My questions about camshaft design

[PedroCG]

Hi, the past month or so i've been diving in this subject, i've read some Race Engine Technology articles by prof Blair and Associates and whats available trough google books of Norton's "Camshaft Design and Manufacturing".
My objective is to develop an accurate enough software to predict valve train dynamics for a direct acting (bucket) system. The software will be used to project a cam for a FSAE car.
I'll use this thread to get some help in some questions.

The first thing i looked at was those 3 arc cams then polynomials and then splines. Splines prove to be the best, all polynomials will work but some will be more troublesome than others to "fine tune".

So my first question is: whats the problem with infinite jerk? 3 arc cams have infine jerk at the inflection points, they also have less area under the displacement for higher acceleration values and those are clear disavantages, but the jerk part i dont understand since its not related directly to force it will not cause loss of contact. i guess

 

[larrylcoyle]

Pedro, that would depend on the type of machine you are using. If it is a manual machine, it typically uses what is called a master lobe that is mounted on the end of the grinder to machine the actual lobe on the camshaft. This master lobe is several times larger than the actual lobe but I do not remember the actual ratio. If it is a CNC grinder, it uses the same data as any other type of CNC machine.

Larry

 

[facty]

There are several different file formats that are recognised by the most common standard types of cam grinding machines: .des, .opn, .cls, .s96, .p and .r files

As LarryCoyle pointed out, there is also the possibility of creating a 'master cam'. The machines that do this are: Berco, Storm Vulcan and Van Norman.

If you are going to write code to produce cam profiles and their corresponding manufacturing data, I would also suggest that you include in your program the facility to output the manufactured cam profile in exactly the same format as would be recorded by a cam profile measuring machine on the manufactured camshaft. You can then check that the manufactured profile is as intended!

 

[larrylcoyle]

The first thing I would do is determine what company was going to manufacture my camshaft. Once that decision is made you can contact them, tell them who you are and the nature of your project. Then you let them tell you the file format required to produce a camshaft on their machine. I have seen Berco and Storm Vulcan machines that originally used a master lobe converted to cnc machines so do not assume anything here. If at all possible, ask to speak to the person who does the camshaft design work as he will most likely be able to answer most of your questions. One thing you will have to expect is that you will probably run into some proprietary issues along the way. One possibility is they might want you to send your general design information to them and they might format it so that it is compatible with their machines.

You need to understand Pedro that you are pretty much doing a one-off camshaft design. This means that you need to make sure you are working with a manufacturer that currently offers something for the engine type you are working with. That will ensure that they have blank cores in stock or readily available. The last thing you want to do is to have to work with what is called a "round lobe" core as this drives the expense up considerably.

 

[PedroCG]

i see. thanks again guys, btw i can see now why most of the teams from Portugal dont toy with the engine alot...

 

[dicer]

If you know machine work you could mill the master on a standard milling machine, with the proper attachments of course. In the past I've done a non engine cam on such machines, and could have done what you are talking about. It would take some major time and no mistakes.

 

[facty]

Pedro,

You will see in the SAE paper I mentioned earlier that they were able to use the existing stock cams and get them reground. A reputable cam grinding shop will be able to regrind existing cams for about 200 pounds (at today's exchange rates that's equal to about 201 Euro!!!).

The fact most of the 600cc engines used in FSAE have a separate cam for exhaust and intake means you can set timings to whatever you like. As a FSAE university you will be able to get any of the 1D performance simulation codes for free to analyse your engine intake (including restrictor) ducting, exhaust ducting and valve lift events, and optimise them for peak performance. If the optimised valve lift profiles fall within the envelope of the stock cams then you are in business - regrind!

 

[larrylcoyle]

Pedro,

facty has a great idea that I totally forgot about. Since I am sure there are certain parameters that you have to remain within, this would probably provide you a very cost effective option. You could probably talk to one of the current cam manufacturers that offer grinds for your engine and ask if they offer regrinds. If so, there is a very good chance they have something very close to what you need. It would involve a few phone calls or emails and it should be worth that.

Good catch facty... As many years as I worked for a cam company, I can't believe this went right over my head.

Larry

 

[PedroCG]

we have tougth of that before, buy a used set of camshafts and have them reground, the new profile will probably be "smaller".

 

[SomptingGuy]

When's your deadline PedroCG?

- Steve

 

[larrylcoyle]

Pedro, what do you mean when you say the new profile will be smaller? If you are thinking that the newly reground used cores will be physically smaller from a dimension standpoint, that is true but most likely more of the material will be removed from the base circle. If you are thinking that the lobe characteristics will be smaller from a performance standpoint, that is false. Regrinding camshafts is a common occurance even when you start out with a new core. This is where the experience of the tech support guys at the cam company can be of service to you. Most likely you will be able to get at least one good regrind out of a set of cast cores for your engine.

 

[PedroCG]

SomptingGuy, the objective is to race the car in 2011 so having everything ready for testing and training in the beginnings of 2010 would be nice.

larrylcoyle, i believe the new profile will have less duration and since the original one must be agressive ish... but i may be wrong we haven't done any simulation on that area yet, mostly chassis and suspension for now.

the project is 8 years old on that school, many people have passed and last year the car was finaly put together, now some things need to be "re-engineerd"... what should be an advantadge (time) proved to be a disavantadge in this case.

