Wood gas vehicles: firewood in the fuel tank

During the Second World War, almost every motorised vehicle in continental Europe was converted to use firewood.

Wood gas cars (also known as producer gas cars) are a not-so-elegant but surprisingly efficient and ecological alternative to their petrol (gasoline) cousins, whilst their range is comparable to that of electric cars.

Rising fuel prices and global warming have caused renewed interest in this almost-forgotten technology: worldwide, dozens of handymen drive around in their home-made woodmobiles.

Wood gasification is a process whereby organic material is converted into a combustible gas under the influence of heat – the process reaches a temperature of 1,400 °C (2,550 °F). The first use of wood gasification dates back to 1870s, when it was used as a forerunner of natural gas for street lighting and cooking.

In the 1920s, German engineer Georges Imbert developed a wood gas generator for mobile use. The gases were cleaned and dried and then fed into the vehicle’s combustion engine, which barely needs to be adapted. The Imbert generator was mass produced from 1931 on. At the end of the 1930s, about 9,000 wood gas vehicles were in use, almost exclusively in Europe.

Second World War

The technology became commonplace in many European countries during the Second World War, as a consequence of the rationing of fossil fuels. In Germany alone, around 500,000 producer gas vehicles were in operation by the
end of the war.

A network of some 3,000 “petrol stations” was set up, where drivers could stock up on firewood. Not only private cars but also trucks, buses, tractors, motorcycles, ships and trains were equipped with a wood gasification unit. Some tanks were driven on wood gas, too, but for military use the Germans preferred the production of liquid synthetic fuels (made out of wood or coal).

In 1942 (when the technology had not yet reached the height of its popularity), there were about 73,000 producer gas vehicles in Sweden, 65,000 in France, 10,000 in Denmark, 9,000 in both Austria and Norway, and almost 8,000 in Switzerland. Finland had 43,000 “woodmobiles” in 1944, of which 30,000 were buses and trucks, 7,000 private vehicles, 4,000 tractors and 600 boats. (source).

Woodmobiles also appeared in the US, Asia and, particularly, Australia, which had 72,000 vehicles running on woodgas (source). Altogether, more than one million producer gas vehicles were used during World War Two.

After the war, with gasoline once again available, the technology fell into oblivion almost instantaneously. At the beginning of the 1950s, the then West-Germany only had some 20,000 woodmobiles left.

Research programme in Sweden

Rising fuel prices and global warming have resulted in renewed interest in firewood as a direct fuel. Dozens of amateur engineers around the world have converted standard production cars into producer gas vehicles, with most of these modern woodmobiles being built in Scandinavia.

In 1957, the Swedish government set up a research programme to prepare for a fast transition to wood gas cars in case of a sudden oil shortage. Sweden has no oil reserves, but it does have vast woodlands it can use for fuel. The goals of this research was to develop an improved, standardised installation that could be adapted for use in all kinds of vehicles.

This investigation, supported by car manufacturer Volvo, led to a great deal of theoretical knowledge and hands-on experience with several road vehicles (one seen above) and tractors over a total distance of more than 100,000 kilometres (62,000 miles). The results are summarized in a FAO document from 1986, which also discusses some experiments in other countries. Swedish (overview) and, particularly Finnish amateur engineers have used this data to further develop the technology (overview, below a vehicle of Juha Sipilä).

A wood gas generator – which looks like a large water heater – can be placed on a trailer (although this makes the vehicle difficult to park), in the boot (trunk) of a car (although this uses up nearly all the luggage space), or on a platform at the front or the back of the vehicle (the most popular option in Europe). In the case of an American pickup, the generator is placed in the truck bed. During WWII, some vehicles were equipped with a built-in generator, entirely hidden from view.

Fuel

The fuel for a wood gas car consists of wood or wood chips (see picture on the left). Charcoal can also be used, but this leads to a 50 percent loss in the available energy contained in the original biomass. On the other hand, charcoal contains more energy, so that the range of the car can be extended. In principle, any organic material can be used. During the Second World War, coal and peat were also used, but wood was the main fuel.

One of the more successful wood gas cars was built last year by Dutch John. While many recent producer gas vehicles seem to come straight out of Mad Max, the Dutchman’s Volvo 240 is equipped with a very modern-looking system made of stainless steel (see the first picture and the two pictures below and then compare to this Volvo, this BMW, this Audi or this Yugo).

“Producing wood gas is not that hard”, says John. “Producing clean wood gas is another thing. I have objections to some woodmobiles. Often, the produced gas is as clean as the appearance of the construction”.

Dutch John strongly believes in wood gas generators, mainly for stationary uses such as heating, electricity generation or even the production of plastics. The Volvo is meant to demonstrate the possibilities of the technology. “Park an Italian sports car next to a wood gas car and the crowd gathers around the woodmobile. Nevertheless, wood gas cars are only for idealists and for times of crisis.”

