Multifuel engine

Multifuel is any type of engine, boiler, or heater or other fuel-burning device which is designed to burn multiple types of fuels in its operation. One common application of multifuel technology is in military settings, where the normally-used diesel or gas turbine fuel might not be available during combat operations for vehicles or heating units. Multifuel engines and boilers have a long history, but the growing need to establish fuel sources other than petroleum for transportation, heating, and other uses has led to increased development of multifuel technology for non-military use as well, leading to many flexible-fuel vehicle designs in recent decades.

A multifuel engine is constructed so that its compression ratio permits firing the lowest octane fuel of the various accepted alternative fuels. A strengthening of the engine is necessary in order to meet these higher demands. Multifuel engines sometimes have switch settings that are set manually to take different octanes, or types, of fuel.

History
Already in 1903, the German engineer Joseph Vollmer presented the first truck of the NAG, the automobile division of AEG, which was powered by a multi-fuel engine. The gasoline engine with 50 hp worked with magneto ignition and a carburetor, which was designed for both gasoline and alcohol.

Since specialized fuels were difficult to obtain in the early days of automobile history, relatively many manufacturers used multi-fuel engines. As the gas station network was expanded, these designs lost importance. Today, multi-fuel engines are particularly popular in the military sector, where the greatest possible independence from specific fuel types is often sought.

Working principle
A mixture engine works according to the otto process, where the combustion is initiated by an ignition or spark from a spark plug. The formation of the combustible mixture takes place outside the combustion chamber; in a carburetor or by fuel injection into the intake manifold.

A mixture of fuel and air is formed outside the combustion chamber. Under normal conditions, air contains 80% nitrogen (N 2) and 20% oxygen (O 2). Due to the oxygen present, this mixture is combustible. The mixture is sucked into the combustion chamber by a negative pressure generated there during the intake stroke. After the inlet stroke the compression stroke follows: the mixture is compressed. After the compression a spark brings the mixture to ignition. This causes a pressure rise which in turn causes a volume increase. The volume increase translates into the work stroke, in this battle work is done on the environment, for example on a vehicle or a pump. In the design of a classic mixture engine, a Carnot process is sought.

The term mixture engine originated from the need to make a distinction with the diesel engine. With a diesel engine, the fuel is only mixed with the air at the end of the compression.

Fuels
The fuel for a mixture engine is usually, but not exclusively, petrol. As a result, incorrectly used gasoline engine is used as a synonym for ottomotor or mixture engine.

Natural gas
Gasoline
E85 or Bio-ethanol
Kerosene
LPG
Nitromethane
Methanol

Motor variants
Multi-fuel engines are generally self-igniting reciprocating engines that operate on the diesel principle. In addition, some engine versions have a spark ignition, since not all fuels ignite properly without them. The variety of fuels such as gasoline, petroleum, kerosene, vegetable oil fuel, ethanol, wood gas or heavy oil leads depending on their properties, for example, the cetane number, octane number and viscosity to different constructions(see also internal combustion engine and overview of the injection technology).

Although multi-fuel engines usually operate on the diesel principle, they differ in their construction of pure diesel engines, which are designed only for diesel fuel. On the one hand, technical solutions must be provided in order to increase the temperature of the mixture so that all the fuels used ignite spontaneously within the permissible ignition delay. This can be done by increasing the compression or preheating the intake air. In turn, intake air preheating can be achieved by charging without an intercooler, exhaust gas recirculation or electric heating in the intake tract. Also supporting is a spark plugor a glow plug used in the combustion chamber.

On the other hand, the injection pump must be connected to the lubricating oil circuit, as some of the fuels used have no lubricating effect. The Lohmann engine does without injector and without carburettor.

When designing all seals, it should also be noted that they are not attacked by the different fuels.

