2stroke diesels

2stroke diesels

Some time ago I had to give lessons in the basics of engines. I came across the issue of 2 stroke diesels. The more I read about them, the more I came to think that they might be a good engine for ultralight airplanes.

If you like to know how a 2 stroke diesel engine works, go see:


If you are not keen on reading that whole page, I can already tell the following:

  • Oil is not mixed with the fuel, it is in a oil-sump under the piston. Lubrication guaranteed.
  • A system (supercharger or similar) gives the intake-air a lot of pressure. So ... when the air enters the chamber it pushes the old gasses out the chamber. Which leads to higher efficiency.
  • No spark-plugs, but injectors. Less maintenance. Less reasons to operate badly.
  • No carburetor, but a diesel-pump. Less maintenance. Less reasons to operate badly.
  • The diesel engines of the old days (no electronic steering) kept turning once they were started. Even a empty battery didn't stop them. You had to turn of the fuel-flow.

After viewing those pages I thought that the reasons to use 2 stroke diesels in ultralights are:

  • More reliability (more details about that after the summary)
  • More power in lesser size
  • Lower rpm, so maybe no need for reductions (resulting in lower weight again) and less wear of the engine parts.
  • In case of crashes, no fear of fire
  • Less fuel consumption

Let me explain the first and most important reason, reliability. First, you need to look at the kind of parts needed. A fuel 2 stroke engine has a sparkplug and a carburetor. Two reasons for in-flight engine failures. A 2 stroke diesel has a injector, which hardly fails, and a fuel pump, which is also so simple in design and almost never fails (only failure of dieselpumps is after tanking the classic fuel instead of diesel). Second, you hardly get reliable info about engine temperature in a 2 stroke fuel engine (in those I saw anyway).
In a 2 stroke diesel engine you can use the temperature of the oil in the oil sump to see the true temperature of the engine. LIVE SAVING DATA!
Another thing that adds to the reliability is the fact that diesels turn at a lower rpm. It has less tear and wear (right saying???). This makes that you need less revisions of your engine. Saving money!

OK, here is your fuel. Just keep me in the air for 2 hours. Pleeeeeaase.It is known that diesels consume less fuel. In other words ... you can have a larger range with the same fuel-tank or ... you can save a few liters (gallons) to have the same range. The latter is a bit of weight-saving in the total take-off weight. Might compensate a possible weight-gain in the engine due to the air-charger.
Another point in my list was "no fear of fire". Maybe it is not directly something you consider as "Well, that was what I always was looking for". But ... just follow this. Each closed or semi-closed airplane has a firewall. Reason: if the engine starts to burn, the fire doesn't get in the cockpit easily.

Finally... it will get there anyway (if it gets the time). Secondly, you need to store your fuel near the airstrip. If you have a large airfield with a fuelpump, that is no problem. But if you need to store it in drums, it is ... less safe.

If you are military ...

... you have a fundamental advantage here. Your airfield-fuel-tank might get shot by gun-fire and it will not explode or burn. Just leak. What do you prefer? Large BOOOOOM or only a cleaning-job?
And ... do not most military trucks use diesel? Why use a different type of fuel? Diesel is in large amounts in the armies, I guess.

Comparing to 4stroke

When you look at the disadvantages of classic 4 stroke diesels in history, you will see mostly noise and vibration. OK, lets look at them in another way. Can a 2 cylinder 2 stroke diesel boxer engine solve the vibrations? I guess it will reduce the vibrations a lot. And noise ... more noise than the screaming 2 stroke fuel engines? I don't think so. Maybe in dB. That can be possible. But ...the tone of the engine is lower. And it is that high tone noise of the 2 stroke fuel engines that is so annoying for the people (who don't fly).

Disadvantages and ...the solutions

The main car-mechanics teacher in my old school told me a disadvantage: paraffin in diesel hardens at low temperatures. But he gave a possible solution. Just place a oil-tube from the oil-sump towards the diesel-tube. Let it run several times around it. Result: when the engine runs some time, the oil in the sump gets hotter. The oil in the oil-tube heats up the diesel in the diesel-tube. All you need to do is to let the engine run a bit on the ground to heat up the oil. The heating on the ground can be done by a glow-plug and a car-battery or ... by putting a electrical blanket over the engine and heat up the engine (just thinking out loud)

A nephew of me, who is also car-mechanic, told me a possible other and better solution. Just place the fuel-filter IN the engine-block. When the engine heats up the diesel gets heated too. Due to the fact of return flow of diesel towards the diesel-tank the diesel in the tank gets heated too.
He mentioned me that most problems of hardened paraffin don't happen when you start the engine, but when the car drove some time. I think the reason must be the cold air running over the diesel-tube. So ... we need to place the diesel-filter on the back of the engine to prevent prop-air running over it. Maybe even a wind-screen over it for all security. 

I got a reply from Sébastien Bolduc who said the following: "Where we live we have two kinds of diesel. Summer and winter version. The winter version is a mix of diesel and kerosine. By adding kerosine, the parafine problem is less. You can run a diesel engine on kerosene (jet A1) if you want. Just add a good amont of addictive to increase lubrication. I know that militairy use special jet fuel for very high altitude. They don't want it to freeze.
Diesel engine were not use in aircraft because of the low hp they generate. But today, with new technology (turbocharge and commond-rail) they are starting to get attention. The price of oil sky-rocket help too."


I recalled to have read that Mr. Sikorsky thought up the first multi-engined airplane after his single engine failed due to a bug blocking the carburetor. It seemed later that the multi-engined airplane was a success as a idea. I keep on wondering why after all those accidents that give as reason "engine-problems at low altitude" no one ever thought about the diesel 2 stroke in the ultralight aviation again. Pleeeeaaase, give us this safer engine.

There are some firms working on 2-stroke diesels. But ... they all work on large engines. The less powered engines are not being considered. It is a hole in the airplane-engine-market!

You can see here a discussion about 2-stroke diesels from the fly5k mailing group at Yahoo Groups

Later I got remarks from a expert in engine-parts. I guess his advice is a true guide to have a prototype working and give it a good chance to have a potential in the light-aviation-market. I wished I could follow his advise and make the engine. But ... I lack the economical talent to start a firm. So, for me, it would be all costs and no gain.

Sadly, I had a computer crash and all data was lost. Even his email-address. I even don't recall his name. Grrrrr, bad memory. If you are that person, pleaaase, recontact me again. I hope to start our discussion again so others might learn as much as I did.