Crankshaft rods cams

[cumminspower665]

So I've never built a small scale engine before. I'm working on a inline 6 with a supercharger. Have questions about crank rod cam sizes if I'm going to big to small?? Hoping someone my have some input on this subject for me. Maybe help with a plunger haha! Here's a picture of what I've been working off of.
Bore 1.4in
Stroke 1.5 in
Rod length is 2.6in
Crank counter weights and all 2.6in
Block is 10.15 long
4.75 wide
Cant seem to add any photos...

 

[cumminspower665]

 

[Longboy]

Not sure what you are asking here looking at the drawing......What will you be operating on the engine at the end of the plunger from a "crank rod cam"?

 

[cumminspower665]

Haha. The plunger is for getting the head out of a$$!
But what I'm asking is..
Am I over sized or undersized. Feel like I go to far on some things not enough on others. Looking for a little insite what someone has found for ratios between these sizes that works good or best.
-Crank journals. 7in
-Piston pin location
(this drawling is older but still used. Pin is closer to center but not all the way)
-Cam .5in diameter
Rods 2.6in

 

[Longboy]

Your guide here is your engine block and cyl length and will determine the rotating diameters and lengths of the moving con rod assembly. Lets go to clearances instead of under / oversize. The engine is not going to care much about ratios of its components. They do run built as long stroke, small bore, deep skirt piston or....... short stroke, big bore, short skirt piston design. Your parameters begin with the crank center line position and end at the deck height of the block. Your piston rise has to end somewhere below the deck as per your illustration. Tell me some more about the cam stock diameter!

 

[cumminspower665]

Stock diameter I set to .75in
Will work from that point.
And yes that's how I got to where I am now.

I guess what I'm looking for is this ok and it will be able withstand the forces
I should just cross that bridge once there.

 

[Longboy]

Not knowing your back round, you should think this over some more Cummins. A red flag in your first post: "Never built a scale model engine before"......... Heading out 1st. with a multi cylinder, you may shelve early or just drop out all together. This thought is backed up by the veteran machinists here. However if you know some design work and machine tools....I would say here that your cam stock, .75 in. diameter.... seems like a large dia. piece of metal for a camshaft on a sub foot long model! Also, you can in the search window find out how to "post photos" on this forum.

 

[awake]

I too am a newbie to model engines (though not to machining), and I took the advice that Longboy suggests - starting with a simple single-cylinder model that is freely available (the Webster). I must say, I am glad I did - the number of small, fiddly parts even for a single-cylinder model is quite astonishing; even though I read through the plans carefully before starting, I have been surprised. Likewise daunting is the number of uncertainties - have I got good sealing in the valves, good compression in the cylinder, etc. - I *think* I have the cylinder and piston sorted, but just getting to the point of seeing whether my valves will seal (and working on them if - maybe more likely when - they don't).

 

[cumminspower665]

Not knowing your backs a round, you should think this over some more Cummins. A red flag in your first post: "Never built a scale model engine before"......... Heading out 1st. with a multi cylinder, you may shelve early or just drop out all together. This thought is backed up by the veteran machinists here. However if you know some design work and machine tools....I would say here that your cam stock, .75 in. diameter.... seems like a large dia. piece of metal for a camshaft on a sub foot long model! Also, you can in the search window find out how to "post photos" on this forum.

I actually work on engines generators digital boards etc. Tiny bit of machine work. At this small of scale things are different. I'll figure it out tho.
Thanks for your help.
I dont plan to shelve it.

 

[cumminspower665]

I too am a newbie to model engines (though not to machining), and I took the advice that Longboy suggests - starting with a simple single-cylinder model that is freely available (the Webster). I must say, I am glad I did - the number of small, fiddly parts even for a single-cylinder model is quite astonishing; even though I read through the plans carefully before starting, I have been surprised. Likewise daunting is the number of uncertainties - have I got good sealing in the valves, good compression in the cylinder, etc. - I *think* I have the cylinder and piston sorted, but just getting to the point of seeing whether my valves will seal (and working on them if - maybe more likely when - they don't).

That is great advice but knowing myself I'd never stop what I'm currently working on unless some major happen.

