How does an opposed piston engine work

Given below is a typical valve timing diagram for a 4 stroke SOHC engine. As stated before, the idea behind the opposed piston engine is to merge two cylinders into one by having the pistons heads move opposite to each other, both acting as the cylinder head for each other.

Additionally, the OP engine does not require a camshaft or valves as demonstrated by the Achates 2. Instead of a camshaft, the OP engine has another crankshaft for the upper set of pistons which routes its power through a large gear to the main crankshaft located at the bottom. The lack of valves is also noticeable. Unlike the conventional engine, the OP engine utilises old-fashioned ports like the 2-stroke engines of the past.

This means that the assembly of an OP engine is simpler and requires fewer parts, which also bodes well for overall reliability. Speaking of which, since the OP engine develops power in every deg rotation of the crankshaft, it is technically a 2-stroke engine. In a conventional engine, if you want to fit 6 pistons you will need 6 cylinders.

That means every cylinder will require cooling and lubrication and will also serve as a hot spot inside the engine.

In the case of an OP engine, that problem is immediately halved since 2 pistons share the same cylinder.

That means fewer galleys for engine oil, fewer water jackets for coolant, and fewer hot spots in the engine, which translates to simpler, sturdier fabrication and higher thermal efficiency. Additionally, since the combustion chamber volume is effectively doubled, a higher amount of charge can also be inducted into the engine which means more power, better scavenging, and much higher volumetric efficiency.

Since the charge will burn better, that also means lower amounts of CO2 and NOx emissions, the importance of which cannot be stressed upon enough in these times. The BMEP numbers are especially interesting.

And here the Achates engine hits an astonishing The issue of harsh NVH levels has plagued modern engines, especially 3 cylinder ones due to the uneven forces exerted on to the crankshaft by the pistons.

Granted, a conventional 4-cylinder engine does not have this problem and some engine variations like the boxer eliminate it completely. However, the OP engine manages to take this even further by having stable NVH levels in a 3 cylinder, 6 piston configuration.

Since the pistons opposing each other have separate crankshafts to exert force onto, the opposing forces are cancelled out and the engine does not violently thrash at idle revs like the usual 3 cylinder variety. Put simply, knocking or detonation happens when the cylinder temperature and pressure are extremely high which causes a secondary flame front to propagate after the spark plug fires and the pressure waves of both fronts collide.

This creates an audible metallic pinging sound that can be heard while both idling and driving. Knocking can severely damage the engine. Commonly, it causes cracks to appear in the cylinder walls which can leak coolant and engine oil into the chamber, and if that happens then the engine is basically scrap.

Coming to pre-ignition; this is similar to knocking but the key difference is that it takes place before the spark plug fires i. Like knocking, it happens due to extremely high temperature and pressure inside the cylinder.

Usually what happens is a stray carbon particle remains in the chamber from the previous cycle and automatically ignites due to high temps as the piston approaches TDC. This means the piston is prematurely pushed down which exerts excessive pressure on it as well as the crankshaft. Pre-ignition is very serious and can blow a hole through the piston head if not fixed in time. The advantages of the OP engine have been discussed in length, but nothing is ever perfect, and the OP engine does have a potentially big flaw.

A conventional engine has a plethora of variations spanning across a diverse range of components. A 4-stroke engine can have 1, 2, 3, 4, 5, 6, 8, 10, 12, and even 16 cylinders. It can use petrol, diesel, ethanol, and even hydrogen as fuel.

The crankshaft and pistons can be arranged in radial, flat, V, straight, inline, X, and W formations. All these configurations can be mixed and matched together and every single permutation and combination impacts the efficiency of the engine. Unfortunately, in the case of OP engines, 2-cylinder and 4-cylinder configurations are instantly ruled out since for 2 cylinders there is too big of a gap between power delivery in each cycle and for 4 cylinders there is too much overlap.

This means that the ideal configuration currently for the OP engine is 3-cylinder, 6-piston. Keeping the above graph in mind, multiples of 3 configurations could also work, but I have no idea whether that would increase the efficiency or offer diminishing returns.

Crankshaft and piston formations are also out of the equation since it would upset the 3-cylinder configuration and be detrimental towards efficiency. And to make matters even sourer, the only fuel type available for the OP engine currently is diesel. Achates has taken this basic design and improved the combustion characteristics, while also providing the ability to tailor the engine's operation to match different conditions and fuels.

Much of the development has focused on diesel versions, but the company is also working on gasoline versions without spark plugs— the HCCI Homogeneous Charge Compression Ignition concept that many are developing but no one has perfected. Theory suggests that an opposed-piston engine has an efficiency advantage because, since there are no cylinder heads, less heat is lost to the cooling system.

Think about a conventional combustion chamber in which the surfaces consist of a piston crown, a short cylindrical circumference, and a cylinder head. In the OP design, each pair of cylinders comes together, so the heads are eliminated. Furthermore, because each piston's combustion chamber connects with another's, the resulting chamber is thicker and less like a thin disc. Achates increases this advantage by using a stroke as much as 1.

Achates carefully shapes the piston crowns to provide swirl and tumble to the intake charge to promote rapid and stable combustion. Two opposing direct injectors—each delivering multiple carefully timed squirts—achieve a thorough distribution of fuel to burn most of the air and to limit particulate generation.

A compression ratio of around The secret sauce to making this work is the ability to control the scavenging in the engine. The supercharger, which provides low-pressure air to scavenge the cylinders, has a two-speed drive and can also be bypassed.

This flexibility allows the cylinder to be less than fully scavenged under certain conditions. For example, during a cold start, partial scavenging leaves plenty of combustion products from the previous cycle internal exhaust-gas recirculation in the cylinder.

That raises the combustion temperature and delivers a hotter exhaust stream to get the catalysts up to operating temperature. Such a hotter combustion environment also helps combustion at low rpm and under light loads, which has been a problem area for conventional HCCI engines. At idle, for example, the retained exhaust gas is about 50 percent. At higher power outputs, the cylinder is more completely scavenged to keep peak combustion temperatures down, thereby limiting nitrogen-oxide NOx emissions.

Also helpful is that, while the engine also has a turbocharger to take over from the supercharger at higher loads and to improve efficiency, the engine does not operate with much boost pressure. Instead the two-stroke design, with twice as many power strokes per revolution, lets the engine match conventional engine power without much boost, even when it is downsized by 20 to 30 percent in displacement.

Although there is no conventional valvetrain in this engine, overall friction is similar to a conventional engine because of the second crankshaft, the robust geartrain connecting the two cranks, and the need to drive the supercharger. The advantages are low emissions thanks to the GCI and that the engine is simpler and cheaper to manufacture than a conventional counterpart, with no cylinder heads or valve trains.

In fact, during testing, the efficiency of the GCI version of the engine approached the low CO2 levels of the diesel. OPEs have been around forever, powering ships, tanks and railway locomotives since around However, this is a direct-injection engine, so the power output will be higher than that of a petrol equivalent. Hydrogen is CO2-neutral at the tailpipe, although burning it in air will still produce NOx. Under the skin: How tyres can be sustainable.

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