The technology of the Semper Vivus
The historic Semper Vivus was one of a kind. It was used by Ferdinand Porsche to practically test his idea of the serial hybrid drive. This is why only a very few design engineering drawings, documents and photographs of the world’s first working full hybrid car remain. They are the basis on which the replica of the Semper Vivus was created.
With its drive concept, the Semper Vivus first implemented a principle which is now scheduled for imminent market launch as an electric vehicle with range extender. The vehicle is driven by electric motors which are primarily supplied by a battery. If this is exhausted, the driver can start two combustion engines which are coupled to generators and which can be used to generate electrical current to continue driving.
Two steered wheel hub motors, which Ferdinand Porsche invented and patented in 1896 or thereabouts, served as driving machines in the hybrid prototype. He mistrusted transmissions of all kinds as sources of power loss and wearing parts. Although their housing is manufactured from cast aluminium, each of these motors weighs around 270 kilograms. They have been designed as so-called external rotor motors in which the resting stator lies in the centre and is circled by the rotor. Flat collectors which are insensitive to shocks and 82 individual armature coils enable the motor’s desired disc form. Electrically, this functions as a series wound motor with eight poles, i.e. the rotor and stator are connected in series. The advantage of this principle is that the motor applies its maximum torque – approximately 300 Newton metres – from a standing start.
The heavy front wheels run on steering knuckles which are connected to the tubular frame without suspension. In order to at least guarantee a minimal level of ride comfort and not pass on all of the shocks caused by the then very uneven road surfaces to the wheel hub motors and occupants without damping, the wheels were fitted with pneumatic tyres with dimensions of 880 x 120 millimetres. The Semper Vivus is steered using a steering knuckle steering system. A worm gear steering system with a ratio of 6:1 transmits the driver’s rotational pulses to the former’s tie rods. Fitted with full rubber tyres measuring 1150 x 100 mm, the rear wheels are located at the ends of a rigid axle, which is also rigidly connected to the frame.
This chassis consists of a simple rectangular circular steel tube frame which is connected using clamps. To keep the superstructure and the centre of gravity comparatively low, Ferdinand Porsche did not mount the battery, which was the heaviest individual component, on the frame, but suspended it in a box in the rectangle using four helical springs. The battery was originally a 420-kilogram lead/acid storage battery with 44 cells. When under no load, this offered a voltage of 88 V and a charge storage capacity of 110 ampere-hours. Due to safety reasons, the Semper Vivus replica is equipped with a lead gel battery with comparable values and dimensions plus a similar weight. This suspension not only protects the energy store’s sensitive lead plates from destructive shocks; the seats, current generator and drive control system are also mounted on the battery box, which extends from the rear axle to beneath the front seat. The steering column projects out of the unsprung driver footwell; the foot brake lever is located next to this. Protected by the knee-high front panelling, the voltmeter and ammeter inform the driver of the energy balance; the petroleum headlight and the two radiators are bolted externally to the canvas-covered wood panelling.
The two single-cylinder engines behind the driver seat made the Semper Vivus a milestone in automotive history. These were not mechanically connected to a drive axle but were instead used exclusively to produce current for the wheel hub motors via generators – the first time the serial hybrid drive principle was implemented. The two identical combustion engines are faithfully reproduced DeDion-Bouton four-stroke engines, each with a displacement of 700 cubic centimetres and an output of approx. 3.5 hp at 1200 revolutions per minute. Their crankcase is manufactured from aluminium, their pistons from grey cast iron. The engines are cooled by a water jacket whose content is circulated by a water pump. In the original, this was mounted on an axle with the crankshaft, and pumped the coolant to the tubular radiators in the vehicle’s front end, whilst the water reservoir was mounted beneath the rear passenger seat. In the replica of the Semper Vivus, small electric pumps which are supplied from the battery are used for the two cooling circuits.
Mixture preparation is undertaken by early spray-nozzle carburettors, whose air flow can be regulated using a small hand lever connected to a throttle valve in the carburettor. The mixture flows laterally through a so-called snifting valve into the combustion chamber, which is opened due to the vacuum during intake and is closed again via spring load prior to the compression stroke. Ignition of the compressed mixture is carried out by a distributor ignition system with a spark plug located laterally between the undivided blind cylinder’s intake and exhaust port. The tin-sized ignition coil operates with a voltage of 12 V, which was usually picked-up at the battery’s sixth cell at the start of the 20th century. Today, this task is undertaken by a modern voltage converter which is supplied by the gel battery. The DeDion-Bouton engines are designed according to the counter flow principle, which is why the exhaust gasses, controlled by a side valve, exit the combustion chamber below the snifting valve.
