Clean-burn engine dodges ever tighter regulations
SOMETHING big is brewing under your car's hood. Car makers around the world are developing technology that promises to combine the high fuel efficiency of a diesel engine with the low nitrogen and soot emissions of a modern petrol (gasoline) engine.
But unlike alternative technologies such as fuel cells, the new engine will involve minimal change to vehicle design. It can be used with both petrol and diesel fuels, and is expected to be on the road within the next few years.
The idea behind the engine, called controlled auto-ignition, or homogeneous charge compression ignition (HCCI), is not a new one. Technical problems have prevented the idea getting off the ground until now, but looming emissions regulations in the US are forcing car makers to speed up development of the technology.
Car makers have always faced trade-offs when designing engines. Is it better to minimise emissions or maximise fuel economy? Should they reduce toxic soot-particle emissions from diesel engines or levels of smog-generating nitrogen oxides (NOx)?
But because new regulations around the world will demand low emissions of all pollutants, trade-offs won't cut it any more. In the US, soot-particle and NOx emissions must be slashed by nearly 90 per cent in heavy-duty diesel engines by 2010, with particle emissions to be reduced from 0.1 grams per brake horsepower in 2004 to 0.01 g, and NOx from 2.5 g to 0.2 g per brake horsepower. Engineers feared the targets could not be met with existing technology.
In Europe, vehicle makers have agreed to increase fuel efficiency to reduce carbon dioxide emissions for new passenger cars, down to 140 grams per kilometre by 2008, a 25 per cent reduction on 1995 levels of 186 grams per kilometre. Later this year the European Commission is planning to introduce proposals to reduce this further to 120 grams per kilometre by 2010, which will require further improvements in fuel efficiency.
HCCI technology is expected to help meet these demands. "This is a renaissance of the internal combustion engine," says John Pinson, head of diesel engine research at General Motors in Warren, Michigan.
The technology combines aspects of diesel and petrol engine design. The concept of homogeneous charge, taken from petrol engines, means fuel and air are well mixed before they ignite, allowing the fuel to burn uniformly and preventing soot particles from forming. In a typical diesel engine, by contrast, fuel is injected into the cylinder at the last possible moment, and the heat generated by compression of the air inside the cylinder then ignites the mixture almost immediately. As the fuel does not have time to mix well before burning, soot emissions are higher.
This "compression ignition" is the diesel engine's contribution to HCCI. Combining this with a homogeneous charge causes ignition to begin at various points throughout the cylinder at once (see Diagram), helping HCCI combustion to beat both types of standard engine in its near-zero emissions of smog-forming NOx. Like a diesel engine, HCCI also operates "fuel-lean", in other words, with more air than is necessary to burn all the fuel. But HCCI can operate at even more dilute levels than diesel, and this keeps peak temperatures low, below 2000 kelvin. In standard engines, poorly mixed fuel creates areas where the temperature is high enough for nitrogen and oxygen in the air to react, forming NOx.
The HCCI engine will also benefit from the higher compression ratios and unthrottled operation of diesel engines, to give 30 to 40 per cent greater fuel efficiency than petrol engines. This is because today's petrol engines use throttle valves to control the amount of air and fuel taken into the engine. But at low loads the valves must be nearly closed, and energy is wasted pulling the mixture through this nearly closed valve. Diesels do not use a throttle, and instead suck air into the cylinder during the induction stroke. Taken together, HCCI combustion results in a high-efficiency engine with near-zero emissions of soot particles.
All scrubbed up
Modern petrol engines are fitted with catalytic converters, which remove the NOx before the exhaust leaves the tailpipe. But the same approach does not work for fuel-lean diesel engines, as the high levels of oxygen in their exhausts make conventional catalysts ineffective.
There are alternatives, such as a tailpipe NOx trap, but these age rapidly and reduce the vehicle's fuel economy by 5 to 10 per cent, which would be an unacceptable penalty. So as NOx emission regulations from diesel engines tighten, industry has been struggling to meet them. "A couple of years ago, we weren't sure it was even possible," says Ford spokesman Nick Twork.
Compared with today's petrol engines, the benefits of HCCI are in the increased fuel economy of operating without a throttle and at higher compression ratios, and the associated reduction in greenhouse gas emissions. HCCI petrol engines would have 20 per cent higher fuel efficiency than these petrol engines, Pinson says.
But getting HCCI on the road has been hampered by difficulties controlling when and how fast each cylinder fires. A compressed mixture of fuel and air ignites when it reaches a critical level of temperature and composition. And that depends on factors such as the fuel quality and the temperature of the cylinders, which varies from cylinder to cylinder within the engine and over time as the engine warms up. What's more, all of these change with different driving conditions, for example whether the driver has the pedal to the metal to climb a hill or is coasting home.
Valve-control technology that dictates when the intake and exhaust valves open and shut, on-board computers and electronic fuel injection systems can now allow an engine to detect and respond to varying conditions to keep the cylinders firing at the appropriate time. That makes HCCI operation possible.
Researchers are modifying these technologies developed for conventional engines to control the HCCI ignition process. These include recirculating exhaust gas to preheat the fuel, controlling the timing and amount of fuel injected into the chamber, and valve-control technology.
Chris Gerdes, an HCCI researcher at Stanford University in Palo Alto, California, has already achieved HCCI combustion in an engine in the laboratory over rapidly changing operating conditions using these techniques. "I'm not saying it is an easy problem," he adds, "but I don't see anything that stands in the way of solving it."
For now though, HCCI has not been made to work at the lowest loads, when there is not enough fuel present within the mixture for it to automatically ignite. It is also unreliable for operating at the highest loads, when the large amounts of fuel required release heat so quickly that it damages the engine.
So the first engines to incorporate the technology will operate in HCCI mode at loads at which it performs best, and switch over to standard diesel, petrol or even battery-power operation at other times. As the technology improves, HCCI should increasingly take over.
Whatever its exact form, HCCI is likely to be in use within the next five years, and its importance will grow. "It is absolutely the single greatest thing that is happening in engine development," says Pinson.
What Will be at the pump?
Engines that operate at lower temperatures are desirable because they emit lower levels of NOx pollution. In principle they could also operate on any fuel. So researchers are looking into which fuels may offer more stable low-temperature combustion than is possible with petrol or diesel HCCI combustion.
"If you're not constrained by gas or diesel, you can do amazing things," says Charles Mueller, who studies fuels at Sandia National Laboratories in Livermore, California. Fuels that contain a lot of oxygen burn more stably at low temperatures than diesel or gasoline, but these are more costly and may not be compatible with some engine parts, as they may react with the rubber seals. Even if a cheap, compatible fuel is found, one big issue remains - changing the established infrastructure for refining oil to produce petrol and diesel.
As renewable fuels such as ethanol and biodiesel gain a footing, and as petrol and diesel increasingly come from heavier crudes and sources such as oil sands and shale oils, refiners will change the fuels they make, Mueller believes.
Another fuel that researchers are investigating for use in the internal combustion engine is hydrogen, which could all but eliminate vehicle emissions.
Hydrogen will burn over a huge composition range, from 4 per cent to 75 per cent hydrogen in air. Like HCCI, this allows hydrogen-burning engines to operate at low temperatures and to avoid NOx emissions.
