Two stroke or four stroke

[Cadence]

It would seem the US and I guess soon Australia would apply some common sense. A polluting 10HP up I can see eliminating as two stroke. A dingy engine?

[LoudMusic]

Originally Posted by Mr O

I used to own 2 strokes which where lighter but way too polluting. Then a 4 stroke 9.9 on a 2.6m RIB. Worked fine but way too heavy to get off the rail. My SO has weak arms and it’s nearly impossible to get the boat up the beach. Now we have an electric 3hp motor and love it! Super light, breaks down into 2 pieces which means it can be easily carried onto dinghy by hand. No stinking exhaust or noise and no fuel to spill. It can sit on the rail or be stashed inside the boat. Charges when I charge the house batteries with genset or with shore power. Solar to come. Slower, yes, but we usually just need to go a short distance to shore or crab pots. We looke at Torqueedo but the EPropulsion Spirit 1.0 won out. Most quiet and less money. Very happy with this setup.

EPropulsion is where I've been looking as well. They seem like great products and a noticeably cheaper as well. Thanks for your report!

[LoudMusic]

Originally Posted by Dockhead

There's lots in the archives on this for the OP.


I have come around to not caring so much anymore about the differences. The current generation of four strokes are not really much if any heavier than two strokes.


I love two strokes, find them (contrary to what someone above posted) to be smoother and easier to start than four strokes. Love their simplicity. But in my experience, my particular four stroke outboards have been vastly more economical than the two strokes I used to use -- I think about half the fuel burned. Maybe my two strokes were all set up rich -- I don't know.


So if I were buying a new outboard, in a place where I had a choice, I think I would just buy what is lighter and/or cheaper providing it's good quality.


Or go electric, depending on the use case. I'm thinking about Torqeedo myself. In addition to an IC powered outboard, but probably what will be used 90% of the time considering how I use my dinghy.

Have you seen the EPropulsion products?

Electric Outboard | ePropulsion

They're a bit cheaper and appear to perform a bit better.

[Dockhead]

Originally Posted by LoudMusic

Have you seen the EPropulsion products?

Electric Outboard | ePropulsion

They're a bit cheaper and appear to perform a bit better.

Yes, I've seen them, and they look interesting, but I would probably go with Torqeedo myself, despite the higher cost -- longer track record and demonstrated German quality.

[Capt.Don]

Originally Posted by Luc bulterys

Normally I keep low profile and rarely react on posts, but some of the above advices really chocked me:
Isn’t it the ultimate experience for us cruisers spending quality time in beautiful places on the water, as far away as possible from exhausts and pollution of all kind … One of the horrific moments in the anchorage you share, is such a two stroke passing by, leaving his toxic fumes behind, all because this offers a few practical advantages.
And stop thinking that it won’t make a difference, or because others pollute even more. So please, at least consider it in your choice, if not for your own peace of mind, do it for your fellow cruisers who do care and just want to fully enjoy the beautiful environment.
And, as some have suggested already, if you’re willing to invest in a better environment and really ease of use, look also for electric. Lots of choices these days.

This is the opinion of a fellow cruiser who’s hoping more and more people will make eventually the changes we need.

Versus the noise of a group of Honda 2.5HP 4 stroke -- they are way noisier than 5HP 2 stroke,

[wsmurdoch]

On the subject of relative weights of 2 and 4-stroke engines, it is easy to compare the weights of Tohatsu's small engines because they used the same block for the same pairs of horsepowers in their 2-stroke engines and in their 4-stroke engines.

The 4-stroke 2.5 and 3.5 engines are both 1 cyl x 55mm bore x 36mm stroke and both weigh 18.4kg. The 2-stroke 2.5 and 3.5 engines were both 1 cyl x 47mm bore x 43mm stroke and weighed 12.5 and 13kg. That makes the 4 strokes 5.9 or 5.4 kg heavier (47 or 42% heavier).

The 4-stroke 8 and 9.8 engines are both 2 cyl x 55mm bore x 44mm stroke and both weigh 37kg. The 2-stroke 8 and 9.8 engines were both 2 cyl x 50mm bore x 43mm stroke and both weighed 27kg. That makes the 4-strokes 10 kg heavier (37% heavier).

Bill

[a64pilot]

Respectfully, there is no way a 2T and a 4T share the same block, way too many differences like where does the cam go, and of course a 2T inhales the intake to the underside of the piston, then as the piston drops, pushes the mix though transfer ports plus the crankshafts are completely different, a two stroke crank is many pieces pressed together so the needle bearings can be fitted, a four stroke is usually a one piece crank, but there are some that are needle bearings or ball bearings, the Kawasaki 900 back in the day had a multi piece crank with needle bearings, a hold over I’m sure from their two stroke days.
But just as specialty road race two stroke motorcycles were replaced with four strokes cause they could be lighter and more powerful, it’s going to happen with outboards too.

