As a rule diesel engines are more fuel efficient than petrol engines. In other words, diesel engines waste less fuel in the form of heat.
Further efficiency can be gained by optimising the fuel dosage and timing. This has led to the current crop of direct-injection diesels as pioneered by the German firm Elsbett.
Another positive development has been the fitting of turbos. With this technology some of the waste heat is used to improve fuel combustion, thereby capturing more of the fuel's energy potential.
Diesel engines used to have a bad name because of the black smoke they emitted. In modern, well-tuned diesels this problem is largely eliminated. New regulations concerning particulate filters will reduce this type of visible pollution even further.
Of course there are also other, non-visible pollutants that leave the exhaust. In broad terms the trick to minimise harmful emissions is to select a clean-burning fuel and ensure that combustion is as complete as possible over a wide range of revs and engine loads.
Apart from maintenance and driving style there is not much a driver can do to optimise combustion. That leaves selecting another fuel.
It was interesting to learn that Rudolf Diesel, the inventor of the diesel engine, envisaged a future for the use of vegetable oils as fuel. Currently I see two main options: biodiesel or straight vegetable oil (SVO).
Biodiesel is made by chemically altering vegetable oils or animal fats. When waste oils or fats are used this seems to make perfect sense. Waste is reduced and biodiesel doesn't require any changes to the engine.
Producing biodiesel from virgin vegetable oils is a different story. There is still a fossil fuel component even in unblended biodiesel and the energy required to make biodiesel seems a waste when straight vegetable oil could be used.
Still, there is definitely a role for biodiesel. Just be aware of its cleaning properties which means that tank deposits can start to clog the fuel filter. But after several filter changes the tank will be clean and the problem over.
Also be aware that the quality of commercially available biodiesel still varies in Australia. Most biodiesel is sold as a blend. E.g. B5 would mean 5% biodiesel and B20 20% biodiesel, the remainder being petro-diesel.
Good biodiesel information, including on how to make it yourself, can be found here.
- Straight vegetable oil (SVO)
The downside of SVO is that it requires changes to the engine. The absolute minimum is pre-heating the oil before it reaches the injection pump. On the upside SVO itself is fossil-free and carbon-neutral. It is even possible to grow oilseeds and produce SVO with only SVO as energy input.
While the earliest diesel engines could run on SVO instead of diesel, but weren't very efficient at either, the technology was later increasingly optimised for petro-diesel.
Only fairly recently did a company start to build engines specifically designed for SVO. Again the German firm Elsbett was the pioneer. Unlike their direct-injection technology, the SVO-optimised engine has not yet been embraced by any major car company. In fact, at some point Elsbett had to cease production of its SVO engine.
What all this did achieve, however, was the establishment of an SVO-fuel infrastructure in Germany. For a period of time with several companies producing conversion kits, straight canola oil being available at many petrol stations, and a relatively favourable tax regime, Germany was noticibly ahead of other countries.
Unfortunately, after the 2009 federal election the German government, in the words of a former Elsbett employee, "started to tax the biofuels heavily, what finally didn't bring money to the government but killed our main business in Germany within months".
Biofuel v. Food
There is a genuine concern that farming for fuels could negatively impact on food production. It doesn't have to.
Some reasons are:
Hunger is more often caused by poverty than by food shortages per se
The growing of oilseed trees has been shown capable of bringing about an increase in food production and farm income
The growing of oilseed trees can curb and even turn around deforestation and erosion
Some oil crops can be grown on land considered marginal or unsuitable for food production
Much work is being done on technologies to bring down the cost of converting agricultural waste into liquid and gaseous fuels
The final verdict is still out. But the need is there and the signs that biofuels can actually help improve land management, food production and farm income are encouraging.
Generally speaking biofuels produce fewer and less harmful emissions than petro-fuels. As mentioned earlier, this also depends on how complete the combustion process is.
With SVO, changes to the engine's fuel delivery system are necessary to achieve good combustion. Any research comparing diesel and SVO emissions in unmodified engines should be ignored.
At a later stage emission comparisons will be posted here.
Next: Elsbett SVO conversion