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Opposed piston engines have been around for years and could be making a comeback.

The dual goals of meeting tight emissions standards and improving fuel consumption have seen some engine development companies look to designs of the past for inspiration. 

‘Flat’ engines aren’t new. The most familiar is Ferdinand Porsche’s opposed-cylinder (OC) design that powered millions of Volkswagen Beetles and Kombis and lives on today in the rear ends of Porsche cars and between the legs of BMW motor cycle riders.

flat engines In the 4WD world the ‘boxer engine – so called because of the back and forth ‘punching’ action of the pistons – sits under millions of Subaru bonnets.

But the boxer engine isn’t the only type of ‘flat’ engine that’s been developed. The opposed piston (OP) engine is one in which the cylinders are through-holes in the block, with a piston at each end and no cylinder head. The crankshaft timing is arranged so that the opposed piston action creates the necessary compression for efficient combustion.

A revolutionary early application of an OP design was in the World War II Junkers Jumo 205 diesel aircraft engine.

This OP engine had two crankshafts
– one at either end of a single bank of cylinders – that were geared to the propeller shaft.

The 1940s Napier Deltic diesel engine had three crankshafts to serve three banks of double-ended cylinders arranged in a  triangle with the crankshafts at the corners, and was used in railway locomotives and to power fast patrol boats.

OP diesel designs by Fairbanks-Morse powered US submarines and diesel locomotives.

The best-known OP engine Down Under is undoubtedly the Rootes TS3 engine that powered Commer trucks from 1954 to 1972. This three-cylinder, six-piston diesel used one crankshaft linked by large rockers to the piston connecting rods. The engine had a distinctive sound, earning it the Australian nickname of ‘Commer Knocker’. flat engines

An improved four-cylinder version, the TS4, was almost ready for production when the English Rootes Group fell into Chrysler’s hands in 1968 and the TS4 prototypes were discarded in favour of more conventional diesels.

During the past 40 years it seemed that the flat engine would be limited to OC types, but a 2005 development of the OP engine surfaced in the USA, at the SAE World Congress.

This light-weight, high-efficiency diesel concept engine was developed by FEV Engine Technology for use in a broad range of military applications.

Developed in conjunction with Advanced Propulsion Technologies (APT), the engine’s innovative design combined an OC engine with an OP engine, eliminating the cylinder head, valves, camshafts and all related drive systems.

fev-apt opoc engine The diesel two-cylinder, four-piston display engine featured an opposed piston pair, connected to a crankshaft. Additional paired rods, also linked to
the crankshaft, pulled and pushed free-floating upper pistons. The moving combustion chambers were sandwiched between the piston pairs.

Piston movement, not valves, opened and shut the inlet and exhaust ports, with cylinder fill performed by an APT electrical/exhaust gas driven turbocharger.

The two-stroke cycle of the opposed-piston, opposed-cylinder (OPOC) engine used uniflow-scavenging, with intake and exhaust ports at opposite ends of the cylinders; asymmetric port timing, with the exhaust opening before intake and exhaust ports closed before intake.

(Uniflow scavenging was successfully used by Nissan Diesel in its two-stroke truck engines and even developed into their heavy truck brand name: UD, an acronym for Uniflow-scavenging Diesel.)

The FEV/ATP concept engine might have been just another engineering curiosity, but not so. The companies were asked by the US Army Tank Automotive Research Development and Engineering Center (TARDEC) to produce an engine for heavy-truck applications.

Performance indicators suggested the 465kW production engine should have a power density of 2kW/kg, a height of only 40cm and a best fuel consumption figure of 206g/kWh.

At the 2008 SAE World Congress TARDEC displayed a smaller prototype that had a claimed output of 240kW at 3800rpm and weighed only 170kg.


Commercial OP engines

In 2008, another company, EcoMotors, sprang out of the FEV/ATP ventures and was aiming to exploit non-military applications of this revolutionary engine.

The company was funded by Vinod Khosla, who founded Sun Microsystems and Microsoft’s Bill Gates, with R&D spearheaded by Peter Hofbauer, former head of engine and transmission development for Volkswagen.

In March 2015 EcoMotors announced a joint venture with a subsidiary of Chinese auto maker FAW Group, outlining plans to begin producing opposed-piston engines.

The joint venture intended to build a plant in China’s northern Shanxi province with the capacity to make 100,000 engines a year.  However, EcoMotors folded in 2017, without producing a single production engine.

Achates Power is another US company
engaged in OP engine development. The company secured its fifth OEM contract in December 2014.

These customers span different applications: passenger vehicle, light commercial vehicle, heavy commercial vehicle, military, marine and stationary power, according to David Johnson, president and CEO of Achates Power.

Interestingly, two of these customers have OP engine backgrounds: Fairbanks Morse and TARDEC.

Achates engines differ from the EcoMotors designs in having two crankshafts. The cranks are geared together and output passes through a single shaft.

Early test engines met EPA 2010, Euro 6 and Tier 3/LEV 3 emissions standards, the company said.

Some challenges that Achates Power faced included finding an effective way to reduce oil consumption; increase piston compression ring life; manage the thermal loads on the piston and liner and support 200+bar cylinder pressures at the gudgeons.

Testing of two different three-cylinder engines has said to have resulted in a 32-percent increase in thermal efficiency, compared with production medium-duty diesel engines.

The design enables an increase in volume to surface ratio within the cylinder, preventing heat losses through the engine walls. Furthermore, pumping losses are reduced because of the two-stroke design.


Achates nearer to production

At the Detroit Motor Show in February 2018, Achates Power displayed a production prototype petrol, 2.7-litre, six-piston, three-cylinder engine in a Ford F-150.

That engine  produced 200kW/650Nm and boasted an EPA combined fuel-economy rating of 37 mpg (US) – around 9L/100km.

The two-stroke Achates powerplant burns a very lean gasoline-air mixture purely through compression, giving it diesel-like characteristics and claimed thermal efficiency of 45 percent.

The company designed and developed this engine with US$9 million in funds from the US Department of Energy’s ARPA-E program, in conjunction with both Argonne National Laboratory and auto-parts maker Delphi.

In Detroit, Achates said it had formed a joint-development agreement with consultant Aramco Services, the Houston-based US subsidiary of Saudi Aramco, the national oil and gas company of Saudi Arabia.

Achates ultimately intends to license its design to vehicle and engine makers throughout the world, said CEO David Johnson, but doesn’t intend to manufacture engines itself.

The company reportedly has nine development programs underway with engine manufacturers; some of which could lead to other automotive applications.


Achates/Cummins military engines

In September 2019 the first fruit of a joint Achates/Cummins engine collaboration was displayed at a military equipment show in the UK.

This new 14.3-litre, four-cylinder, eight piston engine is part of the US Army’s diesel-powered Advanced Combat Engine initiative.

Other engines in this family are a 10.6-litre, three-cylinder, six-piston, 750hp powerplant and a six-cylinder, 12-piston, 1500hp version.

The four-cylinder version is being trialled in the Bradley 6700 fighting vehicle, with around 1000hp and 3253Nm.

A commercial version of the three-cylinder model is being tested for series production in road-going heavy trucks. It is reported that output is 335kW (450hp) and peak torque is 2372Nm.

Achates/Cummins said this engine offers a 15-percent reduction in CO2, compared with the California standards for diesel engines and a 90-percent drop in NOx emissions, compared with US federal standards.






























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