Why could Diesel engines based on a LiquidPiston project actually be a good idea?

Diesel engines have been a mainstay of trucking for decades, and some manufacturers such as Scania used to keep traditional designs and a higher degree of modularity across different generations of their engines. Obviously many technical improvements lead to a distancing between older and newer models, including aspects pertaining to the engine itself and other features of a vehicle as a whole. Considering how long-nose cabins gave way for flat-nose ones in most markets, which eventually makes it harder to overhaul a wet-sleeve engine without removing it from the frame, going quite radical such as resorting to a rotary engine might sound more tempting, yet the Wankel rotaries usually have a poor compression ratio only suitable for spark-ignition and volatile or gaseous fuels. So, maybe a Diesel engine developed following some principles of the LiquidPiston projects may be a better option, even though any chance to succeed commercially would need to go beyond the single-rotor proof-of-concepts to venture into the dual-rotor layout at least, yet a triple-rotor might also be desirable to keep up with current standards for modularity seen on reciprocating engines, even though for heavy-duty trucks they tend to be straight-6.

Just like Ford resorted to Cummins engines of the B series in 4-cylinder versions ranging from 3.9L to 4.5L and 6-cylinders around 5.9L and 6.7L in most Brazilian versions of the Ford Cargo, so modularity was already a key factor, also considering how some degree of parts interchangeability between engines with different amount of cylinders makes it easier pertaining to maintenance, inherently simpler designs devouted to rotary engines may take one step further. Even though engine blocks and cylinder heads for inline engines can be made basically with the same tooling, yet it may not be so easy to simply chop the block and cylinder head of an engine with more cylinder barrels to fit to the crankcase of a similar one with fewer cylinders or adding more cylinders to a larger crankcase, an increased modularity inherent to rotary engines by simply adding or removing one more rotor housing and intermediate plate according to the need like Mazda did to the 2-rotor 13B and 3-rotor 20B engines on road-legal models, and even a 4-rotor Le Mans racing engine, may render it somewhat easy to apply LiquidPiston designs on a Diesel engines range. Of course some parts will still be specific according to the amount of cylinders or rotors in an engine, such as intake and exhaust manifolds, and the crankshaft for a conventional engine or the eccentric shaft for a rotary, while rotaries not featuring a valvetrain may still have a lower total amount of parts and noticeably getting rid of a camshaft.

While the amount of cylinders used to vary more often according to the gross vehicle weight rating of a vehicle, and so a 4-cylinder would be seen as inherently better suited to a minibus and articulated buses would be supposed to always have to resort to a 6-cylinder, nowadays some power ratings above 200hp are also covered by 4-cylinder engines in regions where stricter emission rules favored the downsizing, even though modularity continues to play a pivotal role for commercial vehicle engines and sometimes greater power rating are achieved with the smaller versions within the range of the predecessor engine. Improved efficiency of turbocharging technologies to overcome turbo-lag and a more accurate control of the electronic fuel injections (usually common-rail, yet unit-injector setups still hold their fair market share too) made a difference, even in a conservative market such as Brazil where a 4-cylinder engine is favored because the cost of replacement injectors for electronically-controlled ones is proportionately higher than for their mechanically-governed counterparts. Considering a LiquidPiston engine will have 3 injectors per rotor housing, maybe at a first glance the cost-effectiveness can be brought into question, since a 2-rotor engine would overcome the perceived economy once downsizing took place.

When it comes to long-distance buses/coaches, eventually the promised compactness of a LiquidPiston engine design proportionate to power ratings might be a good option for the double-deckers which are fairly common in the South and Southeast regions of Brazil and also in neighboring countries such as Argentina and Paraguay, which are usually fitted with 6-cylinder engines around a 12-litre displacement range. Sure a difference between such engines with a larger displacement and others with the very same amount of cylinders, which may be capable of reaching similar power ratings at a higher RPM requiring a shorter differential ratio to provide a similar performance, would still be favorable to the perception of an increased durability under such operating conditions, yet it may already lead to a comparison with the LiquidPiston engines being pointed out to achieve performance levels suitable to replace engines up to 5 times their displacement, considering the 2-stroke LiquidPiston XTS-210 design compared to some conventional 4-stroke Diesel engine. Well, as long as the rotors and the eccentric shaft don't reach sonic speed, and how Wankel engines such as the Mazda 13B are pointed out to be much more rev-happy in order to prevent carbon deposits which could harm the apex seals, despite LiquidPiston engines having substantial differences and the inherently lower compression of the Wankel designs being unsuitable to a Diesel engine, the space constraints caused by the location of the air conditioning and the engine too close to the main luggage compartment would be very compelling to a LiquidPiston-designed engine in order to improve luggage capacity, among other advantages pertaining to overall fuel-efficiency.

A more compact engine layout may be specially relevant in South America where front-engined buses are still fairly common, for both urban transit buses and long-distance coaches, and the stricter emission regulations such as the recently-enforced Euro-6 increase the complexity and physical volume taken by aftertreatment devices, which also tend to be installed remotely from the engine and causing clearance issues with other parts of the vehicle. It may not be much of an issue for urban transit buses, despite the requirement for a lift to comply with the Brazilian accessibility regulations for wheelchair users as most front-engined buses have no provision for a stepless entry, while the underfloor space tends to be taken little to no advantage of, in contrast to coaches on which some valuable underfloor space is already too compromised by the frame rails preventing the fitment of pass-through luggage compartments, and the accessibility regulations also charge their toll on newer models. Having to manage one more clearance issue due to the fitment of a more complex aftertreatment module might be another aspect to consideer in favor of a LiquidPiston engine, because the inherently smaller packaging could free up some space to bring the aftertreatment module closer to the engine bay, which may also be advantageous considering a more direct exposure to the exhaust heat which could lead to an improved efficiency and durability of the particulate filter (DPF) and oxidation catalyst (DOC).

Even though some folks may consider Diesel engines somewhat "superfluous" for private vehicles due to the high complexity of modern aftertreatment devices, they may still retain a foothold on commercial vehicles for a long time because factors such as a much better fuel-efficiency and reliability under the most demanding conditions. While professional operators may have a conservative profile and prefer to invest on solutions already tried and tested, with some drivers and fleet managers still taking 4-cylinder engines on medium-duty trucks with a grain of salt after some 20 years for instance, the ever-increasing strictness of emission regulations may open the doors for some technical solutions which may take one step further, such as resorting to a definitely unorthodox LiquidPiston-based design for instance. In the end, as fully-electric vehicles are proving themselves troublesome time and time again despite all those political attempts to ban internal-combustion engines in general, the suitability of LiquidPiston designs to the operating principles of a Diesel engine may become a reason to give them a chance...