I dont think the wikipedia article explains it that well as its missing some of the fundamentals.......so, here goes...
NOx is a bit of a nasty, particularly in cities where the levels can get pretty high so its good to lower it.
You need to consider the fact that when burning any fuel you use the fuel and oxygen to produce the energy but, there is more than just oxygen in air (as we all know).....one of the other elements is Nitrogen and its this that causes the issue.
Under high temperatures and pressure, the Nitrogen reacts with oxygen in the air to produce NOx and a combustion engien has both high temp and high pressure!
So, what can we do to reduce it.
One option is to reduce the pressure by reducing the compression ratio but, this goes against the principle that for most efficient combustion and max power output we want the highest compression ratio possible. This technique (and the use of a more advanced spark and injection control system) was used on the 2.6 and 3.2 engines on the Omega which have a lower compression ratio than the 2.5 and 3.0.....spot the EGR valve on them.....
The other option is to reduce the combustion temperature.....and this is where the EGR system comes in.
Remember that if we want to add anything to the fuel air mix that wont affect the engines running it needs to be inert (i.e. it wont interfere) with respect to the engines operation.....so it must not have any oxygen in it or any fuel.......
Now we know that if the engine is running as it should be then there should be no oxygen or fuel in the exhaust gases (a reasonable assumption as its controlled by the lambda sensors in the exhausts) and hence we can consider the exhaust gases to be inert in this case.
So.....what next.....well
If we take some inert exhaust gases and re-inject them into the inlet we are effetively filling each cylinder on each cycle with something it cant burn which in turn lowers the percentage of fuel air mix in the cylinder.......this gives what is effectively a reduction in the cc of the engine (not an exact way to view it by any means but simple enough to understand) but, more importantly, means that less heat is produced as less fuel air mix is burnt and hence less NOx.
So, to complete the understanding, lets consider some of the throttle position scenarios
Idle - Closed throttle and very little fuel air mix getting in, the engine is barely running and just about has enough energy to overcome the internal losses and drive the ancillaries. If we open the EGR at this point the idle valve would need to open much further to stop the engine stalling and the engine would run rough......and this rough running can be a classic symptom of a stuck EGR valve. To test this the best approach is to use a diag tool to actuate the EGR whilst the engine is idling and listen for the poor running (it IS noticeable), some may have seen me do this using tech2 in the lakes.
Full throttle - WOT (Wide Open Throttle) This is where we want max go, we dont want our fuel air mix contaminated with an inert gas we want max grunt and hence the valve should be closed here also.....this is about the only time you will see a benefit from blanking off a working and not leaking EGR valve....when booting it the throttle response can be slightly improved.
All other throttle positions - the ECU will vary the amount of exhaust gas inserted by modulating the valve and this is likely to be at a maximum when cruising.
Do be aware though that on the Omega power plants the level of EGR is not huge unlike later Vx engines (and most other manufacturers) where blanking off an EGR valve can actualy be detected by the MAF and that the above applies to petrol engines, diesels run EGR very differently!
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Topic: How to clean the EGR on 2.2i (Read 3492 times)