According to new research done by the International Council on Clean Transportation (ICCT), it has been discovered that the newer tier II engines emit higher nitrogen oxide than older tier I engines. The study also found that there is not much statistical difference in NOx emission rates between unregulated Tier 0 engines and Tier II engines. NOx contributes to the formation of smog and acid rain, as well as affecting tropospheric ozone.
A recent working paper examined 615 samples of actual NOx emissions from 545 ships that were operating in Danish waters, specifically between the North Sea and the Baltic Sea, during the year 2019. To obtain these measurements, sniffers were attached to helicopters which were flown into the exhaust plumes of the ships. The data set includes measurements from ships that fall under all tiers. Tier I limits apply to engines on ships constructed between 2000 and 2010. Tier II limits apply to vessels built in the beginning of 2011 and they are set at approximately 15–20% below Tier I. Tier III is set at 80% below Tier I, or approximately 75% below Tier II. Tier III applies to engines on ships constructed in 2016 or later.
According to a recent study, the highest average NOx emissions are observed when ships are operating at below 25% of their main engine load, a part of the duty cycle that is not included in the NOx compliance certification schemes for most engines. The current NOx test cycles assume that marine engines operate at higher engine loads, but this study shows that engines typically operate at lower engine loads. Furthermore, NOx control technologies such as selective catalytic reduction are ineffective at low engine loads. All of this means that when ships are sailing slowly, as they usually do when they are close to the shore, they emit higher rates of air pollution, which amplifies shipping’s negative impacts on coastal air quality and human health.
The think tank based in Washington DC is urging the International Maritime Organisation (IMO) to contemplate the implementation of not-to-exceed (NTE) standards for ship engines, both new and existing. The focus should be on operations at low loads, and a test point below 25% load should also be included.
Dr Bryan Comer, the marine program lead at ICCT, has pointed out that the method used by the International Maritime Organisation (IMO) to regulate NOx is not working as intended. The test cycle for NOx employed by the IMO allows engine manufacturers to optimize their engines for the emissions test in the lab, which results in presenting a low-emission engine. However, this test cycle is misleading because newer engines are emitting more air pollution than older ones in reality.