The Ideal Engine

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The first inspirations for an engine that combined the best qualities of the diesel- and gasoline-powered engines-great fuel economy and low emissions, respectively-grew from observations by Japanese researchers over two decades ago that gas-powered tractors often continued to run despite fouled spark plugs and engines.

In a typical engine, fuel is burned to release energy for motion in an event called internal combustion. When this explosion can be set off rapidly and repeatedly, enough energy can be created to propel a car forward. In engines that run on gasoline, each combustion cycle usually consists of these steps: a mixture of fuel and air is taken into a cylinder or combustion chamber, compressed and ignited with a spark, which causes an explosion that drives the exhaust out of the cylinder and through the tailpipe. The well-mixed fuel blend burns more completely resulting in lower soot emissions. In addition, emissions of smog-forming nitrogen oxides (NOx) are relatively low because advances in techniques for recycling exhaust gas into the combustion chamber lowers the reaction temperature and reduces the formation of NOx. Any rogue emissions are scrubbed clear of the exhaust with catalytic converters. However, the fuel economy-the achievable miles per gallon-of this type of engine leaves much to be desired.



In a diesel engine, on the other hand, the air and fuel are not premixed; air is first injected into the combustion chamber and compressed before the fuel is added. The compressed air provides the heat needed for ignition of the fuel. Since the need for a spark plug is eliminated, a higher compression can be used, which allows the engine to squeeze more economy from its air-fuel mixture. However, higher compression also means higher reaction temperatures. This results in greater NOx emissions, which can't be controlled with conventional catalytic converters because the sulfur in diesel fuel pollutes these devices.

Herein lies the central barrier that engine makers must overcome in designing the ideal engine: when fuel economy is optimized, emissions rise; when emissions are reduced or eliminated, fuel economy crashes. How can this either/or interplay between better fuel economy and lower emissions be overcome? Enter those enigmatic tractors mentioned above.

"It was really a mode of combustion that in many ways was happening by accident," explains Dennis Assanis, chair of the mechanical engineering department at the University of Michigan. "They figured out that somehow you could have ignition happening in an engine as long as you had hot air and hot fuel and the right amount of compression."

These early observations led to what is now called the homogeneous charge compression ignition (HCCI), a type of internal combustion that combines the best of the gas engine and the best of the diesel engine to create an engine with low emissions and high fuel economy. The homogeneous charge part of HCCI comes from the gas engine and the well-mixed blend of fuel and air that is added to the combustion chamber prior to ignition. The compression ignition part of HCCI comes from the diesel engine. In this case, however, the compression of the air-fuel mixture results in a fireworks display of auto-igniting fuel at an array of points within the chamber. "This new mode of combustion has been phenomenal because it can substitute all the spark usage under light-load conditions thereby improving city driving by 25 [percent] to 30 percent or more," says Assanis. "By relying on compression ignition you get good fuel economy."

With over 10 years of serious research into these engines, why aren't vehicles running with HCCI out on the road? Well, a few are, says John Dec, a mechanical engineer and HCCI researcher at Sandia National Laboratories. However, these vehicles aren't running with true HCCI engines. Instead they're more like a partial-HCCI diesel-engine hybrid with significantly lower emissions but not yet at the levels promised by HCCI. "We're seeing more and more manufacturers investigating partial HCCI and methods that extend HCCI over more of the operating range in order to meet the [U.S.] EPA's 2010 diesel engine standards," Dec says. These new regulations require that emissions of NOx and particulates must be reduced to levels comparable to spark-ignition engines with a three-way catalyst. "The need to reach this goal is driving HCCI research because of its potential to achieve these ultra-low NOx and particulate emissions without expensive after-treatment," he says. "On the other hand, automotive manufacturers are working on the development of HCCI engines for their significant fuel-economy benefits over spark-ignition engines," Dec says. "HCCI engines can reduce fuel consumption by 30 percent over a typical driving cycle, and General Motors has recently announced that it will introduce a demonstration vehicle next year."

The obstacles to overcome before the HCCI engine will be widely available include, controlling the timing of combustion and reducing emissions of unburned fuel or hydrocarbons. The timing issue stems from the lack of a trigger for ignition such as a spark plug or fuel injector. The high emissions of unburned fuel results from the lower temperatures of the reaction; the fuel burns more slowly and the fraction that doesn't burn by the end of the cycle gets flushed through the exhaust system. Scientists generally try to solve these problems by one of two approaches: working directly on a prototype engine or developing computer models of the dynamic interactions that occur in the combustion process.

