Bore stroke ratio

[Steamchick]

To add to Nerd1000's comment #40:
35 years ago - when I was working in engine design for a car maker - piston design was changing for emissions and friction improvement. Early 1980s combustion modelling led to the square chamber... bore = stroke, as the ball of flame is near spherical for optimum combustion. Not quite spherically, as it is started by a spark on one side of a flat disc of gas at TDC...., so as this expands with piston moving down the bore the ball is distorted somewhat... returning from a flattish shape to spherical-ish at BDC. Some engines had dished pistons, which helped the shape at BDC, but with tightening emissions laws the unburnt gases became a major issue. Fuel-air mixed gases stop burning as they reach cooler metal walls of the combustion chamber. CO stops burning around 350 degrees C in the combustion chamber. Corners are therefore a big problem, e.g. where the cylinder head meets the bore, or the piston meets the bore. So dished pistons with a flatter bottom to the dish and large radius towards the bore were a development. But then it go more difficult as the top-land above the top ring became longer, and the gap between piston and bore above the top ring trapped more unburned gases. So the dishing was removed ( in the 1990s) to get a minimum top-land of piston above the top ring.
Rings were made narrower - From 3mm wide cast iron to 1mm or less in steel. Piston land between rings was made closer to bore diameter, to reduce unburnt gas storage there... and piston skirts were reduced so today they are more like bikini bottoms than skirts! All to reduce friction and reciprocating mass.
Piston improvements accounted for lots of hydrocarbon reductions to only 1 or 2 % of the unburnt hydrocarbons of a decade earlier. But none of this is relevant to your models, mostly of engines that represent engines from more than 50 years before any emissions considerations.
The main principle is minimise everything!
K2

 

[minh-thanh]

and piston skirts were reduced so today they are more like bikini bottoms than skirts!
K2

 

[Gordon]

Hi Gordon,
On a different part of your model... What is inside the "Chimney"? Is is a baffled silencer, or just a large empty space for expansion of the exhaust gases to reduce the pressure wave at the top?
I haven't previously thought of making a hit n miss traction engine, but of course it was a logical evolution from Steam to IC. I must try and find some plans... (Though I have more than a lifetime full of plans already to fill retirement!).
Hope you can try a "Solid" flywheel, smaller carburettor or something to see what can be done to slow your engine....?
K2

The front tower is for cooling. The engine used oil for coolant because it had a higher boiling point than water. This was before they used pressurized radiators. The cooling oil was circulated through the front cooling tower and the exhaust was routed through the top of the tower to create air flow past the cooling tubes. The idea was to make the engine hot enough so that it would burn kerosene which was much cheaper. The engine was started on gasoline and then run on kerosene once the combustion chamber got hot enough to run on kerosene.

 

[Steamchick]

Interesting idea of the cooling tower, with draught forced by the exhaust.... An obvious development from the steam loco exhaust being used to force draught - but in this case cooling air through a heat exchanger instead of combustion air and exhaust being drawn through the heat exchanger (boiler).
Thanks Gordon.
K2

 

[ajoeiam]

Your information is about 30 years out of date. Modern automotive petrol engines tend towards the highest compression ratio they can get away with (detonation limited). Usually somewhere between 9:1 and 12:1, turbo engines of course will be at the bottom of that range whereas naturally aspirated will be up the top. The catalytic converter (and very precise control of fuel:air ratio by the computer) cleans up the nasty emissions.

Back in the early days of emissions control in the US there was a trend towards lower compression ratios, but that was mostly because of the removal of tetraethyl lead from the fuel (it's nasty in and of itself, plus it clogs up catalytic converters). Modern fuel is much better and engineers have got a lot better at combustion chamber design.

