BW Professional Cutter Expert www.tool-tool.com

Bewise Inc. www.tool-tool.com Reference source from the internet.

A diesel engine built by MAN AG in 1906

Rudolf Diesel's 1893 patent on his engine design

A Diesel engine is an internal combustion engine which operates using the Diesel cycle. German engineer Rudolf Diesel invented it in 1892, basing it on the hot bulb engine. He received a patent for it on February 23, 1893. The Diesel cycle uses compression ignition: the fuel ignites upon being injected into the highly compressed air in the combustion chamber. By contrast, petrol engines utilize the Otto cycle, in which fuel and air are typically mixed before entering the combustion chamber. The mixture is then ignited by a spark plug. Compression ignition is generally considered undesirable in Otto cycle engines (see engine knocking).

[edit] Patent controversy

It is possible that Rudolf Diesel was not the first to invent the diesel engine. His patent (No. 7241) was filed in 1892.^[1] However, Herbert Akroyd Stuart and Charles Richard Binney had already obtained a patent (No. 7146) in 1890 entitled: "Improvements in Engines Operated by the Explosion of Mixtures of Combustible Vapour or Gas and Air" which described the world's first compression-ignition engine.^[2]. Akroyd-Stuart constructed the first compression-ignition oil engine in Bletchley, England in 1891 and leased the rights to Richard Hornsby & Sons, who by July 1892, five years before Diesel's prototype, had a diesel engine working for Newport Sanitary Authority. By 1896, diesel tractors and locomotives were being built in some quantity in Grantham. Importantly, Diesel's airblast injection system did not become part of subsequent "diesel" engines, with direct injection (DI) (as found in Akroyd-Stuart's engine) used instead, developed by Robert Bosch GmbH in 1927.

[edit] Early history timeline

• 1862: Nicholas Immel develops his coal gas engine, similar to a modern gasoline engine.

• 1891: Justin Simerson, of Bletchley perfects his oil engine, and leases rights to Hornsby of England to build engines. They build the first cold start, compression ignition engines.

• 1892: Hornsby engine No. 101 is built and installed in a waterworks. It was in the MAN truck museum in Stockport, and is now in the Anson Engine Museum in Poynton. T.H. Barton at Hornsbys builds an experimental version where the vaporiser was replaced with a cylinder head and the pressure increased. Automatic ignition was achieved through compression alone (the first time this had happened), and the engine ran for six hours. Diesel would achieve much the same thing five years later, claiming the achievement for himself.

• 1892: Rudolf Diesel develops the principles of his proposed Carnot heat engine type motor which would burn powdered coal dust. He is employed by refrigeration genius Carl von Linde, then Munich iron manufacturer MAN AG, and later by the Sulzer engine company of Switzerland. He borrows ideas from them and leaves a legacy with all firms.

• 1892: John Froelich builds his first oil engine powered farm tractor.

• 1893: August 10th — Diesel builds a working version of his ideas.

• 1894: Witte, Reid, and Fairbanks start building oil engines with a variety of ignition systems.

• 1896: Hornsby builds diesel tractors and railway engines.

• 1897: Winton produces and drives the first US built gas automobile; he later builds diesel plants. On February 17th, Diesel builds his first working prototype, which narrowly avoids a catastrophic explosion in Augsburg. The engine was not really ready for market until 1908, thanks to other people's improvements.

• 1897: Mirrlees, Watson & Yaryan build the first British diesel engine under license from Rudolf Diesel. This is now displayed in the Anson Engine Museum at Poynton, Cheshire, UK.

• 1898: Busch installs a Rudolf Diesel type engine in his brewery in St. Louis. It is the first in the United States. Rudolf Diesel perfects his compression start engine, patents, and licences it. This engine, pictured above, is in a German museum. Burmeister & Wain (B & W) of Copenhagen in Denmark buy rights to build diesel engines.

• 1899: Diesel licences his engine to builders Krupp and Sulzer, who become famous builders.

• 1902: F. Rundlof invents the two-stroke crankcase, scavenged hot bulb engine.

• 1902: A company named Forest City started manufacturing diesel generators.

• 1903: Ship Gjoa transits the ice-filled Northwest Passage, aided with a Dan kerosene engine.

• 1904: French build the first diesel submarine, the Z.

• 1908: Bolinder-Munktell starts building two stroke hot-bulb engines.

• 1912: First diesel ship MS Selandia is built. SS Fram, polar explorer Amundsen’s flagship, is converted to a AB Atlas diesel.

