Sport_Pilot

Stay on topic please. Your post was wrong because you did not discuss the topic. This has nothing to do with baffling. This is about running a smaller higher picthed prop to help cool the engine running on the bench during break in.

downunder

Not really a wrong assumption because the tests were done with two different pitched props that gave identical loads so identical revs and needle setting for identical fuel flows. This meant that any internal cooling by fuel was identical so any difference in engine temps would be from external cooling. However I didn't try to hover my test stand so can't comment on 3D flying .

DarZeelon

Brian,


Some people already designed a radio-controlled, John Deere green 'flying lawn mower', that has been flown in various events here in Israel; and I am sure elsewhere too...

I am sure you, of all people, to complete your testing, can design a test-stand that can be flown in a 3-D manner, including some hovering...

wjvail

My Feedback: (3)

I shouldn't have said "wrong assumption". That was not correct of me to post. You didn't really begin with any assumption. You set out to show that what effect prop pitch cooling had on our engines and your tests suggests it has very little effect. In fact, I would suggest your tests support my assertion that airflow cooling is secondary to fuel cooling. In your test, given that fuel flow is constant but air velocity varies by more than 200% and temperature remain constant, only proves my point that our engines are largely fuel cooled.


Bill Vail

wjvail

My Feedback: (3)

I think we are closer to the source of our disagreement... In the above you mention "We cannot run our engines like that because they would detonate as the flame front would travel too fast. But when we richen our fuel we also cause the glow plug to run cooler lean and the timing advances." This suggests that we may HAVE to tune our engines at less than optimal for reasons other than efficiency and power. In that simple step, we agree that we may HAVE to tune to ALLOW our engines to be fuel cooled.

I will concede that the thermodynamics of fuel evaporation do not alone provide all of the cooling; however, we do not operate our engines from a text book. The vagaries of combustion chamber shape, plug temp, nitro, fin area etc all play a part, but I will continue to insist that fuel mixture plays a FAR larger part in engine temperature than any other form of cooling. Looking at our alcohol model engines as a complete system, I can directly control it's temperature in all modes of operation with fuel mixture.... I say again, our engines, in the way we operate them, are fuel cooled.

An example might be tightly cowled scale airplanes that are natorious for over heating. This would seem to be a simple case of cause and effect. Tighly cowled (cause)-> overheating (effect). But as is often the case, it may not be that simple. Some scale airplanes are also overweight and underpowerd and HAVE to be run very near peaked to fly. It is my assertion that the same scale airplane with a larger engine but the same cooling could be safely flown at a richer setting, produce the same power and not overheat.

Bill Vail

DarZeelon

Bill,


You had more or less hit the nail on the head.

My over-emphasis of preferring a high-pitch prop, results from all the so-called 'big names' understanding engine cooling in quite a simple way...

I.e. the simplified approach; "faster air-flow - better cooling"...


There is apparently more to it than this very straight forward equation...

Sport_Pilot

No he did not hit the nail on the head. The heat of evaporation is only 500 BTU/lb and even at the proper rich setting we should get over 8,000 BTU per pound for the heat of combustion, so under normal conditions the fuel is not the primary cooling method. And before someone says its the oil. I haven't found the specific heat of the oil but that is probably less than 1 degree per pound per degree F, so even if its over two then that would be even smaller. But if you run the engine extramely rich then the heat of combustion drops off rapidly and there is more excess fuel to provide cooling. And Dar while Yallaair's test shows that there is a differance in the engine temp with the smaller higher pitched prop, it wasn't that much, less than I would have thought. So for most practical applactions I don't think it makes any differance.

DarZeelon

Hugh,


The latent heat of evaporation of methanol is 473 BTU/lb and it is all taken from the metal surfaces the fuel contacts.

The heat of combustion of methanol is 9,970 BTU/lb and most of it is not absorbed by the engine's surfaces...

About 25% of the fuel's energy actually heats the engine's metal.

The remainder:
Heats the cycled air and exits through the exhaust.
Is turned into motive kinetic energy.
Is lost to reciprocating motion and friction.
Is lost through pumping.

So, only about 2,500 BTU/lb must be gotten rid of and fuel evaporation takes care of nearly 20% of that.
This is pretty much, compared to other engine types.

gkamysz

My Feedback: (19)

I do agree that the the engine cooling due to methanol can't be overlooked. It is documented in engine texts. The other thing to note however is that some of the heat needed to evaporate the fuel comes from the air, which has also been documented. Changing from gasoline to alcohol can improve volumetric efficiency significantly by cooling the inlet air. This is also documented in engine texts. Many methanol fueled racing engines use this to advantage by injecting fuel as close to the engine as possible with short intake tracts, thereby avoiding the intake air heating from the engine casing.

Dar where did you pull those figures from? I'm just curious. Wouldn't losses in friction and motion and pumping losses basically turn into engine heat?

DarZeelon

Greg,


There are may articles on the web and in high-school textbooks, that break down the energy produced by burning fuel, to where it goes... As you know, most four-stroke gasoline engines reside around 30-32% efficiency and real Diesels around 40%.

