CHAPTER 4
The Junkers Jumo 205 Diesel Engine
Junkers Jumo Diesel aircraft engines are the only large engines
functioning on the two-cycle principle which are used in modern
aviation. They are built in Germany and are used extensively in both
civil and military airplanes in that country. The best known model is
the Jumo 205 which has been built by the thousands in a speciallyequipped factory. This engine is not supercharged although it isequipped with a centrifugal-type blower. Like other Jumo Diesels it iswater-cooled and of low frontal area (Fig. 35).
The Jumo 205 has six in-line cylinders and two crankshafts-one at the
top and the other at the bottom of the cylinder block connected by atrain of gear wheels. It also has two opposed pistons and twoconnecting rods in each cylinder so that virtually it is a twelve-cylinder
engine. It has no cylinder heads and the combustion chamber in eachcylinder is formed between the crowns of the two pistons when they
are at the tops of their strokes. This type of Diesel forms an interesting contrast to the opposed-piston gasoline engine
which has one crankshaft and one or more pairs of opposed pistons and cylinders with separate heads and combustion
chambers.
Another feature of the Jumo 205
is its unique valving. In thisengine as in other Junkers Jumo
Diesel aircraft engines, the inlet
valves and exhaust valves andtheir operating mechanism havebeen eliminated and in their place
are inlet ports and exhaust ports
controlled by the movement of thepistons. The inlet ports are aroundthe lower end of the cylinder liner
and the exhaust ports are aroundthe upper end. The movements ofthe pistons up and down in theliner cause the ports to be covered
and uncovered at the right time ina simple yet effective manner.
When the engine is running the air
charge in the cylinder iscompressed between the two
pistons as they approach eachother. The fuel charge is injected in finely atomized form into the combustion chamber just before top dead center. The
fuel ignites quickly in the highly-heated air and the resultant gases of combustion force the two pistons apart on their
expansion strokes. The upper or exhaust piston is timed so that it has a slight lead over the lower or intake piston and theexhaust ports are uncovered first. This timing permits most of the burnt gases to escape before the inlet ports are
uncovered and clean air rushes into the cylinder under slight pressure from the blower (Fig. 36).

Because both the inlet ports and the exhaust ports are open at the same time momentarily, the cylinder is said to havestraight-through scavenging. The volume of air supplied by the blower during each cycle of operations is approximately
fifty per cent greater than the cylinder displacement so that plenty of it is available for scavenging the burnt gases throughthe exhaust ports. Strong rotational swirl is imparted to the outgoing gases and the incoming air by constructing theexhaust ports and the inlet ports tangentially in the cylinder liner.
When the pistons have passed bottom dead center and commence to approach each other once more, they cover first theexhaust ports and then the inlet ports. Compression of the air charge then commences while the strong rotational swirl
imparted to the air is maintained to create good turbulence. This turbulence causes the air and fuel to mix rapidly when thelatter is injected and results in good combustion and high power output. The low fuel consumption obtainable with theengine is due to the good combustion and the straight-through scavenging which prevents pollution of the air charge.
CONSTRUCTION DETAILSThe main part of the Junkers Jumo
205 Diesel which is also the power
section, is the aluminum alloy
cylinder block. This casting contains
integral supports for the bearings forthe two crankshafts and passages forthe cooling water which surrounds thecylinder liners when they are in place.
Six bores are machined in the castingfor these liners with several guides tohold them in correct alignment. The
liners are machined from nitralloysteel forgings and are secured withlocking rings at the bottom of thebores so that they can expand upward.
Several rows of small inlet ports, eightlarge exhaust ports, and numerous
grooves to increase the cooling areaaround the combustion chamber aremachined in the liner which is thenchromium plated on the outside toprevent rust and corrosion (Fig. 37).
THE CRANKSHAFTS

The two crankshafts are of the six-throw type with counterbalances. They are supported in seven lead-bronze bushings oflarge diameter. Spur gear wheels attached to flanges at the front ends of the crankshafts transmit the drive through threeintermediate gear wheels to the propeller shaft. The intermediate gear wheels are mounted on roller bearings and the entire
gear train is enclosed by a cover plate which also contains the bearings for the propeller shaft. A vibration damper of thehydraulic type in which lubricating oil under pressure circulates between the paddles of a damper wheel and recesses in its
housing is mounted inside the top intermediate gear wheel. The damper wheel is connected to the propeller shaft bymeans of a flexible splined shaft.
THE PISTONS

