A New 10.6L Two-Stroke Diesel Engine Has 3 Cylinders And 6 Pistons
Walmart is testing an opposed-piston diesel engine with the intention of replacing its more conventional 13 and 15-litre four-stroke units
We love ourselves an unusual engine here at CT, regardless of the intended application. That’s why a few weeks back we took a look at the world’s first V12 outboard engine, and today, we want to talk about something designed to propel a lorry.
It’s no ordinary power plant, however – this unit from Achates Power is one that sounds increasingly obscure the more you read the description. It’s a 10.6-litre, two-stroke, three-cylinder, six-piston diesel engine. You read that correctly – six pistons in three cylinders since this is an opposed-piston engine.
Unlike a horizontally-opposed ‘boxer’ engine, which mounts the pistons pointing outwards at 180 degrees in separate cylinders, an opposed engine involves a pair of pistons facing each other and sharing a cylinder. Each piston very nearly meets in the middle at top dead centre, at which point ignition sends both back to bottom dead centre. Two crankshafts at either end are linked via a set of gears, providing power to the transmission.
In theory, this is great. Normally, a load of energy is wasted through the cylinder head, but in an opposed-piston engine, it’s shared between the pistons with more minimal losses. Instead of a cylinder head, there are gaps on one side of the engine for exhaust gas flow and some at the opposite end for intake air. Ditching the head also means fewer moving parts, potentially increasing reliability and decreasing manufacturing costs.
This isn’t a new concept – two-stroke opposed-piston engines were about years ago, but it’s only now that a comeback has been proposed. Achates’ ‘OP’ engine isn’t just an idea, either. A 2.7-litre prototype powering a Ford F-150 pick-up laid the groundwork, and from July in California, Walmart will be testing the 10.6-litre version in a Peterbilt 579. John T. Walton, the late Walmart heir, co-founded Achates with physicist James Lemke in 2004.
One inevitable downside of stacking pistons on top of each other is how tall the package ends up being, but according to Achates, this won’t be much of an issue in big Class 8 trucks. All being well, the OP will be a cost-effective alternative to upgrading existing 13 and 15-litre four-stroke engines for stringent Environmental Protection Agency (EPA) rules due to come into force in 2027.
It was announced late last year that the OP will comply with the new regulations. Currently, Achates says its testing has shown a seven per cent drop in CO2 emissions and a whopping 96 per cent NOx reduction compared to a conventional lorry engine. Despite this, the 10.6-litre engine used in Walmart’s demonstration vehicle is good for around 400bhp and 1674lb ft of torque.
Fully electric lorries, of course, would be greener still, but given the hurdles there in terms of range/battery technology, something like the OP could be the perfect interim solution. It’ll be fascinating to see how this pans out.