What Determines the rpm of a engine?

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Alan C

Alan C

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Hello, this is a newbie to building mini engines. I'm currently working on a 173cc V10 engine and I'm confused about what parameters determine the engine's minimum and maximum operating speeds. I want my engine to have a low idle speed, which I know might be challenging.

Initially, I thought the engine's bore-to-stroke ratio determined its RPM, believing that increasing the stroke could lower the RPM. However, I've noticed that many model engines with very realistic idling speeds don't have long piston strokes. Even some aircraft engines with shorter strokes than their bore can idle at very low speeds.

Now, I suspect that the engine's RPM might actually be more influenced by the valve timing mechanism. It seems like reducing the valve overlap angle can lower the engine's RPM. If anyone knows the answer to this, please let me know.

Also, here's a photo of my engine's cylinder block, which is made of 3D-printed aluminum.
 

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I don't have any multi-cylinder experience, however I would offer the following:

1. For the old hit-and-miss engines, the weight of the flywheel, and low friction often determine how slow this type engine will idle.
Auto engine flywheels have become progressively lighter over time, as the number of cylinders increased, and as the engines have become more sophisticated.
Modern auto flywheels are shockingly thin compared to some of the old engines.

2. For multi-cylinder engines, I would guess that if the firing pulses are not symmetrical, then the flywheel may need to be slightly heavier to maintain a smooth idle.

Long-stroke engines I think are build in order to produce more torque at a given rpm than a similar displacement shorter stroke engine.
I think long-stroke engines trade off rpm for increased torque.

I have had some model airplane engines that idle very slowly, with the propeller taking the place of the flywheel to some extent.
Having excellent and precisely adjustable carburetion also helps achieve a slow idle.
And I have found on airplane engines that a pressurized fuel tank really helps a lot to achieve a very stable slow idle.

For a model engine with no load, I would think it could be easy to over-rev it.
From the old auto racing days of the 70's, those racing folks would start replacing things that would deflect under too much force, such as pushrods, valve spring, studs that held the rocker arms, etc.
Roller cams use to be a big deal too.
And people changed to four-bolt mains to prevent over-stressing the bearing cap bolts.

Connecting rods can also start to deflect at a high speed.
Forged crankshaft and connecting rods are much stronger than cast units.

Most of the immediate problems I saw in auto racing was the valves not closing fast enough at high rpm, and thus the valve would contact the piston, with catastrophic failure of the engine.

The higher the rpm of an engine, the more important metalurgy and high-strength metals comes into play.

Hope this helps in some way.

.
 
Last edited:
GreenTwin said:
I don't have any multi-cylinder experience, however I would offer the following:

1. For the old hit-and-miss engines, the weight of the flywheel, and low friction often determine how slow this type engine will idle.
Auto engine flywheels have become progressively lighter over time, as the number of cylinders increased, and as the engines have become more sophisticated.
Modern auto flywheels are shockingly thin compared to some of the old engines.

2. For multi-cylinder engines, I would guess that if the firing pulses are not symmetrical, then the flywheel may need to be slightly heavier to maintain a smooth idle.

Long-stroke engines I think are build in order to produce more torque at a given rpm than a similar displacement shorter stroke engine.
I think long-stroke engines trade off rpm for increased torque.

I have had some model airplane engines that idle very slowly, with the propeller taking the place of the flywheel to some extent.
Having excellent and precisely adjustable carburetion also helps achieve a slow idle.
And I have found on airplane engines that a pressurized fuel tank really helps a lot to achieve a very stable slow idle.

For a model engine with no load, I would think it could be easy to over-rev it.
From the old auto racing days of the 70's, those racing folks would start replacing things that would deflect under too much force, such as pushrods, valve spring, studs that held the rocker arms, etc.
Roller cams use to be a big deal too.
And people changed to four-bolt mains to prevent over-stressing the bearing cap bolts.

Connecting rods can also start to deflect at a high speed.
Forged crankshaft and connecting rods are much stronger than cast units.

Most of the immediate problems I saw in auto racing was the valves not closing fast enough at high rpm, and thus the valve would contact the piston, with catastrophic failure of the engine.

The higher the rpm of an engine, the more important metalurgy and high-strength metals comes into play.

Hope this helps in some way.

