Stage IIIa Diesel Generator Emissions Standards – What You Need to Know

Ever since the debacle with Volkswagen’s intentional thwarting of diesel emissions standards, there has been a great deal of attention on diesel engine emissions, and even questions about whether they deserve to be called a ‘clean’ technology at all. The current standards for internal combustion engines which are not part of a car (Non-Road Mobile Machinery, NRMM) were revised in 2016 to take the Volkswagen case into account. Here are the standards, as they stand now:

Europe’s NMRR emissions standards come in 5 successively more restrictive tiers, called stage 1 through stage 5 Standards. Most important to us is stage IIIa, as we primarily produce tier 3a diesel generators. However, we’ll take a look at all of tiers 1-4 here, and those aspects of stage 5 which have some application (ignoring the regulations for train locomotives and waterway craft engines.


Is there a ‘global’ standard for stage 3a diesel generator emissions?

Yes and no. In many parts of the world, primarily the US, the EU and Japan, there has been a substantial drive to create a harmonious international standard. Much of this pressure has come not from environmental groups, but from the manufacturers of diesel engines and equipment. Many of them manufacture tier 3a gen-sets and other equipment for export all over the world, and it is much more efficient to have a single set of standards in all of their primary markets.

As a result, Europe went to great efforts to make their stage 1 and 2 standards functionally equivalent to its counterparts in US law, at least in part. The greatest levels of congruence, however are seen in the stage 3 and 4 regulations, which are essentially the same for the EU and the US. Tier 5 regulations, however, vary substantially between regions. For example, the US regulations do not require particulate filters for many engine types which do require such filters in the EU.

What this means for our bread and butter, tier 3a diesel generators, is that there effectively is a unified EU and UD standard. Tier 3 emissions standards are essentially identical across most major international markets, and all of our diesel generators are compliant with all major standards, and completely US and EU compliant in particular.


Stages I and II

These were the first regulations promulgated in this set, stage 1 in 1997 and 1999, and tier 2 in 2001 and 2004. These apply to non-road diesel engines, but only within certain power output levels.

The tier 1 and 2 standards cover non-road trucks (including forklifts/loaders), highway excavators, snow plows and other road maintenance equipment, Airport-specific ground support equipment, construction industry wheel loaders, bulldozers, forestry tractors and several types of mobile cranes. The tier 1 and 2 standards originally specifically excluded all generating sets, aircraft, locomotives and watercraft engines.

In 2002, however, Directive 2002/88/EC [2905] was adopted. This amended the above standards to include any ‘spark ignited engines’ with an output of less than 19 kW. Similarly, tier 2 standards were expanded to cover various constant speed engines.

The tier 1 and 2 regulations place specific limits on the amount of 4 different pollutants which an engine can release per kWh of electric power produced. Both tiers are divided across a total of seven engine output categories, each of which has different allowable values.

A stage 1a engine, for example, is allowed to produce up to 5 grams of CO, 1.3 grams of HC, 9.2 grams of NOx and 0.54 grams of PM per kWh. These limits are defined as output before the effects of any exhaust aftertreatment devices are considered. The measurement was to be accomplished according to ISO 8178 using the C1 8-mode cycle, using fuel that had a sulphur content of 0.1% to 0.2% by weight.


Stages III and iV

Tiers 3a and 3b were phased into effect gradually between 2006 and 2013. Tier 4 came into force all at once in 2014. However, they only applied to engines in new machinery. Replacement engines destined for existing machinery needed only to comply with the standards in place when the machinery as a whole was originally marketed.

Of primary importance for our purposes are the tier 3 and 4 standards, as we produce mostly tier 3a diesel generators. Still, you need to understand the tier 3a emissions standards within the context of the regulations as a whole, so we’ll discuss both categories, not just those section that apply to tier 3a gen-sets.

The Tier 3 and 4 emissions standards for non-road engines were formally adopted in 2004, and were made applicable to certain classes of agricultural and forestry equipment a year later. In 2010 the stadards were modified by EU directives 2010/26/EU [2903] and 2010/22/EU [2904], which amended the testing standards for tier 3b, tier 4 and applicable agricultural or forestry engines. The 2010 modifications did not change the tier 3a emissions standards or the way they were measured at all.