Well-known multi-fuel engines include:

Middle spherical motor
Elsbett engine
Lohmann engine
Glow plug motor (Lanz Bulldog)
gas engine
Overview of the fuels
Both fossil and renewable fuels can be used:

LPG, also known as Autogas (LPG = Liquefied Petroleum / Propane gas, even low pressure gas) known
Natural gas (CNG = Compressed Natural Gas or LNG = Liquefied Natural Gas)
Petrol fuels such as gasoline or alcohols
Light oils such as diesel fuel and biodiesel
heavy oils
Coal dust

Advantages and disadvantages
Disadvantages
The disadvantage of a mixture engine is the limitation in the maximum achievable compression ratio, due to the risk of knocking. For petrol the maximum (safe) compression ratio is around 15: 1. Some contemporary, high-performance super sports, motorcycles already have a compression ratio of 14: 1 at the factory, without the addition of special additives to push the knock limit or detonation limit even further. Diesel engines achieve a significantly higher thermal efficiency because the compression ratio can rise to 40: 1. As a result, a mixture engine has a higher fuel consumption.

A second disadvantage is that the fuels are highly flammable and therefore already evaporate at low temperatures. At an outside temperature of 10 ° C, there is already a risk of explosion in an LPG engine. The fuel tank of a mixture engine is heavy-duty.

Benefits
An important advantage of the mixture engine is the lighter version, which benefits the cost price.

The engine is mainly used by the army. If the supply of fuel stagnates, the vehicles can still drive because they could switch relatively easily to another fuel that was available.

Applications
The use of multi-fuel engines is today practically limited to military vehicles, especially tanks. An example is the main battle tank Leopard 2 of the Bundeswehr.

In public and private use are multi-fuel z. As for powering remote farms in cogeneration plants, they provide electrical energy and heat.

Multi-fuel glow-head engines can be found in ancient tractors and marine diesels. The frequently used on ships heavy oil two-stroke diesel engines can also be considered as a technical variant of the multi-fuel engines, even if they are already limited for economic reasons (low fuel costs) mostly on heavy oil.

Military multifuel engines
One common use of this technology is in military vehicles, so that they may run a wide range of alternative fuels such as gasoline or jet fuel. This is seen as desirable in a military setting as enemy action or unit isolation may limit the available fuel supply, and conversely enemy fuel sources, or civilian sources, may become available for usage.

One large use of a military multi-fuel engine was the LD series used in the US M35 2 1⁄2-ton and M54 5-ton trucks built between 1963 and 1970. A military standard design using M.A.N. technology, it was able to use different fuels without preparation. Its primary fuel was Diesel #1, #2, or AP, but 70% to 90% of other fuels could be mixed with diesel, depending on how smooth the engine would run. Low octane commercial and aviation gasoline could be used if motor oil was added, jet fuel Jet A, B, JP-4, 5, 7, and 8 could be used, in an emergency fuel oil #1 and #2 could be used. In practice, they only used diesel fuel, their tactical advantage was never needed, and in time they were replaced with commercial diesel engines.

Currently, a wide range of Russian military vehicles employ multifuel engines, such as the T-72 tank (multifuel diesel) and the T-80 (multifuel gas turbine).

Non-military usage
Many other types of engines and other heat-generating machinery are designed to burn more than one type of fuel. For instance, some heaters and boilers designed for home use can burn wood, pellets, and other fuel sources. These offer fuel flexibility and security, but are more expensive than are standard single fuel engines. Portable stoves are sometimes designed with multifuel functionality, in order to burn whatever fuel is found during an outing.

The movement to establish alternatives to automobiles running solely on gasoline has greatly increased the number of automobiles available which use multifuel engines, such vehicles generally being termed a bi-fuel vehicle or flexible-fuel vehicle.

Underperformance issues
Multifuel engines are not necessarily underpowered, but in practice some engines have had issues with power due to design compromises necessary to burn multiple types of fuel in the same engine. Perhaps the most notorious example from a military perspective is the L60 engine used by the British Chieftain Main Battle Tank, which resulted in a very sluggish performance — in fact, the Mark I Chieftain (used only for training and similar activities) was so underpowered that some were incapable of mounting a tank transporter. An equally serious issue was that changing from one fuel to another often required hours of preparation.

The US LD series had a power output comparible to commercial diesels of the time. It was underpowered for the 5-ton trucks, but that was the engine size itself, the replacement diesel was much larger and more powerful. The LD engines did burn diesel fuel poorly and were very smokey, the final LDT-465 model had a turbocharger largely to clean up the exhaust, there was little power increase.

Source from Wikipedia