Camshaft Design

[bhusted]

I came across this forum while searching the web for help with a problem I'm having. I've found many great resources here concerning cam lobe profiles and suggested machining techniques, but I my question is more fundamental than this. I'm slightly embarrassed to be asking because it seems like I should be able to think my way around this one, but here it goes.

I'm designing a model engine (just for demo purposes and never intended to be run) modeled after the LS1. I chose this for the simple reasons that it is a prolific engine with many dimensions readily available and it's a V8. From all of the info and pictures, I've modeled a scaled and simplified version except for the camshaft. I've made several attempts and it never seems to come out right and I need one of you to point out where I'm going wrong in my thinking. Here it goes...

Being a 90 degree V8, there will be an intake event for every 90 degrees of crank rotation. Of course the camshaft rotates at half the speed of the crank, so there are 360 deg of cam for every 720 deg of crank rotation. The firing order of the LS engines is 1,8,7,2,6,5,4,3 and the cylinders are numbered with odd cylinders on the right when facing the front of the engine and a clockwise rotation of the crank.

When modeling the lobes, I started with the intake on cylinder 1 as 0 deg. The intake lobe of cylinder 2 is therefore 135 deg of cam "behind" cylinder 1. This pair worked out fine, but goes entirely haywire when I try doing any other cylinders. For instance, cylinder 3 is last in the firing order and is 315 deg of cam "behind" cylinder 1. This does not line up with an intake event on cylinder 3 at 630 deg of crank. The same goes for every other cylinder aside from #2. Here's my chart for the firing order, crank and cam angles.

Firing Order: 1 8 7 2 6 5 4 3
Crank Angle: 0 90 180 270 360 450 540 630
Cam Angle: 0 45 90 135 180 225 270 315

I'm not sure what I'm missing. I'm happy to share my models or the entire assembly if it helps (Fusion 360). At this point I'm stuck and have been contemplating buying a stock LS cam off of Craigslist just so that I can pull measurements from it directly. Will someone please point out my mistake? Thanks for the help.

 

[stevehuckss396]

Firing Order: 1 8 7 2 6 5 4 3
The intake follows the exhaust so I always key on the exhaust. Dont know what your lobe seperation is so i'll use 110. As stated there is an event every 90 degrees or 45 cam degrees.

Ex1 = 0 Degrees
In1 = 0 or 360 so 360 = 110 or 250 degrees
Ex8 being next would be 360 – 45cam = 315.
The cylinder being 90 degrees away on the even bank Ex8 = 315 – 90 = 225 Degrees
Ex7 = 360 – 90 = 270 Degrees
Ex2 = 360 – 135 = 225 Even bank so 225 – 90 = 135 Degrees
Ex6 = 360 -180 = 180 Even bank so 180 – 90 = 90 degrees
Ex5= 360 – 225 = 135 Degrees
Ex4 = 360 – 270 = 90 Even Bank so 90 – 90 = 0 Degrees
Ex3 = 360 - 315 = 45 Degrees

Then all the intake lobes lag by the lobe separation (110) so exhaust degree – 110 = intake lobe

E1=0
I1=250
E8=225
I8=115
E7=270
I7=160
E2=135
I2=25
E6=90
I6=340
E5=135
I5=25
E4=0
I4=250
E3=45
I3=315

The degree marks are cam degrees looking at the cam from the front in a clockwise rotation. Please check all the math. I typically model the cam and give it a twirl in the model to verify everything before I cut. I know that you are not doing that but someday you might so double check me and if something is off, come back and it will get figured out.

Your post said " I'm slightly embarrassed to be asking"

Not here. No matter how dumb you think the question is don't feel like you cant or shouldn't ask. If you don't know, you don't know. That's why were here, to help each other. If we don't feel safe asking dumb questions here, there is nothing here for any of us so ask away.

 

[bhusted]

Thanks so much for your quick response. I follow your math and things make sense to me. The piece that I had neglected is the 90 deg offset from the even and odd cylinders. I knew there must be something simple that I was missing. Once I accounted for this 90 deg separation of the cylinder banks, my numbers agree with yours.

To be clear, my intention is not to just have the CAD model of the engine, but also to machine all of the parts. I want to finished engine to be accurate, but I am simplifying things by not dealing with oiling, ignition, fuel, or compression. I am finishing a machinist program and need a capstone project and this seemed like a good way to go. The goal is to learn and machine many of the parts in 4 axis CNC mill or on a live tool CNC lathe. Around here, those things may seem like cheating, but it's all a learning experience for me. If I get approved and once I get to making chips, I'll certainly start a thread here with my progress.

Thanks again for your help and support.

 

[Cogsy]

You will need to consider some sort of oiling if you plan to spin the engine over. It doesn't need to be a pressure system, simple dippers on the big end caps would be fine for splash lubrication. And incorporating piston rings to generate compression is virtually trivial. Ignition will be a piece of cake with hall effect and a CDI(assuming you're planning on adding a mock distributor driven off the cam) and then it's just a matter of adding a carb and it will be a running engine. For all the work you're going to be putting in, I think the little extra to make it run would be worth it.

