Difference Between 2 Stroke vs. 4 Stroke Outboards

How do they work and which is 'Best'?

Whats the difference between 2stroke and 4stroke outboard engines outboard manuals

In this guide we step through the basics of how two stroke and four stroke outboard engines work, the main mechanical differences and which may be best depending on which boat and boating style you have and do.

First, The basics...

Any outboard described as a either 4 stroke or 2 stroke engine is an internal combustion engine, buring injected fuel through an ignition and exaust sequeuce.  These types or variants of the combustion engines are common across outboards, motorbikes and even agricultural and garden machinery!

Both types of engine have the same fundamental components to enable the buring of fuel and subsequent generation of power, which ultimately is transferred to the propeller to provide the driving force. They are:

  1. Piston & Piston Head - Metal rod with circular block at the top that moves up and down the crankcase (connected to the crankshaft at the bottom) - Each movement up OR down is referred to a 'stroke' - hence the term.
  2. Input & Outlet Valves - Holes or ports in the cylinder head through which fuel and/or exhaust gas flows
  3. Cylinder Head - Metal block that sits on top of the crankcase with ports through which valves and ignition devices pass
  4. Crankcase - Metal Block in which the piston and piston head move up and down
  5. Crankshaft - Offset (balanced metal shaft that rotates at the bottom of the crankcase. (Connected to the piston rod)

The general assembly of these components is the same in most outboard engines with all engines having a crankcase, crankshaft and cylinder head.  The numbers of pistons (and piston heads) varies with larger more powerful outboard engines have a more.  Small outboards e.g. 2HP to 5HP will typically have just one.

If you want to learn more about Internal Combustion Engines in general this Wiki article is verg good.



How 4 Stroke Engines Work

A four-stroke outboard motor is a combustion engine where four separate strokes of the piston turn the crankshaft in order to complete a single cycle. These four phases are known as:

  1. Intake - Fuel entering the combustion chamber
  2. Compression - Fuel being compressed in the combustion chamber
  3. Combustion - Ignition of the compressed fuel
  4. Exhaust - Expullsion of burnt combusted fuel

This means you get one combustion stroke or 'power cycle' (#3 in the list above) for every 4 strokes of the piston in the crankcase, i.e.

1 power stroke to 4 strokes = '4  Stroke'

Advantages

A 4 stroke will quieter across the rev range, particularly at idle typically or low levs for instance when pottering to a mooring or trolling a fishing line. 4 stroke engines will provide a smoother and even delivery of power and are usually more fuel efficient and generate less emissions

Disadvantages

4 stoke outboards are heavier than an equivelent 2 stroke with the same power, often due to their larger relative size. 4 strokes are more mechanically complex around power block and head so consequently to a higher level of skill to maintain and repair.

How 2 Stroke Outboards Work

By comparision in a two stroke outboard motor, the end of the 'Combustion' stroke and and beginning of the Compression stroke happen simultaneously as does the 'Intake' & 'Exhaust' strokes.

This means you get one combustion stroke or 'power cycle' for every 2 strokes of the piston in the crankcase, i.e.

1 power stroke to 2 strokes = '2  Stroke'

This clever reduction in crackshaft revolutions required for each power cycle means 2 stroke engines typically generate more power for the equivalent weight of a 4 stroke engine. It also means reduced numbers of moving parts and complexity in the main engine componets and therefore lighter, more portable and easier to maintain. All critical factors in outboard engines!

While 2 stroke outboard motors are typically not maintenance free, servicing is fairly cheap and easy to fix when issues arise.

Advantages

2 stroke outboards are smaller and lighter than their 4 stroke (of an equivalent power). Due to them being mechanically simpler with fewer major parts to degrade or go wrong the are easier to maintain to a basic level.  Two strokes are generally cheaper to buy new.

Disadvantages

Swapping from a 4 stroke engine or getting up close with a 2 stroke for the first time you will notice that they don't run as smoothly as a 4 stroke. They can be described as 'tinny' or 'metalic' particularly at mid to higher rev range.  Older 2 strokes and especially those utilising a Carburettor instead of a more modern fuel injection system will generate more emissions.

So Which is Better?

The short answer is.... It depends!

Both 4 stroke and 2 stroke outboards have different qualities and strengths which match different boat types, the sort of boating you do and your requirements around portability, reliability, serviceability etc.

For instance if your looking for a outboard for a small inflatable tender to get you out to a yacht mooring and then pack and stow both tender and outboard on the yacht then the small size and portable nature of a 2 stroke would suit.

If however you have a larger rigid inflatable and you want to cover more distances perhaps day trips our motoring to a fishing spot then portability is not really a consideration but power and comfort (and noise levels) under power for longer periods of time mean a 4 stroke outboard is probably more suitable.

2 Stroke vs. 4 Stroke - Marks out of 10

In all scenarios a quality outboard made by a reputable company that is well maintained and cared for should provide many years of happy and safe boating. To round up we compare two stroke and four stroke side by side scoring marks out of 10.

Two Stroke Four Stroke
Power 8 10
Weight 9 7
Durability 9 9
Portability 10 7
Emissions 7 9
Mantainability 9 6
Affordability 9 7

Enjoy your boating and be safe on the water.

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2 Comments

  1. Thanks for the comment.

    From a purely emissions perspective,yes. Many burn the oil/fuel mix which often means some unburnt fuel coming out of the exhaust. But 2-strokes mechanical simplicity and less components means less resources go into the initial manufacturing and servicing.

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