Ah ja - - - - gasoline - - - or petrol engines - - - - I was speaking of diesel engines.
40 some years ago 21:1 compression (IIRC) was available in auto engines - - - - today - - IIRC (really not sure) I think they're down approaching 14:1 which is wonderful for producing lower levels of NOs but terrible for efficiency. (Just read of some models much lower even than 14:1 - - - - but I'd bet their efficiency is terrible!!!
Modern fuel is better - - - - hmmmmmmm - - - - I take it that you haven't had to deal with residues in fuel systems yet, or in gunk in fuel systems or the extreme lack of stability and (could likely find a few more areas!!!). The changes wrot in fuels to 'protect the environment' have assisted in such but to the extreme detriment of the actual quality of the fuel.

Gasoline (petrol) engines and I have never really gotten along. Working on a diesel there are a limited number of things to examine - - - - with the other - - - not only more variables but some of those have their own (huge) variability. Then the inefficiency and the lack of real power (torque) - - - - can I just leave it as that I think the light fuel engines work much better from gaseous fuels (propane/natural gas) and aren't really that wonderful when used on liquid fuels. A compression engine, on the other hand, can handle anything you can get atomized/injected into the cylinder at the appropriate time. (Can you tell that I am no fan of spark ignition engines? I'd rather never have to deal with one again - - - that would be sweet!!!)

 

[stewart drummond]

a fly wheel is never going to determine idle speed , dampen crank loads , dull crank acceleration yes ,but will not determine the speed of any thing , combustion quality will give you the slowest operating speed , this will not be so easy with such a small combustion chamber , lower compression will help create a more generous space for flame propagation , ignition quality & timing will also help this , timing valve events correctly (there always seems to be talk of "playing with it till it runs") , plotting the valve events & timing them will save a lot of grief + the thing should run like a watch , one last thing everyone seems to miss is piston pins (gudgeons) are always off set in the direction of rotation (this is to promote reciprocation) and will surely help with your quest for a slow idle

 

[Gordon]

a fly wheel is never going to determine idle speed , dampen crank loads , dull crank acceleration yes ,but will not determine the speed of any thing , combustion quality will give you the slowest operating speed , this will not be so easy with such a small combustion chamber , lower compression will help create a more generous space for flame propagation , ignition quality & timing will also help this , timing valve events correctly (there always seems to be talk of "playing with it till it runs") , plotting the valve events & timing them will save a lot of grief + the thing should run like a watch , one last thing everyone seems to miss is piston pins (gudgeons) are always off set in the direction of rotation (this is to promote reciprocation) and will surely help with your quest for a slow idle

I certainly have learned a lot with this project. I more or less just drew this up based on what looked OK and what other engines I have built have used. I was not actually trying to actually "design" anything. It has been a worthwhile project from an education standpoint.

Presently I have a compression ratio of about 6.9:1. I can easily move the wrist pin (gudgeon) by 1/8" to give a compression ratio of 5:1. This may be worth a try.

Presently I am aiming for a valve timing of intake valve open 20 BTDC and close 20 ABDC. Exhaust open 30 BBDC and close 10 ATDC. I have not actually tried to be too fussy on this. I have mostly just eyeballed it to somewhere in the ball park. Perhaps I should actually put a degree wheel on it to try for a more exact setting.

I am not sure what you mean by the piston pins being offset in the direction of rotation. I have never seen a piston with the pin any place but on the center line of the piston. The distance from the pin to the top is less than the distance from the pin to the bottom of the skirt. In many automobile engines the skirt is very short or as someone said it is actually a bikini.

Some of the remarks seem to think that the engine is a hit and miss but it is not. As you said the heavier flywheel will not affect idle speed.

 

[Nerd1000]

I certainly have learned a lot with this project. I more or less just drew this up based on what looked OK and what other engines I have built have used. I was not actually trying to actually "design" anything. It has been a worthwhile project from an education standpoint.

Presently I have a compression ratio of about 6.9:1. I can easily move the wrist pin (gudgeon) by 1/8" to give a compression ratio of 5:1. This may be worth a try.

