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two stroke/four stroke-which produces more torque?

Posted by indy452 NC Kansas (My Page) on
Mon, Feb 5, 07 at 10:33

I was wondering, do four stroke engines produce more or less torque than a two stroke of similar horsepower?

My brother in law whom sells Honda power equipment says four strokes out torque two strokes, is he just selling his equipment or is he correct?
I like Honda equip just fine but I'll always be a two-stroke kind of guy.
I always thought that the fewer moving parts and the power stroke every other revolution was king.
Any opinions out there?
Neal


Follow-Up Postings:

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RE: two stroke/four stroke-which produces more torque?

I dont know your answer, but I have never been disappointed with the torque of a LawnBoy 2 stroke and been ready to run over plenty of doggy 4 strokes Ive owned. I have had some torque stout 4 strokers too though, like my Robin 6.5 on my Snapper Commercials and a Wisconsin I once had.

All in all though I would take a 2 stroke LB anyday before any typical 4 stroke mower. I mowed with a neighbors Honda once or twice when he was down and I was helping him out. I was not impressed for some reason.


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RE: two stroke/four stroke-which produces more torque?

There is no real solid answer to the question. Both types of engines can be configured many different ways. Both 2 and 4 stroke engines can be made for torque at either high or low rpm. An example of a 2 stroke configured for low speed torque would be a "trials" bike. (Rock crawler) 4 strokes can be made that way as well. Your brother-in-law is trying to sell bikes. Generally the conversations about engines on sales floors of dealerships has little to do with reality.


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RE: two stroke/four stroke-which produces more torque?

These guys are right, that's way too open a question to have a single answer. A few general rules that can be said in comparing the two engine types. First, a two-stroke does generally have less moving parts. Second, it can therefore be lighter and more compact than a 4-stroke of similar power and tractability. Third, it will be much less clean running than a four-stroke. Beyond that there really can't be other generalizations drawn.

To add to what rdaystrom said about sales talk, torque is generally a salesman's topic. Torque doesn't tell you how much work happens at what rate - power does. On a mower, rpm is essentially constant for all 21" models, so that means torque and power are interchangably comparable. If one engine makes 10% more power at 3200rpm than another, it also makes 10% more torque (and vice versa). A 6hp Honda engine makes the same power as a 6hp Duraforce (I'm making up numbers here, not quoting dyno charts). It doesn't matter which cycle each engine produces, each will cut the same number of blades of grass in the same amount of time at their rated rpm.

BTW, as a technical point, Indy452, a two-stroke has an ignition cycle on _every_ revolution, not every other. I'm pretty sure you know this and just mistyped, but just in case... And rdaystrom, I can see your trials bike and trump it for torque and low-rpm power. I'll skip over the Caterpillar diesel engines, and slide by the diesel locomotives that make 3600hp at around 900rpm. I'm going straight to the king: the The Wartsila-Sulzer RTA96-C turbocharged two-stroke diesel engine is the most powerful and most efficient piston engine in the world. It displaces 1,556,002 cubic inches (25,480 liters) and makes 108,920 hp and 5,608,312 lb/ft at 102rpm! It's also incredibly efficient, getting roughly twice the power out of a pound of fuel as a typical car engine. That would cut some grass, but it won't fit in your garage...

Here is a link that might be useful: The ginormous 2-stroke engine


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RE: two stroke/four stroke-which produces more torque?

'diesel locomotives that make 3600hp at around 900rpm'
The GM ones are/were 2-cycles!

Considering we're talking 'apples and oranges' in comparing 2-cycles to 4-cycles, I look at it like this; if the displacements are equal, the 2-cycle has to 'win'. But, the 4-cycle is so much more efficient that the 2-cycle has to be almost as big (rather than half-size) in order to make the same power. My Lawn-Boy 4-cycle 6.5hp Tecumseh is 195cc, and my LB 2-cycle 6.5hp Duraforce is 141cc. My guess is that the Duraforce has more torque at 3200RPM, but it burns a lot more calories to get it.


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RE: two stroke/four stroke-which produces more torque?

"Generally the conversations about engines on sales floors of dealerships has little to do with reality."

rdaystrom, I believe you hit the nail on the head.
He does sell bikes and power equip. It shows how little the consumer really knows about torque and horsepower. With a descent salesman you can sell anything.

