Why don't two cycle / 2 stroke bikes explode?

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Now, i understand with two cycle engines gas and oil vapor enter the crankcase, and considering this is so close to a possible source of ignition and heat or possibly crankcase blow by, it makes me wonder why 2 stroke crankcases don't explode
 
I know its not unheard of for some of the combustion flame to enter the crankcase of an engine as some engines experience explosions from hot oil mist that way, and im pretty sure you wouldn't need compression for it to explode as if you out any combustible vapor gas in an enclosed space it can cause an explosion
 
Bad reeds, lean mixture when often get you a sneeze out the carb, but I've seen more 4 strokes have flaming carb backfires from bad timing.
 
Probably very rich on the fuel side and not so much on the oxygen side in the crankcase too.
 
Study the 2-cycle engine design and it will likely make more sense to the way they function.

There are three types of 2-cycle design:
- reed valve
- piston port
- rotary valve

Each of these designs control the induction of the air/fuel flow differently. Just as it's possible to have a intake fire on a 4-stroke due to backfire/misfire it's also possible to have a intake fire on a 2-stroke. It's usually caused by incorrect ignition timing or faulty reeds/rotary valves. Piston port engines are far less likely to suffer such malodies.
 
Every two stroke engine must have a way to pump the air into the combustion chamber against the positive pressure of the remaining exhaust in there. The more complex 2-stroke engines use a mechanical blower sometimes augmented by a turbocharger. Simple 2-strokers use the piston underside as the pump. Here the crankcase for each cylinder is sealed separate from the crankcase spaces under the other cylinders (multi cylinder outboards a an example, as well as 2 cylinder motorcycles).

The simple gasoline 2-stroke engines mix the gas with the air, pull it into the crankcase when the piston goes up, then push the air & fuel charge into the combustion chamber when the piston goes down. There is no heat source in the crankcase hot enough to ignite the fuel & oil mixture.

Very large two stroke diesels (getting up to almost 9,000 hp per cylinder) use multiple turbochargers, with electric blowers for slow speed. The piston underside is still used as an additional air pump getting the air from the turbocharger & after cooler.
 
In spite of what Hollywood would have you believe,igniting a gasoline/air mix is not that easy.The ratio has to be just right.
pour some gas on the ground and try to light it by throwing a lit match at the puddle.Most of the time the match will hit the fuel and go out.
 
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