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May 21, 2013 8:15 AM, EDT

E&MU: Spark vs. Compression: Natural Gas ABCs

By John Baxter, Contributing Writer

This story appears in the May/June 2013 issue of Equipment & Maintenance Update, a supplement to the May 13 print edition of Transport Topics. Click here to subscribe today.

All natural-gas engines take advantage of the fuel’s lower cost and lower output of regulated emissions than diesel fuel, but there are some key differences in the types of engines available that could affect a truck buyer’s considerations of what type to buy.

The most significant difference is in the way the fuel is ignited in the combustion chamber. Either the engine is built to run solely on natural gas, requiring it — unlike a diesel — to use spark plugs for ignition, or it is dual-fuel, combining the natural gas with a small amount of diesel fuel, which is compressed in the combustion chamber until it ignites, a process known as high-pressure direct injection, or HPDI.

That difference means engines have somewhat different ways of getting the fuel and air into the combustion chamber, and that different process makes for differences in exhaust aftertreatment. It also can determine whether the truck uses liquefied natural gas or compressed natural gas, differences that affect a truck’s range and the fleet owner’s initial costs, both for acquisition and for setting up terminal operations.

But regardless of what choices a fleet buyer makes, natural-gas trucks cost significantly more than vehicles powered with conventional diesel engines, mainly because the fuel tanks cost more.

“The vast majority of the additional cost is in the fuel tank system,” said Jim Arthurs, president of Cummins Westport Inc., one of the principal suppliers of natural-gas engines to the trucking industry. Arthurs said a natural-gas truck costs $30,000 to $70,000 more than a similar diesel unit, depending on how much fuel the truck carries.

While some truck manufacturers have plans to make their own natural-gas engines, all of those now available for heavy trucks are made either by Westport Innovations Inc., based in Vancouver, British Columbia, or by Cummins Westport Inc., a joint venture of Westport Innovations with engine manufacturer Cummins Inc., of Columbus, Ind.

Spark-Ignited

The more common design for natural-gas truck engines is spark-ignited, known to engineers as “stoichiometric” because of the way it precisely meters both fuel and air before the mixture goes into the combustion chamber. It has many carlike components, including spark plugs, and a three-way catalyst instead of the diesel particulate filter and selective catalytic reduction systems used on diesels. But it also incorporates much of the technology of a truck diesel, such as a turbocharger, charge-air cooling and water-cooled exhaust-gas recycling, along with most major structural parts.

Cummins-Westport favors the spark-ignited engine, the design that Cummins Inc. will use for its own natural-gas version of its big-bore ISX engine, slated for release in 2016. It is more efficient than a gasoline engine but 10% to 15% less efficient than modern truck diesels.

The spark-ignited engine has a number of advantages, including lower first cost, virtually maintenance-free exhaust aftertreatment, the ability to use a single fuel and very quiet op­eration — 10 decibels quieter than a diesel, said Mostafa Kamel, director of advanced engine systems for Cummins Westport.

Steve Charlton, vice president and chief technical officer of Cummins’ engine business, explained why the engine maker favors the spark-ignited concept: “We’re using the spark-ignited, stoichiometric natural-gas engine because HPDI requires two fuels, has a far more complex injection system and also requires a DPF and SCR system in place of the simple, maintenance-free three-way catalyst [that] spark-ignited engines use.”

Roe East, general manager of Cummins’ heavy-duty and midrange natural-gas business, added, “Two big advantages are that spark-ignited natural-gas combustion is a cost-competitive solution in terms of first cost, and its lower noise level is a big advantage, especially when running in neighborhoods.”

East said the spark-ignition engine is “agnostic when it comes to which fuel storage system you want to use.” Either LNG or CNG will work because the gaseous fuel needs only to have its pressure regulated down to about 100 pounds per square inch, while the liquid fuel is evaporated using engine coolant heat before it enters the fuel system.

One disadvantage of spark ignition is the need to replace the spark plugs themselves at 67,500 miles or 1,500 hours. But Kamel said that’s a relatively simple job that takes about an hour, and the plugs are pregapped and don’t have to be regapped when installed.

The engine’s spark plugs have coil-on-plug ignition to avoid high-voltage ignition wires, and it employs an intake air throttle so fuel and air can be kept in “stoichiometric” balance, like a gasoline-powered car, and it has a much lower compression ratio than a diesel — in the range of 11:1 to 12:1 rather than 18:1.