Presently I am aiming for a valve timing of intake valve open 20 BTDC and close 20 ABDC. Exhaust open 30 BBDC and close 10 ATDC. I have not actually tried to be too fussy on this. I have mostly just eyeballed it to somewhere in the ball park. Perhaps I should actually put a degree wheel on it to try for a more exact setting.

I am not sure what you mean by the piston pins being offset in the direction of rotation. I have never seen a piston with the pin any place but on the center line of the piston. The distance from the pin to the top is less than the distance from the pin to the bottom of the skirt. In many automobile engines the skirt is very short or as someone said it is actually a bikini.

Some of the remarks seem to think that the engine is a hit and miss but it is not. As you said the heavier flywheel will not affect idle speed.

More flywheel mass should make a difference. To sustain an engine at idle the flywheel needs to have enough kinetic energy to drive the engine through the compression stroke (else the engine will stop before TDC). The energy absorbed in doing this is pretty much independent of engine speed, while the amount of energy stored in the flywheel is proportional to its mass and the square of its speed. Hence an engine that operates at high rpm can have a lighter flywheel.

 

[Steamchick]

Sorry Gordon, Maybe I misled everyone on that. You are quite right in that a constant firing engine needs "less bang" at each firing stoke to run slower. - Hence adjust carburation and delay ignition to reduce idle speed. Lower combustion engines can run relatively slower that higher combustion engines, - that need mere kinetic energy of rotation of the total flywheel masses to get the engine up-and-over compression. There are calculations for the total flywheel mass size needed for this...
However, Whether a hit-and miss or constant running engine, a more massive flywheel/ larger rotating inertia can store the required energy to get up-and-over compression at a slower speed than a lower rotational inertia.
Basically, the speed is a saw-tooth profile. The steep rise part is when the piston is on the firing stroke, putting the chemical => thermal energy into the rotating inertia as rotating kinetic energy. = At 1/2 x Rotational Inertia x the rotational speed squared. Therefore assuming each "bang" is the same energy transferred to the rotational inertia, and is as small as practical to get up-and-over compression, then the "top-of-the-saw-tooth" of rotational speed is lower for a higher rotational inertia. It is physics and cannot be changed.
For a hit and miss engine this is readily seen, as the governor determines the speed at which the engine does a firing stroke. Therefore on that engine, the SLOWEST speed of the saw-tooth is determined by the governor. The highest speed is determined by the power of the bang versus the rotational inertia. The only difference from a constant-firing idle is that there is a governor taking control of the engine speed at firing....
Putting it another way.

• A small flywheel needs a lot more revs to get over TDC at compression than a larger flywheel;
• a smaller power of "Bang" gives a lower top of the saw-tooth of rotational speed than a larger "bang"; (smaller fuel quantity, and lower compression can reduce the size of "bang", but also delaying the firing by retarding the ignition will help.
• A lower compression engine can get over-the-top of compression easier, I.E. slower, than a higher compression engine. - Softer valve springs also help, as does lower friction. But the main reason for rotational inertia is to get "over-the-top" of the compression without stalling, or kick-back.

Sorry for any previous confusion...
K2

 

[kop]

https://www.jvejournals.com/article/20651
The other option for adding mass to the rotating assembly

 

[Steamchick]

Hi Kop. Yes, it does add inertia, as well as countering imbalances. However, I suspect it would be detracting from the originality of the model that already exists. Whereas simply increasing the inertia of the flywheel by installing another of larger diameter and thickness of rim, will have some effect for slowing the idle speed, without detracting too much from the originality of the model.
Many Modern Japanese parallel twin motorcycles have balance shafts and they are very effective. Commonplace since the 1970s if my memory is correct?
Thanks,
K2

 

[stewart drummond]

I certainly have learned a lot with this project. I more or less just drew this up based on what looked OK and what other engines I have built have used. I was not actually trying to actually "design" anything. It has been a worthwhile project from an education standpoint.

