Friday, November 14, 2014

Diesel EGR - Diesel Exhaust Gas Recirculation Systems


Exhaust Gas Recirculation is an emissions control system and or device used to reduce Nitrogen Oxide (NOx) emissions.
Nitrogen Oxide emissions are among the most dangerous and difficult to deal with emissions from internal combustion engines (and other sources).
NOx is created when nitrogen and oxygen (air) react at high temperatures (combustion) and pressures. NOx released through the exhaust into the atmosphere then reacts with Volatile Organic Compounds (VOC’s) and sunlight to form Photochemical Smog, which negatively affects all living things, in particular small children and the elderly with Asthma. This form of smog is very long lived in the atmosphere, breaking down very slowly.


The EGR system takes a portion of an engine’s exhaust gases, which after combustion have very little oxygen and nitrogen left in them and runs them through a cooler or coolers (using engine coolant or other closed loop cooling system fluid) which lowers their temperature and then using a valve, meters that cooled gas back into the air intake system.
This cooled exhaust gas displaces some of the oxygen and nitrogen from the incoming air, which in turn reduces combustion temperature, lowering the amount of NOx the engine produces.
In diesel engines, the amount of EGR (as controlled by the EGR Valve) can vary from near 0% to as high as 50% depending on engine speed, temperature, and load. Engine and emissions computer manage EGR levels.
In North America OEM’s initially tried, to use EGR as the primary or in some cases their only method of controlling NOx.
While EGR offers the positive effect of reducing NOx, trying to use very high levels necessary to lower NOx to the levels demanded by EPA regulations produces several serious negative effects; such as a significant reduction in power output, much lower fuel economy, an increase particulate production, and a number of other maintenance and operational issues.
The effect of taking volatile exhaust gases, which can contain over 1000 chemical compounds and rapidly cooling them, forces creation of condensate, which forms deposits, that are highly acidic, and can wreak havoc with sensors and system components. A soft, light, powdery carbon often plugs coolers, while a rock hard carbon forms in the valve and passage areas.
The high level of particulate production forces the Catalytic Diesel Particulate Filter (DPF) to work harder and regenerate far more often using even more fuel.
The first sign of trouble is often a check engine light showing reduced EGR flow. There are usually accompanying problems such as poor performance, reduced fuel economy, rough running, and several other possibilities.
When under warranty, OEM shops typically first try to replace parts, such as the EGR Valve and or EGR Cooler(s).
While the valve or cooler may well be plugged, and replacing them may temporarily clear the code. Doing this without cleaning all the passages, tubing, sensors, the exhaust side of the Variable Geometry Turbocharger (VGT) found in the entire EGR system is usually a wasted effort.
This type of turbocharger is particularly susceptible to problems with engines producing high levels of particulates (black soot). This soot will attach to the vanes and prevent their proper movement, reducing or eliminating the advantages provided by VGT’s.
Cleaning the valve, the cooler(s) all of the passages on both the exhaust and intake sides as well as the exhaust side of a VGT is vital to restoring full power, fuel economy, and drivability to the engine.
Cleaning this system properly also reduces the load on the DPF, further improving mileage and power output.
With the advent of Selective Catalytic Reduction (SCR) using Diesel Emission Fluid (DEF) (Urea) to form Ammonia, which then reacts with a Zeolite or Precious Metal Catalyst to convert the NOx into Water Vapor, and CO2 it becomes possible to reduce the amount or percentage of EGR.
Using SCR and EGR together produces an engine with very good operational characteristics, acceptable fuel economy, reasonable emissions, and fair maintenance intervals and costs.
That said; all EGR systems have maintenance issues. These issues are made better or worse depending on how the vehicle is used.
Diesel engines are at their best running at a steady state, for example running down the interstate at highway speeds under a load for long periods of time. When operated in this fashion, the EGR, SCR, and DPF systems are most efficient, and will operate with minimal maintenance.
However, when a diesel is operated under low load, stop and go, short runs, or is allowed to idle for extended periods, those same systems struggle to operate correctly. Cold Temperatures, poor quality fuels, and poor maintenance practices exacerbate these problems. Under these conditions, where components are not up to operating temperatures or where they are not run long enough to complete their operational cycles, they can very quickly (and expensively) fail.
There are a number of things that can be done to improve operational characteristics and reduce maintenance. High quality fuel is the most important. Your engine wants and needs clean, water free, contaminant free fuel. Your owner’s manual tells you that you need fuel with a Cetane rating of 45 to 50, yet most fuel sold in North America is only rated at 40. Raising Cetane above 45 will have a huge positive effect as will adding detergents that can remove both internal and external injector deposits. Removing water is vital for pump and injector longevity. Adding a synthetic Lubricity agent as well as thermal and oxidative stability agents. Improving fuel quality is an investment costing only a few cents per gallon that will provide benefits many times its minor cost. In what is both a challenge and an opportunity for dealers and repair facilities, customers need to be educated that adding certain types of high quality supplements to their fuel on a regular and on-going basis, can have a major impact on improving operability and reducing repair costs and down-time.
There are several high quality fuel additive or supplement products available today that can dramatically reduce EGR and DPF problems.
Also, at least three companies are offering professional technician service tools  and chemicals that quickly, safely, and very effectively clean the entire EGR system.
Setting a preventive maintenance program that includes an EGR service and using properly formulated fuel supplements can and will prevent many if not most problems with these systems.
Diesel EGR service together with DPF service are the two biggest problems facing owners of diesel powered vehicles. Repairs made under warranty, especially those repairs made multiple times, still have a negative effect on customer satisfaction. Vehicles “out-of-service” cost money far beyond the actual repair cost.
Diesel will be about 3% of Automobile and Light Truck sales this year (2014) and is expected to reach about 10% by 2017.
In Europe well over 50% of the new passenger, car and light trucks sold are diesel powered, with some countries being over 80%. In many cases, diesels perform as well as hybrids for less money. While here in North America, we are considerably behind in the move toward diesel, there is little doubt that this is the direction in which we are headed.

 

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