Sunday, July 1, 2007

Vegetable Oil Car Conversion

The Car

Above is a picture of the back of the car with my preferred means of transport parked against it. It is a Nissan AD VED 4WD. This has a 17oocc Indirect Injection Diesel Engine.

The Idea
The idea is straightforward enough and well documented elsewhere on the web. In short, a diesel engine will burn any fuel that can be sprayed into the cylinder. The specific properties required of the fuel are somewhat limited by the design of engine you have. In general for vegetable oils, as long as the oil is:
  1. Clean - no particles > 1uM
  2. Dry - no crackles or bubbles when a teaspoon of oil is placed into a smoking hot pan
  3. Hot - above 80 degrees C when it goes into the injector pump
then your small vehicle diesel engine will run on it. So, to convert a car to run on vegetable oil, the usual approach is to start the car on diesel or bio-diesel wait until it is warm and then switch over to oil. Before shutting the car down again the car is switched back to diesel to purge oil from the injector pump and injection system.

The History
Back in 2003 I got interested in making Bio-diesel (B100 , the 100 is a percentage blend with petroleum based fuel) and running my diesel engined car on it. I read a lot of information on the web about various options including running on straight vegetable oil (SVO) or waste vegetable oil (WVO). There are several forums and many web sites with good information and a number of companies in the US and Britain selling conversion kits of components for conversion.

Initially I shied away from WVO as I had read a number of articles relating problems with damaged injector pumps and ring land coking etc. In January 2006 I was in Whangarei, New Zealand for a conference and met a guy there who had the same model car as me, a 1992 Nissan AD Station Wagon (commercial version of the Sentra with 1700cc indirect injection engine). What's more he had successfully converted it to run on WVO and was using it to shuttle conference attendees to the airport. The conference was a Green Party event so this went down well, although the irony of using a carbon neutral transport to the airport was not lost on many. This was a turning point in my thinking though, as I thought, well if he has done it then surely it is okay for my car! Flawed logic I know, but strangely compelling nonetheless.

I started looking more deeply into the whole WVO conversion process and found much conflicting information ranging from: "just pour it in your tank and go" to "any SVO will destroy your engine". Even the more moderate commentary was confusing, should one "loop the return" or not? What temperature should the oil be at? etc. etc. In the end I decided that I'd launch into it in as cheap a fashion as possible and try some ideas out.

Some people see bio-fuels in general as a possible panacea for the worlds (more specifically the western world's) insatiable thirst for energy. I don't believe this. Neither do I hold any illusions that everyone can convert their cars and run them on waste vegetable oil. There simply is not enough waste vegetable oil for this. The first step is to reduce one's consumption. I do this by living near to where I work and cycling everywhere I need to go in the city. All that said, I am more than happy to take what is currently someone's waste and divert it from the landfill into my fuel tank. I think collecting small amounts of vegetable oil for free will be a fairly short lived phenomenon. As the price of fossil fuel goes up due to peak oil and possibly carbon charges the price of just about everything else in our lives will increase too, including vegetable oil. The waste vegetable oil that is now discarded by restaurants will become valuable enough that even small amounts in the order of litres a week will be collected and sold to fuel companies. In the mean time I'm happy to benefit and drive a reduced carbon footprint vehicle at the same time!

The Injector Pump
The first step is to identify your injector pump. Commonly held wisdom seems to say that Lucas CAV type pumps are not really suitable for conversion because they are too vulnerable to damage from cold oil. Fortunately my pump is a Zexel (Bosch licenced Japanese).

Heating The Oil
The main reason for heating the oil before it goes into the injector pump is to reduce its viscosity. Heating most vegetable oil to around 80 degrees C makes it runny enough for your injector pump to deliver to the injectors. There is a bit of information around that says that heating the oil makes it easier for the injectors to spray it. This has been debated at length on the fora and in the end I think the arguments come down on the side that heating the oil before the injection pump or injectors makes little difference to the temperature of the oil as it is injected. The reason is that the oil is travelling through the injectors relatively slowly. It therefore has time to cool or heat to the temperature of the injector. The injector stays at a temperature close to that of the head that it is screwed into. So when the engine is hot the injected oil will be hot. The consensus on the fora is that running on WVO with the engine cold (and therefore the injected oil cold) is not a good idea because the oil does not spray from the injectors properly and unburned oil droplets can land on the cylinder walls and cause serious issues for the engine. The normal way to heat the oil is to utilise some of the waste heat from the engine that is carried away by the cooling water. The cooling water is thermostatically regulated and therefore provides a nicely controlled form of heating. Most diesel cars run at around 80 degrees which is sufficient for the oil heating. The thermostat on my car was faulty when I started the project and the engine was only running at 63 degrees. As a result my initial trials on oil failed and I ended up installing an electric heater for the oil to boost the temperature. After replacing the thermostat this heater is probably not necessary but it is in there now and gives me piece of mind.

