Courtesy of Midwest Engine Company, Indianapolis, Indiana.
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.