indirect and direct injection diesel engine which is best

Indirect injection engines

In an internal combustion engine, the term indirect injection refers to a fuel injection where fuel is not directly injected into the combustion chamber. Gasoline engines are usually equipped with indirect injection systems, wherein a fuel injector delivers the fuel at some point before the intake valve.
An indirect injection diesel engine delivers fuel into a chamber off the combustion chamber, called a prechamber, where combustion begins and then spreads into the main combustion chamber. The prechamber is carefully designed to ensure adequate mixing of the atomized fuel with the compression-heated air. This has the effect of slowing the rate of combustion, which tends to reduce audible noise and softens the shock of combustion and produces lower stresses on the engine components. The addition of a prechamber, however, increases heat loss to the cooling system and thereby lowers engine efficiency and requiring glow plugs for starting. In an indirect injection system the fuel/air mixing occurs with the air moving fast. This simplifies injector design and allows the use of smaller engines and less tightly toleranced designs which are simpler to manufacture and more reliable. Direct injection, by contrast, uses slow-moving air and fast-moving fuel; both the design and manufacture of the injectors is more difficult, the optimisation of the in-cylinder air flow is much more difficult than designing a prechamber, and there is much more integration between the design of the injector and that of the engine it is to be used in.[1] It is for this reason that car diesel engines were almost all indirect injection until the ready availability of powerful CFD simulation systems made the adoption of direct injection practical.[citation needed]
Aside from the above advantages, early diesels often employed indirect injection in order to use simple, flat-top pistons, and made the positioning of the early, bulky diesel injectors easier.[citation needed]
Advantages of indirect injection combustion chambers
Smaller diesels can be produced.
The injection pressure required is low, therefore making the injector cheaper to produce.
The injection direction is of less importance
Indirect injection is much simpler to design and manufacture; less injector development is required and the injection presures are low (1500 psi versus 5000 psi and higher for direct injection)
The lower stresses that indirect injection imposes on internal components means that it is possible to produce petrol and indirect injection diesel versions of the same basic engine- at best such types differ only in the cylinder head and the need to fit a distributor and spark plugs in the petrol version whilst fitting an injection pump and injectors to the diesel. Examples include the BMC A-Series and B-Series engines and the Land Rover 2.25/2.5-litre 4-cylinder types. Such designs allow petrol and diesel versions of the same vehicle to be built with minimal design changes between them.
Higher engine speeds can be reached, since burning continues in the prechamber. The Mercedes-Benz type prechamber is able to achieve a peak power of over 6000rpm in a turbo charged engine.[citation needed]
Disadvantages
Specific fuel consumption is high because of heat loss due to large exposed areas and pressure loss due to air motion through the throats.
Glowplugs are needed for a cold engine start.
Because the heat and pressure of combustion is applied to one specific point on the piston as it exits the precombustion chamber or swirl chamber, such engines are less suited to high specific power outputs (such as turbocharging or tuning) than direct injection diesels. The increased temperature and pressure on one part of the piston crown causes uneven expansion which can lead to cracking, distortion or other damage. This can be solved by designing the pistons to have a slight oval shape so that when heated unevenly they become circular.[citation needed] The higher the power required from a given engine design the greater degree of ovality is required until it becomes impractical.[citation needed] Direct injection engines deliver fuel to the centre of the piston crown, negating these problems.
[edit] Maintenance hazards
Fuel injection introduces potential hazards in engine maintenance due to the high fuel pressures used. Residual pressure can remain in the fuel lines long after an injection-equipped engine has been shut down. This residual pressure must be relieved, and if it is done so by external bleed-off, the fuel must be safely contained. If a high-pressure diesel fuel injector is removed from its seat and operated in open air, there is a risk to the operator of injury by hypodermic jet-injection, even with only 100 psi pressure. [6]. The first known such injury occurred in 1937 during a diesel engine maintenance operation .[7]
Direct injection
Modern Diesel engines make use of one of the following direct injection methods:
Direct injection injectors are mounted in the top of the combustion chamber. The problem with these vehicles was the harsh noise that they made. Fuel consumption was about 15 to 20 percent lower than indirect injection Diesels, which for some buyers was enough to compensate for the extra noise.
