Into The Engine
Make It Hot
Using Waste Oil


Few people realize that vegetable oil can be used for more than frying fast food! Indeed, Rudolph Diesel’s first public exhibition of the internal combustion technology that was to later bear his name featured an engine running on peanut oil. He envisioned freeing small businesses from the monopolistic coal and steam power of the day by using organic fuels in his engine. Unfortunately, it turned out that his engine also lent itself to burning low-grade fractions of petroleum, and the rest is history — diesel engine manufacturers optimized the design for lighter oils, and the use of vegetable oil never really got a chance.

But experimenters throughout the world have been reviving Diesel’s vision, and “Straight Vegetable Oil” (SVO) is finding increasing use, particularly in the US, UK, Germany, and Australia.

Into The Engine

Because of this legacy, the fuel injection pump and the fuel injectors in modern diesel engines won’t work on room temperature vegetable oil, which is too thick. There are three common ways to thin vegetable oil so it can be used in diesel engines:

  1. Blend the vegetable oil with a lighter fuel,
  2. Change the chemical composition of the vegetable oil, or
  3. Heat the vegetable oil until it becomes thin enough.

Blending (#1) is fraught with problems, and although some enthusiasts swear by it, others end up swearing at it, as they damage expensive injection pumps with the heavier fluid. Some have suffered explosions when trying to mix extremely lighter fuels, like gasoline, with vegetable oil. Those reporting success seem to be limited to a mix no more than 50/50 with petro-diesel at no colder than “shirt sleeve” temperatures, on a few engines that have very robust injection pumps.

Changing the chemical composition, (#2) also known as biodiesel, is safe, reliable, reduces pollution, and will work in almost any diesel engine without modification. On the other hand, creating it requires processing with dangerous chemicals, and although the methanol commonly used to make biodiesel can be made in a sustainable manner, most of it comes from fossil fuel today. So today’s biodiesel is not completely sustainable.

When you heat vegetable oil (#3) to about 80°C (or about 180°F), it’s viscosity is only a bit above that of normal diesel fuel, and so can make its way through pumps and injectors without damaging them. This strategy is called “Straight Vegetable Oil” (SVO). But how to heat the oil?

Make It Hot

Fortunately, those wishing to use SVO for fuel generally have a large, hot object nearby — the engine inself! The most efficient diesel engine still produces a lot of waste heat, and has nicely packaged it in a readily available form. All you have to do is insert a “T” into the lines going to the passenger compartment heater, and route the engine coolant to a heat exchanger to warm up the SVO. Most typically, this uses two tanks, one for your “start/purge” (S/P) fuel that you run until the engine is hot, and the other tank for the SVO.

Many SVO enthusiasts supplement engine coolant heat with an electric boost (pictured), just before the fuel hits the engine. This ensures it is really hot enough!

Fluid heat exchangers take many forms. To be efficient, the exchanger needs a large surface area shared between the two fluids. This has led to embedding the fuel line inside the coolant line, or “Hose In Hose” (HIH). This design is fraught with peril if not done carefully; if there is a leak, you’ve got a mess in your radiator, or worse yet, water in your fuel — even a tiny bit can cause expensive engine damage.

For this reason, many tightly bind their fuel lines to the coolant lines, using a “Hose On Hose” (HOH) approach, which transfers less heat, but makes it impossible for the two fluids to mingle.

If it is inconvenient to run thick bundles of hoses around, they can be run separately to a manufactured or home-built heat exchanger, which can be as simple as a spiral of small copper tube for the SVO soldered to a thicker copper pipe for the coolant.

The thermally-coupled fuel and coolant lines have to go to the same place — how convenient! Because the next thing to do is heat the fuel tank. This is generally done by sending the coolant through a coil of copper or aluminum tubing inside the fuel tank, or by using a commercially-made tank heater. In colder weather, this may be a requirement, as some SVO may solidify at fairly high temperatures (compared to diesel fuel), but some report success without a tank heater.

Finally, you need a way to switch between the S/P fuel and the SVO. This takes the form of an electric valve and a toggle switch on the dash. You start your engine on S/P fuel (hopefully, biodiesel), wait for the temperature gauge to come up to normal, then flip the switch to run on pure vegetables! A few minutes before your destination, flip the switch back to your S/P fuel to ensure easy starting once the engine cools down. (Leaving SVO to cool in the engine may make it impossible to start without heating up the engine by some means.) Some commercial kits feature timers and buzzers to remind you to purge.

Using Waste Oil

As with biodiesel, waste vegetable oil can be recovered for engine fuel use, in which case, SVO is commonly called “WVO.” But the similarity ends there!

The process of making biodiesel isolates contaminants as a side-benefit. Food particles have multiple opportunities to leave the fuel, during separation of glycerin, during washing, during filtering. But to use WVO in a diesel engine, you must explicitly remove contaminants that get removed as a side-benefit of biodiesel processing.

