He classified engines in three groups in 1882:

Type I. Explosion acting on piston connected to crank. (No compression.)

Type 2. Compression, with increase of volume after ignition, but at constant pressure.

Type 3. Compression, with increase in pressure after ignition, but at constant volume.

Only those cycles employing compression have remained in use, and the development of the engine can be readily expressed in the steady rise in the ratios used.

The theoretical efficiency curve for different ratios of com- pression of three symmetrical cycles is given in Fig. I.

Types 2 and 3 are those found in use to-day and they are given with their variation from symmetry in Fig. 2.

A diagram is added of a constant volume cycle with a 10 : I ratio.

This indication of the present day possibilities of the mixture engine, with doped fuel, shews the high ratio such a type of engine with suitable liquid fuel has now reached. On the other hand, with waste gas such as is obtained from blast furnaces, a similar ratio is attained.

This diagram may also be taken as a theoretical example of the low ratio injection engine, in which extra heat over that due to the compression is required, and also for the ignition of the fuel at starting.

In actual practice in the injection oil engines of to-day, heat is always added to the cycle at constant pressure as well as constant volume—the so-called dual cycle, and for purposes of safe ignition the compression ratio is generally higher.

There are other cycles that have been used, notably the Atkinson or extended expansion, and the search for higher economies will bring these up again for reconsideration in the near future.

A diagram from this type of engine is given in Fig. 3.


0 8

I Wessur‘c unlbs pcreqb. above Armepbere.

_____-__ __ __-_-_-2ezro_I-_io_egF_P_r~c.ssurw-

Fig. 3. Diagram from Atkinson Cycle Engine.