Daniel, R., Wang, C., Xu, H., Tian, G. et al., "Dual-Injection as a Knock Mitigation Strategy Using Pure Ethanol and Methanol," SAE Int. J. Fuels Lubr. 5(2):772-784, 2012, doi:10.4271/2012-01-1152.
For spark ignition (SI) engines, the optimum spark timing is crucial for maximum efficiency. However, as the spark timing is advanced, so the propensity to knock increases, thus compromising efficiency. One method to suppress knock is to use high octane fuel additives. However, the blend ratio of these additives cannot be varied on demand. Therefore, with the advent of aggressive downsizing, new knock mitigation techniques are required. Fortuitously, there are two well-known lower alcohols which exhibit attractive knock mitigation properties: ethanol and methanol. Both not only have high octane ratings, but also result in greater charge-cooling than with gasoline. In the current work, the authors have exploited these attractive properties with the dual-injection, or the dual-fuel concept (gasoline in PFI and fuel additive in DI) using pure ethanol and methanol. The single-cylinder engine results at 1500 rpm (λ=1) show benefits to indicated efficiency and emissions (HC, CO and CO₂) at almost every load (4.5 bar to 8.5 bar IMEP) compared to GDI. This is because the spark timing can be significantly advanced despite the use of relatively low blends (≤50%, by volume), which lowers the combustion duration and improves the conversion of fuel energy into useful work. Overall, these results reinforce the potential of the dual-injection concept to provide a platform for aggressive downsizing, whilst contributing to a renewable energy economy.