Blending update

[tbird650 Senior Member Find all posts View Profile Close tbird650]

Re: Blending update

There could be a number of possibilities.
First thing about the Hiace is the engine compartment is very, very cramped and there's heatproof insulation preventing heat from soaking into the interior of the van itself. Any heat that's in the engine compartment can't rise and is to a certain extent trapped. I reason that after shutdown, the fuel in the injectors and injector lines will quickly rise to the prevailing engine temp. If there's any volatile component in the blend it may aerate. If this is the case it would explain the hot restart problem. It's nothing that some extra cranking won't fix but the starter must be getting extra workout over time.

I suppose another possible is your petrol could have less volatile additives. That may be more your field?

The issue was more pronounced if the vehicle does more than... say 30mins running and moreso when the ambient temps are higher.

In my case at least, what I can say is that blends without petrol hot re-start significantly better. What I'm doing now is working from the opposite end of the scale and adding progressively more ULP, but starting from scratch. So far I haven't noticed any deterioration with a 1.5% petrol blend. Next I'll try 2 or 2.5%. That'll be probably a couple of weeks away.

I notice you're keen on the experimenting so I was thinking that an experiment on the stabilty of a blend could be done by progressively adding heat and monitoring the boiling or bubbling threshold temps. It would need a bit of careful consideration as to how to capture the data accurately. Just a thought.

[Captaincademan Senior Member Find all posts View Profile Close Captaincademan]

Re: Blending update

I am still amazed that you are getting vapour locks on the pressure side of the IP. I would have thought that the ambient pressure of the fluid would prevent any vapours from forming as they left solution. How many PSI does your IP operate at in the fuel lines?

I know that vapour locks occur in a petrol fuel injected motor on occasions (even then not often on the pressure side of the pump - thats one reason that fuel pumps are located in the fuel tank), but I thought the operating pressures were well south of that of a modern Diesel engine.

A few numbers here to think about. How hot does it really get in the engine bay? I would have thought that it wouldn't climb over 150 on the exhaust side? At the worst case you would be lucky to get 150 degrees on the intake side, but for the sake of the exercise lets assume so. (unless anyone has some numbers they can add). So the question is at what temperature do you need to raise the fuel (blend) to in order to boil it at the ambient pressure inside the fuel lines? Pressure has a dramatic effect on boiling point. At the PSI that should exist inside your fuel lines, I doubt it would boil at 150 degrees.

Has anyone got any data to add to this?

[tbird650 Senior Member Find all posts View Profile Close tbird650]

Re: Blending update

Of course we are speculating that the issue is indeed vapour lock. The extra cranking would appear to fit the symptoms though.

One more thing that happened one time, was I had some thinners which was only marked "thinners mix". I made a small batch of fuel using this along with other distillates. This was particularly hard hot starting but the exact composition will remain a mystery.

Another time I'd overdone the thinners was with toluene, acetone and may be one other (I think it was). On this occasion the van ran quite well for an hour or so.
Then after a slightly difficult start, while driving home, streams and streams of bubbles could be seen in the return line. I was struggling to keep up with traffic!
I should note that this was at a time when I had an inboard fuel tank so wasn't having the benefit of any cooling of fuel lines. I believe the fuel was getting
hotter and hotter because hot fuel was returning to tank, then upon being fed back to the IP, became hotter still. Kinda like a runaway effect.

Just puzzling over the possibilities, after a motor is shut down, I wonder if fuel that's in the injector lines can escape after a while through the injector return lines.
These are the row of banjos on the injectors themselves that re-route the diesel that's left over from the injection process. On my Toyota, these are plumbed to the return
banjo on the IP, then onward back to the tank. If fuel can slowly bleed back through this return line rail, it could explain how air could be in the injector line. It could effectively be displacing the fuel.

[Tony From West Oz Vice Chairperson of WARFA Find all posts View Profile Close Tony From West Oz]

Re: Blending update

The injector return lines return to near atmospheric pressure when the engine is off.
Following each injection, the injection line pressure is released into the return line by the injection element, regardless whether it is an inline or rotary injection pump.
This occurs because the metering of fuel delivery is controlled by a "spill port", which opens when the required amount of fuel has been injected.
shows the operation of the spill port.

When the injector pressure is reduced, it has a valve which opens to release pressure, to prevent dribbling.

When the engine is not running, the pressure in all, or all except one (possibly being at the start of injection?), injection lines will be at atmospheric pressure, due to the action of these valves.

At atmospheric pressures and temperatures of >90°C, petrol will easily boil out of the blend in the injector lines, raising the pressure a little and pushing the fuel into the return lines, severely affecting hot starting of the engine. This does take time to occur, so hot starts after a short stop may be easier than after a longer stop.