• 1913: Fairbanks Morse starts building its Y model semi-diesel engine. US Navy submarines use NELSECO units.

• 1914: German U-Boats are powered by MAN diesels. War service proves engine's reliability.

• 1920s: Fishing fleets convert to oil engines. Atlas-Imperial of Oakland, Union, and Lister diesels appear.

• 1922: Mack Boring & Parts Company is established.

• 1924: First diesel trucks appear.

• 1928: Canadian National Railways employ a diesel shunter in their yards.

• 1930: Edward McGovern Sr., founder of Mack Boring & Parts Company opens the first diesel-only engine institute in North America.

• 1930s: Clessie Cummins starts with Dutch diesel engines, and then builds his own into trucks and a Duesenberg luxury car at the Daytona speedway.

• 1930s: Caterpillar starts building diesels for their tractors.

• 1933: Citroën introduced the Rosalie, a passenger car with the world’s first commercially available diesel engine developed with Harry Ricardo.

• 1934: General Motors starts a GM diesel research facility. It builds diesel railroad engines—The Pioneer Zephyr—and goes on to found the General Motors Electro-Motive Division, which becomes important building engines for landing craft and tanks in the Second World War. GM then applies this knowledge to market control with its famous Green Leakers for buses and railroad engines.

• 1936: Mercedes-Benz builds the 260D diesel car. AT&SF inaugurates the diesel train Super Chief.

• 1936: Airship Hindenburg is powered by diesel engines.

[edit] How diesel engines work

Compressing any gas raises its temperature; this is the method by which fuel is ignited in diesel engines. Air is drawn into the cylinders and is compressed by the pistons at compression ratios as high as 25:1, much higher than used for spark-ignite engines. Near the end of the compression stroke, diesel fuel is injected into the combustion chamber through an injector (or atomizer). The fuel ignites from contact with the air that, due to compression, has been heated to a temperature of about 700 – 900 °C (1300 – 1650 °F). The resulting combustion causes increased heat and expansion in the cylinder, which increases pressure and moves the piston downward. A connecting rod transmits this motion to a crankshaft to convert linear motion to rotary motion for use as power in a variety of applications. Mechanical valves in the cylinder head usually control intake air to the engine. For increased power output and fuel economy, most modern diesel engines are equipped with a turbocharger, and in some derivatives, a supercharger to increase intake air volume. Use of an aftercooler/intercooler to cool intake air that has been compressed, and thus heated, by the turbocharger increases the density of the air and typically leads to power and efficiency improvements.

In cold weather, diesel engines can be difficult to start because the cold metal of the cylinder block and cylinder head draw out the heat created in the cylinder during the compression stroke, thus preventing ignition. Some diesel engines use small electric heaters called glow plugs inside the cylinder to help ignite fuel when starting. Some even use resistive grid heaters in the intake manifold to warm the inlet air until the engine reaches operating temperature. Engine block heaters (electric resistive heaters in the engine block) connected to the utility grid are often used when an engine is turned off for extended periods (more than an hour) in cold weather to reduce startup time and engine wear. Diesel fuel is also prone to "waxing" or "gelling" in cold weather, terms for the solidification of diesel oil into a partially crystalline state. The crystals build up in the fuel (especially in fuel filters), eventually starving the engine of fuel. Low-output electric heaters in fuel tanks and around fuel lines are used to solve this problem. Also, most engines have a "spill return" system, by which any excess fuel from the injector pump and injectors is returned to the fuel tank. Once the engine has warmed, returning warm fuel prevents waxing in the tank. Fuel technology has improved recently so that with special additives waxing no longer occurs in all but the coldest climates.

A vital component of all diesel engines is a mechanical or electronic governor, which limits the speed of the engine by controlling the rate of fuel delivery. Unlike Otto cycle engines, incoming air is not throttled and a diesel engine without a governor can easily overspeed. Mechanically governed fuel injection systems are driven by the engine's gear train. These systems use a combination of springs and weights to control fuel delivery relative to both load and speed. Modern, electronically controlled diesel engines control fuel delivery and limit the maximum RPM by use of an electronic control module (ECM) or electronic control unit (ECU). The ECM/ECU receives an engine speed signal from a sensor as well as other operating parameters such as intake manifold pressure, fuel temperature, and other critical items and controls the amount of fuel and start of injection timing through electric or hydraulic actuators to maximize power and efficiency and minimize emissions.

