Air-blast injection

Design

Atomiser design

Initially, sieve-type atomisers were used for the injections nozzles, until the sieves were widely replaced by discs. Also, ring-type atomisers were used for some engines.

The ring-type atomiser is based on the principle of different air velocities occurring inside the nozzle, which force the fuel to mix up with air. Disc-type atomisers have small perforated discs placed above each other with small gaps in between (as seen in Fig. 6 in the sectional drawing on the right). The discs are slightly misaligned to increase constriction. Depending on the capacity of the engine and, therefore, quantity of injected fuel, either two, three or four dics are used per injection nozzle. The disc material depends on the fuel type. In general, bronze casting and phosphor bronze casting are used; for engines running on coal tar, the discs are usually made from steel.

For engines with disc-type atomisers, the injection pressure should be in sync with the rotational frequency of the crankshaft. This means that with an increase in rotational frequency, the air pressure must be increased as well. Usually, at injection, 97% air and 3% fuel are getting injected through the injection nozzle. The injection pressure is between 5 and 7 MPa which limits the rotational frequency. Also, with increasing engine load, the injection pressure must be reduced to prevent misfire.

Neither disc hole diameter calculation nor the proper size of the discs were known engineering knowledge at the beginning of the 20th Century. The disc designs were usually based on the engineers' experience. While big holes require a lot of compressed air and therefore consume more engine power, holes being too small reduce the engine power output. Julius Magg recommends a disc hole diameter depending on the indicated cylinder power output: ${\displaystyle D^2=0,2N}$. ${\displaystyle D}$ is the hole diameter in millimetres, ${\displaystyle N}$ is the power output in PS.

Nozzle layout

In the early 20th Century, two different injection nozzle designs for air-blast injected engines were common: The open nozzle and the closed nozzle design.

The closed nozzle design was the initial and most common design, it is usually found in vertical engines (such as the Langen & Wolf engine as seen on the right). It can be used for both two- and four-stroke engines. The injection nozzle is supplied with fuel from the fuel-feed-pump while constantly being fed with compressed air from the compressed-air tank. This means that the fuel-feed-pump has to overcome the resistance caused by the injection-air-pressure. A separate cam on the camshaft (as seen in Fig. 5 and on the two-cylinder Johann-Weitzer-engine on the right) would activate the injection valve so the compressed air would then press the fuel into the combustion chamber. Before the injection valve opens, neither fuel nor compressed air can enter the combustion chamber. The closed nozzle design allowed for a good air-fuel-mixture at the time which made it very useful for high capacity engines. This also resulted in lower fuel consumption compared to the open nozzle design. Biggest disadvantages were the higher manufacturing cost and the injection nozzle restrictions that made designing engines with horizontal cylinders considerably difficult, since in horizontal cylinder engines, compressed air can easily enter the cylinder without pressing a sufficient quantity of fuel into the combustion chamber which leads to engine misfire or post-ignition.

The open nozzle design was mostly used for engines with horizontal cylinders and unusual for engines with vertical cylinders. It can only be used for four-stroke engines. Like in the closed nozzle design, the fuel is fed to the injection nozzle. However, the injection valve only prevents compressed air from entering the cylinder; the fuel constantly enters a prechamber above the combustion chamber in the cylinder. Between prechamber and combustion chamber, the disc-type atomisers are placed to separate the chambers from each other. At injection, the compressed air would then press the fuel through the disc-type atomisers into the combustion chamber. Manufacturing engines featuring the open nozzle design was considerably cheaper and easier than making them with a closed nozzle design. It also allows for using tar as fuel. However, fuel supply is insufficient and at the beginning of the injection, too much fuel enters the combustion chamber which causes too much pressure buildup inside the cylinder. This, as well as the problem that it is impossible to supply high capacity engines sufficiently with fuel, means, that the open nozzle design can only be used for smaller engines.