When buying an advanced engine for their boat, vessel owners are often stuck on 2-stroke vs. 4-stroke models. 2-stroke and 4-stroke are two different frameworks used in internal combustion engines. These engines differ in design, operation cycles, and working principles. Moreover, these marine engines are configured for specific power range, torque, and other aspects. There has been an ongoing debate on 2-stroke vs. 4-stroke marine engines regarding their energy efficiency, lifecycle, performance, reliability, and emission issues.
Here, we provide an in-depth comparison of 2-stroke and 4-stroke engines for watercraft. This edition is recommended to help you make informed decisions when upgrading to a revolutionary marine diesel engine.
What is a 2-Stroke Marine Engine?
A 2-stroke diesel engine in marine vessels is an internal combustion engine. With a simple framework, these engines are ideal for small inflatable boats, kayaks, ferries, and light operations. The cylinder wall ports and pressure check valves allow the mechanism to conduct combustion cycles at a high speed. The two-stroke engine performs two piston strokes for a power cycle during one revolution of the crankshaft:
First – Compression Stroke
During the first stroke, the inlet port on the cylinder opens to allow the air-fuel mixture into the chamber. The piston movement from the cylinder’s bottom dead center (BDC) to the top dead center (TDC) to compress the air-fuel mixture. After compressing the contents, a spark plug fires up the mix for combustion.
Second – Power Stroke
After the first stroke, the hot gases exert high pressure on the piston for downward movement. Next, the piston proceeds towards the BDC to eliminate the waste gases pass through the exhaust outlet to empty the cylinder for the next power cycle.
Benefits of 2-Stroke Engines
- Simple design with very few components
- Low maintenance and repair costs
- The lightweight framework offers an enhanced power-to-weight ratio
- Can operate well when using low-grade fuel
Drawbacks of 2-Stroke Engines
- Noisy operations
- High fuel consumption
- Low power efficiency
- High thermal stress and emissions
What is a 4-Stroke Marine Engine?
The 4-stroke marine engine completes one power cycle in four piston strokes during two crankshaft revolutions.
Intake/ Induction/ Suction Stroke
The stroke starts when the intake valve opens to induce the air-fuel mixture into the cylinder. The piston (located at the extreme TDC) moves down to the BDC to create a vacuum and pull the mix rapidly into the cylinder.
Compression Stroke
The second stroke resumes at the BDC immediately after the completion of the intake stroke. Now, the piston moves upwards to the TDC to compress the air-fuel mixture before the ignition (during the power stroke). All inlet and exhaust valves are closed during the compression stroke to maximize pressure before using a spark plug.
Combustion/ Ignition/ Power Stroke
The third stroke starts at the beginning of the second crankshaft revolution. A spark plug or heating regulates the combustion to produce mechanical energy to power the crankshaft. The piston located at the TDC is forced to return to the BDC after the explosion.
Exhaust/ Outlet Stroke
The piston moves from BDC to TDC during the exhaust stroke to release the waste gas. This action expels the gases through the exhaust port and forces the remnants to pass through the open exhaust valve.
