Difference between 4 stroke and 2 Stroke Outboards

[Seajet]

Dylan,

as it worked out we didn't meet when you were around here, not for want of trying on both our parts.

My engine is an efficient 100:1 job and I'm careful both keeping that way and avoiding any spillage, of oil or anything else.

Possibly another effort towards this is stowing the engine and sailing whenever reasonably possible; and something tells me 4-strokes and diesels are not so holy either !

If you knew me you'd know pollution is not alright by me at all; do you know the significance of my boat's name ?

 

[Beamishken]

All this talk about pollution & excessive fuel consumption makes me chuckle,it's a 5hp,they run on fumes you could buy a heck of a lot of fuel for the cost of trading up & if you factor in the extra servicing cost of a 4 stroke they probably cost little difference over a few seasons
As for pollution you probably get as much pollution washing the frying pan after you cooked your bacon sarnie
The amount of pollution from the leisure boater pales into insignificance when you compare to commercial vessels

 

[Binman]

I want to move up from a yam 2b, to a yam Malta 3hp, as I feel the 2b is under powered to push my 9ft dingy, the advantage is the 3hp it has a first gear, will I find a big change in power output.

 

[VicS]

I want to move up from a yam 2b, to a yam Malta 3hp, as I feel the 2b is under powered to push my 9ft dingy, the advantage is the 3hp it has a first gear, will I find a big change in power output.

2hp cant be all that much under powered.

3hp is 1¹/₂ times as powerful. It might just make that all important difference, but at 16.5kg is somewhat heavier than the 10kg 2B.

If you increased the power of your car engine by 50% you'd notice the difference

 

[Seajet]

The Malta engine had a lot of criticism re reliability when first out but that should be sorted by now.

Do check the weight though, I suspect it's a lot more than your 2hp job.

 

[Iliade]

All this talk about pollution & excessive fuel consumption makes me chuckle,it's a 5hp,they run on fumes you could buy a heck of a lot of fuel for the cost of trading up & if you factor in the extra servicing cost of a 4 stroke they probably cost little difference over a few seasons
As for pollution you probably get as much pollution washing the frying pan after you cooked your bacon sarnie
The amount of pollution from the leisure boater pales into insignificance when you compare to commercial vessels

For many years I had a variety of 4-8 hp two strokes pushing my Listang T24 (sim. J24 mass & u/w shape)

Longish passages, such as the 40 Nm home from the IOW on a Sunday were a nightmare if there was no wind. With four cans and two tanks, if there was a slop, I could barely carry enough fuel and had to stop off a few times. The four stroke 9.8hp achieved hull speed at about half revs, using something like a gallon per hour, and easily overcame a bit of a running sea.

I know the issue is also related to power, but...

 

[Seajet]

How come I've managed to motor from Lymington to Guernsey & Guernsey to Salcombe in calms then ?!

22', 4hp Yamaha 2 stroke in those days.

Yes I carried extra 5 litre cans as well as the 10 litre remote tank, but we didn't exactly have to do a Replenishment At Sea with a tanker alongside.

With the 4 or current 5hp 2-stroke I get 2.5 hours at 5 knots from 5 litres of fuel.

 

[VicS]

All this talk about pollution & excessive fuel consumption makes me chuckle,it's a 5hp,they run on fumes you could buy a heck of a lot of fuel for the cost of trading up & if you factor in the extra servicing cost of a 4 stroke they probably cost little difference over a few seasons
As for pollution you probably get as much pollution washing the frying pan after you cooked your bacon sarnie
The amount of pollution from the leisure boater pales into insignificance when you compare to commercial vessels

For many years I had a variety of 4-8 hp two strokes pushing my Listang T24 (sim. J24 mass & u/w shape)

Longish passages, such as the 40 Nm home from the IOW on a Sunday were a nightmare if there was no wind. With four cans and two tanks, if there was a slop, I could barely carry enough fuel and had to stop off a few times. The four stroke 9.8hp achieved hull speed at about half revs, using something like a gallon per hour, and easily overcame a bit of a running sea.

