Learning how to optimize engine efficiency

  • 25-Sep-2013 02:15 EDT
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Fiat’s production Multiair system proves a combination of advanced engine technologies can improve overall engine efficiency, particularly in reducing pumping loss.

Engineering engines for optimum efficiency is more important to automakers than ever before, as fuel economy and emissions regulations intensify. To deliver better engines, designers have access to more gadgets—some old, some new—than ever before.

“It is an alphabet soup of tools one can use,” admits Dr. W. Mark McVea, Chief Technology Officer for Torvec and an instructor for SAE International. These advanced engine components include VCT, VVT, VCR, DI, DoD, and HCCI (see table for full description.)

Tools, combinations, and systems

“What is important is that these tools are often best used in combination with each other. In many cases, some are enablers for others,” Dr. McVea explained. As each of these components become more complex as technologists and suppliers refine them, they provide more benefit to the end user through their ever increasing complex interactions. It is essential to find the right set of components to get the best system performance.

This means engine designers must evolve into systems engineers—to fully understand both the parts (the gadgets) and as well as the complete engine  But it's not just the engine systems engineer who needs to know details about these tools and gadgets.

“Each component engineer needs to know how other tools work to best optimize their individual pieces,” he said, explaining why an injector supplier, for example, might need to know the basics of variable valve timing (VVT) or what goes into designing a variable compression ratio (VCR) engine.

Unlocking the HCCI door

With political pressure on better fuel economy and less emissions, the automotive world has been treated to gasoline supplies that are cleaner and more consistent. This, according to Dr. McVea, will lead to better fuel economy for a couple of reasons. First, engine designers can tune existing technology better without accounting for as much variability in the fuel.

“It also opens the door to HCCI,” he said, referring to the acronym for homogeneous charge compression ignition, a combustion technique that promises diesel-like fuel efficiencies with gasoline. “Previously, the variability in fuel supplies made it too hard to control HCCI, making it impractical,” he explained.

Dr. McVea notes the technology for HCCI has evolved from asking the question “can we make a new HCCI engine" to "how can we most effectively use HCCI?"

“We now know that HCCI is good for midrange power,” he noted, but it's not as practical or efficient for when the engine is operating at low and high rpm, where spark ignition has been employed on many HCCI engines currently in development. 

Key enablers improve

As certain technologies improve, such as VVT or DI, previously difficult engine architectures become more feasible.

“For example, the Holy Grail of engine optimization is the combination of VCR and VVT, where VVT is a key enabler,” he said. That's because conventional gasoline engines avoid knock by limiting compression ratios to about 12:1. However, this limitation is overly strict at most engine speeds. The 12:1 limit is to ensure safe operation at all speeds and loads. If a device could be invented for adjusting the compression ratio, even better fuel economy could be realized.

Computer control and variable combustion chamber geometry, along with VVT to ensure optimum intake and exhaust, are making VCR more practical. Another example is direct injection, which HCCI requires.

To learn more, Dr. McVea teaches an SAE International seminar titled “Internal Combustion Systems: HCCI, DoD, VCT/VVT, DI, and VCR,” ID# C0613 (http://training.sae.org/seminars/c0613/).

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