The first group of this second classification is formed by motors which have developed the idea conceived in 1860 by M. Lenoir. For the first half of the forward stroke the piston draws in a mixture of gas and air; the valves being then closed, and ignition taking place, the explosion drives the piston to the end of the stroke. The return stroke is made use of to expel the gases through the exhaust. Before igniting the gases which have been drawn in they may be compressed either by a separate pump, or in a chamber forming a continuation of the cylinder.

The arrangement is characteristic of the second group. This again can be modified to form the third group, by allowing the gases to burn under constant pressure throughout the stroke instead of violently exploding at the commencement. Engines using this sort of progressive combustion have been designed by Simon and Brayton.

Finally, in the fourth group the explosion is merely used for obtaining a partial vacuum under the piston, and the work is done by the excess of atmospheric pressure acting on its external surface. It is almost unnecessary to state that this method has been completely abandoned, and has been replaced by a sort of combination type, in which the explosion is used in the forward stroke and atmospheric pressure in the return stroke, such a motor as the Bisschop gas engine being therefore practically double-acting.

The table on page 20, which we have borrowed from M. Witz’s very complete work on gas engines, shows at a glance the cycle of operations in the cylinders of the different types: they are arranged in parallel columns, in order to make it more easy for the reader to compare the operations undergone by the gases before and after their combustion. It is necessary to subdivide the motors of the second group into three, according as the cycle of operations is completed in one, two, or three complete revolutions of the fly-wheel. Perhaps this subdivision is somewhat unnecessary, because the employment of a second cylinder for compressing the gases does not alter the character of the cycle, but we think that it will make the classification clearer if we proceed in this manner.

Group I.
Without compression.		Group II.
With compression.		Group III.
Combustion and compression.		Group IV.
Atmospheric.
1. Explosive mixture enters the cylinder at atmospheric pressure1. Explosive gases enter the cylinder at atmospheric pressure1. Explosive mixture enters the cylinder at atmospheric pressure1. Explosive mixture enters the cylinder at atmospheric pressure
2. Compression of the gaseous mixture2. Compression of the gaseous mixture
2. Explosion at constant volume3. Explosion at constant volume3. Combustion at constant pressure3. Explosion at constant volume
3. Expansion of gases in cylinder3. Expansion of gases 3. Piston driven back by the pressure of the atmosphere
4. Products of combustion expelled from the cylinder5. Products of combustion expelled from the cylinder4. Products of combustion expelled from the cylinder4. Products of combustion expelled from the cylinder

Group I.
Explosion without compression.

Group II.
Explosion with compression.

  1. (1) Two-cycle type.
    1. Dugald-Clerk
    2. Koerting-Lieckfeld
    3. Wittig & Hees
    4. Andrews (Stockport)
    5. Benz
    6. Ravel
    7. Baldwin
    8. Taylor (Midland)
    9. Campbell
    10. Bénier
  2. (2) Four-cycle type.
    1. Millon
    2. Otto
    3. Linford
    4. Crossley
    5. Maxim
    6. Martini
    7. Lenoir
    8. Simplex
    9. Koerting-Boulet
    10. Lombard
    11. Durand
    12. Daimler
    13. Varchalouski
    14. Atkinson
    15. Tenting
    16. Diedrichs
    17. Adam
    18. Ragot
    19. Forest
    20. Noël
    21. Charon
    22. Niel
    23. Lablin
    24. Poussant
    25. Roger
    26. Letombe
    27. Lacoin
    28. Cronan
    29. Cadiot
    30. Dürkopp
    31. Brouhot
    32. Levasseur
    33. Fielding
    34. Delahaye
    35. Acmé
    36. Cuinat

Group III.
Combustion with compression.