Your Guide to Outboards: Chapter 2

In Chapter 1 we briefly discussed the differences between conventional two-stroke outboards, direct-fuel-injected (DFI) two-strokes and the "new technology" four-strokes

Understanding Two-Strokes and Four-Strokes

Now we'll take a closer look at how each type of outboard works. A basic understanding of these technologies - and their respective advantages and disadvantages - will help you select the model that best fits your budget and style of boating.

Conventional Two-Strokes

The main advantages of carbureted and EFI two-stroke outboards is their affordability, simplicity, ease of maintenance, light weight and excellent power-to-weight ratio. Their downside is that since they allow up to 30 percent of the fuel/air mix to escape unburned, they are not as economical to operate. They also pollute more than newer-technology outboards, which is why manufacturers are shifting to the cleaner technologies in order to meet emissions standards.
Conventional two-stroke outboards - the type we have known for years - are available in two forms: carbureted and electronic fuel injected (EFI) versions.
Relying on mechanical operation, a carburetor regulates the ratio of fuel and air drawn into the intake. The faster the air passes through the carburetor venturi, the greater the volume of fuel introduced. EFI accomplishes the same thing, except that fuel is introduced into the airstream via a pressurised injector, and the volume is electronically controlled according to manifold pressure, throttle, rpm and temperature.
Both systems draw the fuel/air mix into the crankcase (along with injected oil in outboards that feature automatic oil injection) through one-way reed valves. As the piston goes up, the fuel/air mix is drawn into the crankcase. After the spark plug fires and the piston is in the downstroke, the reed valve closes and the mixture is compressed in the crankcase. During this stage, an exhaust port in the cylinder wall is uncovered and exhaust gasses are vented. Next, the transfer port is uncovered and the compressed fuel/air charge is forced into the combustion chamber. Since the exhaust port is first to open as the piston goes down, it's the last to close as the piston goes up. This allows a portion of the charge to escape before the exhaust port fully closes. This reduces efficiency.

Direct Fuel Injection

DFI outboards deliver better fuel efficiency than conventional two-strokes by injecting fuel directly into the combustion chamber. By not introducing fuel until the exhaust port is closed, emissions are reduced significantly.
Direct-injection adds complexity, however. In a conventional EFI outboard, the fuel system operates at approximately 35psi, and this pressure is developed and maintained with an electric fuel pump. DFI systems operate between 80 and 1000psi and require a mechanical pump to achieve the necessary injection pressures. DFI outboards are lubricated by automatic oil-injection systems similar to those used in conventional two-strokes.
There are currently three types of DFI: low-pressure injection, high-pressure injection and solenoid injection. In a low-pressure system, such as that used on Mercury's OptiMax outboards, a pressure-regulated electric pump delivers fuel to the fuel rail, while a separate belt-driven pump generates air pressures of 80psi that mix with the fuel in a dual-stage injector. This fuel/air mix is injected into the combustion chamber after the exhaust port has closed. The purpose of premixing fuel and air is to provide a fine mist that promotes complete fuel burn. This improves economy while reducing emissions. An advantage to this system is that injection pressures are relatively low, which eases fuel-handling problems under the cowl. The disadvantage is added complexity and plumbing.
A high-pressure DFI system, such as that used by Yamaha's HPDI models, relies on a belt-driven fuel pump to develop pressures up to 1000psi. The advantage to extreme pressure is that it promotes more efficient combustion from 2500 to 5500rpm. A disadvantage is the engine uses a richer fuel mix at idle, resulting in slightly higher emissions. To offset this, at slow speeds the outboard drops from six to four cylinders, which results in a slightly edgier idle.
Solenoid direct-injection, as used on Evinrude outboards, uses an electro-magnetic solenoid at each cylinder. An electric pump feeds the fuel rail at approximately 30psi. The solenoid has an armature core and when the coil is energised, the armature strikes a spring-loaded ball that hammers fuel into the cylinder. Injector pressures operate around 500psi. The advantage of this system is its simplicity. A disadvantage is that the injectors require considerable electrical power to operate, which cuts into the alternator's ability to feed electrical accessories and recharge the battery.

Four-Strokes

Compared to two-strokes, four-stroke outboards are relative newcomers.

Although this outboard technology dates back to the 1960s, it wasn't until 1998 that the 100hp plateau was reached. Since then, four-strokes have been added to virtually every outboard manufacturer's stable.

While four-stroke outboards offer smooth idling, quiet performance, excellent economy and reduced emissions, they also have some downsides.
A four-stroke is more complex than its two-stroke counterpart. Because each cylinder fires only half as often as a two-stroke, it is more challenging to pull horsepower from a small-displacement block. As a result, four-stroke outboards are physically larger and heavier for a given horsepower. To regain some volumetric efficiency, they generally have a higher wide-open-throttle rpm range and cost more to purchase.
Four-stroke technology is often perceived as having a durability advantage over a two-stroke outboard. However, both engines will have equal lifespans if properly cared for.
Servicing a four-stroke is more complicated, as there are valves to adjust and oil to change. A four-stroke outboard has intake and exhaust valves just like an automotive engine. It does not rely on oil injection to lubricate the crankshaft and connecting rod ends as do two-strokes. The crankcase isn't pressurised, and lubrication is by way of a recirculating oil pump. Unlike two-strokes, oil is not burned in the combustion chamber; instead, it must be changed periodically, similar to a car engine.

Your Guide to Outboards

Your Guide to Outboards: Chapter 2

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