This type of engine was transformed by electronic control of the injection pump, pioneered by the FIAT in 1988 (Croma). The injection pressure was still only around 300 bar (30 MPa; 4,400 psi), but the injection timing, fuel quantity, EGR and turbo boost were all electronically controlled. This gave more precise control of these parameters which made refinement more [edit] Unit direct injection
Main article: Unit Injector
Unit direct injection also injects fuel directly into the cylinder of the engine. In this system the injector and the pump are combined into one unit positioned over each cylinder controlled by the camshaft. Each cylinder has its own unit eliminating the high pressure fuel lines, achieving a more consistent injection. 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, Volvo). With recent advancements, the pump pressure has been raised to 2,400 bar (240 MPa; 35,000 psi)[23], allowing injection parameters similar to common rail systems.[24]
[edit] Common rail direct injection
Main article: Common rail
In common rail systems, the separate pulsing high pressure fuel line to each cylinder injector is also eliminated. Instead, a high-pressure pump pressurizes fuel at up to 2,000 bar (200 MPa; 29,000 psi),[25] in a "common rail". The common rail is a tube that supplies each computer-controlled injector containing a precision-machined nozzle and a plunger driven by a solenoid or piezoelectric actuator.
[edit] Types
[edit] Early
Rudolf 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, while 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; maximum speeds 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.[26]
In the early decades of the 20th century, when large Diesel engines were first being used, 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 some manufactures made double-acting two-stroke and four-stroke Diesel engines to increase power output, with combustion taking place on both sides of the piston, with two sets of valve gear and fuel injection. While it produced large amounts of power and was 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, and no more were built. By the 1930s turbochargers were fitted to some engines. Crosshead bearings are still used to reduce the wear on the cylinders in large long-stroke main marine engines.
[edit] Modern

A Yanmar 2GM20 marine Diesel engine, installed in a sailboat.
As with petrol engines, there are two classes of Diesel engines in current use: two-stroke and four-stroke. The four-stroke type is the "classic" version, tracing its lineage back to Rudolf Diesel's prototype. It is also the most commonly used form, 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 power-to-weight ratio, as well as better fuel economy. The most powerful engines in the world are two-stroke Diesels of mammoth dimensions.[27]
Two-stroke Diesel operation is similar to that of petrol 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 traditional two-stroke design relies upon a mechanically driven positive displacement blower to charge the cylinders with air prior to compression and ignition. The charging process also assists in expelling (scavenging) combustion gases remaining from the previous power stroke. The archetype of the modern form of the two-stroke Diesel is the Detroit Diesel engine, in which the blower pressurizes a chamber in the engine block that is 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 a two-stroke Diesel engine, as the cylinder's piston approaches the bottom dead center exhaust ports or valves are opened relieving most of the excess pressure after which a passage between the air box and the cylinder is opened, permitting air flow into the cylinder.[28][29][30] The air flow blows the remaining combustion gases from the cylinder—this is the scavenging process. As the piston passes through 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 detailed coverage of aspiration types and supercharging of 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. Direct reversible two-stroke marine Diesels need at least three cylinders for reliable restarting forwards and reverse. Four-stroke engines need at least six cylinders, providing repeated power strokes at 120 degrees.
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 opposed-piston engine uses two pistons in one cylinder with the combustion cavity in the middle and gas in- and outlets at the ends. This makes a comparatively light, powerful, swiftly running and economic engine suitable for use in aviation. An example is the Junkers Jumo 204/205. 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.
Advantages and disadvantages versus spark-ignition engines
[edit] Power and fuel economy
The MAN S80ME-C7 low speed Diesel engines use 155 gram fuel per kWh for an overall energy conversion efficiency of 54.4%, which is the highest conversion of fuel into power by any internal or external combustion engine.[1] Diesel engines are more efficient than petrol (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 6.2 L/100 km (38 miles per US gallon, 16 km/L) for the 102 bhp (76 kW) petrol engine and 4.4 L/100 km (54 mpg) 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 petrol at 45.8 MJ/kg, liquid Diesel fuel is significantly denser than liquid petrol. This is important because volume of fuel, in addition to mass, is an important consideration in mobile applications. No vehicle has an unlimited volume available for fuel storage.