In the spark-fired engine, fuel is metered into the intake manifold and mixed in a system that functions almost like the carburetor that gasoline engines used to have. Patric Ouellette, chief technical officer of Westport Innovations Inc., explained that the gas is supplied at constant pressure via a regulator. It then flows continuously through an orifice with a regulating needle whose position is constantly adjusted by the engine computer, depending on changes in engine speed and load or oxygen concentration in the exhaust, monitored by sensors there.

The three-way exhaust catalyst consumes any unburned fuel and gets rid of leftover nitrogen oxides. A catalyst of this type is not poisoned by sulfur or other destructive impurities and, if kept within a reasonable temperature range, will last the life of the vehicle. It has the additional advantage that it will not trap ash from the oil and thus won’t need cleaning, as a DPF does.

Compression-Ignited

The other natural-gas engine type is the compression-ignited HPDI engine, which uses a small amount of diesel fuel for ignition along with the LNG that provides most of the energy.

Because it is similar to a diesel, an HPDI engine can run in “limp mode” on diesel alone if natural gas is not available.

It has a number of unique features, but closely resembles a diesel engine in terms of how the fuel is injected and mixed, how it burns, and how the emissions are formed and cleaned up. It can use the same engine-lubricating oil as diesels do, unlike spark-ignited gas engines, which need a special engine oil.

The HPDI engine uses LNG because high pressure is required for injection, and it’s not practical to compress gaseous fuel to a high pressure on the truck. Thus, a cryogenic refrigeration-type fueling station is necessary.

The HPDI engine is favored for its superior, diesel-like fuel economy and its better performance, said Westport Innovations’ Ouellette.

The only HPDI engine available today is the Westport Innovations-modified HPDI ISX, based on the Cummins ISX block, which is called the Westport 15L. It produces up to 450 horsepower and 1,750 pound-feet of torque, Ouellette said.

Cleaning emissions from an HPDI engine, as with diesels, is a more complicated task than with a spark-ignited engine because diesel exhaust, like that of all diesels, contains unused oxygen, so it is not possible for the spark engine’s simple catalyst to remove the oxygen from the NOx to bring it down to compliance levels. Therefore, compression-ignited natural-gas engines need the same type of SCR system that diesels use.

Compression-fired engines also need a DPF. Joel Hiltner, an engineer associate at Hiltner Combustion Systems, Ferndale, Wash., explained that some particulate matter, or soot, is formed when engine oil burns, and some comes from the pilot shot of diesel fuel. Hiltner said that only 5% of the fuel charge may be diesel fuel at full load, but a higher percentage of diesel is needed to ensure ignition at lower loads.

Although natural gas is much less likely to produce soot than diesel fuel, Hiltner said, “We have found that natural gas can produce soot when the mixture ratio is three times as rich as in a stoichiometric engine. Some of the soot likely comes from the gas.”

In the compression engine, the pilot charge of diesel and the natural gas are injected directly into the cylinder, producing what Ouellette called “diffusion combustion,” in which the fuel forms a rich zone for a short period before it mixes fully with the air and can burn, so the system inherently produces more soot. However, further work may allow the use of a smaller-than-standard DPF and possibly even its eventual elimination, he said.

A Footnote on Oil

Using the right engine lubricating oil is critical for spark-ignited natural-gas engines, said Stephen Charlton, chief technical officer of Cummins’ engine business.

Natural gas is very high in hydrogen, containing about twice as much as diesel fuel. Thus, especially when burned in an engine that takes in no more air than is needed for combustion, as the “stoichiometric” spark engine does, “the blowby has a high moisture content,” Charlton said. “Unfortunately, the detergents used to remove soot from diesel oil can grab onto the water and form an emulsion.”

Because there is little soot for detergents to handle, special natural-gas engine oils are formulated without them and are required for use in the spark-ignited engines.

The designers took great pains to keep moisture from the combustion from condensing inside the engine, Charlton said: “This required that we pay special attention to the crankcase ventilation system and make modifications to keep everything warm enough, for example, by insulating the camshaft cover.”

East said that the special oil handles other products of combustion much better than standard diesel engine oil would, making the use of the proper oil essential.