I know the issue is also related to power, but...

Absolutely.

The actual cost of fuel for a small outboard used as an auxiliary in a small sailing boat is pretty small compared with what some people waste on fuel for the zillion hp engines in some mobos and with other berthing and running costs for that matter.

The ability to carry enough fuel is the worry for longish trips. I have upgraded to a 6 gallon tank in place of the original 2 gallon tank for that reason. Its more than adequate for pottering around the Solent. Its enough to motor from one end and back again but if I went further afield, esp cross channel, carrying extra fuel would be a worry.

 

[CreakyDecks]

My old Volvo S80 car burns a lot more fuel during my twenty minute drive to the boat than my 2 stroke Suzuki uses during a day's sailing!

 

[dartmoor]

I think EU legislation and regulations create intended and unintended consequences - some good, some bad. On the positive side, new emissions regs drive innovation and improvement in combustion engine efficiency. I would imagine with no regs, the same outboard models produced in 2006 would still be unchanged today. You may say that's a good thing! But actually, the new small outboards are no doubt more efficient. In 2006 2.5HP engines for example did not have twist grip throttles, and most were direct drive, and burnt maybe 1.5 litres an hour. All 2.5hps now have twist grips, burn less than 1 litre per hour, have a neutral function as well as forward gear, and seem better built (apart from Honda, which remains unreformed from its origins with the same sniff-oxygen-rust away-mild steel fixings and rocker cover). I believe the EU regs also covered things like the amount of vibration delivered down the handle of the engine - which would be a good thing in terms of using outboards. Be interesting if they lowered noise db limits - as I hate Jet skis, I might applaud and like the consequences of that....

 

[Seajet]

With extra 5 litre fuel cans for cross Channel trips - 60-70 miles from here - I don't see any worries as long as the cans are intelligently and securely stowed so as to stay upright, with the outlet uppermost.

I also use a splash-proof funnel when topping up the main tank.

I route all electrics well away from the fuel locker and for what it's worth have an auto extinguisher there.

If I or a crew was a chain smoker, especially while topping up, then I might worry !

 

[Sneaky Pete]

Saw this on youtube it explains one of the differences that you get a power stroke every revolution. Cant understand how they work out the 2 stroke has 25% more power. That's Americans for you.

https://www.youtube.com/watch?v=esnsI440dTg

 

[VicS]

Saw this on youtube it explains one of the differences that you get a power stroke every revolution. Cant understand how they work out the 2 stroke has 25% more power. That's Americans for you.

I think that was qualified in the video by a reference to 3500 rpm.

At their specified max revs 5500?? 6000 ?? I expect you will find both have the same power namely 150hp. What they are doing is using the fact that at some intermediate speed the two stroke is developing 25% more power than the four stroke as an advertising gimmick.

 

[Seajet]

If it's 5 horsepower it's 5 horsepower, whether generated by 2 or 4 stroke, jet turbine, nuclear steam turbine or in an ideal world my ex' on a treadmill !

 

[TSB240]

Quite a useful comparison between 2T and 4T fuel consumption here.

http://www.tohatsu.com/tech_info/fuel_consumpt.html

would be interesting to see an alternative manufacturer who can quote the same.

 

[Lakesailor]

Saw this on youtube it explains one of the differences that you get a power stroke every revolution. Cant understand how they work out the 2 stroke has 25% more power. That's Americans for you.

https://www.youtube.com/watch?v=esnsI440dTg

Don't forget that because of the way a 2 stroke engine works there is an overlap between exhaust evacuation and fresh charge inlet. The fresh fuel mix actually pushes the exhuast gases out. Some of the new charge goes straight out of the exhaust. The measured capacity may be the same as a four stroke, but the 4 stroke probably uses it's ingested fuel/air mix more efficiently.
The 2 stroke probably only works efficiently at a particular point in the rev range.


Look at this animation http://www.animatedengines.com/twostroke.html

 

[Beyondhelp]

This is not a problem because it's irrelevant as far as the OP is concerned. The technology you describe is simply not available on small outboards, from Evinrude or anyone else. The smallest G2 Evinrude would sink his boat.

