The following is reprinted from Internal Combustion Engines,
publish ed by the American Technical Society, Chicago, Illinois,
1937. Sent to us by Walter A. Taubeneck, 11801 52nd Dr. N.E.,
Marysville, Washington 98271-6225.
The sectional view in Figure 44 shows the cylinder heads
arrangement of a popular Hvid (cup) type of engine. The operation
of the cup, F in Fig. 44, is based on the fact that every oil, no
matter how heavy it may be, contains some light hydrocarbons that
will vaporize or distill at a fairly low temperature. The explosion
of these lighter parts of the fuel provides the means whereby the
remainder of the fuel is injected into the engine cylinder in a
finely atomized condition. A further matter of common knowledge is
that the temperature of ignition of an oil is dependent upon the
degree of atomization.
Assume that the engine is on head end dead center with the
piston as shown in Fig. 44, and that a charge of fuel and air is
about to be admitted. As the piston H moves to the right, this is
the first or suction stroke, fresh air for combustion of the fuel
is admitted through valve A, which is the main air intake valve.
Previous to this, fuel oil had been pumped through pipe D by means
of a low-pressure fuel pump; this oil is held above the seat of the
inlet, valve E until the proper time for admission. Soon after the
main valve A has been opened, fuel valve E will also be .
mechanically opened. Fuel oil with a small amount of air is drawn
into the steel fuel cup F, but it is held in the bottom of the cup
and is not drawn into the cylinder. Air only is drawn into the
cylinder. The air intake valve A is held open until near the end of
the suction stroke, as is also the fuel valve E. The amount of oil
drawn into the fuel cup does not depend upon the length of time
that the valve is open, but on the amount pumped by the variable
stroke oil pump connected to the flyball governor by a floating
link.
Fig. 44. Section through cylinder head of Mid-west-Hvid
engine.
The camshaft is parallel to the cylinder and located as shown in
Fig. 45; it operates the intake and exhaust valves, the fuel oil
valve, the air starting valve, the combined governor and variable
stroke fuel pump, and the force-feed lubricator.
The second stroke is the compression stroke. As the piston
returns to the left, Fig. 44, with all valves closed, the pressure
of the air in the cylinder and fuel-oil cup will increase to nearly
500 pounds per square inch at the end of the stroke. The air in the
compression space C cannot cause pre-ignition, as it consists of
air only, since the fuel oil remained in the cup. The interior of
the cup is connected to the compression space by means of three or
four small holes. The number and size varying for different
conditions and fuels. As the pressure rises in compression space C,
it also rises inside of cup F, because of the connection through
the small holes; but the pressure does not rise as rapidly in the
cup as in the cylinder. At 400 pounds pressure the temperature is
sufficient to start ignition of the oil within the cup. The
resultant high pressure (about 700 pounds per square inch) forces
the rest of the oil out through the holes into the combustion space
and cylinder.
Fig. 43. Midwest Hvid 35 horsepower engine.
It is evident that the fuel cup performs two important
functions: first, delays and therefore times the ignition; second,
atomizes the fuel as it is forced through the holes and thereby
aids combustion. The amount of fuel consumed in the cup per cycle
is infinitesimal because there is only a very small amount of air
present in the cup to support combustion. As the fuel in an
atomized and vaporous state comes into contact with the heated air
in the combustion space, very rapid combustion takes place as the
piston starts to the right on the third or expansion stroke. The
fuel oil burns with an in crease in temperature at nearly constant
pressure, and the gases expand and do work on the piston. All
valves remain closed until near the end of the stroke. Then the
exhaust valve B is opened and the gases begin to escape to the
outside air through the exhaust port G. The return or fourth stroke
is called the exhaust stroke. The exhaust valve B remains open
until near the end of the stroke, so that as much of the burned
gases may escape as possible.
