The basic propulsion motor and the battery pack were proven modules from a series of vehicles with motor propulsion and energy stored in a battery. The two-stroke engine was developed for a determined load/speed operation range, the main stages being tuning of the scavenging, adjustment of direct injection, and optimization of the combustion process. The conceived Citroen Saxo Electrique had a DC motor with a power of 20 kW in a speed range of 1600–5500 min1 and a torque of 127 N m at 1600 min1. The battery was formed by 20 Ni-Cd modules.
For a given application, the injection of liquid fuel in a simple and very compact two-stroke engine working as a current generator is less complex than an injection of emulsion, which requires an air compressor, a mixing chamber, and an additional injector. On the other hand, a system for direct injection of liquid fuel with high-pressure modulation is more advantageous than a common rail system, which requires more power than is available in such a compact engine.
A benefit of the Zwickau pressure pulse direct injection system is that the high-pressure course and, thereby, the injection rate modulation are not dependent on the engine speed. The system can be tuned for short fuel atomization, allowing fast vaporization and mixture formation.
Acceleration of the fuel before impact is generated by the difference between the initial fuel pressure and the ambient pressure in a return pipe to the tank. The initial fuel pressure is produced in a module consisting of a low-pressure fuel pump, filter, and pressure control valve, the whole module being integrated in the tank.
This wave is available at the input of an injector, which is placed close to the electromagnetic valve. The injector consists of a needle with a spring and a body. By adjusting the spring, the opening pressure of the injector can be set to values required for this utilization. The injected mass is controlled by the opening duration of the electromagnetic valve, which determines the velocity of the accelerated fuel before impact on the closing valve. The start of an injection event is the start of the opening of the electromagnetic valve, which provokes fuel acceleration. This opening start is easily controllable by the current.
On the other hand, variation of the injection mass has no influence on the droplet size. This is typical of the water hammer effect in the pressure pulse injection system: the atomization of the fuel is mainly determined by the steep pressure rise and not by the pressure peak itself. This behavior is very favorable for mixture formation in an extremely short time in a compact combustion chamber. The combustion chamber design and the positions of the injector and spark plug have been optimized by 3D simulation and experiment and tested on the engine test bench.
For each configuration, the injection and spark start and the injection rate modulation were analyzed. The compression ratio was also adjusted to obtain minimum pollutant emission. The compression ratio achieved a value of 12.13:1. A major problem was the design of the exhaust pipes, which determines the scavenging of a two-stroke engine. The vehicle configuration did not permit the placement of two exhaust pipes of identical length.
About 80 % of the population of Europe, for example, lives in urban areas. Some 50 % of their inhabitants drive in their own car less than 5 km per day. More interesting is the fact that 80 % of the inhabitants of European cities drive less than 50 km per day. For such utilization, a compact car with an acceleration profile that corresponds to the most frequent city driving cycle is the best alternative. Moreover, such a car should have zero emission of CO2, pollutants, and noise in the city centers. Thus, propulsion by motor seems to be the best form under such conditions.
However, fuel cells are more complex than thermal engines, necessitating more room on board; last but not least they are much more expensive than simple engines for the current generation.