Adjusting the numbers to account for the energy density of Diesel fuel, the overall energy efficiency is still about 20% greater for the Diesel version.
While higher compression ratio is helpful in raising efficiency, Diesel engines are much more efficient than petrol (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 loss and destruction of availability on the incoming air, reducing the efficiency of petrol engines at idle. 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 dieselisation).
Weight can be an issue, since Diesel engines are typically heavier than petrol engines of similar power output. This is essentially because the Diesel must operate at lower engine speeds.[32] Even more than that, the high weight of a diesel is mandated by the need to make parts stronger to resist the higher operating pressure from the high compression ratio of the engine, as well as the large amounts of torque generated to the crankshaft. In many instances, diesel engines are also heavier as a result of their intended benefits. Diesels have historically been know for their long lives and reliability, which are important in industrial applications such as the diesel is often used for. The stronger parts last longer than lighter parts, albeit making the average diesel have a poorer power/weight ratio. For most industrial or nautical applications, reliability is considered more important than light weight and high power. Diesel fuel is injected just before the power stroke. As a result of this, the fuel cannot burn completely unless it has a sufficient amount of oxygen. This can result in incomplete combustion and black smoke in the exhaust if more fuel is injected than there is air for it the be burned by. Modern engines with electronic fuel delivery can adjust the timing and amount of fuel delivery (by changing the duration of the injection pulse), and so operate with less waste of fuel. In a mechanical system, the injection timing and duration must be set to be efficient at the anticipated operating RPM's and load, and so the settings are less than ideal when the engine is running at any other RPM than what it is timed for. The electronic injection can "sense" engine revs, load, even boost and temperature, and continuously alter the timing to match the situation. In the petrol engine, air and fuel are mixed for the entire compression stroke, ensuring complete mixing even at higher engine speeds.
Diesel engines usually have longer stroke lengths to achieve the necessary compression ratios. As a result piston and connecting rods are heavier and more force must be transmitted through the connecting rods and crankshaft to change the momentum of the piston. This is another reason that a Diesel engine must be stronger for the same power output.
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 petrol 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 service requirements. These are issues with newer, lighter, high performance Diesel engines which are not "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 petrol 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.
With a Diesel, boost pressure is essentially unlimited. It is literally possible to run as much boost as the engine will physically stand before breaking apart.
The increased fuel economy of the Diesel engine over the petrol 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. Although concerns are now being raised as to the negative effect this is having on the world food supply, as the growing of crops specifically for biofuels takes up land that could be used for food crops and uses water that could be used by both humans and animals. The use of waste vegetable oil, sawmill waste from managed forests in Finland funded by Nokia venture capital, and the development of the production of vegetable oil from algae, demonstrate great promise in providing feed stocks for sustainable biodiesel, that are not in competition with food production.
Diesel engines have lower power output than equivalent size petrol engine because its speed is limited by the time required for combustion.[33] A combination of improved mechanical technology (such as multi-stage injectors which fire a short "pilot charges" of fuel into the cylinder to warm the combustion chamber before delivering the main fuel charge), higher injection pressures that have improved the atomisation of fuel into smaller droplets, and electronic control (which can adjust the timing and length of the injection process to optimise it for all speeds and temperatures), have mostly mitigated these problems in the latest generation of common-rail designs, while greatly improving engine efficiency. Poor power and narrow torque bands have been addressed by the use of superchargers, turbochargers, (especially variable geometry turbochargers), intercoolers, and a large efficiency increase from about 35% for IDI to 45% for the latest engines in the last 15 years.
