How does Veg oil affect timing?

[MikeT]

Does anyone know how/if veg oil alters the engines characteristics, i.e. Timing, flame speed, injection pressure etc.

[CitroJim]

Mike,

My gut feeling is perhaps a little but not by a lot.

If you make genuine bio diesel from veg, the product is, I understand, very similar chemically to proper pump diesel. I may be over-simplyfying things a bit but if I understand the trans-esterification process of making bio, all you are doing at the end of the day is removing the fatty acids from the raw veg oil and turning them into soap (glycerine). So, it follows therefore that a good proportion of veg is proper diesel anyway with added impurities.

So, on that basis, the ignition delay of veg compared to pump diesel should not be that much different. However, given that raw veg comes loaded with chemicals that perhaps do little to assist in rapid combustion, the ignition delay may be a tad or so longer and thus a small amount of advance may well be appropriate.

I'm sure though that cold veg takes a bit longer to get going than pump diesel, even though the cetane ratings are broadly similar. I'd say that the cold advance could usefully be upped by a bit to improve cold starting and running.

It's all academic anyway on a semi-electrnic pump as advance settings are strictly under the control of the ECU and cannot easily be influenced. If you had a fully mechanical pump fitted, some interesting experiments could be done in trying different advance settings, especially in cold weather.

Do you want a mechanical pump?

[CitroJim]

On the subject of injection pressure, this will be influenced by the viscosity of the oil and given that I believe veg is thicker, the base delivery pressure from the pump will be greater. This, if I'm correct, will have a curious effect:

The point at with the injector starts to spray is determined by injection pressure. When the pressure reaches a set figure (as determined by the injector needle spring), the injector starts spraying. Now, if the basic pressure of fuel as delivered from the pump is increased, say due to its viscosity, theory suggests that the injector should start spraying earlier, thus advancing the timing very slightly. A sort of viscosity timing compensation.

That may all be a load of spheres, I'm just thinking out loud

The flame propagation in the combustion chamber is solely influenced by chamber design and injector spray pattern and those two factors are independant of the fuel being used.

What would be interesting though is to observe how differently (if at all) an injector atomises veg compared to pump diesel. You need to get hold of an old injector tester to see that.

[MikeT]

Hi Jim, if the outcome is possibly retarded timing, I'm happy with that as it could actually compliment N2O injection.

I may go for a pump change but will have to discuss that more with you though.

[CitroJim]

Sorry, this thread has got me fired up somewhat

My earlier statement that the timing cannot be influenced on the sem-electronic pump because of the ECU is not entirely correct.

Thinking aloud again, if you were to look at the signal from the needle lift sensor and TDC sensor on a dual trace oscilloscope, triggered from the TDC sensor, it should be possible to determine the timing the ECU has set and calculate how many degrees before TDC injection commences.

If that is so, it should be possible to adjust the timing by inserting an electronic delay between the needle lift sensor and ECU, thus fooling the ECU into thinking the timing is more retarded than it actually is. The ECU should then advance the timing to compensate, resulting in more advance.

I don't know the range of timing adjustment available on the pump but judging by the rack length on the servo and how horribly the engine runs in the absence of a lift signal, it must be quite a bit.

You could, by using a processor and a bit of programming, make a piggy-back ECU that could set your choice of timing for any given set of conditions.

Perhaps this is how these power boosters work?

[CitroJim]

Hi Jim, if the outcome is possibly retarded timing, I'm happy with that as it could actually compliment N2O injection.

Yes, if all we have discussed here holds true then the net result will be slightly retarded timing. This may give rise to the talk I have heard that using veg causes the engine to run hot; a retarded engine generally runs hotter.

If it were a petrol turbo you were N2O injecting, it would make sense to have slightly retarded timing as a safeguard against detonation but in a diesel it is of little consequence as detonation is not nearly such an issue.

I have an idea for how to more simply increase fuelling when running N2O. I shall discuss in due course

[MikeT]

That's what I'm thinking Jim. Injector pressure is raised and the injectors "fire" earlier. Obviously, as the fuel is heated, the viscosity is reduced but at room temperature, I've read it can be appx 7 times thicker than diesel.

