Design Details of The BMW-801A Engine

by Myles V Cave,
Aviation's British Correspondent

First-hand study by an Aviation correspondent shows Germans have built a very fine powerplant with many interesting features, especially in cooling system.

The BMW-801A Fourteen-cylinder twin-row radial engine powering the Dornier Do217 series — and a modification of which powers the Focke-Wulf 190 fighter — represents the farthest production advance of German design in the aircooled field. An 18-cylinder model —802 — developing some 2,000 hp, reported coming into production, is the only Nazi engine to better it.

In layout, conception and compactness the 801 represents a big advance in German design; it is a complete power unit housed in exceptionally close-fitting; cowling. Conforming to modern Luftwaffe practice, it is a direct-fuel-injection engine.

The 14 cylinders are arranged in staggered positions in a triple-split cast steel crankcase with the bottom cylinder of the front row and top cylinder of the rear row being vertical. Characteristic of construction throughout the engine, no expense appears to have been spared to give the most rigid case possible. The whole case is machined all over and, for wartime construction, is very finely finished.

The crankshaft is machined from two steel forgings, coupled in the center by a serrated Hirth joint, the halves of which are threaded coarse and fine in each half of the shaft. To lock the shaft up a solid steel bush, internally splined for the assembly tool, is screwed into the threads bringing the serrations on the coupling into close engagements. The outer circumference of the Hirth coupling acts as the journal for a center hearing for the crankshaft. The bearing itself is a heavy-duty single-row ball hearing; type, while both front and rear crankshaft bearings are of the roller type.

The crankshaft has four heavy webs with counterweight integral, the webs being drilled and lead-filled for balancing.

Master connecting rods are located in the low port cylinder in both front and rear rows. Connecting rod hearings are steel, lead-bronze lined.

The steel cylinder barrels are closely finned, and at the base of each there is a thick flange drilled circumferentially for the retaining studs fixed in the crankcase. Cylinder heads are of aluminum alloy and appear to be screwed and shrunk to the steel cylinder. Each head has two large diameter valves — one inlet and one exhaust, the latter having a. hollow stem and is sodium cooled.

Valves are operated by push-rods from circular cam rings located at the front and rear of the engine, each series of cams operating the valves on the front and rear rows of cylinders. Valve rockers run on needle bearings and are completely enclosed and pressure lubricated closely following current American practice.

A simple epicyclic gear train for driving the front valve cam ring is carried on the front end of the crankshaft in front of which is a steel drum containing the epicyclic star gearing for the propeller reduction gear.

At the front end of the drum is cut a large-diameter spur gear driving a shaft which drives a geared sleeve on the crankshaft center line. This sleeve is flanged forward of the gear teeth and bolted to 2 corresponding flange on the rear of the cooling fan thus forming a drive for the fan.

From the fan drive gears is taken the drive for the Twin Bosch type ZM14 magnetos. Both magnetos are vertical mounted on the aluminum alloy front casing.

Unusual Cooling Fan Used

The cooling fan is mounted directly behind the screwed and splined propeller shaft. Slightly to the rear of the fan and located circumferentially in the forward part of the cowling is the special oil cooler through which cooling air is forced between the cooling gills until it exits thru a controllable peripheral slot near the cowling nose.

Between cylinders there is an annular ring forming a center tracing for the cowl and also acting as a collector ring for cooling air for the rear cylinder heads. Air from the front fan is forced into the ring from which it passes through "scoop-ducts" which are riveted to the ring and which guide the air direct to the cylinder head fins. Front and rear cylinders have these "scoop-ducts."

Around the cylinder barrels and sides of the cylinder heads are elaborately shaped one-piece cooling baffles, the throats of which lead into the center annular collector ring. At the front of each of these skirt baffles are riveted small fin-shaped deflector baffles for directing the flow air to the rear of the engine.

The aluminum alloy supercharger delivery casing is bolted at the rear of the crankcase, in turn having a heavily ribbed magnesium alloy intake case bolted to it. Bolts fixing the two casings together pass through into the crankcase and secure the whole assembly.

