Still Hammering on the 2-4 Cycle Thing

[Texasmark]

Ok, have both engines. Both run about the same rpm's and both have about the same cubes....per the sales brochures.

Torque x rpm's and a little fudge factor = HP.

So if the 2 smacks every rev and the 4 smacks every other rev, and both have the same cubes and rpm's something has to be different.

Ok, my guess is that the 2's torque crapps out half way down the stroke when the exhaust ports get uncovered by the piston. The 4 goes all the way down with it's bang.

So to get the same thing with half the bangs, the torque on the 4 must be 2x the torque on the 2.

Any takers?

Mark

 

[JB]

Re: Still Hammering on the 2-4 Cycle Thing

Sounds right to me.

Two strokes can only use half to 3/4 of the power stroke to make torque, but they have twice the power strokes of a 4 stroke at the same rpm. Sort of a wash.

 

[Paul Moir]

Re: Still Hammering on the 2-4 Cycle Thing

Scavenging efficiency, the percentage of exhaust gasses removed from the cylinder by the intake charge, is no where near 100% too. However, the effective exhaust gas recirculation really does improve the NOx emissions.

 

[itstippy]

Re: Still Hammering on the 2-4 Cycle Thing

You are correct that if a 2-stroke and a 4-stroke produce the same maximum horsepower at the same RPM's then the 4-stroke is producing twice the torque per power stroke. But it only has half the power strokes. The overall torque of the two motors is identical.
The definition of "torque" is where the debate lies. The formula: (Tq X RPM) / 5252 = HP is a generally accepted way to calculate "torque". But to measure torque this way you need to have RPM's. And you can have lots of torque without any RPM's. Torque is supposed to describe POTENTIAL horsepower. Put a torque wrench on a stubborn nut and give 'er heck. You can apply 200 ft/lbs of torque onto that dang stubborn nut without moving it one bit. Using the formula above you have "zero" torque. Nuts to that!
A "torquey" motor is capable of producing lots of horsepower early in its RPM range. It will accelerate a boat or a car quickly because it WANTS to go faster. A 2000 lb dragster, early in its run, throttle wide open, is going 40 MPH at 1500 RPM. Not very impressive measuarable horsepower there. But the torque, the potential horsepower, is awesome. It WANTS to accellerate, to increase its RPM's. You could hook ten plows behind that dragster and it would pull them at 40 MPH and 1500 RPM.

 

[Texasmark]

Re: Still Hammering on the 2-4 Cycle Thing

Well, thanks guys for the comments.

it, I hear you on "potential energy". Funny outboard literature seldom if ever talks about torque ratings at certain rpm's like tractors, cars and trucks. But we all know that you have to have it to get the HP rating at that rpm.

Also, my experience with engines is that the higher the cu in displacement for a given function, the higher the torque; especially when stroke length is considered.

So back to Merc increasing their cubes from 2 to 2.5 liters on their bass boat engines it was (my opinion) to increase torque to help spin up the prop coming out of the hole for a faster hole shot.............course vent holes in the prop will do that too........yes?

Enjoyed the exchange.

Thanks,

Mark

 

[itstippy]

Re: Still Hammering on the 2-4 Cycle Thing

It's possible that increasing the displacement was part of a Merc engineer's design to boost low-end torque. I don't know, and as you say they don't publish torque curves. Simply increasing the bore and/or stroke without redesigning everything else won't get you much though. It's always a trade-off. No free lunch. Increase the torque at low RPM's and lose some at the top. You'll see boats with crummy holeshots become liquid fire once the RPM's hit the sweet spot. Watch the "pad runners" on a calm lake. Some of them stumble at the get-go, kinda fumbling along like a gooney-bird trying to get up to flight speed. But then the magic happens. The tuned exhaust hits its effective RPM level, the timing hits its stride, and the hull starts to lift. It's like the thing got kicked in the asp. It's in its high torque RPM range at last, and it accellerates like a demon. Those things are made for top end.
A 4 cycle is not inherently more "torquey" than a 2 cycle. It's all in how the engineers choose to distribute the torque range. No free lunch.

 

[lowe160]

Re: Still Hammering on the 2-4 Cycle Thing

Also you have to take into account the final gear ratio which will give lots of low or high end torque? The dragster's want out of the hole fast where Nascar wants topend. Just a thought.

Wayne

 

[itstippy]

Re: Still Hammering on the 2-4 Cycle Thing

The motor's (engine's) torque output does not change if you go to a higher or lower gear ratio. You adjust the gear ratio to take advantage of the RPM's where the motor produces the most torque. If a motor puts out its best torque at 3000-5000 RPM's and you want to pull stumps with it you gear it low so that you do your stump pullin' in the 3000-5000 RPM range. If you intend to use the same motor to pass other cars at 70 MPH you gear it higher so you're passing cars in the 3000-5000 RPM range. The motor's torque range doesn't change.

 

[Texasmark]

Re: Still Hammering on the 2-4 Cycle Thing

This just keeps getting better. I'm going to start a new post on gear ratio.

Thanks for your replies.

Mark