Why are 2 strokes unreliable compared to 4 strokes?

[DieselFan]

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

Will try get some docs, however from what I've read in order to improve reliability, components need to be added and the reason they were omitted was due to weight and cost.

[DieselFan]

GreylingR I think you've opened a very INTERESTING topic I've read over 30 websites and once you're past the marketing crap which consistently says "While our competitors use unreliable 2 strokes..." Yet they don't state exactly why that is.

But of the competitors who rant about their 2 strokes they do mention why theirs are now reliable compared to older 2 strokes and these are the 2 most common reasons they believe make them as reliable as 4 strokes

1. AutoLube
2. Direct Injection

The reason for number one (apart from the actual part failing) is that oil build up around the spark plugs is way less as it's more efficiently controlled. This means servicing intervals could be increased and not as serious if overdue by a few hours, whereas without you have to be more religious - this is good as the reliability is almost directly controlled by YOU, dont skip a service. This is one of the reasons 2 strokes got bad names, oil on the spark plugs and oops.

For number 2, this apparently by design reduces the chances of fuel being contaminated and descreases the pollution slightly.

Unfortuantely to post links would be a mission as you have to skim read alot of crap.

Try googling "2 stroke vs 4 stroke unreliable" etc

also
http://www.physicsforums.com/showthread ... 958&page=3

After some reading I struggled to get incidents of broken oil injectors.

http://sportpilot.org/magazine/feature/ ... r%20ON.pdf
good link, the heading is "To inject or not to inject" specifically the 503 it also raises the same point that Morpheus mentioned.

[RudiGreyling]

Hi Dieselfan,

Good stuff, I started reading, but a lot of talk about 2 strokes that we do not use, bikes and boats etc.

Any case I looked up the oil injection for a 503. It looks like it is driven directly from the crank. That means that the oil injected is directly linear to RPM, you get exactly the same effect with premix, more RPM=more air/fuel/oil mixture. So I do not see the advantage of better optimised lubrication yet with injection, it is also linear!

I've read the article on "to inject or not", and the only bad thing they could say about premix is that you have to remember to mix it with the fuel. Duh, , but there is a lot of things to consider on the injection, (Tank location, filters, not auto priming, servicing etc…etc) and only one advantage, you can add fuel without premixing.

Here is a diagram of the 503 oil injection, see the sprocket driven by the crank?


Kind Regards
Rudi

[RudiGreyling]

My searching continues into the reliablilty issues on 2 strokes, still have not found any real figures yet!

If you read this article this guy makes some good arguments to use oil injection. Somehow he says the pump is not linear, changing the mix ratio at lower revs, how I don't know since it is crank driven...

His bottom line is oil injection gives less carbon build up.

Regards
Rudi

[krusty]

...Somehow he says the pump is not linear, changing the mix ration at lower revs, how I don't know since it is crank driven...

From my experience with a 2S bike motor with A/L, I think this is correct. The way that the A/L system on my bike worked was it was linked to the the throttle cable which regulated the amount of oil pumped into the carbs.

[RudiGreyling]

Ditto Krusty,

after some more research I came accross this article: Pro's and Con's of Oil injection
It is 3MB big (though)

A good honest article checking both sides of the coin.

It looks like the pump is connected to the throttle cable. The pump is setup that when the throttle is below 3000 RPM the ratio is 70:1 above 3000 RPM it is your normal 50:1.


In Summary so Far
It is still not the primary reason a 2 stroke is seen as unreliable.

I think the fact is that a 2 stroke needs a bit more tender loving care compared to a 4 stroke because of the unclean combustion and potensial oil faults. You can't treat them as 4 strokes (maintenance wise) then expect them to last the same. They cost much less, but we still want compare them the same.

We can't have our bread buttered on all sides you know.

If you are the type of person that do not want to take care of your engine & fuel then a 2 stroke is not your engine. A 4 stroke is then a little more forgiving, but you pay for it with you wallet.

I wonder how many of the 2 stroke 'failures' can be attributed to engines that have run way past their maintenance schedules. Ignorance is bliss, until it bites.

Kind Regards
Rudi

[RudiGreyling]

Here is another good article comparing all the rotax engines from all angles, power to weight, power to cost, cost to tbo, operating cost, etc etc. It has a lot of comparison graphs.

It is used for people that need to decide what engine to buy!
http://www.800-airwolf.com/pdffiles/ART ... cision.htm

Regards
rudi

[Morph]

If you are the type of person that do not want to take care of your engine & fuel then a 2 stroke is not your engine. A 4 stroke is then a little more forgiving, but you pay for it with you wallet.