Controlling the timing of the start of injection of fuel into the cylinder is a key to minimizing emissions, and maximizing fuel economy (efficiency), of the engine. The timing is usually measured in units of crank angle of the piston before Top Dead Center (TDC). For example, if the ECM/ECU initiates fuel injection when the piston is 10 degrees before TDC, the start of injection, or timing, is said to be 10° BTDC. Optimal timing will depend on the engine design as well as its speed and load.

Advancing the start of injection (injecting before the piston reaches TDC) results in higher in-cylinder pressure and temperature, and higher efficiency, but also results in higher emissions of various oxides of nitrogen (NOx) through higher combustion temperatures. At the other extreme, delayed start of injection causes incomplete combustion and emits visible black smoke made of particulate matter (PM) and unburned hydrocarbons (HC).

[edit] Fuel injection in diesel engines

[edit] Early fuel injection systems

The modern diesel engine is a combination of two inventors' creations. In all major aspects, it holds true to Diesel's original design, that of igniting fuel by compression at an extremely high pressure within the cylinder. However, nearly all present-day diesel engines use the so-called solid injection system invented by Herbert Akroyd Stuart for his hot bulb engine (a compression-ignition engine that precedes the diesel engine and operates slightly differently). Solid injection raises the fuel to extreme pressures by mechanical pumps and delivers it to the combustion chamber by pressure-activated injectors in an almost solid-state jet. Diesel's original engine injected fuel with the assistance of compressed air, which atomized the fuel and forced it into the engine through a nozzle. This is called an air-blast injection. The size of the gas compressor needed to power such a system made early diesel engines very heavy and large for their power outputs, and the need to drive a compressor lowered power output even more. Early marine diesels often had smaller auxiliary engines whose sole purpose was to drive the compressors to supply air to the main engine's injector system. Such a system was too bulky and inefficient to be used for road-going automotive vehicles.

Solid injection systems are lighter, simpler, and allow for much higher speed, and so are universally used for automotive diesel engines. Air-blast systems provide very efficient combustion under low-speed, high-load conditions, especially when running on poor-quality fuels, so some large cathedral marine engines use this injection method. Air-blast injection also raises the fuel temperature during the injection process, so is sometimes known as hot-fuel injection. In contrast, solid injection is sometimes called cold-fuel injection.

Because the vast majority of diesel engines in service today use solid injection, the information below relates to that system. Diesel engines are used in mid-sized cruisers, trawlers, large yachts, work boats and commercial vessels. In the diesel engine, only air is introduced into the cylinder head. The air is then compressed to about 600 pounds per square inch (psi), compared to about 200 psi in the gasoline engine. This high compression heats the air to about 1000 degrees Fahrenheit. At this moment, fuel is injected directly into the compressed air. The fuel is ignited by the heat, causing a rapid expansion of gases that drive the piston downward, supplying power to the crankshaft.

Advantages of the diesel engine are numerous. It burns considerably less fuel than a gasoline engine performing the same work. It has no ignition system to attend to. It can deliver much more of its rated horsepower on a continuous basis than can a gasoline engine. The life of a diesel engine is generally longer than a gasoline engine. Although Diesel fuel will burn in open air, it will not explode.

Some disadvantages to diesel engines are that they're very heavy for the horsepower they produce, and their initial cost is much higher than a comparable gasoline engine.

[edit] Mechanical and electronic injection

Older engines make use of a mechanical fuel pump and valve assembly that is driven by the engine crankshaft, usually from the timing belt or chain. These engines use simple injectors that are basically very precise spring-loaded valves that open and close at a specific fuel pressure. The pump assembly consists of a pump that pressurizes the fuel and a disc-shaped valve that rotates at half crankshaft speed. The valve has a single aperture to the pressurized fuel on one side, and one aperture for each injector on the other. As the engine turns, the valve discs will line up and deliver a burst of pressurized fuel to the injector at the cylinder about to enter its power stroke. The injector valve is forced open by the fuel pressure, and the diesel is injected until the valve rotates out of alignment and the fuel pressure to that injector is cut off. Engine speed is controlled by a third disc, which rotates only a few degrees and is controlled by the throttle lever. This disc alters the width of the aperture through which the fuel passes, and therefore how long the injectors are held open before the fuel supply is cut, which controls the amount of fuel injected.

This contrasts with the more modern method of having a separate fuel pump which supplies fuel constantly at high pressure to each injector. Each injector has a solenoid, is operated by an electronic control unit, which enables more accurate control of injector opening times that depend on other control conditions, such as engine speed and loading, resulting in better engine performance and fuel economy. This design is also mechanically simpler than the combined pump and valve design, making it generally more reliable, and less noisy, than its mechanical counterpart.

