2 Stroke reliability

[imacdog]

For those experienced with ultralights, what are your opinions on operating 2-stroke engines? I am looking into the possibility of a larger 2-seat ultralight-style sport plane and am wondering what the advantages and disadvantages are. Do all 2-strokes need to run on premixed oil, or do some have separate oil tanks? I am trying to decide if it would be worth the peace of mind to invest in a more expensive 4-stroke like a Rotax 912, over a Rotax 582. Thanks for any advice.

 

[UA-RESURRECTED]

2-stroke engines are a blast. They're Lightweight, simple, and produce tons of power compared to your typical putt-putt 4-stroke.

Most modern 2-stroke application will use oil-injection, eliminating the need to pre-mix.

The problem though, is that they typically do not last a long, nor are as reliable as a typical 4-stroke. They also consume far more fuel/oil in a given amount of time, and therefore are more expensive to operate. Given these facts, the price of a 2-stroke engine may seem like a good deal, but once you buy a 4-stroke, you can be assured of longer engine life and reduced operating costs.

It will come down to personal preference. The only reason 2-strokes are still around is because they are lighter and more powerful than 4-strokes. This makes them ideal for snowmobiles, watercraft, and ultralights, where power/weight can be an issue. Buy a 2-stroke it you enjoy instant and plentiful power when you open the throttle. Buy a 4-stroke if you want reliability/fuel efficiency.

 

[ackattacker]

2-strokes have an annoying habit of self-destructing when run lean (siezing or burning holes in pistons). Jetting is problematic. Oil injection is problematic. Go with a 4 stroke for piece of mind...

 

[imacdog]

Thanks, I'll keep that in mind. I fly large RC planes with 2-stroke engines with electronic ignition, and so far they have been extremely reliable. I'll start looking around at the different four-strokes, such as the Rotax and Jaribus, if I can find them. Reliability is especially important to me and I'm willing to sacrafice power-weght to have the more reliable choice.

 

[PUNISHER]

2-strokes have an annoying habit of self-destructing when run lean (siezing or burning holes in pistons). Jetting is problematic. Oil injection is problematic. Go with a 4 stroke for piece of mind...

Ultralight 2-strokes, Sound like a pissed off weedwacker.

 

[avbug]

The two stroke offers some exceptional advantages for certain applications, such as areas in which weight is a concern in light aircraft. Reliability is a tradeoff, as is longevity...but the modern two stroke such as a Hirth is also a lot less expensive to overhaul and maintain.

Many airport rescue vehicles run two stroke; it isn't just for chain saws. The rapid acceleration and light weight is a great advantage, as is size and simplicity.

Modern alloys and metal treatment have reduced the expansion and siezing problems, as well as wear issues that plagued older two strokes.

 

[Huggyu2]

Avbug is right, with respect to the weight issue. However, if you're buying an aircraft where a bit more weight isn't that much of a factor, the extra horsepower and reliability might be worth it.
I'm considering some UL 's, and I'll choose a 912 (or 912S). Why? 300 hour vs 1500 hour TBO is one reason. And while many of us might sell our ultralight after 300-500 hours, I believe the 912 UL will hold the value better. Would you rather buy a 500 hr 2-stroke, with 100 hours to go until TBO? Or a 500 hour 4-stroke with 1000 hours until TBO?
I'm not an expert, but I think if I pay the extra for the 4-stroke up front, then I'll pay less in the long run.

 

[ultrarunner]

Buy a 4-stroke if you want reliability/fuel efficiency.

That's always been big in my book when operating an airplane.

No one really cares when their weed-wacker quits!

 

[avbug]

Four stroke is no gaurantee of reliability; I've had more than a few failures ranging from lifted cylinder heads to catastrauphic engine failures in four stroke engines...more engine failures behind four stroke piston engines ranging from small four cylinder powerplants to large radials, than any other type of engine. Once a two stroke starts running, it's operation is very simple and straightforward.

No oil system to fail in many. No valves to fail in many. Dirt simple carburetion, dirt simple fuel system. Increadibly simple in construction, easy to work on. No timing to worry about. Lightweight...and as far as fuel efficiency, far more efficient than a four stroke engine. For the power produced vs. fuel consumed...the two stroke walks away from the four stroke. That two stroke fires every time the piston cycles. The power response is faster, crisper, more immediate

Two strokes do have their drawbacks, to be sure, from noise to the fact that the engine doesn't burn as much of it's fuel charge...it gets more power out of it pound for pound and gets it faster, but it also spits out unburned fuel and usually oil with it...it's messier. You generally need to mix the fuel and oil when fueling.

Reliability...I've had turbine engines come unglued on me in a major way, and a turbine engine is generally considered to be more reliable than a piston engine. Perhaps, and perhaps one can dredge up hour by hour statistics to show which is more reliable, but those statistics mean exactly squat when one is looking at sagebrush and pinion rushing up beneath the wings because the powerplant has failed. It happens.

Modern lightweight two stroke powerplants sell for two reasons; they weigh very little in applications where weight is critical, and they're affordable.

Which will cost you more in the long run? When you overhaul that four stroke, you're going to be spending thousands, if not tens of thousands of dollars. When you overhaul that two stroke, even if it has a shorter TBO interval, you're going to be spending hundreds. If that. If you have money to burn and have the choice, then four stroke is a great choice...it's also got a lot more going on internally to fail. The more there is to fail, the greater the possibility some might say. You decide.

 

[ackattacker]

For the power produced vs. fuel consumed...the two stroke walks away from the four stroke.

I'm going to disagree with you there. 4 strokes are significantly more fuel efficient in general. I could come up with many examples.

From the wikipedia:

Two-stroke engines have several marked disadvantages that have largely precluded their use in automobiles (although there was some use, such as in historic Saabs and DKWs and until recently in several automobiles produced in the Eastern bloc, including Trabants and Wartburgs, among others) and are reducing their prevalence in the above applications. Firstly, they require much more fuel than a comparably powerful four-stroke engine due to less efficient combustion. The burning oil, and the less efficient combustion, makes their exhaust far smellier and more damaging than a four-stroke engine, thus struggling to meet current emission control laws. They are noisier, partly due to the more penetrating high-frequency buzzing and partly due to the fact that muffling them reduces engine power far more than on a four-stroke engine (high-performance two-stroke engine exhausts are tuned by determining the resonant frequency of the exhaust systems and exploiting it to top-up the fuel air charge just before the cylinder port closes). Finally, they are considered less reliable and durable than four stroke engines