# Engine bore/stroke ratios



## MarkB (May 26, 2014)

Hi all,

I know a little about the theory of IC engines and thir stroke/bore ratios eg: short stroke, high reving low torque - long stroke, low reving high torque. my question is:

Is there any magic or ideal stroke/bore ratios for steam/air engines, or any views on the subject.

Thanks in advance.

Cheers, Mark.


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## dave-in-england (May 27, 2014)

.
Depends whether you want a racing car engine or a heavy duty slogger power engine for pumping water out of mines.

You have to adapt the choice of bore and stroke to suit the application.

.


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## lohring (May 27, 2014)

Internal combustion engine performance is determined by the amount of airflow through the engine.  For piston engines this is displacement times rpm.  Rpm is limited by valve area and the inertia of the reciprocating parts that determines bearing loads.  In four strokes both of these factors mean a short stroke and large bore is heavily favored.  The valve area is closely related to the bore squared and inertia is reduced with a short stroke.

The picture is more complicated tn two strokes.  Here the port area is related to the cylinder wall area or bore times stroke.  Overhead valve two strokes have more limited breathing.  A longer stroke may make scavenging easier as well.  High power small two strokes usually run bores equal to 1.2 times the stroke.   The very large marine two strokes usually run longer strokes.

Lohring Miller


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## Charles Lamont (May 27, 2014)

Steam engines are broadly similar - shorter stroke, higher speed. Other considerations may apply: vertical steam engines in naval vessels were kept as short as possible.


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## purpleknif (May 27, 2014)

Most automobile engines are square engines where the bore and the stroke are equal. A longer stroke relative to the bore will produce more HP and a shorter stroke will give more torque. Horsepower is what keeps you rolling but torque is what gets you rolling.
  That's why the  Chevy 302 in the old Z 28's was such a great motor. It was a de-stroked 327 to be legal for 5 liter Trans-Am.


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## Tin Falcon (May 27, 2014)

> Is there any magic or ideal stroke/bore ratios for steam/air engines, or any views on the subject.



From what I have seen the range is from square for a marine engine to stroke 2x the bore for a walking beam and mill engines around  1.5 and in the middle.
Tin


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## Charles Lamont (May 28, 2014)

Very early engines were often longer. A replica of one of Trevithick's engines
is 7" bore x 30" stroke.


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## TinyTool (May 28, 2014)

Infernal combustication engines work like this.

2 engines.

Identical compression ratio, combustion chambers and piston crowns etc...

10 units of energy (thrust) in the combustion process

Engine A has a 100mm bore and 50mm stroke.

Engine B has a 100mm bore and a 200mm stroke.

Each engine runs at 1000 RPM

Engine A - lower mean piston speed.

Engine B - has a 4 x higher piston speed.


Engine A - The frictional loss from ring scrape and piston sliding occours for 100mm per revolution.

Engine B - The frictional loss from ring scrape and piston sliding occours for 400mm per revolution.

The limiting factor for power is based upon the highest possible pressure inside the cylinder, the most amount of times, per given time period; thus the limit of piston acceleration and side thrust etc., are the limiting factors of the engine putting out many high pressure pulses for period of time.

Mindful of the chemical limits of combustion speed etc., and making reasonable use of the thrust and the time of that thrust in the cylinder..

Thus a big piston with a short stroke will put out more power, than an identical bore with a long stroke.

The long stroke motor will lose a significant amount of power from internal friction in the bore and the reduced amount of strokes for a given limit of piston speed, based upon RPM.


And when one designs engines the efficiency of percentages - of the increases or decreases of power at the crank shaft, based upon stroke and bore dimensions - then taking well established designed based upon proven testing - in enormous amounts of infernal combustication engines and steam engines, accurately translates directly into operating costs of the engine, and what ever the engine has been stuck into, to power.

For instance if one is operating a steam ship or engine, or a truck or trucking fleet - one of the primary overheads is fuel consumption.

While a long stroke engine (except for the modern marine diesels) loses more from internal friction in the bore and the limit of the mean piston speed in meters / minute, limits the RPM and thus the power strokes, but it develops a high torque - a bigger engine with a long stroke weighs more and has a lower power output.

With the short to square stroke motor, the results are a smaller engine, higher RPM, more specific thrust cycles per unit of time, and lower internal friction from the short stroke.

So the balance has to equate purely into the amount of fuel in, to the amount of power out.

If one is using say 100,000 litres / Kg - of fuel per day or month,  then an engine that can drive the ship or trucks the greatest distance with the biggest loads, is kept on the market and the motors that deviate anything more than marginally from the optimum expectation, are eliminated from the market.

The bore and stroke issue comes down to purely thermal efficiency and TCO or Total Cost of Ownership...

Enormous amounts of engineering and testing have established these principles.