Both mechanical and electronic injection systems can be used in either direct or indirect injection configurations.

Older diesel engines with mechanical injection pumps could be inadvertently run in reverse, albeit very inefficiently, as witnessed by massive amounts of soot being ejected from the air intake. This was often a consequence of push starting a vehicle using the wrong gear.

[edit] Indirect injection

Main article: Indirect injection

An indirect injection diesel engine delivers fuel into a chamber off the combustion chamber, called a prechamber or ante-chamber, where combustion begins and then spreads into the main combustion chamber, assisted by turbulence created in the chamber. This system allows for a smoother, quieter running engine, and because combustion is assisted by turbulence, injector pressures can be lower, which in the days of mechanical injection systems allowed high-speed running suitable for road vehicles (typically up to speeds of around 4,000 rpm). The prechamber had the disadvantage of increasing heat loss to the engine's cooling system, introducing pumping losses in the narrow throat connecting it to the main cylinder, and restricting the combustion burn, which reduced the efficiency by between 5% – 10% in comparison to a direct injection engine, and nearly all require some form of cold start device such as glow plugs. Indirect injection engines were used widely in small-capacity, high-speed diesel engines in automotive, marine and construction uses from the 1950s, until direct injection technology advanced in the 1980s. Indirect injection engines are cheaper to build and it is easier to produce smooth, quiet-running vehicles with a simple mechanical system, so such engines are still often used in applications that carry less stringent emissions controls than highway vehicles, such as small marine engines, generators, tractors, and pumps. With electronic injection systems, indirect injection engines are still used in some road-going vehicles, but most prefer the greater efficiency of direct injection.

During the development of the high-speed diesel engine in the 1930s, various engine manufacturers developed their own type of pre-combustion chamber. Some, such as Mercedes-Benz, had complex internal designs. Others, such as Lanova, used a mechanical system to adjust the shape of the chamber for starting and running conditions. However, the most commonly used design turned out to be the "Comet" series of swirl chambers developed by Sir Harry Ricardo, using a two-piece spherical chamber with a narrow "throat" to induce turbulence. Most European manufacturers of high-speed diesel engines used Comet-type chambers or developed their own versions (Mercedes stayed with their own design for many years), and this trend continues with current indirect injection engines.

[edit] Direct injection

Modern diesel engines make use of one of the following direct injection methods:

[edit] Distributor pump direct injection

The first incarnations of direct injection diesels used a rotary pump much like indirect injection diesels; however the injectors were mounted in the top of the combustion chamber rather than in a separate pre-combustion chamber. Examples are vehicles such as the Ford Transit and the Austin Rover Maestro and Montego with their Perkins Prima engine. The problem with these vehicles was the harsh noise that they made and particulate (smoke) emissions. This is the reason that in the main this type of engine was limited to commercial vehicles, the notable exceptions being the Maestro, Montego and Fiat Croma passenger cars. Fuel consumption was about fifteen to twenty percent lower than indirect injection diesels, which for some buyers was enough to compensate for the extra noise.

One of the first small-capacity, mass produced direct injection engines that could be called refined was developed by the Rover Group.^{[citation needed]} The 200Tdi 2.5-litre four-cylinder turbodiesel was used by Land Rover in their vehicles from 1989, and the engine used an aluminum cylinder head, Bosch two-stage injection and multi-phase glow plugs to produce a smooth-running and economical engine while still using mechanical fuel injection.

This type of engine was transformed by electronic control of the injection pump, pioneered by the Volkswagen Group with the Audi 100 TDI introduced in 1989. The injection pressure was still only around 300 bar, but the injection timing, fuel quantity, EGR and turbo boost were all electronically controlled. This gave much more precise control of these parameters which made refinement much more acceptable and emissions acceptably low. Fairly quickly the technology trickled down to more mass market vehicles such as the Mark 3 Golf TDI where it proved to be very popular. These cars were both more economical and more powerful than indirect injection competitors of their day.

[edit] Unit direct injection

Unit direct injection also injects fuel directly into the cylinder of the engine. However, in this system the injector and the pump are combined into one unit positioned over each cylinder. Each cylinder thus has its own pump, feeding its own injector, which prevents pressure fluctuations and allows more consistent injection to be achieved. This type of injection system, also developed by Bosch, is used by Volkswagen AG in cars (where it is called a Pumpe-Düse-System — literally "pump-nozzle system") and by Mercedes Benz (PLD) and most major diesel engine manufacturers in large commercial engines (CAT, Cummins, Detroit Diesel). With recent advancements, the pump pressure has been raised to 2,050 bar (205 MPa), allowing injection parameters similar to common rail systems.

