Design of 4 stroke CI engine from few parameters(bmep,Brake Po,max.mean piston speed)

[marellasunny]

(I have trouble in finding the stroke length and fuel flow rate)

From Heywood,E2.11
You are designing a 4 stroke CI to provide a brake power of 300kW,Naturally aspirated at its max rated speed.Based on typical values for brake mean eff.pressure(bmep){700kPa to 900kPa} and max mean piston speed,estimate the required engine displacement,and the bore and stroke for sensible cylinder geometry and number of engine cylinders. What is the maximum rated engine speed(rpm) for your design? What would be the brake torque and the fuel flow rate(g/h) at this max speed?Assume a max mean piston speed 12 m/s is typical of good engine designs.


I use the usual equations which relate bmep to max power(i.e max rpm) and bmep to max torque(i.e max bmep)


bmep=Pb(max)∗nR∗10^3/Vd∗N(max) ; nR=2 for 4 strokes


bmep(max)=6.28∗nR∗T(max)/Vd


where, Vd is the displacement volume=number of cylinders*3.14/4*B^2*L


where B-bore,L-stroke


The problem I have is that once I have found the bore dimension,I have no other way of finding the stroke length i.e I just have to assume the stroke=bore and continue the problem. Also,to solve for fuel flow rate,I have to assume that the sfc for NA diesels is usually 200(g/kWh) which the question doesn't say I can assume.Any another ways?What answers do you get?I get a bore dia of 388mm.

Correction to my calculations:
I added an unnecessary *10^3 in the formula for bmep. Stupid of me. In that case,the bore dia comes to 115.5mm. The calc. of stroke stays the same.


Note to moderator:I already posted the same on physicsforums but am re-posting here since there were no replies.

[PinkertonD]

An interesting approach, assuming the stroke is equal to the bore. At the risk of stating the obvious, you have not looked in the right places yet to arrive at a Volume. If you had, then the stroke should be easily calculated by you. You do not "get a bore dia of 388mm," what you mean there is "I hope a bore...," or at best "might be a bore..."

To this tired old brain it is pointless to proceed with other calculations until you know the bore and stroke.

As far as I can recall, "assume" is not an Engineering term. Time to hit the books. If you are struggling (and it appears so) with this then you need to back up a few months in the course notes and study the parts you have missed.

[marellasunny]

0334_001.pdfI have attached a scanned note of my calculations on paper. I finally arrive at a realistic value for stroke by assuming it as half of the bore diameter. L=B/2
I conclude this assumption realistic because the max rpm is 1300 for the consideration that it is 6 cylinder and having a 300kW horsepower. Is my methodology correct?
I am clueless to find the stroke length any other way than assuming values for the number of cylinders and max rpm range.

I cannot resort to geometrical calculations either since no relation between bore dia and stroke length exist(atleast not that I know of). I would like to go back and hit the books but haven't found much help beside equations.
Please offer corrections!

[PinkertonD]

I cannot resort to geometrical calculations either since no relation between bore dia and stroke length exist

As I said earlier, "assumptions," are not going to work. It is nuts to proffer up some work with "assume stroke is..." There are specific ratios between ideal bore and stroke relationships. It is that you will have to work on.

Our TOS here precludes helping with homework, but I do not mind trying to point you in the right direction. However, you seem hell bent on using calculations based on assuming the stroke. To make matters worse, you are now suggesting that stroke be half the bore. That highlights to me just how little you are understanding engine design.

Based on that apparent lack of understanding, I am reluctant to proceed, but let me give you three terms, over-square, square and under-square.

Now hit the books and stop wasting time on other calculations until you have a calculated bore and stroke to start with, "assume" is STILL not an Engineering term.

[marellasunny]

Thanks,that was the best advice everver-square,under-square.I realize what I did it wrong the first time. I took the bmep at max Power too low i.e I used the value of 7bar instead of something like 8 bar(which is realistic for NA diesels). But,what makes my design unrealistic is that the bore dia I obtain is 364mm for a 12 cylinder(an engine as large as this doesn't even exist for marine diesels!!).


Anyway,I proceed further to state that this would be a undersquare engine(with stroke greater than the bore dia).I state this design because having a oversquare engine with a rated power of only 300kW isn't realistic,the power rating given in the question is too low for a oversquare engine. So,the only possibility is to have a 'undersquare' design which means I get a stroke length of >350mm. I am astonished by these values. Especially when undersquare engines are meant for engines that have a need for higher peak torque. I arrive at a max rpm value of 791 rpm,again not realistic.Peak torque is a whopping 3600 Nm or 2655 ft-lb.

Have you ever come across such specs in your experience,for a 300kW engine i.e? After all this,I think there is something horrendously wrong with the author stating that this were to be a 300kW engine. Am i wrong?

[PinkertonD]

Well, done, but I have drawn the line on further assistance. There are many other things to consider besides the ultimate output. I am guessing, but with a reasonable (99%) degree of certainty that the exercise was worded such as it is to make you go through the things that lead you to the output figures.

Sorry, but you really have to do this yourself. If it seems incorrect then it probably is. And -- there are generally no single absolute methods for deriving results in engine design. There are always at least two methods to cross check the results. Damn, I think I have said too much already.

Hmmm, (dragging crystal ball to the front of the desk) I see a man/woman with the light on, studying way late into the small hours of the mornings...

[marellasunny]

Correction to my calculations:
I added an unnecessary *10^3 in the formula for bmep. Stupid of me. In that case,the bore dia comes to 115.5mm. The calc. of stroke stays the same.