How a Looper Works va. Cross-Flow

[Skools Out]

TWO STROKE DEFINED
The two-stroke engine completes its power cycle in only one crankshaft revolution with two strokes of the piston. There are no valves, camshafts, springs chains, etc. so the engine is much less complex and lighter. Instead of valves There are a series of strategically located transfer ports - intake and exhaust, cut into the sides of the cylinder wall. The ports are on opposite sides of the cylinder. The transfer ports are opened and closed by the up and down movement of the piston. To accomplish a complete power cycle both sides of the piston are used; consequently several events occur simultaneously during each stroke. They are:

Up Stroke - Intake and Compression:
On the up stroke the top side of the piston is compressing an air/fuel mixture in the cylinder. At the same time the BOTTOM side of the piston pulls another fresh charge of air/fuel mixture into the crankcase thru a one way valve called a reed valve. Near the top of the stroke the compressed air/fuel above the piston is ignited by the spark plug and begins to burn. The rapidly burning fuel expands and begins forcing the piston down.

Down Stroke - Power and Exhaust
On the down"power"stroke the piston is forced towards the crankcase reducing its volume and creating a positive pressure. As it continues downward travel it starts first to uncover the exhaust ports. Exhaust gas begins to rush out of the cylinder. Then the intake ports are uncovered. The fresh air/fuel charge in the crankcase is forced into the cylinder and continues to push the remaining exhaust gases out.

The 2 stroke process of purging exhaust gases from the cylinder and filling it with a fresh air/fuel charge is called scavenging. Two stroke engines use 2 different scavenging methods, cross-scavenging and loop scavenging. Both differing designs have particular advantages.

TWO STROKE CROSS-FLOW
two stroke cross-scavenged engines can be identified by the irregular shape of the top of the piston called a deflector. This deflector directs the incoming air/fuel up, towards the top of the cylinder. This creates a wall or column of fresh mix that sweeps across the cylinder towards the exhaust ports. As the column advances it pushes the spent exhaust gases out of the exhaust ports.
See Picture Below Hope this makes since to all and helps all.



TWO STROKE LOOPER
Pistons in loop scavenged engines are generally near flat. They do not rely on deflectors to aim the fuel/air mix, rather they have shaped intake ports and combustion chambers to control the scavenging of the cylinder. Several intake ports are aimed upwards and arranged such that their combined streams flow upward and then LOOP down toward the exhaust ports.

Cross-Flow engines are better performing at idle and low speed. All older motors of any horsepower are of this design. Until the late 60's it was not economical to try to produce this design in quantity at a reasonable cost.

Looper engines, although having poorer idling characteristics are more fuel efficient and perform better at higher RPM's than crossflow as they have lighter pistons. This lowers the strain on the connecting rods, bearings and crankshaft. OMC created the first US production looper in 1968 with the 3 cylinder 55HP.

[macojoe]

Who Cares!!

As long as its running!! :-/

How do you know whether an engine is a looper or a crossflow without killing it forever or asking somebody who knows? I say killing it forever because of my glorious engine rebuilding attempt on a honda civic, which ended in lots of spilled gasoline and my parents kicking me out of the garage.

[Skools Out]

look at the piston the loopers have flat pistons and cross-flow have that crazy looking piston with a high slope on one side just look up your motors piston as if you were going to rebuild it and that way you can know without removing the heads.

[kamikaze]

Skools:

Is it true that the carbs can also ID a crossflow from alooper. I have been told ganged / dual throated carbs indicate crossflow and single carbs per cylinder indicate looper? Any truth or just a wise tale?

Kamiakze

[Skools Out]

that's true as well in OMC not in some others

[Skools Out]

for the questions on loopers here's ya some info

[THEFERMANATOR]

The short story is a crossflow uses ports directly opposed and uses a deflector shaped dome piston to direct the incoming and outgoing charges. Loopers use a flat top or nearly flat topped piston and uses the port locations to direct the incoming and outgoing air charges. Since loopers rely on airflow to move teh air where it needs to go, they are less efficient at low RPM's where air speed is lower. As teh RPM's increase though, air speed increases and allows teh porting to work as designed in a more efficient manner. Cross flow engines with there design utilizing the piston shape is more efficient at low RPM's than a looper is. Cross flow engines are normally used in applications where they need alot of bottem end torque and good idle characteristics like smaller kicker motors and such. OMC really changed the game when they came out with the first loopers, and revultionized outboards at the time.

Then there is the type of porting used in many chainsaws, but I won't even go into that one.

[smokeonthewater]

can't help ya on the carb question but fyi it isn't a "wise tale". The term is "wife's tale"

as for how to ID the design..... Just the other day I was shining a light into the spark plug holes on my merc and looking at the ski slopes on the pistons

[THEFERMANATOR]

Originally Posted by smokeonthewater

can't help ya on the carb question but fyi it isn't a "wise tale". The term is "wife's tale"

as for how to ID the design..... Just the other day I was shining a light into the spark plug holes on my merc and looking at the ski slopes on the pistons

Most loopers will not have removeable covers over the intake ports whereas every crossflow I have seen will have a removeable cover over the intake ports. Also crossflow engines tend to have a smoother idle with a slight rattle sound to them whereas a looper will have a lope to it like it has a big camshaft(unless it is a direct injected engine).