Are there two-stroke diesel engines?

Yes. Two-stroke diesel engines not only exist, they remain the dominant choice for propelling the world’s largest cargo ships and are still used in certain locomotives, naval and industrial power applications; however, they have largely disappeared from on‑highway vehicles. Modern two-stroke diesels leverage advanced turbocharging, emissions controls, and even alternative fuels like LNG and methanol, while historical automotive and truck two-strokes have been phased out by emissions rules and efficiency gains in four-stroke designs.

Where two-stroke diesels are used in 2025

The sectors below illustrate where two-stroke diesel technology is alive and evolving today, and where it has receded due to regulation or market shifts.

• Deep-sea merchant shipping: Low-speed, two-stroke crosshead engines from MAN Energy Solutions (MAN B&W ME-series) and WinGD (X and X-DF families) power most large container ships, tankers, and bulk carriers.
• Dual-fuel and alternative-fuel vessels: New two-strokes run on LNG, LPG, ethane, and methanol, with ammonia-capable variants entering the market between 2024 and 2026 as owners decarbonize fleets.
• Railway locomotives: Electro-Motive Diesel (EMD/Progress Rail) two-stroke 710-series engines continue in service and production for marine/export and rebuild applications; in the U.S., new Tier 4 freight locomotives shifted to four-stroke designs.
• Naval and industrial power: Fairbanks Morse opposed-piston two-strokes and related large engines provide shipboard and stationary power, especially in defense and heavy-industry roles.
• R&D and demonstrators: Opposed-piston two-stroke diesels from Achates Power (including U.S. Army’s Advanced Combat Engine) are in testing and demonstration phases, targeting higher efficiency and lower emissions.

Taken together, these uses show two-stroke diesel’s persistence where high torque, fuel flexibility, and excellent efficiency at very large scale matter most, even as smaller and on-road markets moved on.

How a two-stroke diesel works

Unlike a four-stroke engine, which completes a power cycle in two crankshaft revolutions, a two-stroke diesel produces power every revolution. Air is forced into the cylinder to sweep out exhaust (scavenging) and fill the cylinder for the next combustion event. Most large marine units use uniflow scavenging: fresh air enters through ports in the cylinder liner while exhaust exits via a poppet exhaust valve in the cylinder head. A blower and turbocharger provide scavenge air, and electronic fuel injection times and meters the charge precisely. Opposed-piston designs achieve uniflow scavenging with two pistons in one cylinder moving toward each other, eliminating the cylinder head entirely.

Why large ships favor two-stroke diesels

Low-speed, two-stroke marine diesels deliver immense torque at 60–120 rpm, allowing direct drive to the propeller without reduction gearing. Their brake thermal efficiency is among the highest of any heat engine in service (approximately 50%, depending on load and model), translating into lower fuel consumption per ton-mile. The architecture also accommodates heavy fuels (where permitted) and now a range of cleaner fuels, with robust reliability over multi-decade service lives.

Notable engine families (past and present)

The following examples show current mainstream two-stroke diesel providers, as well as historic makers that shaped the category and where they stand today.

• MAN Energy Solutions (MAN B&W ME series): Electronically controlled two-strokes (ME-C, ME-GI for LNG, ME-LGIM for methanol, ME-LGIP for LPG). Ammonia-capable variants (ME-LGIA) are moving from testing toward commercial deployment mid-decade.
• WinGD (formerly Wärtsilä two-stroke; now part of CSSC): X (diesel) and X-DF (low-pressure dual-fuel) lines for LNG carriers and wide vessel classes; X-DF2.0 adds enhanced EGR-based emissions control (iCER) for NOx/methane slip reductions.
• EMD/Progress Rail 710 series: Two-stroke medium-speed engines used in locomotives (rebuilds and export) and marine/power markets; U.S. Tier 4 locomotive applications shifted to the four-stroke 1010J.
• Fairbanks Morse opposed-piston: Two-stroke OP engines continue in naval and stationary roles, valued for power density and maintainability.
• Detroit Diesel Series 53/71/92/149 (historic): Iconic uniflow-scavenged two-strokes once common in trucks, buses, and equipment; phased out in the 1990s–2000s by tightening emissions standards. Support and parts persist for legacy fleets.
• Achates Power opposed-piston (development): Two-stroke CI demonstrators for defense and commercial heavy-duty, targeting higher efficiency and lower emissions; progressing through validation but not yet in mass production.

These families underscore a split market: active innovation and large-scale deployment at sea and in niche heavy-duty roles, alongside a legacy footprint in road and off-road equipment that has largely sunsetted.

Environmental and regulatory context

Marine two-strokes comply with IMO Tier II/III NOx rules using strategies such as SCR aftertreatment, EGR, optimized combustion, and increasingly the use of cleaner fuels (LNG, methanol, LPG). The IMO 2020 sulfur cap curtailed high-sulfur fuel oil unless paired with scrubbers, accelerating uptake of low-sulfur fuels. New frameworks like EEXI and CII push operators to improve carbon intensity, prompting orders for methanol- and LNG-capable two-strokes and research into ammonia-fueled models. On land, U.S. EPA Tier 4 locomotive rules made compliance difficult for legacy two-strokes, prompting a shift to four-stroke platforms for new domestic builds, although two-strokes continue in other regions and applications.

Pros and cons of two-stroke diesels

Key advantages and trade-offs help explain where two-stroke diesels thrive and where they do not.

• Advantages: Very high torque at low speed; strong fuel efficiency at large scale; ability to directly drive propellers; simpler gas exchange hardware in some designs (ports vs. full valvetrains); fewer moving parts in opposed-piston architectures.
• Trade-offs: Dependence on scavenge blowers; historically tougher NOx/PM emissions control; oil consumption management; noise and vibration; less suitable at small sizes; regulatory headwinds in some on-road and rail markets.

In practice, these trade-offs favor blue-water shipping and certain heavy-duty roles, while pushing everyday transportation toward four-stroke engines and electrification.

Real-world examples

Modern container ships and LNG carriers frequently run two-stroke diesels: Maersk’s methanol-fueled containerships employ MAN B&W ME-LGIM engines, while LNG carriers commonly use WinGD X-DF two-strokes for dual-fuel operation. In rail, vast fleets of EMD two-strokes remain in service globally through rebuilds and exports, even as new U.S. line-haul locomotives gravitate to four-stroke Tier 4 solutions.

Bottom line

Two-stroke diesel engines are very much real—and essential—today. They dominate large marine propulsion, remain relevant in specific rail, naval, and industrial niches, and are evolving to use cleaner fuels. Their retreat from on-highway use reflects regulatory and market realities rather than obsolescence of the core technology.

Summary

Yes, two-stroke diesel engines exist and are widespread in modern shipping, with ongoing use in select rail, naval, and industrial roles. Marine two-strokes from MAN and WinGD lead the field, now offering dual-fuel and low-emissions variants. Historic road-going two-strokes (e.g., Detroit Diesel) have been phased out, but the technology remains a cornerstone of heavy-duty propulsion where efficiency, torque, and fuel flexibility are paramount.