“The use of plant oil as fuel may seem insignificant today. But such products can in time become just as important as kerosene and these coal-tar-products of today.”
Rudolf Diesel in the year 1912, in his application for a patent.
The Diesel engine was designed by Rudolf Diesel in 1892. The first working engine was built at the Augsburg Maschinenfabrik (now part of the MAN B&W group) in 1897. The single cylinder engine, weighing five tonnes, was used to power stationary machinery. It had a single 10 ft (3 m) iron cylinder with a flywheel at its base, and produced 20 hp at 172 rpm! The engine operated at 26% efficiency, a very significant improvement on the 20% achieved by the best petrol engines of the time.
On February 27, 1892, Diesel filed for a patent at the Imperial Patent Office in Germany. Within a year, he was granted Patent No. 67207 for a “Working Method and Design for Combustion Engines: a new efficient, thermal engine.” With contracts from Frederick Krupp and other machine manufacturers, Diesel began experimenting and building working models of his engine. In 1893, the first model ran under its own power with 26% efficiency, remarkably more than double the efficiency of the steam engines of his day. Finally, in February of 1897, he ran the “first Diesel engine suitable for practical use”, which operated at an unbelievable efficiency of 75%.
Rudolf Diesel literally disappeared in 1913. There is some question over the cause of Diesel’s death. Diesel did not agree with the politics of Germany and was reluctant to see his engine used by their naval fleet. With his political support directed towards France and Britain, he was on his way to England, possibly to arrange for them to use his engine, when he inexplicably disappeared over the side of the ship in the English Channel. Whether by accident, suicide or at the hand of others, the world had lost a brilliant engineer and biofuel visionary.
Development of the Diesel engine
The 1920s brought a new injection pump design, allowing the metering of fuel as it entered the engine without the need for pressurised air and its accompanying tank. The engine was now small enough to be used in vehicles. 1923-1924 saw the first lorries built and shown at the Berlin Motor Fair. In 1936, Mercedes Benz built the first automobile with a Diesel engine, theType 260D.
The modern Diesel engine
The modern Diesel engine works in essentially the same way as an engine from the late 1930s, though there have been many advancements in manufacture and engine efficiency.
A Diesel compresses the air in the cylinder at a ratio of about 22:1. When the atomised fuel is injected into the cylinder it ignites because of the high temperature the air in the cylinder reaches as it is compressed. Petrol engines have a lower compression ratio of about 9:1. A spark from the spark plug ignites the petrol that is atomised in the cylinder. This is the main difference between petrol and diesel-fuelled engines.
It is necessary to heat up the air in the cylinder to initiate the cycle. This is done with a glow plug that is deactivated once the engine has started.
The Diesel cycle
The cycle of a compression-ignition (Diesel) engine
In this diagram of the cycle of the Diesel engine:
- the air is compressed in the cylinder by the rising piston
- the fuel is injected and ignites under compression
- The fuel explodes, forcing the piston down, which propels the crankshaft round
- The crankshaft is connected to the camshaft which in turn opens the exhaust valve
- The rising piston forces the exhaust gases out of the exhaust valve
- The camshaft opens the air intake valve
- The falling piston sucks new air into the cylinder to start the cycle again.
Fuel injection
The major differences between modern Diesel engines are in the way fuel is injected into the cylinder
Indirect injection (IDI)
In an IDI engine the fuel is injected into a pre-chamber or ‘swirl chamber’ before passing into the cylinder. The connecting passage and space can be designed to produce good swirling and mixing of the fuel and air, hence better combustion. This type of control usually allows for a lower overall fuel-to-air ratio and lower pollutant emissions. Until recently it was not feasible to make small direct injection engines, so most Diesel cars over ten years old are IDI. Some of these engines have the ability to use SVO but the fuel system is usually the limiting factor.
Direct injection (DI)
In a DI engine the fuel is directly injected into the combustion chamber, directly above the piston. This design is also referred to as an open chamber engine. The larger of these engines are to be found in ships, trains and tractors amongst others. They usually work at about 1500 rpm. The fuel is dispersed via a set of 8-10 holes in the injectors into a fine mist. The air content of this mist contains enough oxygen to combust. These large DI engines tend to be very fuel tolerant and many will run on SVO.
