Study of opposed piston engine

The concept of an opposed piston engine has existed for the better part of a century. It’s fairly simple to follow. Instead of a cylinder head, the piston head shares its combustion chamber with another piston head. Theoretically, an opposed piston engine runs rings around a conventional engine when it comes to efficiency and it shows. In April of 1904, a French automotive company called Gobron-Brillie tested a car powered by an opposed piston engine and clocked a maximum speed of 152.5 km/h. To put that into perspective, most of the cars we see these days would struggle to reach that speed. Gobron-Brillie set a world record that day, only to break it again a few months later clocking a top speed of 161 km/h. All these numbers surrounding opposed piston engines seem too good to be true, but they aren’t. Clearly Achates Power USA and Calstart think so too, because they are hard at work at reviving what will most probably be the next evolution of the internal combustion engine.

Founded by theoretical physicist Dr. James Lemke in 2004, Achates Power works with engine manufacturing companies such as Cummins and Caterpillar as a design and tool licenser for new engine technologies. Achates started out developing opposed piston engines for heavy duty vehicles. Dr. Lemke believed that with modern fluid computation and engine simulation software, the OP engine can be further optimised for even greater efficiency. Achates has impressively achieved this but to understand how they did it, the inner workings of the OP engine must be known. And to truly appreciate its brilliance, the working of a conventional engine must also be known.

When it comes down to brass tacks, today’s engines work with the help of 4 basic components – piston, valves, spark plug, and crankshaft. These 4 components perform 4 sequential movements or strokes to produce power – intake, compression, combustion, exhaust.

During the intake stroke, the piston moves from its topmost position (top dead centre or TDC) to its lowermost position (bottom dead centre or BDC) and the intake valve opens up to induct a charge of air and fuel into the combustion chamber. The crankshaft angle is at 180 deg at the end of this stroke.

During compression, the piston then moves up from BDC to TDC, thereby compressing the charge inside the chamber and greatly increasing its temperature and pressure. At this point the crankshaft has revolved 360 deg.

As the piston reaches TDC and the charge is almost at its self-ignition temperature, the spark plug lets off a high voltage spark (north of 22000 V) and combusts the charge causing a controlled explosion that pushes the piston down towards BDC. Theoretically, this spark fires when the piston is at TDC. However, due to the miniscule delay in the propagation of the flame front, the spark is fired slightly earlier so that it perfectly hits the piston head as soon as it reaches TDC. This is called ignition timing and it’s a vital part of engine tuning. Anyway, due to the piston being forced down to BDC, the crankshaft also spins faster and this is what creates power. The crank angle at the end of this stroke is 540 deg.

After the piston reaches BDC, it starts to go back up again and the exhaust valve opens up to vent out the remains of the explosion, aided by the piston as it travels upward and also by the fresh charge that the intake valve inducts into the chamber (this is called scavenging) as the cycle perpetually repeats and the crankshaft completes 2 full revolutions i.e. 720 deg.

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These are the very basics of a 4 stroke spark ignition engine. For compression ignition aka diesel engines, the process is similar except instead of a spark plug, it has a diesel injector and the intake valve only inducts air which is compressed to its self-ignition temperature. The diesel is injected into the chamber as the air just begins to ignite. That’s how power is developed in a compression ignition engine.

Another vital part of the conventional engine is the camshaft. This component operates the intake and exhaust valves and is directly linked to the crankshaft via a timing belt. The camshaft has small oblong lobes or ‘followers’ directly milled onto it which push the valve mechanism open as the camshaft revolves. The camshaft rotates once for every 2 revolutions of the crankshaft, which means that over the course of a single power cycle, the crankshaft rotates 720 deg whereas the camshaft rotates 360 deg.

The terms SOHC and DOHC are often listed in car brochures. SOHC means Single Over Head Camshaft and DOHC means Dual Over Head Camshaft. These terms represent the number of camshafts in an engine. Modern engines utilize the DOHC setup which means they have 4 valves per cylinder. Having more valves means more charge in the cylinder which results in more power, higher volumetric efficiency, and better scavenging which is great for overall efficiency.