[edit] Common rail direct injection

Main article: Common rail

In older diesel engines, a distributor-type injection pump, regulated by the engine, supplies bursts of fuel to injectors which are simply nozzles through which the diesel is sprayed into the engine's combustion chamber.

In common rail systems, the distributor injection pump is eliminated. Instead, a high-pressure pump pressurises fuel at up to 2,000 bar (200 MPa, 29,000 psi)^[3], in a "common rail". The common rail is a tube that branches off to computer-controlled injector valves, each of which contains a precision-machined nozzle and a plunger driven by a solenoid or piezoelectric actuators. (For example, Mercedes uses piezoelectric actuators in their high power output 3.0L V6 common rail diesel).

Most European automakers have common rail diesels in their model lineups, even for commercial vehicles. Some Japanese manufacturers, such as Toyota, Nissan and recently Honda, have also developed common rail diesel engines. Some Indian companies have also successfully implemented this technology.

Different car makers refer to their common rail engines by different names, e.g., DaimlerChrysler's CDI, Ford Motor Company's TDCi (most of these engines are manufactured by PSA), Fiat Group's (Fiat, Alfa Romeo and Lancia) JTD, Renault's dCi, GM/Opel's CDTi (most of these engines are manufactured by Fiat, other by Isuzu), Hyundai's CRDi, Mitsubishi's DI-D, PSA Peugeot Citroën's HDi (Engines for commercial diesel vehicles are made by Ford Motor Company), Toyota's D-4D, and so on.Mahindra & Mahindra for their 'Scorpio-CRDe' and Tata Motors for their 'Safari-DICOR'.

[edit] Types of diesel engines

[edit] Early diesel engines

Rudolph Diesel intended his engine to replace the steam engine as the primary power source for industry. As such diesel engines in the late 19th- and early 20th-centuries used the same basic layout and form as industrial steam engines, with long-bore cylinders, external valve gear, cross-head bearings and an open crankshaft connected to a large flywheel. Smaller engines would be built with vertical cylinders, whilst most medium- and large-sized industrial engines were built with horizontal cylinders, just as steam engines had been. Engines could be built with more than one cylinder in both cases. The largest early diesels resembled the triple-expansion reciprocating engine steam engine, being tens of feet high with vertical cylinders arranged in-line. These early engines ran at very slow speeds — partly due to the limitations of their air-blast injector equipment and partly so they would be compatible with the majority of industrial equipment designed for steam engines — speed ranges of between 100 and 300 RPM were common. Engines were usually started by allowing compressed air into the cylinders to turn the engine, although smaller engines could be started by hand.

In the early decades of the 20th century, when large diesel engines were first being fitted to ships, the engines took a form similar to the compound steam engines common at the time, with the piston being connected to the connecting rod via a crosshead bearing. Following steam engine practice, double-acting 4-stroke diesel engines were constructed to increase power output, with combustion taking place on both sides of the piston, with two sets of valve gear and fuel injection. This system also meant that the engine's direction of rotation could be reversed by altering the injector timing. This meant the engine could be coupled directly to the propeller without the need for a gearbox. Whilst producing large amounts of power and being very efficient, the double-acting diesel engine's main problem was producing a good seal where the piston rod passed through the bottom of the lower combustion chamber to the crosshead bearing. By the 1930s it was found easier and more reliable to fit turbochargers to the engines, although crosshead bearings are still used to reduce the stress on the crankshaft bearings, and the wear on the cylinders, in large long-stroke cathedral engines.

[edit] Modern diesel engines

As with gasoline engines, there are two classes of Diesel engines in current use: two-stroke and four-stroke. The four-stoke type is the "classic" version, tracing its lineage back to Dr. Diesel's prototype. It is also the most commonly used type, being the preferred power source for many motor vehicles, especially buses and trucks. Much larger engines, such as used for railroad locomotion and marine propulsion, are often two-stroke units, offering a more favorable horsepower-to-weight ratio, as well as better fuel economy. The most powerful engines in the world are two-cycle Diesels of mammoth proportions. These so-called low speed Diesels are able to achieve thermal efficiencies approaching fifty percent.