For many years technical difficulties meant that Direct Injection engines had to be large and heavy. In the late 1960s, largely through the work of Elsbett, a German engineer also heavily associated with biofuels research, smaller direct injection engines began to be feasible. Since the mid 1990s this has become the dominant type of Diesel engine in vehicles. (TDI, SDI, HDI etc.).
Unfortunately DI engines tend to be less tolerant of differences in fuel viscosity than IDI engines.
Fuel systems
There are a number of different fuel systems on modern Diesel engines. They differ in the way the fuel pump and injectors are connected and controlled.
Mechanically-controlled fuel injection
Fuel is supplied to the injector by the fuel pump, which is mechanically driven from the crankshaft. The drive from the crankshaft is set so that the pump delivers fuel at the correct time in the engine operation cycle. There is a separate fuel pipe from the pump to each injector. Most Diesel engines more than ten years old use a system like this. There are two main types:
- in-line injector pump: in-line pumps have the pipes coming out of the top. These pumps have a separate internal pump for each injector pipe. They are typically better at dealing with viscous fuels. These pumps are only common on larger and more ‘agricultural’ vehicles
- rotary injector pump: rotary pumps are similar in appearance to a petrol engine distributor. A single pumping mechanism rotates and supplies fuel to each cylinder in turn. These vary in their ability to deal with viscous fuels like SVO. As a rule of thumb, injector pumps made by Lucas and CAV tend to be less tolerant of high-viscosity fuels than those made by Bosch or most of the Japanese manufacturers.
Electronically-controlled fuel injection
Recently developed electronically-controlled injectors can provide exact amounts of fuel at very high pressure, very precise timing and even multiple injections within each cycle to give greatly improved combustion, and in turn increased fuel economy, lower engine noise and cleaner emissions. By monitoring the engine using a number of sensors, the electronic controller can modify the fuel injection characteristics to improve combustion. These systems are typically less fuel tolerant than mechanical systems. They are also dependent on fuel sensors, which can easily be damaged by methanol and perhaps other solvents.
- common rail injection systems (CDI): with this system a pump constantly supplies fuel at a very high pressure to the common rail – a tube with thick walls. From the common rail, fuel is supplied to electronically-controlled injectors. The higher pressure injection gives a finer spray and improved combustion.
- unit injector system and unit pump system: unit injector systems combine the pump and injector into one unit. The pump is driven from the engines camshaft. Fuel delivery is timed and metered by electronically-controlled valves. Unit pump systems are similar with the pump and injector separated by a short high-pressure line. Some VW Golf TDIS use this sort of system.
Fuel pipes, filters & seals
There are a number of links in the chain from fuel tank to cylinder. Typically the fuel travels down a pipe under the car to the engine bay. It then passes through the fuel filter that removes particulate matter and water, sometimes assisted by a small and simple lift pump. It then travels to the fuel pump and up to the injectors. Between each point there are pipes that may be made of mild steel, copper, rubber or plastics. If you are using biodiesel for a long period, rubber hoses will fail, as they are not compatible with biodiesel. It is relatively easy to find and replace these hoses with Viton or similar fluoroelastomer hoses. Vehicles made since 1995 will almost certainly have this type of hose already. The lift pump and fuel pump contain seals that may also be made of rubber on older vehicles. This could potentially create problems, as especially in the fuel pump these seals will be hard to replace. However we know of many people who are using biodiesel on older vehicles without problems of this nature.
Turbocharging
Many diesels are fitted with a turbocharger. This takes the energy in the exhaust and uses it to compress the air going into the engine, allowing for a higher efficiency of combustion. An intercooler is fitted to deal with the higher temperatures this causes. There should be no turbo issues with biodiesel or SVO.
