Diesel engine

• 1. Introduction

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  The diesel engine carries the name of Rudolf Diesel, a German engineer who spent years trying to build a machine that could extract far more work from heat than any engine before it. The engine he eventually produced does not ignite its fuel with a spark. Instead it compresses air so violently that the air itself grows hot enough to set diesel fuel alight. That single difference in operating principle has shaped how ships cross oceans, how freight moves by rail, and how trucks haul goods across continents.

  In 1878, Diesel was a student at the Polytechnikum in Munich when his professor Carl von Linde showed him just how wasteful steam engines were. Steam engines of the era converted only six to ten percent of their heat energy into actual work. Linde pointed toward something far more promising: the Carnot cycle, which could theoretically recover a much larger share of that heat. That lecture planted the idea Diesel would spend the rest of his life pursuing.

  The path from that lecture to a working engine took fifteen years and passed through failed prototypes, harsh critics, contested patents, and a design flaw that Diesel himself had to abandon. What he eventually produced was the most thermally efficient practical combustion engine ever built. The questions worth exploring are how that engine actually works, how Diesel won and lost and rebuilt his case for it, and what the century of development after him produced.

• 2. The Inventor and His Struggle

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  Rudolf Diesel published his ideas in 1893 in an essay titled Theory and Construction of a Rational Heat Motor, but the version of the engine he described in that essay had a fundamental flaw. His original design called for isothermal compression, meaning the air would be compressed at a constant temperature. That would have required pressure as high as ninety atmospheres to reach the combustion temperature he needed. As critics quickly noted, such an engine could never actually produce usable work.

  Diesel was attacked for years. Otto Kohler, one of his most prominent critics, had published an essay in 1887 describing an engine similar to what Diesel proposed and had concluded that such a machine could never perform any work. Emil Capitaine, who had built a petroleum engine with glow-tube ignition in the early 1890s, claimed that his own device worked the same way as Diesel's. Capitaine was wrong; he lost the patent lawsuit he brought against Diesel.

  By June 1893, Diesel had recognized that his original constant-temperature cycle would not function. He switched to a constant-pressure cycle, and that revision made the engine buildable. With help from Moritz Schroter, he convinced both Krupp in Essen and the Maschinenfabrik Augsburg to sign contracts in April 1893 and fund prototype construction.

  The first ignition happened on the 10th of August 1893. The fuel used was petrol. The experiment destroyed the indicator instrument but proved that compression alone could trigger combustion. Over the following years Diesel rebuilt and refined the engine through three successive prototypes. On the 17th of February 1897, Moritz Schroter conducted the main public efficiency test. The engine was rated 13.1 kW with a specific fuel consumption of 324 grams per kilowatt-hour, delivering an effective efficiency of 26.2 percent. That was a figure no comparable engine had reached. By 1898, Diesel had become a millionaire. On the 29th of September 1913, he died mysteriously while crossing the English Channel.

• 3. How Compression Ignition Works

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  Compression ratios in a diesel engine typically run between 15:1 and 23:1, a range far above what any petrol engine can tolerate. In a petrol engine, mixing fuel with air before compression means that squeezing the mixture too hard causes it to detonate prematurely. Because a diesel engine compresses only air, that danger disappears entirely. The fuel is injected only when the piston is near the top of its stroke, into air already heated by compression to a temperature high enough to cause spontaneous ignition.

  Once the fuel enters the combustion chamber, it does not ignite all at once. The heat of the compressed air vaporizes fuel from the surface of each droplet. That vapor reaches ignition temperature, and combustion spreads from there. The characteristic knocking sound associated with diesel engines, sometimes called diesel clatter, comes from this process: when vapor reaches ignition temperature there is an abrupt pressure increase above the piston, producing the audible knock.

  Torque control in a diesel engine works differently from in a petrol engine. A diesel does not throttle its incoming air. The volume of air entering the cylinder is kept as large as possible at all times. When the driver demands more or less power, only the quantity of injected fuel changes. That keeps the air-fuel ratio very lean except at peak demand, which is one reason diesel engines achieve higher fuel efficiency than petrol engines under partial load.

  Theoretcally the highest possible efficiency for a diesel engine is 75 percent. In practice, passenger car diesel engines reach up to 43 percent efficiency, large truck and bus engines up to 45 percent, and large two-stroke marine diesel engines up to 55 percent. Low-speed marine engines, which power the world's largest ships, hold the record in practical combustion engine efficiency.

• 4. From Ships to Automobiles

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  The first diesel-powered ships appeared in 1903, when two river and canal vessels, the Vandal naphtha tanker and the Sarmat, were launched. The French launched the first diesel submarine, the Aigrette, in 1904. By the end of World War I, double-acting piston two-stroke engines producing up to 12,200 PS had been built for marine use, and submarine programs had driven diesel technology forward faster than almost any other application.

  On land, the transition took longer. The first locomotive with a diesel engine appeared in 1912 on the Swiss Winterthur to Romanshorn railway. That same year, the MS Selandia became the first ocean-going ship with diesel engines. The first diesel-engined lorry appeared in 1908, but it was not until 1924 that the first mass-production diesel trucks came to market. The first passenger car with a diesel engine was not produced until 1929.

  The turning point for passenger cars came in 1936, when Daimler-Benz, working with Robert Bosch GmbH, began manufacturing the Mercedes-Benz 260 D, the first mass-produced diesel passenger car. Its engine, the Mercedes-Benz OM 138, was rated 45 PS. By the early 1990s, car manufacturers had begun offering luxury vehicles with diesel engines, aided by electronically controlled fuel injection. According to Konrad Reif writing in 2012, diesel cars accounted for roughly half of newly registered cars across the European Union at that time.

