by Gordy Slack
Squeezed by the four-dollar gallon of gas, the threats of climate change, and the dependence on foreign oil, American engineers are searching in new fields for more ways to wring more and cleaner miles out of a growing variety of fuels. One of the most promising recent innovations is the Homogeneous Charge Compression Ignited (HCCI) engine, which combines the efficiency and versatility of diesel engines with the cleanliness of spark-ignition engines. Currently, spark-ignited gasoline engines burn relatively cleanly but not very efficiently. And today’s best diesels are quite efficient, but tend to emit lots of particulates, especially the dreaded NOx.
HCCI engines, however, use 15 percent less fuel than gas engines and emit only 30 percent of the NOx of a typical diesel engine. Thus, they appear to combine the best of both engines. Except for one problem: temperature variations. In the words of UC Berkeley engineering post-doc and biofuel specialist Hunter Mack, “Temperature variations can make HCCI engines a nightmare on wheels.”
The way that HCCI engines work is to premix fuel the way that a spark engine does, but then compression-ignite the fuel in the manner of a diesel engine. It is the compression ignition that gives them their high efficiency but also leaves them vulnerable to temperature variations across the cylinders. The ignition occurs when pressure reaches a certain point within a cylinder, and that point is achieved sooner if temperatures are higher. So, if a load changes on the engine and the coolant gets hotter as it moves through, then the coolant will not thoroughly cool some cylinders, which will cause the ignitions to fall out of synch and misfire. Suddenly, what was a very efficient engine starts to lose power, suck fuel, and spew pollutants.
“These HCCI engines have a lot going for them,” says UC Berkeley engineer Robert Dibble. “They are clean, efficient, and can run on almost anything— including a variety of biofuels, diesel, or gasoline-like mixtures—but before we can really exploit them, we have to solve this [temperature] problem.”
The HCCI engines need to have control, which means they must be properly sensed, evaluated, and then actuated, according to Dibble. In order for these engines to be practical, as their load becomes uneven, each HCCI cylinder has to be monitored and adjusted constantly.
To be efficient, these cylinders need to communicate with each other and coordinate their actions. With support from CITRIS, Dibble is working to make that happen. He and his collaborators are developing sensors and controllers that will keep temperatures constant throughout the engine or compensate for temperature differences by adjusting pressure ratios within the cylinders.
Creating the algorithms to coordinate the firing of cylinders is tricky, notes Mack. “Different cylinders at different temperatures may have only a one percent difference in their compression ratios, but when they are compressing something 20 times, that difference is greatly amplified,” Mack explains. “There are a number of other factors that affect combustion timing too: the fuel-to-air ratio, charge stratification, intake pressure, and a host of other factors need to be monitored and adjusted in order to maximize the power and efficiency. So, this is a multi-variable problem that needs to be solved rapidly.”
Albert Pisano, professor and chair of UC Berkeley’s Department of Mechanical Engineering, is developing wireless sensors that will make the essential communication step possible. Some may be microphones, others will sense temperature or pressure, but many times a second they will report on conditions from inside the cylinders. Once the engine knows that some cylinders have to be a little hotter or cooler or slower or faster to fire in synch with their neighbors, it will take steps to re-adjust them.
While Pisano’s forté is building robust sensors and Mack’s expertise is drawing conclusions from the data they provide, Dibble’s challenge is making the engine act quickly and properly based on the information that it receives. Exhaust, for instance, may be redirected to heat up cooler parts of the engine. Dibble compares the regulation to a water fixture in the shower that can automatically microadjust the ratio of cold water to hot in order to keep the shower’s temperature just right, even if someone has started a load of laundry nearby.
Though the timing issue is HCCI’s biggest problem to be conquered, the Berkeley team is not stopping there. In addition, they are designing wireless relationships for the car that go beyond the engine’s own parts. One of HCCI’s great qualities is its ability to run on a variety of fuels. While diesel engines cannot use gasoline, or gasoline engines use diesel fuel, HCCI engines can run on both types and on a wide range of biofuels as well. Dibble and UC Berkeley engineering professor Van Carey have been talking to oil companies about establishing and taking advantage of wireless communication links between the vehicle and the fuel pump. “The car can pull up to the station and tell the pump that its efficiency is low, for instance,” says Carey. “And the pump responds, perhaps by determining that the engine is having incomplete combustion and needs a higher octane fuel. The pump could also recommend the cheapest, best-running biofuel cocktail available at that time.”
“Fuel does not usually have much brand appeal,” says Mack. “But these fuels would be mixed specifically for your car; the microbrews of gasoline.”
The conversation between the car and the service station could go much further than the subject of fuel. Wireless sensors could also report to the station on the state of the car’s oil, coolant, brakes, air filter, tire pressure, and fan belts, says Carey. Such an instant and effortless diagnostic would keep cars running at their safest and most efficient. And best of all, these sensors would work for any vehicle with something worth talking to the gas station about, not just those with HCCIs.
“Not long ago, when gas only cost people two dollars a gallon, the idea of tailoring your fuel to the car would have seemed over the top,” says Dibble. “But imagine a year from now when gas has continued to skyrocket in price,” he said. “If I tell you that with my HCCI engine and properly-tuned fuel I could increase your fuel economy by 20 percent, you are going to sit up and listen.”