2 Stroke reliability

[avbug]

The reference you cited applies to automotive applications, and what is automotive is not aeromotive...most of that doesn't apply to the use of a two stroke powerplant in an aircraft...and while a two stroke may consume more fuel, it puts out more power pound for pound than a comparable four stroke motor, with far less moving parts and complexity. The biggest reason we don't see them more is emissions, and that could be solved, too.

Efficiency is quantifiable by many parameters, but when speaking of efficiency, does one speak in terms of miles per gallon, gallons per hour, or one of a dozen other possible frames of reference?

The two stroke is a very efficient way of packaging a lot of power in a lightweight, simple mechanism, and consequently is a very efficient method of powering a light aircraft.

 

[ackattacker]

I'm not following you avbug. Find me an aeromotive 2-stroke which is more fuel efficient (specific fuel consumption) than it's comparable 4-stroke. (Not counting diesels).

and while a two stroke may consume more fuel, it puts out more power pound for pound than a comparable four stroke motor, with far less moving parts and complexity.

This I agree with. It puts out more power per pound of motor, not per pound of fuel. This may or may not represent an overall weight savings depending upon a lot of factors, mainly how far do you want to go. Reliability is also a much more complex function than simply number of parts. A modern jetliner is very reliable but is incredibly complex.

The biggest reason we don't see them more is emissions, and that could be solved, too.

I wasn't aware general aviation powerplants had to meet any emissions standards, other than maybe noise. A two-stroke burning regular unleaded gas is certainly less damaging to the environment than a 4-stroke burning 100LL, but that's neither here nor there.

Efficiency is quantifiable by many parameters, but when speaking of efficiency, does one speak in terms of miles per gallon, gallons per hour, or one of a dozen other possible frames of reference?

Normally when one is talking about an engine's fuel efficiency, one talks in terms of specific fuel consumption or SFC (lbs fuel per hour per shaft horsepower).

You previously claimed that two stroke engines were able to extract more power from a pound of fuel:

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

and:

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

I'd just like to see you back up those two statements.

 

[avbug]

Faster response, more torque for the fuel burned. 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.

Again, efficiency may be quantified in many ways. Specific fuel consumption is only one of them.

 

[ackattacker]

Faster response, more torque for the fuel burned. 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.

Again, efficiency may be quantified in many ways. Specific fuel consumption is only one of them.

What measurement do you want to use? SFC is what an engineer would deal with when measuring efficiency of an engine. SFC measures power at the crank. SFC includes the loses for internal friction, so if in fact a 2-stroke was more efficient then it would have a lower SFC than a corresponding 4-stroke. But the opposite is the case.

As for "torque", a much misunderstood word, torque and power are very closely linked by a simple formula. Power = Torque x RPM. Since most aircraft 2-strokes and many small aircraft 4-strokes use a reduction gearbox, the final prop RPM's will be similar. Hence two engines with the same power rating will produce the same torque at the same prop RPM. Take a 2-stroke and a 4-stroke, both producing 100hp, both spinning an identical prop at the same speed... and they produce identical torque. One cannot claim that the 2-stroke has more torque, or is extracting more torque per pound fuel.

It is common use to refer to an engine as "torquey" if it is able to produce significant power at low RPM. In this regard 4 strokes hold the clear advantage since they don't have a pronounced power band as most 2-strokes do. Very few people would describe your typical 2-stroke as "torquey". In fact the more appropriate term is "peaky", as in having a pronounced power peak at a certain (usually high) RPM. Tuning a 2-stroke to be "torquey" at low RPM's means trading off peak horsepower (detuning, usually through means of adjusting the exhaust pipe). Many two-stroke powered aircraft owners are familiar with the problem of getting the perfect propeller pitch. Too high a pitch and the aircraft never gets into the powerband for takeoff, resulting in piss-poor takeoff performance. Give it just a little less pitch and all of a sudden the engine gets "on the pipe" and can easily overrev.

I am not trying to knock 2-strokes. They certainly have their applications in aviation. But fuel-efficiency is just not one of them. Nor is "torque". Engine acceleration gives a slight advantage to the 2-stroke, due to the lower rotating intertia, but in practice this is a small effect since the inertia of the propeller is much greater anyway.