Biggest difference in efficiency is due to a 2T doesn’t scavenge very well, meaning there is a lot of exhaust in the intake charge, and a lot of raw fuel and air mixed in with the exhaust to, plus reversion. A two stroke will blow air out of the carb, along with fuel too, put your hand in front of one to see what I mean.
Over the years there have been many devices fashioned to try to prevent some of this mixing of charges, Suzuki’s rotary valve back in the day, and of course the most prolific is reed valves, essentially just one way check valves on the suck side of the engine to prevent reversion.
But they are inherently inefficient, attempts were made to try to help that, Orbitals high pressure injection for example, but big two stroke outboards died decades ago, it just taking longer for the little ones to go is all. But just as anyone who has run big two stroke outboards before and now runs four strokes will tell you, they don’t miss them.

[Dockhead]

Originally Posted by a64pilot

Respectfully, there is no way a 2T and a 4T share the same block, way too many differences like where does the cam go, and of course a 2T inhales the intake to the underside of the piston, then as the piston drops, pushes the mix though transfer ports plus the crankshafts are completely different, a two stroke crank is many pieces pressed together so the needle bearings can be fitted, a four stroke is usually a one piece crank, but there are some that are needle bearings or ball bearings, the Kawasaki 900 back in the day had a multi piece crank with needle bearings, a hold over I’m sure from their two stroke days.
But just as specialty road race two stroke motorcycles were replaced with four strokes cause they could be lighter and more powerful, it’s going to happen with outboards too.

Biggest difference in efficiency is due to a 2T doesn’t scavenge very well, meaning there is a lot of exhaust in the intake charge, and a lot of raw fuel and air mixed in with the exhaust to, plus reversion. A two stroke will blow air out of the carb, along with fuel too, put your hand in front of one to see what I mean.
Over the years there have been many devices fashioned to try to prevent some of this mixing of charges, Suzuki’s rotary valve back in the day, and of course the most prolific is reed valves, essentially just one way check valves on the suck side of the engine to prevent reversion.
But they are inherently inefficient, attempts were made to try to help that, Orbitals high pressure injection for example, but big two stroke outboards died decades ago, it just taking longer for the little ones to go is all. But just as anyone who has run big two stroke outboards before and now runs four strokes will tell you, they don’t miss them.

I think direct injection solves the efficiency problem, doesn't it? I have a 120 hp 600cc direct injected two stroke snowmobile that seems to use no more fuel than four strokes, while being much lighter and torquier. Direct injection can just about eliminate the passage of unburned fuel, can't it?



Don't forget two strokes have one big efficiency ADvantage -- they get double the power strokes out of the same swept volume at the same RPM. That's why big ship engines are all two strokes.


I wouldn't write them off yet.

[a64pilot]

No, injection does nothing for charge scavenging, and DD’s and big ship motors are all forced induction I assume and therefore have exhaust valves, so they don’t share much in common with a piston port two stroke.
Direct injection is a way to help emissions mostly. It’s from our Aussie friends, and I think Mercury Marine was the first to use it in the Opti-Max engine, which quickly became know as the Opti-Blow
http://archive.maas.museum/australia...ticle_id=10041
Except for rare cases, like maybe snowmobiles where maybe due to ridiculously cold temps an oil lubricated engine isn’t desirable, two strokes are dead.
No two stroke comes even close to the power output of a four stroke.
As an extreme example, Top Fuel is over 12,000 HP from a 500 cu in engine.
That is an extreme example, the formula road race bikes is likely a better one, until not all that long ago there just wasn’t a four stroke formula bike, most of us thought there never would be.
We called four strokes “Diesels” cause they were so heavy, and revved so slow.

That isn’t true anymore, I’ve been out of Road Racing for years now, but bet the only two strokes there are, are raced in Vintage class.

There was even a two stroke Aprillia built around 2000 that I wanted, but I’m not sure why it didn’t make it, but it didn’t.
There were a couple of new technologies tried to save the two stroke outboard 20 yrs or so ago, it didn’t work, the four strokes were just so superior in smoothness and noise level etc. and Mercury solves the out of the hole lack of torque by Supercharging their Verado line of four strokes.

[Reefmagnet]

Horses for courses. There seems to be this notion of two strokes being some kind of smoke belching machine second to only the steam engine in ability to pollute. Some modern outboards use as little as 100:1 fuel oil mix ratios. The days of the Seagull are well and truly past. If pollution really is such a big deal to you then power your dink (and mother ship, for that matter) using only oars, solar or sails.


Electric propulsion might be fine for weekenders. How to charge the battery when away from the dock? Can solar alone keep it charged for anything more than a few minutes to the dock and back? Or do you charge late afternoon into early evening using the generator? Your anchorage neighbour's will love that. What's the range? Do you need to carry spare batteries? What's the energy density of a battery? How efficient is Diesel/Petrol -> generator -> Charger -> Battery?



If you submerge a four stroke in salt water in a remote location, can you confidently get it flushed and running again within an hour with no further work required? Will fuel consumption likely be higher on a four stroke that can't plane it's dinghy compared to a two stroke of the same weight that can, because the two stroke will have more hp and better low end torque?