Our methanol burning two-stroke engines are close, I believe, to 15%... But the majority of the remaining 85% is not wasted on heating the engine's metal...

About the energy and latent heat of most fuels... Google's got many answers.

gkamysz

My Feedback: (19)

Dar, google does turn up many things. I was wondering if you had a specific reference. Thanks anyway.

yallaair

When a two stroke engine is ran very rich, most of the fuel is not even combusted. It's just blown though the engine. To evaporate the extra fuel takes energy. This is what gives the extra cooling when you run it ritch. Energy tranferet to the cyl. liner and cyl. head is also becoming less when the engine is ran rich, due to the late ignition. Hence, the total energy needed to be removed from the cyl. head and liner is less. This is what gives the lower engine temp when more fuel is added to the engine.

Sport_Pilot

It actually varies with temperture, but 500 is a nice round number.

We cannot get that number because we must run it rich, I rounded to about 8000 for a fairly rich mixture.
A good gasoline engine is less than 30% efficient, model glow engines may be less than 20% efficient. The exhaust temp is about 800 degrees? Not sure about that but if so then this indicates that a lot more then 25% is heating the metal.

DarZeelon

Hugh,


As you stated, the exhaust temperature of a glow engine is pretty high.

Actually, one of the reasons for the lower efficiency is the fact that when the exhaust-port is unmasked by the descending piston, the burn inside the cylinder has not yet been completed.

It id not incandescent gasses exiting, or the plug's glow that you see, when you look into the open exhaust of a glow engine running without a muffler. It is an actual flame, like from a military jet engine's afterburner.

A very large part of the heat produced by the combustion of the fuel, leaves the engine with the exhaust gasses. I believe this may be as much as 50% of the energy produced by combustion. The relatively large amount of super-heated, liquid lubricant, allows even more heat to be carried out with the exhaust gasses.


And about efficiency; a four-stroke, spark ignition gas engine can reach a level of 32%, while running at about 80% of its peak torque RPM. It will then be using about 0.45 lb/HP/hour of fuel.
When a two-stroke, spark ignition gas engine is discussed, the consumption number is a much higher ~0.7 lb/HP/hour of fuel.

The diminished efficiency is only 20.5% for this two-stroke engine.

So, our methanol fuel, glow ignition, two-stroke engines are in the vicinity of 12-15% efficiency. That is all.

Red B.

In the proceedings from the 31st National Conference on Fluid Mechanics and Fluid Power, 16-18 December 2004, Kolkata, India I found a measurement of the specific fuel consumption for an OS46LA using 75% methanol 5% nitro and 20% castor oil:

The lowest SFC was 1.8 gram/W/h = 3.0 lb/HP/hour @ 9000 rpm.

You do not want your car to have an SFC like that! By comparing the energy content of the fuel to the power output it is clear that the efficiency for the OS46LA is only 13%. As Dar pointed out this is probably what can be expected from small two-stroke glow engines.

Does anybody out there have any measured data on the SFC for four-stroke glow engines?

gkamysz

My Feedback: (19)

I have a figure of 20% for an OS 1.20 on spark ignition burning JP-8. I've seen some figures as low as 9% for a two stroke model size engine. Efficiency of up to 25% seems to be possible in a model size four stroke. Of course, the richer you set the needle the lower the efficiency goes.

Sport_Pilot

I did not mean to get into specifics about efficiency. The subject is not about just two stroke engines, but I don't think the four stroke engines ar more then a few percent efficient thant the two stoke engines. My point is that if the energy is not being used for work than likely more of may be used to heat the metal.

I think the proper arguement to the above may be that the loss of efficiency is due to lower peak temperature, if so then the loss of efficiency is going out the exhaust. But then again the higher exhaust temp may mean the average combustion temp is higher so the metal is heated with less work output. Not sure.

gkamysz

My Feedback: (19)

That was why I questioned Dar about his figures. I was wondering if it pertained directly to a model aircraft engine or was an estimate from some other source. Glow four stroke engines are almost twice as efficient as glow two strokes. I have read a few DOD proposals which tested existing model engines to compare to. Unfortunately google doesn't turn much up about model engine efficiency and it's stuff I ran across looking for other information. There was one college thesis paper that claimed an OS FS-30 was between .5 and 4.8% efficient. I'm sure that got a failing grade. They were clueless.

DarZeelon

I believe the efficiency of a model four-stroke would relate to that of a model two-stroke engine, in a similar way to the full-size relationship.

Similar, but not identical, since a full size four-stroke engine must also operate an oil-pump, that a model four-stroke does not...

So, a full size four-stroke reaches 32% (while spinning the oil-pump) and a full size two-stroke is at 20.5%.

Respectively, if the model two-stroke is at 13%, the model model four-stroke (NOT spinning an oil-pump) could be about 1.6 times that, or ~21%.


When efficiency is calculated, size does matter - a lot!

The biggest Diesels are much more efficient than small ones, even though they employ identical technology.

So a 21% efficiency for a model four-stroke engine is quite impressive.