Petrol in Diesel Tank

Submitted: Saturday, Jun 12, 2004 at 22:26
ThreadID: 13715 Views:16049 Replies:8 FollowUps:5
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Earlier today I put 40 litres of ULP (Unleaded) into the main tank of my 100 Series Diesel, just before heading out of town for a 4 hour trip.

As I left the servo in the city I switched to the main tank (out of habbit as I always start consuming any new fuel to make sure it's OK) I drove 2 KMS before the engine began to feel down on power and noticed idling 200rpm underneath normal. Just as I managed to pull over to a carpark the engine stalled.

Having realised that it might have been the fuel (and at this point just thinking it might have been dodgy diesel) I checked the lines which were all fine so flicked over to the SUB tank. Managed to get it running again after a few cranks and activating the manual fuel pump.

Drove back to the servo on the SUB tank and found the diesel bouser was of an entirely different in shape/style to the other pumps and didn't fit into my category of a "pump" when doing a quick scan of the options, looked more like a wheelie bin really. Leaded and premium were clearly marked, I'd say I chose ULP as it was nearest in price to diesel and I had just assumed thru a process of elemination that I must have finally stumbled along diesel. Not the case. Did the usual "pay for the Diesel" to the the cashier but he didn't notice it either.

Realising the main tank now composed of 25 litres of Diesel and 40 litres of ULP I consulted post 12124 and post 5824 to work out what to do with the 5 part diesel to 8 part ULP. The posts tended to explain the most you could get away with was 7:3 when temperature below freezing, tho 11:1 being the normal ratio used when needing to to stop diesel freezing. Temp's around here vary from 10-24.

I drained out 30 litres from the bottom of the tank and kept in container to creep into the unleaded car over time - diesel floats on petrol so assume this is richer in ULP than Diesel. To check I then drove for 2 KLM and found that it ran a little better but there was still way too much ULP in the mixture, so rather than stall it just ran groggy (clearly wasn't injecting much diesel).

I drained a further 30 litres which really all I can do is use for degreaser, leaving about 5 litres in the tank (comprising of 3.5 litres of ULP and 1.5 litres of diesel). Added 65 litres of diesel giving a 20:1 ratio. Main tank runs fine, you can't tell there's 3.5 litres of ULP in the tank.

In hindsight perhaps I could have only drained the first 30 litres (leaving 21 litres of ULP and 9 diesel) and simply added 65 litres of diesel to this (1 part ULP to 5 part diesel), however I wasn't keen to mix up what would could potentially be 95 litres of degreeser.

Might change the fuel filter once the ULP has been exhausted.

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Reply By: Lone Wolf - Saturday, Jun 12, 2004 at 22:30

Saturday, Jun 12, 2004 at 22:30
"- diesel floats on petrol "

Are you sure?

When I was a lad on the station, Diesel 44s were WAY lot heavier than Super & Standard........
AnswerID: 62934

Follow Up By: Truckster (Vic) - Saturday, Jun 12, 2004 at 22:46

Saturday, Jun 12, 2004 at 22:46
That doesnt mean anything in properties of the fuels.
FollowupID: 324275

Follow Up By: Lone Wolf - Saturday, Jun 12, 2004 at 22:51

Saturday, Jun 12, 2004 at 22:51
Something about specific weight or someting? Gravity....... I dunno.

I just hope the poor bugger hasn't done any damage.

Those 44 s were heavy for a 15 year old though!
FollowupID: 324277

Reply By: Goran - Sunday, Jun 13, 2004 at 00:42

Sunday, Jun 13, 2004 at 00:42
In 2 to 4 kays you wouldn't do any serious damage to 1HZ motor (or it's variants).
However, you should dispose of entire tank contents in a safe way. Petrol of any kind is HARMFULL to diesel engine. If you did not notice what you have done you will be up to engine rebuild now. Undo drain plug on the main tank and be done with it.
AnswerID: 62946

Reply By: Peter 2 - Sunday, Jun 13, 2004 at 07:57

Sunday, Jun 13, 2004 at 07:57
BTDT ;-)) you will have got most of it without any worries, just keep topping the tank up with diesel and it will be ok. The injectors will be much cleaner too ;-))
If you had done any lasting damage it would already be obvious.
Could be wise to check the filters as the petrol may have shifted some rubbish if the vehicle is a few years old.
AnswerID: 62954

Reply By: Bilbo - Sunday, Jun 13, 2004 at 11:24

Sunday, Jun 13, 2004 at 11:24

I've worked for 30 years in oil refineries and offshore oil & gas - petrol does NOT float on diesel. All petroleum/oil products are soluble in each other. The petrol will mix with diesel, I assure you. If It didn't, then you wouldn't need about 50% of most refinery equipment - distillation towers, those big things that you see in all refineries. There is a slight Specific Gravity or Density difference between the 2 fuels but it is only small. The difference in drum weights above, I would put down to using heavier steel drums for diesel. It's more corrosive in the long term than petrol. The "a**e" falls out of diesel drums much quicker than petrol drums due the diesel's much higher sulphur content.

