Emission standards for off-road diesel engines are set in the U.S. by the Environmental Protection Agency (EPA). The first off-highway diesel emissions standards began in 1996 with the implementation of Tier 1. Since then, increasingly aggressive standards have continued to be implemented to limit the amount of pollutants being released into the atmosphere. Tier 4A requirements for the high-horsepower (above 175 HP) engines went into effect in 2011.

In 2014, the latest emissions requirements, known as Tier 4B or Tier 4 Final, were implemented. Manufacturers are required by law to meet these emissions requirements in the products they sell.

The primary focus of the legislation is the reduction of nitrogen compounds (NOx) and particulate matter (PM). NOx (nitrogen monoxide and nitrogen dioxide) are gases produced during combustion at high temperatures. PM is made up of tiny carbon particles and other pollutants that are the result of incomplete combustion.

When emissionization of off-road diesel engines began, the regulations could be met with inside-the-engine technologies, with no changes apparent to the tractor owner. Since 2011, however, EPA requirements have increased to the extent that manufacturers can no longer control both NOx and PM emissions inside the engine. Only one of them can be controlled internally; the other needs to be managed externally, after combustion.

The industry has used two different solutions to meet the Tier 4A regulations: selective catalytic reduction (SCR) and cooled exhaust gas recirculation (CEGR).


The fundamental difference between SCR and CEGR is that SCR controls the PM internally by allowing the engine to run as efficiently as possible, then converting the NOx in the exhaust to nitrogen and water vapor by treating it with a urea solution known as diesel exhaust fluid (DEF).

CEGR, on the other hand, reduces NOx inside the engine by reducing combustion temperatures. When the intake charge is diluted with exhaust, there is less oxygen in the cylinder to promote combustion, and the temperature is reduced.

However, this reduction produces more particulate matter, which must then be removed by means of a filter. The negative effect of “incomplete combustion” is that it takes away from the efficiency of the engine. More fuel needs to be consumed to get the work done.

To meet the more stringent Tier 4B regulations, there are different solutions the industry will use:

1. SCR only

2. CEGR plus a diesel particulate filter (DPF)

3. A hybrid system which uses CEGR, a DPF and SCR

4. A hybrid system which uses CEGR, no DPF and SCR

New Holland, for example, will continue to use an “SCR=only” system in higher-horsepower tractors but has refined the system to be even more efficient than the SCR system used to attain NOx levels at Tier 4A. This advanced, high-efficiency system is known as Hi-eSCR.

When the engine is set up for maximum efficiency, combustion is fortified with fresh, clean air, and high combustion temperatures result. This produces a low quantity of PM and a relatively higher level of NOx. The after-treatment system is used to clean up the excess of NOx.

With the Hi-eSCR system, the same chemical reduction is used as with Tier 4A but a slightly higher amount of DEF is used in conjunction with improved monitoring in the system. The basic principle is an extension of what was used effectively at Tier 4A but the reduction in NOx has been improved from 80 to 85 percent up to 95 percent.

For Tier 4B in high-horsepower applications, most manufacturers in the agriculture industry have chosen not to use the CEGR/DPF-only strategy. This is due to power density loss and excessive fuel consumption. This has an even larger impact at Tier 4B emissions levels than it did at Tier 4A.

To meet the Tier 4B mandate using only a CEGR/DPF system would require high levels of exhaust gas recirculation, which would adversely affect engine performance and efficiency. The solution to this problem is to add an SCR system to the existing CEGR emissions system, which could recirculate the same or less exhaust gas than what was done at Tier 4A.

The result is a hybrid system that uses CEGR, DPF and SCR. Since a percentage of CEGR is still used, PM formation will occur. A diesel oxidation catalyst/DPF is required to take care of the PM created. Tier 4B now requires an ammonia oxidation catalyst whenever DEF is being used to eliminate any excess ammonia (the working agent in the DEF) from escaping into the atmosphere. PD

Gary Wojcik is a product marketing manager with New Holland. Contact him by email.

Gary Wojcik
Product Marketing Manager
New Holland