Even though Diesel engines have a theoretical fuel efficiency of 75%, in practice it is less. Engines in large Diesel trucks, buses, and newer Diesel cars can achieve peak efficiencies around 45%,[34] and could reach 55% efficiency in the near future.[35] However, average efficiency over a driving cycle is lower than peak efficiency. For example, it might be 37% for an engine with a peak efficiency of 44%.[36]
[edit] Emissions
See also: Diesel particulate matter, Diesel exhaust air contaminants, Diesel fuel#Health effects, Diesel engine#Emissions, and Exhaust gas#Diesel engines
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 caused by local low temperatures where the fuel is not fully atomized. These local low temperatures occur at the cylinder walls and at the outside of large droplets of fuel. At these areas where it is relatively cold, the mixture is rich (contrary to the overall mixture which is lean). The rich mixture has less air to burn and some of the fuel turns into a carbon deposit. Modern car engines use a Diesel particulate filter (DPF) to capture carbon particles and then intermittently burn them using extra fuel injected into the engine.
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. This is only done in specialized applications (such as tractor pulling competitions) 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 mechanical and hydraulic governor control to alter the timing, and multi-phase electrically 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.
All Diesel engine exhaust emissions can be significantly reduced by the use of biodiesel fuel. Oxides of nitrogen do increase from a vehicle using biodiesel, but they too can be reduced to levels below that of fossil fuel Diesel, by changing fuel injection timing.
[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 and 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.
While Diesel engines tend to have more torque at lower engine speeds than petrol engines, Diesel engines tend to have a narrower power band than petrol engines. Naturally-aspirated Diesels tend to lack power and torque at the top of their speed range. This narrow band is a reason why a vehicle such as a truck may have a gearbox with as many as 18 or more gears, to allow the engine's power to be used effectively at all speeds. Turbochargers tend to improve power at high engine speeds; superchargers improve power at lower speeds; and variable geometry turbochargers improve the engine's performance equally by flattening the torque curve.
[edit] Noise
The characteristic noise of a Diesel engine is variably called Diesel clatter, Diesel nailing, or Diesel knock.[37] Diesel clatter is caused largely by the Diesel combustion process, the sudden ignition of the Diesel fuel when injected into the combustion chamber causes a pressure wave. Engine designers can reduce Diesel clatter through: indirect injection; pilot or pre-injection; injection timing; injection rate; compression ratio; turbo boost; and exhaust gas recirculation (EGR).[38] Common rail Diesel injection systems permit multiple pre-injections as an aid to noise reduction. Diesel fuels with a higher cetane rating modify the combustion process and reduce Diesel clatter.[37] CN (Cetane number) can be raised by distilling higher quality crude oil, or by using a cetane improving additive. Some oil companies market high cetane or premium Diesel. Biodiesel has a higher cetane number than petrodiesel, typically 55CN for 100% biodiesel.[citation needed]
A combination of improved mechanical technology such as multi-stage injectors which fire a short "pilot charges" of fuel into the cylinder to initiate combustion before delivering the main fuel charge, higher injection pressures that have improved the atomisation of fuel into smaller droplets, and electronic control (which can adjust the timing and length of the injection process to optimise it for all speeds and temperatures), have mostly mitigated these problems in the latest generation of common-rail designs, while improving engine efficiency.
[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), a benefit that is magnified by the lower rotating speeds in Diesels. Diesel fuel is a better lubricant than petrol so is less harmful to the oil film on piston rings and cylinder bores; it is routine for Diesel engines to cover 250,000 miles (400,000 km) or more without a rebuild.
Due to the greater compression force required and the increased weight of the stronger components, starting a Diesel engine is harder. More torque is required to push the engine through compression.
Either an electrical starter or an air start system is used to start the engine turning. On large engines, pre-lubrication and slow turning of an engine, as well as heating, are required to minimize the amount of engine damage during initial start-up and running. Some smaller military Diesels can be started with an explosive cartridge, called a Coffman starter, which provides the extra power required to get the machine turning. In the past, Caterpillar and John Deere used a small petrol pony motor in their tractors to start the primary Diesel motor. The pony motor heated the Diesel to aid in ignition and utilized a small clutch and transmission to actually spin up the Diesel engine. Even more unusual was an International Harvester design in which the Diesel motor had its own carburetor and ignition system, and started on petrol. Once warmed up, the operator moved two levers to switch the motor to Diesel operation, and work could begin. These engines had very complex cylinder heads, with their own petrol combustion chambers, and in general were vulnerable to expensive damage if special care was not taken (especially in letting the engine cool before turning it off).
