It’s no exaggeration that BMW has always been
known for the excellence of its engines. And one reason for that is the
company’s lineage: it started as a manufacturer of aircraft engines.
Supercharging, turbocharging, smart engine management and variable cam timing –
welcome to the world of pre-WWII BMW aircraft engines!
It was in February 1917 that a new Chief Engineer
joined Rapp-Motorenwerke: Max Friz came to Munich from Daimler
Motoren-Gesellschaft, bringing with him the idea for an aircraft engine he had
not been able to turn into reality in his former job. His concept was to build
an extra-large, extra-high-compression power unit enabling an aircraft to reach
higher altitudes than ever before.
This was the greatest demand made by the military
of engine manufacturers during World War I, since the ability to reach a higher
altitude gave pilots a significant strategic benefit.
With air density decreasing as a function of
height, conventional engines quite literally ran out of air at altitudes above
3,000 metres or about 10,000 feet. An engine with extra-large capacity and a
higher compression ratio, on the other hand, operates in thin air like a
“normal” engine on the ground. But on the other hand such an engine must be
throttled back in its performance when close to the ground in order to avoid any
overload on the components.
Lightweight engineering: pistons and crankcase
made of aluminium
Introducing this concept, Friz took up a proposal
Wilhelm Maybach had made a year before, one which however had not gone into
series production. This was that while high performance was important, it was
far from everything - at the same time engines had to be as robust, light and
aerodynamic as possible. Friz’s particularly innovative approach was to combine
Maybach’s new idea with proven technologies. So on 20 May 1917, the Development
Department registered the first construction drawing for a new engine conceived
by Friz - a low vibration straight-six.
This concept helped to significantly reduce the
structural loads acting on the aircraft, which in those early years was still a
relatively fragile construction. The slender frontal area of the engine and the
compact arrangement of the ancillaries served to reduce air resistance. However,
the substantial engine capacity of 19 litres and the high compression ratio
called for relatively large components. So to keep weight within limits, Friz
opted for a crankcase and pistons made of aluminium.
In Friz’s design the cylinder head and cylinders
formed one inseparable unit, with the steel cylinder liners simply being bolted
into the cylinder head extending far down into the engine block itself. In this
way Friz was able to do without a cylinder head gasket. Dry sump lubrication and
dual ignition gave a reliable and consistent supply of oil and kept the engine
running reliably at all times.
Just how modern this concept was is shown by the
valve control system, with the valves being driven by rocker arms running on an
overhead camshaft that was driven not by a chain, but rather by a vertical
layshaft.
High-altitude carburettor for enhanced output
and fuel economy
One of the most important components of the
innovative power unit designed and built by Friz was the special high-altitude
carburettor – a system made up of three mixing chambers, three air and fuel
supply nozzles in each chamber and five throttle butterflies interacting with
one another to efficiently adjust the fuel/air mixture to suit the respective
altitude. To make the required adjustments, the pilot had two levers for normal
and high-altitude gas; that is a fuel/air mixture control system destined to
give the engine an excellent balance of output and fuel economy.
The design and construction drawings had not even
been completed when a group of specialists from the German Reichwehr visited
Rapp-Motorenwerke in July 1917, seeking to obtain further information on the
project. After Friz had presented and explained his construction, the military
were so convinced that they ordered 600 engines, requesting delivery at the
earliest possible time.
The birthday: 21 July 1917
As a result of this completely surprising success
within a company which so far had not exactly enjoyed a great time in the
market, Rapp-Motorenwerke soon had to be re-organised: Founder Karl Rapp left
the company and the Meeting of Partners decided to give the company a new name –
Bayerische Motoren Werke. Just one day later, on 21 July 1917, the new name of
the company was entered in the trade register, the change then becoming
absolutely official another two days later when, on 23 July, the company’s Top
Management wrote to the Ministry of War: “We hereby inform you that as of today
we have changed the name of our company to Bayerische Motoren Werke.”
Even so, the old name and logo - a horse in the
silhouette of a black chess figure - still appeared at the time on the company’s
letterhead as a symbol for the name Rapp. But, reflecting the new name of the
company, the symbolic knight soon disappeared and was replaced by the
white-and-blue colours of Bavaria. And since the new name was too long, it was
cut back to three short letters: BMW. Then, on 5 October, the German Imperial
Patent Office registered the new logo as the trademark of the company.
Taking off for the first time on 23 December 1917
in the Rumpler C IV biplane, BMW’s first engine bore the model designation
“IIIa”. With its output of 185 horsepower, the engine fulfilled all the
expectations of the military users, who promptly placed an order for 2,500
units. And while not all of these engines were completed by the end of the war,
the engines used at the time quickly gave the BMW brand its great reputation for
reliability, power, and economy.