During cranking, the starting volume of fuel is pumped into the injector lines, but until the vapours are condensed as the pressure rises, the injector cannot reach injector opening pressure. When the injector lines are full of fuel (blend) the injection pressures are reached and the engine will start, but may have a rough start due to different heating of some injector lines having boiled more petrol out of the blend than other lines, thus needing more fuel to be pumped to fill that injector line before the pressure rises enough for that injector to open.

As the engine cools, the condensing of the petrol will reduce the pressures in the injector lines and suck the blend back into the injector lines, either from the IP or from the injector return lines, allowing relatively easy starts.


I hope this helps explain the pressures involved and why it is an issue with hot starts but not cold starts.

Regards,
Tony

[Jeffrey S. Brooks Banned Find all posts View Profile Close Jeffrey S. Brooks]

Re: Blending update

When the engine is not running, the pressure in all, or all except one (possibly being at the start of injection?), injection lines will be at atmospheric pressure, due to the action of these valves.

At atmospheric pressures and temperatures of >90°C, petrol will easily boil out of the blend in the injector lines, raising the pressure a little and pushing the fuel into the return lines, severely affecting hot starting of the engine. This does take time to occur, so hot starts after a short stop may be easier than after a longer stop. Regards,
Tony

I agree with all, but this part of Tony's description of the injection process as it applies to ULP-based blends. I have a rotary IP and I have worked on my injectors and injector pump numerous times, and each time I crack open the injector pipes significant pressure is released, so I doubt that the pressure drops off due to time within the short time of parking a vehicle, unless you have leaky injector pipes.

Tbrid, while you and I have different diesel engines, it seems like there are significant similarities between them. For instance a common problem for the 6.2L Detroit Diesel is a hot restart problem, which has nothing to do with burning alternative diesel fuels. In that case the hot restart problem is a sign the IP is beginning to go. The short term solution is to open the bonnet and dump cold water on the IP. The solution is to replace the IP. So, maybe in your case this is true, but only observed with ULP blends, because ULP in the blend exacerbates the pending hot restart problem.

However, if I recall correctly, you had been measuring the temp of your return line and you had observed temps as high as 60C. Now, I get that you have a cramped, and well insulated engine compartment, but so do I in my 1983 Chevy G-20 van. However, I do not notice boiling returning fuel. So, perhaps there is something different about your fuel line, or the return line, such as: they are in close contact with the engine block, and/or the exhaust pipes and/or manifold?

Your experiments of slowly raising the percentage of ULP in your blend is a good one, and I look forward to reading your reports.

On your hypothesis that there are lighter fractions in the US variety of ULP, verses the Aussie version, I am not sure that would apply; however, there might well be more alcohol in USA ULP, because we have a mandate of 10% in all of our ULP. However, I would not think that the presence of ethanol would substantially raise the boiling point of our ULP, if anything it might lower it.

[tbird650 Senior Member Find all posts View Profile Close tbird650]

Re: Blending update

I hope this helps explain the pressures involved and why it is an issue with hot starts but not cold starts.

Excellent, well reasoned explanation there Tony.
This summarizes the situation quite well.
And, yes the dribble issue would be why pressure needs relieving. It stands to reason.

As mentioned earlier, I'm going on a step-by-step trial to determine where the threshold amount of ULP% is for my vehicle.
I reckon each make and model could behave quite differently. JSB's Chevy appears to get away with a huge amount of blend ratio.

I take my hat off to JSB for taking the time and effort to report findings that might benefit us all in the blending community.
His work pushes the boundaries and tests assumptions. This forum is about helping each other! Thanks

[smithw Senior Member Find all posts View Profile Close smithw]

Re: Blending update

Hi T Bird
toyotas also get had to start when hot if they have a cracked head, might be worth trying a tank of dino and see if the problem is still there.

[tbird650 Senior Member Find all posts View Profile Close tbird650]

Re: Blending update

Hi smithw
Thanks for the thought. The problem isnt there now.
My current blend has only 1.5% ULP and the hot re-starting is quite acceptable.
Cold starts fire up after perhaps 2 compressions, virtually instant.
Hot restarts would add maybe 1 or 2 extra compression strokes before firing.

Re the ULP testing. I'm looking to find the max amount of ULP% that still gives a good hot re-start.
I'll keep upping the % till I get unacceptable deterioration and then back off the amount to the "sweet spot"

[Tony From West Oz Vice Chairperson of WARFA Find all posts View Profile Close Tony From West Oz]

Re: Blending update

Tbird650,
My Mercedes 300D is much quicker at starting when warm or hot. It is on 100% veggie, so ULP is not an issue at all. I believe it fires on the first compression when warm or hot and on the second or third compression, after glowing a suitable time for the ambient temperature, for cold starting.

For a diesel engine in good condition, I believe that this should be the case.

For a diesel engine with poor compression, cold starting is harder because less heat of compression is generated during cranking. the same engine, hot, has the advantage of a higher initial air temperature in the cylinder before compression, resulting in adequate air temperature at injection, for the injected fuel to ignite.

I hope this helps fill some gaps,
Tony