The Advantage of Renewable Fuels in High-Efficiency Engines
Approximately 95% of the energy used in transportation applications comes from fossil fuel sources, so improving the efficiency of engines can substantially reduce their fuel consumption. Furthermore, by making our fuels from renewable sources, such as plants, we can reduce the environmental impact of combustion. However, using new types of fuels in more efficient engines can sometimes produce undesired results such as increased emissions, so basic research backed by systematic engine testing is critical to ensuring that engines and fuels perform optimally. Scientists at the Combustion Energy Frontier Research Center (CEFRC) have developed a more efficient and less polluting engine concept and verified its performance using new fuels through extensive fundamental research.
The engine concept uses multiple fuels that are introduced into the combustion chamber sequentially. By injecting a low-reactivity fuel followed by a high-reactivity fuel, the engine can compress the gases more and achieve higher thermodynamic efficiency than typical engines. Due to their ready availability and large difference in reactivity, gasoline (low reactivity) and diesel (high reactivity) are frequently used. By varying the relative amounts of these fuels, the engine's computer can optimize the emissions and efficiency simultaneously.
In a recent study, the team tested several methods of further optimizing this concept. They compared the efficiency and nitrous oxide (NOx) emissions in four cases, using two fuel combinations in two engine designs. The engine designs were a piston optimized for high efficiency and a stock engine piston. The fuels tested were methanol/diesel and gasoline/diesel, recognizing that methanol is a renewable fuel.
The results showed that substituting methanol for gasoline as the low-reactivity fuel while using the stock piston increased the NOx emissions from the engine. Using the high-efficiency piston increased the NOx emissions with both methanol/diesel and gasoline/diesel fuel combinations compared to the stock piston using the same fuel combination. Higher temperatures were achieved in the engine cylinder with the high-efficiency piston, leading to the higher NOx emissions. Nonetheless, the NOx emissions were still below target emission levels established by the government. This result shows that trade-offs between different objectives are often required when optimizing engine designs.
The experiments also revealed some advantages of the new engine concept. For instance, using the methanol/diesel fuel combination with the stock piston design improved the efficiency of the engine operation by approximately 5% compared to using the gasoline/diesel combination. Similarly, for the high-efficiency piston design, the efficiency of the engine when operating with methanol/diesel fuel combinations was higher than when operating with gasoline/diesel fuel combinations. The researchers found that the efficiency of the engine increased by nearly 10% when using the methanol/diesel fuel combinations with the high-efficiency piston compared to using the gasoline/diesel combinations with the stock piston. Thus, by using methanol with the high-efficiency piston, the efficiency of the engine can be substantially increased leading to a similar increase in gas mileage.
These promising results for methanol suggest that investigating other renewable fuel options may uncover other advantages, perhaps by using renewable fuels for the high-reactivity fuel in addition to the low-reactivity fuel.
The work presented here was supported by the Direct-injection Engine Research Consortium (NR Walker) and the Combustion Energy Frontier Research Center (AD Dempsey), an Energy Frontier Research Center, funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.
Dempsey AB, NR Walker, and R Reitz. 2013. "Effect of Piston Bowl Geometry on Dual Fuel Reactivity Controlled Compression Ignition (RCCI) in a Light-Duty Engine Operated with Gasoline/Diesel and Methanol/Diesel." Society of Automotive Engineers International Journal of Engines 6(1):78-100. DOI: 10.4271/2013-01-0264