COVER STORY
“Many alternative fuels, like ethanol, methanol and hydrogen can be operated in a low or zero carbon manner, but these high-reactivity fuels do not work well with compression ignition because of their high auto ignition temperatures. Consequently, combustion of low reactivity fuels in conventional engines typically requires spark ignition, which is less efficient than compression ignition. The OP engine has a distinct advantage in operating with low reactivity fuels, including hydrogen – and we can operate with efficient compression ignition across the full operating range of the engine. The reason for this is that our OP engine design has much greater control of trapped conditions, including the portion of residual exhaust gas remaining in the cylinder after the combustion event, and therefore trapped temperatures. This means the OP engine will have both greater power density and greater efficiency than conventional engines when operating
with low reactivity fuels. When operating on diesel fuel, it has demonstrated the ability to reduced tailpipe NOx by 90% and CO2
by more than 10% with no
additional emissions control technology, and therefore no increase in cost, complexity, or compliance risk compared with today’s engines. So, the engine architecture has important advantages compared with conventional fuels. And because alternative, low-carbon fuels are more expensive than petroleum-based fuels, this improved efficiency can help drive adoption, so we view this as a forever engine.”
THE NEXT GENERATION To elaborate on what happened next, we talk to Tobias Burek, Chief Engineer at Ricardo. “While modern four-stroke engines
can now surpass OP power density, the fact remains that OP can be very fuel efficient due to the reduced heat transfer. Having two pistons in each cylinder reduces the combustion
The Achates OP engine on the test bed
chamber surface area, resulting in more energy converted to useful torque with a wide efficiency map – the engine operates in a high-efficiency zone for more of its range. Also, OP is very fuel agnostic and high residual heat in the cylinder facilitates compression ignition, which allows us to burn gasoline without a spark.” The interest in gasoline is due to the
US Government’s Advanced Research Projects Agency-Energy (ARPA-E), the body which provided the funding. Burek continues, “A lot of these full-size pickup trucks in the US run on gasoline, so the motivation is to see how efficiently you can burn this as a fuel.” For delivery, a modified high-
pressure common rail diesel system is used, with twin injectors in the middle of the cylinders. On the other side of combustion, there is a Formula One-inspired electrically assisted turbocharger, complete with 48V regen. So, it’s an opposed piston, two-stroke,
The OP engine has an advantage in operating with low reactivity fuels
gasoline compression ignition hybrid engine. “There’s four levels of novelty!” notes Burek. “It’s a unique engine, especially considering some of the other tech that was developed and integrated. Efficiency, weight, emissions and power have been the themes of this project. Taking efficiency as an example we’ve done some new things such as using 3D printed Inconel pistons (a desirable material due to the high pressures) which enabled us to redesign the cooling gallery of the piston to improve oil flow by 29 per cent over the previous iteration of the engine. We also have fully variable piston cooling jet control. This in turn means we can fit a smaller variable oil pump, improving overall
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