Quality of fuels? A diesel engine runs compression ratios of at least 20:1 and higher. A petrol engine runs 6.5:1 up to about 10:1 ratios. Petrol engines can't run much higher ratios than these because the petrol fuel will detonate just from the heat of the compressed air in the cylinder - BEFORE the spark plug ignites it. The result is that the premature detonation tries to push the piston back down again, before it's had a chance to reach the top of the cylinder. This premature pushing, leads to piston crown damage at best, burnt out valves and possible crankshaft and big end bearing damage in extreme cases. It's the nature of petrol fuel - it's far more volatile than diesel.

Now, a diesel engine is designed to run on much less volatile fuel and it doesn't use spark plugs to ignite the fuel. It uses the heat of compressed air, and at 20:1 (and higher) compression ratio that temperature is far in excess of the ignition temperature of petrol fuel. So what happens when you feed mixed fuel to a diesel engine? The petrol in the mixed fuel "detonates" prematurely and behaves as I've mentioned above - pistons gets 'knocked backwards/downwards" BEFORE they've reached the full stroke of the cylinder. Result - damage as above.

DUMP THE TANK AND FORGET IT. There will be some minor damage already, probably not immediately apparent. Don't keep using it. And if you must use, whatever you do, don't put the engine under a high torque, low RPM load. That's when any detonation will make itself known - rattling, pinging etc. An engine rebuild will cost far more than the $50.00 you've lost.

AnswerID: 62970

Follow Up By: rob1 - Sunday, Jun 13, 2004 at 15:20

Sunday, Jun 13, 2004 at 15:20
I agree.....DUMP THE LOT.
I'd rather blow $100 than stuff the motor. Diesel engines are expensive to re-build.
FollowupID: 324301

Follow Up By: ToyMotor - Sunday, Jun 13, 2004 at 17:06

Sunday, Jun 13, 2004 at 17:06
Bilbo writes:
So what happens when you feed mixed fuel to a diesel engine? The petrol in the mixed fuel "detonates" prematurely and behaves as I've mentioned above - pistons gets 'knocked backwards/downwards" BEFORE they've reached the full stroke of the cylinder.

How does this happen? A diesel doesn't get its fuel injected until it's ready to fire, they're not like a petrol engine which draws in its fuel on the downward inlet stroke.

But anyway, I reckon dump the lot as well - petrol doesn't do diesel engines much good at all (understatement of the year)

FollowupID: 324319

Follow Up By: Goona - Saturday, Jun 19, 2004 at 12:39

Saturday, Jun 19, 2004 at 12:39

You seam to know a lot about diesels. Have you heard anthing about mixing lpg with diesel. I have heard up to 20% and the power is 50 -100% beter with a bit of extra PSI diled up on the turbo. Is this detremental to the engine?

FollowupID: 325027

Reply By: Bilbo - Sunday, Jun 13, 2004 at 11:27

Sunday, Jun 13, 2004 at 11:27
PS - prior to working in the oil industry, I was a diesel mechanic and a builder of high performance motor cycle racing engines. DUMP IT

AnswerID: 62972

Reply By: Bilbo - Sunday, Jun 13, 2004 at 17:39

Sunday, Jun 13, 2004 at 17:39

Fuel is injected into a diesel engine before "Top Dead Centre" - the number of degrees at which this occurs varys from engine to engine. The temperature inside the combustion chamber (read 'above the piston') is hot enough to instantaneously iginite the diesel fuel IN A CONTROLLED MANNER know as"the flame front". It doesn't "explode" or detonate. In spite of the speed at which things happen in a combustion chamber, it is a controlled burn.

When petrol is released along with the diesel it will instantaneously explode - not burn. It does not perform a controlled burn or flame front. It just goes "BANG" like dynamite. It's like a the ball end of ball pein hammer hitting the piston many times a minute. And all this occurs as the piston is still yet to reach "Top Dead Centre". Hence it eventually chew tiny pieces off the piston crown. These resemble small pitting of the metal particularly at the edges of the piston where the heat cannot be absorbed by the piston metal. Slowly, the piston edge disintegrates.

In many ways the effect is similar to using low octane petrol fuel in a high compression petrol engine. It "knocks" and you can hear it as a slight rattle or "pinging" under load. That noise is the sound of prematurely exploding (detonating) fuel.

AnswerID: 62996

Reply By: Jeff (Beddo) - Sunday, Jun 13, 2004 at 19:20

Sunday, Jun 13, 2004 at 19:20
I've done similiar silly things I went to the snow in my old Fairmont 1969 model XW 302 V8 - 2am in morning the girl in the servo yells out does your thing run on "diesel". I had put $20 bucks in so filled the rest up with super. Didn't affect the V8 but would be totally different the other way round. Yep the diesel & petrol will mix as we use it as drip torch fuel for fire fighting.
AnswerID: 63006

Reply By: Bilbo - Sunday, Jun 13, 2004 at 22:48

Sunday, Jun 13, 2004 at 22:48

Yer right mate. It would slow that old V8 down a bit but wouldn't do it much harm. Lotsa smoke I would expect?? Those old V8 were not very sophisticated engines, comp ratio about 7:1, but tonnes of torque and for "grunt" it was just a matter of "cubes". Ya can't beat "cubes"!

But put petrol in diesel, BNAG or should that be BANG!

AnswerID: 63034

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