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Automotive Removal of EGR system: performance gain or loss?

  1. Jan 8, 2013 #1
    Hi, I'm trying to put a long open-ended debate to rest.

    For those who don't know, the EGR, Exhaust Gas Recirculation, system takes some of the exhaust gas and reintroduces it into the intake manifold. From what I understand, at 1600°C NOx is formed because air molecules, nitrogen N2 and Oxygen O2, react. By allowing cooled exhaust back into the intake, the combustion temperature of the engine becomes lower than the 1600°C. I should note that at idle and full throttle, the EGR valve is closed, and does not allow any exhaust into the intake.

    The debate is whether or not removing the entire EGR system, and getting a tune to take it out of the ECU will allow for performance gains. Some say there won't be any performance gains and will hurt MPGs, and some say it would result in performance gains. I would think there would be performance gains. Each cylinder can only has a certain amount of displacement. I will be using Ford's 4.6 V8 motor. Each cylinder has .575 liters of space. If the EGR is not functioning then all .575 liters of space will be filled with .575 liter of the air and fuel mixture. If the EGR is functioning then all of the .575 liter of space will not be filled with the maximum amount of the air and fuel mixture as possible. There will still be a small amount of space occupied by the spent exhaust gases in the engine. Because of this, I would think removing the EGR system would increase performance. I have heard that because of the higher cylinder temperature, there will be pre-detonation. I don't understand why this is because at full throttle, when the EGR system is off, the cylinder temperatures would be at a high temperature than when at slight throttle with the EGR system on.

    I would greatly appreciate anyone's input.

    http://www.hitachi-c-m.com/asia/images/products/excavator/wheel/zx170w-3/KS_EN040_03.jpg [Broken]
    Last edited by a moderator: May 6, 2017
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  3. Jan 8, 2013 #2


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    Higher cylinder temperature can lead to pre-det. The fuel air mixture can ignite before the piston causes max compression and the plug sparks if it is too hot, much like a diesel engine runs w/o spark plugs.

    If you are going to disconnect the EGR, make sure you don't also disconnect the knock sensors.
  4. Jan 8, 2013 #3
    Yes I understand that, but why would an engine under 35% throttle without EGR, running hotter than an engine with EGR, be hotter than an engine under 100% throttle, which wouldn't have EGR on regardless if you remove it or not?
  5. Jan 9, 2013 #4


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    What effect does disconnecting the EGR have on the ECM (or PCM) when it is calculating the air-fuel ratio for the engine? If the ECM leans out the mixture, that could cause the engine to run hotter.
  6. Jan 9, 2013 #5
    EGR delete without a remap of the ECU will give no performance benefit but will increase emissions. But it does simplify things greatly.

    A remap is unlikely to give collossal gains in power, but there will be some.

    Either way it's not an engine killer.
  7. Jan 9, 2013 #6
    Here is an example of why EGR at part throttle should make and engine more efficient. The below image is a curve that shows the thermal efficiency for an Otto cycle engine.


    Say I have a 10:1 compression engine. At part throttle the engine only ingests 20%. This would make an efficiency of 17%. Now open the EGR and say that it can also flow an additional 20% inert air into the cylinder. This brings the compression ratio to 40% and the thermal efficiency bumps up to 37%.

    These are approximate numbers but hopefully one can see that ANY increase in compression ratio will increase thermal efficiency.
  8. Jan 10, 2013 #7


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    A naturally aspirated engine is going to take only so much air into the cylinder. What you are talking about (I think) could only be accomplished by supercharging (forcing more air into the cylinder).

    I do not follow 2milehi's explanation about increasing compression ratio by having the EGR disconnected. In any event, if such an increase in CR were to occur with the engine having a nominal 10:1 CR, detonation would be a serious concern.

    If you look at the same engine design and compare the CR for the naturally aspirated version versus a version which is supercharged or turbocharged, generally the boosted engine has a lower nominal CR to help prevent detonation.
  9. Jan 10, 2013 #8
    In the OPs example, if his 575cc cylinder contained 575cc of charge, he would be at WOT and the EGR would not be functioning. At partial throttle where it is working you'd maybe have 345cc of charge (60% VE) and 115cc of EGR (20%). I think the idea is that you are forced to use more throttle opening which reduces pumping losses, and without using more fuel to keep the AFR at stoich, it raises efficiency.