There is one more interesting aspect of engine tuning that is related to the camshaft. Valve timing is one of the biggest make-or-break aspects of an engine’s efficiency. Put simply, valve timing is the alteration of the cam angle at which the intake and exhaust valves open. This is extremely important to the functioning of the engine. If the timing of the valves is even 1 degree off, it could severely impact the efficiency and in some cases even the durability of the engine. In more extreme cases, if the intake valve is timed incorrectly and it remains open as the piston approaches TDC, it could break the valve head and make it fall in the cylinder, effectively rendering the entire engine unusable. Just another example of how incredibly tight and complex the mechanism of an engine is. Given below is a typical valve timing diagram for a 4 stroke SOHC engine.

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Now that we know the working of a conventional engine, it’s time to dive into the opposed piston 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. The advantage of a mechanism like this is that you would save a lot of space that would’ve otherwise been wasted if you the pistons were in separate cylinders. Additionally, the OP engine does not require a camshaft or valves as demonstrated by the Achates 2.7L unit.

Image credit: Achates Power

Image credit: Achates Power

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 360 deg rotation of the crankshaft, it is technically a 2-stroke engine. However, unlike the 2-strokes we know, it’s actually much more efficient than a 4–stroke.

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 table given below provides some basic performance and efficiency figures from Achates’ 2.7L engine across a variety of loads. The BMEP numbers are especially interesting. The current highest BMEP for a commercial vehicle belongs to the Ferrari 458’s 4.5L V8 with a maximum of 14 bar@9000 rpm. And here the Achates engine hits an astonishing 12.8 bar at full load in the first test, which is even more impressive when you consider that the tech is experimental and still in its early days.

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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.

Reliability is also a big plus point in the OP engine’s favour. The absence of valves, camshaft, and all the bearings and sensors associated with them means there’s fewer parts to break or malfunction. Moreover, because the cylinder pressure doesn’t need to be as high as in a conventional engine, the possibility of knocking or pre-ignition are also greatly reduced.

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. That’s one problem the OP engine won’t face because it will have significantly higher overhead for cylinder pressure, so it won’t get hot enough to allow a second flame front to ignite within.

Coming to pre-ignition; this is similar to knocking but the key difference is that it takes place before the spark plug fires i.e. during the compression stroke. 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. Moreover, because flame front doesn’t uniformly propagate, the charge doesn’t combust properly which means fuel is wasted. Pre-ignition is very serious and can blow a hole through the piston head if not fixed in time. Yet another issue the OP engine won’t deal with.

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.

The flaw comes in the form of the engine’s flexibility and scaling i.e. the impact that different layouts and variations will have on the efficiency of the engine. 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.

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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.

That’s a far cry from the sort of diversity we’ve come to expect from conventional engines, but this is the cost of pioneering new technology. I have no doubt that with time these issues will be ironed out not only by Achates, but a large number of automobile manufacturers. As of 2017, 9 manufacturers have pledged to look into opposed piston technology for integration into their lineup of vehicles, and one manufacturer has even began the tooling process to start building OP engines.

Achates Power meanwhile are already dipping their toes into the market. Their latest 2.7L turbocharged 3-cylinder engine is currently seeing use in a number of medium and heavy duty commercial vehicles travelling across the USA. Achates claim that their engines can effectively halve the fuel bill for truck companies and emit upto 90% fewer NOx and CO2 emissions with 55% overall efficiency. In fact, the engine is so efficient that it meets the EPA’s 2027 emission regulations! That’s absolutely unbelievable for a diesel engine. Moreover, Achates have contracts with the US military to supply OP engines for their vehicles and in 2018, they tested a Ford F-150 installed with their engine which showed promising results. Achates is working their hardest to drive up adoption for OP engines and there’s a very good chance that they turn out to be the saving grace for internal combustion in the future. In the very likely event where that happens, there is only one person to thank – Dr. James Lemke.