Dumb question time .....

What's the difference?

What's the difference in fuels?

Am I being totally stupid?

diesel is a type of fuel. the kind used in mass transportation most often because it is very cheap and efficient. the rc plane version is diesel kerosene which is more flamable nad smells really wierd. Two strokes are a type of engine. two stroke means that there are two movements of the piston before the cycle is complete. this is not probably the most accurate explanation but i am going to sacrifice some accuracy to make it simple as it would take a page or more to be accurate. on the piston's upstroke a little rotating hole in the hollow crankshaft under the carburator opens up and the upward motion of the piston sucks in air. meanwhile, as the piston reaches tdc (top dead center) the crankshaft rotates and shuts off the opening so the fuel air mixture is trapped. you get combustion from the previous cycle and the downward motion of the piston compresses the trapped mixture in the crankcase. the piston hits the exhaust port and the hot gasses are released. jsut past the exhaust port is the intake port where the compressed mixture is forced into the chamber and the cycle goes on.

Space clam has most of it. Diesel is a mix of kerosine,oil and I believe ether or something similar. A glow engine (which looks like an RC diesel BTW) uses methanol and a chemical reaction with platinum in the glow plug to ignite the fuel.

Diesel however use heat from compressing the fuel air mixture in the cylinder. Diesel RC engines have a screw in the center of the head rather than a glow plug. This is the compression release (I think that's what they call it). You use this to move a button inside the engines cylinder to raise and lower the compression. You back off the compression to start it. Then screw the button in to increase the compression to normal operating levels.

Diesels have tons of torque. They can swing a much larger prop than an equivelant glow engine. They are supposed to be quieter ( Just what I have heard) and also are supposedly more fuel efficient. There are companies out there, i believe Davis Diesel is one, that sells conversion heads for popular 2 stroke engines. Diesels stink and make quite a mess I am told.

I hope this helps. I am not a diesel expert. I hope someone else can chime in here.

Mark Shuman

diesels are not quieter. they are actually really loud. for all the extra torque they have they are a pain in the wahoozi

Model diesel and glow engines are both 2-stroke cycle engines. They differ in the ignition that initiates combustion and the fuel used. Glow engines use methanol (mixed with oil for lubrication) while the diesel uses a mixture of kerosene and ether (again mixed with oil for lubrication). The ignition in the glow engine is a hot platinum wire that glows in contact with methanol to initiate ignition upon compression. Ignition in a diesel is the heat of compression igniting the ether which in turn ignites the kerosene. The following is a comparison I wrote in the diesel forum a while back based on my own experience.

I have spent the last several months learning about diesels. I too read a lot of the hype from diesel aficionados, and wanted to see for myself if it was all true. What I have concluded is that some of it is and some of it isn't. Here are some examples"

Pros:

1. Diesels are more powerful for the same displacement. This is basically true. Kerosene has more energy per ounce than methanol (so does gasoline for that matter) so burning it will produce more power per displacement.

2. Diesels burn less fuel. True, see above. Since kerosene has more energy per ounce, the difference can be taken as less fuel burn at the same power instead of more power at the same burn rate.

3. Diesels never quit. Not quite true. If the mixture (and compression) is carefully set a diesel will run as long as fuel is supplied. But the same is true of a glow engine. Most of the problems, including many flame outs with my PAW were traced to fuel mixture problems. Of course there is no glow plug to burn out but I’ve never had that happen in flight. The diesel, usually having a smaller intake and a stronger fuel draw, isn’t as sensitive to tank position or acceleration forces, but a properly designed and installed fuel system with a glow engine is just as dependable. Of course line leaks, pinholes etc. are just as likely to occur no matter the type engine involved.

4. Diesels will swing a larger propeller. This is mostly true. Because you can control the ignition timing by adjusting the compression, any reasonable sized propeller can be used. That doesn’t mean the engine will necessarily develop its rated power with a big prop. That depends on other design factors. With a glow engine shims must be used under the head to adjust compression for operating with a larger prop.

Cons:

1. Engines built originally, as diesels are generally heavier for the same displacement than a glow engine. To develop more power from the same displacement usually requires more robust construction to resist the added load.

2. Diesels are more complex to adjust and operate. The are many combinations of compression and mixture at which a diesel will run. The compression must generally be increased to start, then as the engine warms up, retarded to avoid an over compressed condition. If the mixture is a little lean, the compression must be less and if a little rich the compression must be a little more. Juggling the compression and mixture to find an optimum setting is where those used to glow engines have the most problem. Even the most experienced with diesels have two rather than a single adjustment to make. The adjustments can easily take several times that required to launch a glow powered aircraft.

3. Diesels stink. The exhaust smells like a big diesel truck that is wet stacking. That burnt smell gets into everything and seems to persist forever. They are messy. The oil from the exhaust is dark and thick (Variously described as honey or syrup colored) and harder to clean off than the mostly clear residue from a glow engine.

