A Comprehensive Study on the Effect of Compression Ratio and Pilot Injection Strategy on Alcohol Compression Ignition

<div>Achieving compression ignition (CI) with ethanol, a renewable fuel, comes with challenges because of its much lower cetane number compared to diesel. Additionally, ethanol’s high cooling potential and high volatility compared to diesel also offer challenges and opportunities to achieving robust, high-efficiency CI. Increasing the compression ratio (CR) and expanding the injection strategy beyond a conventional close-coupled pilot-main diesel injection strategy can help overcome these challenges.</div> <div>This work experimentally tested ethanol CI with several different injection strategies with CRs ranging from 16.3 to 22.3. The results showed that in homogeneous charge CI (HCCI), increasing the CR improved thermal efficiency but incurred a combustion efficiency penalty. In any CI concept, increasing the CR lowered the required intake temperature to achieve ignition. Using close-coupled pilot injections is an effective way to achieve ethanol CI, but it was also shown that HCCI-like intake stroke “pilot” injections offer a new avenue of ethanol CI. With a 25% pilot injection during the intake stroke, stable ethanol CI was achieved at 6 bar IMEPg with an intake temperature of 330 K using a CR of 20.0. There was a ~1 percentage point thermal efficiency benefit and ~50% reduction in NOx, though there was also a 1 percentage point combustion efficiency penalty. At lower loads, it was more beneficial to run with more fuel in the intake stroke pilot.</div> <div>Finally, experiments showed that the NOx emissions decreased from 5.75 g/kWh to 3.43 g/kWh at 6 bar IMEPg by increasing the CR from 16.3 to 20.0 and reducing the intake temperature by 60 K. Even with matched intake temperature, the engine-out NOx was 4.57 g/kWh with a CR of 20.0. CFD simulations showed that this was due to the higher CR having a more rapid expansion process, cooling the diffusion flames more rapidly.</div>