Low RPM engine question

[kadora]

Hello friends
I am collecting information for my next project - small boat powered by single cylinder four stroke engine.
I would like to machine a kind of low RPM engine.
So my question is What should I take to consideration to build low RPM four stroke engine .
What valve timing is recomended if is better flat head or overhead valve train and other tricks what you know.
Thank you guys for your help.

 

[PopAlexandra]

I would go for an overhead valve if I were you.

 

[TonyM]

I have been looking at just this type of project. I have a thread in work in progress

At tickover you are looking at the minimum amount of power required in the power stroke to put enough inertia into the flywheel rotation to perform the exhaust stroke and the following compression stroke in order to initiate the next power stroke. The other variables that help you achieve that are are fuel intake and ignition timing.

The speed of rotation of the flywheel is mostly dependent upon the diameter of the flywheel and the weight of the flywheel, the mass being as close to the circumference as possible. Low compression to provide enough power to turn the wheel and present less resistance during the compression stroke. The old stationary engines probably ran at 4:1 compression ratio and long stroke. Even when you watch those engines ticking over you will see that the smaller the flywheel the faster the tickover. Too slow and the flywheels do not have enough ineria to overcome the following compression stroke. Also the ignition timing becomes relevant. To minimise RPM you will need to retard ignition even up to TDC or slightly after. But that compromises the power stroke to get it over the next compression stroke. You can also vary the volume of fuel.

The engine I am building is 1/6 scale to fit the same scale boat which should turn out around 1.5 - 2mtr long. The original engine is a very slow runner with around 100/150 RPM tickover. I would love to achieve something similar but its not physically possible. The engine is 25mm bore 32mm stroke and 60mm long conrod. Compression ratio of 4.2:1
The flywheel is CI 100mm diameter at scale but I may increase to 125

 

[Rustkolector]

Kadora,
TonyM has enumerated the problems with slow speed model engines rather completely. I don't think it is possible to duplicate real slow speed engine flywheel performance in any scale model engine, or the friction level, or the breathing performance of the full scale engine. Realistically, you can probably build an engine in the 400-500 RPM range with as small as a 1" bore, but it won't be a scale model in all respects. What are your desired engine size and performance specs, and what are you willing to compromise?

Jeff

 

[grapegro]

Hello All, I am thinking of building an open crank horizontal engine similar to the Crossley, Blackstone type around the late 1800 or early 1900. I intend looking at making it from stock material. Has anyone any knowledge of kits for this type of engine. I am thinking in the 150 to 300 rev range. Norm

 

[kadora]

Thank you guys a lot for your advices.

TonyM my engine will have stroke 30 mm /bore 20 mm.
//I hope con rod will not be too tiny for such small bore//
My intention was to use glow plug ignition but is practically
impossible to run glow engine with such low compression ratio.
I have to reconsider ignition issue.
I am going to machine cam shaft with 0 degree valve overlap
and probably OHV valve train.
My boat hull is 750 mm long only so I hope i will "press" all gear there.
Yesterday I machined cylinder with water chamber inside.
Thank you.

 

[Hopper]

Longer stroke engines tend to rev lower.

 

[TonyM]

That's going to be a reasonably tall engine assuming that its a vertical. You could easily fit a heavy 80-100 Dia flywheel without it looking out of proportion. As Rustkolector says, don't expect anything below 500-600 RPM. For mine I would be very happy to get near that with a bit of power in hand. Anything below will be a bonus. That will give me a reasonable cruising speed using a 70mm prop. I will either gear it down and/or use a smaller prop to get a good speed range for a scale fishing vessel.

 

[deverett]

TonyM my engine will have stroke 30 mm /bore 20 mm.

My intention was to use glow plug ignition but is practically impossible to run glow engine with such low compression ratio.
I have to reconsider ignition issue.

Thank you.

My Hubbard engine, which is a 2 stroke, uses a glow plug for ignition with a compression ratio of about 5:1. The glow plug has to be connected to the battery continuously because of the low running speed of about 400 rpm.

Dave
The Emerald Isle

 

[kadora]

Thank you friends
deverett I read on internet that glow plug does not ignite fuel under 8:1 compression ratio so I am surprised you run your engine on such low ratio.
Very useful information.
TonyM I will be happy to reach stabile 600 RPM.
Yesterday I had time to machine some new engine parts.
This engine will be probably my the ugliest one but I am going to
machine it from scrap metal and left overs from my previous projects.
For example crankcase is piece of worn air cylinder.