Two-stroke Diesel operation is similar to that of gasoline counterparts, except that fuel is not mixed with air prior to induction, and the crankcase does not take an active role in the cycle. The two-stroke Diesel depends upon forced aspiration to charge the cylinders with air and to scavenge exhaust gasses. The traditional two-stroke design relies upon a mechanically driven, positive displacement blower to charge the cylinders prior to compression and ignition. The archetype of this design is the Detroit Diesel engine, in which the blower pressurizes a chamber in the engine block often referred to as the "air box." The (much larger) Electromotive prime mover utilized in EMD Diesel-electric locomotives is built to the same principle.

In the EMD prime mover the movement of the piston in its cycle uncovers direct openings through the cylinder shaft into intake and exhaust manifolds; which through their blower generated higher air pressure either charge or scavenge the internal volume of each cylinder, at the correct time in the piston stroke. That is, at the end of exhaust, through intake, and prior to the compression stroke. Then at the end of the power stroke and through the exhaust stroke. Thus in the EMD two-stroke cycle engine there is no separate mechanical cylinder valving or other complex appratus which controls access to the cylinder(s) for feeding a cylinder its air charge or scavenging a cylinder of its waste gases. This mechanical simplicity greatly enhanced the early acceptance of this type of engine.

When the cylinder's piston approaches bottom dead center in a two-stroke Diesel, a passage between the air box and the cylinder is opened, permitting air flow into the cylinder. During this time, the exhaust valves are opened and some of the air flow forces the remaining combustion gasses from the cylinder. As the piston passes bottom center and starts upward, the passage is closed and compression commences, culminating in fuel injection and ignition. Refer to two-stroke Diesel engines for more discussion concerning aspiration issues with a two-stroke engine.

Normally, the number of cylinders are used in multiples of two, although any number of cylinders can be used as long as the load on the crankshaft is counterbalanced to prevent excessive vibration. The inline-six cylinder design is the most prolific in light to medium-duty engines, though small V8 and larger inline-four displacement engines are also common. Small-capacity engines (generally considered to be those below five litres in capacity) are generally four or six cylinder types, with the four cylinder being the most common type found in automotive uses. Five cylinder diesel engines have also been produced, being a compromise between the smooth running of the six cylinder and the space-efficient dimensions of the four cylinder. Diesel engines for smaller plant machinery, boats, tractors, generators and pumps may be four, three or two cylinder types, with the single cylinder Diesel engine remaining for light stationary work.

The desire to improve the diesel engine's power-to-weight ratio produced several novel cylinder arrangements to extract more power from a given capacity. The Napier Deltic engine, with three cylinders arranged in a triangular formation, each containing two opposed-action pistons, the whole engine having three crankshafts, is one of the better known. The Commer van company of the United Kingdom used a similar design for road vehicles,designed by Tillings-Stevens,member of the Rootes Group,the TS3. The Commer TS3 engine had 3 horizontal in-line cylinders,each with two opposed action pistons that worked through rocker arms,to connecting rods and had one crankshaft. While both these designs succeeded in producing greater power for a given capacity, they were complex and expensive to produce and operate, and when turbocharger technology improved in the 1960s this was found to be a much more reliable and simple way of extracting more power.

As a footnote, prior to 1949, Sulzer started experimenting with two-stroke engines with boost pressures as high as 6 atmospheres, in which all of the output power was taken from an exhaust turbine. The two-stroke pistons directly drove air compressor pistons to make a positive displacement gas generator. Opposed pistons were connected by linkages instead of crankshafts. Several of these units could be connected together to provide power gas to one large output turbine. The overall thermal efficiency was roughly twice that of a simple gas turbine. (Source Modern High-Speed Oil Engines, Volume II by C. W. Chapman, published by The Caxton Publishing Co. Ltd. Reprinted in July 1949)

[edit] Carbureted compression ignition model engines

Simple compression ignition engines are made for model propulsion. This is quite similar to the typical glow-plug engine that runs on a mixture of methanol (methyl alcohol) and lubricant (typically castor oil) (and occasionally nitro-methane to improve performance) with a hot wire filament to provide ignition. Rather than containing a glow plug the head has an adjustable contra piston above the piston, forming the upper surface of the combustion chamber. This contra piston is restrained by an adjusting screw controlled by an external lever (or sometimes by a removable hex key). The fuel used contains ether, which is highly volatile and has an extremely low flash point, combined with kerosene and a lubricant plus a very small proportion (typically 2%) of ignition improver such as Amyl nitrate or preferably Isopropyl nitrate nowadays. The engine is started by reducing the compression and setting the spray bar mixture rich with the adjustable needle valve, gradually increasing the compression while cranking the engine. The compression is increased until the engine starts running. The mixture can then be leaned out and the compression increased. Compared to glow plug engines, model diesel engines exhibit much higher fuel economy, thus increasing endurance for the amount of fuel carried. They also exhibit higher torque, enabling the turning of a larger or higher pitched propeller at slower speed. Since the combustion occurs well before the exhaust port is uncovered, these engines are also considerably quieter (when unmuffled) than glow-plug engines of similar displacement. Compared to glow plug engines, model diesels are more difficult to throttle over a wide range of powers, making them less suitable for radio control models than either two or four stroke glow-plug engines although this difference is claimed to be less noticeable with the use of modern schneurle-ported engines.

