Why are 2 strokes unreliable compared to 4 strokes?

[RudiGreyling]

Hi Guys,

just wondering why is everyone for ever knocking 2 stroke engines saying they are unreliable?

Old engineering principle says a system with less components should be more reliable. A 4 stroke has much more components with valves, camshaft, lifters, oil pump, sump, which a 2 stroke doesn't.

So I ask again what makes 2 strokes "unreliable"?

Thanks
Rudi

[Junkie]

Im also curious as to how peeps figure theyr more unreliable, I'm also considering a motor upgrade and probably go with another 2stroke unless someone can convince me otherwise....

Maybe ill just dump the Rotax for a Hirth

[Aerosan]

There is a couple of reasons behind this. I think the major ones are the fact that 2-strokes need a added lube whereas a 4-stroke does not. Also 4-strokes rev much lower than 2-strokes and therefore
"do less work"

There is a hell of a lot more but my 2c worth

[Arnulf]

Hi,

I am not an engine boffin, I would think that because the combustion is not so clean, coking up of the motor could be a problem. Next potential problem is the carbs gumming up (not applicable to oil injection).
The chance of fuel contamination is also bigger, because two different liquids are used (see the Castrol saga).

However personally I think the 2 Strokes have a bad reputation, because initially they were not so reliable, they only had single ignition. If the 4 strokes also only had single ignition, their reputation would probably also be not so great.

Regards,
Arnulf

[ZULU1]

Nothing gives me greater pleasure than flying over the ocean, beautiful morning...oil pressure at 4 bar, oil temp at 100 degrees, lovely sound of a thumper...they have got real oil in them. Whereas your two tempos have to have oil added in somewhere. Just study the schoolboys text books on the basic technology.

Oh and as for overlap...half the unburnt gases going shooting out the exhaust, the balance washes lubricant off the rest of the bits that go round.

A classic case of one of the most vicious two strokes and I may add the best sounding (terrrible handling) was the old Kawasaki Mach1 500cc it did back in I think 1973 18 mpg because of the induction exhaust overlap. But the East Germans made another beast in the form of the Trabant (I actually bought 12 of them in cabrio format for DM500 each)...this had a 600cc motor with a semi auto gearbox and paper mache components. With the wall coming down and emission controls they couldnt recycle them. Its only a matter of time before two tempos get wiped out with emmission controls.
But they produce a lot of power and are light, simple and effective. I bought a two stroke diesel a few years back on a genset I bought in E Germany..this thing also turned fuel into noise...

I dont know how a Lumberjack would use a 4 stroke chainsaw ???

ZULU1 (HKS 700E !!!) and I bet this is going to start some responses..

[DieselFan]

1. 2 strokes can have an oil sump and dedicated lube system - Caterpillars.
2. There are diesel 2 strokes once again check the big caterpillars.
3. 4 strokes generally run cooler therefore need less oil to keep cool as they fire less often - less heat build up.
4. 4 strokes more torque less kw / hp - solved by using a diesel 2 stroke.
5. 4 stroke more complicated doesn't mean unreliable, just normally more expensive as you pay for more parts.
6. 4 stroke rev lower, cooler, longer life??
7. Due to the lack of a dedicated lube system in common 2 strokes more wear and tear.
8. 2 strokes have lots more pollution - burn oil and air flow is not as efficient and therefore heavier on fuel somtimes 2-3x.
9. When standing over long periods of time 2 strokes struggle to start - varnish type film over the carburetor jets.
10. 4 stroke generally quieter and smoother.

2 stroke motors can do most of what a 4 stroke can but it was chosen to either ommit the sump, etc and make use of premix for whatever design reasons like weight, cost etc.

So yes 2 strokes could be more reliable BUT the ones we use for ML's compared to the 4 stroke - well lets just say I haven't found one that has a dedicated lub system like the 4 strokes and over time the overall maintenance cost should be lower on 4 strokes, enough to offset the initial cost?

[RudiGreyling]

So in Summary so far:

It seems that the 'unreliability' comparison has more to do with the way a 2 cycle works, than actual part count in the engines we fly..

OK so I buy the following statements about our little 2 stroke engines:
1) 2 strokes work harder, rev higher, hence more friction and parts work harder thus means needs replacement sooner, if not replaced they can break.
2) due to 'unclean' combustion compared to 4 strokes it may lead to gumming up and carboning up and this leads to engine problems.

I Do NOT buy the following (yet?):
1) Need dedicated lube system. Why if one premix your fuel correct it is always perfectly lubbed or not? Is having dedicated separate engine oil in sump of engine better than to add it with the fuel (Premix or autolube?)

Am I on the right track so far?

I am trying to see how I can eliminate as far as possible some of the things that lead to unreliability in 2 strokes, so that I can fly safer more comfortable on my current 503.

Any other comments welcome, thank you.