From what I've learnt, ECU's can be re-programmed, intercepted and hacked in all manner of clever ways, depending on original design and sensor inputs. Where to get in seems to be the trickiest part, the rest is pure logic presumably 8)

[elma]

I follow what you're saying. But why would thicker fuel make a higher pressure? Pressure is about collisions between particles of the fuel and the container per unit time. I don't see how viscosity would effect this.
Think if you had a footpump and a pair of perfectly sealed mcdonalds cups.
One contains milkshake and the other coke.
If you pumped 20 psi down the straw of each, with the footpump, then both would be at 20 psi.
Just because the milkshake is thicker does not mean it will be at higher pressure.
I may have missed the point here, again, but this doesn't compute for me.

[MikeT]

...if...will be slightly retarded timing. This may give rise to the talk I have heard that using veg causes the engine to run hot; a retarded engine generally runs hotter.

All I know is this car is very hot under the bonnet compared to previous petrol engines I've owned.

If it were a petrol turbo you were N2O injecting, it would make sense to have slightly retarded timing as a safeguard against detonation but in a diesel it is of little consequence as detonation is not nearly such an issue.

I've been told N2O increases flame speed, hence my enquiry. I've learnt that with lower doses of nitrous, pre-detonation actually isn't a concern due to the low boiling temperature of the liquid to a gas (injection) and followed by the release of nitrogen (combustion).

I have an idea for how to more simply increase fuelling when running N2O. I shall discuss in due course

I've learnt a little more how the afore mentioned "fuel plunger solenoid" works but I bet you're there already

[MikeT]

elma, the fuel isn't pressurised by air

I could only think of a crude analogy to make the distinction but as this is the 'net I shall refrain from talking about taking a dump

[admiral51]

elma
if you think along the lines of a garden hose without a nozzle and then with a nozzle the pressure is induced by trying to fit a given volume of water through a smaller exit hence why it travels further with the nozzle

if the liquid is thicker the greater the pressure created in trying to force it through or at least thats how i understand it

mike
i admire your restraint

colin

[CitroJim]

I follow what you're saying. But why would thicker fuel make a higher pressure?

I see what you're saying Elma. I was thinking of how an engine, when cold has a lot more oil pressure than when hot, due to oil viscosity. I was imagining much the same happening in an injection pump handling thicker oil.

As I say, I'm thinking aloud and may well be talking a load of old spheres...

[elma]

if the liquid is thicker the greater the pressure created in trying to force it through or at least thats how i understand it

I do believe you are correct. If you look at my analogy backwards the milkshake is far harder to suck through the straw than the cola.

Still not totally sure if this causes a pressure rise though, both are pulled up the straw because they want to equalise with the vacuum caused by sucking. I will think about it again tomorrow when I'm not hung over. I have a feeling we may be confusing pressure with surface tension or specific gravity or another scientific term.

[admiral51]

if the liquid is thicker the greater the pressure created in trying to force it through or at least thats how i understand it

I do believe you are correct. If you look at my analogy backwards the milkshake is far harder to suck through the straw than the cola.

Still not totally sure if this causes a pressure rise though, both are pulled up the straw because they want to equalise with the vacuum caused by sucking. I will think about it again tomorrow when I'm not hung over. I have a feeling we may be confusing pressure with surface tension or specific gravity or another scientific term.

if i remember correctly 1 litre of water =1 kg which is the base figure for specific gravity(SG). 1 litre of derv = 750g hence a SG of 1:0.75 for derv water has a SG of 1:1.
the higher the SG the more force(pressure) is needed to move it a given distance. as the derv is being moved through the pipes continuously it has a constant pressure on it which should increase only at the narrowist point the injectors.
my assumtion is that as veg oil is heavier than derv it requires a greater force to move it and therefore the pressure at the injectors will get the same as for derv but only quicker with less volume of liquid. if the injectors/timing is set at a certain pressure/volume then the "mixture" for burning will be to lean (sorry thats a petrol term but heyho) and run slightly lumpy and increase engine heat.
i would say it would be very difficult to detect the lumpiness as the "mixture" would be only fractionally out

now that lot has just hurt my brain so im off to tackle mr j smith and friends and await the above to be disected by people who really know what they are talking about

and im sure surface tension is involved as well

colin

[elma]

Ooh, I really should know this. I've spent the last 3 years studying physics, maths, chemistry and optics. I'm claiming hang over and I'll know what I'm on about tomorrow.