Solutions - Engine Technologies
The basic strength of the two-stroke engine is that every second stroke is a power stroke. Only one in four strokes is a power stroke in a four-stroke engine. This fundamental difference provides a significant power-to-weight advantage for the two-stroke engine.
Two-stroke engine are ideally suited to conventional snowmobiles. They provide the high power output and light weight necessary for driving a tracked vehicle through heavy snow, and they start easily in temperatures as low as –40°C.
The two-stroke engine does not need a camshaft to operate intake and exhaust valves, nor does it require complex valves.
In the two-stroke engine, the piston itself, moving past intake and exhaust ports in the cylinder wall, acts as a kind of sleeve valve. Lubrication is provided in vaporized form with the oil injected into the fuel, rather than from an oil reservoir and pump system. Its light weight and simple design combine to provide an exceptionally low manufacturing cost. It is also relatively easy to maintain and repair. These virtues have made two-stroke engines the near-universal choice in scooters used in road transportation.
Power-to-weight advantages and responsiveness of performance have kept two-stroke engines popular for recreational uses in racing motorcycles, personal watercraft, recreational boat engines and snowmobiles.
But when emissions are considered, the only advantage for the two-stroke engine is lower production of oxides of nitrogen (NOx) compared with four-stroke engines. It also has disadvantages.
In the two-stroke engine, exhaust gases are forced out of the cylinder in the same stroke that admits the air/fuel/lubrication charge for the next ignition and power stroke. The use of the incoming charge to help expel exhaust gases is a turbulent and imperfect process, resulting in two undesirable consequences:
- Retention of a portion of the exhaust gas in the cylinder, diluting the fresh charge and leading to ignition problems.
- A portion of the air/fuel/lubricant charge escapes directly to the atmosphere with the combustion products, producing poor fuel economy and releasing high levels of hydrocarbons as air pollutants. This phenomenon is known as "short circuiting."
In addition, two-stroke engines use lubricants that are mixed with the fuel, and normally produce particulate emissions as they are burned with the fuel. So, in addition to direct release of lubricant by short-circuiting, burned lubricant creates additional emissions.
Because of the need for exhaust expansion as part of the tuning of two-cycle engines, noise levels are higher for two-cycle than for four-cycle engines. Expanding the size of the port can provide a degree of freedom in exhaust management, and modern systems show some improvements in noise levels, as compared with earlier systems.
These weaknesses may be fixed by various improvements but cannot be entirely overcome with known technology.