4. Diesels are expensive. As an example, a PAW 19, two bearings,(.5 hp at 18,000 rpm) cost $100. While a MDS .18 (also two bearings - .52 hp at 22,000 rpm) cost $60. As far as conversions are concerned, a Davis diesel conversion head for .40-.46 size engines cost $56. This is as much as the original cost of some engines in this range, and a substantial proportion of the others. To realize the full benefit of the conversion, Davis recommends operating without the muffler (not acceptable at many fields) or buying one of his low loss units at an additional cost of another $44-50. The cost of a converted engine is at least $136 and can be as much as $190 compared to the new $80 glow engine.

5. Early diesel engines have rather primitive carburetors and transition can be a problem. My experience with the PAW indicates that even new from the manufacturer is no guarantee. As far as conversions go carbs designed for methanol have a larger throat area and are designed to meter more fuel. This doesn’t usually present a problem as the throttle is simply not opened all the way at maximum power. However it does result in less fuel draw than a carb on an original diesel design.

6. It is generally recommended that electric starters not be used with diesels, since they are more likely to hydraulic lock than the lower compression glow engine. This isn’t much of a problem for very experienced diesel users, but for a beginner, finding that just right combination of prime and compression to start can seem as though it requires some “sixth� sense. Be prepared to build some muscles in your flipping hand.


These are just some of the pros, and cons that come to mind. The list is not necessarily complete, but serves to highlight the major ones.

The pros are real. In many ways the diesel is a better engine than glow. However the cons are true too and an individual must also consider them seriously when making a decision.

If you enjoy tinkering around with engines and spending time adjusting things, and don’t mind paying a premium to do so, diesels are definitely for you. On the other hand if you mainly enjoy flying and want an engine that requires a minimum of fuss at a lower cost, you should stick to the glow powered ones.

Diesel in WA state gas stations is more expensive than regular gasoline. Is like this everywhere?

Nilo

i think it is about $.10 to $.15 lower on average nationwide. But... the diesel that is mentioned here isn't pump diesel. it is ROUGHLY ~20% castor, ~50% kerosene and ~30% ether, and is actually just as expensive as regular glow fuel. (check the price for davis diesel fuel, about $20 for a gallon, $13 for a quart. (not saying pump diesel won't work, but it isn't recommended by davis diesel, which of course sells the recommended fuel!

http://www.davisdieseldevelopment.com/orderform1002.pdf

Cool, I am an inveterate tinkerer, and my PAW 19 (2nd hand) should provide me with hours of entertainment assuming it's actually in running order, but hey it was pocket change.

Edit -----

And thanks for not answering my 3rd question folks

To give you an idea as to how the PAW19 compares to a glow 2-stroke and a 4-stroke, this is some test data for one of each that I flew in the same airplane. The figures are all with the same propeller:

The first was a K&B Sportsman 20.

Installed weight = 9.6 ounces
Full throttle = 11,790 rpm
Idle = 2,850 rpm

The second was the PAW 19.

Installed weight = 6.8 ounces
Full throttle = 9,600 rpm
Idle = 2,800 rpm

The last was an ASP 30 four stroke.

Installed weight 8.5 ounces
Full throttle = 10,800
Idle = 2,650 rpm


10-5 master prop

Performance of the airplane was similar with each engine. The engines were tuned for flight which in each case is just a little rich of peak rpm. It was a little more spritely with the Sportsman 20 as you might guess from the figures.

The K&B Sportsman >20 is 85% more powerful than the Chinese >.30 4-stroke and is (I am sure) of a similar degree of quietness, and has a 20% better Max/Idle rpm ratio. These factors ought to put to rest the frequently stated poor opinions about K&B sportsman engines.

Add the 50 or so bucks for a Davis Diesel cylinder head and you might reasonably never consider a (simple and inexpensive?) 4-stroke over a "dirty" Diesel again!



Morty.

Im not sure if i agree with you on that one. Certainly when a glow and a diesel are spinning the same prop on the same displacement the sound will be comprable but the glow will have a slightly higher pitch which seems louder to me. However, one of the main benifits of a diesel is its ability to swing larger props, thus lowering the rpm and dBs (props are a lot of the noise). This lower rpm and added thrust from the prop also provide for the added fuel efficency of the diesel (I like mine @ ~7000-8000 max rpm). Another benifit is the ability to run an exhaust extension (longer than a yard if you desire) on a diesel because of the tolerance to backpressure, somthing its glow counterpart would be severly hampered by.

That being said, I have found that diesels are definately more complicated and take some time to getting used to the costs are close to glow with the inital engine cost being more for the diesel. I echo LouW; in that if you want less fuss, go with the glow, but if you want to try something different diesels may be for you.