Presently I have a compression ratio of about 6.9:1. I can easily move the wrist pin (gudgeon) by 1/8" to give a compression ratio of 5:1. This may be worth a try.

Presently I am aiming for a valve timing of intake valve open 20 BTDC and close 20 ABDC. Exhaust open 30 BBDC and close 10 ATDC. I have not actually tried to be too fussy on this. I have mostly just eyeballed it to somewhere in the ball park. Perhaps I should actually put a degree wheel on it to try for a more exact setting.

I am not sure what you mean by the piston pins being offset in the direction of rotation. I have never seen a piston with the pin any place but on the center line of the piston. The distance from the pin to the top is less than the distance from the pin to the bottom of the skirt. In many automobile engines the skirt is very short or as someone said it is actually a bikini.

Some of the remarks seem to think that the engine is a hit and miss but it is not. As you said the heavier flywheel will not affect idle speed.

wrist pin/gugeon pins are always offset to rotation direction (clockwise rotation - pin toward right-hand side) so off-centre -yes , pistons (commecially made i.e. car ,m/cycle , industrial ,garden equip , etc) have a direction marked on them , this is for (sometimes) valve reliefs,porting & so -on , but also pin off-set , the offset is only small but is always present , like i said it helps promote reciprocation & a smoother action as well ,the only bi-directional motor ive come across was a golf cart 2 stroke(it just ran backwards for reverse, seemed coarse & noisey compared to most other two strokes ive dealt with) , pulling down high mileage motors you tend to see wear more toward the rotation /thrust side this is in part from this off-set

 

[Steamchick]

Thanks Stewart, good explanation, but - in case I mis-understand what you mean - when you say "you tend to see wear more toward the rotation /thrust side" I think you mean that "this is what the off-set is trying to reduce"...? I am trying to imagine the forces during a firing stroke: at "mid-stroke", or thereabouts, the down-thrust of the gas pressure on the piston, is pushing the crank-pin down, but there is a side trust on the crankshaft caused by the rod not being vertical. Therefore the reaction trust from the con-rod is to the opposite face of the piston (skirt). The pin off-set is towards the "crank-pin side" to make the rod a bit more vertical to reduce this side-thrust... I think?
Anyway, my ramblings don't affect anything - I am sure you are right with all your experience.
Thanks,
K2

 

[Kasey]

Thanks Stewart, good explanation, but - in case I mis-understand what you mean - when you say "you tend to see wear more toward the rotation /thrust side" I think you mean that "this is what the off-set is trying to reduce"...? I am trying to imagine the forces during a firing stroke: at "mid-stroke", or thereabouts, the down-thrust of the gas pressure on the piston, is pushing the crank-pin down, but there is a side trust on the crankshaft caused by the rod not being vertical. Therefore the reaction trust from the con-rod is to the opposite face of the piston (skirt). The pin off-set is towards the "crank-pin side" to make the rod a bit more vertical to reduce this side-thrust... I think?
Anyway, my ramblings don't affect anything - I am sure you are right with all your experience.
Thanks,
K2

Surely the offset is to give the con rod a small amount of leverage at ignition earlier in the rotation.

 

[TonyM]

The small offset gives a smoother travel during the power stroke and reduced friction. I have seen it compared to dropping a weight on a piece of string straight down to dropping it in a slight arc. The shock at the end is much less when dropped in an arc. However I cannot see this making much difference on slow revving engines when trying to limit the RPM.

