DOH Cam Head Design 3

[MRBB]

A student I am working with is designing a four valve head for a 3.5 Hp Briggs engine. We are planning to make the head in three parts. The parting lines join in the combustion chamber. How could these parts be joined and still have compression? Does anyone have any good ideas?

 

[MRBB]

Thanks for all the great ideas and information. My student and I are firming up some ideas on this head design. We still have some questions. We think we understand the idea of squishing off the intake side, but would rather use the flat top piston designed for our block. Could we change the shape of our combustion chamber to allow less room on the intake side than the exhaust side? This would seem to do the same thing. We like the idea of placing two spark plugs on the exhaust valve side and plan to use a engine management system to control both our fuel injection and ignition, so we do not need two mags. We like the idea of closing the intake valves before BDC to allow less CR on the intake stroke than the exhaust stroke. We also like the idea of opening one of the intake valves before the other to control swirl. I hope I correctly understand this idea. Does opening the exhaust valves in the same sequence as the intake valves make any difference? My student is designing roller followers that use ceramic bearings. We are also testing an engine with the middle piston ring missing. The idea is to see if we can get enough compression, with less internal resistance. As far as our original question of joining three machined parts together, we have a solution we can handle. We have access to a CNC machine, but not a foundry, so we are planning to mill the combustion chamber from one slab, and the four ports from three more. We felt it was more important to not have seems in the combustion chamber than the ports. Our goal again, is to increase fuel mileage and not power. Thanks for your help and please respond if you see large pit falls in our crazy ideas.
Dan

 

[SWB]

You should open and close the exhausts at the same time. Send me an E-mail at a_racerr@hotmail.com and I will send you a line drawing.

Sean

 

[ivymike]

Are you going to use a "gapless" top ring, or the standard one? Oil consumption may suffer, as the second ring generally does a great deal of oil scraping.

 

[Marquis]

Hello Sean, sorry It's been a while for me to get back to you- I've been a little tied up!

Regarding EGR, I don't know who started it, but alot of manufacturers use it now, including Jaguar.
External EGR is when there is plumbing and some sort of vacuum actuation device that allows the EGR to be ducted during part load operation to get the fuel economy. In the old days before 3 way catalysts mixtures were simple leaned off to get fuel ecconomy-now a days we don't have that luxury and at the moment a stoichiometric mixture must be maintained- that is why EGR is used for fuel economy by several manufacturers.In these cases it's quite important for the throttled volume ( the volume from the throttle plates to the intake valves) to be kept small -so that when there is a change of condition or EGR level required there isn't a need for many engine cycles to purge the throttled volume.

The other method is internal EGR where the engines overlap is intentionally used to allow some degree of residual gas reversion. At full load /WOT the tendency happens to be less EGR at WOT anyway. This method has the advantage of lowering HC emissions slightly and the charge being recylcled is warm.


I'm quite familiar with the May Fireball head. Over here on 5 star 100 octane fuel that engine was supplied with a 12.5 : 1 CR in production!
In retrospect however we've moved on: although that chamber had good initial burn characteristics, (0-10 %), later in the cycle it didn't burn all that fast. Also, that engines CR made is supremely suitable for part load operation, but it was quite knock limited all the way up the rev range even with quite modest VEs at WOT. For after market performance applications of this engine I reccommend using the flat head at the ports flow better then thge swirl inducing May ones.It should be noted that one CAN have too much of a fast initial burn that leads to knock also!

I often like to emphasize the importance of low surface to volume ratios in combustion chamber design. Nothing new there, you might be thining, however, this includes the piston head design. I predict this will become more important again as some engines go toward GDi and we see weirdly shaped pistons once more, to try and get motion for charge stratification, where the surface to voume ratio factor has been neglected.

"Taking the idea to the extreme, we find a chamber that is very wide and very thin with the spark plug in the center. This I'm told by varying experts would be "ideal!" What we have now is analogous to a flat sheet of paper! Try burning a flat sheet of paper and see how long it takes to get to the edges.
"

This is true WHEN taken to extremes, which is why Bore to stroke ratios play such an important part of modern engine design as related to COMBUSTION.

I have a few comments on the stuff you've sent me which I'll endeavor to email you on next week.

 

[MRBB]

We found another little problem, or rather question. We went to the RB Racing web site. They have a calculator for RSR Inlet Runner and Peak Torque Calculator, you all have probably seen it, or maybe even wrote the thing for all I know. When we plug our numbers in, the calculator tells us that our peak torque will be around 8K. Just a touch after the connecting rod beats our car to the finish line. The question is, can we have to much intake valve and inlet runner for our engine? We can fit .8835 inches square of inlet runner with two valves. The engine volume is 9.018 CI. The way these cars run, is they burn the engine for about 3 seconds then coast for 40-60 seconds with the engine OFF. They accelerate from 9mph to 21mph. The clutch engages at 2000 RPM and max RPM is around 6 grand. What do you think?

 

[SWB]

Try this,

Mean piston speed in feet per second = 2 * desired peak torque RPM * stroke in feet / 60.

Assume valve throat diameter 87% of valve head diameter and figure the respective cross sectional area.

Then, Mean port velocity in feet per second = piston area in square feet / valve throat cross section in square feet * mean piston velocity in feet per second.

Try to size the valve throats so that you obtain about 260 FPS through them (collectively) at the peak torque RPM.

As Marquis has pointed out, this is not a definite way to predict peak torque, but it has worked very well for me in the past and will get you in the ballpark. You will have to adjust inlet runner length for optimum pressure tuning. That you will have to figure out on your own.

If you find the assumption that 87% does not allow a workable valve size, make adjustments along the lines of my previous post in this thread. Also see the post on rod-ratio Vrs airflow down the list.


Sean