Bore stroke ratio

[Gordon]

What is the result of making a larger or smaller bore/stroke ratio. Presently I have an engine with 1.25 bore with 1.5 stroke. I want this engine to run slower. As is usually the case with these small engines power is not a large factor. We want it to run smoothly.

I can quite easily change stroke to 1.75 without too much trouble. The rule seems to be that the longer stroke will run slower. Engines I have built have a bore /stroke ratio of .5 to .9. The engine which presently runs the slowest has a .9375 ratio. The engine with .5 runs fast.

Am I going to gain enough by remaking the crankshaft to increase the stroke or am I wrong that the longer stroke will run slower?

 

[Brian Rupnow]

In general, longer strokes will run slower. I wouldn't go much beyond a full stroke of 1 5/8".

 

[mu38&Bg#]

Bore/stroke does have some effect on the torque curve and efficiency. In my opinion. the following is far more important. More flywheel inertia will allow slower running. Why does a "hit and miss" fire after several strokes? There must be enough energy stored in the flywheel to get it through compression when necessary. As RPM is reduced, an engine needs much more mass in the flywheel to achieve reliable running. Flywheel question The other thing to be looking at is the carb and it's ability to meter fuel correctly.

 

[Jasonb]

As well as a decent size flywheel you can play about with the cam profiles and also ignition timing, he timing can make a lot of difference. But all will depend on whether you are just making the engine to tick over slowly for display or want it to be doing work at low speed.

 

[TonyM]

It's all a bit hit and miss if you'll excuse the pun. Increasing the stroke will increase the compression ratio, potentially increasing speed, unless you compensate at the head or by moving the centre of the crank back, plus there is a possibility to change the length of the conrod, at the same time ensuring clearance at the bottom of the cylinder. You may want to consider reducing the bore rather than increasing the stroke as well as the things Jasonb mentions. It's all part of what makes this hobby fun. But generally as Brian says it's usually longer stroke slower revs.

 

[ajoeiam]

Looking at lots of torque curves it would seem that long stroke would tend to give a flatter torque curve and a bigger bore gives perhaps greater power but a stronger 'hump' for a torque curve.

Its fascinating to compare North American developed gasoline engines and European/Japanese ones. The two tend to be quite different - - - at least imo.

IIRC a higher compression ratio tends toward cleaner burning but also produces higher levels of NOs and with the cleaner burning higher levels of CO2 and CO - - - which today are real real bad. So today we are told to want engines that waste more fuel so that we create less CO2 or CO. It would seem that someone's forgotten that if you use a LOT less fuel less byproducts are created - - - - but - - hey - - - - who said that the public hysteria re: global warming and all of the miasma surrounding it was logical.

 

[Gordon]

I am trying to get the engine to run slower. I have plenty of power but it runs too fast. I am building a Rumley tractor and it moves too fast. I am running out of ways to add gear reduction. Presently I have a compression ratio of 6.7 so perhaps I should try to lower that. I have remade the cams to give a better cycle. I have tried a couple of different carburetors and I have played around with ignition timing.

I basically just kind of designed a four cycle engine by what looked right without a lot of calculating the various variables. Now I am going back and looking at the things which I should have looked at before I started. I know, cart before the horse but I have learned quite a bit in the process. The engine runs OK but I am not happy with the speed.

 

[Rocket Man]

Engineering books say the most efficient is, stroke should be the same as bore diameter. 2" bore should be 2" stoke. But stroke should also take into account engine RPM. High RPM car engines have a 4" bore with 3.5" stroke.

 

[lohring]

In high performance, naturally aspirated engines, breathing is what limits power. Racing engines are usually limited in displacement for a particular class. Valve area in a four stroke is limited by the bore diameter so F1 engines have big bores and short strokes for a given displacement. In two strokes the port area is proportional to the cylinder wall area. That means for the biggest ports two strokes should have long strokes. Other factors like scavenging efficiency also enter in so racing two strokes have close to equal bores and strokes.

Lohring Miller

 

[tornitore45]

From the carb side of the picture reduce the throat diameter. Air speed should be between 1/2 and one speed of sound to vaporize the fuel. A mall carb will lower the top power but help the low RPM.

 

[Gordon]

From the carb side of the picture reduce the throat diameter. Air speed should be between 1/2 and one speed of sound to vaporize the fuel. A mall carb will lower the top power but help the low RPM.

I remember that there is a rule of thumb about carb throat size but I cannot remember what it is and I cannot find it in a search. It was some percentage of bore size.

