Misadventures in diesel forensics

Early in the orchard project I resolved to give up large scale burning of brush, both to keep the carbon out of the atmosphere for a bit longer and to recycle the biomass into orchard soil fertility. Accordingly, last Sunday I planned a session with a large rented chipper, to reduce the large windrow of brush from the two rows of apple trees we added this spring to chips that can be used to mulch the trees and the perimeter fence around the orchard. These large diesel-powered wood chippers are fantastic and fearsome machines; the one we rented has a 14 inch diameter capacity. They are heavy and difficult to move, requiring a 3/4 ton pickup to tow them safely, and a sizable crew of energetic folks to feed them. I prevailed on my dad to pick up the machine Saturday afternoon and drop it off on Monday, and convinced some friends to come help feed the thing. Sunday morning I checked the fuel level, which was half a tank, so I poured in about six gallons of diesel, which took it to about 3/4 full, hitched the chipper to the big Kubota tractor, and pulled it out into the orchard. We set to work, and in an hour or so we had ground up the first big pile of brush into a sizeable heap of chips. We took a short break and shifted the chipper up to the next pile, which involved changing direction, to where we were sloped slightly to the right, where we had previously been sloped slightly to the left. We restarted the machine, and started chipping the next pile.

But after a couple of minutes, the machine abruptly started to bog down, and produce clouds of black smoke. It was clearly not right, so we shut it down, scratched our heads, checked the fuel (about half a tank), and tried it again (same result), and called the rental company, which was closing in about an hour – too soon to get it back and swap for another. They didn’t have any suggestions, so we cleaned up, pulled the chipper out of the orchard, and salved our disappointment by going fishing in the afternoon.

Monday morning the chipper went back to the rental shop, and shortly thereafter a phone call – the tank was half-full of gasoline! As soon as I heard that I went and checked the two cans that I had poured into the tank – the dregs in the cans was clearly diesel, by smell, feel, and color. And the chipper was operating normally with good power for the first hour or two – though in retrospect a quarter of a tank consumption did seem too much for the amount of chipping we did. To be double sure I checked the funnel I used, which conveniently was the sort that has screw caps on top and bottom to keep dust out – it surely contained diesel. Piecing the story together, this is what must have happened:

When we received the chipper, it had a half a tank of gasoline (or a gas-rich mix) in it. Before using it, I added a quarter tank of diesel fuel. I poured the fuel in fast and used a funnel with a long tube with a sort of nozzle on the tip, and diesel is significantly denser than gasoline. Though diesel is obviously miscible in gasoline, it must have flowed to the bottom with relatively minimal mixing and formed a pool there – otherwise it could not have run the machine for over an hour. There must have been enough mixing to cause an increase in fuel consumption; otherwise there’s no way it would have burned a quarter tank in less than 2 hours, but not enough to cause a noticeable loss of performance. But why did the machine stall so soon after we moved it? I think the key is in the lay of the land – by the fuel gage we had burned a volume nearly equal to the amount of diesel I added, so the layer of diesel (or mostly-diesel) would have been very thin. I didn’t notice which side of the tank the intake tube was on, but I would lay odds that it was on the side of the machine that was downhill when we started. Then when we turned it around the dwindling pool of mostly-diesel ended up on the opposite side of the tank, and as soon as we burned up the fuel that was in the lines and filter (which allowed us to chip a few armloads of brush in the new configuration) it started pulling the mostly-gas from the upper level of the tank, shutting down the works.

As best I can tell online, running a diesel on a mix of diesel and gasoline causes loss of power (check), dark smoke (check) and may cause excessive wear in the fuel system (since gasoline is much less lubricious than diesel oil). Hopefully we didn’t run the machine for long enough to do any permanent damage. The policy of the rental company is that the machine should be returned to the shop with the same amount of fuel as it left with, so it is likely that the previous renters got the machine with half a tank of fuel, used it for a while, and then stopped at a gas station on the return trip, and accidentally filled the machine back to half full with gas instead of diesel. Obviously we could be making this entire story up, and the rental company doesn’t have much basis on which to judge our story versus that of the previous renters (who would likely deny that they could ever have put in gasoline), but our family has a history of renting equipment from that shop without destroying it, and they haven’t made any effort to get us to pay for repairs, if that’s any indication. So, hopefully no permanent harm done, but getting the crew and the equipment together to finally finish the job isn’t something I relish.

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