What is the difference between a 2-stroke and 4-stroke outboard motor? As the question points out, one main difference is the number of movements (strokes) it takes for the piston to complete a cycle. For example, a cycle on a 4-stroke engine begins with air and fuel intake to the compression stroke, where ignition of fuel and air occurs. So far, that is just two strokes. The third stroke brings the piston back to the bottom, and the fourth stroke presses out the exhaust as it raises to the top. That’s four strokes.
The Two-Stroke Outboard Cycle
A two-stroke outboard requires only two piston movements to complete a cycle. Whereas in a 4-stroke outboard engine, the fuel and air enter at the top of the engine, a two-stroke engine allows fuel to enter the bottom of the piston housing. As the piston turns, the fuel and air are compressed and then ignited in one stroke. On the downstroke, an exhaust valve opens near the bottom of the combustion chamber, and as the piston continues its cycle, it forces gas and air into the combustion chamber, which forces out the exhaust.
So the first difference is the mechanical design of both engines. One advantage of a 2-stroke engine is that it weighs less than a comparable 4-stroke outboard motor. That is because there are fewer moving parts in a 2-stroke engine than inside a 4-stroke engine. A con of 2-stroke engines is an increase in pollution and often an increase in fuel consumption. That is because of how the ignition and exhaust strokes work on a 2-stroke engine. As fresh fuel and air fill the combustion chamber, the exhaust exits via the exhaust valve. Along with the exhaust is some unburned fuel. That is a fundamental problem with 2-stroke engines, though new advancements in marine engine tech are changing thanks to direct fuel injection and sometimes the inclusion of a catalytic converter.
Stricter environmental regulations nearly killed off the 2-stroke engine because of significant hydrocarbon emissions from 2-stroke boat motors.
The Engine Oil Issue
With a 2-stroke engine, the oil supplied to the engine’s moving parts can be handled in two ways.
- As part of the engine fuel, which would include a percentage of engine oil, such as 50:1 ratios.
- Oil pumps from an oil reservoir into the intake tract of the engine.
Interestingly enough, once the oil is in the gas and the gas is in the fuel tank, the engine has oil. This is because an oil injection system pushes a specific amount of oil into the engine. If that part fails, the engine is without oil, which often means a catastrophic engine event.
Pros and Cons of 2-stroke and 4-stroke Outboard Motors
We have gone over some pros and cons of both engines, but here we go:
- If you want better fuel efficiency from your outboard motor with better emissions, you most likely want a four-stroke engine. 4-stroke engines mix oil and fuel separately for better exhaust emissions and zero raw fuel pollution.
- If mixing gas is not your cup of tea, then a 4-stroke engine is for you. You add only gas to the gas tank and oil to the reservoir. A 2-stroke engine uses mixed gas/oil in a specific ratio that can vary from one engine brand to the next.
- 4-stroke engines are welcomed in more places than are 2-stroke engines. Why? Well, the improved emission standards for most 4-stroke engines allow you to fish in environmentally sensitive areas. Plus, 4-stroke engines are generally quieter than equal HP 2-stroke engines.
- If you think you changed the oil and maintained the boat motor, you probably want a 2-stroke engine. One plus of 2-stroke engines is that they require less maintenance than a 4-stroke engine. As there are fewer moving parts, there are also few opportunities for something to break down. 2-strokes that mix oil in the gas does not require an oil sump injector or pump to lubricate the engine, which means you never have to check the oil.
- If you’ve filled the engine with water, you want a 2-stroke engine. Two-stroke outboards are much easier to revive once flooded with water. Part of this is because the pistons pump out anything in the chamber to continue the exhaust stroke.
- A 2-stroke engine is considerably lighter than its equivalent 4-stroke counterpart. If you need a lightweight engine, then a 2-stroke engine is what you need. A four-stroke engine has more internal parts, which is part of the reason it is heavier.
Conclusion — 2-stroke vs. 4-stroke marine engines
Many new marine engines are 4-stroke because they offer better emission ratings over older 2-stroke boat motors. That, however, is not a universal truth, rather a generalization in the evolution of boat motors. Many physical engine changes occur due to regulations at the state level, which also vary from one state to the next. These regulations have created an opportunity for anglers with 4-stroke engines to fish or boat in places where anglers or boaters with 2-stroke engines are no longer permitted.
Is one engine better than the other? It is a highly subjective answer, and much of how you answer that question comes down to how you use your boat, where you want to boat, and how diligent you are with outboard motor maintenance.
There are as many positives for each motor as there are negatives. One gets better fuel economy but is 30-50 percent heavier. One is not very environmental but is lighter and easier to maintain. The list continues. Should you buy a 2-stroke motor for your boat? Should you opt for the 4-stroke engine instead? Personally, a 4-stroke makes more sense if you are an angler. A healthier environment means better fishing opportunities. If you want to go farther on a tank of gas, then the 4-stroke is a good choice as it has better fuel economy. If lifting a heavy boat motor is not for you, a 2-stroke is an option, and you can find certain brands of 2-stroke boat motors with excellent emission standards.