A Case for Small Displacement Twin Cylinder Engines ??? - Snowblower Forum : Snow Blower Forums
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post #1 of 17 Old 03-17-2017, 10:33 AM Thread Starter
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A Case for Small Displacement Twin Cylinder Engines ???

My experience with re-powering and using snowblowers has got me thinking . . . (and that can be dangerous )

Anyway, I do my fair share of boating once the snow melts and work on my various boat engines, etc. One thing I notice about the 2 small outboard motors that I have (4 HP Johnson and 7.5 HP Mercury) is that they both have 2 cylinders, even at that low power.

Certainly the twin cylinder design give a more even power and can handle loads better (barring the fact that they are 2 stoke as well).

Taking that concept of 2 cylinders at small displacement into the snowblower world, it seems like a twin cylinder approach should yield smoother operation and better ability to handle snow throwing loads versus our single cylinder engines that are so common.

Thoughts, ideas . . . is there such a machine, like a 250-300cc twin cylinder snowblower engine?

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post #2 of 17 Old 03-17-2017, 10:45 AM
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I'd love to see such an engine. It would vibrate less, and probably be easier to pull-start since the effort to get past each compression stroke would be half.

Of course there are downsides too. I'd expect a 2-cylinder engine to take up more space than a single of the same displacement. The ignition system becomes more complex, and you need "real" intake and exhaust manifolds (or two carbs or mufflers).

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post #3 of 17 Old 03-17-2017, 11:11 AM
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I think the size alone would make it hard to fit with out moving handlebars. The harbour freight twin horizontal is a big engine


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post #4 of 17 Old 03-17-2017, 02:04 PM Thread Starter
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Originally Posted by e.fisher26 View Post
I think the size alone would make it hard to fit with out moving handlebars. The harbour freight twin horizontal is a big engine
the thing about most twin cyl horizontal shaft engines today are that they are twice the size (600cc ish) and twice the power (20+ HP) than we need. Wondering if there is such a thing as a twin cylinder in the 250-300cc range . . . I think small dirt bikes might have twins in that size range (maybe)?

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post #5 of 17 Old 03-17-2017, 02:16 PM
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Besides the need to add cooling and figure out shaft size


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post #6 of 17 Old 03-17-2017, 02:53 PM
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Would the single cylinder have more torque than the same cc size as two cylinders?

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post #7 of 17 Old 03-17-2017, 02:56 PM
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One larger cylinder compared to 2 smaller ones.... I don't know


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post #8 of 17 Old 03-17-2017, 03:05 PM
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The single-cylinder would have twice the peak torque (during a power stroke) as the two-cylinder, all other things being equal. But the two-cylinder engine would have twice as many torque pulses during any given amount of time (I'm talking minutes/hours here not milliseconds) so the work done would be equal.

I suspect the major barrier to two-cylinder engines is cost. For example since most of the cost is machining, a piston for a 150 cc engine costs nearly the same as a piston for a 300 cc engine. So the cost of pistons for your 300 cc twin will be nearly double that of the single. And that goes for a lot of other things like cylinder heads, the cylinders themselves, and valvetrain components. So you'd end up having to charge a lot more for your 2-cylinder engine that makes X horsepower than a single that makes the same horsepower.

I imagine that's why twins are so much more common in the next power bracket up. It makes sense to people that a 20 HP engine would cost twice as much as a 10 HP engine, even though the cost is related more to the component count than the actual horsepower output.

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post #9 of 17 Old 03-17-2017, 03:54 PM Thread Starter
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Quote:
Originally Posted by JLawrence08648 View Post
Would the single cylinder have more torque than the same cc size as two cylinders?
Not really, hard to tell . . .

The big difference that I see is that with a single cylinder (4-stroke) engine, you are getting 1 power stroke for every 2 rotations of the engine. The engine is relying on momentum for 1-1/2 revolutions and power on the remaining 1/2 revolution.

With a twin cylinder you get 1 power stroke for each revolution of the engine, each being 1/2 of the revolution. . . therefore smoother operation, less reliance on momentum.

That may add up to similar or even more torque . . . the mass of the flywheel would even have some effect, I would think.

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post #10 of 17 Old 03-17-2017, 04:33 PM
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Quote:
Originally Posted by tpenfield View Post
Not really, hard to tell . . .

The big difference that I see is that with a single cylinder (4-stroke) engine, you are getting 1 power stroke for every 2 rotations of the engine. The engine is relying on momentum for 1-1/2 revolutions and power on the remaining 1/2 revolution.

With a twin cylinder you get 1 power stroke for each revolution of the engine, each being 1/2 of the revolution. . . therefore smoother operation, less reliance on momentum.

That may add up to similar or even more torque . . . the mass of the flywheel would even have some effect, I would think.
That all sounds tight to me.

Other then for smoothness, I'm not sure a two cylinder would do anything other then take up more space and cost more.

I certainly appreciate my twin cylinder tractor over my single cylinder tractor rated at similar power.

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