I wonder how many of the 2 stroke 'failures' can be attributed to engines that have run way past their maintenance schedules. Ignorance is bliss, until it bites.

I think if you are in the first category, you are going to have problems anyway. If I think back to all the problems I have and others in the club have had over the past year they would be
1. Fuel and fuel systems related - blocked filters, insufficient air ventilation in the tank, cracked pipes
2. Breaks in the drive/gearbox function to the prop. Bolts shearing off, gearboxes falling off ala Junkie, Rubber coupling shearing off, prop strikes
3. Near misses with incorrect pref light inspections. Being distracted or talking while doing preflight. Covers left on airfilters, tops left off coolant reservoir bottles on radiators, causing overheating due to loss of liquid, tops left off the oil reservoir

These can happen to both 2 stroke and 4-stroke.

Things like carb icing can happen to both etc etc

I know of 2 instances where incorrect/no maintenance on a 2-stroke caused the engine to quit. One the 503 SCSI had been standing for years, untouched, unstarted unmaintained and the guy came to the field, threw fuel in, started and took off, only to have the whirly bits stop and neccesitated an emergency landing.

The second is a 503, that had previously seized due to incorrect fuel mixing, was not repaired properly and later, 100 or so hours TTSN, the crank, bearings the works seized up solid. It was found that the bearings had suffered damage at the first seizure and not repaired properly.

One we still don't have an answer for was Marcel's crash in the skyfox. Flying low level, engine stopped, resulting in crash. After the crash the engine was started again and ran perfectly. In talking to Marcel I deduce it had to be intermittent electrical fault cutting the ignition. It was not a heat seizure or fuel starvation as CAA would have it. The motor ran and then stopped.

BTW, I love a 4-stroke, at least you don't have to regularly fork out money to pay for major services (although thinking of it at my rate of 100 hours a year it's once every 3 years I would have to spend a few thousand on maintenance) but I will only look at this sort of option when I can afford it, it will be a long term investment, i.e. 10 years or more, and I don't have the 450kg weight restriction that limits my carrying capacity to 170kg, Pilot, PAX and fuel. I want to be able to carrya payload of Pilot, PAX, fuel and some luggage. So I need to be able to carry 250KG legally.

[DieselFan]

Ditto Krusty,
I think the fact is that a 2 stroke needs a bit more tender loving care compared to a 4 stroke because of the unclean combustion and potensial oil faults. You can't treat them as 4 strokes (maintenance wise) then expect them to last the same. They cost much less, but we still want compare them the same.

whereas without you have to be more religious - this is good as the reliability is almost directly controlled by YOU, dont skip a service

This was largely a reason why diesel engines were given a bad name. Because it's easier to put crap in they will die later and rather unexpectedly.

Human error

I'm glad you started this topic! As I feel a lot better about 2 strokes whether it be bikes or mls.

[Griffin]

Has any one done a comparison of the reliability of a 503 vs a 582?? I do not like the 582 (may have something to do with haveing my only engine out with one). 503 just seem bomb proof. Any opinions??

[Pumba]

Interesting topic this. A few comments on what others have mentioned here: Someone alluded to 2 strokes having oil in a sump. If you study the working principle of your normal spark ignition (Petrol) 2 stroke engine, this is not possible as they make use of what is called crankcase induction. As the air / fuel mixture is actually introduced into the crankcase, this would constantly dilute the oil.

To explain the principle: The piston moving upwards creates a lower pressure in the crankcase, and this sucks the fresh change of air / fuel mixture into the crankcase. At the same time it compresses the mixture contained in the cavity above it, and near the top of the stroke, the sparkplug ignites the mixture, forcing the piston down. About 2/3 the way down on the power stroke, first the exhaust and then the inlet ports are uncovered. A part of the expanding exhaust gasses are pushed out through the exhaust port. The downward motion of the piston raises the pressure in the crankcase, forcing the fresh charge through the inlet ports, and this inrush of fresh charge displaces more of the exhaust gasses out of the cylinder through the exhaust port. Reed valves or a rotary valve prevents the charge from being pushed back through the carb when the piston moves downwards.

A few important things to note here: You never achieve perfect scavenging where all the exhaust gasses are displaced by the fresh charge, and some of the fresh charge is actually also pushed out of the exhaust port.

