Advanced Combustion Strategies

The Transportation Technologies Office (TTO) funds research focused on developing a greater understanding of engine combustion and how emissions form within engine cylinders as well as how combustion and emissions depend on factors such as fuel spray characteristics, in-cylinder air motion, and type of fuel. This greater understanding will help researchers develop higher efficiency advanced combustion engines strategies that produce less pollutants like nitrogen oxides (NOx) and particulate matter compared to conventional engines.

Research is focused on three major combustion strategies:

• Low temperature combustion
• Dilute (or lean-burn) gasoline combustion
• Clean diesel combustion

All the combustion approaches and associated critical technical issues TTO addresses are compatible with the industry trend toward engine downsizing and boosting to improve vehicle fuel economy. In addition, it also supports research on materials that can withstand high operating temperatures and pressures needed to capitalize on these engines' potential benefits.

Low-temperature combustion

Low temperature combustion (LTC) is a flameless, staged burning of the fuel (gasoline, diesel, or biofuel) in an engine's combustion chamber at temperatures that are lower than what occurs during conventional engine combustion. Research suggests that LTC has the potential for a 20% efficiency improvement over current diesel engines. The lower temperature, flameless combustion results from compression of a fuel-air mixture that has been diluted with either excess air or recirculated exhaust gas. This process raises the density and temperature of the dilute mixture, and causes it to autoignite (a process known as compression ignition).

With the LTC process, the engine compresses a dilute fuel-air mixture, raising its density and temperature. This process, known as compression ignition, causes the fuel-air mixture to autoignite. To dilute the fuel-air mixture so that it has a lower proportion of fuel in it than conventional combustion would, the engine uses either excess intake air or recirculated exhaust gas.

Staged burning, the other key element of LTC, is achieved by controlling the timing of the autoignition and rate of heat release. This process works to eliminate excessive combustion rates that can cause engine noise and structural damage, especially at higher loads.

TTO is researching a number of forms of LTC, including homogeneous-charge compression ignition (HCCI), premixed-charge compression ignition (PCCI), and reactivity controlled compression ignition (RCCI).

LTC offers a number of advantages over today's engines:

• The fuel/air mixture and combustion product properties enable the engine to be more efficient than conventional combustion engines.
• Because of the lower combustion temperature, the engine loses less energy through the cylinder walls to the environment. Some of this reduced energy loss allows the cylinder to maintain higher pressure for a longer period of time, enabling the engine to do more work. Some of the energy appears in the form of higher exhaust energy that turbocharging can partly capture.
• Gasoline-based LTC does not need to throttle intake air to control load, which is a major cause of inefficiency in today's gasoline spark-ignition engines.
• LTC is not restricted by "knock" (explosive, uncontrolled combustion) in the same way gasoline spark-ignition engines are. As a result, LTC allows gasoline engines to have high compression ratios similar to diesels, increasing their fuel economy.
• LTC may be able to achieve ultra-low exhaust emissions, which could greatly reduce aftertreatment requirements, cost, and fuel economy penalties.

With a combustion strategy that utilizes LTC, a vehicle fuel economy improvement of 19.4% (over a model year 2015 baseline) was demonstrated in FY19. The details of this evaluation can be found here.

TTO is supporting work to address a number of critical challenges facing the development of low-temperature combustion, such as:

• Difficulty controlling the start of combustion, because of the lack of a spark or fuel injection
• Expanding the range of engine loads
• Managing the heat release rate
• Reducing the lack of control during transient events, such as changing loads and acceleration
• Reducing potentially higher hydrocarbon (HC) and carbon monoxide (CO) emissions
• Understanding if LTC can be more fully effective when combined with fuel that has different specifications than gasoline and diesel

Back to Top

Dilute (or lean-burn) gasoline combustion

In dilute gasoline combustion, a flame moves through either premixed or non-premixed (i.e., stratified) mixtures of fuel and air. In this process, the engine dilutes the fuel with either more air than is required to burn it (excess intake air) or recirculated exhaust gases. TTO's research focuses on the non-premixed (stratified) version because it offers the highest potential to improve efficiency. These engines can operate on current gasoline and gasoline/ethanol blends and are primarily for automotive and light truck applications. This combustion technology can offer fuel economy improvements of up to 35% relative to a 2009 baseline gasoline vehicle.

In the stratified version of the process, the vehicle injects fuel directly into the cylinder. It times it so that a properly stratified combustible fuel-air mixture occurs near the spark plug at the time of spark.

Dilute gasoline combustion results in fuel economy improvements because:

• The engine uses the amount of fuel injected to control the load rather than restricting the intake air flow (throttling) to control it. Most gasoline vehicles on the road have port-fuel-injected (PFI) gasoline engines that use throttling, which is far less efficient.
• At part load, the combustion products allow the engine to carry out work more efficiently compared to conventional engines.
• The engine has a lower combustion product temperature at partial loads than a conventional engine would and as a result, loses less heat.

TTO is supporting work to address critical challenges which include:

• Determining the most efficient fuel-air mixing strategies, which involve issues with port configurations, fuel-spray characteristics, and mixing characteristics
• Initiating ignition and propagating a flame in stratified mixtures
• Facing challenges with stochastic misfire and knock (explosive, uncontrolled combustion)
• Reducing emissions that are different from those that occur with conventional (PFI) engines

Back to Top

Low-emission diesel combustion

In low-emission diesel combustion, the burning process takes place in a process fairly similar to conventional diesel combustion. In conventional diesel combustion (also known as diffusion combustion), the rate at which the fuel spray mixes with air inside the cylinder before it reaches the flame determines the rate at which the fuel and air burn in the flame. In low-emission diesel combustion, more fuel-air mixing occurs prior to the flame. This enables cleaner combustion that produces less soot as well as retains or improves the high efficiency of diesel engines. Adding recirculated exhaust gas to the intake air stream dilutes the fuel-air mixture, resulting in lower combustion temperatures and reducing the formation of NOx. Because fewer emissions form inside the cylinder, these engines do not have to rely as heavily on aftertreatment technologies to further reduce emissions.

TTO is supporting research to further improve low-emission diesel combustion and make it cost-competitive for all passenger and commercial vehicles. This requires pushing the state-of-the-art of technologies such as computer-control, multi-pulse fuel injection, high-pressure fuel injection, use of exhaust gas recirculation, and manipulation of in-cylinder gas flows.

TTO's research on low-emission diesel combustion engines for passenger and commercial vehicles addresses critical challenges which include:

• Controlling the amount and temperature of the exhaust gas used for exhaust gas recirculation to minimize emissions
• Improving the fuel injectors, injection pressure, and control over the fuel spray and spray types in high-pressure and multi-pulse injection
• Improving lifted-flame combustion, which is when the flame that lifts off of the fuel nozzle stabilizes downstream of the fuel jet. Clean diesel engines must maintain auto-ignition of the fuel-lean mixture that is immediately upstream of the flame base.
• Improving post-combustion injections for reducing emissions both in-cylinder and through aftertreatment