My Solution

The car I converted is not worth an awful lot and so the idea of shelling out $1000 or so on a conversion kit did not appeal to me. Instead I spent a fair bit of time researching what I needed to do and figuring out cheap ways of doing it. The main components I needed to procure or build were:
  • fuel drums
  • pipe fittings
  • fuel transfer pump
  • header tank
  • oil float switch
  • heated fuel filter
  • two three port valves
  • fuel tubing
  • electric oil heater

Fuel Drums
I found a source of 20 litre plastic drums from The oil I collect sometimes comes in similar drums but these often have holes cut into the top to allow easier pouring. In any case it seems that few plastic drums are refilled or recycled these days so it should be easy to find a source.

Pipe Fittings
I used a number of barbs and reducers in various parts of the system. These come in various sizes and shapes. I mainly used products from purchased from a local engineering supplies shop.

Fuel Transfer Pump
This pump needs to be able to pump cold oil through a few metres of hose at a low flow rate of about a litre per minute or so. I used an after market electric diaphragm pump but my friend has had good luck with a high powered immersed centrifugal petrol pump from a wrecked car. The main problem with a centrifugal pump in this case is that they are not generally self-priming and the pump will need to be lowered into the drum of oil. The diaphragm pump on the other hand is a displacement pump and can be installed anywhere in the line.

Header Tank
The header tank was problematic as the space under my bonnet is very limited. I managed to find a spot that would squeeze in a couple of litres of tank but then finding an existing tank to fit was not possible. In the end an old steel computer case was cut up and brazed together to form a tank of the required shape. A U shaped piece of 20mm steel pipe was brazed into the bottom of the tank to form a simple heat exchanger for the engine cooling water to pass through. 8mm steel pipe was brazed in to form inlet, outlet and vent pipes. A pipe was also put in for the return fuel from the injector pump but this was not used in the final set up. Brazing is difficult for sheet metal like the tank and the tank ended up a bit warped in places. There were also a few false starts caused by leaks in the tank. In hind sight I think it would have been better to weld it or get it made by an engineering firm.

Float Switch
The fuel transfer pump needs to be controlled to keep the level of oil in the tank at the correct point. I hunted around and found a source of industrial float switches from The OLH-32 is the one I bought. I found that this didn't actually work that well as the oil is a bit sticky. In the end I replaced the float with a cork and magnet connected by a lever this has worked well ever since and has the extra benefit of being nicely adjustable.

Heated Fuel Filter
The fuel filter for the oil must be included in the system to prevent any particulate matter from entering the injector pump. I was given an old diesel filter holder by a friend and mounted this on the fire wall of the car. In order to reduce the pressure drop across the filter to a level that the injector pump can manage (maximum of about 10 inches of mercury for my car) it is necessary to heat the filter, usually with cooling water. After some investigation into wrapping the filter in rubber hose or making a custom filter holder, I settled on using a copper tube wrapping. You need about 2 metres to wrap a standard sized filter cartridge. It is very easy to work with and holds its shape nicely. The spiral of copper tube is clamped in position on the filter cartridge using large hose clamps.

On a diesel engine there is usually a flow of fuel from the bypass of the injector pump and injectors that is returned to the tank. The implication of this is that we need two three-port valves to switch between oil and diesel: one for the supply and one for the return. Solenoid or motorised valves are the ideal as they are easily controlled from the cabin of the car. However these valves are quite pricey. In the end my friend Bevan recommended using standard 3/8" brass ball valves. These are fairly cheap and are very robust. I mounted two of them on an aluminium plate and ganged the levers together so that they operated in unison. To actuate them from the cabin of the car I use a bicycle brake cable with a home made handle. A tension spring acts to return them to the other position. I had plans to use a central locking actuator or a windscreen wiper motor to actuate them electrically but there was not enough room under the bonnet and cable actuation has the advantage that you always know what is happening.

I needed quite a bit of tubing for fuel and for the coolant circuit. Under the bonnet it is all standard rubber fuel and coolant tubing form a car parts shop. This stuff is fairly pricey though so for the cold oil tube that runs from the rear of the car to the header tank in the bonnet I tried using a piece of steel tube. My friend Bevan again came to my aid to bend this tube up and help me mount it under the car. This worked after a fashion but the flow resistance was a bit too high for my fuel pump so I swapped the steel tube out for a larger diameter industrial nylon air hose. This works very well and is easy to install and tough enough for under the car.

