine builder usually states the minimum amount advisable to use under the best conditions, it follows that the operator, in limit ing the quantity of oil to this figure, did not use enough. The piston and 'cylinder wear rapidly because the water washes away the oil that is supplied. All reports on cylinder wear make mention of the dry condition of the piston, showing the absence of any lubricant.
The Method of Water has much influence on the degree of cylinder wear. Figure 251 shows a cross-section of an engine' using a simple bleeder valve. As shown, the water Connection enters the cylinder, although the majority of engines using water have the water line leading into the air passage. The water is injected solely by the difference between the pressure existing in the engine cooling jacket and the pressure in the cylinder or air passage. The intention of the designer is to cause the water to be injected at the moment the piston uncovers the exhaust port, the cylinder pressure dropping to zero at this point. If the water entered at this time, the heat in the cylinder would transform it into steam and probably no cylinder cutting would occur. It happens that on the compression stroke the air pressure in the crankcase, or front end of the cylinder, drops below the atmospheric pressure. The pressure difference will cause the water to drip into the cylinder, or air passage, and run down onto the piston. This will evidently wash away ahy oil film. With an automatic pump arrangement the water is injected at a fixed point when the piston has uncovered the exhaust and air ports. The existing cylinder temperature is suffi ciently high to vaporize the water into steam. Under these conditions there should be but little effect shown on the piston lubrication.
Mietz and Weiss Water Injection Mietz and Weiss horizontal oil engines make use of a unique design of water injection. The' cylinder water jacket is provided with a float box A, Fig. 328. The water line to the cylinder jacket is con nected through the float box, the. float of which maintains the water in the jacket at a level about two-thirds fiom the top.
The heat in the cylinder evaporates this water, forming it into steam at atmospheric pressure, or a few pounds gage. This steam is led into the air passage through the steam dome B. When the pressure in the air passage drops below the pressure in the jacket, there is a flow of steam into the air passage and the engine cylinder. At the time the exhaust port opens the cylinder pressure drops to atmospheric, and a supply of steam rushes in with the air charge. This assists in cleansing the cylinder of the exhaust gases and also serves to lower the compresssion temperature, due to the absorption of heat by the steam, which has a greater specific heat capacity than has air.