How superyacht diesel engines work

The vast majority of yachts today are powered by diesel engines. The basic technology has been around for more than a century, but today’s diesels have evolved considerably in just the past two decades. Today’s diesel engines run cleaner, burn less fuel and produce more power per pound than ever before. In the face of these improvements, and all the new power plant schemes coming on line, diesels remain the gold standard in yachting.

There is no question that the power plant of choice for the vast majority of yachts, and more recently for ships as well, is the diesel engine. Its overwhelming popularity is the result of a combination of technical, economic and practical advantages unmatched by any other alternative.

The marine diesel engine is one of the most versatile power sources available for marine propulsion and auxiliary use. The range of sizes is remarkable, ranging from one of the smallest, a Yanmar single-cylinder, 8hp (6KW) engine in a small catboat, to what is (according to Wikipedia) the world’s largest: a Wartsila Sulzer propulsion engine of over 113,000hp (84,000KW). Some diesel engines operate in excess of 4,000rpm, while others run at less than 100rpm. Whatever your need, it seems, there’s an engine for your application, or a supplier willing to build one.

Diesel's history

All combustion power, whether internal combustion, as with a diesel or petrol/gasoline engine, or external combustion, as with a steam engine or, in part, some gas turbines, depends on burning fuel to create heat. The expansion resulting from the heat is then used to create pressure, and the pressure is used to move something, generally a piston or turbine blade. That movement, through a variety of mechanisms, is converted to rotation that eventually finds its way to an output shaft.

For many years, even after Rudolf Diesel introduced the engine that bears his name in 1893, steam remained the first choice for marine propulsion. It was a mature technology that worked, and it could be fuelled by anything that would burn in a boiler. Coal was a common choice, but river steamboats often scavenged driftwood and fallen trees as supplemental fuel along their routes.

When petroleum oil was discovered and refining began, one of the nearly unusable by-products was Bunker C, a tar-like sludge that virtually defines 'the bottom of the barrel'. It was so thick that it had to be heated before being pumped and burned, but being cheap and plentiful, it was perfect for marine steam plants.

Rise of diesel

A number of factors – economy, safety, reliability, longevity, versatility and environmental concerns – have conspired to allow diesels to gain dominance over steam engines as well as over other internal combustion engines. When used in an engine, diesel fuel yields a high thermal efficiency, meaning that you get a lot of caloric heat, for more power per litre/gallon.

As new refining techniques have been developed, suppliers have been able to squeeze out greater quantities of higher-grade fuels per barrel. This increased the percentage of light diesel fuel and reduced the percentage of each barrel that ended up as Bunker C, with the economic law of supply and demand narrowing the cost gap.

Diesel fuel is cleaner to burn and easier to store and handle than Bunker C or coal, and it is safer than more volatile fuels including petrol/gasoline and gaseous fuels such as propane (LPG) or methane (LNG). The latter fuels can easily be ignited by a spark, and engines that use these fuels are referred to as spark-ignition. The problem is that stray sparks outside the engine, such as static electricity, a cigar or a mobile phone, can also ignite the fuel unintentionally, so the precautions for storage are considerably more restrictive.

By contrast, it is relatively difficult to ignite diesel fuel by spark, so much so that another method of igniting the fuel inside the engine was needed, and that’s where Rudolf Diesel came in. He found that the solution was a compression-ignition engine. As air is compressed, its temperature rises. Compress it enough, and mix in a little diesel fuel, and the mixture will combust spontaneously.

Simple in concept, but difficult in execution, the evolving art of compressing air, injecting fuel, and extracting power still continues to improve today, some 120 years later. Because the amount of compression needed to heat the air to the ignition temperature of diesel fuel is so high, diesel engines are generally built more stoutly than other types, simply to withstand the physical stresses imposed. That rugged construction has the added benefits of reliability and longevity.

Diesel engine features

Various engine configurations have been tried over the years, the V and in-line being the most common today. In the past, however, multiple engines were attached to one reduction gear, or engine blocks were coupled together in series, for additional power. One interesting variant, used in hundreds of World War II landing craft, was the opposed piston arrangement, which had both upper and lower crankshafts and no cylinder heads, as the two pistons in each cylinder fired against each other.

It is important to keep in mind that power output from an engine is largely determined by the amount of fuel burned. With few exceptions, a modern diesel engine will yield about one horsepower for each 0.05 to 0.055 gallons (0.19 to 0.21 litres) of fuel burned each hour. That equates to 50-55 gallons per hour (190 to 210 litres per hour) for a 1,000 horsepower engine operating at full power. If you can’t remember all of that, and you have a twin-engine yacht, there’s a simple rule of thumb: the maximum fuel burn in gallons per hour is going to be about one-tenth of the horsepower of one engine, i.e. twin 1,000 horsepower engines, operating at full power, burn about 100 gallons of fuel per hour total.

Now, understanding that power output is directly related to the amount of fuel burned, it is obvious that the more fuel you can burn, the more power you can get out of a given engine. That’s advantageous because it allows the use of a smaller, lighter engine, improving the all-important power-to-weight-ratio, to meet the needs of a given yacht.

To burn more fuel, you also need more air, so various schemes are employed to increase the amount of air going into the engine. The most well-established is the use of a turbocharger, which uses the engine’s exhaust to spin a compact air pump, compressing the incoming air, allowing more of it, at higher pressure, to pass through the intakes into the cylinders.

It’s also a law of physics that cooler air is denser than hot air, so there are more grams per litre/pounds per cubic foot, and it’s the weight, not the volume, that matters in combustion. That’s why manufacturers of high-performance engines like to see intake air drawn directly from the outside, rather than from within a hot engine room. It’s also why engine performance specifications rate the horsepower at a certain temperature, with lower power outputs as the air temperature increases.

To that end, water-cooled intercoolers and aftercoolers are often fitted in conjunction with turbochargers so that the incoming air is not only compressed, but cooled as well. This maximises the power developed for a given engine displacement – displacement being the cross-sectional area of a cylinder, times the stroke (distance travelled by the piston in the cylinder), times the number of cylinders

Spark-ignition vs compression-ignition

In a spark-ignition engine, such as a petrol/gasoline engine, the fuel and air are mixed before entering the cylinder, then compressed and ignited by a spark plug at just the right moment. This is done by adjusting the timing to maximize power and avoid damaging pre-ignition or post-ignition.

In a compression-ignition engine such as a diesel, there is no spark plug. The air enters the cylinder and is compressed by the upstroke of the piston. The fuel is then sprayed as a fine mist, through an injector, into the top of the cylinder where it is instantly ignited by the compressed, superheated air. The explosion sends the piston downward in what is called the power stroke.

Traditionally, fuel was supplied to injectors by an injector pump, which had a number of outlets equal to the number of cylinders. Each outlet port had a steel tube leading to one injector, through which it sent an appropriate amount of fuel, under high pressure, at just the right time to spray through the injector tip as the piston reached the top of the cylinder. A later alternative was to have a constant supply of fuel under high pressure, timing the opening of mechanical injectors via a camshaft.

Today, many engines utilise a common-rail injector system that employs electronically controlled injectors. The control system for electronic injectors can be programmed to vary each injector’s timing based on engine speed and power demands, improving both performance and fuel economy, and reducing exhaust pollutants.

Diesel engines have improved with each passing decade and there is no reason to expect that the trend will not continue. Propulsion systems continue to evolve as well, with hybrids, diesel-electric and pods at the forefront, but in each, the diesel remains a key component and will remain so for the foreseeable future.