Assanis and his team of researchers at the University of Michigan recently began experimenting with biodiesel in HCCI engines. "Biodiesel is very attractive because we can grow it locally and it has no sulfur by definition … this is a huge advantage in helping with emissions because catalysts in the engine would work much better," Assanis explains. "Another advantage of biodiesel is that it has more oxygen in the molecule, which helps a number of things with respect to diesel combustion such as releasing less soot … and oxidizing hydrocarbons and carbon monoxide," he explains. "In theory you can get all your emissions to zero by using biodiesel in an HCCI engine with the right set of catalysts."

However, there are two unfavorable properties of biodiesel for HCCI: it has a high cetane number, which represents how quickly the fuel auto-ignites and it has low volatility, which makes it harder to vaporize-a phase-change that must occur before a fuel can be burned, Assanis says. "The combination of this low volatility and high cetane number means that we might actually ignite something that has not mixed well," he says. "But I think we have ways to address that."

One of the strategies his team is using to try to delay ignition of biodiesel is to recycle up to 50 percent of the exhaust of the engine back into the chamber. "Because we're using something that's already burned it's not going to ignite as easily. We hope this will be the solution to slow down how quickly the biodiesel will ignite," Assanis explains.

In terms of control, Assanis also explains that a cross between HCCI and diesel combustion or HCCI and spark-ignition combustion may be the more realistic solution. "In reality, for better fuel economy and ultra-low emissions internal combustion engines will rely on a hybrid combustion strategy," he says. We don't make the fuel-air mixture perfectly homogeneous. In that respect, we retain some control over how fast the fuel burns. We sacrifice a little bit of the benefit but we have a little bit of the control. That's the catch, to play the devil and to basically still get enough controllability without sacrificing all the benefits."

Another approach to making HCCI engines road-ready is to build computer simulations of combustion under various conditions including the use of different fuels. To that end, Charles Westbrook, an expert in combustion chemistry modeling and president-elect of the Combustion Institute, is developing chemical models for biodiesel from soy and canola as well as for ethanol and butanol fuels. The structural features of these compounds determine the factors related to ignition like cetane number for diesel fuels or octane number for fuels used in spark-ignition engines. "There are no existing computer simulation models for the large molecules characteristic of biodiesel fuels," Westbrook says. "My computational models can be used to predict HCCI ignition timings for any of these cases."

In a second modeling approach, Gregory Shaver, a mechanical engineer at Purdue University, and his team of researchers developed a model that tracks the performance of an HCCI engine from one combustion cycle to the next. "A key part of our work is being able to, with models, capture this cycle-to-cycle coupling," Shaver says. "We were the first to do that."

This type of model informs the design of combustion control strategies because knowing what happened on a previous cycle determines how to adjust factors like the injection of fuel, air and exhaust for subsequent cycles thereby allowing for more efficient combustion. "It's an interesting challenge," Shaver says. "The benefit for both traditional gasoline engines and diesel engines really merits this level of attention."

Another key aspect of Shaver's work is the achievement of HCCI through variable-valve actuation (VVA), a system that allows for the re-induction or trapping of hot exhaust back into the cylinder. This differs from re-circulating exhaust because in the latter case, the gases are cooled before they're mixed with fuel and air. With VVA, the hot gases are directly mixed with the fuel-air blend to achieve HCCI with modest compression, which helps with efficiency, Shaver says. "We like re-induction and trapping because it allows us to elevate the precombustion temperature and also get the benefit of diluting the reactant gas to bring down NOx emissions."

Shaver hasn't tested the flexible-valving with biodiesel yet, but says, "The idea of using some of the same things that VVA allows you to do with HCCI with alternative fuels is a big deal."

Although it's unclear how soon HCCI engines will become a common fixture in cars and trucks, it's clear that this is an increasingly popular and important area of research. "There's no doubt that this direction improves fuel economy, reduces greenhouse emissions and is very compatible with alternative fuels…we're going to see more and more developments along these lines," Assanis says.

Jessica Ebert is a Biodiesel Magazine staff writer. Reach her at jebert@bbibiofuels.com or (701) 746-8385.