 

[tornitore45]

Percentage of bore works as a rule of thumb for "normal" configuration engine. One parameter that keep fairly constant after scaling an engine is the piston speed therefore the air intake speed is in the inverse ratio of Bore/Throat from which the rule of thumb come from. I can't remember the typical piston speed but can always remember the speed of sound in standard air as 340 m/s
The only thing that really count inside the Venturi is air velocity which define the pressure drop which is what "suck" the fuel. We want low pressure to suck fuel from a tiny hole to atomize the fuel well.

 

[Phil Haldenby]

Larger bore than stroke length is called over square and is usually high revving. Bore and stroke the same is called a square engine. Smaller bore than the stroke is called under square an is usually considered to be a higher torque engine.

 

[awake]

Some time back when I was thinking about a similar question, I found this Wikipedia article to be helpful: Stroke ratio - Wikipedia - obviously, Wikipedia can be "hit and miss" in terms of accuracy, but as best I can tell this one seems to be well researched and clearly written.

 

[Gordon]

Percentage of bore works as a rule of thumb for "normal" configuration engine. One parameter that keep fairly constant after scaling an engine is the piston speed therefore the air intake speed is in the inverse ratio of Bore/Throat from which the rule of thumb come from. I can't remember the typical piston speed but can always remember the speed of sound in standard air as 340 m/s
The only thing that really count inside the Venturi is air velocity which define the pressure drop which is what "suck" the fuel. We want low pressure to suck fuel from a tiny hole to atomize the fuel well.

That is fine except how do I calculate the air velocity? I envision some rather complex calculations based on speed of piston and bore.

 

[Ozwes007]

A much better option is to put as long a rod in as you can. 2 to 1 rod length to stroke length is a good starter.

 

[johwen]

I am trying to get the engine to run slower. I have plenty of power but it runs too fast. I am building a Rumley tractor and it moves too fast. I am running out of ways to add gear reduction. Presently I have a compression ratio of 6.7 so perhaps I should try to lower that. I have remade the cams to give a better cycle. I have tried a couple of different carburetors and I have played around with ignition timing.

I basically just kind of designed a four cycle engine by what looked right without a lot of calculating the various variables. Now I am going back and looking at the things which I should have looked at before I started. I know, cart before the horse but I have learned quite a bit in the process. The engine runs OK but I am not happy with the speed.

I would reduce the choke size of the carburetor to slow the engine speed down adjusting the mixture accordingly. Good luck Johwen

 

[mu38&Bg#]

One parameter that keep fairly constant after scaling an engine is the piston speed.

You'll want to look at the math for this. Piston speed scales proportionally.

I agree with the idea about carb flow velocity. This is the most overlooked concept in model engines, especially scaled from full scale. The percentage of bore rule is inadequate. Full scale carbs were often rated in CFM and this is important to understand.

 

[tornitore45]

Max Piston velocity at the middle of the stroke = Omega x R = 2 x pi x f x R
R = 0.5 x Stroke f = RPM/60
Velocity = 2 x pi x 0.5 x Stroke x RPM / 60 = pi x Stroke x RPM / 60
Stroke in meters give Piston speed in m/s

The Air speed in the Venturi is Piston Speed x (Bore Area/Venturi Area)Squared

Here comes the trick The Venturi Area is not exactly given by the diameter because the Spraybar reduce the Air passage Area therefore the Bore/Venturi diameter ratio is just an approximation when the Spraybar occupy a sizeable portion of the Venturi as it may be particularly true in small engines.

On the other hand I have seen model engine run with the largest carburetor twice the diameter of the smaller one.
It all depends what parameter one wants to favor.

 

[Steamchick]

Hi Brian, the main reason longer strokes run slower is piston speed. F1 engines have more than 2 : 1 bore to stroke but run at 18000rpm... limited by piston speed.
Also, the first engines - From Mr Otto, et al. - were based on what worked for steam engines.... which were at their most economical with longer strokes and partial cut-off. So that thinking biased the infernal engines.... that you aim to copy.
You can have twice the stroke of the bore diameter. The real way to get a slow runner is low compression and a massive flywheel. A longer stroke smaller bore may help, but carburation and valve timing for a tiny bang should be the real control. As you will know, on typical hit-n-miss engines, the bang is so big that you get many revs before the engine slows and asks for another charge of fuel and air to fire. A more massive flywheel will not go so fast for the same size of bang, whatever bore and stroke you have. So try that first?
K2