[Editors] MIT works toward fuel-efficient engines

[Elizabeth Thomson]

MIT News Office
Massachusetts Institute of Technology
Room 11-400
77 Massachusetts Avenue
Cambridge, MA  02139-4307
Phone: 617-253-2700
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MIT researchers work toward spark-free, fuel-efficient engines
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For Immediate Release
MONDAY, JULY 23, 2007
Contact: Elizabeth A. Thomson, MIT News Office
Phone: 617-258-5402
Email: thomson at mit.edu

PHOTO, DIAGRAM AVAILABLE

STORY ONLINE AT: http://web.mit.edu/newsoffice/2007/engine-0723.html

CAMBRIDGE, Mass.--In an advance that could help curb global demand  
for oil, MIT researchers have demonstrated how ordinary spark- 
ignition automobile engines can, under certain driving conditions,  
move into a spark-free operating mode that is more fuel-efficient and  
just as clean.

The mode-switching capability could appear in production models  
within a few years, improving fuel economy by several miles per  
gallon in millions of new cars each year. Over time, that change  
could cut oil demand in the United States alone by a million barrels  
a day. Currently, the U.S. consumes more than 20 million barrels of  
oil a day.

The MIT team presented their latest results on July 23 at the Japan  
Society of Automotive Engineers (JSAE)/Society of Automotive  
Engineers (SAE) 2007 International Fuel and Lubricants Meeting.

Many researchers are studying a new way of operating an internal  
combustion engine known as “homogeneous charge compression  
ignition” (HCCI). Switching a spark-ignition (SI) engine to HCCI mode  
pushes up its fuel efficiency.

In an HCCI engine, fuel and air are mixed together and injected into  
the cylinder. The piston compresses the mixture until spontaneous  
combustion occurs. The engine thus combines fuel-and-air premixing  
(as in an SI engine) with spontaneous ignition (as in a diesel  
engine). The result is the HCCI's distinctive feature: combustion  
occurs simultaneously at many locations throughout the combustion  
chamber.

That behavior has advantages. In both SI and diesel engines, the fuel  
must burn hot to ensure that the flame spreads rapidly through the  
combustion chamber before a new “charge” enters. In an HCCI engine,  
there is no need for a quickly spreading flame because combustion  
occurs throughout the combustion chamber. As a result, combustion  
temperatures can be lower, so emissions of nitrogen pollutants are  
negligible. The fuel is spread in low concentrations throughout the  
cylinder, so the soot emissions from fuel-rich regions in diesels are  
not present.

Perhaps most important, the HCCI engine is not locked into having  
just enough air to burn the available fuel, as is the SI engine. When  
the fuel coming into an SI engine is reduced to cut power, the  
incoming air must also be constrained-a major source of wasted energy.

However, it is difficult to control exactly when ignition occurs in  
an HCCI engine. And if it does not begin when the piston is  
positioned for the power stroke, the engine will not run right.

  “It's like when you push a kid on a swing,” said Professor William  
H. Green, Jr., of the Department of Chemical Engineering. “You have  
to push when the swing is all the way back and about to go. If you  
push at the wrong time, the kid will twist around and not go  
anywhere. The same thing happens to your engine.”

According to Green, ignition timing in an HCCI engine depends on two  
factors: the temperature of the mixture and the detailed chemistry of  
the fuel. Both are hard to predict and control. So while the HCCI  
engine performs well under controlled conditions in the laboratory,  
it is difficult to predict at this time what will happen in the real  
world.

Green, along with Professor Wai K. Cheng of the Department of  
Mechanical Engineering, and colleagues in MIT's Sloan Automotive  
Laboratory and MIT's Laboratory for Energy and the Environment have  
been working to find the answer.

  A large part of their research has utilized an engine modified to  
run in either HCCI or SI operating mode. For the past two years,  
Morgan Andreae (MIT PhD 2006) and graduate student John Angelos of  
chemical engineering have been studying the engine's behavior as the  
inlet temperature and type of fuel are changed.

Not surprisingly, the range of conditions suitable for HCCI operation  
is far smaller than the range for SI mode. Variations in temperature  
had a noticeable but not overwhelming effect on when the HCCI mode  
worked. Fuel composition had a greater impact, but it was not as much  
of a showstopper as the researchers expected.

Using the results of their engine tests as a guide, the researchers  
developed an inexpensive technique that should enable a single engine  
to run in SI mode but switch to HCCI mode whenever possible. A simple  
temperature sensor determines whether the upcoming cycle should be in  
SI or HCCI mode (assuming a constant fuel).

To estimate potential fuel savings from the mode-switching scheme,  
Andreae determined when an SI engine would switch into HCCI mode  
under simulated urban driving conditions. Over the course of the  
simulated trip, HCCI mode operates about 40 percent of the time.

The researchers estimate that the increase in fuel efficiency would  
be a few miles per gallon. “That may not seem like an impressive  
improvement,” said Green. “But if all the cars in the US today  
improved that much, it might be worth a million barrels of oil per  
day-and that's a lot.”

This research was supported by Ford Motor Company and the Ford-MIT  
Alliance, with additional support from BP.

--MIT--