    The compression ratio/efficiency is I think irrelevant in this scenario, the EGR does not change the CR it and the relation to efficiency is more to do with the expansion ratio, and how much work can be extracted from the expanding gases. Expand a pressurised container to twice it's size, and you'll get less work extracted than expanding it to ten times it's size.
  10. Jan 10, 2013 #9
    At part throttle, there is a vacuum in the intake manifold. Also the exhaust gas is at positve pressure. So when the EGR valve opens, exhaust gas is easily drawn into the intake manifold.
  11. Jan 21, 2013 #10
    Running EGR allows you to run more timing at light cruise loading.
  12. Jan 22, 2013 #11

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    Whoa up here boys...having been told on this forum I am so old I was around when Nikolaus Otto ( aka Quick Nick) invented the internal combustion engine, I was also around when we started hanging smog pumps on the big cube engines in the early 1970s. We need to review what makes the IC engine run. Gasoline is mixed with air and sucked into the combustion chamber via intake manifold, valve opening events and is compressed. The fuel/air mix is spark ignited and rapidly burns/ expands thus driving the piston down ( mechanical action I will not review in detail..another discussion later maybe), The spent mix is pushed out of the cylinder to the exhaust manifold/ exhaust pipe etc...
    We all can agree on this action. When you introduce outside air into the system you make the engine a lean burn system and you have to be very aware of detonation when you lean down the mix. When you add more air (O2) then fuel you lean the mix. This will give you more MPG , all other things being equal. You will impact horsepower as well since the amount of fuel is diluted for every amount of intake charge used by the IC engine. More fuel means more HP....More air means more MPG.

    When we add the EGR we do the above but add in spent fuel and all the evil bad pollutants to the intake mix. We are running the IC engine on dirty air when at speed. If we had not compensated for spark event the with a tightly controlled ignition we would have detonation and melted pistons, bent con rods..etc...
    So what happens when we remove the EGR? My guess is MPG drops and we may pick up a slight performance gain. Why slight?
    In the old days when we ran carburetors simply bolting on a bigger carb got you more headaches than you needed. The intake , cam shaft and exhaust were all tuned to handle the best fuel/air ration for a given operating band ( sweet spot in the RPM range). Adding a bigger carb met you may pick up some high RPM power but idle and low to mid RPM range operation would be too rich and you would have fuel puddeling . sooty spark plug fouling, carbon build up, wasted gasoline ( all other things being equal).
    You see todays engines are designed to handle that very lean dirty fuel air mix. Intake manifolds are designed to keep fuel suspended in the dirty air when turning corners on the way to the combustion chamber. The combustion chamber is designed to swirl the dirty leaned out mix for maximum packing of the cylinder. The exhaust is designed to handle so much spent mixture and don’t forget a portion of the is mix is being recycled and NOT going out the pipe. You will need to increase the exhaust header size or you will have excessive back pressure.

    Summary- Removing the EGR ( which I hate ) will cause minimal performance because you will be adding more clean air to the fuel/air mix that will not stay suspended and which will cling to the intake manifold walls, puddle on the combustion chamber walls due to being too heavy to swirl, and cause over rich combustion. This over rich mix once lit, will have to exit thru a high back pressure ( under sized ) exhaust system. And we have not even talked about cam shaft design, valve events, ignition timing etc...
    Last edited: Jan 22, 2013
  13. Feb 12, 2013 #12
    I stumbled across this post through a random search and had to sign up to respond!

    Exhaust gasses act inertly. This cools the burn by SLOWING the oxidation process. At lower throttle angles, this can be tuned for with ignition timing advance. Minor issue.

    Exhaust gasses may have portions of CO and HC pollutants, but primarily constitute 13.6% CO2, 18.2% H2O, and approximately 68% N2. The ambient air typically contains 78% nitrogen. Water vapor expands at approximately 12X per BTU input over nitrogen, and carbon dioxide has an even greater expansion ratio. One of the benefits of EGR (when properly done) is more bang for the BTU.

    Remember, oxidizing fuel only generates HEAT. This heat must act upon the gasses in the cylinder by expanding them generating PRESSURE. It is the pressure that forces downward on the piston. If the same amount of heat can generate more pressure, you have more power. EGR has been shown to slightly improve MPG. Too much EGR and the flame propagation characteristics suffer past any possible gains from the "souped-up" expansion medium.

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