The pistons are machined from aluminum alloy forgings. They haveunusually long skirts with a length-to-diameter ratio of approximately
2:1. A fire-plate of special heat-resisting steel is attached to each pistoncrown by four long anchor bolts fitted with strong compression rings tocompensate for the unequal expansion of the two metals. Fourcompression rings are fitted above the piston pin and an oil scraper
ring is located at the bottom of the skirt. In addition there is what is
known as a fire-ring around the lower edge of the fire-plate with its

lower edge resting on a Niresist insert cast in the piston wall. The fire-ring is of L-section so that the gases of combustion
tend to force it outward and seal it against the cylinder wall. Its object is to protect the compression rings from the flaminggases of combustion and at the same time prevent gas leakage past them (Fig. 38).
THE CONNECTING RODS

The connecting rods are made of chrome nickel steel and are of I-section. Aluminum alloy bushings are used in the bigends and needle or quill bearings in the small-ends. The piston pin is hollow and of the fixed type. The propeller shaft is
made of chrome nickel .steel and is supported at its mid-point by a roller bearing and a ball thrust bearing and at its innerend by a needle bearing. The gear train provides the propeller shaft with a reduction drive of 1.58:1.
THE BLOWER

The blower which supplies the air for scavenging the cylinders and filling them with their air charge is attached to the rearof the cylinder block where it is driven through gearing from the lower crankshaft. The blower is of the centrifugal typeand contains a disc-shaped impeller with blades on both sides tangential to the hub. The air is drawn in through a large
screened intake, compressed to a pressure of 5 lb. per sq. in. and forced through manifolds along both sides of the cylinder
block to the inlet ports. The output of the blower can be regulated by means of a shutter in its intake.
A slipping clutch is provided between the drive shaft and the impeller of the blower to protect the latter from sudden
overloads. If the engine crankshaft speed increases very rapidly when the throttle is opened, the clutch slips until a safe
ratio between the two rotating masses has been reached. The same protective action occurs when the engine throttle isclosed and this is important in a Diesel which stops very quickly due to its high compression.
FUEL INJECTION PUMPSThe fuel injection system comprises two
high-pressure injection pumps and four
injectors for each cylinder of the engine.
The pumps are arranged in sets of six alongeach side of the cylinder block where they
are actuated by two camshafts driven fromthe center intermediate gear wheel of thegear trim. The pumps operate at crankshaft
speed to suit the two-cycle functioning of
the engine. Each set is an independent unit
capable of supplying the engine at fullload. The pumps and the camshaft covers
on which they are mounted can be removed quickly for servicing (Fig. 39).
The two injectors are screwed through the cylinder block into the cylinder liner with an angle of 90 degrees between them.
The two injectors on the other side of the cylinder block are arranged in a similar manner so that even distribution of fuelis obtained. The injectors have two-hole nozzles which spray the fuel fan-wise into the combustion chamber.
Strenuous duties are imposed upon the injection pumps. The two pumps and their four injectors for each cylinder operate
simultaneously and when the engine is running at 2,200 r.p.m., they deliver approximately 37 injections each second.
During each injection period the pressure of the fuel in the pumps is raised from approximately zero to 8,000 lb. per sq. in.
and then returned to zero (A pressure of 5 lb. per sq. in. is maintained in the supply lines to the injection pumps by the fueltransfer pumps on the engine). At the same time the small fuel charge is metered accurately and its timing is controlled.
Both precision and ruggedness are built into the injection pumps and they run for 800 hours between overhauls.
THE LUBRICATION SYSTEM

