Technologies That Fight Back

Add RCCI to our possible choices of future prime mover.

Engine
What does the future hold for internal combustion? University of Wisconsin Professor Rolf Reitz has played a leading role in a pioneering effort to create a highly efficient, low-emissions, dual-fuel engine technology known as Reactivity Controlled Compression Ignition (RCCI).University of Wisconsin-Madison, Alumni Research Foundation (WARF)

Back in the early 1990s, I had an interesting conversation with an Öhlins engineer at Daytona. Alternative motorcycle front ends had been a big excitement in the ’80s, with Honda spending millions backing a series of Elf Honda “forkless” racers. Aside from slightly shorter braking distances, however, the radical front ends did not demonstrate clear superiority. And, as the Öhlins man pointed out, “Each time the alternative guys announced a shorter braking distance, we fought back by cutting stiction; we increased the hardness and smoothness of our fork-leg coatings.”

And so today we see, gleaming on the forks of late-model bikes, the black of super-hard diamond-like carbon (DLC) and the gold of titanium nitride.

Is the telescopic fork, with its cantilevered load path, structurally inferior to Bimota’s radical Tesi hub-steerer? Apparently not so as you would notice: Bikes with telescopic forks dominate the market, and they dominate competition. When more stiffness was needed, makers of telescopics fought back by putting the bigger, stiffer tube at the top—where bending moment is greatest—creating the now-common upside-down fork.

Not so long ago, the automotive world seemed to be rushing into an aluminum future. Audi’s A8 and Honda’s NSX featured all-aluminum structure, and major makers worldwide were trying aluminum hoods and trunk lids. Why? Because vehicle weight is a major element in achieving new mandated fuel economy standards. Aluminum, one-third the density of steel, offered an answer.

But the steel industry fought back. Whatever you may believe about “the miracle of disruptive capitalism,” established industries dislike seeing major investment in plant and process suddenly devalued, and their stockholders agree. So suddenly new, affordable high-strength, low-alloy steels (HSLA) were developed, and the engineering magazines presented ads touting the new lightness of steel. High-strength materials can be very difficult to form in thin gauges into the complex shapes required for auto and light truck sheet metal. No problem: New processes were developed by which the steel was delivered soft enough to form but then hardened as it was pressed between heated dies. In all, steel showed so much versatility with low cost that the “aluminum tide” receded.

Similar vitality is being shown by the internal-combustion engine, which continues to increase in efficiency while showing ability to meet tightening exhaust emissions limits. I recently read about a new variation on the concept of HCCI, which is homogeneous charge compression ignition. Homogeneous charge means that the engine’s cylinders take in a uniform mixture of air and fuel, as opposed to the pure air that diesels take in. Instead of a spark igniting this mixture, it is so heated by a combination of compression and some recirculated exhaust gas that it auto-ignites in a great many places at once. The result is a hybrid form of lean combustion that combines the efficiency of a diesel with low nitrogen oxides (NOx) and particulates.

HCCI’s big problems are that it needs spark assist in conditions in which not enough heat is present to produce ignition: at idle/low throttle and at or near full throttle. It is also difficult to time HCCI combustion to reach peak pressure at the desired point, just as the piston begins to accelerate downward on its power stroke.

Combining differing fuel types
Combining differing fuel types has led to enhanced efficiency and reduced emissions in test engines. Wisconsin Engine Research Consultants co-founder Reitz has dedicated much of his professional life to exploring new engine technologies.University of Wisconsin-Madison, Alumni Research Foundation (WARF)

Researchers have used a variety of variables to better regulate HCCI, such as controlling the volume of hot exhaust gas recirculated, varying the compression ratio, etc. So far, no clear answer has appeared, though Mazda has a promising HCCI process of its own.

Yet at the same time we know that heavy-duty power in big trucks, rail locomotives, and marine diesels is under increasing pressure to “clean up.” The high cost of doing so has motivated automakers to sometimes rely on “defeat devices,” software elements that enable an “almost-clean” engine to pass government emissions testing. It’s popular to disapprove of Volkswagen because of “Dieselgate,” but the only differences between that company and the other makers who have had their wrists slapped is that 1) they may have relied upon such cheating in the long term, and 2) they were caught doing so.

What next? We can’t do without the transportation of goods provided by diesel power, but no easy, affordable alternative can be seen.

I found myself reading about an HCCI variant based upon a fresh variable: the reactivity of the fuel itself. Gasoline, which has been developed to resist auto-ignition and detonation, is a low-reactivity fuel. Diesel, whose cetane number quantifies the ease with which it can be made to auto-ignite, is a high-reactivity fuel.