Benefits of 4-Stroke Engines for Ships
- Efficient fuel burning
- Low fuel wastage
- Minimizes unburnt hydrocarbon gas release from the exhaust port
Drawbacks of 4-Stroke Diesel Engines
- Large and heavyweight compared to 2-stroke marine engines
- Heavy and intricate components may raise repair expenses
- Requires frequent maintenance to keep the parts in good shape
- A single shot of power in every four strokes makes these engines less powerful than 2-stroke modules
2-Stroke vs. 4-Stroke Marine Engines – Comparison
| Features | 2-Stroke Engines | 4-Stroke Engines |
| Pistons Strokes | Takes 2 piston strokes to complete the combustion cycle | Completes the combustion cycle in 4 piston strokes |
| Horse Power Output | 1.5 -1.8 times than that produced in 4 stroke engines | Less than 2-stroke engine |
| Thermal Stress | The combustion chamber walls always receive extreme thermal stress due to high temperature and pressure | Performs combustion cycle in low temperature and pressure conditions to reduce the thermal stress on the cylinder walls |
| Volumetric Efficiency | Low efficiency and incompetent scavenging | Remarkable efficiency and scavenging |
| Fuel Economy | High fuel consumption | Consumes less fuel than 2-stroke modules |
| Power Output | More power output | Generates less power than 2-stroke engines |
| Headroom Space | More headroom space required | Performs well in compact headroom space |
| Torque | Can produce high torque using a small flywheel at higher rpm | Requires heavy flywheel to maintain balance due to irregular torque at low rpm |
| Best Suited for | High-powered low-speed diesel propulsion engines | Vessels operating in medium-speed power output |
| Crankshaft Revolutions | Completes one power stroke per crankshaft revolution | Requires two crankshaft revolutions to complete a power stroke |
| Camshaft to Crankshaft Ratio | 1:1 | 1:2 |
| Fuel Quality | Low-speed engine operating on cheap fuel offers more time for scavenging and exhaust | Requires high-quality fuel for efficiency and performance |
| Lube Oil Cost | Lower than that in 4-stroke marine engines | More than that in 2-stroke models |
| Power to Weight Ratio | High | Low |
| Operational Requirements | Gets overheated quickly due to high thermal stress so continuous cooling and lubrication is essential | Requires less lubrication and cooling due to low thermal stress |
2-Stroke or 4-Stroke – Which is the Better Option for My Vessel
The simplicity of design, lightweight, and low repair costs are some favorable aspects of 2-stroke engines. However, when reliability, emission control, and fuel efficiency come to the scene, four-stroke marine engines are the ultimate choice for vessel owners.
Furthermore, high thermal stress in two-stroke engines makes them vulnerable to wear and tear. Besides raising the cost of cooling systems, prolonged thermal stress reduces equipment lifecycle and causes damage to the adjacent machinery in the engine room.
Silent or minimal engine sound is a desired feature for hassle-free operations in luxury yachts and other pleasure crafts. Noisy 2-stroke engines may entail poor customer satisfaction and loss of business for ferry vessels.
Purchase New Marine Engines at Competitive Prices Online
If you want a modern ship engine with cutting-edge features, visit our engine webpage to explore the extensive collections from top brands. Alfa Marine Spare Parts offers umpteen choices of 2-stroke and 4-stroke marine engines at different rpm to match diverse vessel powering needs. We sell genuine engines from Detroit Diesel, Cat, Cummins, Volvo Penta, Wartsila, Doosan, Yanmar, Komatsu, John Deere, Perkins, Scania, and other brands.
Here are the top-selling items in our product gallery for those who are seeking top recommendations from marine engineers:
C7.1 Commercial Propulsion Engine – Cat 4-stroke diesel engine
B4.5 Maine Propulsion – Cummins 4-stroke engine
4045AFM85 Marine Generator Drive Engine – John Deere 4-stroke engine
S6R2-T2MPTK – Mitsubishi 4-stroke marine engine
D 2.9 L4 – Deutz 4-cylinder marine engine
Please feel free to contact us to check out our entire collection of marine engines. If you are looking for marine engine spare parts, the stocks in our warehouse are sufficient to fulfill orders of any size. We are happy to serve an ever-growing customer base across various international sea ports for diverse needs, from equipment supply in overhaul ports to shipbuilding units.
Here are the top-selling engine parts in our inventory:
A063L211 Cummins Control Engine
A049S678 Adapter for Cummins Marine Engine
A048S478 Cummins Engine Pulley System
A040C187 Cummins Engine Installation Accessories
0501-0687-04 Line Engine Vent for Cummins
410673 Cummins Engine Temperature Gauge
0338-2607 K50 Cummins Engine Harness
