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Possible new directions for the 2 stroke, looking to overcome reliability concerns ?

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KeithO

Well-Known Member
Joined
Nov 29, 2009
Messages
1,608
Location
Jackson, MI
I just read Lehmans thread on his crankcase supercharged 4 stroke engine https://www.homebuiltairplanes.com/...irect-drive-engine-replace-277-477-rotax.html and couldn't help thinking that many of us would be served going in the opposite direction:

In otherwords:
Taking the 2 stroke and giving it a conventional crank case with proper pressurized lubrication.
There have not been many engines like this built, but the Detroit diesels and many of the very large marine engines are built this way.
So one would be adding the weight of sump oil and a gear pump, but that should make a dramatic improvement to engine life expectancy.

Removing crankcase pumping means that (before considering the load imposed by a secondary blower) one has also removed the pumping losses from the engine.
What else is needed for reliability ? We will not be able to remove the ports, thus the rings will have to remain pinned. Going to regular valves and cams seems to just add a lot of weight and complexity and more things to go wrong.

OK, now one needs to add back a pump. There are a few different options possible.
A roots type 250px-Roots_blower_-_2_lobes.svg.png

The claw type hqdefault.jpg

Or a centrifugal type blower, like the type used in turbochargers Rotrex-C8.jpg Made by Rotrex Rotrex A/S

There are some more sophisticated versions of the Roots blower available in which the rotors are twisted like the product offered by Eaton
Eaton TVS.jpg

Of all the above, the original Roots design is the worst, it has poor efficiency and a low pressure ratio and it is noisy. The claw style pump is better, but not produced by many manufacturers. I believe the patents date back to the 1930's or even earlier. The Eaton type is most familiar because it is most commonly used in automotive applications. One concern is that application volume is on larger engines, so we would be looking for a relatively tiny example.

Potentially most interesting is the Rotrex. View attachment Rotrex Technical Datasheet C8T Range.pdf
Practically, it has a peak efficiency of about 75%, which is very good compared to the Roots style blowers. It is very compact and the air outlet can be re-configured to point in multiple different directions. It contains its own lubrication system and its weight is listed as 8lb and we would not need the clutch since it would be in operation all of the time the engine is running. It achieves full output at 4000rpm input shaft speed, which makes it possible to either consider a direct drive (no belt) or a geared drive with a modest ratio.

If we now want to manage the one remaining historical problem (fuel consumption), we need to implement direct injection.
Assuming it is possible to integrate an injector from a current automotive application (Honda Fit), then all that remains is to come up with the high pressure fuel pump of the right capacity and an engine control system for the solenoid type DI injectors. Did I say "all" ???

So lets think of the scope again:
Comparing to the Rotax 582: 582cc @ 6500rpm producing about 60hp
Thus, assuming one plans to lower rpm a little in the interest of extended life it looks like one would have to increase displacement from 582cc to something closer to 940cc to still be moving the same amount of fuel and air at a lower speed. I don't know how essential it is to reduce the RPM, I know HKS run their 60hp engines at 5800 rpm continuous rating. Right now they are claiming a 1000hr TBO, but the engine still costs nearly $10k and weighs 127lb. Theoretically one can keep adding cylinder capacity and lowering RPM's, it will just depend on how the cylinder and piston weight ads up. There is no great complexity in these parts, but ideally you find at least connecting rods and pistons to work with that are "off the shelf" performance parts. Pistons may have to be modified to pin the rings. Crank and crank case will be custom anyhow. The heads and cylinders could potentially be made in 1 piece, given the absence of valvetrain.

Any thoughts ? Is the 2 stroke really dead, or does it just need to be re-incarnated in a better form ? The only thing still in my mind is a vague recollection that bearing loading in a 2 stroke is different since there is basically always a thrust down onto the connecting rod and apparently maintaining lubrication can be a problem. But perhaps this was primarily an issue with 2 stroke diesels with much higher compression ratios.
 
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