The Search for Increased Engine Thermal Efficiency

All segments of the commercial transportation industry seek methods by which to move goods while consuming less energy per ton. The search for increased efficiency is ongoing, and the maritime sector has responded by developing larger ships that move massive amounts of cargo while consuming minimal amounts of energy per ton or unit volume. A complementary method of reducing energy consumption and related costs involves engines of greater thermal efficiency, and it is perhaps no coincidence that the Wärtsilä-Sulzer 2-stroke diesel is the world’s most thermally efficient mover of super-sized ships.

Tug Barges

An alternative method of improving overall ship efficiency has been the development of the tug-barge. A barge will carry a greater payload than a ship of equal size because of the additional cargo space resulting from the removal of the engine and fuel tanks. In coastal and inland waterway service, the increased payload offsets higher fuel consumption that results from the increased combined tonnage of tug and barge when operating at full load. When operating at light load, a barge can ride higher in the water while the tug couples to the barge at a lower relative elevation.

While the tug’s propeller rides relatively deep in the water, the empty barge can ride high and sail with greatly reduced hydraulic resistance. A self-powered ship would have to carry ballast water to sail at a depth and with greater hydraulic resistance at the bow so as to assure efficient propulsive operation of the propeller. When operating at light load, the diesel engine will function at reduced engine thermal efficiency. In some forms of inland and coastal maritime service, vessels may sail fully laden in one direction and partially laden in the opposite direction.

Inland vessels often sail at high power when moving upstream against a river current and at part power when sailing downstream with the current. While vessel operators often seek to carry freight in both directions, such operation is not always possible. Operators would optimally want to sail their vessels with the lowest possible energy consumption regardless of direction or the payload being carried. Over the past several decades, the engine industry has continually developed many innovations that have steadily improved engine thermal efficiency in the maritime, railway and truck industries. 

Engine Features

One of the early methods of enhancing efficiency was to greatly increase the speed at which fuel is injected into the cylinder near the top of the compression stroke. Electronically activated solenoid-driven injectors achieve such a result far more effectively than mechanically driven injectors. Variable timing of the inlet and exhaust valves also enhances efficiency with some engines now being available with electrically operated valves. “Unloading” cylinders will allow fewer cylinders to operate at full load and yield greater efficiency than all cylinders operating at very light load.

Many commercial engines operate on the “Miller cycle,” in which the equivalent compression stroke is shorter than the power stroke. Such operation reduces the amount of work the engine does to compress air within the cylinders while yielding usable power output. Modern engine technology includes multiple turbochargers and can allow an engine on the Miller cycle to also operate on fewer cylinders during light-load conditions. Engine concepts that were successful during an earlier time but discarded due to market conditions may be reintroduced to improve efficiency.

Multi-Stage Expansion

Multi-stage expansion of the working gas was popular during the steam era when many boats were powered by the 3-stage expansion Skinner “Unaflow” steam piston engine that involved low, intermediate and high-pressure cylinders. Many years ago in rural Australia, some 2-stroke diesel engines were converted to steam operation and included a 2-stage variant. An American company seeking to convert a 2-stroke locomotive diesel to 2-stage steam expansion explored using four cylinders to expand high-pressure steam and eight or twelve cylinders to expand low-pressure steam. Gas turbine engines typically expand combustion gases in two or three stages.

Several decades ago, Puch and Husqvarna built small-scale piston engines that expanded a single charge of combustible gases over two cylinders, the parallel twin-cylinder engines having intake and exhaust ports on different cylinders. A private inventor subsequently used the Puch and Husqvarna precedent to adapt the 4-stroke parallel twin-cylinder engine to compress air and fuel in one cylinder and expand the combustion gases over two cylinders, a variation of the efficient Miller cycle. A former 500-cc 2-cylinder engine operated as a 250-cc 4-stroke single could deliver the output of an engine of 300 to 350-cc. 

But fuel prices were low at the time, and there was little market interest in the concept of compressing air in one cylinder and expanding the gases over two cylinders. Today, high fuel prices warrant re-examination of that old concept, perhaps using a retired V-12 or V-16 engine as the basis for such research. During light-load operation, modern engine technology in the cylinder head and valve operation could allow the V-16 to produce power in eight cylinders with combustion gases also entering each next-door cylinder.  

The combination of single cylinder combustion and two-cylinder expansion offers potential to increase part-load engine efficiency. Combustion could occur in eight cylinders of a V-16 engine with further expansion in each next-door cylinder to yield the power output of 10 or 11 cylinders. During light-load operation, a V-12 engine could operate on four cylinders with further expansion occurring in each next-door cylinder to yield the equivalent output of a 5-cylinder engine. A second variation would be to combine combustion in four cylinders with low-pressure expansion spread across the remaining eight and perhaps achieve the equivalent output of six cylinders.

Leverage

Mechanical leverage between con-rod and crank rapidly diminishes as the combustion piston approaches bottom-dead center. In 4-stroke V-12 engines, the next-door cylinder would be 120 degrees behind the combustion cylinder. At 120 degrees after top dead center (TDC), an interconnecting pipe would open between the combustion cylinder and its companion that would be at TDC. As the combustion cylinder reaches 150 degrees after TDC, mechanical leverage between con-rod and crank rapidly diminishes. At 30 degrees after TDC, leverage between con-rod and crank rapidly increases on the trailing cylinder that could extract greater usable work from the low-pressure gas it receives from the combustion cylinder.

The lead cylinder of a V-12 engine would supply gas to the trailing cylinder for up to 135 degrees before TDC, when it would transition into the exhaust stroke. In a V-16 engine, the trailing cylinder would follow the lead cylinder by 90 degrees. The lead cylinder would supply gas to the trailing cylinder between 90 degrees after TDC and up to 150 degrees before TDC, or 120-degrees of crank rotation. At 150 degrees before TDC, the lead cylinder would transition to the exhaust stroke. Compound cylinder expansion during light-load operation promises to take the Miller cycle to a higher level of efficiency.

Recycled Engines

There may be scope to rebuild older-generation V-12 and V-16 diesel engines from the maritime and railway industries, modifying the cylinder heads for more versatile operation that would include compound expansion. An engine may operate all cylinders during full-power operation. During part-load operation, it could operate on fewer cylinders and expand the combustion gases in each next-door cylinder to achieve greater efficiency. Technical staff may make appropriate modifications to existing, re-useable cylinder heads or develop new, more versatile cylinder heads so that existing functional engine blocks may operate in compound combustion.

Technical ingenuity applied to the cylinder heads could allow the same engine to fire on all cylinders during heavy-load operation and switch over to compound expansion during light-load operation. Many older ships that are withdrawn from service end up in India’s recycling yards. There is much technical talent available across India. Some of that talent could develop more versatile cylinder heads for maritime and railway engines.

In some tug-barge operations, it may be feasible to install a rebuilt, oversized engine block adapted to compound expansion and use it for propulsion. While it may be practical in a tug assigned to tug-barge operation, it may otherwise be impractical in a self-powered vessel of equivalent size to the barge. The possibilities are many and worth exploring.  – MarEx

Mr. Valentine has a degree in mechanical engineering from Carleton University and has worked as a technical journalist for the past 10 years. He is a frequent contributor to the newsletter. Contact him at [email protected].