Smarter combustion01 June 2011
Andrew English quizzes Professor Hongming Xu, chair of Energy and Automotive Engineering at Birmingham University, about the future of the internal combustion engine
Homogenous Charge Combustion Ignition (HCCI)? You'd be forgiven for thinking it has turned into the great South Sea Bubble of the automotive industry. From rave reviews at the beginning of the century, this sparkless technique of compressing well mixed fuel/air mixtures, until they ignite, has almost disappeared in recent years. To the extent that, when BMW recently unveiled its next generation of three-, four- and six-cylinder engines, HCCI wasn't even mentioned.
Professor Hongming Xu, chair of Energy and Automotive Engineering at Birmingham University, is the man most likely to explain what has happened to HCCI.
So, is HCCI a busted flush? Xu cautiously agrees that the heat has dropped in research. "From my first job at JLR in 2000, I was working on this technology and also at Birmingham University. The most crucial time for the research was between 2005 and 2007." He points to more than 200 publications of original research in SAE papers from that period. "Now publications have fallen off to a third of that," he says, "and, in my opinion, there is very little new stuff coming out at the present."
What's gone wrong? "In the early years of research, the car industry was overwhelmed with advantages, but now the disadvantages have been fully exposed," he points out.
He says the production realities have now sunk in, such as the limited operating region where an engine can be run in HCCI mode and the fact that the engine has to be turbocharged or supercharged, in order to extend that operating region.
"The benefits of HCCI are greater in a gasoline engine, where you gain fuel economy," says Xu. "A diesel doesn't benefit from much greater economy, but HCCI does reduce emissions of oxides of nitrogen and particulates simultaneously."
He states that the economy benefits in petrol engines accrue mainly from the throttle-less operation, more so than the precise control of fast combustion. Combustion efficiency is lower in a HCCI engine, because the combustion temperature is lower."
"There were claims of between 20 and 30% economy gains," he says, "but that was only at certain operating periods. Those gains were nearer to four per cent at a vehicle level, although the adoption of turbocharging can make the operating window larger and thus the gain bigger. "Up to 10%," he says before cautioning, "you have to be very careful with the figures." This is because advances in other areas such as cam profile switching (CPS) techniques will have contributed their own fuel savings. "You can't give all the credit to HCCI," he says.
It was cost that also caused the industry to rethink its HCCI plans. Xu reckons the on-costs of CPS to be about €350 an engine unit, depending on the numbers of cylinders.
Then there's a reliability factor and the effects of early detonation on the cylinder block. Mercedes Benz has only just sorted the problems in its 'Diesotto' F700 1.8-litre, four-cylinder HCCI engine, first shown in Frankfurt in 2007. "We went silent on the project, because we were having problems," said Professor Herbert Kohler (head of 'e-drive & Future Mobility', chief environmental officer, Daimler AG) earlier this year. But we now have a totally different design of crankcase, which is highly modified from standard. Now we need to work on the question of costs and results."
He is confident that the Diesotto system could be on the market in as little as two years, although the modified crankcases would need a new generation of engines that dictates when they could be introduced. "It all depends when we modify the crankcase," he says. "This is not an easy or a cheap investment."
So does Xu think that it's all over for HCCI? Absolutely not. "There is continuous research in this area, although in smaller scales." He offers an example of recent progress. "A joint research group of Tsinghua University and Chery in China has just demonstrated they can now run a HCCI engine in a real passenger car model, with attractive fuel economy benefit, and this appears to be a step ahead among very few others towards the practical application of HCCI.
"HCCI is now a generic name covering all manner of technologies involving premixed compression ignition combustion," he says. "The endless name list ranges from PCCI [Premixed Charge Combustion Ignition, used in diesel engines where multiple injections of fuel control the release of combustion heat and therefore soot] and PPCCI [Partially Premixed Charge Combustion Ignition], they are all derivatives of HCCI technology."
And as modern petrol engines start to sprout turbochargers, variable cam phasing and direct fuel injection to meet forthcoming emissions legislation, HCCI is looking increasingly possible and feasible. Another example is the complex, fragile and expensive sensors required to monitor the combustion process in each cylinder on HCCI engines. Tomohiko Kawanabe, Honda's R&D director, recently told us that, as new engines are likely to require these sensors merely to meet forthcoming emissions standards, so HCCI, which his company is still working on, would be even more viable.
With HCCI derivatives, we have new ways of continuing the technology and people are working on it. It's difficult technology, but the benefits are clear."
In fact, Xu goes even further, predicting a new smart type of injector, similar in operation to an ink-jet printer, which could be used to control minutely the combustion process. He suggests that, in future, internal combustion engines should have both port and direct fuel injection. "We are already working on this," he says. "Just imagine going to a fuel pump and filling up, using a coaxial fuel line with diesel, gasoline, maybe biofuel, or maybe some other type of fuel property modifier. The label would be 'Hydrocarbon Fuel'; you wouldn't have to know what it is, but the engine would identify it and burn it in the most efficient way, according to the operating conditions. It's my dream and hopefully I will see it before I am finished with this business."
Whether we will need such multifuel engines capable of burning fuels across a wide range of temperatures, speeds and loads, is a moot point. Hybridisation and the development of smart continuously variable transmissions mean engines wouldn't have to deliver that sort of flexibility and could work at an almost constant speed and load. This seems unlikely, however and a much more credible future is where both poles of development continue and that includes HCCI and its derivatives.
"For a long period in the future, research on the internal combustion engine will continue," says Xu. You can take that to the bank.
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