Thanks everyone.
Neal


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RE: two stroke/four stroke-which produces more torque?

I believe that generally an engine with a longer stroke will have more torque than one with a shorter stroke. It is all about mechanical advantage. It's possible that a 4 stroke of similar horsepower will in fact have more torque because the 4 stroke has more displacement and therefore a longer stroke.


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RE: two stroke/four stroke-which produces more torque?

"I believe that generally an engine with a longer stroke will have more torque than one with a shorter stroke. It is all about mechanical advantage."

Well, stroke is one part of the equation of the torque, so is cylinder pressure's force. A bigger bore with the same cylinder pressure as the short-stroke engine will generate more total force (total force being cylinder pressure x piston surface area). So it's not at all a given that longer stoke gives more torque. We all intuitively think that way because most long-stroke engines are tuned this way. All else being equal, they have to be because the longer the stroke, the higher the piston speeds and side forces on the piston (and the less skirt area to absorb it if the piston is smaller). So long-stroke engines tend to be tuned for lower-rpm powerbands. IOW, it's more about mechanical stress limitations than theoretical power-making ability.

Ultimately, guys, torque is kind of a made up number for a piston engine. That's because we try to talk about it as a unit that is averaged over time, the same way we talk about power (power = force x distance / time). But force or torque is actually an instantaneous measurement without a time factor. And a piston engine doesn't really produce a smooth torque curve as is represented on a dyno graph. It really makes a series of wildly swinging torque impulses that rise and fall with the firing sequence. Single-cylinder engines would be the worst about this, and they very well might produce _negative_ torque at some point in the intake or compression cycle. And since you can't average torque over time, really, then the "peak" torque at some rpm that we talk about doesn't really exist. Since force is really supposed to be measured statically, we never can accurately and correctly measure it with an internal combustion piston engine. We'd have to stall the motor against a load and then measure the stalling load, but of course if you stall an i/c piston engine then the combustion stops and there is no torque at all. OTOH, you can do this with an electric motor and still get a torque reading with it being stalled.

Probably more than you wanted to know, huh?


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RE: two stroke/four stroke-which produces more torque?

  • Posted by bogi cMN (My Page) on
    Sat, Feb 10, 07 at 16:29

I am going to borrow a concept from the real estate industry.

"Application, application, application."

On paper, 4 strokes tend to be the winner. They have very broad power curves on the graphs, producing more usable power throughout the power band. In the real world, they fall short when used in the wrong application. Look at the motorcycle racing world as an example. Back when I was in those circles, 750cc 4 stroke race bikes were allowed to race with 500cc 2 strokes. Seems like an unfair race right?
Power to weight ratios were considered, and HP per liter were another factor. The 4 strokes were deficient in both catagories. Hence allowing larger displacement. The same thing is going on in the motocross sector. 400cc bikes running with the 250cc.

In the 80's, Honda tried to compete with the 500cc 2 strokes by building a 4 stroke with the caracteristics of a 2 stroke. It was an "oval" pistoned v-4, two rods per wrist pin. The engine produced tremendous power, but within a narrow power curve. 18,000- 24,000 rpm. With a stroke of 50mm.......those pistons had to be seemingly standing still, at those engine speeds. It turned out to be too complex and costly to maintain,(NAPA didn't stock piston rings:), and no one built a tire that could handle the power of the engine when it came on to it's power curve.
Fuel efficiency was a concern also.

The engine that mattv21 mentioned is an incredible engine in many respects, for the intended purpose. The efficency in fuel is remarkable. In contrast to it's HP/liter of displacement, it falls way short. That engine is producing just a hair over 4.25 HP per liter of displacement. The Rotax in my snowmobile is pushing about 186 HP per liter. Big difference, no doubt. But, both engines will be completely inadequate if their applications are swapped.
HP is the final word in a sense. Torque is but one function of that equation. The rpm at which that torque is available is determined by the need of the application. Performance junkies tend to go the RPM route as long as the torque is sufficent with practical gearing. Typical homeowner power products don't put too many demands on this catagory.....so it comes down to personal preference in most instances.

Proper application, user needs/habits dictate which type of engine output are best. Unfortunately, the Feds and EPA are going to have the final say in our available choices.


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