What is the advantage of a 2 stroke engine?

Question

I used to ride my Dad's old 1972 Kawasaki 750 3 cylinder motorcycle. It was surprisingly quick for it's weight and age. It was a 2 cycle. What are the advantages and disadvantages of a 2 stroke compared to a 4 stroke? Are they more powerful or something? Thanx!

Answer 1

Pro's:

Lighter weight

More horse power per cc

Easily rebuilt top end

Con's:

Pre mix or injector ,2 stroke oil

Shorter top end life

Erratic power band and torque as compared to a 4 stroke.

Answer 2

2 stroke they a snap yer but if you run it out of gas you will blow the engine 4 stroke you can run them out of gas and have no worrys

Answer 3

Are they more powerful or something? Yeah, they are.  The advantage of two stroke is that it gives you more power.  A 'stroke' in an engine is everytime the piston goes up and down.  A "power stroke" is the stroke when the gasoline in the cylinder fires, and produces power, which is transferred (eventually to the wheels).  In a "two stroke" engine, every second stroke is a "power stroke" (where the piston is burning gas and making power) and the other stroke is an "exhaust stroke" where the burned up gasses are cleaned from the cylinder.  On a "four stroke" bike, every fourth stroke is a "power stroke" and the three other strokes are "exhaust strokes".  So a two stroke will be more powerful because it is firing twice as many times as a four stroke. To figure out the power of a two stroke relative to a four stroke, multiply a two stroke engine by 2.  So your dad's (incredibly cool because you can't get it anymore and I bet it's fun to ride) 3 cylinder 750 cc 2 stroke, puts out as much power as a 1500 cc 6 cylinder 4 stroke engine.  Yeah, that's why it is surprisingly quick!

The big disadvantage, and why you can't really get them anymore is because they pollute.  The three other strokes in a four stroke engine are to reduce emissions.  Most countries and states in the USA have banned new two stroke bikes.  Even in California, two stroke bikes OFFROAD dirtbikes are not allowed to ride certain times of the year (we have the "green sticker" and "red sticker" thing).  

The other disadvantage is that on a 2 stroke, you have to mix the oil in the gasoline, and the engine will burn oil. (Another reason they are big polluters.)  The newer two strokes have a separate resevoir for oil, and the engine automatically mixes it for you.  But a two stroke engine is still designed to burn oil, more than a 4 stroke.  

Answer 4

The other answers listed either do not answer the question or they are basically incorrect. Here it is from a mechanic:

4-stroke engines have 4 strokes: intake, compression, power, and exhaust. The crankshaft only gets 1 power stroke for every 2 revolutions.

2-stroke engine has 2 strokes. Besides what is going on inside the cylinder, it also relies on the opposite pressure inside the crankcase. Think about this- as the piston goes down the cylinder becomes larger. At the same time, the crankcase becomes smaller. The rising pressure inside the crankcase is used to blow the burnt gasses out of the crankcase. Now back to the explaination....

The first stroke begins with the power portion about half way down, until the piston clears the openings in the cylinder wall. Then, after the openings are exposed, the crankcase pressure enters the cylinder to begin the exhaust portion of that downward stroke. As the piston begins the upward stroke, the fresh fuel and air enters the cylinder through a passage on the opposite side. About half way up the cylinder the piston covers the passages, and the compression portion of the upward stroke begins. So yes, there is a power stroke every time the piston goes down in the cylinder; twice as many as in the 4-stroke.

Another very important point to consider is that there are no valves in a 2-stroke. The job of the valves is accomplished by the passages on the sides of the cylinder. Valves must open and close, therefore they must change directions, completely stopping and starting, with each cycle of the engine. Now consider what CLOSES the valves. The valves are closed by springs, which have limitations on the speed they can do their job. The faster a valve has to work, the less efficient it becomes. That is why 2-stroke engines have much better performance at higher RPMs.

Removing the valves, cams, and chains or gears that drive them, and you also lase weight and the horsepower it takes just to run the valve system.

I hope you enjoy reading my response as much as I enjoyed writing it!

~Vulcan

Answer 5

two stroke more directly fast in short track or if u ride with 2 stroke engine in traffic city

Answer 6

Your Answer:

One advantage is that they do accelerate faster in a very short distance, which is one reason free style guys use them. They generally have a very short run at the ramp in smaller venues. They are also lighter, but not by a lot, another reason FMX guys prefer them. Some pro level woods riders also like the 2 stroke because you can quickly loft the front end to get over obstacles that tend to come out of nowhere in woods events. Now, concerning motocross...2 strokes are at a huge disadvantage in motocross. It is rare to even see a 2 stroke larger than an 85cc racing anymore. Although they accelerate faster in a very short distance, a 4 stroke (that races the same class obviously) will eat the 2 stroke alive on the hole shot. Another issue they have is lack of traction. The abrupt power delivery of a 2 stroke engine is not conducive to proper traction. While the 2 stroke is breaking loose and all over the track coming out of a corner the 4 stroke is hooked up and moving forward, spinning tires don't win races. It does take a 4 stroke of about twice the displacement to compete against a 2 stroke, but who cares. That is the way the AMA rules are written so it is not an issue. Just one clarification on this whole polution thing. People do not race 4 strokes to be eco friendly, they race them because they want to win. Yes, 4 stroke motocross bike were developed to have better emissions standards, but what peolple found out as a result of this was that the 4 stroke made a better race bike. Also there are still almost as many 2 strokes available now as there ever were. A couple companies are dropping their 125 MX bike because it cannot compete with the 250F and folks just aren't buying them, it has nothing to do with the EPA at this point. There have been no laws passed as of yet banning the 2 stroke motocross bikes from racing.