Kind Regards
Rudi

[DieselFan]

See if you can get an Oil Injector. It adjusts the mix on the fly from 100+:1 at low rpm to 35-20 at much higher rpm. Remember you'll need to tweak it for your rev settings, they're also known as "autolube" pumps. Check with Solowings I think their Aquilla 582's have them or something similiar fitted.

I personally plan on buying a 503, fitting an Oil Injector and eventually upgrade to a HKS for maintenance and fuel savings, plus no 2 stroke oil depending on how much I fly

[Morph]

I have heard of 2 strokes running reliably all they way through to 1200 hours with regular checkups, decoking where neccessary, and bearing and crank replacements only if absolutely neccessary. The 503 is a bullet-proof motor. My little motor has 540 hours on it, and has not had an engine related problem since day 1. I premix, more reliable to me than oil injection, and I fly regularly. I have had a fuel starvation problem, where the fuel-filter blocked up and the only thing that saved the day was an electric fuel pump. This would have happened to any engine. Other wise, hassle free

They do require more maintenance but, unless you are willing to keep your fourstroke motor for longer than 1050 to 1200 hours, the 2 stroke is more cost effective than the fourstroke. I have a spreadsheet where I worked out the costs of owning and running the 2 vs 4 strokes. If you take a 582 and compare it to a 912UL(80hp) in a Bushbaby for example, the 582 gives a better power-to-weight-ratio and is much more cost effective until that majical 1050 to 1200hour mark. I have factored in a complete engine replacement at 1200hours for the 2 strokes. The 4-stroke is a huge initial investment with weight disadvantages on planes limited to 450kg. It would be different once they open it up to 600Kg.

However that be said I would love to have a 912ULS for that added power in a bushbaby if I had the higher weight limit and I could afford it.

Here is my spreadsheet, I used engine and parts pricing from a Canadian website and used the relevant Canadian$ to R conversion. I believe it is fairly accurate and an interesting look. I have not factored in any inflationary costs as both the 4-stroke and 2-stroke would experience the same level of inflation. http://www.gregperkins.co.za/flying/ima ... arison.xls

Look at the accumulated costs and see how much of an initial investment the 4-stroke is. BTW the costs of the overall on the stroke motor are based on a conversation I had with Demon re the overall on his 912ULS a while ago. Using that as a guidline, it worked out to be about 66% of the new engine pricing. I then used this to determine guestimate for the other engines. This is only relevant if you do the service at 1500, in that case the cost of the 4-stroke leapfrogs away from the 2 stroke again.

[RudiGreyling]

Hi Morp, very interesting, you've got PM,

Regards,
Rudi

[DarkHelmet]

To clarify what Dieselfan said.

Pre-mix is not 100% accurate. At low revs you need less oil to oil the motor and as the revs increase the need for lubrication increases.

With an autolube device/pump this mixture is adjusted accordingly (but one more thing to break )

[RudiGreyling]

To clarify what Dieselfan said.

Pre-mix is not 100% accurate. At low revs you need less oil to oil the motor and as the revs increase the need for lubrication increases.

With an autolube device/pump this mixture is adjusted accordingly (but one more thing to break )

Hi Darkhelmet,

Sorry to keep on, but I learn a lot, but I still don't buy it, I still don't see the benefit of Autolubbe yet w.r.t. lubrication properties.

With Premix at low revs, less volume of air/fuel and OIL travels through the combustion chamber during a given time (say 1 minute), so "less" lubrication IS achieved.

With Premix at High revs, more volume of air/fuel and OIL travels through the combustion chamber during the same duration of time (say 1 minute), so "more" lubrication IS achieved.

So it looks to me that with Premix you always have perfect lubrication, if you mixed correctly.

Or am I missing some fundamental design parameter for lubrication on 2 strokes??? Are you saying that at low RPM you only need a 100:1 mix ratio Fuel to Oil, while at a high RPM you need a 20:1 ratio. Can you point me to some documentation somewhere on the net where this gets discussed for 2 cycles??

Thanks,
Rudi

[Morph]

The secret with reliability on a 2-stroke is simplicity. Keep it simple. the autolube mechanism is just one more thing to worry about both for mechanical failure and running dry during flight. You have a far greater chance of destoying your motor completely with autolube than without even with running slightly rich at times due to the fixed 50:1 fuel oil ratio. Your worst case is scenario is you give the motor a little more oil than it needs. The autolube worstcase scenario is 100% lean and silence and $$.

I know that the premixed motors show a bit more wear and tear, but how much is this, I know of 503's that have run to 1200 hours without problems, so how do you measure the reduction in reliability.

[RudiGreyling]

The secret with reliability on a 2-stroke is simplicity. Keep it simple. the autolube mechanism is just one more thing to worry about <SNIP>

Morph, Ditto on your sentiments at this stage.

I still would like to learn some more so I am open for discussion and opinions with some documentation as backup.

Regards
Rudi

[Morph]

Food for thought

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