[edit] Quality and variety of fuels
Petrol/gasoline engines are limited in the variety and quality of the fuels they can burn. Older petrol engines fitted with a carburetor required a volatile fuel that would vaporize easily to create the necessary fuel/air mix for combustion. Because both air and fuel are admitted to the cylinder, if the compression ratio of the engine is too high or the fuel too volatile (with too low an octane rating), the fuel will ignite under compression, as in a Diesel engine, before the piston reaches the top of its stroke. This pre-ignition causes a power loss and over time major damage to the piston and cylinder. The need for a fuel that is volatile enough to vaporize but not too volatile (to avoid pre-ignition) means that petrol engines will only run on a narrow range of fuels. There has been some success at dual-fuel engines that use petrol/ethanol, petrol/propane, and petrol/methane.
In Diesel engines, a mechanical injector system vaporizes the fuel directly into the combustion chamber or a pre-combustion chamber (as opposed to a Venturi jet in a carburetor, or a Fuel injector in a fuel injection system vaporizing fuel into the intake manifold or intake runners as in a petrol engine). This forced vaporisation means that less-volatile fuels can be used. More crucially, because only air is inducted into the cylinder in a Diesel engine, the compression ratio can be much higher as there is no risk of pre-ignition provided the injection process is accurately timed. This means that cylinder temperatures are much higher in a Diesel engine than a petrol engine, allowing less volatile fuels to be used.
Diesel fuel is a form of light fuel oil, very similar to kerosene/paraffin, but Diesel engines, especially older or simple designs that lack precision electronic injection systems, can run on a wide variety of other fuels. Some of the most common alternatives are Jet A-1 or vegetable oil from a very wide variety of plants. Some engines can be run on vegetable oil without modification, and most others require fairly basic alterations. Biodiesel is a pure Diesel-like fuel refined from vegetable oil and can be used in nearly all Diesel engines. The only limits on the fuels used in Diesel engines are the ability of the fuel to flow along the fuel lines and the ability of the fuel to lubricate the injector pump and injectors adequately. In general terms, inline mechanical injector pumps tolerate poor-quality or bio-fuels better than distributor-type pumps. Also, indirect injection engines generally run more satisfactorily on bio-fuels than direct injection engines. This is partly because an indirect injection engine has a much greater 'swirl' effect, improving vaporisation and combustion of fuel, and also because (in the case of vegetable oil-type fuels) lipid depositions can condense on the cylinder walls of a direct-injection engine if combustion temperatures are too low (such as starting the engine from cold).
It is often reported that Diesel designed his engine to run on peanut oil, but this is not the case. Diesel stated in his published papers, "at the Pairs Exhibition in 1900 (Exposition Universelle) there was shown by the Otto Company a small Diesel engine, which, at the request of the French Government ran on Arachide (earth-nut or pea-nut) oil (see biodiesel), and worked so smoothly that only a few people were aware of it. The engine was constructed for using mineral oil, and was then worked on vegetable oil without any alterations being made. The French Government at the time thought of testing the applicability to power production of the Arachide, or earth-nut, which grows in considerable quantities in their African colonies, and can easily be cultivated there." Diesel himself later conducted related tests and appeared supportive of the idea.[39]
Most large marine Diesels (often called cathedral engines due to their size) run on heavy fuel oil (sometimes called "bunker oil"), which is a thick, viscous and almost un-flammable fuel which is very safe to store and cheap to buy in bulk as it is a waste product from the petroleum refining industry. The fuel must be heated to thin it out (often by the exhaust header) and is often passed through multiple injection stages to vaporize it.
***best is direct injection engine******