Cylinder head and valve choices

[digiex-chris]

Can someone point me in the right direction for valve design for a 4 stroke IC engine? interested in valve diameter, port diameter, and valve lift. Generalities and rules of thumb for my first engine design would be fine I think but I'd like to try to make some informed decisions instead of "that looks about right".

For example, I've come up with a 1" bore, 1" stroke. I decided to scale down the valve diameters using the ratio of bore diameter to valve diameter of a motorcycle I'm familiar with, which gives me an intake of 0.49 and exhaust of 0.42". I have no idea what the lift is, and would appreciate some guidance on any of this. Considering the scaling issues, I'm probably way off.

Also, it appears that at some point both valves will be open. How much of the lift is taken when both valves are open the same amount? I have a somewhat conical head in order to cram it all in there, so I want to figure out how shallow of a cone I need to keep the valves from hitting each other.

Willing to do math if someone can help me find formulas!

 

[stevehuckss396]

Hello digiex-chris!

Personally, unless i am trying to replicate an existing design in which case I do the exact match of the existing, I use the looks about right method. Works good. If you are building an engine for fun and not replicating an existing engine, make both valves the same size and both cam profiles the same. No need to over complicate things for little to no advantage.

 

[petertha]

Semi related to your question: post 16. Not really engineering, just model comparisons I happen to be looking at. I had a different flavor of this relating valve dia to piston dia or displacement. You might have to do a search here, its been discussed a few times me thinks

http://www.homemodelenginemachinist.com/showthread.php?t=23909&page=2

 

[t.l.a.r. eng]

Generalities and rules of thumb for my first engine design would be fine I think but I'd like to try to make some informed decisions instead of "that looks about right".

The beauty of model machining is you can go as complex and technical as you want or by using a reasonably educated decisions you can get it into the ball park and have the engine run just fine as they usually don't have to perform under load or for a long time.

An old t.l.a.r. method that works fine for smaller engines is the 1/4 diameter of the bore for valve head size. Have used bigger, and used smaller, the engines run just fine, depends on the application. Smaller valve area equals faster flow, larger area equals slower flow. All really depends on camshaft specs, compression, carburation, what the engines intended rpm range is, what the engine application is, etc.

 

[gbritnell]

Chris,
Like Steve said, unless you're trying to replicate a full size engine there are no hard and fast rules.
First off you can't really use a hemispherical combustion chamber because it splays the valve angle out to the point of cocking the pushrods at a steep angle. What I tend to use is a pent roof type chamber or one that has curved sides with a flat top where the valve seats can be machined flat. As far as diameter goes you can follow automotive practice and make the intake a little larger than the exhaust. One thing to keep in mind is that without a means to measure flows, both intake and exhaust, there is no positive way to calibrate the optimum diameters.
What you're asking about both valves being open at the same time is known as overlap and has more to do with cam timing than actual lift because one valve is closing while the other is opening.
It's best when designing a cam not to get too radical with valve timing or lift. Sometimes it's hard enough to get these things running without throwing in extremes.
gbritnell

 

[Swifty]

Chris, on a 25mm bore engine I've used 10mm dia valve heads with a 3mm stem. Don't forget to allow room for the spark plug hole as well. Valve lift was 2mm.

Paul.

 

[digiex-chris]

Ah I see. The valves are passing each other in opposite directions so it should be reasonably easy to see when they'll collide. One of those trees I missed while looking at the forest.

Thanks guys, I'll abandon the mismatched valves and make them the same. I can always make another head when this one runs. I'm thinking a Perry model aircraft carb, and targeting about 8000 rpm. Does that mean a sharper cam profile than one designed to run at 2000? Higher rpm = sharper, lower rpm = softer?

Hmm, biggest valve area isn't necessarily the goal. Interesting. That does make me feel better about not filling up all available space. Maybe I can exploit that to make a methanol carb run better on gasoline?

George, do you happen to have a photo of those head styles you mentioned, curved and pent roof? I'm having trouble visualizing how to make that. Would a double overhead cam change your preferences at all? I can cant the base that the cam bearing blocks bolt to at an angle to match the valve angle, so each valve could be at a different angle to each other fairly easily as long as they lean directly opposite each other along the same plane going through the center of the cone. I haven't figured out how to make a valve seat that isn't a wave shape though, so you're right, it's probably going to have to be a flat roof section at some point.

Swifty, what kind of engine was that on? I'm trying to gather a few examples and choose something that way.

petertha, thanks a bunch, that kind of data is quite helpful. Where did you find the data on the Saito engines? Would you happen to have valve data on the Saito FA-91 or 82? That's fairly close to the type of engine I'm trying to build except DOHC instead of pushrod so I can more easily play with cam timing.

 

[Swifty]

Hi Chris, the valves that I mentioned were from my Nemmet Lynx, its pictured in my avitar. Here is a link to the build http://www.homemodelenginemachinist.com/showthread.php?t=23065&page=16

Paul.

 

[tornitore45]

Lift = D/4 give the same open area as the valve head, Since the duct has the stem the area through the duct is even smaller.
There is no gain in providing more lift than D/4.

 

[digiex-chris]

Lift = D/4 give the same open area as the valve head, Since the duct has the stem the area through the duct is even smaller.
There is no gain in providing more lift than D/4.

Hmm I see what you mean. That looks like a good rule to follow, thanks!

 

[petertha]

Here's is a quicky table of valve sizes I put together from some engine plans I have. I'm not suggesting any engineering insight, just reflecting dimensions the original designers chose. Likely lots of caveats:

- these are all valve intake = exhaust dia type (maybe a compromise to I/O but easier to make). Recognize many RC model 4S engines are non equal diameter,

- different fuel types (I segregated)

- different valve orientation angles & combustion chamber shape & CR (not listed)

- I'm not even sure the %ratio of valve/bore area makes sense without lift & duration also specified, but it was an easy thing to calculate on this simple summary, so there it is FWIW

I have a copy of 1986 'RC 4-Stroke Engines' by Clarence Lee. He goes into spec detail of several 4S engines of that era, even to the degree of re-measuring manufacturers bore/stroke specs & I/O valve timing. The sad part is didn't put the calipers across the valve face or I could have added more commercial engines to this list.

If anyone has an engine they want me to add, I'll happily encompass the values. I have more engine plans through SIC mags but thought I'd kick this out for now.

 

[petertha]

Re-reading the posts I see some folks are referencing valve diameter to bore (vs. area) so I added that row for reference.

 

[digiex-chris]

Awesome thanks! The numbers do help me adjust my "right" part of "that looks about right"

 

[lohring]

Recommendations by Sir Harry Ricardo for a moderate performance, 2 valve, flat combustion chamber engine designed for high torque:

Intake valve flow area needs to be 50% greater than the exhaust.
Inlet valve throat diameter = 40% of bore giving an area of 16% of the piston area
Intake valve lift at least 30% of throat diameter
The above gives a gas velocity through the valve 6.25 times piston speed.

A velocity of around 120 to 160 feet per second gives best volumetric efficiency with "normal " timing. That means a piston speed of around 1100 to 1500 feet per minute. This can be shifted to higher flows and piston speeds with hotter timing at the expense of low end torque. Higher performance engines get more intake area with "hemi" heads and more than 2 valves. They can often run over 5000 feet per minute piston speed.

Lohring Miller