Since you claim SFC is not representative of overall efficiency, let's examine a typical installation. I choose the Murphy Renegade as typical of a light sport-plane, on the criteria that I really like it. And it has factory performance specs for 2-stroke and 4-stroke engines.

http://murphyair.com/Product_Info/Renagade/Performance.htm

Using a 2-stroke Rotax 582 (65 hp) the aircraft has an empty weight of "420 to 500" pounds. Lets take the average, 465. Gross weight is 950 lbs, leaving 485 lbs as useful load.

Using a 4-stroke Rotax 912 (80 hp) the aircraft has an empty weight of "460 to 520" pounds. Again use the average, 490. Useful load is now 460 pounds, or 25 pounds less.

So it doesn't look good, our 4-stroke airplane is heavier. So it looks like we won't be able to carry as much. BUT, let's assume we want to fly somewhere 200 miles away. Our 2-stroke version cruises two-up at 75% power at only 72mph, while burning 4.5 gallons per hour (16 mpg). The 4-stroke version cruises two-up at 75% power at 80 mph and only burns 4 gallons per hour in cruise (20 mpg)

Leaving a 30 minute VFR reserve, The two-stroke version requires 14.75 gallons... which is .75 gallons more than it holds. So you can't take the trip at all, while the 4-stroke version only requires 12 gallons and so can make it handily, while burning 16.5 pounds LESS gas, nearly making up for the extra weight of the engine.

 

[avbug]

What measurement do you want to use? SFC is what an engineer would deal with when measuring efficiency of an engine.

As the issue is a small engine in an ultralight aircraft, all the numbers don't really mean squat...the point is mute. Efficiency is being able to power an aircraft in this case with a very light engine. You could probably make it happen with a really big engine, heavier structure, carry more fuel, blah, blah, blah...but that would make it...less efficient. Get it?

Since you claim SFC is not representative of overall efficiency, let's examine a typical installation.

I made no such claim...I did however correctly assert that efficiency may be defined in many ways.

If a finely designed four stroke engine with the inner workings of a swiss watch is too heavy for the structure in which it is targeted, then it just ain't that efficient. Period. Efficient is what is light enough to fit in that aircraft and deliver the power required.

As for your comments regarding engine RPM and torque, they do not apply, except in a fixed pitch installation for comparitive purposes. Torque becomes an issue in a constant speed, or variable pitch application, which is inclusive of some ultralightor light airplanes utilizing two stroke engines with flexible propellers, or adjustable propellers. As pitch is increased on a propeller (as it is moved toward coarse), more torque is required to generate the same RPM for any given airspeed...torque is more than merely propeller RPM. It must consider propeller loading, which includes other factors such as blade angle and angle of attack, which in flight may be considered a function of pitch and airspeed, among other things.

Clearly there's more to the topic. Fact is that for the same size (eg mass, or weight), a two stroke can put out more power, particularly with regard to small light aircooled airplane engines. A heavier four stroke engine will be required to put out more power to sustain the same flight condition, because the aircraft is heavier. In so doing, any comparitive difference in fuel efficiency is lost...the point of diminishing returns has been reached, and one may well find that one burns less fuel with the two stroke as less fuel is required to accomplish the same thing for a given parameter...time, speed, distance, etc.

Exactly what the numbers are will of course, vary with the aircraft and flight conditions (eg, density altitude, operating weight, etc).

Two strokes are cheaper to operate, cheaper to overhaul, cheaper to purchase, lighter, put out more power per pound than compartive four stroke engines of the same mass, are much more simple in construction, more tolerant of change in operation, and in many cases, ideally suited for the light aircraft they power. Getting into the lighter aircraft, a 1/2 vw still can't compete with a typical Hirth or Rotax...and even if you could wrap it up enough to produce the same power, it would be burning considerably more fuel and would have become...you guessed it...less efficient. Including fuel efficient.

 

[ackattacker]

As the issue is a small engine in an ultralight aircraft, all the numbers don't really mean squat...the point is mute. Efficiency is being able to power an aircraft in this case with a very light engine. You could probably make it happen with a really big engine, heavier structure, carry more fuel, blah, blah, blah...but that would make it...less efficient. Get it?

This is not an issue I have debated. Any engine which is not suitable for the intended installation is just that, not suitable. If your intended use cannot tolerate the weight of a 4-stroke engine then your powerplant decision is an easy one. I was addressing your claim that a 2-stroke engine extracts more *power* from *fuel*, otherwise known as "fuel efficiency".

Efficient is what is light enough to fit in that aircraft and deliver the power required.

I would propose that what you are calling efficiency is more akin to "suitability". A 2-stroke may very well be a more suitable engine for certain ultralights. Other ultralights may be designed so that a 4-stroke is equally or more suitable.

