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Purpose of an EGR System: Lowering Combustion Temperatures

Moses Ludel

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Note...In an older (2016) forum exchange at https://forums.4wdmechanix.com/topic/917-ram-67l-delete-the-def-and-egr/, a late Ram/Cummins 6.7L diesel owner was curious about removing an EGR system.  In that lengthy discussion, I included this reference:  'EGR (exhaust gas re-circulation) is a system designed to lower the upper cylinder temperatures in an internal combustion engine.  Above 2500 degrees F, the engine's combustion process produces nitrogen oxide (NOx).  So, the intent of EGR is to lower combustion temperatures and cool the upper cylinders.  In a gasoline engine without EGR, temperatures can soar above 4800 degrees F, enough heat to melt many metals.  For that reason, I have always cautioned against removing an EGR system or valve from a gasoline engine.  The same logic should apply to a diesel engine.  You're already concerned about engine/coolant temperatures rising under load on grades, eliminating the EGR valve could create more of an issue here"...A forum member questioned my reference to "4800-degree F"  and higher combustion temperatures in non-EGR gasoline engines.  Below, I further clarify combustion temperatures, EGR systems, pollution control, NOx and the effects of combustion temperatures:

The above reference is to combustion temperatures in a gasoline engine.  My reference came from an emissions presentation by an engineer in the early 1980s.  In the discussion, he emphasized that emissions NOx devices, particularly a gasoline engine EGR system, have the role of lowering combustion temperatures below 2500 degrees F to prevent undesirable NOx formation. 

My copy of the California Motor Vehicle Pollution Control Handbook of the early 1980s indicates that the formation of NOx occurs above 2700 degrees F.  The book describes EGR as the means for bringing combustion temperatures below this threshold to eliminate tailpipe NOx, the purpose of exhaust gas recirculation (EGR).  An EGR system dilutes the incoming air/fuel stream with spent fuel/exhaust, which effectively lowers combustion temperatures to prevent the production of NOx.

The lecturer emphasized that without EGR, temperatures can reach as high as 4800 degrees F.  This is combustion temperature (not the cylinder block, cylinder head, valve or cooling system temperature).  The lecturer noted that if the combustion heat is sustained, it could melt a piston.  There are a number of factors that can raise combustion temperatures.  Removing an EGR system is one.

Normal piston operating temperature in a properly tuned gasoline engine is around 300-degrees C or 570 degrees F;  normal gasoline engine exhaust temperature (not the combustion temperature) is around 650-degrees C or 1,200-degrees F with an EGR system, EFI and emissions tuning.  An aluminum alloy racing piston can melt around 995-1200 degrees F, somewhat higher for a production engine piston. 

The piston is not static or sitting under blow torch heat.  Pistons don't melt under normal conditions because the combustion temperature quickly cools as the gases expand and the piston moves downward in the cylinder.  Another cooling effect is the incoming intake stream of air/fuel, which can be close to ambient temperature or even inter-cooled.  The engine's cooling system also draws off heat;  a typical coolant thermostat holds coolant around the 190-205 degree F range, hotter when under load. 

As pollution control techs, we were concerned about the impact of tampering with emissions equipment.  The effect of removing an EGR system is obvious.  This raises combustion and upper cylinder temperatures. That heat can be exaggerated by an engine running low octane unleaded gasoline, a leaner air/fuel mixture, raised compression (from carbon buildup or performance pistons) and too much spark timing advance.  Add this up for high risk of piston meltdown.

If you'd like to read other cake takers' comments specifically related to diesel engine combustion, see the forum member entry (below) at the Turbo Diesel Register forum.  The temperature cited there is 5000 degrees F in a diesel combustion process.  There are instances where sustained high temperatures can melt an alloy piston in any internal combustion engine.  In the exchange below, factors cited include injector fuel flow.  Injection timing and other variables can also weigh into the risk:


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