Off-highway diesel engines meet Tier 4 emissions regulations

Emissions guidelines for diesel engines in off-highway equipment continue to get tougher. The introduction this January of EPA Tier 4 interim (Tier 4i) regulations in North America (and E.U. Stage IIIB in Europe) marks a significant tightening from Tier 3 limits which have been in place since 2006.

Although specific limits vary somewhat depending on engine size, they’re all headed toward near-zero emission levels. For example, for engines rated from 175 to 750 hp, nitrogen-oxide (NOx) emissions must be cut in half and particulates by 90%, compared to previous levels. And starting in 2014, Tier 4 final (Tier 4f) takes effect. For engines under 750 hp, this means an additional 80% cut in NOx, compared with Tier 4i.

According to officials from MTU Detroit Diesel, Detroit, the first step in complying with ever-tighter emissions regulations involves upgrading processes inside the engine — for example, by increasing injection pressure, fine-tuning combustion, and using exhaust-gas recirculation (EGR) to keep raw emissions as low as possible.

These measures can significantly reduce the complexity and cost of exhaust aftertreatment and, in some cases, make aftertreatment unnecessary. This, in turn, means simpler designs that require less installation space and maintenance.

Unfortunately, there is an inherent conflict between lowering particulate levels and reducing NOx, say MTU officials. Few in-engine techniques reduce both pollutants simultaneously. More often, an improvement in one area worsens the other.

Where emissions limits cannot be met using in-cylinder techniques, exhaust aftertreatment is needed. These include selective-catalytic reduction (SCR) and dieselparticulate filters (DPF) to reduce NOx and particulates. Here’s how some major engine manufacturers are attacking the problem.