As for your comments regarding engine RPM and torque, they do not apply, except in a fixed pitch installation for comparitive purposes. Torque becomes an issue in a constant speed, or variable pitch application, which is inclusive of some ultralightor light airplanes utilizing two stroke engines with flexible propellers, or adjustable propellers. As pitch is increased on a propeller (as it is moved toward coarse), more torque is required to generate the same RPM for any given airspeed...torque is more than merely propeller RPM. It must consider propeller loading, which includes other factors such as blade angle and angle of attack, which in flight may be considered a function of pitch and airspeed, among other things.

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.

You are correct that more torque is required to maintain RPM as pitch increases. But that's the same thing as saying more power is required to maintain RPM as pitch increases. It's one and the same. It takes more power to turn a coarser propeller because a coarser propeller is doing more work. In a constant speed application of course the cause/effect is reversed. Adding more power causes the propeller to coarsen, not the other way around.

The point I am driving at is that a 100 hp 2-stroke engine and a 100 hp 4-stroke engine hooked up to indentical constant speed propellers will behave exactly identically and at all airspeeds. They are both inputing to the propeller the same torque and at the same rpm, the propeller can't tell the difference.

Clearly there's more to the topic. Fact is that for the same size (eg mass, or weight), a two stroke can put out more power, particularly with regard to small light aircooled airplane engines. A heavier four stroke engine will be required to put out more power to sustain the same flight condition, because the aircraft is heavier. In so doing, any comparitive difference in fuel efficiency is lost...the point of diminishing returns has been reached, and one may well find that one burns less fuel with the two stroke as less fuel is required to accomplish the same thing for a given parameter...time, speed, distance, etc.

Exactly what the numbers are will of course, vary with the aircraft and flight conditions (eg, density altitude, operating weight, etc).

Two strokes are cheaper to operate, cheaper to overhaul, cheaper to purchase, lighter, put out more power per pound than compartive four stroke engines of the same mass, are much more simple in construction, more tolerant of change in operation, and in many cases, ideally suited for the light aircraft they power. Getting into the lighter aircraft, a 1/2 vw still can't compete with a typical Hirth or Rotax...and even if you could wrap it up enough to produce the same power, it would be burning considerably more fuel and would have become...you guessed it...less efficient. Including fuel efficient.

This is all correct... but I would propose that the "point of diminishing return" for overall fuel efficiency is a very light aircraft indeed.

 

[avbug]

This is all correct... but I would propose that the "point of diminishing return" for overall fuel efficiency is a very light aircraft indeed.

Which is precisely what will be using a small two stroke engine, and why this thread is in the ultralight forum.

 

[ackattacker]

Which is precisely what will be using a small two stroke engine, and why this thread is in the ultralight forum.

I meant so small that it would probably not be able to carry a normal sized person. Even large RC planes show better fuel economy generally with 4-strokes. Show me a design which is capable of using either a modern 2-stroke or modern 4-stroke engine of similar power, and which demonstrates better overall fuel economy with the 2-stroke engine. Use any measure of fuel efficiency you wish.

In order to reach the point of diminishing returns the increased induced drag from the increased weight must offset the improved fuel efficiency of the 4-stroke engine. Because the 4-stroke engine is significantly more efficient, you're going to need a lot of extra drag. I don't think you'll be able to come up with one (although I admit it's a theoretical possibility, if your design operates very far up the back side of the power curve).

 

[ackattacker]

Also, consider this comparison:

A 60 hp Hirth 2-stroke engine weighs 79 lbs, plus 19 lbs for gearbox, plus radiatior and coolant (about another 10 lbs) for a total of about 108 lbs. At 75% power (45 hp) it burns 3.15 gallons / hour.

A 60 hp HKS 4-stroke engine weighs 108 lbs including gearbox and electric start. Add 7 lbs for the exhaust, 6 lbs for oil tank and cooler, and 3 liters of oil (about 6 lbs) and you get a total system weight of 127 lbs. At 75% power (45 hp) it burns 2.38 gallons per hour.

So in this example the 4-stroke installation weighs 19 lbs more but burns 76% of the fuel per hour. In order for the overall aircraft efficiency to be equivalent that 19 pound weight penalty would have to cause a 25% increase in total airframe drag.

Not saying it's impossible, just not very likely...

 

[avbug]

Take it a step further...the weight may mean that the engine can't be mounted on the airframe at all...efficiency is entirely dependent upon perspective.