Most modern two-strokes require 89 octane gasolines to prevent detonation. And the fuel should not be more than a few weeks old. As gasoline ages, the light hydrocarbons evaporate, leaving varnish and gums behind. A fuel stabilizer, such as Briggs & Stratton Fresh Start, will preserve gasoline for as long as 24 months. However, stabilizers [...]
Near the end of the eighteenth century, James Watt observed that a mine pony tethered to a turnstile could lift 550 lb one foot per second, or 33,000 lb per minute. Horsepower was a brilliant sales tool that put steam engines into a context that potential customers could understand. In metric notation, one horsepower equals [...]
The compression ratio (CR) describes the amount of “squeeze” the piston applies to the air-fuel mixture prior to combustion. It is arrived at by dividing total cylinder volume, that is, the volume with piston at BDC, by the volume that remains when the piston rises to TDC. The latter figure is the clearance volume. Normally [...]
We class ships by tonnage, houses by square footage, and engines by the volume the piston displaces as it moves between centers. All things equal, an engine should develop power in proportion to its displacement. Displacement = bore × bore × number of cylinders × stroke × 0.7858 For example, the Tanaka series TBC-2501 has [...]
Current practice, based on work carried on in Germany during the 1920s, is to use loop, or Schn_rle, scavenging. Multiple transfer ports are arranged around the cylinder periphery with their exit ramps angled to impart swirl to the charge (Figs. 1-8 and 1-9). The miniature cyclone fills the whole combustion chamber, sweeping exhaust gases out [...]
Readers with long memories may recall the deflector pistons that were once standard ware on these engines (Fig. 1-7). The fuel charge enters through a single transfer port, rebounds upward off the deflector, and drives residual exhaust gases out the exhaust port. While this design works well at moderate speeds, at high speeds, the deflector [...]
Short-circuiting is the term for the way incoming fuel escapes out the exhaust, as if one were trying to fill a leaky bucket. Most of the leak can be laid to symmetrical timing. Because piston motion controls port timing, the timing is symmetrical around BDC. For example, an exhaust port that opens 60 crankshaft degrees [...]
When a high-pressure wave encounters a solid obstacle or an abrupt change in direction in the exhaust plumbing, it rebounds back to the exhaust port. These waves oscillate at the speed of sound and at a frequency determined by engine rpm. Where space permits, the length of the exhaust system can be tuned to reflect [...]
As the piston falls, it first uncovers the exhaust port and then, 5° or 10° of crankshaft rotation later, the transfer port. Blowdown occurs during this brief period that, at wide-open throttle, occupies no more than one or two thousandths of a second. In spite of its brevity, the blowdown phase is the primary mechanism [...]
Although third-port engines are still encountered, many manufacturers prefer to control crankcase filling with a reed valve installed between the carburetor and crankcase. The valve, similar to the reed on musical instruments, opens and closes in response to crankcase pressure (Fig. 1-2). Utility engines make do with a single reed, or pedal, athwart the intake [...]
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