How does a multi fuel engine work ?


Okay, in militaries or as emergency backup in rural areas, multi fuel engines are used because of the ability to run on any fuel source such as diesel or kerosene.

How does such a engine work without the problems of running on different fuel sources such as engine knocking and spluttering (the classic trying to run petrol onto a diesel engine problem)…

In: 1

Nearly every reciprocating engine that’s designed to run on diesel will run fine on jet fuel or kerosene. They are practically the same thing except with different additive packages. Modern diesel engines aren’t designed for this because their injectors rely on diesel fuel to lubricate them. Jet engines are designed to still flow freely in an aircraft cruising at 50,000 feet with an ambient temperature of -40° outside and at the same time be heated up to temperatures found inside a jet engine without coking.

Usually by sacrificing efficiency and going simpler …

Modern engines have precise high compression, electronically controled fuel mixing and supply etc…

If you want to have simple reliable engine, look at garden tools. There are simple 2 or 4 stroke engines, that don’t need computer… They need carburator and spark plug.

If you scale it up and build it with better materials than cheap lawnmower, you get really tough engine, that can run on anything.

For example: I have German generator from ww2… It can run on gasoline, ethanol, diesel, jet fuel etc… But it consumes 20l/h and is extremely loud

To start off diesel engines are usually quite forgiving with the fuel. To prevent knocking when running on more volatile fuels you can retard the injection point. This will make it less efficient so it use more diesel but it will still run on a lot of different fuels. Most slow diesel engines set up this way can run on a wide variety of fuels such as kerosine, jet fuel, and even high octane petrol. The problem with the later is that diesels use the fuel as a lubricant so the diesel pumps and injectors will wear out faster. These components can be built with better tolerances which help but generally you want to use a two stroke mix by mixing lubricating oil with the petrol.

There is however a different engine design used in some military vehicles which can run on both diesel, petrol and even alcohol. It is not quite right to call it a diesel cycle even though it resembles it. Instead of using a high pressure injector to atomize the fuel into the combustion chamber, something which would cause petrol to instantly detonate, the injector is instead spraying the fuel into a cup on the cylinder head. The heat of the cylinder head will then boil the fuel slowly allowing it to burn. This helps prevent petrol and alcohol from detonating. It also allows diesel to burn at lower compression ratios and allows the fuel pressure to be lower so the fuel pump tolerances can be higher requiring less lubrication by the fuel. The disadvantage is that it does not run well on any fuel source and waste a lot of fuel.

Another type of multifuel engine is the gas turbine. These inject the fuel after the compressor, kind of like setting the injection timing late in a diesel engine. So they can run on a number of different fuels. Preferably you would run them on jet fuel, which is a tight specification kerosine. But by building components heavier and stronger and with loser tolerances you can make them run on anything. The disadvantage is just the maintenance cost. But the multifuel capabilities is mostly considered an emergency feature where the cost of maintenance can be justified.

Military engines tend to have relatively loose tolerances and are built very robustly. They also don’t have to deal with emissions, which takes a lot of the headaches out of retuning the engine on-the-fly if need be.

Of note, though; diesel and kerosene really aren’t that different. Any diesel engine can run on kerosene or jet fuel with essentially no modifications for the short term, and in the long term the only real issue is due to differences in fuel lubricity and sulfur poisoning any exhaust catalysts (which, again, isn’t there on a military engine).