Lubrication of the engine is effected by means of a pressure pump and a duplex scavenge pump functioning on the dry
sump principle. The two scavenge pump units suck the oil out of the lower crankcase sump and force it through a cooler
into the oil tank. From the latter it feeds through a wire-mesh filter into the pressure pump which forces it through a metalelement filter on the front of the engine into the high-pressure lines. Oil is supplied to the vibration damper at a pressureof 50 lb. per sq. in. but its pressure is reduced by a regulator valve to 25 lb. per sq. in. for the other working parts of theengine. Constant oil pressure is maintained in the crankshaft and connecting rod big-end bearings by automatic overflow
valves. The gear wheels in the gear train are lubricated through small nozzles which spray the oil onto the teeth where
they mesh. Splash lubrication is used for the pistons and the connecting rod small-ends.
THE COOLING SYSTEM AND ACCESSORIESThe water used for cooling the engine is circulated bya centrifugal-type pump. It is pumped into passages atthe bottom of the cylinder liners and circulates upward
to passages connected with outlets on both sides of thecylinder block. Any steam which forms passes througha pipe to an auxiliary tank where it condenses and is
fed back into the system. The weight of the coolingsystem for a 700 h.p. Jumo 205-E Diesel (including
radiator, auxiliary tank and 732 gallons of water) adds
approximately 290 lb. to the installation weight of theengine. The dry weight of the engine is 1,260 lb.
Various drives for accessories are provided at the rearof the engine. The upper crankshaft has outlets for
attaching two tachometers and provision is made formounting an electric-inertia starter or a cartridge-typestarter (Fig. 40). Drives for an electric generator andair compressors or vacuum pumps are provided from a
tail-shaft attached to the lower crankshaft. The engine
starts quickly with a Bosch electric inertia starter
similar to the Eclipse starter used in the United States.
A small atomizer attachment can be used to spray a
special mixture of gasoline, ether and lubricating oil
into one of the air intake manifolds when starting theengine in cold weather.
FUEL INJECTION SYSTEM

As has been mentioned previously, the fuel injection
pumps on the Junkers Jumo 205 Diesel are of theindividual single-unit type. The most important parts of thepump are the housing, the detachable barrel, the plunger,
the roller cam arm for actuating the plunger, and the two
fuel discharge valves at the top of the pressure chamber inthe barrel. There is also a simple control mechanism
whereby the plunger can be rotated slightly in the barrel toregulate the output of the pump (Fig. 41).
The stroke of the plunger is constant and the pump is
designed so that the quantity of fuel admitted into thepressure chamber prior to compression can be varied. The
plunger is constructed with an angular or sloping groove
cut in its upper end and an annular groove around it
slightly lower down. The annular groove is connected withthe top surface of the plunger by a vertical hole and anumber of small horizontal holes. The pressure chamber inthe upper part of the barrel has a row of inlet ports around
its lower end which communicate with the suction
chamber of the housing. There are a number of outlet ports
lower down in the barrel which also communicate with thesuction chamber.

The full-load operation of the pump is as follows: When the plunger approaches the bottom of its stroke it uncovers theinlet ports and fuel flows from the suction chamber along the angular groove up into the pressure chamber. The annulargroove in the plunger is below the level of the outlet ports and so the fuel cannot run out. When the plunger rises, intakeof fuel ceases as soon as the angular groove is above the inlet ports. The fuel in the pressure chamber then is compressed
and forced out through the spring-loaded discharge valves into the high-pressure lines to the two injectors. When theplunger is near the top of its stroke the annular groove comes into communication with the outlet ports and the remainderof the fuel in the pressure chamber by-passes down through the holes in the plunger into the suction chamber. Dischargeof fuel then ceases and the discharge valves snap to their seats.
The output of the pump is reduced by rotating the plunger slightly so that the angular groove does not cut off
communication with the inlet ports so soon when it commences to rise in the barrel. Thus, a portion of the fuel in thecompression chamber is forced back through the inlet ports and a smaller fuel charge is compressed and discharged to theinjectors in order to reduce the output of the pump to zero and thereby stop the engine, the plunger is rotated until theangular groove permits communication between the pressure chamber and the inlet ports at all positions of plunger stroke.
As rotation of the plunger changes the moment of commencement of fuel intake the functions of throttle control andtiming control are combined in the one control lever.
Junkers fuel injectors are different from those in other Diesel
aircraft engines in that they are of the open type without
hydraulically-operated or mechanically-operated nozzle
valves. Open-type injectors function quite satisfactorily inhigh-speed Diesels provided there is a slight reduction ofpressure and volume in the high-pressure line when fueldischarge ceases. The nozzle in the Junkers injector has two
small holes or slots in its end so that when the two jets of fuelemerge under high pressure they impinge upon each other andform a fan-shaped spray as in a carbide lamp (Fig. 42).
PRODUCING THE ENGINESThe production of aircraft engines usually proceeds in threestages. First, one or two experimental engines are built for test
purposes and every effort is made to "get the bugs out of them"on the ground and in the air. Next comes small-series
production which may involve the building of a few engines ora hundred or more depending upon the need. The final stage ofmass production necessitates equipping the existing factory or a new one with special machinery and tools so thatthousands of the engines can be built by the latest flow-line production methods.
In 1938, mass production of Jumo 205 Diesels with power outputs ranging from 700 h.p. to 880 h.p. was in full swing in
the Junkers factories in Germany. Most of the engines were for the Luftwaffe or German Air Force although a few ofthem were for Deutsche Lufthansa, Germany's national airline. Mass production of Jumo Diesel aircraft engines isparticularly interesting to watch although the procedure is similar to that followed for water-cooled and liquid-cooled
gasoline aircraft engines. The same methods are used in Germany as in the United States.
Beginning with the largest part of the engine, the cylinder block is one of the most complicated parts to machine. It has tobe faced at the back and the front and also at the top and the bottom, and then have bores for the six cylinder liners
machined in it with great accuracy. The last-mentioned operation is performed on a specially-designed horizontal boringmachine equipped with three boring bars by moving the cylinder block horizontally in the saddle after the first three bores
have been completed. The cylinder liners are machined from hollow steel forgings and have slots milled in them for theexhaust ports and a multitude of small holes drilled in them for the inlet ports. Spiral grooves are milled around the center
part of the liner to increase the area exposed to the cooling water (Fig. 43).

