Low-Temp Gasoline Combustion Research Could Boost Engine Efficiency

Trapping some exhaust in an engine's combustion chamber could lead to auto-ignition designs that replace spark plugs. But ensuring compression stability at low loads remains a challenge.

Leah Scully

Jan. 25, 2017

3 min read

Sandia researchers Isaac Ekoto pictured and Benjamin Wold are part of a team researching automotive engines that replace sparkplugs with autoignition devices The research could help meet automotive engine goals for cleaner emissions and a 545 miles per gallon fuel economy by 2025 — Sandia researchers Isaac Ekoto (pictured) and Benjamin Wold are part of a team researching automotive engines that replace sparkplugs with auto-ignition devices. The research could help meet automotive engine goals for cleaner emissions and a 54.5 miles per gallon fuel economy by 2025.

Low-temperature gasoline combustion (LTGC) is an attractive topic for engine research because the reduction in heat transfer improves engine performance and reduces nitrous oxide emissions. At the Combustion Researc h Facility (CRF) at Sandia National Labs, researchers conduct a range of investigations to understand the chemistry of combustion in LTGC engines. They work with industry partners and other national labs to explore clean engine designs for various fuels. The ultimate goal of the research at CRF is to design auto-ignition methods for future automotive engines that could be used to replace spark plugs found in conventional internal combustion motors.

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2. This shows a 4-stroke internal combustion engine. A crank shaft pulls down the piston so that fuel enters through the inlet valve. The crank shaft continues to turn for the compression stroke, which causes a spark plug to ignite the injected fuel. Energy from combustion causes the cylinder to expand for the power stroke. Exhaust exits through the exhaust valve.

âA recent project explores a negative valve overlap (NVO) system with modified valve timings to trap some exhaust in the combustion chamber at the end of each duty cycle. The exhaust mixes with an injected fuel stream to create a charged product that ignites at low flame temperatures. During the engine's compression stroke, heat transfer causes the new dilute charged mixture to ignite. The released energy drives the power stroke. A shortened exhaust stroke and delayed opening of the intake valve allows for a portion of the exhaust to remain in the chamber again for the next cycle. The team designed the NVO system to try and improve combustion stability at low loads. This instability is one of the reasons that LTGC is challenging for engine designs.

The scientists use a variety of tests and modeling tools to try and understand the chemical combustion reaction in dilute exhaust/fuel mixtures. Understanding the chemistry and constituents of mixtures (in addition to thermodynamic properties) over the duty cycle may help to control combustion in all conditions. Recently, CRF announced that it has formed a Spray Combustion Consortium to pool resources that can be used to explore alternate fuel injection methods.

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3. This image from national instruments shows the difference between a diesel combustion engine, a spark-ignition gasoline engine, and a LTGC engine. LTGCs are still being researched for better combustion stability.

The scientists tested six different types of fuels to determine their constituents after combustion and their reaction with the exhaust. They tested iso-octane, n-heptane, ethanol, cyclohexane, toluene, and 1-hexene to represent components found in commercial gasoline. Research-grade gasoline and a surrogate with a known composition were also tested using single-cylinder research engines. A dump sampling valve was set up to send samples in the combustion chamber to a gas chromotograph, which identifies light hydrocarbon components in the mixture.

To distinguish between hydrocarbon species with similar molecular composition but different effects on the chemistry of combustion, a photo-ionization mass spectroscopy technique was performed using Lawrence Berkeley National Laboratory's Advanced Light Source. The team found that during compression, the mixture's high temperatures cause the fuel to decompose into a reactive “reformate” mixture made up of highly combustible constituents like hydrogen, carbon monoxide, and small hydrocarbons like methane, acetylene, and ethylene.

LTGC is also known as Homogeneous Charge Compression Ignition (HCCI) and Premixed Charge Compression Ignition (PCCI).

About the Author

Leah Scully

Associate Content Producer

Leah Scully is a graduate of The College of New Jersey. She has a BS degree in Biomedical Engineering with a mechanical specialization. Leah is responsible for Machine Design’s news items that cover industry trends, research, and applied science and engineering, along with product galleries. Visit her on Facebook, or view her profile on LinkedIn.