Here's a random sample of short leg 2 stroke weights:

3.3 hp two stroke = 13.5 kg
5 hp two stroke = 21 kg
9.8 hp two stroke 26 kg.

[Cadence]

Originally Posted by Dockhead

I think direct injection solves the efficiency problem, doesn't it? I have a 120 hp 600cc direct injected two stroke snowmobile that seems to use no more fuel than four strokes, while being much lighter and torquier. Direct injection can just about eliminate the passage of unburned fuel, can't it?



Don't forget two strokes have one big efficiency ADvantage -- they get double the power strokes out of the same swept volume at the same RPM. That's why big ship engines are all two strokes.


I wouldn't write them off yet.

I don't think they get full advantage of the double firing strokes but do get some. One advantage is fewer moving parts and reduced weight. For a small tender application I think they make sense.
As far as ships go that is entirely a different diesel story not a small gas engine.

[44'cruisingcat]

Originally Posted by a64pilot

But just as specialty road race two stroke motorcycles were replaced with four strokes cause they could be lighter and more powerful, it’s going to happen with outboards ....

Sorry, this is simply not correct. Almost universally, 4 strokes have replaced 2 strokes in racing because the rules have forced the change. 4 strokes are not lighter and more powerful.

In GP motorcycle racing, the 500cc 2 strokes were replaced by 1000 cc four strokes.

250cc motocross bikes became 450cc four strokes. 125 two strokes became 250 four strokes. And the 4 strokes were still down on power at first. And MUCH heavier. 250's had a 98kg FIM minimum weight, which they easily met even with steel frames. At first the 4 strokes were oveweight even with alloy frames.

4 stroke outboards are cleaner, more fuel efficient yes. But for equivalent HP they're heavier.

The BIG four stroke fallacy has always been about torque, and low revs. The truth is, 2 strokes make more torque, and rev lower. They have much longer strokes for one thing, due to the need to pressurise the crank and transfer fuel/ air. They also fire every cylinder every revolution.

[44'cruisingcat]

Originally Posted by Dockhead

Yes, I've seen them, and they look interesting, but I would probably go with Torqeedo myself, despite the higher cost -- longer track record and demonstrated German quality.

Well, the ones I've seen weren't too impressive quality wise. Gearboxes which had broken due to the prop hitting a jellyfish.

[sailstoo]

Loved the small two-strokes. Yeah, they're smoky and not very fuel efficient and y'gotta mix oil with the fuel but they're lighter (much) and with the four strokes if you lay them down wrong the lubricating oil drains out of the crank case resulting in one burned up power head. So, for a small cruising or racing keelboat (J24, J22, J70, Melges anything, Sonar) where you pull the engine off and lay it inside the cabin/cuddy, I vote two stroke all the way...and there's plenty of low-hour used ones around.

[wsmurdoch]

Originally Posted by a64pilot

Respectfully, there is no way a 2T and a 4T share the same block, way too many differences like where does the cam go, and of course a 2T inhales the intake to the underside of the piston, then as the piston drops, pushes the mix though transfer ports plus the crankshafts are completely different, a two stroke crank is many pieces pressed together so the needle bearings can be fitted, a four stroke is usually a one piece crank, but there are some that are needle bearings or ball bearings,...

I'm sorry, I guess my first sentence was not as clear as I would have hoped. You are correct, it would be difficult to convert a 2-stroke block to a 4-stroke block.

Tohatsu's engine lineup makes it easy to compare the weights of 2- and 4-stroke engines because they have manufactured engines of identical horsepower in identical pairs of down-rated and down-rated engines. It avoids the error of comparing a up-rated 4-stroke engine weight to a down-rated 2-stroke engine weight... an error I have seen in some comparisons.

The Tohatsu 4-stroke 2.5 and the four stroke 3.5 share the same block. (I listed the cylinder count, bore, and stroke to illustrate that.) The 2-stroke 2.5 and 3.5 also share the same block. The 4-stroke engines weigh more... appreciably more.

Similarly, the Tohatsu 4-stroke 8 and the 4-stroke 9.8 share the same block, and the 2-stroke 8 and 2-stroke 9.8 share the same block. Again the four stroke engines weigh appreciably more.

Last year we bought a 4-stroke Yamaha 9.9 to replace our 2-stroke Tohatsu (Nissan) 8. After 5 months in the Bahamas, my wife and I both hated the Yamaha's weight. This year we left the Yamaha at home and took both the 2-stroke Tohatsu 8 and a 2-stroke Tohatsu 3.5 to the Bahamas. Truth be told, we mostly used the little 27 lb 3.5. I can pick it up with one hand.

That little 3.5 has, over the years, been swimming twice. Both times I flushed it with fresh water, shot WD40 into it, and replaced the gas. I am not so sure that a 4-stroke could survive that mistreatment.

Bill