Four-stroke Fuel Efficiency
The Age
Friday October 28, 1994
FOUR-stroke outboard engines are not new but Mercury has introduced a couple of new ones for the coming season, a two cylinder and a four cylinder.
Cost has been one of the handicaps of four-stroke engines, the extra whirling parts in a four-stroke adding to their manufacturing and component costs.
The international marketing vice-president of Mercury, Mr Roger Miller, launching the new models, estimated that the extra cost at the retail level to be ``between 20 and 25 per cent", which means about $6000 instead of $5000.
For the extra cost, the buyer gets several advantages, the first being lower fuel use. Mercury says it does not have comparative figures between its 37.3 kW two-stroke and four-stroke models. You can be sure the sums have been exhaustively worked on in Mercury's research and development department.
There is a rough graph which indicates that the fuel saving would be about 20 per cent, although in some circumstances the fuel use would be down by at least a third.
There have been marked advances in two-stroke technology with better ignition and fuel injection, so the fuel use of two strokes has been considerably reduced.
The four stroke's other big advantage is its smoothness. It is inherently a much smoother engine since it has more even firing impulses and it also runs more quietly.
Trying the four-cylinder four stroke at Carrum Water Sports Centre, it was possible to wonder if the engine was running or not when it was idling in neutral.
Four strokes are also quieter when running - an environmental benefit that adds to the comfort of those in the boat.
The first of the four-stroke, four- cylinder engines in an outboard was the old Bearcat sold eventually by Homelite. It was a single- overhead- camshaft four cylinder that did not run too well when canted for shallow water. The new Mercury, which also has a single overhead camshaft, runs as well as a two stroke when angled for shallow water drive because of a deep sump in the outboard leg.
The main benefit of the four stroke, however, is that it will run smoothly all day at idle, an important point for houseboats and professional fishermen.
The maintenance on the four stroke is complicated by replacing the belt drive to the camshaft once a year or every 1000 hours, but oil changes are simple enough.
Output is nothing special at 37.3 kW from 935 cc at 5500 rpm. Car engines of similar capacity, with the same older two-valve-a-cylinder technology, used by Mercury, develop more power.
Power use in a car is more transient than in a boat, however, explaining Mercury's more conservative engineering.
© 1994 The Age