Piston Design!
Discussion
Hello all!
I currently have a few questions to ask and see if anyone could help me.
I have currently been given the task to design a new piston including the connectong rod and gujon pin.
I have been given certain specification which are:
it must be 500cc
total power between 7500 to 14000 rpm
120 bar combustion pressure
bore
I was just wondering if anyone could recomend some suitable materials which i could construct the piston and the connecting rod from.
also if anyone has done any kind of engine component design before would you be kind enough telling me how you went about it?
Thanks A.Pitman
I currently have a few questions to ask and see if anyone could help me.
I have currently been given the task to design a new piston including the connectong rod and gujon pin.
I have been given certain specification which are:
it must be 500cc
total power between 7500 to 14000 rpm
120 bar combustion pressure
bore
I was just wondering if anyone could recomend some suitable materials which i could construct the piston and the connecting rod from.
also if anyone has done any kind of engine component design before would you be kind enough telling me how you went about it?
Thanks A.Pitman
You are going to need a little more information before you can get a design together. Firstly, you need to know the stroke of the crank and the length of the bore (actually the length from the centre of the crank main journal to the face of the block).
If it is going to be revving to 14000 rpm you are going to have to have a pretty short stroke or it simply won't work. The design of the piston and rod must then centre around the overall length and available rod / piston combination. It's going to get technical now!
The gudgeon pin will need to be set in the piston with enough piston material above it to maintain strength ... this is entirely dependant on the material .. use alluminium and it will need to be thicker than say titanium. Then you have to fit rings into the measurement, once you have decided upon these you can start thinking about rod lengths. How many cylinders? what block? what bore diameter? what block configuration (straight, V etc.)? Answer all of this first and then figures can start being inserted into sums!
If it is going to be revving to 14000 rpm you are going to have to have a pretty short stroke or it simply won't work. The design of the piston and rod must then centre around the overall length and available rod / piston combination. It's going to get technical now!
The gudgeon pin will need to be set in the piston with enough piston material above it to maintain strength ... this is entirely dependant on the material .. use alluminium and it will need to be thicker than say titanium. Then you have to fit rings into the measurement, once you have decided upon these you can start thinking about rod lengths. How many cylinders? what block? what bore diameter? what block configuration (straight, V etc.)? Answer all of this first and then figures can start being inserted into sums!
Configuration:- it is a short stroke engine? Must keep the mass down, looking at combustion chamber shape to minimise the piston dish/weight.
Materials:-Look at how materials cost/vary/wear, most pistons are cast, squeeze cast is good, forged are super strong but don't wear so well (material is too hard, for high milage use).
Rods:- Hige revs=some serious inertia for pistons and the conrods can actually stretch a fair bit, hence bumping clearance (the gap you need in order for the pistons not to crash into the head!) Lightness generally=stretchyness, look at forged steel, billet steel (high forging pressure) means you can keep the weight down, how about titanium? Expensive! For your data look for CRC Handbook of Chemistry and Physics (massive data book) or Handbook of engineering (if you can find it) for expansion coefficients.
What they are asking you to do is to balance the -ve factors out with +ve factors, the two are generally antagonistic, you have to balance the load with strength, come up with a viable figure and say why, again with the chamber, what fuel, how's it burn? What dish do you need? How does it's weight effect the system. YOu have to balance this with material choice and consider the reliability of the whole system.
I'm a race engine enthusiast and reliability is not my highest priority. However, I will do my best to help. Marquis Rex is the man you want to speak to. Any questions about fuels/oils/chemistry/materials give me a shout, that's my expertise.
Stu
Materials:-Look at how materials cost/vary/wear, most pistons are cast, squeeze cast is good, forged are super strong but don't wear so well (material is too hard, for high milage use).
Rods:- Hige revs=some serious inertia for pistons and the conrods can actually stretch a fair bit, hence bumping clearance (the gap you need in order for the pistons not to crash into the head!) Lightness generally=stretchyness, look at forged steel, billet steel (high forging pressure) means you can keep the weight down, how about titanium? Expensive! For your data look for CRC Handbook of Chemistry and Physics (massive data book) or Handbook of engineering (if you can find it) for expansion coefficients.
What they are asking you to do is to balance the -ve factors out with +ve factors, the two are generally antagonistic, you have to balance the load with strength, come up with a viable figure and say why, again with the chamber, what fuel, how's it burn? What dish do you need? How does it's weight effect the system. YOu have to balance this with material choice and consider the reliability of the whole system.
I'm a race engine enthusiast and reliability is not my highest priority. However, I will do my best to help. Marquis Rex is the man you want to speak to. Any questions about fuels/oils/chemistry/materials give me a shout, that's my expertise.