Range

The Volvo reaches a maximum speed of 120 kilometres per hour (75 mph) and can maintain a cruising speed of 110 km/h (68 mph). The “fuel tank” can contain 30 kilograms (66 pounds) of wood, good for a range of 100 kilometres (62 miles), comparable to that of an electric car.

If the back seat is loaded with sacks of wood, the range is extended to 400 kilometres (250 miles). Again, this is comparable to the range of an electric car if the passenger space is sacrificed for a larger battery, as is the case with the Tesla Roadster or the electric Mini Cooper. The difference is, of course, that John has to stop regularly to grab a sack of wood from the back seat and refill the tank.

Trailer

As is the case with other cars, the range of a wood gas car is also dependent on the vehicle itself. This is shown by the different cars that were converted by Vesa Mikkonen. The Fin places all his generators on a trailer. His most recently-converted car is a 1979 Lincoln Continental Mark V, a large, heavy American coupe. It consumes 50 kilograms (110 pounds) of wood every 100 kilometres (62 miles) and is thus considerably less efficient than John’s Volvo. Mikkonen has also converted a Toyota Camry, a much more efficient car. This vehicle only consumes 20 kilograms (44 pounds) of wood over the same distance. However, the trailer is almost as large as the car itself.

The range of electric cars can be considerably improved by making them smaller and lighter. However, this is not an option with their wood gas cousins because of the weight and the volume of the machinery. The smaller cars from World War Two only had a range of 20 to 50 kilometres (12 to 31 miles), in spite of their much lower speed and acceleration.

Freedom

Enlarging the “fuel tank” is the only option to improve the range further (except for reducing one’s speed, of course, but that is another story). American Dave Nichols (the man who shows the wood on one of the pictures above) can load 180 kilograms (400 pounds) of wood into the back of his 1989 Ford pickup truck. This takes him 965 kilometres (600 miles) far, a range that is comparable to a fossil fuel powered car. The merit of this is discussable, of course, as to do this Nichols has to stop regularly to refill the tank: if he loaded the back of his pickup with gasoline, then he could drive even further.

According to Nichols, one pound of wood (half a kilogram) is sufficient to drive 1 mile (1.6 kilometres), which tallies with the Volvo’s 30 kilograms of wood per 100 kilometres. The American has set up a company (21st Century Motor Works) and plans to sell his technology on a larger scale. When he arrives home, he uses his truck to heat his house and generate electricity. His story has caught on in the US, and the reason can be summed up by his license plate: “Freedom”.

“You can go around the world with a saw and an axe”, as John Dutch puts it. His compatriot, Joost Conijn, grabbed this opportunity to take a two-month trip throughout Europe, without worrying about the proximity of the closest of gas stations (which are not always easy to find in a country like Romania).

The locals gave him wood to continue his journey – with the supply stored on a trailer. Conijn not only used wood as a fuel, but also as a construction material for the car itself (picture above – video here). For another trip with a wood gas car, see “Around Sweden with wood in the tank“.

Does the woodmobile have a future?

During the 1990s, hydrogen was seen as the alternative fuel of the future. Then, biofuels and compressed air took over its mantle role, whilst today all the attention is focused on electric cars. If this technology fails, too (and we have expressed our doubts about it several times), can we go back to the wood gas car?

Despite its industrial appearance, a wood gas car scores rather well from an ecological viewpoint when compared to other alternative fuels. Wood gasification is slightly more effiicient than wood burning, as only 25 percent of the energy content of the fuel is lost. The energy consumption of a woodmobile is around 1.5 times higher than the energy consumption of a similar car powered by gasoline (including the energy lost during the pre-heating of the system and the extra weight of the machinery). If the energy required to mine, transport and refine oil is also taken into account, however, then wood gas is at least as efficient as gasoline. And, of course, wood is a renewable fuel. Gasoline is not.

The advantages of wood gas cars

The greatest advantage of producer gas vehicles is that an accessible and renewable fuel can be used directly without any previous treatment. Converting biomass to a liquid fuel like ethanol or biodiesel can consume more energy (and CO2) than the fuel delivers. In the case of a wood gas car, no further energy is used in producing or refining the fuel, except for the felling and cutting of the wood. This means that a woodmobile is practically carbon neutral, especially when the felling and cutting is done by hand.

Moreover, a wood gas car does not require a chemical battery, and this is an important advantage over an electric car. All too often, the embodied energy of the latter’s enormous battery is forgotten. In fact, in the case of a producer gas vehicle, the wood behaves like a natural battery. There is no need for high-tech recycling: the ash that remains, can be used as a fertilizer.