The assembly of the engine is carried out with the cylinder
block mounted in a rotatable stand so that it can be turned tothe most convenient position for the work to be performed.
The assembly operations include securing the cylinder liners intheir bores, inserting the piston and connecting rod assemblies,
mounting the crankshafts in their bearings, and mounting theblower at the rear of the cylinder block.
At definite intervals the engine stand is advanced along theflow line and sub-assemblies and pieces of equipment areadded until the engine reaches the end of the line and iscomplete (Fig. 44). Then it is removed from the assembly line,
its controls are adjusted, and it is taken to one of the engine
test houses. After its tests it is disassembled to check for wear
and then reassembled and given a short test run before being
shipped to its destination.
ENGINE TESTING

The methods used for testing Diesel aircraft engines are similar
to those used for gasoline aircraft engines. Sometimes theengine is mounted on an electric dynamometer stand where its
power is absorbed in driving an electric generator, or it iscoupled to a water brake which absorbs the load. A morecommon procedure is to run the engine in a test cell or in the
open with a short test club of coarse pitch on the propeller
shaft (Fig. 45). The engineers running the test and recording the readings of the various instruments are located in a roomwith shatter-proof windows which is made as sound-proof as possible. The exhaust gases are led away through flexible
metallic pipes of large diameter so that they are silenced to an appreciable extent.
BENCH TESTSThe bench tests which a Junkers 205 Diesel must passconsist of runs of several hours' duration at gradually
increasing throttle opening and load until the rated
output of the engine is attained. There are also shortruns of a few minutes' duration at full throttle to checkthe take-off output of the engine. In the case of
military engines an overload run sometimes is
specified. Fuel and lubricating oil consumption aremeasured carefully during the test runs, together withthe temperature of the cooling water and the
lubricating oil and various pressures in the engine.
PERFORMANCE STATISTICS

The Jumo 205-C Diesel which was used extensively in1936 and 1937, was rated at 600 h.p. at 2,200 r.p.m.
and cruised at 510 h.p. at 2,100 r.p.m. Since then its
power output has been raised by increasing its r.p.m., until now the Jumo 205-E model is rated at 700 h.p. at 2,600 r.p.m.
The Jumo 205-D, which is a military engine, develops a maximum output of 880 h.p. at 3,000 r.p.m. The specific fuelconsumption of these three engines is approximately the same, namely, 0.35 lb. per h.p. per hour at cruising speed and0.37 lb. per h.p. per hour at full load (Fig. 46). These performances are obtained without a supercharger. When the engine
is supercharged with an exhaust-driven supercharger the gear-driven blower is retained for starting purposes. The Jumo
207, as the engine then is known, develops 1,000 h.p. for take-off and maintains this output at an altitude of 20,000 feet.