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Figure above shows cylinder pressure versus crank angle traces of cycles of different operating conditions. As suggested the above figure indicated four consecutive plots of cylinder pressure against crank-angle, ignition timing is three degrees apart between each trace. Knock primarily occurs under wide-open-throttle (WOT) operating conditions. It is thus a direct constraint on engine performance. It also constraint engine efficiency, since by effectively limiting the temperature and pressure of the end-gas, it limits the engine compression ratio or boost pressure on a forced induction engine. Apart from using higher octane fuel or additives such as alcohols, lead alkyls. Another factor affecting the engine's tendency to knock lies upon the ability of the engine designer to achieve the normal combustion behavior while holding the engine's propensity to knock at a minimum. As seen on the figure, the green trace represents the normal combustion cycle. If the ignition is advanced by some three degrees, a slight knock occurs on the yellow trace, indicates the engine has reached its limit. Further timing increase produced servere knock (red trace), this level of knock is audiable to the human ears, it should be prevented from happening for prolonged periods, as permanent engine damage can occur shortly afterwards. The blue trace is placed approximately between the red trace and the yellow trace, ignition timing is around 6 degrees advanced of the normal green trace, but this time water is being injected into the engine. Before TDC, the blue trace followed a predictable path except soon after the TDC, the pressure begins to flatten. As the water within the combustion chamber started to evaporate and absorbed a large amount of the heat and prevented the pressure and temperature reaching the point of detonation. The trace now follows the green trace until the next cycle. Note the area under the blue trace is much larger than the green trace indicating torque increase. More gains are to be expected on the turbocharged engine. |
| We are preparing the next part of this page to calculate mathematically how effective is water for charge-air cooling. |
