4. Increasing the Compression.—The plan of increasing the compression has received considerable attention but is not com mercially attractive. The Diesel engine under present designs works with as great a pressure as is advisable. The increase of compression pressure would augment the operating difficulties and is dangerous. Furthermore, many tests of engines with varying compressions appear to bear out the belief of practical engineers that a compression of 500 to 550 lbs. per sq. inch gives the engine the greatest possible efficiency and is as high as is practical. The injection-air pressure would necessarily be increased a corresponding amount, thus increasing the engine losses. In these tests referred to, the engine smoked badly at loads below half rating.
5. Application of a Light Oil for Primary Ignition.—This plan is the only one offered that possesses merit and has been employed to a large extent in Germany and in a few English installations. The fuel-injection valve, which, in most cases, is of the open type, is equipped with two oil lines. Figure 133 illustrates the
Korting tar-oil valve. All of the tar-oil valves are designed to allow the light ignition oil to be blown into the cylinder ahead of the tar-oil charge. Being lighter, this primary charge immediately ignites and supplies the additional heat required to ignite the heavy tar oil. The time interval for this action is short even in a slow-speed engine. Consequently, it cannot be expected that an engine above 200 r.p.m. will successfully handle tar oils even under this system. The fuel valve becomes foul in a short time. A thorough cleaning is necessary at least once a week. The tar coats the piston and combustion-chamber walls with a thick hard scale that requires constant attention.
6. C atalytical A ction .—No commercial engine has been operated under this method of tar-oil ignition. A catalytic agent is required that will produce combustion even when the tar is in fairly large particles. The cost of the agent and its life are matters of importance. It is highly improbable that it will ever be commercially successful.