The tier 3 and tier 4 emissions standards essentially follow the same format as tiers 1 and 2. Tier 3 engines are divided into 2 categories, tier 3a and tier 3b. Tier 3a is further divided into category H, I, J and K engines, again ranked by size. Tier 3b is divided into categories L, M, N, O and P. Tier 4 engines consist of only 2 categories, Q and R.

It is worth noting that in addition to the CO, HC, NOx and PM emissions limits, tier 3a emissions (as well as tier 3b and tier 4, of course) are also limited to emitting 25 ppm of ammonia over the test cycle.


Stages IIIb engines and particulate matter (PM) filters

All categories of tier 3b engines were given a ‘hard limit’ of 0.025 on the PM scale. This is a substantial change, as tier 3a engines are permitted PM emissions of between 0.2 and 0.6, depending on their size. Tier 3b also instituted separate measurements of HC and NOx for all categories except P, whereas all type 3a emissions standards tracked the combined levels of HC and NOx as a single measurement. This has led to widespread use of NOx aftertreatment systems on almost all tier 3b category L, M and Q engines.



Tier 5 non-road mobile machinery standards in the EU apply to any diesel engines rated at 56 kW or less, and to all engine types of more than a 56 kW rating, but less than 560 Kw. It also applies to diesel engines used in gen-sets which are rated above 560 kW.

The European tier 5 standards depart notably from the US equivalents as they have strict PN limits for most categories of engine. Between this limit and the mass-based PM measurement also applied, all compliant engines have a very low particulate matter output. Most if not all achieve this by using wall-flow particulate filters or similarly modern technologies.


Modern Testing Standards – Defeating the Defeat Devices

A great deal of effort has been spent to make sure engine testing now simulates real usage conditions more exactly. This is intended to prevent engines from being engineered so as to meet emissions standards during testing, yet exceed them in the interest of higher performance in actual use.

Newer ‘transient’ testing procedures use the NRTC (non-road transient cycle), which was developed in partnership between the US and EU environmental authorities. It is essentially run two times, once from a cold start and once with a hot start. The results are combined, but the hot start is weighted to represent 90% of the measurement, and 10% for the cold start. This is now run in parallel with the NRSC (non-road steady cycle) procedure set out I ISO 8178 C1.


Defeat Devices

Stage 3 and 4 treat attempts to ‘defeat’ or fool the testing procedure differently than tier 5.  

For the purposes of emissions testing of stage 3a, 3b and 4 engines, a defeat device is considered to be any device which “measures, senses or responds to operating variables for the purpose of activating, modulating, delaying or deactivating the operation of any component or function of the emission control system such that the effectiveness of the control system is reduced under conditions encountered during the normal non-road mobile machinery use unless the use of such a device is substantially included in the applied emission test certification procedure”.

Essentially, that means that any device which causes the engine to perform differently during an emissions test than in actual operation is prohibited. Such devices cannot be a part of any tier 3a diesel generator or tier 3a gen-set.

Tier 5 emissions regulations define prohibited ‘defeat strategies’.

A defeat strategy, in these terms, is any attempt to engineer the engine such that it reduces the effectiveness of any emissions control system the engine may have under real working conditions, or in conditions other than approved testing procedures. It is intended to do the same thing, but in a very different way.


Irrational Control Strategies

Irrational control strategies are also prohibited, as a kind of ‘catch all’ for any trickery which avoids the legal definition of ‘defeat devices’ or ‘defeat strategies’. It makes illegal any measure that makes the actual emissions of the vehicle worse in performance than in testing.


A note on implementation dates

The EU standards effectively have 2 different implementation dates each. There are ‘type approval’ dates, by which time only newly approved equipment models may begin production, and ‘market placement’ dates, after which all new engines actually placed on the market must comply with the new regulations. This article uses market placement dates exclusively. The type approval dates were almost always one year ahead of the market placement dates.