# Ideal 40 hp/3000 rpm Direct drive inverted Two-stroke single cylinder engine?

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#### llemon

##### Well-Known Member
Its a list of general aviation engines, its almost entirely inline and boxer types with only a few radials. I don't see how their age changes the basics since you are targeting a MEP of just 7.7 bar, many of the engines in that list operate quite a bit higher.

I still don't get why you are using a Hirth 504 as a reference. A 2-stroke would be a more direct comparison. And do you have any evidence that a longer stroke is advantageous weight wise?

#### MACOWA

##### Well-Known Member
Supporting Member
On another thread it is discussed if a half VW is the ideal 40 hp direct drive fourstroke,and I think it is.
Very close to at any rate and unbeatable in price
A single cylinder two stroke that makes 40hp/3000 rpm will be just as unpleasant torque variation wise and lower mass,so let us dream .
Yamaha,Evinrude,Suzuki and Mercury has stopped making big two stroke outboard with fuel injection for various reasons.
We can find realistic numbers for achieveable piston velocities, mean effective pressure etc.
Most modern and biggest Yamaha I know of has cylinders of 93*82mm (557ccm) and makes 50 propshaft hp at 5000 motor rpm
This is Mean effectivePressure of ca 8.1 bar.
This is very high and possible because the exhaust from three cylinders in a bank interact.
Engine can run at piston speed of 15m per sec if wanted.
Combined with 3000 rpm it means maximum stroke can be 150mm.
The bore must then be at least 100 for clearance between cylinder and conrod.
This is 1.2 liter that combined with 50 rps and 8.1 bar gives 65 horsepower.
We ask for 40 and a single cylinder MEP of 8.1 is unrealistic unless we accept a tuned exhaust pipe or turbo.
Lets try 7bar.
This needs 855ccm and can be made by a 90 mm times 135mm cylinder.
One exhaust valve of 45mm diameter,two of 32mm or four of 23mm will pass exhaust from 40 hp.

#### MACOWA

##### Well-Known Member
Supporting Member
Holy Grail ! One can crunch as many VE calculations as one likes but a high output direct drive low rpm 2 stroke of a weight and power pulse configuration suitable for aero use is one tough nut to crack. The conversion of outboard engines mentioned previously is interesting to me. Thier modification and repair has been an income source for many years. I have tried a couple of conversions and have never been able to establish proper cooling. The design coolant temp at block inlet is about 40 to 60 Deg. F. Numerous additional inlets at strategic places has little effect. I know of no radiator with an outlet temperature below its operating air temperature. 2SI / Cuyuna built a 3 cyl a while back. A simple exhaust manifold used with this configuration eliminates the need for a tuned exhaust. The 3 cyl Evenrude / Johnson types in the 70 / 80 Hp range can be real screamers and will give the piped out Merc's a run for their money.

#### Niels

##### Well-Known Member
Holy Grail ! One can crunch as many VE calculations as one likes but a high output direct drive low rpm 2 stroke of a weight and power pulse configuration suitable for aero use is one tough nut to crack. The conversion of outboard engines mentioned previously is interesting to me. Thier modification and repair has been an income source for many years. I have tried a couple of conversions and have never been able to establish proper cooling. The design coolant temp at block inlet is about 40 to 60 Deg. F. Numerous additional inlets at strategic places has little effect. I know of no radiator with an outlet temperature below its operating air temperature. 2SI / Cuyuna built a 3 cyl a while back. A simple exhaust manifold used with this configuration eliminates the need for a tuned exhaust. The 3 cyl Evenrude / Johnson types in the 70 / 80 Hp range can be real screamers and will give the piped out Merc's a run for their money.
A single cylinder ,inverted two stroke with one 45mm exhaust valve or two of 32mm each or four of 23mm diameter can be very short and will have torque variations as a two -cylinder VW .
It has second order vibrations up and down and the VW has not. It can be Rotax cooled that is barrel with air and cylinderhead/valves with liquid.Fewer parts,lower temperatures and maximum pressures.
A better engine if there was not all these VW aftermarket parts around.
What is safer in your expirience; two stroke roller cranks or four stroke pressure lubricated?

