Figure 326 shows an indicator card where the water injection was cut out. This reveals a very marked preignition considerably before dead-center. Figure 327 is a card from an engine using water injection. Even at full load there was no premature combustion; in fact, the explosion occurred slightly after dead center was reached.
Another use of water injection is in the control of the varying ignition temperature of different oils. Oils show marked differ ences in the temperature at which auto-ignition occurs. If the compression pressure is one suitable for a heavy oil, a change • to a lighter gravity oil necessitates a lowering of the compression temperature. If the change be a permanent one, the clearance volume of the engine could be altered, but the most successful method is the use of water injection to lower the temperature of the cylinder,during the compression stroke.
A few low-pressure oil engines do not use water injection, thus proving that it is not absolutely necessary for the prevention of premature explosions. If water is not used, it follows that the oil must not be injected early in the compression stroke; 35 degrees ahead of dead-center is as early as is feasible, and even at this late injection point the stratification in the cylinder must be good or preignitions will occur on full load. From considerable experience with these, so-called, "dry" engines it can be stated that they will display a tendency to preignite if operated at full load for a number of hours and, if operated at a low load factor, will likely possess the fault of missing explosions.
Of late years, due to certain objectionable features of water injection and a hesitancy of acknowledging that it is used to correct defects of design, other claims of the virtues of water injection have been set up. Among the uses of water injection, it is now considered to assist in better combustion and to keep down carbon deposits.
It is practically impossible to establish exactly what occurs in the engine cylinder. All that it is possible to prove is that, under conditions such as are presumed to exist in the engine cylinder, certain events will take place, and this basis deduce that these same events do occur in the engine.
Crude oil or petroleum is made up of an extremely complex mixture of carbon and hydrogen, known as hydrocarbon. The chemical composition of the crude oil in each field shows different characteristics. In fact, even in a single field the oil from
different wells may belong to an entirely different hydrocarbon series. There are five of these hydrocarbon series that arc usually met with. They are = Paraffine Series CnH2. = Ethylene Series C,1112.-2 = Napthalene Series CnH2.-4 = Terpene Series C„H2,1-6 = Benzene Series although hydrocarbons of a series as low as CJI2n-24 have been encountered. These hydrocarbons have a characteristic that has given considerable trouble to the engine builder. They are subject to what is known as "cracking," whereby at a giver temperature the hydrocarbon will break up into a more simple compound. As example, at a certain temperature the paraffin( series C151142 breaks up as follows: + Heat = + C.
This "cracking" temperature is considerably lower than the temperture of auto-ignition.
It was found that, when such an oil was "cracked" and they burned in the presence of water vapor, the free carbon C disap peared, evidently consumed in connection with the water vapor The explanation of this phenomenon is as follows: The fre? carbon atom will not unite with the oxygen of the air except at t temperature much higher than that existing in the engine. Om the other hand, the cylinder temperature, during combustion is sufficient to disassociate the hydrogen and oxygen of the water This nascent oxygen will unite with the carbon at the cylinde temperature. We have then the following reactions: (1) + Heat = (2) C21116 + = 7002 + 8H2.
(3) + Heat = 2H2 + 02.
(4) C 02 + Heat = CO2.
(5) + = 21120.
Reaction No. 1 occurs as soon as the oil is injected and before ignition occurs, and is the " cracking " process. When the cylin der temperature, assisted by the hot bulb, ignites the vaporized oil, reaction No. 2 takes place. The temperature which now exists in the cylinder is around 3000' Fahrenheit, sufficient to sepa rate the water into its hydrogen and oxygen atoms. Conse quently the hydrogen released by No. 2 reaction does not unite with the oxygen of the air. As already stated, this water oxygen will unite with the carbon and reaction No. 4 occurs. As the piston moves outward, the temperature falls below the disassocia tion point of water, and the hydrogen set free by No. 2 reaction unites with the oxygen freed by No. 3 reaction.