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Student's Corner
stroke to bore ratio MarineTalk Discussion Forums
All dates are given in mm/dd/yyyy format. Student's Corner
  stroke to bore ratio
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  Member: raghu Post Date: 1/15/2005  
stroke to bore ratio is given consideration in 2 stroke engines? what are the advantages of an increased stroke to bore ratio? why not consideration is given to stroke/bore ratio in case of 4 stroke engines.
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  Member: Lars G. Jacobsen Post Date: 1/17/2005  
As 2-stroke engines normally are directly coupled to the propeller the engine layout has to take into consideration the principal engine application when determining the engine speed. The possible engine speed is limited by the mean piston speed obtainable, while at the same time ensuring that the piston rings and cylinder liners can withstand years of operation. Thus long stroke engines, and hence low engine speed, is used in e.g. tankers, while short stroke engines and higher engine speeds are used in e.g. container vessels.

Lars G. Jacobsen
MAN B&W Diesel A/S
Copenhagen
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  Member: Ritinkar Sen Post Date: 2/8/2005  
Will it be correct to say that in determining stroke / bore ratio, the starting point must be the propeller design to give best possible propulsive efficiency, which in turn, will depend on hull form and the required ship’s speed?

In the case of a large size tanker or bulk carrier with a relatively lower requirement for ship’s speed, it is of advantage to have a large diameter slow turning propeller. And if the engine turns slower, the stroke must be longer in order to maintain optimum piston speed

For a container ship with a fairly high speed requirement, a smaller diameter fast turning propeller will be more efficient – hence we need a higher rpm engine, so stroke must be lower in order to avoid excessive piston speed that can cause rapid wear and thermal load.

Any comments?

Ritinkar Sen

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  Member: Lars G. Jacobsen Post Date: 2/8/2005  
Yes, without going into too much detail about propeller design that is the reason for the choices that have to be made, when designing a ship.

Lars Gotved Jacobsen
MAN B&W Diesel A/S
Copenhagen
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  Member: anonymous Post Date: 2/8/2005  
I agree to the view point of Mr Lars and Mr Sen and would like to add that low grade fuel oil can be burnt efficiently in long stroke engines only as long stroke engine will operate at lower RPM(for the same given power) and combustion can be completed around TDC(better thermal efficiency!).
Large dia , low RPM propellers have maximum propulsive efficiency as compared other designs, hence that also goes well with long stroke engine without necessitating the use of reduction gear
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  Member: Ritinkar Sen Post Date: 2/8/2005  
I have a question.....

It is said that large stroke / bore ratio engine has a better thermal efficiency i.e. a lower specific fuel consumption. Why?

Is it because due to slow turning, more time is available for combustion? If we assume that at a particular load, the total fuel injection angle is 20 degrees, then at 120 rpm, this will be accomplished in 1/36 second. If the rpm is 90 the fuel injection period will be 1/27 second. As more time is available at low rpm for fuel injection (and burning), the combustion is more complete hance more efficient.

Any coments?

Regards

Ritinkar Sen
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  Member: Ritinkar Sen Post Date: 2/8/2005  
Further to my posting, I have found the following material on the subject which may be of interest to readers:

"For a cylinder of given swept volume, the lower the stroke/bore ratio, the greater the length of the circumference of the piston ring and therefore the greater the potential leakage of air during the compression stroke. Also, the surface area to volume ratio becomes over the upper portion of the compression stroke, where most of the heat loss from the air occurs."

(Starting gear and starting aids -D. Hodge)

and ........

"As with the choice of the combustion chamber system, the choice of the stroke/bore ratio differs from manufacturer to manufacturer. Many conflicting factors arise when designing the engine, one of of which is the stroke/bore ratio. The question is how important is this factor in relation to all the other factors which have to be considered. The final decision concerning the engine design will probably be based on a combination of reasons rather than on one or two factors only; i.e. cylinder arrangements affect the engine dimensions. In-line engines are sensitive to bore diameter because it affects overall length. V-type engines on the other hand, where the demand for sufficient bearing area and, to a certain extent, also the length of the camshaft, has an important effect on the length of the engine, normally have space for large cylinder diameters. Together with a request for a smaller engine width this means that a low stroke/bore ratio is the natural choice. With shorter stroke the engine speed can be increased, not least because space for large valve and ports will be available thanks to the larger cylinder diameter. For a given swept volume this provides the means for increasing the power output.

Historically, the diesel engine is a long stroke engine, probably initially owing to the demand to limit the piston area to avoid excessive gas load on the ports involved. Other reasons may have been that a smaller bore diameter provides better means of controlling the tolerances for volume above the piston, and at the same time provides smaller heat loss areas. However, the high gas loading in a diesel engine requires an increased crank pin area, which in turn means that the connecting-rod big end must be larger. If the engine is to be kept reasonably compact and easy to service it is necessary to be able to pass the big end of the connecting-rod up through the cylinder. The choice of cylinder diameter thus tends to dictate the size of the crank pin or vice versa. Consequently these two factors depend strongly on each another.

The tendency is towards a smaller stroke bore ratio, especially for V-type engines. At least one engine in production has a ratio of 0.8. Direct injection in-line engines generally have 1.1-1.2 stroke/bore ratio. The author’s opinion and experience are that the choice of stroke/bore ratio is not one of the most important factors in designing an engine."

(Truck and bus engines – L.R.C. Lilly)

Regards

Ritinkar Sen

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  Member: anonymous Post Date: 2/8/2005  
If all the heat addition(or fuel combustion) can be completed at constant volume(ie when the piston is at TDC), the engine cycle will have maximum thermal efficiency.Although it is practically not possible to inject and complete the fuel combustion instantaneously when the piston is at TDC for so many reasons. In long stroke engines operating at low RPM fuel combustion will be complete earlier in the power stroke even for a low grade fuel oil, hence better thermal effieciency.
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  Member: aqualove Post Date: 4/7/2005  
i would like to add another point if considering increment in bore:stroke ratio to increase power outputs for the sake or an argument. a larger bore means a larger piston and the the loading on the bearings plus the additional strengthening of the the con rod, piston rod and the journals will not at all be a wise proposition.

( all this in addition to excessive air leakage past the piston rings with a larger bore.. as stated by mr. sen)

any comments?

thanks!

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