Issues with using biodiesel
Mileage
It is generally reported that using biodiesel can reduce range (the distance you can travel on a full tank) by up to 3%. However, our course participants who have been using 100% biodiesel tell us that they get more miles per gallon (or should we say kilometres per litre?). It probably depends on the feedstock, the biodiesel-making method, and the engine. It seems that it’s a pretty small effect in any case. Using biodiesel in blends with petrodiesel can increase the efficiency of combustion of the whole mix. Maybe that’s why diesel from pumps in France always seems to last forever!
Cold starting
Petrodiesel is at the heavier end of the spectrum of fuels obtained from crude petroleum oil, and so there is a problem that in very cold weather, it will start to gel at higher temperatures than petrol. There are several ways to avoid this problem. The obvious one is to park your diesel vehicle in a garage, but you could also add a little kerosene, or other special cold start additives (winterising agents), or have some sort of heating element in the fuel tank, fuel filter or somewhere on the fuel line. Many of the SVO conversion kits use equipment that was designed for petrodiesel in the cold.
In the UK, we don’t experience the kind of temperature extremes found in North America or continental Europe, and so there is rarely a problem starting petrodiesel vehicles throughout the winter. Petrodiesel bought in winter is usually pre-blended with winterising agents.
Depending on feedstock, biodiesel can be similar in cold-weather performance to petrodiesel or a lot worse. Feedstock that is highly saturated makes biodiesel that can turn solid above 0ºC. There are a number of solutions to this problem. You could make ‘summer’ and ‘winter’ biodiesel with different grades of WVO. Many people just add petrodiesel if they are experiencing starting difficulties.
Trials in the US have found B20 (a blend with 20% biodiesel) to have the same or better cold starting properties as mineral diesel, and that this blend worked down to temperatures of –32ºC. You could also decide to leave methanol in the fuel if you are sure your engine can handle it.
Biodiesel produced for sale generally contains winterising agents (as does petrodiesel sold in cold countries). Suppliers are listed in the resources section.
Two terms are used in the literature regarding starting in extremely cold temperatures – the first is cloud point, which means the temperature at which small solid particles are first seen to gel as the fuel is cooled. The second is cold filter plugging point, which means the temperature at which the fuel filter becomes blocked due to the accumulation of solids in the fuel.
Biodiesel acting as a solvent
Biodiesel is an excellent solvent, and so could potentially deposit materials picked up from storage containers if they are not completely clean. Also, deposits may build up from mineral diesel in your fuel tank. Biodiesel could possibly remove them and deposit them in the fuel filter. It’s a good idea to check your fuel filter reasonably regularly when first using biodiesel. It will probably need replacing fairly early on. If you continue to use biodiesel, it won’t be a recurring problem.
Also, be sure to wipe biodiesel off any painted surfaces, because its solvent effect can mean that it could remove paint.
Fuel quality
Quite a few years ago now industry was at last forced to remove the naturally-occurring sulphur from diesel for environmental reasons (the sulphur turned into sulphur dioxide in the engine, escaped into the atmosphere, and eventually came down as acid rain). The problem with this is that engines have been wearing far more quickly because of the lack of lubrication: the sulphur in the fuel increased the lubricity of the oil (like tetra-ethyl lead added to petrol). By using biodiesel in a blend you are not only improving the ignitability of the petrodiesel (increasing efficiency) – the increased lubricity helps reduce wear of the engine components.
There are issues around the quality of the biodiesel that you make. If the quality is too poor, then it may contain water if you do not de-water properly after water washing. This can cause pitting of cylinders, and eventually engine failure. Also, residual methanol can cause de-laminating of sensors in new cars. Any excess catalyst can cause deposits on the injectors and cylinders (coking).
Impurities, especially glycerine and unfiltered gums can cause injector coking which can lead to an uneven fuel spray, uneven heating of the cylinder, and possible engine failure. Straight vegetable oil used in a normal unmodified Diesel engine will certainly cause these deposits due to the glycerine content.
Luckily it is not hard to make biodiesel which is of sufficient quality to avoid these issues.
Biodiesel contains very slightly less energy per litre than mineral diesel, but you shouldn’t notice any difference in the performance of your car, except maybe that it’s quieter.