In terms of the ultra long stroke marine diesels, these are infernal combustication engines with many special features - such as water injection - to create a secondary thrust by operating the engine as a combustion engine and a steam engine... 

The pistons tend to only contact the walls around the ring lands and on bearing pads, instead of the full length of the piston and they also run in a purely vertical motion with the side thrust from the crank shaft and connecting rod, taken up with the cross head.

So the bore friction is reduced to a small fraction of a piston engine that has the piston coupled directly to the crank shaft.

In terms of eking out the residual thrust from the expanding combustion and steaming gasses, the ultra long stroke marine diesels really do it.

But this is based upon their design and the fact that they do move enormous ships with enormous loads and the fuel consumption - even though it's very efficiently done, it's still very high and it's more cost effective to have an big, tall, low RPM engine that runs at a constant speed and gets the most amount of cargo delivered, over the greatest distances, per ton of fuel.


So when considering the engine design - many factors come into play in the design.

While I understand the efficiency of a short stroke motor, the appeal of building a single cylinder engine for a motorbike, with a 3" bore and a 6" stroke, and a big flywheel, for loping through the country lanes on - has enormous amounts of appeal.

But anyway...

Life goes on.


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## lohring (May 28, 2014)

There are a lot of misconceptions in this thread.  Let's start with physics using British engineering units.    Engines were created to do work.  Work is defined as moving a weight a distance, measured in foot (the distance) pounds (the weight).  Power is how fast this work is done or foot-pounds per second.  One horsepower is 550 foot-pounds per second.  In racing, power is the basic measure of engine merit we want to look at.  The torque in foot pounds or amount of work an engine can do is found at a particular rpm by dividing the horsepower by that rpm and multiplying the result by 5252.  This torque can always be increased with gearing to match your needs, but the power will not increase.   Therefore, the work won&#8217;t get done any faster.


There are a lot of considerations in engine design that affect the bore to stroke ratio like time between overhauls, dimensions, cost, weight, emissions, smooth running, and many others.  In racing classes, the least restrictive rules limit displacement, fuel, and amount of supercharge.  Formula I engines run very large bores for their displacement to get the most power possible.  Here valve train design is the limiting factor.  Most other racing engines are derived from stock versions where power wasn't the main concern.  Two strokes have even more different design limitations.



Where racing rules don't apply, bores tend to be close to the stroke.  Power, weight, and costs are all related to the displacement and number of cylinders.  Modern car engines are smaller with fewer cylinders than in the 1960s.  They can also run at higher rpm and develop the same power from less size, weight, and displacement due to material and manufacturing improvements.  Computer engine management also helps.


Lohring Miller


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## ennech (May 28, 2014)

In the UK  there was a formula used to determine the HP of an internal combustion engine for taxation purposes based solely on bore size and ignoring the stroke.  This resulted in the introduction of small bore long stroke engines.  It was only after this method of taxation was scrapped that square and oversquare engines appeared on the scene enabling larger valve sizes to be used.


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## Till (May 28, 2014)

MarkB said:


> Is there any magic or ideal stroke/bore ratios for steam/air engines, or any views on the subject.



Lubrication oil and manufacturing techniques made great improvements over the centuries.

  Steam engines were built for industrial work and had to be reliable for a long time. Lubrication was a great problem for decades.
  To tackle this problem in a piston engine, youll have to reduce the side forces (piston -> cylinder wall). An approach on this will result in a long stroke, long con rods, long piston skirts and maybe (more expensive) a crosshead.
  And... Suddenly, your sketch looks like an old stationary engine 



  Many applications demand high torque at low rpm, but gearing power down for more torque is very difficult at a large and reliable scale, even today. Luckily, steam engines can supply lots of torque at very low rpm without the need of a transmission.

  At low rpm, the losses caused by less valve  area in small bores are very small. 
  High rpm and large chunky low tech castings wont go together very well, either.
  The long stroke with a small bore allows a short crankshaft and a short frame for  multicylinder engines, too. (and this is still very important in modern passenger cars. Lots of cost savings with short blocks)




lohring said:


> There are a lot of misconceptions in this thread.



Yeah, this thread obviously started with steam and air engines in mind


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## lohring (May 29, 2014)

Thar's all true, but at automotive sizes and smaller gearing is the standard way to match engine characteristics to the load.  Even electric motors sometimes need this despite huge starting torque.  We geared up a brushed electric motor (rules again, not to mention costs) to get the output rpm to be similar to the rpm of the internal combustion engine it was replacing.  We used standard quick change gears to allow fine adjustments to match the motor to the characteristics of the propellers we had.  The results were worth it.

[ame]https://www.youtube.com/watch?v=-yNu2_LlO9s[/ame]

Mike Bontoft, the 1/2 owner and driver, built the boat and gear box.

This is now seriously off the topic of bore to stroke ratios of IC engines.

Lohring Miller


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