Im simply clearing up misconceptions that all 2 stroke engines are inferior to 4 stroke when this simply is not true. Also 2 stroke engines have a significant torque per cc advantage.

 

[macd]

Substantially true, Lakey, but in a well-designed two-stroke, harmonic waves in the exhaust system do much to 'pull' spent exhaust gases out, as well as holding unburnt charge in the combustion chamber until combustion has taken place. Generally speaking, the more effective these processes are, the more peaky it will also be, as you wrote (and also more powerful). There are several measures which can reduce this peakiness (and fuel inefficiency), but none that are likely to appear in a small outboard any time soon, if ever.

Regarding SneakyPete's comments, I've no idea of the numbers for high-end modern 2-stroke outboards, but ever since the above harmonics were harnessed they've had a substantial hp-per-cc advantage over four-strokes (which is why Honda spent untold millions developing a quasi 8-cylinder, 21,000rpm, 500cc four-stroke to compete against them: in three seasons it scored not a solitary grand prix point). It's also one of the reasons several global car companies spent even more money developing direct injection two-stroke technology, albeit ultimately to no avail. Some of that technology is now in the likes of Evinrude's G2 range.

Generally I'd say that a 25% peak power advantage for two strokes is credible (which isn't the same as saying it necessarily exists in any current engine). Over 300bhp/litre was common in motorcycle racing two decades ago. It isn't more, mainly because they can't rev as fast nor tolerate the compression of four-strokes (much more than 7.2:1 from exhaust port closing and the pistons melt. That's one of the other practical disadvantages of two-strokes: compared to a four-stroke, one power stroke in every two gives two fewer during which cooling can take place.

Most of this isn't especially relevant to our tiddly outboards which are in a very mild state of tune...and consequently acceptably flexible.

 

[LONG_KEELER]

I think EU legislation and regulations create intended and unintended consequences - some good, some bad. On the positive side, new emissions regs drive innovation and improvement in combustion engine efficiency. I would imagine with no regs, the same outboard models produced in 2006 would still be unchanged today. You may say that's a good thing! But actually, the new small outboards are no doubt more efficient. In 2006 2.5HP engines for example did not have twist grip throttles, and most were direct drive, and burnt maybe 1.5 litres an hour. All 2.5hps now have twist grips, burn less than 1 litre per hour, have a neutral function as well as forward gear, and seem better built (apart from Honda, which remains unreformed from its origins with the same sniff-oxygen-rust away-mild steel fixings and rocker cover). I believe the EU regs also covered things like the amount of vibration delivered down the handle of the engine - which would be a good thing in terms of using outboards. Be interesting if they lowered noise db limits - as I hate Jet skis, I might applaud and like the consequences of that....

I remember the Ariel Arrow which could out accelerate any other motorbike up to a certain speed - if you could actually see the bike behind through the smoke that is. We have certainly come a long way since then.

This season I have found myself in possession of two outboards of identical hp but differ by being 4 stroke and 2 stroke. I'm looking forward to see how I get on.

I loved my 2hp Suzuki two stroke to bits. Powerful, very light and simple. Ironically, it was destroyed by laying it flat in a locker and the seawater flowed into the cylinder .

I recently found out that container ships use a format of 2 stroke diesels for primary power. I can only assume that it must serve them when at optimum speed and revs.

 

[CreakyDecks]

If you don't like 4 strokes then blame the jet skiers. It is they who insisted on using massive two stroke engines in America's freshwater lakes, with with half their fuel, gallons a day, going unburned straight into the lakes. That is what I understand started the two strokes bad, four strokes good campaign. For what we do with them in the sea there is probably no significant reason to use one or the other. If you are a lake sailor then two strokes are not really acceptable. If people insist on using them then the inevitable consequence is a ban on freshwater use of all petrol powered outboards. As an angler I can tell you that electric motors are great for trolling but useless as a means of getting from A to B.