Experimental Study of a Pre-Chamber Jet Igniter in a Turbocharged Rotax 914 Aircraft Engine

An experimental study is performed to investigate the possibility of relaxing the octane requirement of a Rotax 914 engine equipped with a pre-chamber jet ignition system. A pre-chamber jet igniter with no auxiliary fuel addition is designed to replace the spark plug in cylinder two of the test engine and is evaluated across engine speeds ranging from 2500 to 5500 RPM. Experiments are performed across both normally aspirated and boosted configurations using regular 87 AKI gasoline fuel. Normally aspirated results at 98 kPa manifold absolute pressure show a 7-10° burn rate improvement with the jet ignition combustion system. Tests to determine the maximum load at optimal combustion phasing (no spark retard) are then conducted by increasing boost pressure up to maximum knock limits. Boosted jet ignition results demonstrate that 17 bar IMEPn can be achieved using 87 AKI gasoline fuel, which is the highest documented load achieved with this combustion system at stoichiometric conditions without dilution. A ~ 3 bar IMEPn increase over spark ignition combustion is also observed in the same test engine, corresponding to a ~25 kPa boost pressure increase across the speed range. When comparing the 87 AKI jet ignition results to those of the baseline spark ignition OEM engine which requires 100 low lead (≻ 99.5 MON) aviation gasoline, experimental results highlight that peak torque at 4500 RPM can be matched, however peak power is slightly reduced by 9% at 5500 RPM. Hence, it is estimated that this particular jet igniter offers a ≻ 10 octane number improvement over the baseline spark ignition system. This demonstrates that retrofitted pre-chamber jet igniter technology offers the general aviation industry a potential means of relaxing engine octane requirement. Experimental burn rate results and visual evidence of the jet impingement on the piston crown confirm that further pre-chamber nozzle and jet optimization are required to achieve the full knock limit benefits of this combustion system as demonstrated in the literature. However, results thus far are very encouraging for future lead-free gasoline aviation engines as well as boosted automotive powertrains as engine downsizing grows in popularity.