At the 2017 Frankfurt Technology forum in August, Mazda announced its plans for the first commercial compression-ignition gasoline engine, which the company is calling Skyactiv-X. The technology is part of the automakers plan aimed at cutting emissions and maintaining growth through 2030. An article in Automotive News covering the announcement notes that Mazda is struggling to stay competitive against a market that is moving rapidly to electrified cars.
This is not the first time Mazda has ventured off the beaten path. In 1963 at the Tokyo Motor Show, Mazdas predecessor, Toyo-Kogyo, drew a great deal of attention when it introduced the first Wankel type rotary engine. The original engine produced only 70 horsepower, but the novelty of the design caught car buffs imaginations. In 1978, Mazda introduced the RX-7 with a rotary engine, which delivered 115 horsepower. This engine continued to evolve until 2003, when the last model was powered by a 275-horsepower rotary. RX-8 engines were then introduced and used through 2012. The biggest drawbacks to these rotary engines were their relatively poor fuel economy and exhaust emissions compared to traditional piston type engines.
Mazda holds a small part of the North American market at 1.7 percent of car and light truck sales, Automotive News reports. That puts them on par with BMW, Volkswagen and Audi. By contrast, General Motors has 16.6 percent, Ford 15.1 percent, Toyota 14 percent, Chrysler 12.2 percent and Nissan 9.6 percent. The OEM hopes to make Skyactiv-X its next push into a greater segment of the marketplace.
Mazda is not alone in researching the possibility of homogenous composition compression ignition for gasoline engines. GM went public with their HCCI engine concept in 2007, but the technology is not yet commercially produced. Forbes reported last year that Daimler, Honda, Nissan and others have also been experimenting with the technology.
How It Works
Compression ignition gets more useable power from the combustion process and, through the proper mix of fuel and oxidizer (in this case, air), better fuel economy and reduced emissions. Mazdas spark-controlled compression-ignition (SPCCI) engine embodies the well-known technology of HCCI, with some interesting wrinkles.
When we look at modern engines, there are only two widely used fuels: gasoline and diesel. Gasoline-fueled engines have historically been spark-ignited, which results in the fuel-air mix being burned along a flame front. Diesel-fueled engines use compression ignition, which heats the air by compressing it and then ignites a fuel charge by injecting it at high pressure into the hot air mix. Gasoline-fueled engines average about 25 percent efficiency in converting the energy produced into useable power. Diesel engines are around 40 percent efficient.
Mazdas innovation is to combine the best features of both spark- and compression-ignition engines. The SPCCI engine uses a traditional spark plug to begin the combustion process, using the pressure rise from the resulting flame kernel to trigger compression ignition in the remainder of the cylinder, its website explains.
In a video series on YouTube, mechanical engineer Jason Fenske reviews the advantages of HCCI engines. In these videos, which Mazda used in its announcements, he shows that an HCCI engine can capture as much as 55 percent of the useful work in the fuel.
Not only is there more useful work derived, measured as fuel economy, Fenske continues, emissions are also reduced. Fuel efficiency is achieved by the very lean air-to-fuel ratio (18:1) of an HCCI engine versus a gasoline fueled, spark-ignited engine (14:1). In fact, Mazda says their SPCCI engine can operate at up to a 30:1 ratio. The equivalence ratio is essentially the actual air-to-fuel ratio compared to the ideal air-to-fuel ratio. The higher the equivalence ratio, the leaner the air-fuel mixture and the better the fuel economy.
The low-temperature combustion process also results in emissions being significantly reduced, especially nitrogen oxides. Control of emissions could result in a reduction in the complexity of emissions systems. Particulates would not be a significant problem, and NOx sensors might not be required. Catalytic converters would still be required but possibly would be simpler.
Exhaust gas recirculation could actually be used as a means to control combustion chamber temperatures, since the operation of an HCCI engine depends on controlling the combustion temperature. Fenske explains that it is necessary to keep the temperature within a relatively narrow range in order to maximize the benefits of such engines.
This is where the engine operation becomes tricky. He points out that, in order to maintain the combustion chamber gas temperature, some sophisticated technologies are required. If the temperature is too low, the compression-ignition process will not occur. That means a spark plug must be included for start-up and low-temperature operation. Fortunately, spark ignition is old hat to engine designers.
On the other hand, if high-temperature operation is encountered, it will be necessary to cool things down. Fenske suggests that could be addressed with a type of variable valve timing to allow for some exhaust gas to remain in the combustion chamber, partially quenching the next combustion event. Variable valve timing is already available, so that, too, will be possible.
Since additional air must be introduced to achieve the high compression ratios, it stands to reason that superchargers or turbochargers will be required. Again, these are known technologies and their use is well established.
Effects on Lubes
The question of engine oil and what, if any, modifications would be required to meet the needs of SPCCI engines is an important consideration for the lubricants industry. With the delay in developing the ILSAC GF-6 passenger car engine oil specification and the rush to create an interim API SN Plus specification, there are bound to be questions about available engine oil quality. Some modifications may be necessary for oils used in SPCCI engines.
The preferred viscosity of engine oils has been going down for some time. Since oil viscosity for the newest conventional engine designs is now at levels approaching SAE 0W-16 and even lower, that will likely be the range of the oil for Mazdas engine.
Given that API Group II and Group III are now the standard base stocks for top-of-the-line engine oils, it is reasonable to assume that will be the case for oils meeting SPCCI needs. In fact, Group IV polyalphaolefin base oils could be a part of this mix.
Additive chemistry could undergo some modification. While it is not clear what happens in the engine with a modified combustion process, the oils could lean toward an API CK-4 type heavy-duty diesel engine oil chemistry, although GF-5 specifications will still drive the composition.
Other possible characteristics of these engine oils:
There might be a requirement for slightly higher phosphorus content due to the high compression ratios, which will likely increase the stress on connecting rod and wrist pin bearing surfaces.
The lack of soot formation could ease the requirements for dispersancy to levels closer to GF-5 specifications.
Oxidation control will be important due to sustained heat and possible blow-by gases entering the crankcase.
Deposit control will be important due to the need to maintain good piston-ring freedom. Deposits can cause piston rings to stick.
Given that fuel economy is improved through engine design and operation, friction modification may become less critical.
Viscosity modifiers may need to be more shear-resistant due to the type of forces applied during engine operation.
In the end, there are pros and cons of compression-ignition engines. Advantages include:
Lean combustion that returns a 20 to 30 percent increase in fuel efficiency over a conventional spark-ignition engine, according to Mazda;
Cleaner combustion and lower emissions (especially NOx) than a conventional spark-ignition engine;
Compatibility with gasoline as well as E85 (ethanol) fuel;
Quicker fuel burn at lower temperatures, reducing heat energy loss compared to a conventional gasoline engine;
Throttleless induction system that eliminates frictional pumping losses incurred in traditional throttle body spark engines.
High cylinder pressures that require stronger, more expensive engine construction;
More limited power range than a conventional spark engine;
Many phases of combustion characteristics, which are difficult and more expensive to control.
The technological advances in the auto industry over the last 50 years, in response to the call for better fuel economy and reduced emissions, include among others the widespread use of fuel injection, turbochargers and superchargers, resulting in increased horsepower per cubic inch. Of course, the advent of on-board computers has revolutionized the entire process. Any doubts about the future of the internal combustion engine as a primary power source need to be weighed against the history of continuous improvement and the massive infrastructure of the automotive industry that supports it. The Mazda Skyactiv-X engine is just the latest development, and its certain that there will be additional innovations as the market requires them.