Achates Power, the developer of a two-stroke, compression-ignition (CI) opposed-piston (OP) engine (earlier post) is now “concept-ready”, David Johnson, Achates Power CEO, told GCC in an interview at SAE 2011 World Congress in Detroit. Currently, Achates has demonstrated a single-cylinder CI OP two-stroke engine that matches the engine-out emissions of an advanced 2010 medium-duty diesel while achieving specific oil consumption of less than 0.1% of fuel over the majority of the operating range.
Achates has been running single-cylinder research engines in its lab since 2005, racking up some 1,700 hours, Johnson said, noting that the company has demonstrated all of the attributes of the engine, including the ability to meet emissions requirements and controlling oil consumption. “We’re ready now to develop multi-cylinder engines for demonstration against other multi-cylinder engines in applications,” Johnson said.
We’re now at that phase where we have something to say. We can present the data that demonstrates that our solution is real. To do in an opposed-piston engine, and particularly a two-stroke compression-ignition engine—it’s like hard, hard and hard—to do with that formula what is the price of entry in the marketplace today, which is to meet emissions. We’ve had breakthroughs in the past year to 18 months that have allowed us to demonstrate that basically for any emissions standard around the world...we can meet emissions, and not just at one point where it is a novelty of science, but across the entire speed map.
We can do it [meet emissions requirements] a number of different ways, and now you’re talking about the business. We’ve done studies to do it in-cylinder, and we’ve done studies to do it with aftertreatment. When we do that, then the thing is how much fuel efficiency can we demonstrate versus a relevant comparator.—David Johnson
Johnson said that currently, Achates can deliver a 12-16% reduction in fuel consumption compared to the best diesel, and about 50% against gasoline, and with a cost and size reduction. Those are “development-type” numbers, he said, noting that there is still opportunity in that.
The Achates engine. As part of its emergence from stealth mode, Achates is beginning to publish. In January, Johnson presented a paper at the Symposium on International Automotive Technology (SIAT 2011) in India that begins to provide more technical detail behind the engine as well as emissions and fuel consumption test results.
The opposed-piston, two-stroke diesel engine has a long history, and has set records for fuel efficiency and power density; one of the largest obstacles facing commercialization of such an architecture recently has been the advent of modern emissions regulations.
Since the OP arrangement has no cylinder head, the fuel injector must be installed in the cylinder liner. Historically, this has created a technical challenge compared to the common crank-slider arrangement with the fuel injector located in the center of the cylinder head. The large distances in the fuel-spray direction (i.e. across the diameter of the cylinder) combined with low fuel injection pressure made accessing all of the available air in the combustion chamber difficult, resulting in inefficient combustion with relatively high NOx and soot formation.
Additionally, the interaction between the fuel spray and the in-cylinder fresh-charge motion with traditionally high swirl resulted in combustion occuring near the combustion-chamber surfaces causing increased thermal loading of the piston, piston rings, and cylinder liner, and leading to reduced thermal efficiency and increased cooling requirements.
Thanks to modern development tools and advanced fuel systems, the OP arrangement with a liner-mounted injector has turned from a technical challenge into a unique opportunity.—Johnson et al.
The Achates single-cylinder research engine described in the SIAT paper has a trapped compression ratio of 17.4, a bore of 80mm, and a stroke of 212.8mm, resulting in a displaced volume of 1.06L.
The fixed liner geometry creates fixed swirl ratio and port timings, and the piston geometry and injection spray pattern has been specified based on combustion simulation results. A common-rail fuel injection system supports injection pressures up to 2000 bar, and can deliver multiple injection events per engine cycle. Maximum cylinder pressure is limited to 160 bar, and the maximum liner temperature is limited to 200 °C.
An external air compressor feeds compressed air to the conditioning unit where it is mixed with exhaust gas. An EGR pump, required because of the necessary pressure difference across the cylinder, pulls the exhaust through a gas-to-water heat exchanger before delivering the gas to the intake stream. The EGR rate is controlled by the EGR pump speed and a ball valve located downstream of the pump. The combined intake gas flows through a second heat exchanger followed by a heater to precisely control the intake manifold temperature.
Opposed-piston two-stroke engines breathe through ports on both ends of the cylinder liner, implying that the compression rings must traverse the ports. To achieve a low level of oil consumption, the oil control rings in the Achates engine are not mounted on the piston, but are stationary in the liner, and wipe the piston skirt. This method required the use of sulfur-free fuel and lubricating oil with known sulfur content. (The sulfur-free fuel and lubricating oil with known sulfur content was used to measure oil consumption in the development process. Production engines will use standard and widely available fuel and oil.) Fuel-specific oil consumption is below 0.1% across a large portion of the operating map.
Testing reported in the paper showed that Euro IV NOx emissions are achievable in-cylinder, obviating the need for aftertreatment in certain markets. SCR aftertreatment can be fitted with further fuel efficiency benefits and lower NOx levels at the tailpipe.
Testing reported also shoed fuel consumption for this particular engine calibration of 13% lower than a 2010 medium-duty diesel at the same engine-out NOx levels. A cost comparison showed an 11% cost improvement for the OP engine compared to a four-stroke diesel of equivalent power.
Advanced technologies. Just prior to the SAE World Congress, Johnson and colleagues from Achates had attended the SAE 2011 High Efficiency IC Engines Symposium, also in Detroit.
One thing that I enjoyed about the conference over the last two days, is that when I thought about each of the new technologies, almost all of them were complementary...additive...to what we are doing. If you think about Rolf Reitz from the University of Wisconsin, and his reactivity controlled compression ignition [RCCI, earlier post], this is a dual fuel way, basically the industry has looked at the academic studies around dual fuel and said, really cool. I understand, I get it, but man, you’re really blowing my mind in terms of actually implementing this. But if you think about that, and I think he made some pretty compelling arguments, the thing about our technology, because we have these opposed pistons, we can put multiple fuel injectors around the cylinder much easier than a conventional engine can put multiple injectors in the cylinder head. So let’s say we develop that technology...I think we can apply it more easily in our engine. So sure, if that offers 3% more or 5% more, sure, bring it on.—David Johnson
David Johnson, Michael Wahl, Fabien Redon, Eric Dion, Shauna McIntrye, Gerhard Regner and Randy Herold (2011) Opposed-Piston Two-Stroke Diesel Engine–A Renaissance (SIAT-2011)