The supercharger has a two-speed gear train — the drive to both gear ratios being made through a main drive gear with a hydraulic-operated clutch mechanism. The supercharger impeller is a little more than 13-in diameter and has 24 blades. It is machined from an aluminum forging. Alternate blades have a curl-over center portion to direct the air to the blade tips from the center intake. The rear half of the supercharger casing is a good example of fine German magnesium casting technique. This cover carries flanged bosses for the generator, vacuum pump, gasoline delivery pump and the elaborate 14-cylinder injection pump.

The Gasoline Injection System

This injection pump is interesting in that it is the first multi-plunger high-pressure pump built for production in radial form. The pump is made by Friedrick Deckel and, typical of that firm's products, is real precision engineering.

The 14 radially-disposed plungers and sleeves are moved back and forth by a cam ring with three cams driven from the supercharger drive at 1/6 engine speed. Each plunger is moved by a spring loaded roller cam follower which is in constant running contact with the base of the cam ring.

Each pump plunger-sleeve carries a gear ring at its lower end and in slots in this gear — with its flat sided wings — slides the plunger. Rotation of the gear rotates the plunger altering the cutoff angle of the quantity control which is helically cut into the head of the plunger. The whole set-up is similar in design to the familiar Bosch diesel pump.

The quantity control mechanism for the 14 plungers is ingenious. As stated, each plunger sleeve has a gear at the inner end; this gear and other plunger gears are inter-meshed by a circular gear ring which rotates each plunger. Control to the pilot is made by a short shaft geared at one end and meshing with the top plunger sleeve gear.

Gasoline is fed to the injection pump plungers through a, circumferential core in the pump casing from the supply pump located close-by on the rear supercharger case. Provision is made for any air accumulation in the liquid to be diverted through a series of cored passages to an air collector located in the injection pump body within the circumference of the ring of plungers.

Plunger design closely follows standard diesel practice. Each plunger head is helically cut and grooved to provide a suitable angle face to the intake and cutoff ports in the sleeve to allow for varying throttle condition Every pumping element is provided with a sliding-type non-return valve on the delivery side. This is in no way similar to the famous Bosch-Atlas valve but no doubt fulfills the same purpose as far as is necessary on a lower pressure injection system.

The injector which is located between the valves in the cylinder head is of interesting design. In contrast to Mercedes-Benz design, the injector orifice is of the open type and the head of the needle closely resembles the Bosch pintle-type injector. Diameter of the pintle injector needle is 0.050 in.

Directly inside the injector body at the injection end the needle diameter is increased and a three-start sharp-cut thread is used both as a fuel passage and also to impart a swirling action to the gasoline when entering the cylinder. The remainder of the injector design follows standard practice. A small circular filter is incorporated within the body of the screw-in gasoline inlet plug located at the top of the injector.

Lubrication System

The lubrication system in the BMW motor is interesting in view of its complexity and thoroughness. No fewer than six pumps are used — two pressure and four scavenging pumps.

Reference to the accompanying diagram showing this lubricating system layout readily shows the oil flow.

The pump A in the drawing delivers at 175 psi, and a special check valve is provided between the outlet and the inlet side of the pump to prevent excessive buildup in the air-cooling system. The pump B delivers through a rotating vane type filter driven by gearing from the pump. Pumps A and B are coupled on the delivery side and the common outlet is fitted with an automatic pressure control valve which maintains a constant oil delivery when hot at a little in excess of 100 psi. This valve also serves to provide excessive oil pressure when the motor is cold.

There is a lead-off through a pressure-reducing valve to the valve gearing which is lubricated at much lower pressure. The valve gear is scavenged by the pump B.

Drainage and scavenging from the whole of the front of the engine including the fan drive gear, propeller reduction gear and front cam epicyclic gearing are scavenged by the two pumps E and F. Drainage from the lower half center crankcase and the accessory drives located on the rear supercharger casing are all drained and scavenged through pump C.

Very thorough arrangements have been made for de-icing and heating, all of which are taken from the exhaust system. In the Dornier 217E the effectiveness of this heating set-up is shown by a plate on the instrument panel giving the pilot instructions as to what to do when the cabin and other parts get too hot. Air is led across the finned exhaust pipes to such places as the wing leading edges, the front bomb compartment and the cabin. A special multi-drilled tube runs round the transparent cockpit cover through which hot air is distributed over the wind shield.

Specifications are as follows: