06/22/2004
Overlap was heavily used in high power aircraft engines in WWII
including dual stage compound turbo/supercharged engines.  Here is some
reasons why.  Basically, major head cooling, including valves and
pistons at the sacrifice of some charge - similar to running extremely
rich, the elimination of exhaust residual gasses, and the major charge
cooling effect on capacity and end gas temperature of both losing the
residual exhaust and not retaining the superheated intitial charge.

Actual testing - not internet mythology.  No computer models were either
injured nor killed in the process.



7. The use of large valve overlap in scavenging a supercharged
spark-ignition engine using fuel injection 
Oscar W. Schey , Alfred W. Young , NACA Langley Aeronautical Laboratory 
NACA TN 406, 18 pp. , 1932 
This investigation was conducted to determine the effect of more
complete scavenging on the full throttle power and the fuel consumption
of a four-stroke-cycle engine. The NACA single-cylinder universal test
engine equipped with both a fuel-injection system and a carburetor was
used. The engine was scavenged by using a large valve overlap and
maintaining a pressure in the inlet manifold of 2 inches of mercury
above atmospheric. The maximum valve overlap used was 112 degrees. Tests
were conducted for a range of compression ratios from 5.5 to 8.5. Except
for variable speed tests, all tests were conducted at an engine speed of
1,500 r.p.m. The results of the tests show that the clearance volume of
an engine can be scavenged by using a large valve overlap and about 2 to
5 inches of mercury pressure difference between the inlet and exhaust
valve. With a fuel-injection system when the clearance volume was
scavenged, a b.m.e.p. of over 185 pounds per square inch and a fuel
consumption of 9.45 pound per brake horsepower per hour were obtained
with a 6.5 compression ratio. An increase of approximately 10 pounds per
square inch b.m.e.p. was obtained with a fuel-injection system over that
with a carburetor. 
http://naca.larc.nasa.gov/reports/1932/naca-tn-406/ 
 

6. Increasing the air charge and scavenging the clearance volume of a
compression-ignition engine 
Spanogle, J A Hicks, C W Foster, H H , NACA Langley Memorial
Aeronautical Laboratory (Langley Field, Va, United States) 
NACA Report 469, 10 pp. , 1934 
The object of the investigation presented in this report was to
determine the effects of increasing the air charge and scavenging the
clearance volume of a 4-stroke-cycle compression-ignition engine having
a vertical-disk form combustion chamber. Boosting the inlet-air pressure
with normal valve timing increased the indicated engine power in
proportion to the additional air inducted and resulted in smoother
engine operation with less combustion shock. Scavenging the clearance
volume by using a valve overlap of 145 degrees and an inlet-air boost
pressure of approximately 2 1/2 inches of mercury produced a net
increase in performance for clear exhaust operation of 33 percent over
that obtained with normal valve timing and the same boost pressure. The
improved combustion characteristics result in lower specific fuel
consumption, and a clearer exhaust. 
http://naca.larc.nasa.gov/reports/1934/naca-report-469/ 
 

1. An investigation of valve-overlap scavenging over a wide range of
inlet and exhaust pressures 
Creagh, John W R Hartmann, Melvin J Arthur, W Lewis, Jr , NACA Flight
Propulsion Research Laboratory (Cleveland, Ohio., United States) 
NACA TN-1475, 24 pp. , 1947 
No Abstract Available 
http://naca.larc.nasa.gov/reports/1947/naca-tn-1475/ 

  
2. The tracer gas method of determining the charging efficiency of
two-stroke-cycle diesel engines 
Schweitzer, P H (The Pennsylvania State College) Deluca, Frank, Jr (The
Pennsylvania State College) , NACA 
NACA TN-838, 42 pp. , 1942 
A convenient method has been developed for determining the scavenging
efficiency or the charging efficiency of two-stroke-cycle engines. The
method consists of introducing a suitable tracer gas into the inlet air
of the running engine and measuring chemically its concentration both in
the inlet and exhaust gas. Monomethylamine CH(sub 3)NH(sub 2) was found
suitable for the purpose as it burns almost completely during
combustion, whereas the 'short-circuited' portion does not burn at all
and can be determined quantitatively in the exhaust. The method was
tested both on four-stroke and on two-stroke engines and is considered
accurate within 1 percent. 
http://naca.larc.nasa.gov/reports/1942/naca-tn-838 


6. An investigation of valve-overlap scavenging over a wide range of
inlet and exhaust pressures 
Creagh, John W R Hartmann, Melvin J Arthur, W Lewis, Jr , NACA Flight
Propulsion Research Laboratory (Cleveland, Ohio., United States) 
NACA TN-1475, 24 pp. , 1947 
No Abstract Available 
http://naca.larc.nasa.gov/reports/1947/naca-tn-1475/ 
 

5. Effect of exhaust pressure on the performance of an 18-cylinder
air-cooled radial engine with a valve overlap of 40 degrees 
Boman, David S Nagey, Tibor F Doyle, Ronald B , NACA Aircraft Engine
Research Laboratory (Cleveland, Ohio., United States) 
NACA TN-1220, 54 pp. , 1947 
No Abstract Available 
http://naca.larc.nasa.gov/reports/1947/naca-tn-1220/ 
 

4. Effect of reducing valve overlap on engine and compound-power-plant
performance 
Boman, David S Kaufman, Samuel J , NACA Flight Propulsion Research
Laboratory (Cleveland, Ohio., United States) 
NACA TN-1612, 32 pp. , 1948 
No Abstract Available 
http://naca.larc.nasa.gov/reports/1948/naca-tn-1612/ 
 

3. Comparison of computed performance of composite power plants using
18-cylinder aircraft engines with 62 degrees and 40 degrees valve
overlap 
Kaufman, Samuel J Boman, David S , NACA Flight Propulsion Research
Laboratory (Cleveland, Ohio., United States) 
NACA TN-1500, 32 pp. , 1948 
No Abstract Available 
http://naca.larc.nasa.gov/reports/1948/naca-tn-1500/ 

2. Effect of valve overlap and compression ratio on variation of
measured performance with exhaust pressure of aircraft cylinder and on
computed performance of compound power plant 
Carroll S. Eian , NACA Lewis Flight Propulsion Laboratory 
NACA TN 2025, 41 pp. , 1950 
An investigation was conducted on an aircraft liquid-cooled
single-cylinder test engine (four-stroke-cycle spark-ignition type)
equipped with direct fuel injection to determine the effect of valve
overlap and compression ratio on variation of engine power and specific
fuel consumption with exhaust pressure. 
http://naca.larc.nasa.gov/reports/1950/naca-tn-2025/