I too am a newbie to model engines (though not to machining), and I took the advice that Longboy suggests - starting with a simple single-cylinder model that is freely available (the Webster). I must say, I am glad I did - the number of small, fiddly parts even for a single-cylinder model is quite astonishing; even though I read through the plans carefully before starting, I have been surprised. Likewise daunting is the number of uncertainties - have I got good sealing in the valves, good compression in the cylinder, etc. - I *think* I have the cylinder and piston sorted, but just getting to the point of seeing whether my valves will seal (and working on them if - maybe more likely when - they don't).

Well I look forward to checking it out once you do.
Valves are a bridge I haven't yet crossed. I've mocked up everything. Assembling everything now and setting the tolerance to see if I get the engine to bind.

Knowing myself dont think I'll be able to stop working with current project. I tend to not let things go easy and the harder it is to figure out the more I want to do it. Haha!

Making sure I have that seal is a concern of mine as well. What i've done in my engine is adding of more head studs 6 per cylinder. Also im making this a dry deck block. Cylinder head will get coolant from the backside of the block directly from main galo. Feeding to the front and meeting cylinder coolant before heading out to radiator. Same for the oil. Dry decks are great for turbos chargers compression. Gasket should be easier to make as well.
Fire ring the cylinder .025 protrude from steel liners. Use of the copper gasket here will make for a great spacer with Hope's of bringing that to .002. Should be enough clamping force to hold it down. Well see tho.

 

[awake]

Likewise, I look forward to seeing what you come up with. Onward!

 

[kuhncw]

Hi Cumminspower,

Please tell us more about your engine configuration. You mentioned supercharged/turbocharged. Spark ignited or compression? Two or four valves per cylinder? Overhead cam or cam in block? The dry deck is a good choice.


Regards,

Chuck

 

[cumminspower665]

Hi Cumminspower,

Please tell us more about your engine configuration. You mentioned supercharged/turbocharged. Spark ignited or compression? Two or four valves per cylinder? Overhead cam or cam in block? The dry deck is a good choice.


Regards,

Chuck

Sorry it took so long for a reply I've been kindof busy.
Compression, 24 valve, Direct injection.
6cylinders
DOHC
I'd like to do a supercharged roots type but I want to make the engine turn first then cross that bridge.

 

[cumminspower665]

 

[cumminspower665]

It's not 100% yet but this is the injector. Will finish it tonight. Ordered some c300 to start building the bodies and needles.

 

[kuhncw]

Hi Cumminspower,

Thanks for the details. Good luck and I'll be watching your progress.

Chuck

 

[Ken I]

Cumminspower.

With regard to scaling engines there are certain rules relating to scale particularly with respect to rotating and torsionally loaded parts which get weaker (relatively speaking) as they are scaled down.

Below I have re-posted a prior response on this subject...

Generally you scale everything the same. However........

You should keep in mind that scaling up or down introduces problems.

Areas increase/decrease to the square of the scale, volumes and mass to the cube of the scale and polar and mass moments to the 4th power - this last item is the most trouble.

Example:- If you take a 2 litre motor and double its dimensions - the volume / mass is 2x2x2 = 8 times bigger - so if the original engine was 2 litre and weighed 200kgs - your scaled up engine to twice as big (dimensionally) will become 16 litres and weigh 1.6 tonnes !

By the same token if you scaled it down 2:1 (half size) it would become 250cc and weigh 25kg.

In the case of the 2 litre - if it made 200 Horsepower (a hundred horsepower per litre - high performance) - scaling is going to have a strange effect - the double size engine is going to have the same mean effective pressure applied to 4 times the area and twice the stroke so the torque is going to be 8 times greater - however the mean piston speed cannot be increased (limits of lubrication capability & flamespeed were already at maximum on the 2 litre engine) so if our 2 litre motor was capable of 6000rpm our 16 litre motor will only be capable of 3000rpm so overall our power only increases 4 times to 800 horsepower not 1600 as you might have expected. So it achieves only half the specific horsepower - 50 horsepower per litre.