.
Click to expand...
Thank you for your response. Increasing the flywheel weight might indeed help lower the engine's idle speed. I recall seeing a video where the creator managed to reduce the engine's idle speed to just 1000 RPM by installing a massive flywheel. Perhaps increasing the engine's load is the ultimate solution to this problem. I will try different flywheel weights once my engine is finished.
 
A heavier flywheel slows the acceleration of the engine, such as with auto racing engines, and thus one reason for the lowest-mass flywheel possible.

I installed a much heavier flywheel on my CR500, and it had absolutely no noticeable effect at at either idle or racing speed.
I ended up re-installing the original CR500 flywheel.

A heavy flywheel on the end of a shaft, or on the end of an engine crankshaft can make the shaft act like a torsion bar, and can twist the crankshaft to the point of failure. The inertial of the flywheel will resist the crankshaft forces, and the tension will reverse when you change from acceleration to decelleration.
You may need stronger crankshaft bearings and crankshaft with a heavier flywheel.

As I recall, one of the problems of the straight-six engines was potential excessive twisting of the crankshaft.

It is generally easy enough to make two or more different mass flywheels, and try each, I would think.

Edit:
If a crankshaft is not rigid enough, and begins to twist, it will act like a spring, and you can have harmonic problems, such as getting into a resonant frequency condition, which could cause a crankshaft to fail (most likely on a full sized engine, perhaps not so much on a model engine).

.
 
Last edited:
Valve timing has some effect but I also find that ignition timing has a greater effect so make many of my engines with a means of advancing and retarding the ignition timing while running as I like a nice slow tick over. Although a large flywheel can help keep an engine going at low reves it is not ideal when you want to blip the throttle to show off the engine as it is slow to change speed

Although not a 4-stroke this video of one of Nick Rowland's engines gives a good idea of how an engine speed can be affected by changes in ignition timing, the long lever behind the flywheel is the timing adjuster, the smaller one near the head is just being used to make small adjustments to the air/fuel mix.

 
Similar to their full sized counterparts a lot of things can affect engine rpm, slow and fast. Take for instance flywheels. On a sprint type cars there is little or no flywheel for quick engine response. On a model engine they won't operate without a flywheel. You can't scale centrifugal force. The centrifugal force is required to get the piston past the compression stroke. Cam design. Just like a full size engine the cam will determine the engine's operating range. Hand in hand with the camshaft is port design. This will control air flow in and out of the engine. On models you are restricted by how you can machine the ports. Ignition timing for the most part is fixed, no advance, so a setting that is reasonable for both low and higher rpm is needed. Higher rpm requires more advance but then the engine will kick back at idle with too much Ignition timing. On small engines we are also limited by carburetor design. We are limited by scale to make an optimally performing carb. Full size carbs are çomplex and that complexity is hard to scale down.. so to answer your question it all depends on how you want your particular engine to perform.
 
An important factor to consider is the acceleration of the piston as it moves up and down in the cylinder. A longer stroke has higher acceleration as compared to a shorter stroke engine at any given RPM. So you have to consider the forces getting the piston/rod moving and stopping at each end of the stroke. The longer the stroke, the heavier the crankshaft has to be to absorb those forces. Larger engines have limits on RPM because to the forces involved in accelerating the piston in the cylinder. And it gets to be a real challenge, make the crankshaft heavier to handle the forces, increase the rod size to handle the forces, and then the moving mass gets larger so the forces get larger... Many years ago (1960's I believe) Honda developed a 6 cylinder 125 engine for racing that ran at ridiculous RPMs, probably limited by what they could do for ignition at the time. Very small cylinders, very small pistons, very little mass, capable of lots of RPM.
 
Vietti said:
Though I've never printed anything but paper copies, I am very impressed with your printed crank case. I hope you will continue to post updates and maybe some details re. the printing process.
Click to expand...

Wait, what......I thought I was looking at a full sized engine block.
I better go back and read that again.

.
 
I had to go back and look at it myself. That's a nice looking block, but you've got to admit that not everybody as access to the kind of equipment required to do that.
 
This is my 302 V-8 engine. It has a scale flywheel so it will fit within the scale bell housing. It uses a simple air bleed carburetor with no type of accelerator pump. The timing is fixed at what I found was optimum for all around running, 35 BTDC. The idle is about 1500 rpm although by adjusting the timing I can get it down around 1000 but then it won't run well at higher rpm's. Right now it will rev to 7800 rpm.
 
