AW: [Diy_efi] Timing Advance Curve? ---> a few words about knock ;-)

[Schaffer Klaus Maria]

Hi guys!

A hard fight about knock last days ;-). I hope the mind has been =
qietened.
My opinion and the opinion of the most engine researchers today:

The reason for knock is, that at the beginning of inflammation (ignition =
is a little bit earlier) the pressure in the whole burning chamber rises =
cause of the higher bulk density of the hot burnt gas. The higher =
pressure increases the energy level of the residuary unburnt =
air/fuel-mixture to a value at which selfignition will start.
There are some ways to occur the knock problematic:

1) Nonprogressive: Reduce the compression ratio: this will reduce the =
energy level of the air/fuel-mixture at the time of inflammation - so =
there is more space between increase of pressure and exceed of the =
critical energy level.

2) Nonprogressiv too: use a fuel with lower propensity to self ignition =
- a fuel with higher RON. Without any other additives this fuel have not =
more heat value and not really a different flame speed. Maybe a better =
evaporation.

3) Progressiv: you have to ensure that the flame reaches the unburnt =
regions of the chamber befor selfignition starts. This could be reached:
a) the mechanical way: make a good design: short ways for the flame - =
central spark plug - small bore, high stroke (which is limited by piston =
speed and therefor accompanied by lower speedlimit and lethargic =
response).
b) the thermodynamic way: you have to increase the flame speed (yes =
increase !!!). The most significant impact to flame speed is the =
turbulent kinetic energy of the air/fuel mixture. Near others, the best =
way to increase the turbulence is to design the intake ports in a way =
that results in a "tumble" flow inside of the cylinder. When this will =
be done good, it costs you a little bit of filling but that will be =
overbalanced by better efficiency.

(I ignored the ordinary knock phenomen of selfignition at hot surfaces - =
should be easier to understand)

Closing the circle I started last week:

Newer engine concepts use the "bad" selfignition for better emissions: =
burning all the fuel by selfignition at the end of compression stroke =
without spark results in a nearly zero emission engine cycle (the =
critical emission part NOx needs time to formation which will not be =
available in such a combustion concept). In the self ignition engine =
(HCCI - homogeneous charge compression ignition) the inflammation starts =
in the whole combustion chamber at the same time. It results also in =
high pressure rising rates (with could be heard ;-). This cycle is very =
hard to control. Best control mechanism is EGR with EGR-rates more than =
50% (the system works only at part loads which are important to the =
emissions measurement at the approbation). This high EGR-Rates could =
only be generated in a fast way with adequat valve timing as "internal =
EGR". Variable valve train design is necessary for such an engine. For =
the controlling of this engine concepts an onboard heat release rate =
calculator will be needed. For the calculation of the heat release rate =
we need a low cost incylinder pressure sensor.
I'm confident that in at least 5 years there will be such a pressure =
transducer on the market and we can start a DIY-heat release rate =
calculator project which enables us to optimize the ignition advance as =
we can do it on the fuel side with the DIY-WB today.

			--- so long --- and merry christmas, klaus.

--=20
************************************************************
 klaus maria schaffer
 Institute of Internal Combustion Engines & Thermodynamics
 University of Technology, Graz - Austria
 A-8010 GRAZ, Kopernikusgasse 24/II
------------------------------------------------------------=20
 email: schaffer at vkma.tu-graz.ac.at
 phone: +43-316-873-7724
 fax:   +43-316-873-7712
************************************************************
  =20

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