The use of direct injection (DI) of a second fuel, ethanol or methanol (or their concentrated blends), is explored, via simulation, as a means of avoiding knock in turbocharged, high compression ratio spark-ignited engines that could replace diesels in certain vocational applications. The Ethanol Turbo Boost ™ concept uses the second fuel only under conditions of high torque to avoid knock, while using only conventional gasoline throughout the rest of the engine operating range. This approach is an attractive alternative for heavy duty vehicles that operate intermittently at high torque and within a confined locale, reducing the logistical issues of supplying the knock-suppressing fuel.
The combination of GT-Power for engine calculations and a sophisticated chemical kinetics code for predicting knock were used in the study. After benchmarking the engine model against experimental data from an 11-liter heavy duty diesel engine, the 13-mode emissions speed/load points were investigated operating in ethanol boosted SI mode at a compression ratio of 14:1. For the baseline 11-liter case, two additional points were added: one with a higher, knock-free torque at the B-rpm (limited to 190 bar cylinder pressure); and one at 2100 rpm to achieve higher horsepower.
The ethanol boosting technology produces very high specific output, as the engine operates stoichiometrically (with three way catalyst for emission control) and can also operate at higher maximum engine speed. For a given engine size, it was determined that the ethanol-boosted SI engine could operate with about twice the torque and at higher horsepower than the baseline diesel engine. Consequently, a scaled, downsized 7-liter engine was simulated, which exhibited higher predicted efficiency than the baseline 11- liter diesel engine over most of the B-rpm torque range of the 13-mode test.
The paper evaluates E85 and methanol as the knock-suppressing fuels. Relative to E85, the use of methanol reduces the storage requirements of the second fuel by about a factor of two. Further, the SI engine can operate with premium gasoline at the B50 point without any knock-suppressing fuel (e.g., if it is exhausted) with the use of DI gasoline and aggressive spark retard.
