Breathe Easy: Tier IV emissions control made simpleJune 8, 2018
Photos and Charts: John Deere and the author.
In New Jersey where I farm, the television and radio stations broadcast from either New York City or Philadelphia, since we are sandwiched between those metropolitan areas. Many years back a men’s clothier used to run ad spots with the tagline, “The educated consumer is our best customer.” Mr. Syms knew that if the person was knowledgeable about haberdashery, his suits would sell themselves.
In this premiere issue of the Ag PhD Insider magazine and the Iron Talk section, I will exercise poetic license, taking a cue from Mr. Syms to proclaim, “The educated farmer is the most profitable.” Continuous and not random success on the farm is only possible through education in all aspects of your business.
To this cause, it is imperative that you become learned on the basic function of Tier IV emissions control and the necessary service protocols to avoid costly repairs. As a farmer and an engineer, I apply a crop yield metaphor to machinery: How much crop value will an avoidable repair cost me? Tier IV engines can be very efficient and trouble-free once you have a basic understanding of their operation.
Much akin to reading and understanding a soil test, any discussion on engine emissions is based on combustion theory and chemistry. If you remain openminded the technology at the level that we need to understand it to is quite simple, so consider this primer Emissions 101!
Both gasoline and diesel fuel are hydrocarbon-based; they contain hydrogen and carbon. When burned the byproducts of the combustion process are called emissions. When discussing an internal combustion engine, the major emissions of concern are:
• Hydrocarbons (HC): Unburned fuel
• Carbon monoxide (CO): Partially burned fuel
• Oxides of nitrogen (NOx): A pollutant created through pressure, heat and exposure time. Sometimes referred to as nitrogen oxides.
• Particulate matter (PM): Soot found in the exhaust
A gasoline engine by nature of the fuel and its combustion process, predominantly creates CO and a lesser amount of the other three. In contrast diesel exhaust is very biased toward NOx and PM. Thus, the Tier IV program is mainly focused on NOx and PM with less concern for CO and HC.
PM is the visible black smoke we see from a diesel exhaust while NOx is invisible.
The slow flame speed and high cylinder pressure in a diesel is responsible for the elevated rate of NOx production as determined by the Zeldovich Equation. If either the cylinder pressure, heat or flame speed are altered correctly, the resulting effect will be a reduction in NOx.
The methods to control emissions can be qualified as either in-cylinder or aftertreatment. The first describes those that are internal to the engine such as the combustion chamber shape and design, the camshaft profile, the method used to inject fuel, and exhaust gas recirculation (EGR). Aftertreatments are any process or device that are exposed to the exhaust gas after it leaves the engine such as a diesel oxidation catalyst (DOC), diesel particulate filter (DPF), and selective catalytic reduction (SCR). Tier IV engines employ a combination of in-cylinder and aftertreatment to meet their goal of reduced emissions.
The definition of a catalyst is something that speeds up or alters a chemical reaction without itself being consumed.
It is important to understand that the tiered approach was taken by the EPA to allow every aspect of the industry to gradually integrate the necessary changes to the engine, fuel, and operator/repair strategies. As each tier was implemented the level of emissions was further reduced. Due to the drastic reduction in PM and NOx that Tier IV mandated, it was evoked in two steps; interim (or A) and final (or B). In 2019, Tier V will begin and it is predominately focused on the physical size of the PM, but we will not be concerned with that now.
The metric (measurement scale) used by the EPA for diesel emissions is grams per horsepower hour (g/hp- hr). It is determined by weighing the emissions from the engine in a special bag connected to the exhaust, often called a bag test.
Due to this there is not one standard under Tier IV for all engines. The horsepower, use, displacement and estimated time under load were all considered for the different standards. On a farm where you have myriad of diesel engines, uses, power levels and displacements, it is possible that the methods and systems employed to meet the Tier IV requirements will be different on each. A 100 HP Tier IV skid steer will have a different emissions reduction strategy than a 400 HP combine or the pick-up truck you drive to town. This is why you need to understand how these systems work and why an engine may have more or less complexity to it.
In addition, each manufacturer based on their specific engine’s emissions output, may employ only some or all of the control strategies. The EPA only cares about what exits the tailpipe and has no mandate on how to accomplish that.
Emissions Control Methods
The following are used to meet Tier IV and the emission(s) they impact:
Low sulphur fuel: reduction in PM
Updated (API CJ-4) engine oil with low ash content: reduction in PM
Exhaust gas recirculation (EGR): reduction in NOx
Diesel oxidation catalyst: reduction in CO, HC, some impact on PM
Diesel particulate filter (DPF): converts PM to ash
Selective catalytic reduction (SCR): major reduction in NOx when employed with diesel exhaust fluid (DEF)
High pressure common rail fuel injection (HPCR): electronic in lieu of mechanical control of the fuel events (impacts all four emissions)
How These Methods Work
Following the Zeldovich Equation, if exhaust gas (EGR) is introduced into the cylinder it acts as a filler, limiting the volume of combustible mixture. Even though the exhaust gas is hot, it effectively reduces the combustion temperature since it is considered inert (will not burn). If the leading-edge flame temperature in the bore is kept below 2,500 F degrees, the amount of NOx produced is greatly reduced. Most if not all Tier IV engines employ cooled EGR; where engine coolant is used to lower the introduced exhaust gas temperature to further impact NOx. Production of NOx often follows a bell-curve; it climbs with engine load and peaks when engine torque crests. After peak torque NOx ramps down slightly.