Electric Oil Heater
To give the oil a final boost in temperature before it enters the injector pump I built a simple electric oil heater. There are some purpose built parts out there on the web but the are generally very expensive. All that is needed to make my heater is an aluminium block, some threaded pipe barbs, a glow plug, a thermostat and a relay. A hole is drilled right through the block to carry the oil. Next to this, another hole is drilled part way through to fit the glow plug. When I mounted the glow plug in the hole I put some solder in the hole first to act as a thermal transfer compound and ensure good heat flow from the glow plug to the block. The glow plug draws 20 amps so it gets its own fuse and is controlled via the relay by a switch in the cabin and the thermostat mounted on the aluminium block. I chose a 100 degree thermostat to keep the oil nice and hot. One extra feature I added to the block was a third pipe barb to attach a vacuum gauge to. The vacuum gauge resides in the cabin and indicates how blocked the filter is by the pressure drop across it. This is a very useful diagnostic tool, actually I would say essential.

Schematic Diagram

Notice that when the vehicle is switched to run on oil, the return flow from the pump is not directed back to the oil header tank but instead is teed into the inlet. This has two benefits and one draw back. The benefits are that the flow rate through the oil filter is much reduced and the oil circulating around the injector pump stays hotter. The draw back is that if any air gets into the system is circulates with the oil and causes misfiring problems. The drawback is minor and it is an easy matter to momentarily switch to diesel to purge any air from the system if it does get in.

Apart from the vacuum gauge mentioned earlier. I have an LED indicating when the electric heater is on and another indicating when the transfer pump is switched on. These are both very useful to tell what is going on in the system when I am driving. Under normal running it is reassuring to see them switching on and off periodically. I started to build myself a multichannel thermometer to monitor the temperature at various points in the system but in the end that project has languished and is not really needed. I used my friends non-contact thermometer to measure temperatures during the development stage and now I have a fairly good idea about how long it takes to get up to temperature. I mainly use the car for long trips so an early switch over to oil is not really an issue for me.

As is to be expected in a project such as this I had a number of teething issues.

The first problem encountered was simply that the engine was not getting hot enough. I initially got around this by making the electric heater and recently I changed the thermostat on the car, which is probably what I should have done in the first place.

The next issue was leaks. Leaks in the cooling system, in the hose fittings and in the brazing of home made tee fittings. These were soon tracked down and fixed by tweaking hose clamps or re-brazing the joins. There were also leaks in the oil system which were fixed in a similar fashion. The trouble with the oil leaks is that they are very difficult to isolate as most of the oil system is under slight negative pressure with respect to ambient so leaks simply suck air in and cause mysterious misfiring and rough running. This becomes particularly evident when the filter starts to block and the vacuum on the gauge starts to climb.

A particularly tricky problem I encountered was that for some strange reason oil would drain fill the header tank and overflow. I lost a few litres (lucky it is only vegetable oil) on a couple of occasions and suspected that the float switch was sticking. It took me a while to click that what was happening was that oil was syphoning from the drum in the boot to the header tank. To try to prevent this I made a check valve with some brass fittings and a spring loaded ball bearing. The check valve is installed so that the pump must open the valve against the spring pressure in order to move the oil. This seemed to work but as a precaution I also remove the pick up wand from the tank if I am leaving a full drum in the car. In practise a full drum is usually soon emptied !

UPDATE (November 2007)
I recently took the car for a warrant of fitness check and wouldn't you know it, I left a full tank of vege oil in the car. The check valve I had made had stopped working and the car leaked oil on the mechanic's floor. Needless to say they failed the warrant. So I was forced to remedy the problem properly. The solution was to install a solenoid valve in the line after the pump. Lucky for me my friend Bevan had a bag of washing machine valves. I bought a couple of brass fittings and installed the valve. It it wired in parallel to the pump so when the pump is on the valve is open. The mechanics approved of this and I got the warrant.

Oil Processing
As the oil comes from your favourite restaurant it may be less then ideal as fuel. It will be full of bits of food of a range of sizes and dead flies etc. It may also have a significant amount of water in it. These contaminants must be removed before you pop it in your car. I built a stand with a old 25 litre hot water cylinder on top, a filter rig underneath and finally the collection drum on the bottom. I recalibrated the thermostat on the hot water cylinder to allow it to get to about 140 degrees. To get the oil into the cylinder I cut a 120mm diameter hole in the top. I pour or pump the oil through a coarse sieve into the hot water cylinder and heat it up until it stops bubbling and just starts to smoke. After it has cooled somewhat I run it from a tap on the bottom of the cylinder through a kitchen oil filter and a 5uM bag filter. That's it, it is ready to use.

The End.

Unfortunately on Easter Sunday 2011 we were out for a drive and the cam belt failed on the car prematurely. The belt had only done about 80,000km so it should have gone for at least another 20,000. In any case the engine is a write off and a replacement engine installed would cost more than the car is worth. I sold the oil conversion components on Trademe. Total vege oil kilometers = 8163.