12/29/2003

Valve Overlap

>From Bios 1612 - intelligence reports ripped from the throat of
"reluctant" WWII Axis of Weasels Engineering concerning octane
requirements and overlap.  

Just remember, the Group Wisdom is that overlap is bad, and besides, has
no value in a turbocharged engine.  See - I have been listening to you
and now defecate in the Corky Bell shine appropriately.


"B.) Prevention of Knock. Phenomena Occurring with High Valve Overlap 

In his previously cited paper Dreyhaupt......

Considerable research was made on the knock behaviour of fuels in
engines with large valve overlap. The general effect of valve overlap in
raising and flattening the knock limit curve was fully appreciated by
the Germans. Valve overlaps as great as 120? were used in some aero
engines and enabled very good weak mixture performance even with highly
aromatic fuels. 

The advantageous  effects of valve overlap resulted from scavenging of
the residual gases and cooling of the cylinder walls, valves, etc. Use
of high valve overlaps, however, introduced the problem of how far it
was possible to apply the knock limit curves obtained by the D.V.L.
supercharge testing method to main engines with large valve overlaps.
Often, instead of the conventional knock limit curve with its minimum
near stoichiometric mixture strength, a knock limit curve was obtained
with its minimum far in the rich region. Although most experiments, in
which this type of curve was obtained, were made by the DB 601 engine,
it was recognised that such a curve is not necessarily a characteristic
of or peculiar to liquid cooled engines. Indeed, Penzig, in his
experiment using the DB 601 engine with a valve overlap of 113?, seldom
observed such curves. 

The knock limit curves having their minimum in the rich region were also
found to have a reverse temperature sensitivity; i.e. knock free
performance was actually improved by raising the boost air temperature.
With highly aromatic or alcoholic fuels the phenomenon of a knock limit
curve minimum in the rich region was not observed. 

Franke has explained the formation of the knock limit curve with its
minimum in the rich region and allied phenomena by invoking Callendar's
theory for the formation of peroxides on droplets. In the DB 601 engine
the mixture formation had been reported to be poor. This poor mixture
formation would result in the formation of unvapourised fuel droplets
increasing in proportion with increase in the mixture strength, and
these droplets in most instances would be conducive to peroxide
formation. Due to this peroxide formation the knock limit curve would be
expected to decline continuously as the mixture is enriched. The
conventional type of knock limit curve, would be expected with aromatic
and alcoholic fuels, since aromatics are little affected by peroxidation
in the presence of droplets; and alcohols; although easily peroxidised,
require very little energy of activation for further oxidation. Their
resulting products of oxidation do not lead to rapid reactions which can
produce knocking because of their low energy of dissociation. The
reversed temperature effect is readily explained; the increase of the
temperature will hinder droplet formation, and thus the knock limit
curves will be raised and their minima displaces further into the rich
region. Indeed, above a certain temperature no droplets will appear for
a certain range of mixture strength, and so a minimum will appear around
the stoichiometric mixture strength. A second minimum, however, will
appear in the extreme rich region. Fig. 9 is a three dimensional
representation showing how the knock limit curve is influenced by this
effect of temperature on the mixture formation.  

It was shown by Franke that with regard  to the characteristics of the
knock limit curve, neglecting the position of the curve, a relationship
existed between boost air temperature and valve overlap. Thus, as far as
the shape of the curves is concerned, the effect of valve overlap can be
compensated by a corresponding alteration of the boost air temperature.
As would be expected, the temperature change required to compensate for
additional increase in valve overlap is smaller than that required for
the first increase, since once scavenging is complete increase of valve
overlap has a smaller effect in reducing internal thermal stress.  

In the same paper were reported a series of tests made to ascertain the
influence of valve overlap on the anti-knock value of fuels; i.e. on the
position of the knock limit curve. These tests were made at high boost
air temperatures, so that in all cases a minimum occurred near
stoichiometric mixture strength. This facilitated comparison of results.
Fig. 10 shows the change of the knock limit at the minimum with varying
valve overlap. Due mainly to the decrease in internal thermal stress, it
is seen that the antiknock value of all fuels tested increased up to 80?
valve overlap. The effects of further increase in valve overlap were
irregular, in spite of the fact that thermal internal stress must have
been further decreased. The anti-knock value of the fuels with high
thermal sensitivity, i.e. benzol and alcohol blends, actually decreased.
This outweighing of the effect of decrease of internal thermal stress
was attributed to the fact that with these fuels, residual gases
favourably affect the kinetics of reaction influencing the knock
behaviour. Thus increased scavenging reduced the residual gas quantities
until they were no longer of any importance, and so their favourable
influence on the knock limit was lost. Whether or not decrease of
internal thermal stress outweighed the loss of residual gas was
dependent on the type of fuel. 

In a later paper Franke described a series of tests to ascertain if
correlation with rating by the D.V.L supercharge method could be
obtained from knock limit curves having their minima in the rich region.
These tests were made on numerous fuels of differing chemical
constitution, in the BMW 132 N, DB 601 A and DB 601 E engines. 

Use of two valve overlaps (40? and 120?) with the DB engine gave the two
types of knock limit curves, from which it could be seen whether any
variation in rating was due to differences in engines or solely to the
different shapes of the knock limit curves. Knock limit curves obtained
from values of the mean effective pressure were completely temperature
insensitive in the rich region when low aromatic content fuels were
used. Thus the knock limit curves were plotted using absolute boost air
pressure values. 

Three graphs were plotted:-  

(I) Minima of the knock limit curves for the various fuels at 1.05 for
the DB 601 A vs. those for the BMW 132 N.  

(II) Anti-knock values at 1.05 for the DB 601 E vs. the minima at 1.05
for the new BMW 132 N. 

With a few exceptions the ratings seemed superficially to be in good
arrangement. For the results obtained from the two engines to be in
complete agreement, however, it would be necessary for the graph curves
to be straight lines passing through the origin at 45?. Although this
was approximately true in one graph, others differed in slope. Each
difference in gradient results in the requirement of a different
constant for the conversion to the BMW 132 N values. Thus it was evident
that fuel rating by determining the anti-knock values according to the
D.V.L supercharge method in the BMW 132 N engine could not be used for
engines with knock limit curves of different characteristics. Two engine
types were recognised- the one where the anti-knock value of fuels rose
with increasing boost air temperature, and the other where it fell. The
fact that it might be possible to choose a temperature where test and
theoretical curves agreed would not permit the fuel rating in one engine
type to be applied to the other engine type. It was found that, quite
accidentally, the value of 130?C for the boost air temperature of the
D.V.L supercharge test gave a good agreement between the anti-knock
values in the DB 601 E and BMW 132 N, provided only the value at 1.05
was considered. 

It was concluded that where only a general evaluation of the anti-knock
value of any fuel was required, the normal D.V.L. supercharge method
with the BMW 132 N cylinder could be used. Even for this purpose,
determination of the order of rating for fuels in engines giving knock
limit curves differing from those obtained with the BMW 132 N cylinder
is only permissible for fuels with a low aromatic content. Compared with
the DB 601 E cylinder, the BMW 132 N cylinder underrated gasoline-benzol
blends but, surprisingly enough, rated mixtures with a high
isopropylbenzene content too favourably. For the temperature
characteristics and the extent of the knock region, however, it is
essential to test the fuels in engines which give knock limit curves of
a similar kind over the whole operating range. "