>One thing that surprised me is the two stroke engine was invented after the four stroke (Ottoman cycle) engine. It does have advantages like being able to run

>One thing that surprised me is the two stroke engine was invented after the four stroke (Ottoman cycle) engine. It does have advantages like being able to run completely upside down since it doesn't have a sump where the oil needs to drain. They also have a higher power to weight ratio. Its major disadvantage is due to lack of valves is that for a short period of time the intake port and exhaust port are both open allowing up to 30% of the unburned air-fuel mixture to pass right through the engine as pollution. This also makes it nearly impossible to add further emissions controls in the exhaust system such as a catalytic converter, not that its really feasible to due to in such a small engine. Because of this small two-stoke engine like those in a leaf blower can put out much more pollution that a large automobile with a 4-stroke engine with modern emissions controls. There are more advanced two-stroke designs (BRP E-TEC) that use direct fuel injection to avoid this, but they are far too complicated to use in a small lawn tool.