I noticed that the you said the block was 3d printed aluminum. Did you print this yourself or did you send it out to have it printed? If you printed it what make and model of printer did you use?

Rich
 
Forgive me for the aside Gentlemen-
Kf2qd, you were almost right- in the mid-60's Honda raced 50cc twin cylinder, 125cc 5 cylinder, 250cc 6 cylinder and 350cc (actually 297cc- a stretched 250) 6 cylinder engines. Exquisite pieces of machinery, revving to around 20 to 22,000 rpm and with a sound not easily forgotten even nearly 60 years later! Even the memory of it gives me goose pimples.
I also remember that if you exceeded around 4000ft/second mean piston speed, you finished up with daylight-seeking conrods back then. Such were the limitations of the available materials & design.
 
More overlap in the cam can allow a lower idle. The previously mentioned heavy flywheel and retarding the timing also will help. Of course anything you do to help at lower idle will be detrimental to top end performance and vise versa. Best thing to do is replicate a camshaft that came in your type of engine to get a real sounding exhaust note. In my "american style V8" I replicated a cam that would have been purchased aftermarket by a weekend warrior who wanted performance and still make it to work and back. Gives me the rough idle and good sound up to about 6000rpm.


 
the number one issue with low RPM in model engines is flywheel weight, the heavier the slower, as a certain minimum amount of flywheel inertia is required to get through the compression stroke. (also flywheel inertia doesn't scale linearly so smaller scale engines need larger than scale flywheels to have acceptable idle)

the number one issue with high RPM in model engines is valves, not their timing but the actual amount of airflow they can allow - the fully open valve is actually a choke point for high speed air flow and limits the intake, at higher RPM you're getting less air per cycle, hence limits the power, hence limits what is available to overcome friction, even at wide-open-throttle

my Hansen A-frame (gasoline) engine flywheel is over 7" diameter and over 2 pounds so it will run at around 200~300 RPM, and its valve ports are only .203" diameter so it can't run any faster than around 300~400 RPM ! (my diesel flywheel is over 4 pounds, that one is _very_ hard to turn over with 20:1 compression)

theoretically there are stress limits to high RPM, but in model engines this is rarely an issue. Paul Denham (BAEM Club) had a conrod bolt fail at high RPM and has since replaced all such bolts with Unbrako or similar high alloy steel bolts and not had any repeat failures.
 
Alan C said:
Hello, this is a newbie to building mini engines. I'm currently working on a 173cc V10 engine and I'm confused about what parameters determine the engine's minimum and maximum operating speeds.........
Click to expand...
....That is a very interesting opening line Alan. Can you tell us something about your engineering background?
 
An interesting point on engine design and camshaft choice is the same cam profile on a large engine used on a small engine produces an entirely different power curve. When a cam is designed the physical size of the engine in capacity will determine the cam angle settings. You can’t take a cam spec for a small engine and use it on a large engine and expect the same characteristics. The reason is flow dynamics and inertial load. Another limiting factor is the thickness of the rings in radial and axial dimensions. Get that wrong and you get ring float, loss of seal and eventually it will chop the top of the piston off. Piston acceleration is also a factor, get that wrong and holes appear in all he wrong places in the engine. Most of these things have calculations used to produce reasonably accurate results. Mr Bell in his books on the subject uses many of these calculations to prove modification and there effects on engine design. The Legend Smoky Yunick (not sure if that’s right) references Mr Bells calculations frequently in his books on the subject. One interesting point with two stroke engines is the choice of bearing manufacturers can influence the achievable RPMs they are capable off due to the number of balls or needles in the bearings, because of the rolling resistance and inertial loads. One in particular I know off has a difference in peak revs of over 1000 rpm by nothing more than main bearing manufacturer choice. RPM limiting factors are very diverse, but they can be quantified by following the math. 😀
 
Another factor affecting idle can be port size, more so intake port than exhaust. A large port is great for high rpm but at low rpm gas velocity can drop too low, droplets of fuel fall out and puddle etc etc. In full sized engines at least, engines designed to have better running at lower rpm use smaller ports. Port design is a dark art and many whole books have been written on the subject. Plenty available relating to hotting up car engines for drag racing etc by the likes of Dave "The Wizard" Vizard. But the focus is on performance rather that slow idle so bigger is usually better -- up to a point. Likewise a smaller bore carburettor is often used where lower rpm performance is more important than top speed. Ditto valves.
 
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