We don't fly these but just for interest: A 2 stroke compression ignition (Diesel) engine works slightly differently. These have inlet ports at the bottom of each cylinder and an exhaust valve at the top. At the bottom of the stroke fresh air is forced through the ports (under pressure from a supercharger) while the exhaust valve is open. This displaces the exhaust gasses out of the cylinder through the exhaust valve, the exhaust valve closes, the air is compressed by the upward moving piston and at the top of the stroke, atomized fuel is injected into the highly pressurized hot air mixture, and it ignites to give you the powerstroke. Diesel 2 strokes are lubricated in the same way as 4 stroke engines as the fuel does not have the same contact and chances of contaminating the lubricant.

So what then makes a 2 stroke unreliable? To start with, this statement is actually somewhat flawed. Any machine can actually be more or less reliable, depending on how well it was designed and made, and thereafter how well it has been taken care of and operated. All machines (4 stroke engines included) wear and will eventually break down.

Two strokes do have some attributes that make them more susceptable to problems than 4 strokes:

1. The lubricant needs to be introduced with the air / fuel ratio, and is actually marginal to start with. The purpose of any lubricant is to keep the metal surfaces separated from each other, so if this breaks down, you have metal to metal contact and high wear and possible seizure. Whether you opt to use premix or an oil injection system, make sure that the oil gets to where it needs to do its work.

2. The whole set up is more complex than what it appears. Back pressure in the exhaust system is critical for proper scavenging to take place. Anything blocking up or going out of whack is more of an issue than on a four stroke.

3. Every downward stroke is a power stroke, so almost twice as much heat is generated / introduced in the engine. All components are working at much higher temperatures than in an equivalent four stroke.

4. The lubricant in the air / fuel mixture is burnt, and can lead to carbon build up. If a piece of carbon gets lodged between the electrodes of a sparkplug it shorts it out. Carbon also builds up on the sides and underneath the piston rings, making them stick in their grooves. With the ring partially exposed by the ports on every upward and downward stroke, a stuck ring can be broken causing severe damage to the engine.

5. Two strokes are very sensitive to running lean. Air / fuel ratios are far more critical than in four strokes, and incorrect jetting can easily lead to seizure.

6. In a four stroke oil is pumped under pressure and splashed around all over the "wetted parts" of the engine. This removes a lot of heat and ensures proper lubrication. In a two stroke, there is no oil to remove heat, and this makes the engine cooling system that much more important.

All of these factors combined convinces me that in equally created and maintained engines, the four stroke would be inherently more reliable than the 2 stroke. Where the 2 stroke scores highly and beats the 4 stroke hands down is power to weight ratio. It theoretically generates close to twice the power of a similar displacement 4 stroke, and does not have camshaft, valves, oil pump and sump and all the other gear required to make a four stroke work. In practice, this is closer to around 70%.

If you study all the engine failures and engines outs, 2 or 4 stroke, it will be interesting to see what the major contributors are. I would guess the number one cause would be pilot error or neglect, manifesting itself in fuel starvation. (Dirty or clogged filters, water in the fuel etc.) You can't blame the engine, only the pilot.

The number of engine outs on Microlights are also not a good indication of the inherent reliability of a 2 vs 4 stroke engine. There are simply many more 2 strokes on MLs and most engine failures are not really the engines fault to start with and if neglected, 4 strokes will fail as surely as a 2 stroke.

My 2c worth.

[ZULU1]

I think this article above sums it very well...anyway another factor..My HKS has done 100 hrs in 10 months mainly for headset testing. Many circuits:

My last trike a 582, first crank 189 hrs, second 380 hrs, sure I was flying in Mozambique etc. and the coast. Anyway I have flown a 503 a lot in Ladysmith and know of some approaching 1300hrs..Enough was enough.

My fuel consumption never better than 16 ltrs per hour. HKS is about 6-7ltrs per hour, oil change at 100hrs (4 litres of Shell Ultra synthetic) plug change (Iridium) at 200 hrs, oil filter ZAR 43,00.

So approx saving 10 trs per hour (excluding oil) = ZAR 5,000.00 and this was not the main reason to change.

And back to my comments..its nice to watch the oil temp and pressure gauges..not missed a beat and is a real 60hp. On rotation it gives a strange gyroscopic kick with the torque reaction. Noise pollution is very low as it sounds like a 1/2 Cessna !!

I dont think it will perform so well in Gauteng, but coast very happy. I still like the 503 though.

eish Zulu1