Improving diesel engine performance

Kettering University’s Dr. Bassem Ramadan and researchers at the U.S. Environmental Protection Agency (EPA) and Michigan State University (MSU) are literally looking inside a diesel engine cylinder to determine the best way to configure the fuel injector’s spray nozzle, piston bowl shape, and air motion for optimal combustion.

Sound complicated? It is, but that’s exactly what Ramadan likes about the project. “The complexity of the project makes it interesting,” he said. “Diesel engines have a bowl shaped cavity in the piston that helps disperse the fuel as it is injected into the cylinder prior to combustion. We are looking for the best shape for the diesel piston bowl/cavity,” said Ramadan, professor of Mechanical Engineering.

This “bowl” is where fuel flows into the piston, and sometimes small changes in the shape of this bowl can make a difference in performance, he explained. Adding to the complexity of efficient combustion in diesel pistons is the fuel injector spray pattern.

“Even though it sounds simple, there are different features in the fuel injector nozzle that can be changed, like the number of orifices in the injector and the diameter of the orifices, both of which can affect performance,” Ramadan said.

He and his colleagues at MSU and EPA are trying to identify how the design of the injector and the piston bowl coupled with the in-cylinder air motion affect the combustion process. “So far we have found that generally, the smaller the orifice the better,” he said, adding “We need to be able to match the bowl shape to the spray pattern to optimize the combustion process.”

What they have found so far is when an injector has too many orifices the combustion event doesn’t work very well because there is too much interference between the spray jets. They have also determined that when there are more holes, the holes need to be smaller to get the same amount of fuel into the piston as that delivered by fewer and larger holes.

Ramadan is performing computer simulations using different hole patterns and sizes to virtually test them before collaborators at EPA and MSU build physical prototypes.

Once prototypes are created, the MSU researchers are taking video of the actual spray event inside a diesel engine using a high speed camera utilizing an infrared technique that captures the combustion event. To see inside the engine they removed an intake valve and inserted a sapphire window in its place. In another case, they used a sapphire window insert in the piston.  Ramadan does the mathematical modeling attempting to design a bowl and injectors that utilize the entire volume of air inside the piston to reduce soot emissions.

Currently, the diesel engine they are researching runs rich at the outside of the piston, so it creates more soot. “Why we are looking at more holes and smaller holes is to achieve better fuel atomization and mixing in the piston in an effort to achieve combustion in the entire piston cavity,” Ramadan said. “This will utilize the entire volume of air in the piston more efficiently.”

“Most engines work well on low speeds or low loads. At higher loads and engine speeds more soot is formed because the engine is not burning the fuel efficiently,” he explained. Their goal is to increase efficiency of diesel engines operating at higher speeds and loads.

Previous research by Ramadan and co-researchers at MSU also included a focus on diesel engines, specifically diesel trucks, in comparison with small Homogeneous Charge Compression Ignition (HCCI) gasoline engines without spark plugs (gasoline engines that combine how gas and diesel engines work).

In an HCCI engine, fuel is injected in the intake system and so has time to evaporate and mix during the intake and compression strokes, resulting in a uniform mixture where compression causes spontaneous combustion everywhere in the combustion chamber, explained Ramadan. So, HCCI combines the benefits of an SI engine (uniform mixing) and a CI engine (spontaneous combustion at multiple points).

The goal of this project is to explore methods to produce a more uniform burn in a diesel engine by controlling the flow of fuel and mixing in the combustion chamber, Ramadan said.

With a gasoline engine it is necessary to have a spark plug to ignite the fuel-air mixture. After ignition, a flame is formed and propagates into the mixture. In diesel engines, the fuel does not mix uniformly and results in non-uniform burn. Diesel engines combustion produces “hot” and “not so hot” spots in the engine, he said.

“Uniform burn is also the goal with HCCI, however the difficulty is how to control the start of the ignition process?” he said. Ramadan will use computer simulations to try to establish a way to control the combustion process.

“Through this grant from the EPA we will also look at ways to achieve the desired flow structure through the design of the combustion chamber and not the intake system,” said Ramadan.

This project is a continuation of previous research Ramadan has done for the EPA. “The continuity of working with EPA and MSU maximizes the knowledge base developed between Kettering, EPA and MSU, building on existing capabilities,” said Ramadan of the long-term research relationship that investigates making diesel and combustion engines more fuel efficient.

Contact: Dawn Hibbard