By the same token our half size 250cc model can do 12000 rpm and generate 50 horsepower or 200 horsepower per litre - that is why high performance engines have more smaller cylinders.

O.K. I'm talking theoretically here - in practice I don't think the 250cc would be that good or the 16 litre that bad - but always have these ratios in your mind. (Because of aspiration and carburation issues - atoms don't scale ! Flame speed and flame propagation in the engine remain the same regardless of its size etc. etc.)

Now we come to the tricky bit - the polar moment - the ability of a shaft to resist torque - is to the 4th power - so in the case of our 2 litre scaled up to 16 litre the torque increased by 8 times but the ability of the crank to resist the torque went up 2x2x2x2 = 16 times - so the crank becomes over-designed for the application and the journals could be reduced.

The opposite is going to happen with our 250cc scaled down engine - the torque is going to be 1/8th but the ability of the rotating parts to handle it is going to be 1/16th and therefore be much more highly stressed and is bound to fail.

You can see this on large cranks from marine engines - they look relatively "skinny" when compared to our normal frame of reference - a crankshaft from a car engine.
If you radically scale down such a large engine - say 1/10th scale you are going to end up with a crank that is effectively only 10% of the original torsional design strength relative to its new size.

We generally don't want to actually derive extreme performance from a model - so you can get away with it - but be careful.

Assuming you want to make allowance - lets say the 2 litre's main crank bearings were Ø60mm and the big end journals Ø40mm then our simply scaled sizes would be Ø30mm and Ø20mm which is actually too small for the design.
The true (compensated) scale for the Ø60 should be the 4th root of (60^4)/8 which scales down to Ø35.67 (not Ø30 as you might presume).and the Ø40 scales down to Ø23.8 (not Ø20 as you might presume). So we would round up to Ø36 & Ø24

(A simple way to calculate the 4th root is to take the square root twice.)

So torsionally stressed parts need to be slightly larger than scale on scaled down motors.

What you can also see is that a small change in diameter makes a big difference to polar moment - so if you scale down a shaft to say Ø6.9 then round it up to Ø8 - or apply the calculation - never round down. (The Ø8 shaft would be 80% torsionally stronger than Ø6.9).

You don't have to slavishly follow such "Strength Of Materials" type calculations but you should always have these rules at the back of your mind. Also bear in mind the actual strength of the materials you have chosen etc. etc.

In most cases, scaling down works to your favour in terms of strength in everything except torsionally loaded parts.

The inertia of a flywheel is also to the fourth power - so scaled down flywheels have considerably less inertia relative to the scaled down engine - err upwards on diameter and thickness when scaling down flywheels.

A final comment - atoms don't scale - so things like lubrication clearances remain the same and effectively scale up leakage, by-pass etc. in a model that has been scaled down. Hence frequent problems with compression and carburation etc. on small scale motors.

Regards, Ken

 

[cumminspower665]

Cumminspower.

With regard to scaling engines there are certain rules relating to scale particularly with respect to rotating and torsionally loaded parts which get weaker (relatively speaking) as they are scaled down.

Below I have re-posted a prior response on this subject...

Generally you scale everything the same. However........

You should keep in mind that scaling up or down introduces problems.

Areas increase/decrease to the square of the scale, volumes and mass to the cube of the scale and polar and mass moments to the 4th power - this last item is the most trouble.

Example:- If you take a 2 litre motor and double its dimensions - the volume / mass is 2x2x2 = 8 times bigger - so if the original engine was 2 litre and weighed 200kgs - your scaled up engine to twice as big (dimensionally) will become 16 litres and weigh 1.6 tonnes !

By the same token if you scaled it down 2:1 (half size) it would become 250cc and weigh 25kg.

In the case of the 2 litre - if it made 200 Horsepower (a hundred horsepower per litre - high performance) - scaling is going to have a strange effect - the double size engine is going to have the same mean effective pressure applied to 4 times the area and twice the stroke so the torque is going to be 8 times greater - however the mean piston speed cannot be increased (limits of lubrication capability & flamespeed were already at maximum on the 2 litre engine) so if our 2 litre motor was capable of 6000rpm our 16 litre motor will only be capable of 3000rpm so overall our power only increases 4 times to 800 horsepower not 1600 as you might have expected. So it achieves only half the specific horsepower - 50 horsepower per litre.

