Is there such a thing as an environmental engine? Can something that emits exhaust gas, CO2 and noise while depleting fuel resources ever be described as ‘environmental’?

The answer is a qualified ‘yes’. Engines cannot be described as environmentally friendly. Yet there is a clear difference between an engine built with ecological performance in mind and one that is not. As tighter regulations are imposed on engine output it is becoming more important to identify and understand an engine’s environmental credentials.

As the world’s largest engine manufacturer, Honda has an unrivalled moral obligation towards the environment. Honda’s determination to fulfil this responsibility is exemplified in Soichiro Honda’s pioneering decision is the 1960s to only build 4-stroke power equipment engines, including marine engines. As the enlightened Mr. Honda said, “Because the water raises rice and the fishes live in the water, I don’t want to contaminate it.”

In 1964, Honda produced its first marine engine, the GB30, simultaneously inventing the 4-stroke outboard. In 2004, exactly 40 years later, the rest of the world caught on, with the UK selling more 4-strokes than 2-strokes for the first time – a 52% market share. In Europe the 4-stroke bias was more pronounced at 62.9% and the 4-stroke trend continues worldwide.

2-stroke vs. 4-stroke
So why are 4-stroke engines more environmental than 2-strokes?

A 4-stroke engine burns a mixture of air and petrol to drive a piston and rotate a crankshaft. There are four separate strokes – induction, compression, combustion and exhaust. A 4-stroke engine therefore fires on every other revolution of the crankshaft.

A 2-stroke engine burns a mixture of air, petrol and oil to drive a piston and rotate a crankshaft. It performs all the functions of a 4-stroke engine (induction, compression, combustion and exhaust) using only two strokes – combustion and compression. A 2-stroke engine therefore fires on every revolution of the crankshaft.

The differences between the engines give the 2-stroke four clear advantages. 2-stroke engines do not have valves which simplifies their construction, lowering their weight. They are traditionally more powerful than 4-strokes as they fire once every revolution rather than once every other revolution. 2-strokes can be operated in any orientation because they lack the oil sump of a 4-stroke engine, which has limited orientation if oil is to be retained in the sump. Finally 2-strokes are less expensive because of their simpler design.

However, 2-strokes have crucial disadvantages that an increasing number of consumers and manufacturers believe outweigh the advantages. For a start, they are considerably less durable than their 4-stroke counterparts. The lack of a dedicated lubrication system means that the parts of a 2-stroke engine wear a lot faster. Their simpler design makes them fuel inefficient, resulting in poorer mileage than a 4-stroke engine (Honda outboards consume about 50% less fuel than 2-strokes). Refuelling costs are higher still as expensive 2-stroke oil must be added to every tank of petrol. 2-stroke engines also generate a lot of pollution. The combustion of the oil makes the engine smoky while part of the fuel mix leaks through the exhaust port each time a new charge of fuel is loaded into the combustion chamber. This results in the sheen of oil that can be seen on the water around any 2-stroke outboard.