Tea Tree Oil as an Oxygenated Diesel Blend Fuel: Chemical Characterisation, Droplet Combustion, Performance and Emission Analysis

Depleting fossil reserves and tightening emission limits have driven interest in plant-derived diesel blend fuels. This work evaluates tea tree oil (Melaleuca alternifolia) at 5, 10, 15 and 20 vol% in diesel through GC–MS characterisation, suspended-droplet combustion and single-cylinder engine testing. GC–MS resolved between 182 and 204 peaks per sample; eight monoterpene constituents were tracked, of which terpinen-4-ol was the largest peak in the neat oil at 21.81 area %. The oxygenated fraction rose from 19.07 to 20.85 area % between TTD5 and TTD20, raising fuel-bound oxygen from approximately 0.28 to 1.13% by weight, while the derived cetane number of the blends fell from 43.2 to 36.6. Droplet tests produced stable cone-shaped flames with ignition delays of 2–3 s and burn durations of 4–6 s, with micro-explosive burning from TTD10 onwards attributed to the volatility differential between the terpene and diesel fractions. In the engine, the longer ignition delay displaced combustion later in the cycle: CA10 referenced to injection rose from 8.7° for neat diesel to 18.4–20.5° for the blends, peak net heat release from 59.9 to 94.7 J/°CA and peak cylinder pressure from 72 to 88.7 bar, while the CA10–CA90 duration remained constant at 27–30 °CA. Brake thermal efficiency rose from 28.9 to 30.1% at full load and by 2.6 percentage points at part load. Brake-specific CO and hydrocarbons fell by 30.5 and 27.8%, and smoke by 24%, while NOx rose by 17.4%. A normalised multi-criteria assessment identified TTD15 as the preferred blend.