[thedoc (OP)]

Vineet tripathi asked the Naked Scientists:
   
What happens when diesel is used in a petrol engine?

What do you think?

[CliffordK]

A little diesel, and it would likely start smoking a bit, perhaps foul the plugs.  Too much diesel and one may not be able to start the engine cold.

The older engines (Ford Model T, Model A), were supposed to run somewhat on diesel if the engine was already warm.  I saw a diagram of a Diesel pre-heater for the Model A, I think.  I've heard that some of the older tractors could also start on gasoline, then switch over to Diesel.

If you accidentally put some diesel in the tank, either siphon it, or perhaps try to dilute the diesel as much as possible depending on how much you put in.  Then fill with gas, and it should be fine.  If I had a few cans of mixed fuel, I'd probably slowly add them back to get about a 10% mix or so.  Or, find someone running straight vegetable oil in a Diesel and ask them if they want the mixed fuel to mix with their veggie oil.

[chris]

The trouble is that diesel engines these days are really high-functioning with low tolerances compared with the burn-anything-you-like generations that went before them. I'd be very nervous about shoving anything less than top quality fuel through mine; but if I had an old tractor or land rover, I'd happily chuck vegetable oil in it...

[neilep]

What happens when you put petrol/gas in a diesel engine ?

[CliffordK]

I looked up on the web...  everyone else seems to agree that diesel in a gas engine will cause it to run poorly, of fail to start, smoke (blue smoke), and perhaps foul the spark plugs.

It was noted that usually Diesel nozzles will generally not fit into the new unleaded gasoline filling spouts (perhaps they would fit in the old leaded gas filling spouts).

What happens when you put petrol/gas in a diesel engine ?

As mentioned, some people will thin their vegetable oil using regular unleaded gasoline, apparently without serious problems. 

However, for a concentrated gasoline amount, it apparently causes serious problems. 

Diesel provides some lubrication to the injection pump and valves of the Diesel engine.  Gasoline doesn't provide lubrication, and may even have more ring blowby.  I'm not sure how serious of an issue this is for a short distance as the Diesel would not normally be in contact with the valve stems.  But, it may vary by manufacturer. 

The big issue is pre-detonation which can cause serious engine stress and problems, at least in theory. 

If one realizes the "issue", one should not try to drive home, but should stop and resolve the problem immediately, even if it means an expensive tow bill.

Perhaps gasoline in Diesel tanks is one of the problems with Oregon's "no self service" laws.  I would imagine it is less common for individuals to put the wrong fuel into their own vehicle.  And, the individual would have the liability of putting in the wrong fuel at a self-service station.

[chris]

Someone who lives with me but shall remain nameless put a whole load of unleaded in a diesel van we were using to move house; luckily I spotted what she was up to before she'd put more than a third of a tank in. We filled it up the rest of the way with diesel and it ran fine... lucky escape I think...

[evan_au]

Gasoline is a mixture of many different hydrocarbons. However, on average:

• Normal Cars: "100 Octane fuel" will burn similar to Octane, an 8-carbon chain, http://en.wikipedia.org/wiki/Octane
• Diesel Cars and Trucks: Burn a fuel similar to Decane, a 10-carbon chain, http://en.wikipedia.org/wiki/Decane
  

Comparing the chemical behaviours of these two chemicals:

• Octane has a lower boiling point, and a higher vapour pressure. It will ignite more readily. Diesel fuel may not vaporise properly in a car, and may fail to ignite in an engine tuned for octane.
• The mix of carbon and hydrogen in the fuel is different, so an engine tuned to burn one fuel will mix in the wrong amount of oxygen from the air, resulting in less efficient burning, lower power and increased pollution
• Diesel engines compress the fuel/air mixture more strongly than a gasoline engine, resulting in pre-ignition, or engine knock. In extreme cases the fuel could ignite while the piston is still compressing the gas, potentially fracturing the engine block.
• A similar problem is experienced if gasoline engines run on low-octane fuel, which is more like heptane than octane. See http://en.wikipedia.org/wiki/Octane_rating

[damocles]

Evan, it is a bit more complicated than that -- the "octane" of petrol engines is in fact a highly branched octane isomer:

   _\__
     /    \__        (carbon atom framework of molecular structure)
           |

The decane standard for diesel engines is straight chain n-decane.

A petrol engine needs fuels that burn evenly in a smooth radical chain reaction. Fuels that make explosions are bad -- poor co-ordination of expansion phase and shock waves stress the cylinders pistons etc.  Branched chain good, aromatics good, straight chain bad.

A diesel engine, on the other hand, works best with a fuel that will explode at the peak of the compression cycle -- straight chain good, branched chain bad.

The mix of carbon and hydrogen in the fuel is not hugely relevant -- octane (any isomer) is C₈H₁₈ -- a ratio of 2.25 H/C on an atomic basis. As a comparison decane is C₁₀H₂₂ -- ratio 2.20 H/C.

It would only take a tiny proportion of toluene (actually a major component of a good quality petrol) formula C₇H₈ ratio 1.14 H/C to bring the 2.25 of octane down to the 2.20 of decane.

[CZARCAR]

you blow the rings around the pistons. gasoline burns faster then diesel

[CliffordK]

The ping is caused by not enough fuel or to much advancement of the timing. The ping is the diesel effect. It is very bad for gasoline cars, there connecting rods, the rods that connect the pistons to the crank shaft are not built for that kind of firing, so many degrees before top dead center. So most car manufacturers do not tweak the amount of timing advancement very much. It is easier to just use a safe setting. Spark plugs can actually melt, if the engine pings to much.

"Dieseling" is when you shut off the key of a gasoline engine, and the engine keeps running due to compression ignition like a Diesel engine.  It is caused by a hot engine and high compression.  I suppose it would only happen with the key off with a carbureted engine, and not an injected engine, and I haven't heard it happen for years. 

The Dieseling could also occur while the engine is running in certain situations causing a "ping".  It is independent of the engine timing as the fuel is ignited without the spark. 

The higher octane rating of the fuel would tend to prevent this compression related pre-ignition, or dieseling which is why it is used in high performance cars.

Ethanol (CH3-CH2-OH) behaves much like high octane fuel, and is often used in race cars.  However, it is in fact much less volatile (evaporates less readily) than most of the gasoline.

Diesel in a gasoline engine would be much less likely to cause compression ignition as the in cylinder pressures aren't high enough.

Intercoolers are sometimes used with turbos because the turbo compresses the air making it hotter (like a heat pump or refrigerator).  The intercooler then cools the air which would effectively increase the air density and increase the amount of air being pushed into the cylinder. 

I don't know if fuel atomization/vaporization would also affect the air capacity per stroke.  Certainly it would only make a difference if the fuel entered the system prior to, or during the intake stroke, but not if it was injected during the compression stroke.

[imatfaal]

MOD NOTE

William - please immediately stop these off-topic rambling pronouncements.