#### sotaro

##### Well-Known Member
A single cylinder ,inverted two stroke with one 45mm exhaust valve or two of 32mm each or four of 23mm diameter can be very short and will have torque variations as a two -cylinder VW .
It has second order vibrations up and down and the VW has not. It can be Rotax cooled that is barrel with air and cylinderhead/valves with liquid.Fewer parts,lower temperatures and maximum pressures.
A better engine if there was not all these VW aftermarket parts around.
What is safer in your expirience; two stroke roller cranks or four stroke pressure lubricated?
The plain bearings with a high pressure lubrication system seems to be the most dependable. It worked for Detroit Diesel with their turbo and supercharged V-8 uniflow scavenged engines.

#### Niels

##### Well-Known Member
The plain bearings with a high pressure lubrication system seems to be the most dependable. It worked for Detroit Diesel with their turbo and supercharged V-8 uniflow scavenged engines.
Maybach MD870 diesels were used for many years in european diesel locomotives and main bearings were big roller bearings
Maybach

#### sotaro

##### Well-Known Member
Maybach MD870 diesels were used for many years in european diesel locomotives and main bearings were big roller bearings
Maybach
This is the reason why roller bearings were used. "The crankshaft was set into a 'tunnel' crankcase. No split bearings to loosen or separate under load. Roller races were set into the webs of the crankshaft." I anticipate that you are thinking about a conventional crank with normal sized bearings. 99% of all cars and 98% of all four stroke motorcycles are made with plain bearings, to say nothing of all the other four stroke engines out there. If you can have a crank and bearing set up not used for crankcase compression, why not use plain bearings. It is true that many english motorcycles used roller bearings, in the 60's.

#### challenger_II

##### Well-Known Member
I would think plain bearings would be preferable, due to weight and simplicity.

PMD

#### Niels

##### Well-Known Member
Plain bearings are not an option in my dream engines that are crancase scavenged but it is my impression that roller bearing cranks gives lot of warnings before disaster where plain bearings are more on of.

Mr Macowa has repaired outboards and may have an opinion.

Chinese toy heli drones have a triaxial tuned fork sensor selling for 7 $in China and it will be easier to make an accelerometer with analysing software that gives warning when roller bearings moan. And what bearing it is. #### MACOWA ##### Well-Known Member Supporting Member Plain bearings are not an option in my dream engines that are crancase scavenged but it is my impression that roller bearing cranks gives lot of warnings before disaster where plain bearings are more on of. Mr Macowa has repaired outboards and may have an opinion. Chinese toy heli drones have a triaxial tuned fork sensor selling for 7$ in China and it will be easier to make an accelerometer with analysing software that gives warning when roller bearings moan.

And what bearing it is.
How right you are. Plain bearings are unsuitable in a crankcase scavenged engine as they must pass a LOT of oil to maintain proper clearances and cooling to prevent scuffing. I have found that "warnings" appear in many forms. one will probably notice a "rod knocking" in a plain bearing engine long before a failure. Unchecked roller bottom ends can and do disintegrate instantly. An easy check is to put a fingertip on the prop tip and rock back & forth feeling the free slack bearing play, (cold engine) if this is found to be increasing over time trouble is not far away.

#### sotaro

##### Well-Known Member
Sorry Niels, I had forgotten that you were wanting crankcase scavenging. You don't have many choices when the lubrication system is so limited. However, keep in mind that sealed bearings are popular on two stroke scooter engines, made by reputable companies. I think even Bombardier/Rotax used them on their ETEC 850cc direct injected twins, at least for the center bearing. I believe there have been some complaints about just that aspect... I would look carefully at what Bombardier is doing. They have their own seasonal shutdown program for their ski doo engines.
My understanding of roller bearings is that they may not be the most durable in intermittent use, and experimental aviation is the definition of intermittent.

#### Urquiola

##### Well-Known Member
I guess this firma offers single cylinder 2-Stroke engines with power below your stated needs, HE Paramotores – Motores y paramotores
but it is interesting.
I've seen an updated version of Henri Mignet 'Sky Louse', tandem wings with increased clearance from upper to lower, thus reducing chances of aerodynamic coupling inducing a deadly dive, a light single beam fuselage, and a very small motor in center of upper wing leading edge.
Blessings +

#### blane.c

##### Well-Known Member
Supporting Member
This in a "Cub clone" and whoa!

#### PMD

##### Well-Known Member
This in a "Cub clone" and whoa!

View attachment 128100
Quite do-able at Rotax 912 weights. OPOC engines are extremely simple and can be very light. Also perfectly suited to compression ignition thus none of that silly gasoline and spark stuff to deal with.