This was made possible by combining a number of combustion process development measures. A novel 4+1 injector concept played a key role: A fifth injector is utilized to enhance the traditional four injection valves. However, this does not inject the gas selectively into the cylinders but rather into the central air intake. This enables the delivery of the quantities of gas required for the high cylinder performance even in the event of strongly fluctuating gas quality.

In addition, the MBtech specialists also took over the development of additional engine electronics—thus the CNG SLK also possesses an innovative sensor/actuator unit in addition to the gasoline control unit. In combination with the pistons specially adapted to the specific characteristics of CNG fuel and an adapted turbocharger translation, this ensures that the CNG engine runs smoothly and with optimized performance.

CO₂ emissions for the CNG SLK are 150 g/km (NEDC), with fuel consumption of 5.6 kg CNG/100km. The natural gas SLK not only demonstrates fuel consumption far below that of a comparable gasoline engine in the moderate speed ranges of the NEDC consumption cycle, but also at full throttle.

The natural gas engine developed by MBtech omits high-consumption component protection typically utilized in gasoline engines at full throttle, consisting of additionally injected fuel which prevents components such as the catalytic converter from overheating, as a result of the combustion process designed for compact combustion.

As a result of the tight space requirements in a sport convertible, the engineers had to cope with a drastically reduced muffler volume. Extensive detail optimizations enabled the development of a single-pipe exhaust system which fulfills both the NVH and exhaust gas back pressures requirements.

The engineers also took over the system integration of all of the additional natural gas components such as the tanks, gas lines and pressure regulators. In order to create sufficient space in the roadster’s trunk for the carbon composite CNG tanks, the folding roof and the associated kinematic elements were replaced with a fixed carbon roof.

These lightweight design measures for the roof and additional tanks compensate for the otherwise typical additional weight arising from gas tanks in CNG vehicles. The lighter roof also lowers the vehicle’s center of gravity which in turn results in better driving dynamics. One of the rear skirts adapted to the new exhaust system and a CNG filling connection on the driver’s side complete the body measures.

The individual technical solutions MBtech has developed for the CNG demonstrator—such as the injector concept and the exhaust system—can also be transferred to other vehicle applications.

With the CNG demonstrator we have realized an uncompromisingly sporty and powerful vehicle engine with substantially reduced CO₂ emission.

—Andreas Biemelt, the Team Leader for Otto Engine Combustion Process Development at MBtech Powertrain and the Head of the CNG demonstrator project