Most WVO enthusiasts use a combination of gravity settling and pumped filtering. If you let the WVO sit for days to weeks, the heavier food particles settle to the bottom, and you can suck the clean oil off the top. But it still isn’t ready for use, and must be cleaned even more. Diesel engines want fuel that has been filtered to ten microns or less — it can take a lot of work to get something that has been sitting in a dumpster behind a restaurant that clean!

Some report success with a filter-only strategy, by using progressively finer filters to avoid heavily loading — and replacing — the finer filters. For example, a 100 micron strainer keeps out the cigarette butts, a 30 micron filter catches most food particles, and a final 10 micron filter makes it engine-ready. Such a strategy can be used “on the road” when you don’t have time for days to weeks of gravity settling.

Since restaurants currently pay to have their WVO disposed, what works best is to develop a relationship with a restaurant, whereby you agree to remove it on a regular schedule, and they agree to put it back in the original containers and keep it from getting contaminated with other waste. Some even report that they are able to talk restaurants into pouring their waste oil through a paper filter before putting it back in containers! Having such a “contract” with the restaurant also dodges the issue of ownership and theft — some grease recyclers consider the oil “theirs” once it goes in the dumpster they provide, even if you have permission from the restaurant to take the oil.


SVO shares most of the advantages of biodiesel: better engine lubrication, high energy content, improved safety, comparable performance to petro-diesel, and reduced pollution.

Although SVO’s emissions have not had as much study as biodiesel’s, SVO appears to have similar emission characteristics: no sulphur, much lower unburnt hydrocarbons, somewhat lower carbon monoxide and particulates, and slightly elevated nitrogen oxides.

An additional advantage is that an SVO vehicle can be more easily justified as being a “Zero CO2” vehicle, since it doesn’t have even the small amount of petroleum-derived methanol that commercial biodiesel has, except when starting or purging. An organic farm that used SVO in its tractors to produce oil crops to produce SVO could truly be called “CO2-neutral,” since today’s engine emissions are consumed by tomorrow’s oilcrop plants. There are very few transportation fuels that can claim that!

Few disposables are produced. The filtered bits of food from WVO make great fireplace starters! Biodiesel production has by-products that must be further processed or recycled carefully, since burning them at fireplace temperatures produces toxic chemicals.

An additional benefit over biodiesel is that once the engine conversion is paid for, there are no extra chemicals to pay for. You’re down to the cost of the oil itself, which may be free.


Like biodiesel, SVO use is limited to diesel engines, which hold a tiny market share in BC. Diesel vehicles tend to be more expensive than gas engines, but this is offset by their longer life.

Vegetable oil, whether made into biodiesel or not, cannot replace petroleum at current consumption levels. The ethics of switching food producing cropland to transportation fuel when there are hungry people in the world must be questioned. However, the current waste oil stream could supply as much as 10% of all diesel fuel, or perhaps 1% of total current petroleum use. Given that future energy sources may be distributed and diversified, SVO/WVO could be an important niche energy source.

Today, WVO can be harvested for free from restaurants, but if biodiesel and WVO become popular fuels, this “waste” product may in fact approach the cost of diesel fuel.

If you’re going to “roll your own” one way on the other, and you have more than one diesel vehicle, biodiesel may be the way to go, because you’ll have to modify each car to run SVO. On the other hand, you can probably commercially modify two vehicles to burn SVO for the cost of one commercially available biodiesel processor.

SVO/WVO is considered an experimental fuel, and is not without some risk to the engine. Almost without exception, those who have had trouble with SVO don’t get it hot enough, or use poor-quality WVO. Just because the engine temperature gauge says it’s up to temp doesn’t mean the oil is hot! Sometimes, the hot oil flows into a cooler injection pump (which are often mounted away from the engine, and thus thermally isolated), where it cools down and gels, causing damage.

Even if your injection pump doesn’t self-destruct, cool or dirty oil may be causing hidden damage. Injectors can collect carbon deposits (“coke”) as can the piston ring lands on the cylinder wall. Coking eventually causes poor dispersal from the injectors, which can lead to premature wear or even piston failure.

Some injection pumps seem to be better suited for SVO than others. People report good results with Bosche pumps, but tend to have problems with Lucas or Standyne pumps. Also, older direct-injection (DI) engines seem to have more problems than indirect-injection (IDI) engines. But with properly heated and cleaned oil, essentially any diesel engine will work with SVO.


This all may sound scary, but if you install a commercial kit according to directions, and stick to high-quality oil, there is little potential for engine damage. If you do choose to “roll your own” from scratch, it may be worth it to buy a temperature probe to make sure things are as hot as they are supposed to be, since the engine may appear to be running fine, yet still getting coked-up.

And by all means, educate yourself about the process! There are thousands of Internet resources on SVO/WVO; a few starting points are below. The information on this web page is an introduction, and is not intended to be enough for you to do a conversion!

Keeping in mind some risk of engine damage, there is nothing quite like pulling up to a stop light, having your exhaust catch up to you, taking a deep breath, and smiling and thinking “ahhh… that smells good!” and knowing there is absolutely no fossil fuel in that exhaust!


Listings in bold are BC-based companies.


Written by Jan Steinman, for the BC Sustainable Energy Association
Last updated December 22nd, 2004