[edit] Advantages and disadvantages versus spark-ignition engines

[edit] Power and fuel economy

Diesel engines are more efficient than gasoline (petrol) engines of the same power, resulting in lower fuel consumption. A common margin is 40% more miles per gallon for an efficient turbodiesel. For example, the current model Škoda Octavia, using Volkswagen Group engines, has a combined Euro rating of 38 miles per US gallon (6.2 L/100 km) for the 102 bhp (76 kW) petrol engine and 54 mpg (4.4 L/100 km) for the 105 bhp (78 kW) diesel engine. However, such a comparison doesn't take into account that diesel fuel is denser and contains about 15% more energy by volume. Although the calorific value of the fuel is slightly lower at 45.3 MJ/kg (megajoules per kilogram) than gasoline at 45.8 MJ/kg, liquid diesel fuel is significantly denser than liquid gasoline. When this is taken into account, diesel fuel has a higher energy density than petrol; this volumetric measure is the main concern of many people, as diesel fuel is sold by volume, not weight, and must be transported and stored in tanks of fixed size.

Adjusting the numbers to account for the energy density of diesel fuel, one finds the overall energy efficiency of the aforementioned paragraph is still about 20% greater for the diesel version, despite the weight penalty of the diesel engine. When comparing engines of relatively low power for the vehicle's weight (such as the 75 hp VW Golf), the diesel's overall energy efficiency advantage is reduced further but still between 10 and 15 percent.

While higher compression ratio is helpful in raising efficiency, diesel engines are much more economical than gasoline (petrol) engines when at low power and at engine idle. Unlike the petrol engine, diesels lack a butterfly valve (throttle) in the inlet system, which closes at idle. This creates parasitic drag on the incoming air, reducing the efficiency of petrol/gasoline engines at idle. Due to their lower heat losses, diesel engines have a lower risk of gradually overheating if left idling for long periods of time. In many applications, such as marine, agriculture, and railways, diesels are left idling unattended for many hours or sometimes days. These advantages are especially attractive in locomotives (see dieselization).

Naturally aspirated diesel engines are heavier than gasoline engines of the same power for two reasons. The first is that it takes a larger displacement diesel engine to produce the same power as a gasoline engine. This is essentially because the diesel must operate at lower engine speeds.^[4] Diesel fuel is injected just before ignition, leaving the fuel little time to reach all the oxygen in the cylinder. In the gasoline engine, air and fuel are mixed for the entire compression stroke, ensuring complete mixing even at higher engine speeds. The second reason for the greater weight of a diesel engine is it must be stronger to withstand the higher combustion pressures needed for ignition, and the shock loading from the detonation of the ignition mixture. As a result, the reciprocating mass (the piston and connecting rod), and the resultant forces to accelerate and to decelerate these masses, are substantially higher the heavier, the bigger and the stronger the part, and the laws of diminishing returns of component strength, mass of component and inertia — all come into play to create a balance of offsets, of optimal mean power output, weight and durability.

Yet it is this same build quality that has allowed some enthusiasts to acquire significant power increases with turbocharged engines through fairly simple and inexpensive modifications. A gasoline engine of similar size cannot put out a comparable power increase without extensive alterations because the stock components would not be able to withstand the higher stresses placed upon them. Since a diesel engine is already built to withstand higher levels of stress, it makes an ideal candidate for performance tuning with little expense. However, it should be said that any modification that raises the amount of fuel and air put through a diesel engine will increase its operating temperature which will reduce its life and increase its service interval requirements. These are issues with newer, lighter, high performance diesel engines which aren't "overbuilt" to the degree of older engines and are being pushed to provide greater power in smaller engines.