 

[stevehuckss396]

What scale will this engine be modeled in?

 

[retailer]

Hello bhusted

How far have you got with your design ? I'm also desgining a V8 model using autocad and fusion360 to model it, so far I've finished the block, crank, cam, liners, pistons and part of one head, probably enough to make a start but I'm going to finish my current T&C grinder project to the point where it is operational before I start any engine machining. I don't have any CNC gear so all of my machining will be done the old way, I also had the same question about the cam and I did spend an evenings work on it before it dawned on me that what I had was actually two 4 cylinder engines timed 90 deg apart.

I plan to make mine a runner and I see the project as being in 3 parts - design, construction and then getting it to run, the design and construction can be done somewhat concurrently. I have only ever built one engine before a steam wobbler - I'm assuming (hoping) that what ever skill I lack will be picked up on the way. I don't mind sharing my fusion 360 model with you if you are interested it's always good to see how somone else has tackled something, although I have to keep in mind my lack of CNC gear so complex shapes are out of the question. I've found from previous projects that when designing something you have to keep in mind the machining part of it as well.

There are a lot of friendly knowledgeable people on this forum as you've found - if you have questions there is always someone ready to help.

 

[bhusted]

I appreciate all of the help. With the change that stevehuckss396 suggested, the cam looks like all of the lobes are in the correct places.

Since this will be a project for a class, I need to be able to set a goal of completion that is attainable for the duration of the quarter (~8 weeks). To simplify things, I plan to make the pistons and some other components from Delrin. The block, rods, and heads (if I can get that far...) will be aluminum, with the crank and cam being steel. To lubricate the mains and rods, I was thinking of just using silicone grease during assembly. The only rotation it will see is less than 10 RPM with small DC motor that I intend to use as a "starter".

The scale of the model is approximately 1/3. I say approximately because I've changed the sizes of some components to simplify the machining. I made a Stirling engine last year that I love and it got me started down this path. I was inspired to machine the V8 from the video of John Saunders at NYC CNC on his 5 axis mill.

Here are a couple of screen captures of my model thus far:

I'm happy to share the model if anyone is interested. I don't know if I'll work on the intake manifold or not. If I get that far, the exhaust manifolds will probably just be 3D printed. Thanks again for the assistance.

 

[retailer]

Looks as if you have just about finished the 3D modelling, it looks impressive. I'm not that far advanced - 3d modelling is quite new to me I only started using fusion360 a few months ago and what should take me a few minutes often ends up taking an hour or so, I'm more comfortable with 2D cad. Here's a pic or two of my efforts so far

 

[bhusted]

It looks like you're doing a great job at modeling. I wanted to have everything in my model figured out before I made any chips. You obviously have other considerations to work on since you're making a "runner". I am not an expert by any stretch of the imagination, but I have become fairly comfortable with Fusion360 and I have really enjoyed the CAM suite much better than MasterCAM.

 

[Ivan Winters]

You say 'If I get that far, the exhaust manifolds will probably just be 3D printed. Thanks again for the assistance. '

Does the technical college you are, apparently, doing your course at have a 3D metal printing machine complete with a sintering furnace. The fact is, if you make a working model of this engine the highest temperatures will be in the exhaust manifolds. Anything other than high grade metals will vaporize if the engine is even run for a few seconds combusting fuel in the cylinders !!

 

[bhusted]

This engine is only meant to be a model and not intended to run. No hot exhaust issues. You can see the construction of mine here:
https://www.homemodelenginemachinist.com/threads/1-3-scale-ls1.31430/

 

[peterl95124]

not only do you have to be meticulous about getting the angles right for all your lobes, when you actually go to make a model cam shaft you should paint on the cam blank where the lobes go, that way you can double check that you've got your master cam properly indexed for the lobe you're about to grind (don't ask me how I know that !!!)

 

[bluejets]

not only do you have to be meticulous about getting the angles right for all your lobes

Making cams is not all that big a deal.
Run around 110 degrees duration with overlap between the inlet and exhaust, make sure the valves seal on compression stroke and you will be ok.
As long as the cams have lumps on them, model engines will run successfully.

 

[peterl95124]

Making cams is not all that big a deal.
Run around 110 degrees duration with overlap between the inlet and exhaust, make sure the valves seal on compression stroke and you will be ok.
As long as the cams have lumps on them, model engines will run successfully.

Bluejets, I agree with the lobe shape not being a big deal, mine are about 120 degrees with no overlap between intake and exhaust, what I was referring to was the need to be meticulous when making a cam shaft for a multi-cylinder engine, my first cam shaft for a four-cylinder was "backwards" (intake leading rather than exhaust leading) so had to do-over, then I had to make cam shafts for my Merlin V-12 and didn't want to have any do-overs, hence the need to be meticulous.