[Mgs] Engine maths...and spare time

[Richard Lindsay]

As I wrote in the thread's original post, "I love dynamic systems!" Even
more fascinating when multi-dimensional.

R

On Thu, Apr 2, 2020, 4:31 PM Richard Ewald <richard.ewald at gmail.com> wrote:

> Remember the intake valves stay open for a bit past BDC as the piston
> starts up, also to get the mixture to go from flammable to flat out
> explosive the charge has to be compressed.  so those factors tend to limit
> how much effective timing you can have.
> Going a little deeper when you are using a mechanical distributor with
> weights there is only so much refinement you can get in your advance
> curve.  Once you get into fully electronic ignition systems, there are
> systems that have upward to 70 degrees of range from fully retard to full
> advance (they may not use it in all applications, but the capability is
> there in the electronics)
> Rick
>
> On Thu, Apr 2, 2020 at 11:33 AM Max Heim <mvheim at sonic.net> wrote:
>
>> Well, I mean, it would be true at some extreme — for example, igniting
>> the spark at the very beginning of the compression stroke. But I suspect
>> Barney was getting at something else.
>>
>> --
>> Max Heim
>> '66 MGB
>>
>> On Apr 2, 2020, at 11:24 AM, Richard Lindsay <richardolindsay at gmail.com>
>> wrote:
>>
>> Max that might be true if the ignition pressure from the combustion
>> didn't build so long after the spark...within reasonable limits.
>>
>> On Thu, Apr 2, 2020, 12:11 PM Max Heim <mvheim at sonic.net> wrote:
>>
>>> Well, it seems to me that if you advance the spark to a point on the
>>> compression stroke where the piston is still coming up, the expanding
>>> ignited mixture would be fighting the piston, and it would actually reduce
>>> power. I’m not sure if that is what you are talking about, though.
>>>
>>> --
>>> Max Heim
>>> '66 MGB
>>>
>>> On Apr 1, 2020, at 8:17 PM, Barney Gaylord via Mgs <mgs at autox.team.net>
>>> wrote:
>>>
>>> Okay, let's see if anyone knows the answer to this one.  Why is it that
>>> "You do eventually get to a point where no more advance will help".  Why do
>>> we stop spark advance at ~32d BTDC at road speed, and keep the advance the
>>> same for all higher engine speeds?
>>>
>>>
>>> At 07:56 PM 4/1/2020, Richard Ewald via Mgs wrote:
>>>
>>> It takes about 3ms to burn the fuel in a cylinder from when the spark
>>> ignites it.  The engine designers want the peak pressure to occur at
>>> between 5-10 ATDC.
>>> At idle speed 3ms is about 15 degrees. Â
>>> So if you want your peak pressure at 5 ATDC, your base timing is 10
>>> BTDC, if  you want  your peak pressure at 10 ATDC your timing is 5 BTDC.
>>> If you look at tune up specs for non smog controlled engines, most have
>>> base timing in the 5-10 BTDC range.
>>> Now if you double the engine speed, it still take 3ms to burn, but to
>>> keep the max pressure at 5-10 ATDC, you have to light the mixture sooner.
>>> You do eventually get to a point where no more advance will help.Â
>>>  That point varies with the engine design,  cam profile, combustion
>>> chamber design, headers etc.  Â
>>> Rick
>>> ....
>>>
>>>
>>> _______________________________________________
>>>
>>> Mgs at autox.team.net
>>> Donate: http://www.team.net/donate.html
>>> Suggested annual donation  $12.75
>>>
>>> Archive: http://www.team.net/pipermail/mgs http://autox.team.net/archive
>>>
>>> Unsubscribe: http://autox.team.net/mailman/options/mgs/mvheim@sonic.net
>>>
>>>
>>>
>>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://autox.team.net/pipermail/mgs/attachments/20200402/e45d4600/attachment.htm>