Stu
i wonder if blue peter could make one from a used fairy liquid bottle, some "tacky blue stuff" some sticky tape....and some cardboard ?
any other magic ingredients ??
Im sure there is more to your question than you ask. But as most pistons are made from aluminium, and rods from steel products, that part is easy answered.
any other magic ingredients ??
Im sure there is more to your question than you ask. But as most pistons are made from aluminium, and rods from steel products, that part is easy answered.
I heard of a guy complaining that his bike wasnt going too well either, and torturing guys at a local motor factors, even though he already knew everything himself.. So the guys in the shop ( jokingly, as the customer was an idiot ) told him he needed to get a long drill bit, and drill through the spark plug hole, and into the piston, to lower the compression a bit.
Needless to say the idiot did it...
Needless to say the idiot did it...
basically its a job for a company they asked me to come up with piston design. they given me specifications. if i do a good job of it they mite offer me a job.
I use pistonheads alot and i know theres a lot of much more experienced guys on here than me. I never done anything like thos before so i was just asking if anyone would give me advice on certain things and basically how to start such a project.
I use pistonheads alot and i know theres a lot of much more experienced guys on here than me. I never done anything like thos before so i was just asking if anyone would give me advice on certain things and basically how to start such a project.
pittersuk said:
basically its a job for a company they asked me to come up with piston design. they given me specifications. if i do a good job of it they mite offer me a job.
I use pistonheads alot and i know theres a lot of much more experienced guys on here than me. I never done anything like thos before so i was just asking if anyone would give me advice on certain things and basically how to start such a project.
So this is a test to find out whether you know enough to design a piston? I may be reading too much into your original post, but I got the impression that you didn't know how to go about it or what issues you needed to consider. If that is the case and you're hoping to get enough help from PH to blag your way through the test, I think that would be a mistake.
Do you have a target block height that will pretty much determine what you can get in anyway. If its a rhetorical study and package isn't an issue you need to start from somewhere and try and compare as much as possible with other engines as you go. I'm by no means an authority on this, but here goes:
You need a starter and piston speed is very good place to start because it is an easy comparitor with other engines of the same application. Piston speed is a 'derived' limiting factor in engine design principally due to conventional material wear limits. The Honda S2000 is the benchmark for mean piston speed in roadcar at ~25m/s. Actual max piston speed and acceleration is a function of rod length, stroke & speed.
For a max revs of between 7500 and 14000rpm this would suggest this is very fast racing engine and wear and fuel economy probably isn't an issue. On a fast engine like this inertial loading from the reciprocating masses are going to be your biggest stresses rather than gas loading, but you can't workout inertial loading until you have stroke, speed, masses and rod length. Playing safe for a starter assume mean piston speed is ~28m/s and your max revs is 11000rpm. This would give a stroke of ~76mm from interpolation (MPS = 2 x stroke x revs). Conversely this gives you then a bore of ~91mm from interpolation of you 500cc limit.
Generally rod lengths are about 3-4 times crank radius (half stroke). Assume a rod length of say 3.5 because its a very common ratio. Materials for the rod on this type of engine tend to be very strength steel like EN24 or possibly titanium. Maybe a machined with an H shank for high speed engines.
You now have a bore and stroke and rod length to play with. With the bore and max combution pressure you can now calculate the max gas force acting on the piston, which then also acts on the gudgeon pin and the conrod. Now you can calculate bearing stresses on piston, pin and rod due to gas load. Use the gas load and a bit and design the rod shank cross sectiontional area to a percentage of the proof stress of the rod material, determined from the S-N curve for how long you want it to live.
Piston mass is generally a function of bore size because of manufacturability and material. Try to see whats available. Forged aluminium pistons are used in racing.
You now have very rough masses, stroke, rod length, and speed. Work out your inertial loads and see if your initial designs work. If not tweek the factors iteratively until you something that may just work.
This is certainly not comprehensive and could go on but I would be telling you how to do it and I'm not certain this is the best method anyway. Buy "The Internal-Combustion Engine in Theory and Practice Volume 2" by Charles Fayette Taylor which is an excellent book which covers factors in engine design and will help many of these calculations.
>> Edited by wheeljack888 on Sunday 6th February 16:36
You need a starter and piston speed is very good place to start because it is an easy comparitor with other engines of the same application. Piston speed is a 'derived' limiting factor in engine design principally due to conventional material wear limits. The Honda S2000 is the benchmark for mean piston speed in roadcar at ~25m/s. Actual max piston speed and acceleration is a function of rod length, stroke & speed.
For a max revs of between 7500 and 14000rpm this would suggest this is very fast racing engine and wear and fuel economy probably isn't an issue. On a fast engine like this inertial loading from the reciprocating masses are going to be your biggest stresses rather than gas loading, but you can't workout inertial loading until you have stroke, speed, masses and rod length. Playing safe for a starter assume mean piston speed is ~28m/s and your max revs is 11000rpm. This would give a stroke of ~76mm from interpolation (MPS = 2 x stroke x revs). Conversely this gives you then a bore of ~91mm from interpolation of you 500cc limit.