 

[Tmoose]

About 3 posts down on this Packard site is A recollection by one of the principals of HO Racing, a respected Pontiac specialist outfit in the 70s.

http://www.packardinfo.com/xoops/html/modules/newbb/viewtopic.php?post_id=22843

 

[BigVlad]

I hate to be practical but why are you attempting to design a cam? Presumably the engine is from a motorcycle - you must be able to find a suitable grind available off-the-shelf which is almost certainly going to be better than anything you can do yourself (or anybody can probably do themselves). If you want to be different, build a system like that shown in the "Helical camshaft" thread.
You don't need to worry too much about cast cores etc. - motorbike cams are fairly small and a blank can be fairly easily and cheaply machined from 4140 (or similar) and nitrided after grinding.
Rothbart's book "Cam Design Handbook" could be useful.

 

[facty]

The FSAE engine is usually a 600cc motorcycle engine taken from one of the sportsbikes - as you say. BUT the engine has a single 20mm restrictor in its intake ducting. You'll find that there are no "off-the-shelf" parts designed for this purpose, and anyway, this is a university design project were the students are being prepared for a possible carreer in engine design with a racing team/company etc so it is essential that they are exposed to the practices involved in this design process. Understanding unsteady gas dynamics and their effect on engine tuning is key to this process. Then being able to design a cam to provide the optimum valve lift events for the tuned engine with the dynamic constraints imposed by the mechanical valvetrain is the ultimate learning goal for the students. It will let them appreciate why the cams "on the shelf" are designed the way they are.

 

[BigVlad]

Thank you for your answer Facty. I did more-or-less realise that the students were probably expected to build their own gear rather than buy it and this was why PedroCG was designing his own profile. The students may learn a lot but the chances of actually designing a really useful profile are (I think) very slim. I didn't know about the 20mm restrictor. I honestly don't know how the restrictor would affect the type of cam profile needed. It would be interesting to know what other people have done. Given enough time/money I think my approach would be basically trial-and-error using existing profiles from other bikes or even cars.

 

[facty]

The students are being introduced to engineering science and analytical design techniques so that, as graduate professional engineers, they don't have to practise "trial and error", but instead will have the necessary skills and understanding to "optimise" a design problem and save time and resources at the testing and development stage.

Earlier on this thread I posted a link to an SAE paper where the students successfully designed valve lift profiles for the exhaust and intake, and consequently cams, that produced a significant increase in performance of their engine - so it is indeed possible. What you'll find is that this type of successful project will probably be supervised by a member of faculty with experience in engine design!

 

[BigVlad]

Facty - I understand what you are on about - FSAE cars etc. are student training exercices - not necessarily the best way to do things. If a professional engineer was given the job by his employer to come up with a cam to suit Pedro's needs I think his first move would not be to sit down and design one from scratch. He would first look for an existing suitable profile - if nothing was available (and there always is something) he (or she) would then think about designing one. You yourself rightly advised Pedro on 11 March that designing his own cam was an enormous job and it was not wise to attempt such a thing. I think a lot of FSAE design in general is a bit unnecessaryly complicated - but once again you can justify pretty much anything on the grounds of student training. By unnecessary I mean F1 style carbon fibre chassis and spindly suspension arms, inboard bellcrank layouts, excessive electronics etc. I think it would be very interesting after the FSAE competition is run and won for the best FSAE cars then to face a competition against cars built by professional racing teams and amateur (home-built) cars. I have a feeling the student/academic cars would not win - even against the home-builts - but I could be wrong. Maybe there should be a separate racing class for FSAE cars - they are occasionally seen in hillclimbs etc. Possibly my overall point could be that students should not be trained to immediately go for the most complex and academic solution to a problem - simple and effective should be considered as well.

 

[facty]

BigVlad,

If you think any serious racing team, whether it be from F1, Inycar, NASCAR, LeMans Series, MotoGP, Superbikes, MX1...., I could go on ad infinitum, design their "engines off the shelf" then you are seriously wrong - they do it in-house and then get a reputable Cam Manufacturer/Grinder to make it. Yes, home-builders or back-yard-specialists utilise exiting parts, so I have no argument there.

I advised Pedro NOT to sit down and develop his own "cam design software" from scratch!! - there are several very good software applications out there (used by the aforementioned race teams) that already do this and which FSAE teams can get for free or at a great discound due to their academic status!

If a professional engineer at an engine design company was asked to design a cam to suit a specific purpose then I doubt he/she would be satisfied with what was "out-there". Engine design, and specifically optimising performance, is all about understanding the unsteady gas dynamics dynamics as far as pure engineering is concerned. The job of the engineer/designer is to produce the most optimised design before the inevitable testing and development phase. If the engineer has the understanding and tools/applications then a great deal of resource and therefore expense can be saved as there will be less 'suck-it-and-see' testing required.

Most of the great advances in engine performance over the last thirty years has come from a better understanding of the engineering science underlying the engine - the unsteady gas dynamics and the combustion.

Don't get me wrong, experienced engine builders and testers play a massive role in engine design through application of this experience and "know-how", but the real advances come from a better "understanding" of the fundamentals.

You are totally right in sayin that engineering students should not be trained to go for the most complex analyses, but they surely need to be aware of all the possibilities if they are to become successful professional engineers.

 

[PedroCG]

well facty just answered the question for me... i just want to add that if one is to be called an engineer one must be an engineer and not just some guy that spend 3 or more years in the university, thats what i think.

 

[PedroCG]

i forgot one thing, i think the helical camshaft would be very unpratical if not impossible for this aplication, its a direct acting (bucket) system so the contact path is probably to wide.