A properly-operating wood gas generator also produces less air pollution than a gasoline or diesel powered car. Wood gasification is considerably cleaner than wood burning: emissions are comparable to those of burning natural gas. An electric car has the potential to do better, but then the energy it uses should be generated by renewable sources, which is not a realistic scenario.

The drawbacks of wood gas cars

In spite of all these advantages, it takes just one look at a woodmobile to realize that it is anything but an ideal solution. The mobile gas factory takes up a lot of space and can easily weigh a few hundred kilograms – empty. The size of the equipment is due to the fact that wood gas has a low energy content. The energetic value of of wood gas is around 5.7 MJ per kg, compared to 44 MJ/kg for gasoline and 56 MJ/kg for natural gas (source).

Furthermore, the use of wood gas limits the output of the combustion engine, which means that the speed and acceleration of the converted car are cut. Wood gas consists roughly of 50 percent nitrogen, 20 percent carbon monoxide, 18 percent hydrogen, 8 percent carbon dioxide and 4 percent methane. Nitrogen does not contribute to the combustion, while coal monoxide is a slow burning gas. Because of this high nitrogen content, the engine receives less fuel, which leads to a 35 to 50 percent lower output. Because the gas burns slowly, a high number of revolutions is not possible. A producer gas vehicle is no sports car.

Even though some smaller cars have been equipped with wood gas generators (see for instance this Opel Kadett), the technology is better suited to a larger, heavier car with a powerful engine. If not, engine output and range might not be sufficient. Even though the installation can be made smaller for a smaller vehicle, its size and weight do not decrease proportionately with the decreasing size and weight of the car. Some have built wood gas-powered motorcycles, but their range is limited (a motorcycle with sidecar does better, though). Of course, the weight and size of the mobile gas factory is less an issue with buses, trucks, trains or ships.

Ease of use

Another problem of wood gas cars is that they are not particularly user-friendly, although this has improved compared to the technology used in the Second World War. See the second part of this pdf document (page 17 and further) for a description of what it was like to drive a wood gas car back then:

“…experience at the Wurlitzer organ could be a distinct advantage”.

Still, in spite of the improvements, even a modern woodmobile requires up to 10 minutes to get up to working temperature, so you cannot jump in your car and drive away immediately. Furthermore, before every refill, the ashes of the last gasification process have to be shovelled out. The forming of tar in the installation is less problematic than it was 70 years ago, but the filters still have to be cleaned regularly. And then there is the limited range of the vehicle. All in all, it is a far cry from the familiar ease of use of a gasoline car.

The large amount of (deadly) carbon monoxide produced calls for some precautions, too, since a leak in the piping is not impossible. If the machinery is placed in the trunk, the instalment of a CO-detector in the passenger compartment is by no means a luxury. Moreover, a wood gas car must not be parked in an enclosed space unless the gas is flared first (picture above).

Mass-produced woodmobiles

Of course, all the vehicles described above are built by amateur engineers. If we built cars especially designed to be powered by wood, and produce them in factories, chances are that the drawbacks would become somewhat less significant and the advantages would become even greater. Such woodmobiles would also look more elegant.

The Volkswagen Beetles that rolled off the assembly line during World War Two had the whole wood gasification mechanism built in (sources: 1 / 2 / 3). From the outside, the wood gas generator and the rest of the installation was invisible. Refilling was done through a hole in the bonnet (hood).

The same is true for this Mercedes-Benz, in which the installation is completely hidden in the trunk (source).

Deforestation

Unfortunately, wood gas shares an important disadvantage with other biofuels. Mass producing woodmobiles would not solve this. Quite the contrary, in fact: if we were to convert every vehicle, or even just a significant number, to wood gas, all the trees in the world would be gone and we would die of hunger because all agricultural land would be sacrificed for energy crops. Indeed, the woodmobile caused severe deforestation in France during the Second World War (source). Just as with many other biofuels, the technology is not scalable.

Yet, while biofuel-powered car is as user-friendly as a gasoline rival, wood gas has to be the most user-unfriendly alternative fuel that exists. This can be an advantage: a switch to wood gas cars can only mean that we would drive less, and that would of course be a good thing from an environmental viewpoint. If you need to preheat your car for 10 minutes, chances are you will decide not to use it to drive a few miles to pick up some groceries. A bicycle would do the job faster. If you had to cut wood for three hours just to make a trip to the beach, you would probably decide to take the train.

In any case, the woodmobile demonstrates (again) that the modern car is a product of fossil fuels. Whatever alternative fuel you believe in, none of them comes even close to the convenience of gasoline or diesel. If, one day, the availability of (cheap) oil comes to an end, the omnipresence of the automobile will be history. But the individual vehicle will never die.