By the same token our half size 250cc model can do 12000 rpm and generate 50 horsepower or 200 horsepower per litre - that is why high performance engines have more smaller cylinders.

O.K. I'm talking theoretically here - in practice I don't think the 250cc would be that good or the 16 litre that bad - but always have these ratios in your mind. (Because of aspiration and carburation issues - atoms don't scale ! Flame speed and flame propagation in the engine remain the same regardless of its size etc. etc.)

Now we come to the tricky bit - the polar moment - the ability of a shaft to resist torque - is to the 4th power - so in the case of our 2 litre scaled up to 16 litre the torque increased by 8 times but the ability of the crank to resist the torque went up 2x2x2x2 = 16 times - so the crank becomes over-designed for the application and the journals could be reduced.

The opposite is going to happen with our 250cc scaled down engine - the torque is going to be 1/8th but the ability of the rotating parts to handle it is going to be 1/16th and therefore be much more highly stressed and is bound to fail.

You can see this on large cranks from marine engines - they look relatively "skinny" when compared to our normal frame of reference - a crankshaft from a car engine.
If you radically scale down such a large engine - say 1/10th scale you are going to end up with a crank that is effectively only 10% of the original torsional design strength relative to its new size.

We generally don't want to actually derive extreme performance from a model - so you can get away with it - but be careful.

Assuming you want to make allowance - lets say the 2 litre's main crank bearings were Ø60mm and the big end journals Ø40mm then our simply scaled sizes would be Ø30mm and Ø20mm which is actually too small for the design.
The true (compensated) scale for the Ø60 should be the 4th root of (60^4)/8 which scales down to Ø35.67 (not Ø30 as you might presume).and the Ø40 scales down to Ø23.8 (not Ø20 as you might presume). So we would round up to Ø36 & Ø24

(A simple way to calculate the 4th root is to take the square root twice.)

So torsionally stressed parts need to be slightly larger than scale on scaled down motors.

What you can also see is that a small change in diameter makes a big difference to polar moment - so if you scale down a shaft to say Ø6.9 then round it up to Ø8 - or apply the calculation - never round down. (The Ø8 shaft would be 80% torsionally stronger than Ø6.9).

You don't have to slavishly follow such "Strength Of Materials" type calculations but you should always have these rules at the back of your mind. Also bear in mind the actual strength of the materials you have chosen etc. etc.

In most cases, scaling down works to your favour in terms of strength in everything except torsionally loaded parts.

The inertia of a flywheel is also to the fourth power - so scaled down flywheels have considerably less inertia relative to the scaled down engine - err upwards on diameter and thickness when scaling down flywheels.

A final comment - atoms don't scale - so things like lubrication clearances remain the same and effectively scale up leakage, by-pass etc. in a model that has been scaled down. Hence frequent problems with compression and carburation etc. on small scale motors.

Regards, Ken

Thank you very much. This answers my question. I figure this was going to be an issue with torsion parts and in turn is why I over did it. My common sense was telling me wow this is tiny I could break this with my finger tips. I will make my crank cam pins and even my rods and pistons oversized.
I would say this holds true for types of injectors as well. Not by torsion but by pressures. I went from .34x1" to .52x1.87".

 

[cumminspower665]

I am really focused heavy on materials. Injectors are being machined mostly out of c300. crank type of4340 4140 300m or 1144. Any suggestions for that would be appreciated. Rods aluminum. Pistons unfortunately steel.. the expansion of the aluminium and the compression in the cylinder is to much for me to use aluminum. 893 bronze bearings.
If any of that seems unsuitable plz let me know.

 

[mrehmus]

Interesting. I would point out that the 1/3rd scale V-8 and 4-cylinder engines in our club can get up to 10,000 RPM and as far as I know, only one crankshaft has ever broken and that was on an old Challenger. We tend to make the crankshafts of 4130 and the con rods of stronger aluminum like 7074 and 2024. Everything else except gears can be made from the softer aluminum's.