 

[Steamchick]

Whatever the reason, the effect of any offset will be negligable on a small model engine. Whatever I remember the company Piston designer discussing with the Hepworth and Grandage engineer about optimising side thrust is irrelevant without complex modelling and understanding the combustion pressures on the geometry of the engine.
Reducing mass is the first step to reducing stresses and friction...
K2

 

[gbritnell]

Gentlemen,
I have been reading along and enjoying the various inputs about engine performance numbers. The problem is most of the replies kind of got off-track because they are all related to full sized engine characteristics. Not that those numbers don't apply to the model engine world somewhat but in regard to the OP's question, the bore and stroke numbers won't change the performance as far as slow running.
Reducing a carb size (venturi) won't slow the engine down but will only limit the rpm range. The engine will accelerate until it runs out of air and then stay at that speed.
One important factor would be the camshaft numbers. For slow running you don't need a lot of overlap, possibly from 0 to maybe 15 degrees (crank degrees) with about 220 degrees duration with a symmetrical cam.
The compression ration should be in the middle range, 6-8:1. If the cylinder is sealed well, rings and valves, then these numbers are more than adequate.
When building a miniature engine the flywheel generally gets scaled down. this is where most of the slow speed operation is lost. The result of too much compression is that the flywheel doesn't have enough inertia to overcome the compression and the only way to keep the engine running is to increase the rpm.'s
Even an engine with more cylinders (V-8) will still run faster with a small flywheel or scale flywheel.
One of the best examples of a slow running model engines are the Holt or Fairbanks. These engines have large flywheels and in the case of the Holt it has 2 of them, one large and one smaller. My Holt will (on a good day) run down to about 700 rpm but it probably has very little power at that speed.
To get your Rumley to operate at a slow speed you will have to gear it down as much as possible and then accept the fact that it's maybe a little faster than you want.
gbritnell

 

[Gordon]

Gentlemen,
I have been reading along and enjoying the various inputs about engine performance numbers. The problem is most of the replies kind of got off-track because they are all related to full sized engine characteristics. Not that those numbers don't apply to the model engine world somewhat but in regard to the OP's question, the bore and stroke numbers won't change the performance as far as slow running.
Reducing a carb size (venturi) won't slow the engine down but will only limit the rpm range. The engine will accelerate until it runs out of air and then stay at that speed.
One important factor would be the camshaft numbers. For slow running you don't need a lot of overlap, possibly from 0 to maybe 15 degrees (crank degrees) with about 220 degrees duration with a symmetrical cam.
The compression ration should be in the middle range, 6-8:1. If the cylinder is sealed well, rings and valves, then these numbers are more than adequate.
When building a miniature engine the flywheel generally gets scaled down. this is where most of the slow speed operation is lost. The result of too much compression is that the flywheel doesn't have enough inertia to overcome the compression and the only way to keep the engine running is to increase the rpm.'s
Even an engine with more cylinders (V-8) will still run faster with a small flywheel or scale flywheel.
One of the best examples of a slow running model engines are the Holt or Fairbanks. These engines have large flywheels and in the case of the Holt it has 2 of them, one large and one smaller. My Holt will (on a good day) run down to about 700 rpm but it probably has very little power at that speed.
To get your Rumley to operate at a slow speed you will have to gear it down as much as possible and then accept the fact that it's maybe a little faster than you want.
gbritnell

Thank you: That is pretty much what I have gotten from this discussion. There are several things which I can do to make the engine run slightly slower but nothing is going to make any major difference. Getting it to run at 950 RPM instead of 1000 RPM is not going to make enough difference to make it worth all of the work involved to attain a slight speed reduction. I am probably just going to have to find a way to add more gear reduction in the drive train. Actually I have not yet tried to actually run the tractor under load. It is possible that under load it is going to travel at an acceptable speed. At this point I seem to have a jinxed project. I keep running into minor problems so that I cannot get things running long enough to actually test the actual operation.

As I said previously it has been a learning experience and I have learned why some things are done the way they are done and found the actual design criteria involved. I am not in this to make money or impress the world so if I am learning something it is all good.

 

[TonyM]

Reposted. seems the link doen't work

 

[TonyM]

I'd love to know how the low speed tickover is achieved on this masterpiece. Starts around 1min 50

https://www.homemodelenginemachinist.com/threads/wow.31776/#post-331605