Two-stroke Conventional Wisdom


Everyone in the marine industry and most of its environmental critics are aware that there are now at least four manufacturers of two-cycle outboards with DFI, direct fuel injection: systems that put the fuel directly into the combustion chamber after the intake and exhaust ports close. Such systems eliminate loss of some of the incoming fuel charge out the exhaust ports along with the scavenged products of combustion that occurs with carbureted or EFI manifold injection systems. Nevertheless, conventional wisdom tells most of the critics of the two-cycle engine that it will never be as clean as a four-cycle engine. For this reason they would eventually advocate banning the two-cycle engine from the waterways on environmental protection grounds.

There are even more manufacturers producing four-cycle outboards, including the same manufacturers that make the DFI two-cycle engines. They must seemingly follow down both roads for self preservation, as part of the outboard market is definitely leaning in the four-cycle direction, driven that way by hype, environmental concerns, and certain perceived advantages. We have already considered the ramifications of the increased engine weight for the four-cycles, the potential effect on boat trim, and the possible inability to float the boat level when swamped, as required by federal regulations for outboard boats less than 20-feet long. Then there is also the increased cost and complexity involved with four-cycle power, to be offset by savings realized in fuel consumed and elimination of smoke and oil slicks.

This may be the price of progress, they say. But, is it possible to "have your cake and eat it, too?" Some recent tests run comparing 2002 model two-cycle DFI outboards with four-cycle outboards of equal power rating, mounted on the same boat, would seem to indicate such things are really possible. Comparison tests of two brands of four-cycle 225-hp outboards were made with a current state-of the-art DFI two-cycle 225. On identical 20'7" boats one four-cycle brand produced a best mileage of 4.7 mpg at 27.7 mph while the two-cycle gave a best 4.5 mpg at 28.6 mph. Very close. But, the two-cycle had a top speed of 59.8 mph against 52.4 mph for the four-cycle. At the same 52-mph speed the two-cycle gave better mileage to the tune of 3.2 mpg to 2.7 mpg for the four-cycle. The two-cycle produced better fuel mileage at every speed from 34 mph up and was also better at trolling speeds of 4-7 mph.

When tested against the other 225-hp, four-cycle brand on identical 24' boats, the DFI two-cycle again prevailed overall, delivering a matching best 3.15 mpg at 32 mph. This outran the four-cycle 49.3 to 45.7 mph, getting better mileage (2.58 mpg) at its top speed than the four-cycle (2.44 mpg) at its top speed. It also produced far better mileage in the trolling speed range from 3.5-8 mph.

A third set of tests compared a 135-hp, two-cycle DFI outboard against a 130-hp, four-cycle outboard on identical 20' boats. The two-cycle delivered 4.25 mpg at 20.8 mph against a best 3.97 mpg at 20.4 mph for the four-cycle. Best economy for the two-cycle was achieved at 27.9 mph: 4.45 mpg. It also bested the four-cycle in the 3-8 mph trolling speed range and beat it in top speed 43 mph/3.54 mpg to 37 mph/2.97 mpg.

"Bah, humbug!" you might say. But there are sound engineering internal combustion engine principles for this surprising result. It is true that the typical four-cycle engine may have an inherent advantage in fuel consumed per horsepower. But not when the engine must be designed to produce very high horsepower per cubic inch of displacement at high engine speeds, as it must to achieve even the already heavier weight seen when compared to its two-cycle competitor.

In order to achieve this high-power output, while firing only every other revolution of the crankshaft, the camshaft valve timing must develop considerable overlap between intake and exhaust valve openings and closings, which means it begins to suffer some of the same raw fuel loss out the exhaust problems as the carbureted or manifold injected two-cycle engine. It only has manifold injection, so the fuel and air must mix in the manifold and enter together past the intake valve into the combustion chamber while the exhaust valve is still partly open. The result is Some loss in fuel economy.

Since the four-cycle engine has the same radical valve timing at low engine speeds, it suffers even more when compared to the two-cycle DFI engine at trolling speeds. The only way to fix this problem in the four-cycle engine is to go to direct fuel injection into the combustion chamber after the valves close, like the DFI two-cycle, or have a system providing variable valve timing with engine speed, conservative timing at lower speed and radical timing at higher speed. Such systems are now being developed for future automobile engines. Such things would add complexity, cost and weight, to an already more expensive and heavier product.

Then there is the factor of acceleration from idle to planing speed. On the 241 boat the 225-hp, two-cycle DFI went from zero to 150 feet in 7.06 seconds while the four-cycle took 7.76 seconds. On the 20' boat the 135-hp, two-cycle DFI went zero to 150 feet in 6.2 seconds while the four-cycle took 8.7 seconds for the same distance. Acceleration from zero to 30 mph on the 20'7" boat for the 225-hp two-cycle DFI took 5.77 seconds compared to 10.7 seconds for the 225-hp four cycle. This demonstrates the better low-end torque and fast-rising power curve of the two-cycle, firing every revolution of the crankshaft. The four-cycles are quieter at low engine speeds, but this advantage goes away at the higher engine speeds.

So, the conclusions are that the state of the art two-cycle DFI outboard can match or beat the four-cycle in fuel economy, top speed, and acceleration. What about exhaust emissions, which brought on the whole move to four-cycle outboards in the first place? These two-cycle engines can match or beat the four-cycles there, as well. It matches pretty much with the fuel economy story. The more fuel the engine consumes at a given boat speed, the more exhaust emissions that come out the other end. With precise microprocessor control and direct injection of the fuel into the combustion chamber after the ports close, the two-cycle DFI can better the most stringent exhaust emission requirements now proposed out to 2007. The four-cycle can do no better.

After more than five years of testing and field experience the 2002 two-cycle DFI outboards have been developed to have quality durability, economy and environmental friendliness to match or beat the four-cycles, and at lower weight and cost. Both can exist and be successful in the marine market but no one should sell the two-cycle engine short on its ability to survive and prosper long into the future. It just has too many good things going for it. You might even see it on some future stern drives.

Ralph Lambrecht is an engineer with more than 50 years of experience in the marine industry and marine safety standards development.

Lambrecht, Ralph. 2002. “Two-stroke conventional wisdom.” Boat & Motor Dealer. April. 34-37