Pointing in the direction of travel, each combustion engine’s front crankshaft stub drives the rotor of one shunt generator via an elastic clutch disc. In this model, the exciter coil and armature are electrically connected in parallel. As only part of the armature current is required for the field coil, a shunt generator operates smoothly even under alternating loads. Designed in this manner, the direct current generator can therefore be used to drive the wheel hub motors and to charge the battery with the necessary overvoltage. However, the Semper Vivus is unable to simultaneously drive with generator current and charge the battery – that was not possible with the technology handed down by the original hybrid vehicle. The replica is therefore designed so that the battery can only be charged when the vehicle is stationary due to the precise adherence to the charge voltage which is required. Of course, electricity from the mains via a charger can also be used for this as an alternative to current from the generator. 110 years ago, this was also possible without further ado from the urban 110 V direct current mains with the use of resistors to adapt the voltage.
Each of the two current generator packages weighs 140 kilograms, amounting to a total of approximately 280 kilograms. Each of the generators simultaneously functions as a starter for its combustion engine. Like all of the other controls for the combination drive units, the starter switch on the rear of the driver seat is actuated from the rear seat. Once the combustion engine has started, the generator is switched to current generation, producing around 20 amperes with a voltage of 90 V at 1200 revolutions per minute. As both generators are connected in parallel, this current supply therefore supplies approximately 3.6 kW.
Driving with the world’s first full hybrid car is completely different to driving a conventional, modern car. The speed is not regulated using an accelerator pedal but with a lateral control lever which switches the six-speed controller. This was already called a controller back in the year 1900. After pulling out the main switch, the driver engages the first speed to move off. The controller – an actuating wheel with contacts for different current circuits – then connects both the field and the armature coils of both wheel hub motors in series. Up to 80 amperes then flow through the wheel hub motors, and the Semper Vivus rolls off gently. In the event that e.g. an obstacle prevents moving off, a series resistor in this current circuit ensures that the system does not overheat. However, the hybrid pioneer is perfectly able to cope with a gradient of around seven percent.
This first speed is sufficient up to approximately 10 km/h (6 mph); the current consumption has then fallen to around 20 amperes, and the driver has to shift to continue accelerating. In speed two, the controller continues to connect the two field coils in series, but the armatures are supplied with current in parallel. The wheel hub motor torque which is generated by the armature current increases again due to the lower armature resistance, leading to a further increase in speed. At around 20 km/h (12 mph), it is then the turn of the third speed; this time, the controller switches the wheel hub motors’ field and armature coils in parallel. Finally, at around 35 kilometres per hour (22 mph) on the flat, the Semper Vivus achieves its top speed.
If the driver does not wish to drive at full throttle – or to be more accurate, full current – he simply switches the current supply on and off periodically in the selected speed by gently depressing the brake pedal, which is combined with a switch. If the driver makes use of the vehicle’s top speed, one battery charge will carry him approximately 50 kilometres on the flat in the Semper Vivus, which weighs 1.7 tonnes. He can then start the generators and continue driving using the energy of 40 litres of petrol in the fuel tank – range (as yet) unknown.
At some point, however, the driver also has to brake. To do this, he again initially reaches for the lateral lever for the controller and switches to deceleration speed. Both motors are now disconnected from the current supply and are short-circuited via an eight-ohm resistor; this converts the current generated from the inertia of the moved vehicle mass into heat in the opposite direction. Returning the energy to the battery was not yet possible with the technology available in 1900. If the wheel hub motors’ braking torque is insufficient, the driver also firmly depresses the foot pedal, which acts on two external band brakes on the rear wheels. For safety purposes, the Semper Vivus replica is additionally equipped with an external band parking brake, which was presumably not fitted on the original. Added to this is a hill-holder facility in the form of lock pawls; on ascending hills, these are triggered and engage in internal toothing in the brake drums.
The controller’s fifth speed reverses the motor’s rotational direction and the Semper Vivus moves backwards. Speed six is actually speed 0 – the neutral position in which the petrol engines can be started when the hand brake is applied.
Driving with the Semper Vivus is an experience which is as impressive as it is exhausting. With a front axle load of 1060 kilograms – and 830 kg at the rear – steering without power assistance is hard work. To compensate, the individual seat positioned over two metres above the road offers the driver a superior overview – the highest point, the top edge of the driver seat backrest, projects exactly 1830 millimetres above the road surface. In contrast, the world’s first serial hybrid car just covers the floor area of a modern compact car with its length of 2.64 metres and its wheelbase of 2260 mm plus its track of 1370 millimetres and 1540 millimetres at the rear.