Generally rod lengths are about 3-4 times crank radius (half stroke). Assume a rod length of say 3.5 because its a very common ratio. Materials for the rod on this type of engine tend to be very strength steel like EN24 or possibly titanium. Maybe a machined with an H shank for high speed engines.
You now have a bore and stroke and rod length to play with. With the bore and max combution pressure you can now calculate the max gas force acting on the piston, which then also acts on the gudgeon pin and the conrod. Now you can calculate bearing stresses on piston, pin and rod due to gas load. Use the gas load and a bit and design the rod shank cross sectiontional area to a percentage of the proof stress of the rod material, determined from the S-N curve for how long you want it to live.
Piston mass is generally a function of bore size because of manufacturability and material. Try to see whats available. Forged aluminium pistons are used in racing.
You now have very rough masses, stroke, rod length, and speed. Work out your inertial loads and see if your initial designs work. If not tweek the factors iteratively until you something that may just work.
This is certainly not comprehensive and could go on but I would be telling you how to do it and I'm not certain this is the best method anyway. Buy "The Internal-Combustion Engine in Theory and Practice Volume 2" by Charles Fayette Taylor which is an excellent book which covers factors in engine design and will help many of these calculations.
>> Edited by wheeljack888 on Sunday 6th February 16:36
nighthawk said:
And all this is before you even think about flame front propagation and piston cooling too
It's just such a massive subject and why the industry employs 1000's of R&D people in just this country alone. To conceive of every single factor of engine design alone just boggles the mind.
GreenV8S said:
pittersuk said:
basically its a job for a company they asked me to come up with piston design. they given me specifications. if i do a good job of it they mite offer me a job.
I use pistonheads alot and i know theres a lot of much more experienced guys on here than me. I never done anything like thos before so i was just asking if anyone would give me advice on certain things and basically how to start such a project.
So this is a test to find out whether you know enough to design a piston? I may be reading too much into your original post, but I got the impression that you didn't know how to go about it or what issues you needed to consider. If that is the case and you're hoping to get enough help from PH to blag your way through the test, I think that would be a mistake.
The fact that you cant even spell "gudgeon pin" and the specifications are vague as a vague thing with a big dollop of vagueness on it tells me you know absolutley nothing about the subject. Would suggest doing some extensive reading and perhapps even pulling a scrap engine apart to actually see what were talking about.
>> Edited by eliot on Sunday 6th February 20:27
GreenV8S and eliot, give the guy a break.
We all have to start somewhere, and a subject like this is just too huge to go it alone. I've been working in the engine industry for just over 7 years and the only way to learn when starting off is from the experience of your mentors and then doing it for yourself.
The question set sounds like the typically unforgiving question asked by a Principal Engineer whos been working in the business for 30 years. They sometimes forget that engineering undergraduates have a hard enough time at university just learning the fundamentals. Fundamentals that probably most Principal Engineers forgot about long ago.
>> Edited by wheeljack888 on Sunday 6th February 22:18
We all have to start somewhere, and a subject like this is just too huge to go it alone. I've been working in the engine industry for just over 7 years and the only way to learn when starting off is from the experience of your mentors and then doing it for yourself.
The question set sounds like the typically unforgiving question asked by a Principal Engineer whos been working in the business for 30 years. They sometimes forget that engineering undergraduates have a hard enough time at university just learning the fundamentals. Fundamentals that probably most Principal Engineers forgot about long ago.
>> Edited by wheeljack888 on Sunday 6th February 22:18
I always got the impression that something like engine design is a trial and error learning process anyway.
You know, you do it based on something else, adding in some tweaks. Some work out, some don't, but you don't compromise the whole to the point where the NRE is a total loss.
I think that's one of the sad things about the lack of investment in engineering in this country. It's great for the accountants, but what they never really get a handle on is the importance of the knowledge that is retained by having the right people in the right place just moving forward sensibly.
It all seems to be so cut and thrust. You develop something, it's pretty good, but could have been better. So the management reap the profits and turn their staff into pen pushers. Anyone with usefull knowledge moves on.
I suppose my point is that having the right team and product together is so important. The bean counters think they're right, but they aint.
You know, you do it based on something else, adding in some tweaks. Some work out, some don't, but you don't compromise the whole to the point where the NRE is a total loss.
I think that's one of the sad things about the lack of investment in engineering in this country. It's great for the accountants, but what they never really get a handle on is the importance of the knowledge that is retained by having the right people in the right place just moving forward sensibly.
