Diesel engines are highly controversial among some self-declared "eco-conscious" car buyers, sparking some fierce arguments regarding its long-term "environmental performance", especially when compared to hybrids withspark-ignited engines (usually gasoline-powered, altough a few ones in Korea use LPG/AutoGas instead of gasoline/petrol). Due to some visible black smoke (particulate matter), Diesels are regarded as more pollutant, in spite of their higher thermal efficiency and enhanced adaptability to alternative fuels.
Leaving the Diesel vs. hybrid issue behind, one aspect that brings some controversies regarding Diesels and alternative fuels is the usage of either biodiesel or pure plant oils, often also quoted as straight vegetable oils (or waste vegetable oils when they're previoulsy used for other purposes, usually cooking). Some people actually confused vegetable oils for the chemically-modified biodiesel, but there are differences. Biodiesel is made out of some vegetable oil or animal fats, but it's mixed with some alcohol (usually methanol, but ethanol can also be used) and has the glycerin removed, usually with Sodium Hydroxide (lye) as a catalyst. It's a way more energy-intensive process, but the resulting fuel might not require major modifications into a Diesel-powered vehicle to operate in a safe way, altough newer ones might present some issues with biodiesel blends higher than 20% (the so-called B20) due to its vaporizing into the Diesel Particulate Filter (DPF) at the forced-regeneration process not so easy as with regular Diesel fuel. It also tollerates better the lower combustion temperatures into modern direct-injection engines than straight vegetable oils which are more suitable to operate in the higher temperatures of an old-school indirect-injection Diesel engine.
Indirect injection has been left behind in newer Diesel engines, alleggedly because it's not so effective in order to meet more stringent emissions regulations, which are set having regular petroleum-based Diesel fuel as the reference. However, their combustion process which occurs in two steps since it starts into a prechamber (often also quoted as swirl-chamber) is deemed more accurate, leaving fewer unburnt (or partially-burnt) residues. Even the glycerin naturally-found in the pure plant oil is burnt more easily, reducing issues related to glycerin polymers sticking to the piston rings or contaminating the lube oil. Another interesting aspect regarding the usage of plant oils as a fuel is that indirect-injection engines, in spite of their lower efficiency while operating with regular Diesel fuel, actually have a slightly better performance and a considerable increase in fuel-efficiency. Nowadays the only high-volume market where indirect-injection Diesels are easily available for light-duty vehicles is India, altough they're also available for certain less-regulated markets in Latin America, Africa and Middle East at a lower extent.
A good point in favor of the usage of pure plant oils is the easier availability, since it can be sourced from many feedstocks suitable to the specific conditions from different regions while it doesn't depend on other chemicals which production is more concentrated. We shall also consider that it's easier to upfit the vehicle only ONCE (or to make it already factory-fitted to run on SVO/PPO) instead of chemically transforming the oils into biodiesel EVERYTIME. The other issue is pre-heating the oil, in order to decrease the viscosity and increase the flow, and it can be done in different ways, either thru an electric heating element fitted to the tank (or the lift pump) or thru a heat-exchanger connected to the engine cooling, altough this one would probably require the engine to be started in regular Diesel fuel (or biodiesel) from a secondary tank until there is enough heat build-up and before the engine shut-off. It's more critical for direct-injection engines, while indirect-injection ones are less sensible to that. Keeping the glowplugs (or the grid-heater) in a good shape is often enought to ensure a good old IDI is safe to operate with vegetable oils as their fuel.
The real-world efficiency of some emissions-control devices such as DPF, NOx Trap, EGR and SCR, everytime more complex and expensive is still somehow questionable because, in spite of cutting some tailpipe emissions from the Diesel-powered vehicles, they end up increasing the manufacturing footprint and purchasing cost while decreasing the fuel-savings, thus requiring more petroleum to be drilled and refined, then more Diesel fuel transported to the refuelling stations, and it goes. Meanwhile, some engines that would be cheaper to manufacture without all that stuff and more resillient to operate with a fuel that is often deemed unconventional but is renewable-sourced, carbon-neutral, among other clear advantages in the environmental aspect, are left behind because of politics. Leftards would probably call me a "racist", "bigot" or "islamophobic" for what I'm gonna say, and I really don't care for what that bunch of bastards may think, but it also makes more sense to support local farmers instead of giving money away for the Arab oil sheikhs or the Marxist-oriented dictatorships in Venezuela and some African countries which are also major players in the international petroleum trade...