2 Stroke reliability

[sqwkvfr]

Trabants

Does anyone else remember the "Car and Driver" contest a few years back where they were gonna give away a POS "Trabby" to the winner?

Someone from the EPA read the article and the agency prompty seized the car and crushed it.

I'm not joking...apparently it was imported illegally and wasn't even close to meeting US emission standards.

 

[ackattacker]

I had a german friend who owned a Trabant. One person could lift the back of the car off the ground.

He crashed it into a ditch and the car didn't have a scratch. With the plastic body, a crash either did no damage or broke the car in half. No middle ground.

A man stops his Trabant at a garage and says: "Two windscreen wipers for my Trabant". The garage owner thinks for a moment, then replies "OK, that's a fair deal".

 

[UA-RESURRECTED]

and as far as fuel efficiency, far more efficient than a four stroke engine.

 

[UA-RESURRECTED]

Avbug,

Not withstanding small, kit-type aircraft, why does nearly every single piston-engine airplane come from the factory with 4-stroke engines?

Answer: Because they are more reliable, and more fuel-efficient.

 

[UA-RESURRECTED]

...more engine failures behind four stroke piston engines ranging from small four cylinder powerplants to large radials, than any other type of engine.

Duh, that's because 4-strokes USED FAR MORE OFTEN than any other type of [piston] engine.

 

[avbug]

Not withstanding small, kit-type aircraft, why does nearly every single piston-engine airplane come from the factory with 4-stroke engines?

Think. How many certificated two-stroke engines are available for use in production aircraft? Have you any idea the cost involved in certificating an engine for production use in normal category or utility category operations with a standard airworthiness certificate?

Answer: Because they are more reliable, and more fuel-efficient.

Good guess, but no.

Duh, that's because 4-strokes USED FAR MORE OFTEN than any other type of [piston] engine

You're responding to a half-quote taken out of context, to make a point. You failed to include the entire quote, which specifically referred to my experiences with engine failures...truth is just as I correctly stated; I've experienced more engine failures on four stroke pistons, ranging from flat norizontally opposed to large radials, than any other type of powerplant...and no, I haven't used four stroke engines far more often than any other type of piston engine. If you're going to quote to make a point, either include the full quote, or ensure you keep it in context.

They also consume far more fuel/oil in a given amount of time, and therefore are more expensive to operate.

Fuel burn is the least expensive part of operating an aircraft engine. On a per hour basis, maintenance accounts for more of the overall engine operating costs than fuel and oil combined, often a lot more.

As for oil consumption...ever run a radial engine?

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.

You may not be assured of a longer life; there are no gaurantees. Certainly not reduce operating costs. Operation of an A65 or an 0320 is never going to be as inexpensive as operating a light two stroke, particularly when overhaul costs and maintenance costs are factored in. Not by a long shot.

Now a great deal has been made of my discussion regarding specific fuel consumption, and a lot of effort has gone into disproving my claim that two stroke engines are more fuel efficient. Problem is, I never made any such claim. Nor did I introduce specific fuel consumption. I did say pound for pount the two stroke puts out more power...I didn't say pound for pound of fuel, but pound for pound of engine weight, and that's true.

Engine efficiency is subjective, and pound for pound, dollar for dollar, and on the basis of available selection of powerplants for light aircraft, the two stroke offers more options, lower costs, and more power per pound of engine weight. In that respect, the two stroke is more efficient to the application, and certainly a more suitable choice in many cases. In many, it may be the only choice.

My project, a Sorrell Guppy, was originally run on a 18 hp cushman engine, but with a two stroke reliable hirth, I can stay within the engine weight limits and preserve the balance of the design at 50 hp. It won't cruise much faster, but certainly will climb better, and offers far more advantages than disadvantages for the application.

2-strokes have an annoying habit of self-destructing when run lean (siezing or burning holes in pistons).

So do four stroke engines, particularly with respect to detonation and mixture management. The two stroke is not necessarily any more susceptable, especially considering modern metal alloys and treatments.

