Diesel Engine A/F Ratio
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Posted by: matin ® 10/21/2006, 02:26:16 Author Profile eMail author Edit

How can excessive air can damage the engine components of the combustion chamber. Both the intake and exhaust valves gets burnt out in the same fashion like someone has bored a hole in them. Secondly onething more same engines crankcase door studs frequently breaks studs are genuine there is no excessive vibration what could be the reason??????????? Engine Wartsila SW 18 V 38 rated at 11.3 MW

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Re: Diesel Engine A/F Ratio
Re: Diesel Engine A/F Ratio -- matin	Post Reply	Top of thread	Forum

Posted by: Kelly Bramble ® 10/21/2006, 09:48:48 Author Profile eMail author Edit

Matin, I think you have already asked this question before, however I was too busy to give you my best answer. Over twenty years ago I worked in an exhaust emissions testing and research laboratory, so my descriptions are stale (memory). First combustion is very complex; when fuel first enters the combustion chamber a process called "cracking" (I think) must first occur for the fuel to be burned. As you know vaporization of the fuel must also occur. Vaporization and cracking the fuel are endothermic processes, meaning they must absorb heat energy too occur. A common experience to best describe this (evaporation) is when you first get out of a swimming pool you are abnormally cold. This is due to the water on your body changing from a liquid to a gas. For water to change from a liquid to a gas at typical ambient temperatures the liquid water has to absorb a little heat energy from your body and the surrounding air. Thus, you feel cold. So, the first thing to happen to your fuel is it must change from a liquid to a gas, second the fuel must be cracked and oxygen must be present before it can burn. Cracking and evaporatization processes both absorb a significant amounts of heat energy to complete. Cracking fuel is a process of breaking down the complex fuel compound into several other compounds and elements. The rate of cracking is dependant on pressure and temperature. Now, as the fuel cracks into combustible compounds, such as CO and HC there is more fuel available to burn and create more heat energy. This is a accelerating and escalating process, however oxygen is required for the final combustion to occur. The more oxygen you have then the more complete the combustion will be and the higher the peak combustion temperature and pressure. At normal A/F there is more cracked fuel available than oxygen, so the continuing endothermic process of vaporization and cracking tends to lower the combustion chamber peak temperature and pressure. Additionally, the less oxygen also prevents the complete burning of the cracked fuel. As strange as this sounds, and maybe opposite of common logic, you will actually have a lower peak combustion temperature with more fuel than less. I have personally observed this in exhaust emissions testing. This is due to depletion of the oxygen during combustion. You have to have oxygen to have combustion of a hydrocarbon. If you can monitor your exhaust emissions, you will observe the following. Lean mixtures burn hotter and are more apt to form oxides of nitrogen. Oxides of nitrogen, also called NOx can only occur at very high temperatures - or lean burn A/F ratios. In a rich burn case, there will be an excess of un-burned fuel, such as CO and H. The peak combustion temperature will also be lower. The issues you are seeing with burned valves and possibly broken studs are due to the materials not being able to handle the continuous higher peak temperature and possibly pressures created by a lean burn environment. Material used in valves just cannot handle the very high temperatures caused by the lean burn environment. I know lean burn technology has been pursued for many years. Search the internet for "lean burn" "Cracking fuel". Combustion engineers, feel free to clarify or correct as required... Modified by Kelly Bramble at Sat, Oct 21, 2006, 11:30:57

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