Aluminium fuel tanks for motorcycles
Matthias Oesterreich
Abstract
The BMW S1000RR is BMW’s first superbike that has to face the challenge in
this highly competitive segment. Motorcycles in this segment have fuel tanks in
the visible area painted in vehicle color. Aluminium fuel tanks are usually only
found in racing bikes or as optional equipment. The competitors use either steel
or polyamide fuel tanks in their series models. The fuel tank provides ergonomic
contact surfaces for the rider and contributes to riding comfort and handling of
the motorcycle. The type of material used for the fuel tank depends on various
factors. Apart from weight, cost and volume, complex geometries and design
features are often only possible with the polyamide rotation tank.
1. Introduction
The fuel tank of the S1000RR is one of the rare visible metal sheet fuel tanks of
BMW Motorcycle. In the past, design and cost requirements resulted in plastic
fuel tank concepts including polyethylene fuel tanks. For the S1000RR concept
marketing and superbike package requirements were important. Additionally,
the increasing standards concerning HC emissions for motorcycles had to be
fulfilled. With this bike concept only a metal sheet fuel tank fulfilled the volume
demand of 17.5 l usable fuel. When focussing on light weight design you have
to think about aluminium. Additionally, steel and plastic fuel tank concepts were
studied.
In cooperation with the departments of design and metal-sheet method
development the concept resulted in a successful “form follows function”
threepart body.
The costs for the fuel tank were kept low because the design department
considered all manufacturing requirements and possibilities. In this way BMW
Motorcycle has been breaking new ground. An innovative welding method and
the process of punch riveting were applied.
During the concept stages and series development a low concept risk was
realised by using various types of simulation such as stamping simulation and
FEM analysis.
2. Requirements for motorcycle fuel tanks
2.1 97/24/EG
97/24/EG dictates a pressure test with twice the relative operating pressure, but
at least 300 mbar with independent pressure dissipation via a suitable facility
such as a roll-over valve. BMW especially developed a roll-over vent with an
emergency opening pressure of 120mbar to 150mbar.
In the roll-over test with a rotation upside-down a maximum of 30g fuel/minute
may escape. The rotation is stopped when the leakage starts.
For metal sheet fuel tanks no impact tests are required. Plastic fuel tanks have
to fullfill an impact test with 30Nm energy induction at the knee point or
additional critical hitting points which could be defined by the TÜV. The impact
test is performed by a 15kg test body designed as an equilateral pyramid with
R3 at its tip. The test temperature averages –15°C. The tank is filled to rated
capacity with a mixture of 50% water and 50% ethylene glycol.
Plastic fuel tanks have to fulfil a fire resistance test with the determination of the
combustion speed and a high-temperature test. After storage of the fuel tank for
60min. at 70 °C no permanent deformation is permissible.
Plastic fuel tanks are also subjected to a fuel permeability test. The tank is filled
to 50% of its rated capacity with test fuel and stored for at least 4 weeks at a
temperature of 40°C. Then the fuel tank is drained and filled 50% with test fuel
and stored for a further 8 weeks. The mean fuel loss which occurs may be max.
20 g/24 hrs.
2.2 SAE J1241 guideline
SAE J1241 is a guideline and recommendation but its fulfilment is not
compulsory. The test is valid for all types of fuel tanks traditionally located
between the steering crown and saddle.
The tests include a lateral impact test with increased impact energy. The impact
is performed with 450Nm energy induction at the knee point by a test body of
36kg to 74kg. The test body looks like a ball with a diameter of 89mm. The fuel
tank is filled up to 75% of the rated capacity with liquid fuel substitute. Steel
tanks are tested at room temperature and plastic tanks at –20°C. Furthermore a
longitudinal slide test has to be fulfilled. The slide is accelerated and
progressively stopped up to –27g.
Fig. 1 Side impact of the F800R rear located fuel tank
High temperature, pressure and leakage tests are also included. They are
similar to the 97/24/EG regulation.
The impact tests according to the SAE J1241 are the most critical test
procedure of this regulation. The tests are used also to verify the type of fixing
of the fuel tank on the bike. The fuel tank has to remain on the bike. BMW
Motorcycle tests all fuel tanks according to SAE J1241 including rear fuel tanks
(Fig.1).
3. BMW Motorcycle aluminium fuel tank designs
In 1983 the BMW K100 was launched (Fig.2). The bike has a 22l aluminium fuel
tank. The fuel tank is compact and made of three shells, which are welded
together also in the visible area at the 0Y line. For realising a class-A surface in
the visible area, the welding has to be reworked. The fuel tank is painted in
body color. An expensive pre-painting process by reworking different prepainting layers is necessary. Its weight averages 3300g with a wall thickness of
1,5mm.
Fig. 2 Fuel tank of BMW K100, K1, K1100
A further series aluminium fuel tank was the 18l fuel tank of the BMW R1100S.
The bike was launched in 1998. The fuel tank is completely covered. There are
no requirements regarding class-A surface or painting. The weight of the fuel
tank averages 3900g with a wall thickness of 1.5mm because of its saddle
geometry. The weight of the cover parts around the fuel tank is approximately
1000g.
Fig. 3 Fuel tank of the BMW R1100S
In 2010 we have a typical, partially covered superbike fuel tank concept. The
fuel tank of the S1000RR is also made of three shells without any welding in the
visible class-A area. Its weight averages 2200g with a wall thickness of 1.5mm
and a volume of 17.5l.
