25AHX diesel engine goes to market

IHI Power Systems (IPS) has received its first order for its medium-speed 25AHX diesel engine.

The engine was developed in part to meet stricter emissions requirements; for example limits and measurement methods for exhaust pollutants from marine engines ”GB15097-2016” shifted from Stage-I to Stage-II in July of 2022, requiring a significant reduction of NOx emissions—nearly 30% compared to the IMO Tier II NOx regulations for engines with an output of less than 2000 kW.

Overview of the 6L25AHX engine. (Image: IPS)

Selective catalytic reduction (SCR) is one method to reduce NOx emissions, but it needs installation space for the equipment and running costs for the reducing agent, so IPS set out to develop an engine that did not require SCR.

The AHX series engines use Miller cycle to reduce compression air temperature in-cylinder in order to reduce NOx, and high-pressure ratio turbocharger is necessary to supply sufficient air. It is equipped with a variable intake valve timing system (VIVT) that can set the suitable intake valve timing according to the load, the company said. The 25AHX engine is also designed to suppresses the rise in combustion temperature by retarding fuel injection timing in order to reduce NOx, the company said. 

The 25AHX diesel engine has a cylinder bore of 250 mm and a piston stroke of 370 mm for an output of 250 kW/cylinder at 750 min -1 engine speed. In addition, the engine size is designed to replace the conventional engine, and the design also takes into consideration the ease of maintenance of each part.

The 25AHX engine was certified by the China Classification Society (CCS) in July 2022. The 25AHX engine has received orders for some engines including the first commercial engine, so they will be delivered to the Chinese market in 2023.