The addition of a turbocharger or supercharger to the engine greatly assists in increasing fuel economy and power output, mitigating the fuel-air intake speed limit mentioned above for a given engine displacement. Boost pressures can be higher on diesels than gasoline engines, due to the latter's susceptibility to knock, and the higher compression ratio allows a diesel engine to be more efficient than a comparable spark ignition engine. Because the burned gases are expanded further in a diesel engine cylinder, the exhaust gas is cooler, meaning turbochargers require less cooling, and can be more reliable, than on spark-ignition engines.

The increased fuel economy of the diesel engine over the gasoline engine means that the diesel produces less carbon dioxide (CO₂) per unit distance. Recently, advances in production and changes in the political climate have increased the availability and awareness of biodiesel, an alternative to petroleum-derived diesel fuel with a much lower net-sum emission of CO₂, due to the absorption of CO₂ by plants used to produce the fuel.

The two main factors that held diesel engine back in private vehicles until quite recently were their low power outputs and high noise levels, characterised by knock or clatter, especially at low speeds and when cold. This noise was caused by the sudden ignition of the diesel fuel when injected into the combustion chamber. This noise was a product of the sudden temperature change, hence it was more pronounced at low engine temperatures. A combination of improved mechanical technology (such as two-stage injectors which fire a short "pilot charge" of fuel into the cylinder to warm the combustion chamber before delivering the main fuel charge) and electronic control (which can adjust the timing and length of the injection process to optimise it for all speeds and temperatures) have partially mitigated these problems in the latest generation of common-rail designs. Poor power and narrow torque bands have been helped by the use of turbochargers and intercoolers.

[edit] Emissions

Diesel engines produce very little carbon monoxide as they burn the fuel in excess air even at full load, at which point the quantity of fuel injected per cycle is still about 50% lean of stoichiometric. However, they can produce black soot (or more specifically diesel particulate matter) from their exhaust, which consists of unburned carbon compounds. This is often caused by worn injectors, which do not atomize the fuel sufficiently, or a faulty engine management system, allowing more fuel to be injected than can be burned completely in the available time.

The full load limit of a diesel engine in normal service is defined by the "black smoke limit", beyond which point the fuel cannot be completely combusted; as the "black smoke limit" is still considerably lean of stoichiometric it is possible to obtain more power by exceeding it, but the resultant inefficient combustion means that the extra power comes at the price of reduced combustion efficiency, high fuel consumption and dense clouds of smoke, so this is only done in specialised applications (such as tractor pulling) where these disadvantages are of little concern.

Likewise, when starting from cold, the engine's combustion efficiency is reduced because the cold engine block draws heat out of the cylinder in the compression stroke. The result is that fuel is not combusted fully, resulting in blue/white smoke and lower power outputs until the engine has warmed through. This is especially the case with indirect injection engines, which are less thermally efficient. With electronic injection, the timing and length of the injection sequence can be altered to compensate for this. Older engines with mechanical injection can have manual control to alter the timing, or multi-phase electronically-controlled glow plugs, that stay on for a period after start-up to ensure clean combustion — the plugs are automatically switched to a lower power to prevent them burning out.

Particles of the size normally called PM10 (particles of 10 micrometres or smaller) have been implicated in health problems, especially in cities. Some modern diesel engines feature diesel particulate filters, which catch the black soot and when saturated are automatically regenerated by burning the particles. Other problems associated with the exhaust gases (nitrogen oxides, sulfur oxides) can be mitigated with further investment and equipment; some diesel cars now have catalytic converters in the exhaust.

[edit] Power and torque

For commercial uses requiring towing, load carrying and other tractive tasks, diesel engines tend to have better torque characteristics. Diesel engines tend to have their torque peak quite low in their speed range (usually between 1600 – 2000 rpm for a small-capacity unit, lower for a larger engine used in a truck). This provides smoother control over heavy loads when starting from rest, and, crucially, allows the diesel engine to be given higher loads at low speeds than a petrol engine, making them much more economical for these applications. This characteristic is not so desirable in private cars, so most modern diesels used in such vehicles use electronic control, variable geometry turbochargers and shorter piston strokes to achieve a wider spread of torque over the engine's speed range, typically peaking at around 2500 – 3000 rpm.

[edit] Reliability

The lack of an electrical ignition system greatly improves the reliability. The high durability of a diesel engine is also due to its overbuilt nature (see above) as well as the diesel's combustion cycle, which creates less-violent changes in pressure when compared to a spark-ignition engine, a benefit that is magnified by the lower rotating speeds in diesels. Diesel fuel is a better lubricant than gasoline so is less harmful to the oil film on piston rings and cylinder bores; it is routine for diesel engines to cover 250,000