Cat's HCCI achieves 75% load, ultra-low emissions using ordinary diesel fuel

Diesel Fuel News, Sept 2, 2003 by Jack Peckham

Newport, R.I. -- In an industry first, Caterpillar has achieved over 75% load on a multi-cylinder homogenous charge compression ignition (HCCI) prototype diesel engine while producing ultra-low emissions of nitrogen oxides (NOx) and "smoke" (or particulate) emissions.

While this result was achieved at steady-state rather than in transient operation -and plenty of research & development (R&D) work remains--most engineers at the Diesel Engine Emissions Reduction (DEER) conference here said they believe it's a remarkable achievement with potentially profound consequences for the diesel industry. It's especially significant since the results were achieved by a major engine company with keen commercialization interests.

"We're getting proper air-fuel mixing now, and going to higher loads," Cat engineer Kevin Duffy reported to the DEER conference here. "We've achieved 1,600 kPA brake mean effective pressure, and stable combustion at idle and light load. Now we have a full-court press on controls and we're moving to transients--a big challenge.

"If our results continue to be positive, then we will keep developing this technology (along with others) with an eye towards 2010" EPA compliance. "All results to date have been with conventional diesel fuel We wanted to do this first with diesel fuel" rather than with a more gasoline-like fuel as many HCCI developers suggest would seem necessary for possible full-load operation.

However, some engineers at DEER conference speculated here that Cat might not need a full-load HCCI engine to be commercially viable, nor a change from ordinary diesel fuel. One theory is that if Cat can maintain 80% load HCCI and figure out how to achieve full transient operation, then it could simply up-size an HCCI engine by 20% rather than "wasting" money and space on alternative schemes such as urea-SCR or NOx traps.

Other engineers speculated here that if Cat can achieve such incredibly low HCCI combustion emissions with ordinary diesel fuel, then the push by automakers for 10-ppm sulfur ULSD fuel and high cetane would seem unnecessary, at least for some future fleets.

However, that doesn't answer the problem of what to do about current fleets that need lower-sulfur fuels for emissions control. Most automakers and engine makers are counting upon ULSD to enable catalyzed exhaust systems starting in 2007.

What's more: Cat officials told us that ULSD fuel would be needed for HCCI as well, in order to allow "aggressive" oxidation catalyst formulations to handle the increase in hydrocarbon (HC) and carbon monoxide (CO) emissions that are typical of HCCI operation. It's also possible that a sulfur-sensitive diesel particulate filter (DPF) might be required even with HCCI, perhaps if the engine has to modulate between part-load HCCI and full-load regular diesel combustion, Cat told us.

Meantime, in contrast to reports of other HCCI developers finding great difficulty moving from single-cylinder to multi-cylinder operation, Cat seems to have conquered some critical problems with controlling combustion phasing and cylinder-pressure rise rates, at least in steady-state operation. An "advanced injector tip" helps explain some of the recent HCCI progress, and Cat now finds that it can even run HCCI at idle.

Meantime, the immense complexity of controlling an HCCI engine through transients might be overcome with neural-net based computer systems that "learn" to relate seemingly random parameters, including changes in timing, boost and other factors, Duffy explained.

"We think it will take some form of advanced, non map-based controls to do transient HCCI," he said.

While HCCI test results to date are with ordinary diesel fuel, Cat is also teaming with an undisclosed "major oil partner" to investigate 12 different gasoline and diesel fuels of varying ignition quality and volatility, he said.

In tests so far, "cool-flame chemistry" required for ultra-low NOx in HCCI can be dramatically penalized by boosting cetane by five numbers, Duffy showed. With an ordinary diesel fuel (around 45 cetane), cool-flame chemistry happens at about 12[degrees] before top dead center, while a higher-cetane fuel starts at 25[degrees] btdc--a "dramatic change in heat release and not the way we want to go," he said.

"We want combustion closer to top dead center, for improved thermal efficiency and to keep rise rates low--so a lower cetane number is better," he said.

As for fuel economy, this HCCI engine looks very competitive compared to other emissions-control schemes, Duffy showed. Cat's new 2007 emissions reduction technology (demonstrated here at the conference in a Class 8 truck) claims to cut NOx by 50% with the same fuel economy as current production "Acert" engines.

Yet HCCI could achieve less than 0.2 grams/bhp-hr NOx as well as ultra-low PM emissions with even better fuel economy than the current "Acert" engine at certain operating conditions, he said. A relatively low-cost oxidation catalyst probably could handle the expected increase in HC/CO emissions typical of HCCI combustion. Duffy points out that while significant challenges remain to make this a production-ready technology by 2010, Caterpillar is excited by progress to date.