  One fifth of all passenger cars worldwide have diesel engines as of recent data, with approximately 47 percent of European passenger cars running on diesel. In markets such as India, South Korea, and Japan, the share was still growing as of 2018.

• 5. Aviation and Unlikely Altitude

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  Piper Cherokee co-designer Karl H. Bergey wrote in 1978 that "the likelihood of a general aviation diesel in the near future is remote." He turned out to be wrong, though it took two more decades for the technology to arrive in force.

  The Packard Motor Company had actually developed America's first aircraft diesel engine, the Packard DR-980, as early as 1929. It was an air-cooled nine-cylinder radial engine that flew in record-breaking distance and endurance aircraft and enabled the first successful ground-to-air radio telephone communication, which had previously been impossible in aircraft powered by spark-ignition engines because electromagnetic interference made voice radio unintelligible. The engine received its Approved Type Certificate from the U.S. Department of Commerce on the 6th of March 1930. But noxious exhaust, cold-start problems, vibration failures, the death of its developer, and the economic contraction of the Great Depression ended the program.

  From the late 1970s onward, pressure for greater fuel economy and concerns about leaded aviation fuel revived interest. High-compression piston aircraft engines require the addition of tetraethyl lead to aviation gasoline to prevent pre-ignition; diesel engines do not require leaded fuel at all. Frank Thielert and his Austrian enterprise began developing diesel engines as direct replacements for the 100 hp to 350 hp gasoline engines common in light aircraft. The Diamond DA42 Twin Star became the first notable production aircraft to use this technology and demonstrated exceptional fuel efficiency.

  Between 2002 and 2018, over 5,000 aircraft diesel engines were delivered worldwide. By 2007, according to the U.S. Federal Aviation Administration, various jet-fueled piston aircraft had logged well over 600,000 hours of service.

• 6. Fuel Flexibility and Emissions

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  Rudolf Diesel himself considered coal dust as a possible fuel when he wrote Theory and Construction of a Rational Heat Motor in 1893. His actual prototype ran on petrol, then kerosene, then ligroin, before testing settled on petroleum products. Before diesel fuel was standardized, engines ran on petrol, kerosene, gas oil, vegetable oil, mineral oil, and mixtures of all of these.

  The ability to burn a wide range of fuels comes from how the diesel engine handles combustion. Because only air is compressed and fuel is injected at a carefully timed moment, the cylinder temperature is high enough to ignite fuels far less volatile than petrol. Biodiesel, produced through transesterification of plant or animal oils, can run directly in many diesel engines without modification. KHD's AD-System, introduced in 1972, went further: AD-diesels could operate on virtually any liquid fuel, fitted with an auxiliary spark plug that fired only when the fuel's ignition quality fell too low for compression alone.

  The emissions picture is more complicated. Diesel exhaust has been classified as a Group 1 carcinogen by the International Agency for Research on Cancer. It causes lung cancer and is associated with increased risk of bladder cancer. Particulate matter from diesel engines is linked to heart and respiratory disease. About 90 percent of pollutants can be removed by exhaust gas treatment technology, and motor vehicle diesel fuel has been sulfur-free since 2003, eliminating sulfur dioxide from road vehicle exhausts.

  In 2015, the Volkswagen emissions scandal revealed that the company had programmed turbocharged direct injection diesel engines to activate emissions controls only during laboratory testing. The U.S. Environmental Protection Agency issued a notice of violation of the Clean Air Act to Volkswagen Group, a case that cast a shadow over diesel's reputation in markets where it had been gaining ground.

• 7. A Century of Technical Progress

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  Bosch introduced the first inline injection pump for motor vehicle diesel engines in 1927. That pump enabled reliable fuel metering in road vehicles and set the stage for diesel cars to become practical. The next major step came with common rail injection, whose development began at the ETH Zurich in 1976. Common rail systems store fuel in a shared high-pressure reservoir and supply each injector from it, allowing precise electronic control of injection timing and quantity. Modern common rail systems operate at pressures ranging from 140 MPa to 270 MPa. In 2015, systems working at 2,500 bar entered production.

  Turbocharging transformed diesel performance. Daimler-Benz produced the first turbocharged passenger car diesel engine in 1978, the Mercedes-Benz OM617. In 1989, the Audi 100 became the first passenger car in the world with a turbocharged, intercooled, direct-injected, and electronically controlled diesel engine; it achieved a brake-specific fuel consumption of 198 grams per kilowatt-hour. BMW introduced dual-stage turbocharging with the BMW M57 engine in 2004, and in 2012 expanded that to three turbochargers with the N57 engine.

  At the extreme end of the scale, the largest diesel engines ever built are 14-cylinder two-stroke marine units producing peak power of almost 100 MW each. The Wartsila-Sulzer RTA96-C, produced in 2006, was rated 80,080 kW, making it the most powerful diesel engine in the world at that time.

  The 1998 24 Hours Nurburgring race offered a different kind of proof. A modified BMW 320d powered by a two-litre straight-four diesel engine with direct injection won the race and recorded a fuel consumption of 23 litres per 100 km, approximately half the consumption of a comparable petrol-powered car. Eight years later, in 2006, the Audi R10 TDI equipped with a 5.5-litre V12-TDI engine rated 476 kW won the 24 Hours of Le Mans outright.