The diesel oxidation catalyst (DOC) is first in line from the engine. It looks like a muffler. An internal substrate that resembles a honeycomb that is made of precious metals reacts with the CO and HC from the exhaust and renders them benign. It also creates a very minute reduction in PM. For the DOC to function it must reach a minimum operating temperature of 600 F degrees which is called lite-off. In most engine operating states, the internal temperature of the DOC is closer to 800 F degrees. The higher the temperature, the greater the conversion rate measured in percent (%).
The DOC also serves an additional function to create heat for the diesel particulate filter (DPF) to clean itself.
Once the exhaust leaves the DOC it enters the DPF which can be considered a filter or trap (some literature refers to the DPF as a soot trap). The DPF holds the soot in its substrate and based on the application will evoke a process identified as regeneration or regen for short. During regeneration the internal temperature of the DPF will reach nearly 1,200 F degrees. The PM will be reduced to ash and stored in the DPF.
The final step on the exhaust gas journey to the tailpipe is the selective catalytic reduction system (SCR). Here the NOx will be converted to harmless components. It accomplishes this by exposing the exhaust to a unique substrate of rare metals in conjunction with diesel exhaust fluid (DEF) that is 32.5% automotive-grade urea mixed with 67.5% deionized water.
The engine management system monitors the operating conditions of the engine and when NOx production is high, the DEF is injected into the SCR for the reaction to occur. A good rule is that for every 100 gallons of diesel fuel the engine uses, the SCR will consume around 3.5 gallons of DEF, or 3.5% of the fuel consumption.
High pressure common rail fuel injection (HPCR) in simple terms provides almost infinite control over the introduction of the fuel to the cylinder. A mechanical injection system (pump-line-nozzle) worked well but the fuel delivery in regard to the combustion event was only optimized in a small window of parameters. The level of tuneability was limited. In engineering this is called degrees of freedom (DOF). With HPCR the fuel timing, duration and quantity based on inputs from engine sensors allows for extremely precise combustion events. This is why the modern diesel engine is so quiet compared to older versions. The combustion event is highly managed.
Regeneration of the DPF
Since the DPF stores PM it then needs to be converted to ash to take up less space. This process is identified as regeneration and is responsible for most of the owner dissatisfaction with Tier IV engines since it uses fuel to convert the PM to ash.
Regen can be evoked either by a rise in pressure in the DPF that identifies that it is soot-loaded or in some applications with an algorithm based on running hours, fuel used and time under engine load. The best method is to monitor pressure in and out of the DPF.
Based on the PM load, regeneration can be evoked three different ways:
Passive: The internal temperature of the DPF is high enough due to the engine operating state (a combine under high load) to raise the internal temperature to 1,000 F degrees and convert the PM to ash. The caveat being these conditions need to be seen for around one hour or the process cannot be completed and will be aborted. This is the most benign regeneration cycle.
Active: The passive conditions are not met. Diesel fuel is then injected (HC doser) into the DOC to act like a furnace to create enough heat to raise the temperature of the DPF and convert the PM to ash. This will take about one hour and if the engine is shut off, will abort the cycle and it will be evoked once the proper conditions are met. An active regeneration uses a great deal of fuel but the vehicle/machine can still be used though it may be down on power.
Forced: This is a regeneration cycle that requires the equipment to be parked and either through a user prompt from the cab’s dashboard or via a scan tool, the process is instituted.
The goal is to the satisfy the system with passive regeneration cycles which are invisible to the way the equipment is operated. If this cannot be accomplished then an active cycle will automatically be evoked. Depending on the application this may impact how the equipment is used during that time and most certainly the fuel consumption.
After the DPF sees around 5,000 hours of use or 500,000 miles for a road vehicle, the DPF may become loaded with ash and will need to be removed and mechanically cleaned through a special procedure. Under certain operating states or engine conditions this may occur sooner.
Problems occur with short duty-cycle uses such as a truck that is only driven a short distance or a tractor that is employed to feed cattle. These engine operating states have a propensity to build soot at a higher rate and also do not allow for the conditions to be met to evoke a passive regeneration or even an active cycle. The engine controller will then command a forced regeneration cycle that can take up to one hour or longer and may not allow the vehicle or machine to be used during that time. In addition, the forced cycle has the potential to use a good deal of fuel.
The complexity of the Tier IV system requires not only the basic knowledge supplied in this primer but a rethinking of the service mindset.
The following steps will go a long way to keep your newer engines carefree and limit repairs:
WHEN POSSIBLE OPEN THE HOOD TO LET THE HEAT OUT:
Due to the high rate of EGR, multiple turbochargers, and the possibility of packaging some of the aftertreatment systems under the hood, extreme heat soak will occur when the engine is shut down. This impacts all castings, gaskets, electronics, seals, hoses, etc. By simply opening the hood at the end of the work day or during break time, will greatly limit the thermal excursion and the baking effect of the high temperatures.
USE THE RECOMMENDED ENGINE OIL AND COOLANT:
Not adhering to these standards will impact the engine and component life. High ash oil will create more PM and NOx that will degrade the DPF and cause the SCR to dramatically increase the amount of DEF used and may even set a trouble code in the engine management system.
The proper coolant, renewed at the specified intervals, is imperative due to the high thermal load created by the EGR cooling system and the rest of the engine.