The Basics of Motorcycle Engines
ByIn the old days, there were three basic types of motorcycle engines: four-stroke, two-stroke and rotary. Alas, the two-stroke is all but gone for street bikes. Stricter noise and emission regulations have made the two-stroke uneconomical or impossible to manufacture for street use. This apparently has changed manufacturers’ engine choices for most of the other street/dirt bikes in their line-ups as well. On a recent trip to the bike shop, two-stroke engines were found only on jet skis, snowmobiles and some motocross racing bikes.
The rotary engine never caught on, and although in production, it is almost extinct. Poor marketing strategies and rider unfamiliarity caused the rotary’s unpopularity. The last producer, Norton, fell victim to Thatcherism, the leveraged buy-out greed of the ’80s, and Reaganomics. It seems that only the four-stroke is able to survive decade after decade.
The name stems from the fact that the combustion happens internally. This is contrasted to, say, an old steam engine where coal was burned externally beneath a closed tank of water to produce steam power. The first internal combustion engines were built in the 1820s, but the concept of combustion under high pressure was not theorized until 1838 (by William Barnett). In 1876, the German firm of Otto and Langen began producing a “silent engine,” based on Alphonse Beau de Rochas’s 1862 theory of a four-stroke engine. This was the first modern four-stroke engine. Hence, the four-stroke engine is often called an Otto engine. Otto’s engine, like most of the era, burned coal gas.
In 1878 Dougald Clerk developed a two-stroke engine. Clerk’s design used a secondary piston and cylinder that would take in a charge of fresh fuel vapor. It would pass this charge to the working cylinder, scavenging the working cylinder of burned fuel and providing fresh fuel. It was not until 18 91, when Joseph Day simplified the two-stroke engine by using the crankcase to perform the duties of the secondary cylinder, that the modern two-stroke was born. In Day’s design, the fresh charge is drawn into the crankcase, rather than a secondary cylinder.
To round out our discussion of engine history, in 1892, Rudolf Diesel patented an engine in which very high compression (V^5th of original volume) resulted in high enough temperatures (538° Centigrade) to ignite fuel sprayed into the cylinder. This is, of course, the Diesel internal combustion engine.
Internal combustion engines are wonderfully simple in concept. An internal combustion engine requires three things to run: air, fuel and spark.
- Air – A colorless, odorless, tasteless gaseous mixture, containing 78% nitrogen, 21% oxygen, with small amounts of argon, carbon dioxide, neon, helium and other trace gases.
- Fuel – Usually gasoline or gasoline mixed with oil or alcohol. Fuel can also be more exotic. Internal combustion engines run on diesel, propane and other combustible substances as odd as fumes from decomposing chicken droppings (methane). The best economy is obtained when one part fuel is mixed with 17 parts air (notated as 1:17). For power, a richer 1:12 mixture is required. For n. cold engine, an extraordinarily rich mixture is required.
- Spark – In gasoline engines, detonation is caused by a 20,000 to 30,000 volt electric spark. The spark plug provides a gap between electrodes across which the high tension voltage jumps, which is the spark. Detonation is caused by the heat of extreme compression in diesel engines.
These are combined via compression to produce an associative event called combustion, which produces power, as defined below:
- Compression—Usually attained by pushing a piston up a closed-ended tube (i.e., the cylinder) toward the closed end (a.k.a. combustion chamber) to squeeze the fuel and air to a density that will permit detonation via spark.
- Combustion—The rapid oxidation of fuel, or the conversion of gasoline and oxygen into carbon dioxide and water.
When all these factors are present in the proper quantities, and delivered at precisely the correct time, you have a running internal combustion engine. Of course, the engine won’t run for long if you do not have other systems dealing with many residual by-products of the detonation, such as a cooling system for excess heat, a lubricating system to keep metal parts from grinding themselves into shavings, an exhaust system to control noise and fumes, etc. It is all quite clever.
Knowledge of air, fuel and spark is an important trouble shooting tool. A motorcycle mechanic will begin a dead-engine diagnosis by searching for air, fuel and spark. Finding all three operating correctly, a check for compression comes next. The absence of one of these is an immediate clue to what is ailing the engine, and directs the mechanic where to focus his attention.
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