Although plastics have been used in small engines for decades, there has not been such heavy reliance on plastics and polymers in the past such as there is today. Manufacturers faced many marketing problems with the small engine on handheld equipment because their motor units were too heavy and bulky. Marketing departments realized that a significant segment of the population bought inadequate machines from cheaper manufactures because they were lightweight, not because of their quality standards. In other areas, heavier four-cycle engines were underpowered and inefficient.
Engineers turned to plastics and polymers to provide the weight savings that they required, while still providing rigidity and strength in structural components. They sought to strengthen internal parts using ceramics and advanced metals that would also extend the life of the engine.
Designers discovered side advantages over metals in small engine design. The effects of weathering on units that caused corrosion and fatigue were reduced. Safety guards on trimmers that would previously build up with rotting grass and rust away would now remain intact and unaffected. Some new plastic shields proved even stronger than metal with virtually bullet-proof resistance to projectiles. Parts could now merely snap together instead of using superfluous hardware. The most revolutionary design changes, though, occurred in the motor units themselves.
Two-cycle engines have been used in a large variety of applications since their introduction in the late 1800's. Used on everything from wood saws to farm machinery to aircraft, they remain one of the most versatile engines available today. The increased power-to-weight ratio over four-stroke engines makes them more desirable for smaller use on equipment like saws and trimmers. The disadvantages always prohibited use for larger applications however, such as a shorter life expectancy and the latest issue of emissions, which will be examined later in more detail. Two-stroke engines have always been rather popular due to their relative light weight as a result of simple design and few parts.
Although two-cycle engines have been around for more than a century, they have been constructed in more revolutionary and advanced designs only recently. Past designers used either a heavy, reliable design, or a lightweight and unreliable one. Manufacturers have found it difficult to design a long-lasting, lightweight, and efficient two-cycle engine. With the aid of modern polymers and ceramics, engineers are now able to find a favorable balance between all three variables.
Part of the weight-saving solution is the use of high-strength polymers in applications where metal may be replaced. Parts that were formerly stamped from steel sheets are now cast in plastic. Even fuel tanks and caps are completely constructed of polymers. Although lightweight alloys of magnesium and titanium are being introduced in crankcases and pistons, these metals are expensive both to cast and machine.
Because of the light weight and rigidity of modern polymers, nearly all of the external parts on two-cycle gas engines are constructed of plastics. Triggers, switches, guards, shrouds, caps, filters, and levers are all made entirely of some sort of polymer. Select internal parts such as gears, clutches, sprockets, and high-stress parts are sometimes made of ceramics for increased strength and performance. These parts are more commonly a metal coated with a hard ceramic layer to resist wear. This capability adds little or no weight to the engine and drastically increases the lifespan.
The incredible reduction of weight due to extensive use of polymers has made small engines more marketable in the modern economy. Use of advanced materials and ceramics has nearly doubled the lifespan of two-cycle engines. Engines that previously were rated for 600 hours may now last up to 1,200 hours.
The issue of efficiency is addressed by the newest addition of catalytic converters. Though not a complete solution, it is a start to a challenging area of small engine design. With worries increasing about emissions and pollution, efforts are being made to increase the efficiency of engines in differing ways. Most of these methods try to more efficiently control the air-fuel ratio and thus burn most of the potential pollutants that would normally merely be exhausted to the atmosphere.
In larger engines, the way to achieve lower emissions is to work for an almost exact air-fuel ratio where nearly all the fuel is consumed in the combustion process, and further run through a catalyst system to purify the remaining exhaust. Most small engines, if operated at an exact 14:1 mixture, tend to knock, misfire, and overheat. Thus, mechanical modifications must be made to obtain very low emissions output in small engines. Valves and ports can be redesigned or added to reduce scavenging losses, and more refined carburetion can be added. Advanced carburetors like the Intellicarb™ from Stihl® that sense airflow can compensate for clogged or dirty air filters that increase emissions (Madsen’s, 2001). Exhaust treatment through catalysis is becoming more popular, and is now mandatory in California.
Unfortunately, all two-stroke engines burn an oil and gasoline mixture. There is a resultant increased level of unwanted emissions because of the severe consequences involved with burning any type of lubricant designed specifically to withstand the most severe conditions. Because of this, two-cycle engines will never be as efficient as four-cycle engines outfitted with similar environmental protection devices.
The ultimate step is to increase the use of cleaner-burning four-cycle engines in small equipment. Until recently, the major barrier has been the weight restrictions for the added parts such as valves, ports, camshafts, rocker arms, valve covers, and many other small parts that must all be constructed of heavy-duty material. But with the introduction of modern polymers and plastics, many of these parts can be constructed from lightweight materials.
Several manufactures have decided that it is simpler to redesign a small four-cycle engine unit using lightweight materials than it is to keep adding devices and weight to two-cycle units to decrease emissions. These companies have relied heavily on polymers for a lightweight and compact design that can compete with two-cycle engines for the upper level of the small engine market on handheld equipment.
Leading manufacturers such as Ryobi®, Honda®, and Stihl® produce lightweight four-cycle engines with the heavy use of plastics and ceramics. All of the non-critical parts are cast or formed from polymers. In applications such as catalytic converters and select other areas, ceramics are making an appearance. Although these engines still weigh slightly more than the two-cycle versions, they are more powerful, reliable, longer-lasting, and have lower emission levels.
In the larger equipment market, four-cycle engines are being redesigned to take advantage of new stronger metal and ceramic technologies in machines such as tillers and lawn tractors. Pistons and piston rings are made from stronger alloys that resist wear. More powerful engines are flooding the market as efficiency of design allows greater performance increases without serious side effects like extra weight from cooling systems.
The body panels of lawn tractors are now designed from lightweight plastics, while newer mower decks like Honda’s® Xenoy™ composite decks are revolutionizing push mowers. Inside the engines themselves, camshafts and gears made from high-strength polymers operate the valve trains. Polymer gaskets seal the engine covers and bolt recesses. Ceramic coated parts resist wear longer than the bare metal they replace. Flywheel vanes, handles, levers, switches, knobs, recoil assemblies, air filters, fuel filters, hoses, shrouds, and many other myriad parts are all made from plastics (GE Plastics, 2003).
As the limits of technology expand, the potential for the small gas engine expands with the lighter and stronger materials that become available to a constantly improving marketplace. High quality manufacturers have managed to become competitive in the lightweight equipment market because of advanced polymers and ceramics that provide maximum performance at minimum weight.
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