Performance of a single-cylinder diesel engine using oxygen-enriched intake air at simulated high-altitude conditions

A single-cylinder, naturally aspirated, air-cooled, direct-injected diesel engine was used to study the effects of oxygen enrichment of intake air on engine performance at simulated high altitude conditions. Altitudes up to 5200 m (17,000 ft) were simulated by using a throttle valve to restrict the mass air flow to the engine and reduce intake pressure to represent the atmospheric pressures observed at high altitudes. This altitude ceiling is relevant to current technology UAVs used by the military, such as the RQ-7A/B Shadow 200 and the MQ-1 Predator. The oxygen volume fraction in the combustion air was increased by mixing pure oxygen from gas bottles into the intake air. The response surface methodology (RSM) was used to determine the relationships between fuel injection timing (FIT), engine load, simulated altitude, and oxygen volume fraction to parameters of engine performance, such as power output, brake-specific fuel consumption and fuel conversion efficiency. It was found that power output depended mainly on engine load and was not improved by the use of oxygen-enriched air, but it did not decrease significantly for altitudes up to 2600 m (8500 ft). The use of oxygen-enriched air was also effective to prevent the deterioration in brake-specific fuel consumption when increasing simulated altitude, an effect that was observed on fuel conversion efficiency, peak cylinder pressures and maximum rates of heat release. Peak combustion temperatures were significantly affected by simulated altitude and oxygen volume fraction, but the effect of simulated altitude was of larger magnitude than the effect of oxygen volume fraction.