8 PHYSICS AND THE INTERNAL COMBUSTION ENGINE

The Humphrey Pump is perhaps the only example of this type at work to-day.

By reason of the limitations set up by the water surface upon which the explosion takes place, the compression ratio used is

low, and yet the gain from the extended expansion is considerable. See Fig. 4.

THERMAL. EFFICIENCN— PER CENT

Fig. 4. Practical Efficiency Curves 0-71 of Air Standard.

Compression Pressure in lb. per sq. in. above Atmosphere (Humphrey).

With the foregoing in mind, a suggested diagram from‘ an extended expansion engine, but working with injected fuel at a suitable compression ratio (self-ignition) and heat added at constant volume and constant pressure, is given in Fig. 5.

Returning to the general consideration of the efficiency curve given in Fig. I, the efficiency at first improves rapidly with rise of compression ratio, gradually falling as the ratio is increased; hence, we find that 15 : 1 is rarely exceeded.

Few, if any, engines have actually been built with such a high ratio as 20, yet progress in materials and in mechanical accuracy may allow development to proceed again in this direction.

The original compression engines worked at constant volume, except the Brayton. As ratios were low, the increased economy from raising them was very considerable.

Naturally the advantages to be gained by increased compression, as they became known, spread rapidly and were reflected in