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.
