Engines of the "Lenoir" type are the simplest in idea and construction; in them a mixture of gas and air is made in the cylinder during the first half of the piston stroke, air being taken from the atmosphere and drawn into the cylinder by the forward movement of the piston. At the same time gas entering by a number of holes, and streaming into the air to form an explosive mixture, the movement of a valve cuts off the supply, and brings the igniting arrangement into action. The pressure produced by the explosion acting upon the piston makes it complete its stroke, when the exhaust valve opens exactly as in the steam engine. The Lenoir and Hugon engines, the earlier forms of this type, were double acting, receiving two impulses for every revolution of the crank, the impulse differing from that in a high pressure steam engine in commencing at half stroke.
The Lenoir igniting arrangement was complicated and troublesome. I have it upon the table; the mixture was ignited at the proper time by the electric spark produced from a primary battery and Ruhmkorff coil.
The Hugon engine was an advance in this respect, using a flame ignited, and securing greater certainty of action in a comparatively simple manner.
It is really a modification of Barnett's lighting cock described in his patent of 1838.
Other difficulties were found in using these engines; the pistons became exceedingly hot. In the case of the Lenoir larger engines, it sometimes became red hot, and caused complete ruin of the cylinder by scoring and cutting up. Hugon to prevent this injected some water.
In the all important question of economy, these engines were found grievously wanting, Lenoir consuming 95 cubic feet per I.H.P. per hour; Hugon consuming 85 cubic feet per I.H.P. per hour.
The surviving engines of this type are only used for very small powers, from one to four man power, or ⅛ to ½ horse, the most widely known of this kind being the "Bischoff," which is very largely used; its consumption of gas is even greater than the Lenoir, being 110 cubic feet per horse power per hour, as tested with a half-horse engine at a late exhibition of gas apparatus at Stockport.
So large a consumption of gas prevented these engines coming into extended use for engines of moderate power, and led inventors to work to obtain better results. The force generated by the explosion of a mixture of gas and air is very short lived, and if it is to be fully utilized must be used quickly; a high pressure is produced, but it very quickly disappears.
The quicker the piston moves after the maximum pressure is reached, the less will be the loss of heat to the sides of the cylinder. The flame which fills the cylinder and causes the increase of pressure rapidly loses heat, and the pressure falls.
The idea of using a free piston was proposed as a remedy; it was thought that a piston connected to a crank in the ordinary manner could not move fast enough to utilize the pressure before it was lost. Many inventors proposed to perform work upon a piston free from any direct connection with the crank or shaft of the engine; the explosion after attaining its maximum pressure expends its force in giving velocity to a piston; the velocity so acquired carries it on against atmospheric pressure until the energy is all absorbed, and a vacuum or deficit of pressure exists in the cylinder instead of an excess of pressure. The return stroke is accomplished by the atmospheric pressure, and the work is now done upon the engine shaft on the return only. The method of connecting on the return stroke while leaving the piston free on the out stroke varies, but in many engines the principle was the same.