Fig. 4 Fuel tank of the BMW S1000RR (patented)
4. Development and testing of the S1000RR fuel tank
4.1 Concept
The S1000RR fuel tank is made of three main shells, a fuel cap flange including
the pipes for venting and overflow as well as different brackets for the fuel tank,
the fuel level sensor and the external cover parts. The welding is carried out
completely automatically by a welding robot at the Berlin plant. All welds are
situated in covered areas.
Fig. 5 Punch riveting, rivset® 5,3x5,5 and the thickness of the remaining
bottom
The front brakets are fixed by punch riveting a type of cold joining (Fig.5). For
fixing the 2mm thick bracket on the 1.5mm thick shell a 5.3x5,5 rivset® is used.
Special punch riveting tongs are required. The force is applied by a pneumatic
cylinder. In the event of a crash, the bottom of the fuel tank will be not
influenced. The connection by only one revset® allows a force of aproximately
3000N in radial and 2000N in horizontal pulling direction. The connection has to
be tested according to BMW Group Standard GS 96001.
Fig. 6 Fuel tank of the BMW S1000RR explosion view
Fig. 7 Fuel pump flange with rivkle® M5, with O-Ring, 3D section
Fig. 8 S1000RR fuel tank explosion bottom view
The fixings for the fuel pump and the air box cover are realized with M5 rivkles®
fitted with a 1mm O-ring (Fig. 7). For this purpose the holes have to be prepared
with a special outer rim called “Dimpelung”. The holes will be formed directly in
the stamping or cutting process of the shell part. This special geometry assures
correct compression of the o-ring in the space provided. The O-ring is made of
FPM because of the temperature near the fuel tank bottom and the use of
ethanol based fuels. Traditionally on aluminium and steel fuel tanks such
threads are welded. This is time consuming and expensive.
The main gaskets for fuel pump and fuel cap are made of NBR.
4.2 Design and stamping process
The upper shell of the fuel tank, which is entirely in the visible class A category
consists of one part only and represents a great challenge for the
thermoforming process. Consequently, the welding seams do not have to be
reworked. This saves costs and does not impair the strength of the weld seams.
The tooling design was determined with extensive thermoforming simulation
studies and the simulation tool “Autoform”. “Pampstamp” was used for
validation. The simulation was carried out for all shell elements.
Fig. 9 method stamping simulation by autoform, strength and thinning
4.3 Impact simulation
Great attention was paid to the requirements of the impact test of SAE J1241
guideline and BMW endurance test procedure. In the design and development
phase, an FEM model of the fuel tank (Fig. 10) was prepared to optimize fuel
tank design for the use of aluminium with regard to joining and mounting
concepts. The quality of the simulation was evaluated on other metal sheet fuel
tanks in advance. For this purpose, previous simulation models were compared
in a real test. The illustration and simulation of the punch rivet joints could also
be evaluated in advance. For example, the number of rivets on the front fuel
tank bracket could be reduced by one rivet to a total of 2 and a one-piece
riveted fuel tank bracket on the rear could be eliminated.
Fig. 10 SAE J1241 Impact simulation, modelling and strength
4.4 Aluminium corrosion
Fig. 11 Aluminium corrosion on the BMW K1100 fuel tank
New types of gasoline contain more and more ethanol. Presently up to 5%
ethanol is permitted. In future, the ethanol content will rise to 10%. Various
countries already have gasoline containing more than 22% ethanol e.g. Brazil or
USA. The fuel in Brazil is often heavily contaminated. It is up to the
manufacturer to specify the permitted fuel type and ethanol content. Fig. 11
shows the corrosion on a BMW K1100 aluminium fuel tank from Brazil. The fuel
tank was in operation for about 15 years and 19000 km. No critical corrosion
points were found. The dissolved fractions of aluminium of the fuel tank as well
as zinc and iron fractions of the fuel pump and fuel level sensor could be
detected in the analysis of the fuel filter.
Conclusion
The visible metal sheet fuel tank is regarded as classical and therefore proven
especially in the USA and Japan. It is used particularly frequently in the roadster
and cruiser segment. Japanese OEM`s exclusively use steel fuel tanks except
for offroad vehicles.
At BMW Motorcycle the challenge for future steel fuel tanks is the omission of
the stoving paint. Alternatives could be electrolytic zinc-plating, zinc-nickelplating, hot dip aluminized metal sheets or metal sheets with magnesium
coating.
Aluminium remains generally interesting as a lightweight material.
•
A further challenge is the concentration of BMW MOTORRAD on
established extrusion technologies with the relevant tank manufacturing
processes. In this context, the focus is on motorcycle specific suppliers
rather than the big automotives with their PE-HD-COEX technologies.
For this reason new polyamide blow molding and polyamide
rotomolding materials will be analyzed. Such solutions may make
painted plastic fuel tanks affordable again for BMW MOTORRAD.
All materials for fuel tanks are analyzed with regard to fuel with an ethanol level
of 25% and more. Due to lean packages, cost and weight saving, BMW
MOTORRAD concepts are shifting from the saddle tank to compact tank
geometries.
References
1
97/24/EG
Section 6, 18.08.1987
2
SAE J1241 Issued 1978-09, revised 1999-11
3
Matthias Oesterreich, Execution “Fuelsystems for Motorcycles”, 2nd
International CTI – Conference / Forum - Fuelsystems
4
Matthias Oesterreich, Execution “Aluminiumfueltanks for Motorcycles”,
6th International CTI – Conference / Forum - Fuelsystems