What Are Stage Emission Classes?

There is a phrase that appears more and more often in generator specifications, especially in tender documents: “The engine shall be at least Stage II emission class.” Some projects ask for Stage IIIA or Stage V. These phrases refer to the legal limit stages set for harmful gases and particles in engine exhaust. In this article we explain in plain language what the Stage classes mean, which generators fall within their scope and the technical consequences of the choice.

What do the Stage classes limit?

The Stage levels are the emission limits set by the European Union for engines used in non-road mobile machinery (NRMM). The current framework is laid down by Regulation (EU) 2016/1628. Four main pollutants are limited:

• CO – carbon monoxide
• HC – unburnt hydrocarbons
• NOx – nitrogen oxides
• PM – particulate matter (soot); with Stage V, also particle number (PN)

Each new stage lowers these values compared with the previous one. The limit values are defined separately for different power bands according to engine power.

The stages in brief

Stage	Introduction period	General feature
Stage I	Around 1999	First common limits
Stage II	2001–2004	Significant reduction in CO, HC, NOx and PM
Stage IIIA	2006–2008	Stricter NOx and HC limits
Stage IIIB	2011–2013	Large reduction in PM; electronic fuel systems became widespread
Stage IV	2014	Large reduction in NOx; exhaust aftertreatment on most engines
Stage V	2019–2020	Particle number limit; particulate filter in most power bands

Dates vary by power band; the table gives a general framework. In the USA, a similar system is applied through the EPA Tier levels (Tier 1 to Tier 4 Final); in export projects the two systems should not be confused.

Which generators are covered?

A frequently asked question is: “Does a stationary standby generator also have to be Stage V?” The general rule is:

• Mobile and portable sets (for example trailer-mounted sets or construction site sets moved from site to site) fall within the scope of this regulation in the EU.
• Permanently installed sets (standby sets for buildings, hospitals, factories) are generally outside the scope of this regulation. For these sets, the country’s environmental legislation, rules on combustion plants for large installed capacities and the project’s own specification are decisive.

Countries outside the EU apply their own rules; some take the EU stages as a reference, some use the EPA system, and some have no emission requirement for generators. The required class should therefore be checked separately on each project according to the country of installation and the type of use.

Technical consequences: engine, fuel and maintenance

A higher emission stage brings certain differences to the engine and its maintenance:

• Fuel system: higher stages use electronically controlled, high-pressure fuel systems (common rail). These systems are more sensitive to water and contamination in the fuel.
• Exhaust aftertreatment: Stage IV and V engines may use a diesel particulate filter (DPF), a diesel oxidation catalyst (DOC) and selective catalytic reduction (SCR) for NOx. An SCR system also needs AdBlue (urea solution) to be topped up.
• Fuel quality: engines with particulate filters require ultra-low sulphur diesel. In markets where high-sulphur fuel is used, these engines can cause problems.
• Low-load running: when engines with particulate filters run at low load for long periods, filter regeneration is needed. This should be taken into account for standby sets that run few hours a year at low load.

The approach that “the highest stage is always best” is therefore not correct. Unless the specification or legislation requires it, the choice should also take fuel quality and the operating profile into account.

How is the emission class confirmed?

The emission class is determined by the exact engine model, not by the engine make. One engine of a make may be certified to an EU Stage while another engine of the same make is not emission-certified, or is classified under a non-EU system (for example EPA Tier or China III). If your specification requires a particular stage, state it in your quotation request. The emission class of the model we offer is stated model by model in the quotation and datasheet; no stage is declared for models whose class cannot be verified from the engine manufacturer’s documents.

For a general comparison of engine makes, see our article generator engine brands compared, and for other points to watch on export projects, our article on export generators.

When writing a specification

Stating the emission requirement clearly in the specification makes quotations comparable:

1. Specify the required stage and system: “EU Stage II or stricter” or “EPA Tier 3”.
2. State whether the set will be used as stationary or mobile.
3. Define the document requirement: the engine manufacturer’s emission conformity certificate.
4. State the fuel quality on site (sulphur content).
5. If Stage IV/V is required, add an operating plan for AdBlue refilling and particulate filter maintenance.

This article is for general information. For exact requirements, the legislation in force in the country of installation and the relevant authorities should be consulted.