Reduce engine size.

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minh-thanh

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Hi ALL !
Reduce engine size
I am designing an engine
Reducing the engine size, do you keep the ratio of all parts: gears, cylinder diameter, stroke, ... all structural details.? Or are you just trying to keep the shape of the engine the same as the large size engine?
Thanks.
 

Ken I

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Generally you keep 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 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 an 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 scale 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
 
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Ken I

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You're welcome - I edited in an additional comment on flywheels - also a fourth power problem.

Regards, Ken
 

KenC

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And don't forget that if you half all dimensions of an i/c engine the displacement is 1/8 of the original, yet half the choke diameter of the carburettor inlet only reduces the air intake by 1/4 and not by 1/8, so halving all dimensions is not in every case workable. The carb needs to have 1/8 of the area of the original design.
Ken
 
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