It all seems to be so cut and thrust. You develop something, it's pretty good, but could have been better. So the management reap the profits and turn their staff into pen pushers. Anyone with usefull knowledge moves on.
I suppose my point is that having the right team and product together is so important. The bean counters think they're right, but they aint.
wheeljack888 said:
GreenV8S and eliot, give the guy a break.
I accept your point, but I think it may be a mistake to take the question at face value. Do they actually want a piston design, or do they want to find out whether he is capable of tackling this a design challenge of this complexity, or do they want to find out how he responds when given a challenge that is beyond him?
I'm not sure whether he has been asked to source an appropriate piston, or to do the sizing and stress calcs and design a piston from first principles. It sounds like the latter, which is a bit strange. No disrespect intended but if I've read his posts right, then he doesn't know how to do it or what is involved.
Now with help from people like you he could probably come up with a decent outline design and use all the right buzz words but thirty seconds of questioning would show he doesn't know what he is talking about. If this is a part of a recruitment test then I think being caught bluffing would pretty much seal his fate. Better to be honest from the outset, clear about what he does and doesn't know, by all means explain how he would go about tackling the problem from first principles but don't try to fool anyone into thinking he can do something when he can't. Being ignorant is acceptable, if you're willing to learn. Being a liar isn't. That's my take on it, anyway.
GreenV8S said:
GreenV8S and eliot, give the guy a break.
I accept your point, but I think it may be a mistake to take the question at face value. Do they actually want a piston design, or do they want to find out whether he is capable of tackling this a design challenge of this complexity, or do they want to find out how he responds when given a challenge that is beyond him?
I'm not sure whether he has been asked to source an appropriate piston, or to do the sizing and stress calcs and design a piston from first principles. It sounds like the latter, which is a bit strange. No disrespect intended but if I've read his posts right, then he doesn't know how to do it or what is involved.
Now with help from people like you he could probably come up with a decent outline design and use all the right buzz words but thirty seconds of questioning would show he doesn't know what he is talking about. If this is a part of a recruitment test then I think being caught bluffing would pretty much seal his fate. Better to be honest from the outset, clear about what he does and doesn't know, by all means explain how he would go about tackling the problem from first principles but don't try to fool anyone into thinking he can do something when he can't. Being ignorant is acceptable, if you're willing to learn. Being a liar isn't. That's my take on it, anyway.
I agree there is that risk. It's also a very harsh open ended question to ask an undergraduate. Perhaps a disclaimer is required:
Mr Pitman if you’re really serious about this buy the Fayette-Taylor book or an equivalent that maybe suggested by your lecturers. Ask your lecturers what they think you should do. The spiel I produced earlier is just my opinion outlining just a few of the factors involved treat them as such.
I'm teaching myself engine design, I have a design for a cylinder head which I am going to cast myself. I have read dozens of books on the matter and the interdependence of factors is immense. Flame speed/compression ratio/cam figures/etc, etc. The considerations are immense, I expect that "the right answer" is in the discussion leading up to a wrong answer. The processing of information demonstrating the knowledge (or appreciation of) the interdependent factors.
I've done a degree which included some of the nastiest maths possible. None of it boggled my mind more than reading David Vizards book on engine tuning and asking myself "Why? and How?".You can't just break it up into components and treat them in isolation. A lot of books on the subject do not discuss why, they just say "You need some forged Omegas" I'm getting there with my knowledge and I admire anyone who has taken up the challenge.
The lad needs a bit of positive input.
You can't cheat on a question like this.
Good Luck.....Stu
It has been my greatest lesson in "big picture thinking" so far.
I've done a degree which included some of the nastiest maths possible. None of it boggled my mind more than reading David Vizards book on engine tuning and asking myself "Why? and How?".You can't just break it up into components and treat them in isolation. A lot of books on the subject do not discuss why, they just say "You need some forged Omegas" I'm getting there with my knowledge and I admire anyone who has taken up the challenge.
The lad needs a bit of positive input.
You can't cheat on a question like this.
Good Luck.....Stu
It has been my greatest lesson in "big picture thinking" so far.
Look guys! im studying engineering not english! i made one spelling mistake. All the best engineers in the world make mistakes!!!
I only asked this type of question as i know there alot more experienced guys on here than me and im only looking for helpful advice to get started on such a project.
I appreciate all the helpful feedback so far has given me a great idea on how to get started!
Just bit annoyed at a few people have had ago at me for asking the basic question.
Im only learning and i just need some help which i appreciate cheers guys!
I only asked this type of question as i know there alot more experienced guys on here than me and im only looking for helpful advice to get started on such a project.
I appreciate all the helpful feedback so far has given me a great idea on how to get started!
Just bit annoyed at a few people have had ago at me for asking the basic question.
Im only learning and i just need some help which i appreciate cheers guys!
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