World high-altitude record: 9,760 metres
(32,013 feet)
Proceeding from this successful engine, the
engineers at BMW built further variants in the last few months of the war, among
them the even larger 250hp BMW IV. This was the engine that on 9 June 1919 took
test pilot Zeno Diemer to an unprecedented altitude of 9,760 metres or 32,013
feet. Never before had anybody reached an altitude of this kind – meaning that
Max Friz had impressively proven the potential of his innovative engine
concept.
Twelve-cylinder with magnesium crankcase for
the “Rail Zeppelin”.
With post World War I the restrictions imposed on
German aviation being gradually loosened in the mid-‘20s, the six-cylinder again
provided the starting point for the ongoing development of BMW aircraft engines.
What was needed at the time were large engines able to develop a high level of
consistent output over a long period. So again, the engineers optimised a proven
basic design and added both trendsetting and reliable new concepts.
The engineers at BMW put together two units of the
BMW IV six-cylinder in 1924, creating a 12-cylinder V-engine delivering 580hp.
And to save weight, they used not only aluminium on the new engine, but in some
cases even magnesium on the crankcase. This powerful engine was destined to
become famous as the BMW VI quickly became the benchmark of its time, numerous
aircraft relying on the BMW V12 on both their maiden and record-breaking
flights. And indeed, this unique power unit quickly proved its merits not only
in the air, but also in a particularly spectacular manner in the early ‘30s in
the German “Rail Zeppelin” a high speed train driven by an air propeller at the
rear. The BMW power unit accelerated this streamlined railcar to a speed of no
less than 230 km/h or 143 mph, again setting a new world record.
The final highlight was that the engine was not
only delivered from Munich to customers all over the world, but was also built
by licensees in Czechoslovakia, Japan and Russia.
In the late ’20s BMW further expanded its leading
position as a manufacturer of aircraft engines, focusing on the production of
air-cooled radial power units. To gain adequate experience with this new
technology, the specialists in Munich started building Pratt & Whitney
Hornet engines under a licence agreement as of 1929. With its maximum output of
450 hp, the Hornet engine was not as powerful as BMW’s best-selling12-cylinder
BMW VI power unit but the radial engine was significantly lighter.
Radial engine with direct gasoline
injection
Once again, BMW’s development engineers struck out
to optimise proven technology, increasing engine output to 690hp while leaving
engine capacity unchanged and adding only a little extra weight. To achieve this
improvement, both the crankcase and cylinder heads of the new BMW 132 were made
of aluminium, with a supercharger to provide extra boost at high altitudes being
fitted directly on the rear end of the crankshaft.
Becoming a great success, this superior radial
engine gained fame particularly as the power unit featured in the three-engined
Junkers Ju 52. Indeed, in its process of ongoing development, the BMW 132
quickly became the spearhead for other, innovative technologies: The BMW 132F,
for example, was BMW’s first power unit with direct gasoline injection. And in
the mid-30s, the development engineers converted the nine-cylinder to diesel
combustion, added water cooling on certain components, and gave the new engine
the designation BMW 114.
Running on special fuel and with multi-stage
supercharging, the radial engine in its last stage of development even broke the
1,000-horsepower barrier, at least for short spells and bursts of power. But
soon even this was not enough: in late 1938 the specialists in Munich started
developing a double-radial engine with no less than 14 cylinders, that is, two
radial units with seven cylinders each behind one another. To ensure an adequate
flow of air for the cylinders at the rear, the “basic” nine cylinder was cut
back accordingly and the two radial units were slightly offset from one another.
Displacing no less than 42 litres, the BMW 801 aircraft engine weighing
approximately one tonne and developed a continuous output of no less than
1,500hp.
The command unit: the first mechanical
“on-board computer”
Introducing an innovative engine control concept,
BMW’s engineers significantly facilitated and streamlined the process of
operating the engine. The “Command Unit” cut back the conventional array of
levers for the pilot to one single control unit, thus making the job of
controlling the engine and flying the plane much easier and smoother than
before. Ensuring supreme reliability, this miracle in sophisticated mechanics
automatically controlled the fuel/air mixture and air charge process as a
function of load and height, as well as the ignition timing and setting of the
propeller. The result was a reduction of fuel consumption and an increase in
operating reliability.
In its basic configuration, the BMW 801 came with
direct gasoline injection and a mechanical supercharger. But then, in the early
’40s, the latter was slowly but surely replaced by a more sophisticated
alternative - turbocharging. This created a radial engine with turbocharger
technology entering series production as the first aircraft engine of its kind
in 1944.
VANOS even back then: an 18-cylinder with
variable control timing
To further increase engine output and performance,
the engineers at BMW increased the number of cylinders in the BMW 802 aircraft
engine to no less than 18. Cooling air plates made sure in this case that
despite the small spaces between cylinders, enough cooling air was still able to
reach the points subject to high thermal loads and temperatures.
The most particular feature on this 2,500-hp power
unit was however the valve timing, with both the intake and outlet valves being
masterminded by cam plates able to turn in opposite directions while the engine
was running. So it is fair to say that as far back as in 1942, the BMW 802
already came with an early type of VANOS camshaft control now found on modern
BMW car engines!