 

[ackattacker]

Now a great deal has been made of my discussion regarding specific fuel consumption, and a lot of effort has gone into disproving my claim that two stroke engines are more fuel efficient. Problem is, I never made any such claim. Nor did I introduce specific fuel consumption. I did say pound for pount the two stroke puts out more power...I didn't say pound for pound of fuel, but pound for pound of engine weight, and that's true.

the engine doesn't burn as much of it's fuel charge...it gets more power out of it pound for pound

I guess I misunderstood this sentence. I assumed "it" was fuel charge. What was "it"?

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

I guess I misunderstood this also. I assumed "walks away from" meant "produced more power for the fuel consumed"

Not having to drive an accessory section, valve train, etc, means more power goes out the crankshaft...in a four stroke engine, a considerable amount of the power produced is used overcoming internal friction and resistance of all the components in the engine. For the same power produced, in a two stroke, more goes to the end user and not the engine, and it is therefore, more efficient.

I must have misunderstood this also. You appear to be claiming that the internal friction of a 4-stroke engine makes it less efficient than the 2-stroke. I guess I was wrong.

Faster response, more torque for the fuel burned.

Here I thought you claimed a 2-stroke produced more torque for the fuel burned than a 4-stroke. I guess I misunderstood you there also.

 

[avbug]

I guess I misunderstood this sentence. I assumed "it" was fuel charge. What was "it"?

I guess you did. This happens when we don't seek clarification. Pound for pound, a two stroke engine is generally lighter than a four stroke engine, both due to the materials of construction, and the lack of all the weighty parts that go into a four stroke, such as a valve train, accessory section, etc.

While the two stroke engine disperses more unburned fuel in it's exhaust than a four stroke engine, the fact that it's lighter and the fact that it doesn't need to produce excess power to overcome it's own internal friction, and it needs to make less revoloutions per piston to accomplish the same functions (eg, power stroke is also exhaust stroke) means a smaller, lighter engine produces the same output as a larger heavier engine. Pound for pound, that small two stroke is putting out more power, and it's not wasting it on enternal needs. Less wasted energy means something is operating more efficiently.

I guess I misunderstood this also. I assumed "walks away from" meant "produced more power for the fuel consumed"

That would really depend on the power setting and opeating conditions, as I've described previously...the specific operating conditions do indeed impact the issue. The following link discusses this on a power boat, and the person making the observations notes that at wide open throttle, or full throttle, the four stroke outboard boat engine in his comparison increases in fuel economy enough to offset the cost of the diffference between the engines over a set period of operation...however, he notes that those are not real operating conditions...and under real operating conditions, the two stroke burns less fuel per hour with the same miles per gallon, on the same boat under the same conditions.

http://continuouswave.com/ubb/Forum4/HTML/001019.html

http://continuouswave.com/ubb/Forum4/HTML/001078.html

Bearing in mind that we're talking about small engines, the discussions and arguements become skewed and very greyed when we move into larger engines. Large power equipment, trains, power stations, large generators, cruise ships, rapid response fire engines, etc, use two stroke engines, for good reasons. In many cases, these engines are turbocharged, and may also have valves. However, they still only make one revoloution per piston cycle, and still put out more power for their weight, than a four stroke. These engines put out considerably more power per pound of engine and per cubic inch of displacement, than a four stroke.

Seems as the engines get smaller and smaller, two stroke is far more efficient for the intended use, and as the engines get larger and larger, two stroke is more efficient and more common in use, than the four stroke. Why do you suppose that is? The larger two stroke engines are diesel engines, and do not experience the environmental issues that face typical light two stroke gasoline engines. Big or small, the two stroke has the potential to produce twice the power for a given RPM than a four stroke.

Simple advances such as loop scavenging also contribute to greatly inproved efficiency, in terms of emissions, fuel consumption, and power in the two stroke engine.

The Wartsila-Sulzer RTA96-C turbocharged two-stroke diesel engine is the most powerful and most efficient piston power plant in the world today and is used in ships. And that is in terms of specific fuel consumption, as well as power vs. weight.

I must have misunderstood this also. You appear to be claiming that the internal friction of a 4-stroke engine makes it less efficient than the 2-stroke. I guess I was wrong.

No, you were correct. But again, that would depend on the engine. If we're talking about small two strokes, then yes, less internal friction and less internal power wasteage...if a 60 hp engine must produce 70 or 80 hp to output that 60 hp, then it's less efficient than an engine which does not need to waste it's power overcoming it's own internal needs.

In theory, a two stroke engine should perform at twice the power output for a given cubic capacity, than a four stroke, though it's often more like half that with current design...about fifty percent more efficient with respect to power output. This is mitigated and brought up with exhaust tuning, induction boosting, etc.

If you really want efficiency, forget either of these engines and go for a wankel engine such as a mazda rotary...which might be described as a "three stroke" or a one-stroke, due to it's continuous power implulse.

Here I thought you claimed a 2-stroke produced more torque for the fuel burned than a 4-stroke. I guess I misunderstood you there also.

Asked and answered, under real world conditions, including the world's most efficient engine...yes.

I'm scratching my head on this one, as I'm not sure what you're getting at. In a constant speed propeller installation, power is torque and torque is power, since RPM is constant. Tell me the RPM and torque on your propeller and I'll tell you the HP of your engine. Tell me the HP your engine is producing and propeller RPM and I'll tell you the torque. I don't need to know the blade angle, airspeed, angle of attack or any of that. It doesn't make any difference if it's 2-stroke, 4-stroke, or turbine - power is power, and power = torque x rpm.

"horsepower" in aircraft engines is a sales term, but yes, it may be used in a mathematical comparison. However, RPM in operation is not a direct indication of available torque. An engine with a constant speed propeller will experience a torque increase as the RPM decreases as blade angle is increased, as the load is increased on the crank or drive shaft. Two engines producing the same RPM in level flight may not be able to produce the same RPM under a load, such as in a climb. Under a load, one engine may be able to maintain torque, whereas another might lose it. The two stroke engine does better under a load.

A four stroke engine has limitations with respect to RPM and efficiency, as well as operational capability. These range from harmonic issues to detonation issues, to valve float. These are not problems normally asociated with a two stroke powerplant.

Arguements may be made for, or against, and have been for many decades. Much like pitch for power or visa versa, or the subject of downwind turns...it's an unwinnable discussion as both two stroke and four stroke powerplants have their drawbacks and advantages. When discussing both, however, it's important to stay away from nonsensical arguements such as why more aircraft engines are four stroke vs. two stroke, etc...and stick to salient points of interest.

For the subject of the thread, light powerplants in an ultralight aircraft, little option exists beyond any of the excellent two stroke designs available today. Certainly some wonderful advances in four stroke small engines are available such as the Jaiburu and HKS, but even these are more expensive and too heavy for light airplane applications that require engines such as Rotax or Hirth two strokes. Economically, the two strokes win hands-down; they're cheaper to buy, run, maintain, overhaul, and operate.

While a slow evoloution in aircraft piston engines has seen gradual and minimal increases in technology for four stroke installations, we can generally see that we're flying behind powerplants that aren't much more advanced that what powered airplanes in 1930...nearly eighty years later. The technology is there, but won't be utilized in the face of the cost of certification and the very limited aviation market.

 

[Huggyu2]

As someone who's not a mechanic, and wants minimum hassels with an engine, I'll opt for a Rotax 912 over a 582 (or other two stroke) for any UL-type aircraft I get. With a 1500 hour TBO compared to 300 in the two-strokes, it's worth the extra cost to me.

 

[rtmcfi]

Pound for pound, a two stroke engine is generally lighter than a four stroke engine

Which weighs more, a ton of lead or a ton of cotton?

Face it avbug, a two stroke burns more fuel in order to produce the same amount of work.

Two strokes have some efficiencies, but fuel consumption (the generally accepted measure of efficiency) is not one of them.