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.