How do you control fuel reactivity? You do it by shifting their proportions, something that’s relatively easy to do, thanks to highly developed port injection and common-rail diesel direct injection.

In Reactivity Controlled Compression Ignition (RCCI), the engine operates unthrottled, just as diesels do. Gasoline is added to air entering the cylinder by a conventional port-injection system such as is found on all modern cars and motorcycles. In the cylinder head is a second injector, but of the type found in diesels. There is no spark plug. Engine compression ratio is high—16:1—far too high for knock-free operation in a spark-ignited gasoline engine.

As the piston rises on compression and is nearing TDC, a small amount of high-reactivity diesel fuel is injected, mixing into the charge of gasoline and air. This small injection of diesel becomes hot enough to auto-ignite, adding an amount of heat to the overall mix that is enough to result in HCCI’s famous “thousand points of light” combustion. But because the amount of injected diesel is small and well-dispersed, no diesel-like zone of hot, chemically correct combustion takes place. Because this engine’s intake is unthrottled, at anything less than full throttle its overall mixture is lean—most of the time, too lean to be spark-ignited. But the presence of the extra heat added by diesel injection, plus the engine’s high compression ratio, results in widespread ignition. Over the engine’s range of operation, the ratio between gasoline and diesel fuel is varied to obtain efficient, low-emissions combustion.

How can RCCI show higher efficiency than diesel? The major difference is in RCCI’s lower combustion temperature, which rejects less heat to coolant.

Because both the timing and amount of the diesel injection can be finely controlled, so can the point of peak combustion pressure. Because lean combustion naturally occurs at lower temperature, the production of NOx can be essentially stopped (it has a temperature threshold). This was naturally the case in classic crankcase-scavenged two-stroke motorcycle engines of the 1960s and ’70s because they had a lot of natural exhaust-gas recirculation (they had other problems, namely high unburned hydrocarbons from fuel-air mixture short-circuiting to the exhaust port).

Another advantage of cool, lean combustion is that less energy is wasted out the exhaust in the form of molecular vibrations or rotation; nearly all of the energy of combustion appears in the form of molecular velocity, the form that best pushes pistons.

The bottom line? Results published by a research group at University of Wisconsin under Rolf Reitz indicate that this combustion system can operate at efficiency as much as 20 percent higher than that of existing diesels. In graphs presented, efficiency is never below 49 percent and peaks at 58 percent.

How can RCCI show higher efficiency than diesel? The major difference is in RCCI’s lower combustion temperature, which rejects less heat to coolant. Instead, that heat does work upon the piston. It was even possible in test running to operate without the normal, for diesels, piston internal oil cooling.

cylinder injection and fuel distribution for gasoline and diesel
This diagram shows cylinder injection and fuel distribution for gasoline and diesel. RCCI’s low combustion temperature is said to dramatically reduce, if not eliminate, nitrogen oxides (NOx). Broad power and torque characteristics are also claimed.University of Wisconsin-Madison, Alumni Research Foundation (WARF)

The resulting engine—a single-cylinder unit of heavy-truck bore and stroke—is able to pass 2010 US emissions standards now and does not require exhaust-gas after-treatment. A major part of the cost of new diesel engines now is that of the after-treatment equipment: the SCR/urea fluid system for reducing NOx to molecular nitrogen; the diesel particulate filter that must periodically be “burned off”; and the conventional oxidizing cat for completing the combustion of UHC. A small four-cylinder engine of economy-car size was shown to be capable of similar results.

Yes, the requirement to carry two fuels is a drawback, but the bottom line is important: The internal-combustion engine has a lot of potential life left in it. It remains a dynamic technology that is capable of fighting back against economic or socially mandated alternatives.

Isn’t this all irrelevant in view of the expected electric-car revolution? Look at the numbers: 64 percent of US electricity in 2018 came from natural gas and coal. If we start with a highly efficient combined-cycle gas-turbine electric plant, running on natural gas at 58 percent efficiency, and delivering power through an average 89-percent-efficient transformer-and-transmission line efficiency, then we have to factor in the battery-electric car’s charge-discharge efficiency of 70 to 80 percent, the transistor power supply’s 90 percent, and the electric motor’s 94 to 96 percent efficiency. Multiply together to get 0.58 x 0.89 x 0.75 x 0.90 x 0.95 = 33 percent overall system efficiency.

That makes 49 to 58 percent efficiency from an RCCI internal-combustion engine look pretty good.