Running a direct injection engine on 100% WVO

[2776ian]

As I mentioned in the "Bio Diesel" thread, I have been doing some comparative tests, running a direct injection engine on filtered waste vegetable oil. This forms part of my final year project for my mechanical engineering degree course.

The engine I used for this test is the university's Petter single cylinder water cooled unit, originally fitted to a cement mixed and built in the 1960s. It is connected to a generator that is loaded to the tune of a few kW by a bank of electric heaters. The only thing I can measure is fuel consumption. Everything else is subjective!

The engine was horribly neglected and I had to buy a workshop manual so that I could sort out the injection quantity and timing. Then, having dismantled and thoroughly cleaned the engine I ran it for a total of 12 hours at a constant speed under full load, dismantled it again and had a look at the location and severity of carbon build-up. Now, 12 hours may not sound like a lot, but with an engine in such poor condition as this one, 12 hours makes a lot of soot!

Now I had a benchmark I was able to progress to stage 2: 12 hours of running on filtered 100% WVO. This stuff is a really good cleaning agent! The amount of crap it dislodged from the tiny 5 litre fuel tank was astonishing.
I ran the tests in reasonably short bursts of a couple of hours at a time to get in some cold starts. I was surprised at how easily it started from cold on veg oil, having been told to expect some difficulty.

After 7 1/2 hours, the engine seized. The "post mortem" revealed a seized big-end bearing. This is not fuel-related: it's just because the engine was knackered. There is pretty much no chance of the university fixing this any time soon, certainly not in time for me to conclude the testing. I will have to take what I can from what I have seen so far.

Without going into massive detail, the engine was noticeably smoother and possibly a little quieter running on WVO than on diesel. Fuel consumption was up, from slightly under 1.4 l/hour to slightly over 1.5 l/hour. This is reasonable, bearing in mind the lower calorific value of vegetable oils compared against diesel. (About 37 MJ/kg for WVO against 43 MJ/kg for diesel, so you need to burn more WVO to do the same amount of work.)
Looking at the insides, there was no evidence of unburnt fuel around the piston's circumference and no fowling of the piston rings. This surprised me a little, but the rings were all perfectly clean and moving freely in their grooves.
Elsewhere, the carbon build-up around the exhaust valve was much reduced. I think this is due to the combustion of the oil happening later in the cycle than with diesel. The higher flashpoint of veg oils and slower burn-time (which contributes to the smoother running) also means there is more heat available when the exhaust valve opens, so more of the soot gets burnt off.
The only obvious down side I saw was 3 small areas of increased deposits in the piston bowl, corresponding to the positions of the nozzles on the fuel injector. This is indicative of poor atomisation. It is possible that these could create hot-spots on the piston and ultimately burn through, giving holed pistons. Not very likely in an engine rated at 6.6 hp with a piston that weighs about 2 kg, but possible in a car engine!
Other than that, I would say that the emissions were slightly more noticeable in quantity, though much paler in colour and smelt a whole lot better!

I had planned to repeat the tests using WVO heated to about 80C, hoping to see evidence of improved atomisation of the fuel, but clearly this will not be happening.

I hope this is of interest to all (potential) veg-oilers. I was hoping to post photos but they're too big to upload. I can email them to anyone who is interested.
Feel free to ask questions or pick holes in my conclusions!

Ian

[gavin.]

Interesting conclusions, thanks for taking the time to post them!

[webber1982]

very interesting shame the engine threw its hand in. we buy biofuel at work in 1000 litre ibc's and run our wagons on it somtimes we try to mix it but it usually ends up running 100% bio they run smoother and quieter for sure.